Catalytic Reaction of Carbon Dioxide with Methane on Supported Noble Metal Catalysts

: The conversion of CO 2 and CH 4 , the main components of the greenhouse gases, into synthesis gas are in the focus of academic and industrial research. In this review, the activity and stability of different supported noble metal catalysts were compared in the CO 2 + CH 4 reaction on. It was found that the efﬁciency of the catalysts depends not only on the metal and on the support but on the particle size, the metal support interface, the carbon deposition and the reactivity of carbon also inﬂuences the activity and stability of the catalysts. The possibility of the activation and dissociation of CO 2 and CH 4 on clean and on supported noble metals were discussed separately. CO 2 could dissociate on metal surfaces, this reaction could proceed via the formation of carbonate on the support, or on the metal–support interface but in the reaction the hydrogen assisted dissociation of CO 2 was also suggested. The decrease in the activity of the catalysts was generally attributed to carbon deposition, which can be formed from CH 4 while others suggest that the source of the surface carbon is CO 2 . Carbon can occur in different forms on the surface, which can be transformed into each other depending on the temperature and the time elapsed since their formation. Basically, two reaction mechanisms was proposed, according to the mono-functional mechanism the activation of both CO 2 and CH 4 occurs on the metal sites, but in the bi-functional mechanism the CO 2 is activated on the support or on the metal–support interface and the CH 4 on the metal.


Introduction
Nowadays it is no longer necessary to emphasize that the increase of the CO 2 -one of the main greenhouse gases-concentrations in the atmosphere and what kind of dangerous changes it can cause. To date, fossil fuels as the main sources of energy, the relatively new technologies (cement production, iron, steel industry etc.), the revolution in transportation, and the rapid growth in global population have resulted in the dramatic increase of CO 2 emission. Despite different international agreements CO 2 emissions have continued to increase. The CO 2 concentration in the atmosphere in the last few 100,000 years always remained below 300 ppm, but now the CO 2 concentration is over 415 ppm (in May 2020 the monthly mean value was 417 ppm) [1]. According to lots of meteorologists and climate experts, higher CO 2 concentration in the atmosphere results in climate change, the rise in temperature. The global annual mean temperature increased in the last 100 years by 1.4 degrees [2]. The climate change seems to have become generally accepted-although there are always those who disagree with it-and this has resulted in a worldwide increase in interest in reducing greenhouse gases, particularly CO 2 , emissions.
If we want to prevent the increase of CO 2 concentration in the atmosphere we have to (i) hinder the enhancement of the carbon dioxide emission; (ii) use CO 2 capture and storage (CCS) technologies; or (iii) use CO 2 capture and utilization (CCU) technologies.
There is an ongoing debate as to whether CO 2 is a waste, or a wealth as feedstock. If CCS technology is used, CO 2 is considered waste, but if CO 2 is converted into a more valuable compound in a chemical reaction, it is not a waste but a feedstock.
The total amount of products that can be produced using technologies available today for CO 2 conversion is only a few hundred Mt, while CO 2 emissions from large stationary sources alone exceed 10 Gt. This means that traditional products and technologies do not really affect the CO 2 concentration in the atmosphere. One of the highest carbon emission sectors is transportation, so we have to look for the solution in these fields.
The reaction of carbon dioxide with methane, and dry reforming in particular utilizes two abundantly available greenhouse gases, to produce the industrially important syngas (CO + H 2 mixture). From the CO + H 2 mixture both fuel and methanol or plenty of other products can be produced using available technologies. This process could constitute a viable first step towards efficient CO 2 capture and utilization. The main problem is that dry reforming is a highly endothermic and equilibrium reaction (see later). However, CO hydrogenation is an exothermic process so these technologies could help in the storage of electricity because the greatest problem with renewable energy (wind and solar) production is the fluctuation, and the energy consumption is also uneven.
Opponents of the CO 2 + CH 4 reaction argue that carrying out this reaction requires a significant amount of energy. It is true that endothermic reaction consume energy. Synthesis gas is now produced worldwide by the steam reforming of CH 4 , which is also an endothermic process and the dry reforming requires only 20% more energy, but converts greenhouse gases into a chemical feedstock. The disadvantage of steam reforming is that the H 2 /CO ratio is too high to be used for methanol synthesis, although in the case of the CO 2 + CH 4 reaction this value is too low. The correct H 2 /CO ratio could be obtained by reforming of methane with CO 2 + H 2 O mixture.
Extensive research work has been conducted on different catalytic systems for the conversion of CO 2 + CH 4 , various catalysts have been developed but there have been only a few attempts to industrialize this reaction, so it is not surprising that the reaction of carbon dioxide with methane is in the focus of academic and industrial research.
The present review attempts to summarize the results obtained on supported noble metal catalysts. The main aim is to show the different results obtained on the similar catalysts, the different theories regarding the interaction of the reactants and the catalysts and the reaction mechanism.

The Thermodynamics of the CO 2 + CH 4 Reaction
Carbon dioxide reforming of methane is a highly endothermic reaction (1). Thermodynamic calculation indicates that this process is not spontaneous at 1 atm pressure below 906 K. When the reaction takes place between the products and between the products and the reactants, a number of secondary reactions can occur (2)- (7). CO The calculated equilibrium conversions of methane and carbon dioxide at different temperatures are collected in Table 1 [3]. Similar results were obtained by Bradford and Vannice [4] for a feed stream at 1 atm total pressure (CH 4 /CO 2 /He = 1/1/1.8). The CO 2 conversion was always higher than that of methane, due to the reverse water gas shift reaction. The methanation (5) and the reverse reaction of steam reforming (2) may also have contributed to the lower methane conversion. Different equilibrium conversions were calculated by Istadi and Amin [5] and by Nikoo and Amin [6]. The equilibrium methane conversion at 873 K was about 42% [5] while in the other work a value of 82% was reported [6]. The high CH 4 conversion in the latter case was explained by stating that the CH 4 decomposition (3) is the predominant reaction. Figure 1 shows the equilibrium product distribution for the CO 2 + CH 4 reaction using CO 2 /CH 4 feed ratio of 1/1 at a pressure of 1 atm [7]. This calculation cannot take into consideration the possibility of carbon deposition. According to this calculation water is formed up to 1173 K in the reaction and the CO/H 2 ratio is greater than one. These results indicate that the reverse water gas shift reaction also takes place.  The reactions responsible for carbon formation are the methane decomposition (3) and the Boudouard reaction (6). Carbon formation cannot be ruled out in the hydrogenation of CO and CO 2 . It was found [6] that of these processes only the methane decomposition is favored at high temperatures; while the other reactions are favored when the reaction takes place at lower than 800 K temperature.
When the carbon formation, i.e., the methane decomposition (3) and the Boudouard reactions (6), were taken into account in the calculation entirely different equilibrium composition was found ( Figure 2) [8]. In this case the amount of H 2 is more than that of CO, the H 2 /CO ratio is far greater than unity because the increased carbon formation lowers the amount of CO formed and hence increases the H 2 /CO ratio.  According to Wang et al. [9] the reverse water-gas shift (4) and the Boudouard reaction (6) will not occur at above 1093 K, and the Boudouard reaction and methane decomposition (3) will be mostly responsible for the formation of carbon at temperatures ranging from 830 K to 973 K.

Comparing the Activity of Different Catalysts in the CO2 + CH4 Reaction
The possibility of a reaction between methane and carbon dioxide was already mentioned by Lang [10] in the late 19th century, and in 1928, Fischer and Tropsch [11] showed that different metals have different activities for the dry reforming of methane and that the group VIII metals show appreciable activity for this reaction. However, intensive research did not begin until the second half of the 20th century. Since then, a variety of different supported metals were used as catalysts. Most of the group VIII metals, except Os, are more or less active in the CH4 + CO2 reaction. The results obtained are summarized in different reviews [4,[12][13][14][15]. The biggest problem was in most cases the According to Wang et al. [9] the reverse water-gas shift (4) and the Boudouard reaction (6) will not occur at above 1093 K, and the Boudouard reaction and methane decomposition (3) will be mostly responsible for the formation of carbon at temperatures ranging from 830 K to 973 K.

Comparing the Activity of Different Catalysts in the CO 2 + CH 4 Reaction
The possibility of a reaction between methane and carbon dioxide was already mentioned by Lang [10] in the late 19th century, and in 1928, Fischer and Tropsch [11] showed that different metals have different activities for the dry reforming of methane and that the group VIII metals show appreciable activity for this reaction. However, intensive research did not begin until the second half of the 20th century. Since then, a variety of different supported metals were used as catalysts. Most of the group VIII metals, except Os, are more or less active in the CH 4 + CO 2 reaction. The results obtained are summarized in different reviews [4,[12][13][14][15]. The biggest problem was in most cases the rapid deactivation of the catalysts due to the significant carbon deposit. This may also be the reason of different activity orders published by different research groups. Catalysts based on noble metals are less sensitive to coking than Ni containing samples.
Theoretically the H 2 /CO ratio is unit, but due to the reverse water gas shift reaction and to the other secondary reactions this ratio is often less.
The reforming reaction of CH 4 with CO 2 has been studied over different supported transition metals. It was found that at 1073 K the turnover frequencies (TOF) on Al 2 O 3 supported catalysts (Ru, Rh, Pt, Ir, Ni) were within the same order of magnitude. The value obtained for the best Ru (4.0 s −1 ) was only slightly more than twice as for the weakest Ni catalyst (1.8 s −1 ) [16]. This order deviate from those found by Rostrup-Nielsen and Bak Hansen [17] who obtained Ru > Rh ≥ Ni > Ir > Pt > Pd order (Table 2). No significant differences were found at 1050 K in the activities of the alumina supported metal catalysts [21,22] only in the cases of Ru and Pd were significantly lower conversions observed; in the other cases they approached the equilibrium values. The Ni and Pd catalysts were very active at first (especially Ni), then rapidly showed a decline in conversions, and a considerable deposition (up to 40%) of carbon occurred within a few hours. The Ir/Al 2 O 3 catalyst maintained its very high performance for over 200 h, with no evidence for deactivation, and only less than 0.3% carbon was observed on the used catalyst.
The order of catalytic activity at 723 K on turnover frequency basis was found to be Rh > Ni > Ir> Ru~Pt > Co over the alumina supported catalysts [20] (Table 2). A different sequence was found for SiO 2 supported metals: Ni > Ru > Rh~Ir > Co~Pt [20]. This indicates that the support has a great influence on the specific activity of the catalysts.
Over different MgO supported metal catalysts the activities depended upon the metals in the sequence Ni, Ru, Pt >> Rh > Pd [23]. The amounts of CO were greater than those of H 2 possibly due to the reverse water gas shift reaction. Different results were obtained at 1073 K on MgO supported noble metals [24]. The methane conversion was the highest on Rh and the activity decreased following the order Rh > Ru > Ir > Pt > Pd. CO 2 reforming of CH 4 has been also studied on catalysts containing less than a monolayer of Rh, Ru, and Ir deposited on α-Al 2 O 3 , MgO, CeO 2 , La 2 O 3 , and TiO 2 [25]. It was found that the activity, the turnover frequency of CH 4 consumption on Rh/α-Al 2 O 3 was four times higher than on Ru and Ir. The high stability of the noble metal catalyst comparing with the Ni was attributed to the high oxidative properties of compounds formed on the catalyst surface, which inhibit the carbon growth reactions [25].
The turnover frequency for CO formation in the dry reforming of CH 4 on SiO 2 and TiO 2 supported transition metals depends on both the support and the %-d character of the metal [26]; however, different trends were observed for metals on SiO 2 and TiO 2 . The activities of the TiO 2 supported metals were higher than those of SiO 2 based catalysts in all cases. Pt/SiO 2 has an outstanding activity among the silica supported samples and it decreased in the following order at 723 K: Pt >> Pd > Ni > Rh > Fe. Among the TiO 2 supported catalysts the Rh activity was the highest. In this case the turnover frequency for CO formation decreased in the Rh > Ir > Ru > Pt > Pd > Ni > Fe~Co~Cu order [26].
The methane conversion at 923 K decreased on pillared clay impregnated by different metals along the 3% Rh > 10% Ni > 3% Pd > 3% Ni > 3% Ce sequence [27]. The H 2 /CO ratio was 1.1 for Rh and 0.85 for Pd which is metallic at the temperature of the reaction but it was even lower for Ni and Ce.
Comparing the activities of the ZnLaAlO 4 supported Ru, Ni, and Pt catalysts reveals that the best performance was observed for 3% Ru/ZnLaAlO 4 while the 3% Pt shows the lowest activity. These results were attributed to the synergistic effect of the interaction between the support and the active metals [28].
Only the activities of noble metals have been compared in several publications [18,[29][30][31][32][33] and great differences were found in their catalytic behavior as concerns the CH 4 + CO 2 Catalysts 2021, 11, 159 6 of 30 reaction. The reaction occurred at the highest rate on Rh, followed by Pt, Pd, Ru, and Ir. Little or no deactivation of the catalyst occurred during the reaction at 823 K.
The conversion of CO 2 exceeded that of CH 4 for every catalyst sample indicating that the dry reforming reaction was followed by several secondary processes, including the methanation of CO 2 and CO, reverse water gas shift reaction and the Boudouard reaction. The turnover frequency for CH 4 or CO 2 consumption at 823 K on the alumina supported 1 wt% of noble metals decreased in the sequence Ru > Pd > Rh > Pt > Ir [18] (Table 2). This is practically the same as the order for CO methanation [34], and the sequence for CO 2 methanation differed only in that Pd was the least active sample [35]. In contrast at 923 K the order Rh > Ru > Ir >> Pd, Pt was determined for catalysts with 0.5% metal supported on Al 2 O 3 [17] (Table 2) but others found that the activity decreased in the order Rh > Pd > Pt >> Ru [29]. Rh, Ru, and Ir catalysts showed stable long term activity but Pd and Pt containing catalysts deactivated rapidly. On alumina-stabilized magnesia and Mg-Al mixed oxide supported Pt metals at 973 K a different sequence was observed [30,32,33] (Ru > Rh > Ir > Pt > Pd).
The order of activities for ZrO 2 -supported catalysts was Rh > Pt > Ir, Pd > Ru [36]. This sequence is different from that reported for MgO-supported materials [19,24]. The observed differences suggest that the basicity of the support may have some significant effect.
Stable CH 4 and CO 2 conversion was observed for 50 h on Ru substituted LaAlO 3 (LaAl 0.98 Ru 0.02 O 3−δ ) catalyst. This indicates that it is a highly stable and coke resistant catalyst. It has been suggested that the thermal stability of the sample is due to a strong interaction between Ru and Al. In contrast, Pt and Pd incorporated into LaAlO 3 lost their activities due to sintering and carbon deposition [37].
The activities of Rh, and Pt supported α-Al 2 O 3 doped with La 2 O 3 and BaO were compared in the dry reforming of methane [38]. It was found that Rh is more active than Pt independently of the support composition. La incorporation induces a rate enhancement in the conversion of methane in all cases, but Ba incorporation in Pt/Al 2 O 3 leads to loss of activity because there is a preferential interaction between Pt and Ba, in the case of Rh this had no effect.
Ru-and Rh-catalysts supported on Mg-Al mixed oxides revealed activity in the combined partial oxidation and dry reforming of methane to synthesis gas at 1123 K which was markedly higher than that of Pt. Ru and Rh approached their maximal activity which was limited by the equilibrium [39].
Ru and Ir supported on Eu 2 O 3 were active in the dry reforming reaction, but Ru showed an onset of activity around 673 K, whereas the Ir showed initial activity at a much higher temperature, around 823 K [40].
The activity of BaZr (1−x) Me x O 3 (Me: Ru, Rh, Pt) perovskite type oxides [41] decreased with the active species as Rh > Ru > Pt. The better activity of Rh-and Ru-containing samples was linked to their highl-reducible nature.
The brief summary above clearly shows that noble metals are effective in the CO 2 + CH 4 reaction. Despite the high costs of noble metals, their superior catalytic properties at low temperature still make them indispensable to understand the interaction between the metal and the support, to check the surface compounds formed during the reaction and to show the elementary steps of the reaction, and as additives appear to be unavoidable for the development of an effective and stable catalyst.

Supported Rhodium
The above summary clearly shows that Rh is one of the best and most stable catalysts of the reaction.
The specific activity of Rh catalysts was found to strongly depend on the support [42][43][44][45]. It decreased in the order yttria-stabilized zirconia (YSZ) > Al 2 O 3 > MgO > TiO 2 > SiO 2 [43,44]. This order correlates directly with the acidic character of the support [43]. A strong dependence was observed on the specific activity of the methane reforming reaction and the rate of deactivation of the catalysts on rhodium particle size in the range 1-6 nm. Both activity and rate of deactivation decrease with increasing metal particle size. The degree of these dependencies was largely affected also by the nature of the support. Relatively high deactivation rates were observed over Rh/TiO 2 and on Rh/MgO, but on Rh/YSZ and on Rh/SiO 2 no deactivation was observed. Three kinds of factors were suggested which contribute to the catalyst deactivation carbon deposition, metal sintering, and the poisoning of surface Rh sites by species originating from the carrier [43]. The TOF for 0.5% Rh/YSZ at 923 K was about five times higher than for the less active SiO 2 or TiO 2 supported catalysts in the steady state [44].
Others found no significant difference in the catalytic activity at 773 K on 1% Rh supported on A1 2 O 3 , TiO 2 , SiO 2 , MgO (Table 3) and no decay in the activity was experienced and small amount if any carbon deposit was observed [46]. In contrast a significant effect of the support was found on the reaction rate on Rh supported on the same oxides, where the order was Rh/Al 2 O 3 > Rh/TiO 2 > Rh/SiO 2 . The catalytic activity of Rh/SiO 2 increased significantly by physically mixing the catalyst with metal oxides such as A1 2 O 3 , TiO 2 , and MgO, indicating a synergistic effect. The role of the metal oxides used in mixture according to the authors may be ascribed to promoting in dissociation CO 2 dissociation on the surface of Rh, since the CH 4 + CO 2 reaction is first order in the pressure of CO 2 , and −0.6 for CH 4 , suggesting that CO 2 dissociation is the rate-determining step [50,51].
The reforming of methane with carbon dioxide was investigated on Rh supported on reducible (CeO 2 , Nb 2 O 5 , Ta 2 O 5 , TiO 2 , and ZrO 2 ) and on irreducible (γ-Al 2 O 3 , La 2 O 3 , MgO, SiO 2 , and Y 2 O 3 ) oxides [45]. The reducible oxides supported Rh provided much lower conversion of CH 4 or CO 2 than the irreducible one. One possible explanation is that the Rh particles were partially covered by partially reduced oxide species (SMSI effect). Among the irreducible oxides supported catalysts, γ-Al 2 O 3 , La 2 O 3 , and MgO showed stable activities, and it increased in the sequence: La 2 O 3 < MgO ≈ γ-Al 2 O 3 . Possible explanations for stability, based on the strong interactions between Rh 2 O 3 and support were proposed. Among the reducible oxides, Nb 2 O 5 supported Rh has a low activity, Rh/ZrO 2 and Rh/CeO 2 exhibited a very long activation period, and deactivation occurred over Ta 2 O 5 and TiO 2 . MgO and γ-Al 2 O 3 supported Rh are the most promising catalysts; they provided a stable high activity with a CO yield of 83-85% and a H 2 yield of 76-79% at the high space velocity of 60 000 mL/h/g [45].
The turnover frequency for methane conversion in the CO 2 + CH 4 reaction on different oxide supported Rh catalysts decreased in the following order: Rh/TiO 2 > Rh/La 2 O 3 = Rh/CeO 2 > Rh/ZrO 2 = Rh/MgO = Rh/SiO 2 = Rh/MCM-41 > Rh/Al 2 O 3 . The outstanding activity of Rh/TiO 2 was attributed to the fact that Rh remained in metallic state during the reaction, while on Rh/Al 2 O 3 according to XANES characterization cationic Rh was generated [42].
The highest yield in hydrogen in the dry reforming of CH 4 was obtained on differently doped alumina supported Rh in the following order: Rh/NiO-Al 2 O 3 > Rh/MgO- The high activity of Rh/NiO-Al 2 O 3 was explained by the presence of NiAl 2 O 4 spinel preventing the deactivation of Rh migration in alumina and the high dispersion of Rh favored by the presence of Ni particles at the support [52].
Rh/NaY effectively catalyzes the conversion in the CO 2 + CH 4 reaction to CO and H 2 with a H 2 /CO ratio near unity even with low metal loadings [53,54]. Remarkable is the stability of these catalysts under operating conditions. The reason for this, according to the authors is the total absence of carbonaceous deposits. The Rh particles are quite uniform, having a size of~4 nm [53]. These particles are available in the early stages of the catalysis and the zeolite framework protects the encaged particles against further growth. Therefore, catalyst deactivation is not observed under these conditions which are significant for Rh on amorphous supports [53,54].
Unlike other results [54,55] no carbon formation was observed on Rh/Al 2 O 3 in the temperature range of 873-1073 K [56]. Although the activity of the catalyst decreased slightly over time, this was attributed to the sintering because the catalysts could not be regenerated. This catalyst has been successfully tested in large scale experiments in solar panels. The reforming reaction at 923 K over Rh/Al 2 O 3 results in the accumulation of three kinds of carbon species. The ratio of different carbon species depended on the reaction temperature and on the time of exposure. These results support the earlier finding that the transformation of the active form into a less active form occurs with increasing reaction time and the temperature [57]. The amount of carbon deposit decreased at temperatures higher than 923 K probably because of the prevalence of the carbon gasification (8) and the reverse Boudouard reaction (6). It was found that the origin of carbon is mainly the CO 2 molecule, with a small contribution from the CH 4 molecule [44,55].
Bitter et al. [49] also found that the activity of supported Rh catalysts for CO 2 + CH 4 reaction is mainly determined by the availability of Rh irrespective of the support. Contrary to this, Portugal et al. [57] found that the turnover frequency for CO formation over supported Rh at 723 K is strongly affected by the support; it decreased in the following order: Rh/Al 2 O 3 > Rh/Nb 2 O 5 > Rh/TiO 2 > Rh/NaY (Table 3). The higher activity of the oxide-supported catalysts was explained by the higher degree of contribution of the reverse water-gas shift reaction. Similarly, a dominant role of the carrier was observed, the turnover frequency for CH 4 or CO 2 consumption at 1023 K decreased in the Rh/α-Al 2 O 3 > Rh/La 2 O 3 > RhCeO 2 > Rh/MgO > Rh/TiO 2 order [25] (Table 3).
Alumina supported Rh catalysts with a wide range of dispersions (25.1-69.0%) and Rh particle size (1.4-4.0 nm) were prepared by varying the metal content (0.1-1.6 wt%) and the thermal treatment temperature (923-1123 K) [58]. It was found that the turnover rates increased monotonically with increasing Rh dispersion. A similar result was obtained on different NaY zeolite supported Rh, too [47]. This observation suggests that coordinative unsaturated Rh surface atoms, which are more common in small clusters, are more active than those in low-index surface planes on larger Rh crystallites. Edge and corner atoms appear to bind CH x and H more strongly and to decrease the energy required to form relevant transition states for the initial C-H bond activation. They assume that supports can influence metal dispersion and, only in this manner, affect forward turnover rates [58]. It should be noted that a reduction of Rh particle size result in an enhanced rate of deactivation in the case of supported Rh [43].
Contrary to the above findings, it was found that over a wide range of metal content of the catalysts (0.5-5%) and supports (α-and γ-Al 2 O 3 , ZrO 2 , TiO 2 , SiO 2 ), the turnover frequency for CH 4 consumption at 1023 K does not depend significantly on dispersion, indicating that CO 2 reforming at high temperatures is structure insensitive. No direct influence of the microstructure of the support on the reaction rates was detected. The effect of the support seems limited to the stabilization of the metal surface area, which is responsible for the activity of the catalyst [17].
Rh/Al 2 O 3 promoted with V 2 O 5 or TiO 2 were more active in the CO 2 + CH 4 reaction at 773 K than the unpromoted catalyst [59]. The increase of the conversion was attributed to the oxygen vacancies which formed on the promotors during the pretreatment of the catalysts. Vanadium oxide promotion enhances the activity of Rh/SiO 2 too and decreases the deactivation by carbon deposition [48]. This effect was attributed to the formation of VO x overlayer on the Rh which thereby reduces the size of accessible ensembles of Rh atoms required for coke formation. The new sites at the Rh-VO x interface were considered to be active for the dissociation of CO 2 .
The activity and coke resistance of Rh/Al 2 O 3 were enhanced by the addition of CeO 2 . It was suggested that this effect is associated with the presence of two redox couples, which favors the activation of both CH 4 and CO 2 [60]. A Rh-CeO 2 interaction was induced by high temperature reduction, which resulted in the formation of oxygen vacancies in ceria. The electrons could transfer from Rh to CeO 2 on the perimeter. The Ce 4+ /Ce 3+ and Rh 0 /Rh δ+ redox couples were found to coexist in Rh-CeO 2 /Al 2 O 3 catalyst. CH 4 decomposition on Rh released electrons, whereas CO 2 fill up the oxygen vacancies by accepting electrons. The Ce 3+ species promote CO 2 dissociation into CO and surface oxygen, which reacts with CH x to form CO [60].
Slow deactivation of 2% Rh/CeO 2 was observed at 1023 K during 70 h of the reaction, the methane conversion decreased from the initial value of 48.6 to 44.1%., which was attributed to catalyst sintering [61,62], the role in the deactivation of the coking was neglected.
Rh/ZSM-5 zeolite was found to be an active catalyst for dry reforming of methane, the reaction at 773 K was approaching the equilibrium product distribution, but adding 4-5% ethane to the reacting gas mixture lead to carbon deposition and to the deactivation of the catalyst [63].
Surprisingly, only a few studies reported on the effect of sulfur-containing compounds on the CO 2 + CH 4 reaction, although natural sources like natural gases and bio gases as possible candidates for being starting materials in this reaction, often contain more or less sulfur-containing compounds. Prichard et al. [64] did not observe any deactivation for 0.5% Rh/Al 2 O 3 catalyst at 1068 K in the presence of 543 ppm dimethyl sulfide. In their opinion, this is caused by the high temperature, because at this temperature the interaction between the sulfur and the catalyst was limited.
It was found that H 2 S suppresses the formation of CO and H 2 in all cases, but on Rh/TiO 2 and on Rh/ZrO 2 the CO/H 2 ratio increased in time, indicating that H 2 S poisons the secondary processes of the reaction to different extents. Therefore, the effect of H 2 S on the CO 2 hydrogenation and the steam reforming of methane were investigated. It was found that the rate of methane formation in the CO 2 + H 2 reaction increased on TiO 2 and on CeO 2 supported metals (Ru, Rh, Pd), but on all other supported catalysts or when the H 2 S content was higher than 22 ppm the reaction was poisoned [66]. The promotion effect of H 2 S was explained by the formation of new centers at the metal-support interface. gases as possible candidates for being starting materials in this reaction, often contain more or less sulfur-containing compounds. Prichard et al. [64] did not observe any deactivation for 0.5% Rh/Al2O3 catalyst at 1068 K in the presence of 543 ppm dimethyl sulfide. In their opinion, this is caused by the high temperature, because at this temperature the interaction between the sulfur and the catalyst was limited.
The effect of H2S (22 ppm) on the reaction of CO2 with CH4 at 773 K over TiO2, SiO2, Al2O3, ZrO2, and CeO2-supported Rh catalysts was examined [65] (Figure 3). It was found that H2S suppresses the formation of CO and H2 in all cases, but on Rh/TiO2 and on Rh/ZrO2 the CO/H2 ratio increased in time, indicating that H2S poisons the secondary processes of the reaction to different extents. Therefore, the effect of H2S on the CO2 hydrogenation and the steam reforming of methane were investigated. It was found that the rate of methane formation in the CO2 + H2 reaction increased on TiO2 and on CeO2 supported metals (Ru, Rh, Pd), but on all other supported catalysts or when the H2S content was higher than 22 ppm the reaction was poisoned [66]. The promotion effect of H2S was explained by the formation of new centers at the metal-support interface.
Rh supported on La stabilized γ-Al2O3 has an outstanding catalytic activity for dry reforming of methane at 1073 K, but it was severely poisoned by even small amounts (below1 ppmv) of sulfur in the feed [67]. The reverse water-gas shift reaction, occurring simultaneously with dry reforming, was only slightly affected by sulfur poisoning, approaching the equilibrium even at high S-levels. Sulfur inhibition was reversible and the Rh-catalyst slowly recovered its initial activity after the removal of sulfur from the feed.

Supported Ruthenium
Supported Ru catalyst is the best in some comparisons, while other studies show that it is one of the less active catalysts.
The support affected significantly the catalytic activity of supported Ru in the CO2 reforming of methane at low temperature. With 0.5% Ru/La2O3 at 873 K CH4 conversion reached 28.7% in stoichiometric CH4 and CO2 (1:1) mixture in a continuous flow reactor. Rh supported on La stabilized γ-Al 2 O 3 has an outstanding catalytic activity for dry reforming of methane at 1073 K, but it was severely poisoned by even small amounts (below 1 ppmv) of sulfur in the feed [67]. The reverse water-gas shift reaction, occurring simultaneously with dry reforming, was only slightly affected by sulfur poisoning, approaching the equilibrium even at high S-levels. Sulfur inhibition was reversible and the Rh-catalyst slowly recovered its initial activity after the removal of sulfur from the feed.

Supported Ruthenium
Supported Ru catalyst is the best in some comparisons, while other studies show that it is one of the less active catalysts.
The support affected significantly the catalytic activity of supported Ru in the CO 2 reforming of methane at low temperature. With 0.5% Ru/La 2 O 3 at 873 K CH 4 conversion reached 28.7% in stoichiometric CH 4 and CO 2 (1:1) mixture in a continuous flow reactor. On 0.5% Ru/Al 2 O 3 , it was only 5.7%. Y 2 O 3 and ZrO 2 exhibited higher catalytic activities in this reaction (25-29%), but SiO 2 or TiO 2 gave lower conversions of CH 4 (less than 12%) [68].
On a turnover frequency basis, the activity decreased in the order of Ru/TiO 2 > Ru/Al 2 O 3 >> Ru/C under conditions where heat and mass transfer limitations do not exist [69]. The catalytic activity under 0.1 MPa at 1023 K depends on the basicity of the supports (Ru/MgO > Ru/Al 2 O 3 > Ru/TiO 2 > Ru/SiO 2 ) indicating that the CO 2 adsorption ability, the formation of carbonates on the supports determine the efficiency of the catalysts. Similar conclusion was reached on supported Rh catalysts [50]. Under 2 MPa pressure the activity correlated with the BET surface area, the activity order was Ru/SiO 2 > Ru/Al 2 O 3 > Ru/MgO > Ru/TiO 2 . On Ru/TiO 2 carbon deposition was not observed, the lowest activity of this catalyst was attributed to the SMSI effect [70].
Comparing the activity of Ru/Al 2 O 3 , Ru/SiO 2 , and Ru/graphite [71] at 823 K it was found that at the beginning of the reaction the alumina supported catalyst has the highest activity, but it decreased continuously. The graphite-supported sample exhibits a rather stable activity, in contrast with the oxide-supported catalysts. The fastest deactivation rate is observed for Ru/SiO 2 , its activity decays dramatically during the first 5 h. For the explanation of this observation, it was supposed that the activation of both reactants takes place on the metal surface and the accumulation of carbon species formed from CH 4 decomposition on the metal finally hinders CO 2 dissociation and results in a rapid deactivation of this catalyst [72]. The outstanding activity of Ru/TiO 2 compared to Ru/Al 2 O 3 or Ru/C was attributed to the presence of interfacial Ru-Ti n+ -O x species which markedly enhanced the activation of the reactants.
Several research groups investigated the activity of Ru/Al 2 O 3 in the CO 2 + CH 4 reaction and obtained quite different results [17,18,25,31,62,69]. Direct comparison of the catalytic activities published in different papers is difficult due to the different reaction conditions used. Bradford and Vannice [69] extrapolated the results of each author to the same conditions using kinetic equations and it was found that there are two magnitude differences between the calculated turnover frequencies. These values correlate with the space velocity, the lower the space velocity the lower the TOF value. This observation was explained by the fact that the space velocity, the residence time influence reverse and secondary reactions and these result in differences in TOF values. The other possibility of the great differences in the turnover number is the effect of chlorine. Most of the catalysts were prepared using RuCl 3 and Cl could be detected with XPS on the surface. The Ru/Al 2 O 3 catalyst with the lowest TOF value was prepared with Ru 3 (CO) 12 [25]. Finally, it was concluded that it seems possible that Cl could promote the dry reforming of methane over supported Ru by facilitating CH 4 activation [69]. The particle size also influenced the activity of Ru. Silica supported Ru with lower Ru loading performs a better activity in comparison to the sample with higher Ru content [73].
Ru/NaY activity was higher than that of Ru/SiO 2 , but unlike Ru/SiO 2 did not deactivate during the reaction, since the charge compensation protons caused dealumination accompanied by the local destruction of the zeolite structure [74].
Three percent Ru/Al 2 O 3 exhibited good stability during the 70 h exposure to undiluted equimolar CH 4 /CO 2 gas stream at 1023 K but the deactivation of the catalyst was observed during temperature programmed activity tests in the examined range of operating conditions, which was caused by metallic cluster sintering. A general trend was observed, namely that increasing CH 4 /CO 2 ratio in the feed stream positively influenced carbon deposition, which revealed CH 4 as the primary carbon source [62].
The catalytic activities of 5 wt% Ru supported on alumina, ceria, and ceria/alumina with different proportions of ceria were studied for the methane reforming reaction with CO 2 in the temperature range of 673-1073 K. Ru/Al 2 O 3 was a stable catalyst presenting no coke deposition. The addition of ceria on alumina can enhance the activity of rutheniumbased catalyst by the presence of Ru-Ce interaction [75].
2% Ru supported on Mg and/or Al oxides including γ-Al 2 O 3 , MgAl 2 O4, Mg 3 (Al)O (prepared from Mg-Al layered double hydroxide), and MgO showed great differences in the catalytic performance for the CH 4 + CO 2 reaction [76]. The activities of Ru/MgO and Ru/Mg 3 (Al)O were higher than those of Ru/MgAl 2 O 4 and Ru/γ-Al 2 O 3 , which might be related to the strong base intensity of the support and to the more accessible surface Ru 0 atoms, respectively. In a catalytic test at 1023 K for 30 h, Ru/Mg 3 (Al)O showed excellent stability, while a significant deactivation was observed on Ru/γ-Al 2 O 3 , Ru/MgAl 2 O 4 , and Ru/MgO. It was demonstrated that the sintering of Ru particles was mainly responsible for the catalyst deactivation.
In contrast with it, it was found that the activity of 4% Ru/MgAl 2 O 4 in the 723-1023 K temperature range was stable in the CO 2 + CH 4 reaction, although the carbon dioxide and methane ratio was 3:1 [77]. The Ru particle size was found to remain stable during repeated methane reforming experiments, thus preserving the nanostructure.
A 0.15% Ru/MgAl 2 O 4 catalyst prepared by physical vapor deposition method exhibits 1 or 2 orders of magnitude higher activity than Ru catalysts prepared by incipient wetness impregnation [78]. This sample, instead of deactivation or coke deposition, demonstratec an activity increase of 1.5 times after reaction at 1123 K for 600 h. Ru initially was dispersed as isolated atoms on the support, then condensed into clusters (1.1 nm), and finally retained as faceted nanoparticles (2.7 nm) during the reaction, becoming more and more active by decreasing the barrier of breaking the C−H bond of methane.
Ru/La 2 O 3 catalyst remained stable for more than 80 h in the 823-903 K temperature range when the CO 2 /CH 4 ratio was equal to unity. However, a significant deactivation was observed at 823 K when the reaction was carried out in CO 2 rich stream. It was demonstrated by XPS that the partial re-oxidation of metallic Ru could be one of the factors that produces the catalyst deactivation [79]. In the case of Ru supported Al 2 O 3 promoted with different oxides negative temperature dependence was observed, which was explained by the loss of structure of the solids and complete oxidation of ruthenium species at the reaction temperature [80].
Substitution of just 0.5 atom% of Ru in La 2 O 3 (La 1.99 Ru 0.01 O 3 ) enhanced the conversion by 20 times when compared to the activity exhibited by La 2 O 3 [81]. The oxygen storage capacity of the Ru doped sample was considerably higher than that of clean La 2 O 3 , which resulted in higher conversions of CH 4 and CO 2 . When the Ru concentration was higher (1 atom% La 1.98 Ru 0.02 O 3 ), the activity decreased considerably, indicating the role of the ionic nature of substituted Ru at low dopant concentrations. DRIFTS studies demonstrated the role of a specific type of carbonates in promoting the activity of the catalyst [81].
It was shown that a very small amount of Ru (0.13 wt%) supported on ZrO 2 -SiO 2 is very active and stable for the dry reforming of methane and the coke formation was suppressed at 1073 K while Ru/SiO 2 and ZrO 2 -SiO 2 showed poor activity. When Ru was deposited at the SiO 2 , Ru particle size was relatively large at 6.3 nm, but the Ru particles significantly decreased to 1.4 nm when Ru was deposited at the ZrO 2 -SiO 2 support [82].

Supported Platinum
The reaction of CH 4 + CO 2 on Pt wire up to 1373 K produced mainly CO even at a CH 4 /CO 2 ratio of 4.3, but if coke is also present on the Pt the dominant reaction becomes the pyrolysis of CH 4 to form C 2 H 2 and C 6 H 6 [83].
The activities of different supported Pt catalysts were initially similar at 875 K [84,85]. However, the Pt/γ-Al 2 O 3 deactivated almost completely in the first 10 h of the reaction. Pt/TiO 2 lost 80% of its activity within 50 h, whereas the Pt/ZrO 2 showed only a 5% deactivation within 50 h on stream [84]. For all catalysts, the decrease in activity is caused by carbon formation, which blocks the active metal sites for reaction since the carbon formation rate decreased in the order Pt/γ-Al 2 O 3 >> Pt/TiO 2 > Pt/ZrO 2 [85]. Evidences were presented that deactivation is caused by carbon formed on the metal and it is associated with overgrowth of the catalytic active perimeter between the support and the metal. Bradford and Vannice [86] also found that Pt/ZrO 2 and Pt/TiO 2 exhibited high stability even after 80 to 100 h on-stream although the Pt/SiO 2 and Pt/Cr 2 O 3 catalysts deactivated significantly. The differences in the stability of the catalysts were explained in the case of Pt/SiO 2 and Pt/Cr 2 O 3 by the apparent absence of any remarkable metal-support interaction which resulted in significant carbon deposition and so rapid deactivation. Pt on TiO 2 suppresses the carbon deposition, due to an ensemble effect of TiO x species migrating onto the Pt surface. The low carbon deposition on Pt/ZrO 2 was explained by an interaction of Pt with Zr n+ centers, the participation of lattice oxygen species at the metal-support interface, or by the partial coverage of Pt by ZrO x species, as occurs with titania [86]. The same activity order was found by van Keulen et al. [36] and they interpreted the results in similar way, but the lower carbon deposit on Pt/ZrO 2 was explained that the carbon formed on the catalyst is gasified by reaction with CO 2 .
The TOF value of CO 2 consumption at 925 K for Pt/ZrO 2 , Pt/TiO 2 , and Pt/γ-Al 2 O 3 was two orders of magnitude higher compared to Pt-black or Pt/SiO 2 [87]. From these observations it was concluded that in supported Pt catalysts not all the surface Pt atoms contribute equally to the activity of the catalyst; Pt atoms on the support-metal perimeter determine the activity. This can be explained by the fact that the CO 2 activation occurs via carbonate species on the support that must be in the surroundings of the Pt particles to react with the methane which is activated on the Pt.
The catalytic efficiency of Pt/ZrO 2 in the CO 2 + CH 4 reaction was studied by different research groups [36,[87][88][89][90][91][92][93][94]. It was observed that on Pt/ZrO 2 both CH 4 and CO 2 dissociate independently of each other in the CO 2 + CH 4 reaction [94]. The dissociation of carbon dioxide produces oxygen, while the decomposition products of methane react with the oxygen. When there is a large quantity of oxygen on the surface, the CH 4 can be oxidized to CO 2 . CO 2 dissociation and its reverse reaction is also effected by the amount of surface oxygen CO 2ads CO ads + O ads .
The activity and stability of Pt/Al 2 O 3 and Pt/ZrO 2 was compared in different papers [89,90,[95][96][97]. The activity decrease for Pt/Al 2 O 3 was rather slow and minor above 1070 K, while it was fast and almost complete at low temperature (< 875 K) [89,95]. Others found that Pt/Al 2 O 3 deactivated significantly at 1073 K (deactivation rate was 4%/h) [90,97]. The cause of deactivation on Pt/Al 2 O 3 was attributed to coke deposition on Pt. On the other hand, coke hardly deposits on Pt/ZrO 2 which resulted in the high stability of this sample. It was found that the reactivity of coke on Pt with hydrogen was higher on Pt/Al 2 O 3 than on Pt/ZrO 2 but the reactivity of carbon with CO 2 was the opposite. These results and the observation that the methane decomposition is slower on Pt/ZrO 2 than on Pt/Al 2 O 3 , resulted in the small carbon deposition on Pt/ZrO 2 under reaction conditions [96]. The main difference between the two samples was found in the CO 2 dissociation. On Pt/Al 2 O 3 this step was assisted by hydrogen, while on Pt/ZrO 2 the dissociation takes place over the support involving the oxygen vacancies.
The reduction of carbon deposition over zirconia-alumina catalyst was related to Pt-Zr n+ interactions and to the high oxygen mobility on zirconia [90,97].
Ballarini et al. [93] also found that Pt/Al 2 O 3 lost its activity at 1073 K due to the carbon deposition. Na or K addition to the catalyst enhances the stability since they provide basic sites, which promote the dissociation reaction of CO 2 into CO and O, and the O species can react with the surface carbon deposited on the Pt, thus cleaning the metallic phase of the catalysts. A similar effect can be achieved if at least 1% of CeO 2 is added to the Pt/Al 2 O 3 catalyst [98] since CeO 2 promote the gasification of coke deposition, because the dispersity and thus the metal-support interfacial region will be higher.
Comparing the activity of Pt/ZrO 2 and Pt/SiO 2 it was found that the Pt/ZrO 2 was the best sample due to the ability of ZrO 2 to promote the CO 2 dissociation [99] which aids in the removal of carbon deposit from the metal. High reforming activity (CO 2 conversion was 96.4% at 1073 K) was observed for the Pt/ZrO 2 /SiO 2 (4:1) sample among all the prepared ZrO 2 /SiO 2 mixed oxides with different ratios (2:1 to 4:1) supported Pt [91]. The better catalytic properties of the Pt/ZrO 2 /SiO 2 (4:1) were explained by higher Pt dispersion due to the absence of remaining free silica and the presence of higher amount amorphous ZrSiO 4 .
The dry reforming of methane was compared on different alumina supported Pt catalysts [100]. It was observed that the morphological differences between the supports affect in general the catalyst performance. Pt is present as Pt 0 on all catalysts, but the dispersion and stability was higher on Pt supported on a nanofibrous alumina, which presents a better behavior in terms of reactivity. It was found that CO 2 activation is promoted by the CH X species, which probably initiates the reforming reaction.
Deactivation of Pt/Al 2 O 3 was prevented by the addition of Pr-, Zr-, or Nb-doped CeO 2 to the Pt/Al 2 O 3 catalyst [101]. The presence of dopants decreased the carbon accumulation on Pt surface. It was stated that during the reduction of the catalysts the Ce 4+ /Ce 3+ ratio decreased on the surface-which was more significant for Pt/CePr/Al 2 O 3 -this resulted in the increase of oxygen storage/release capacity, which promotes the carbon removal from metallic surface.
High conversions and close to equal H 2 /CO ratios were observed in the dry reforming of methane on 4% Ru/TiO 2 and 4% Pt/TiO 2 but on the Pt-Ru/TiO 2 bimetallic samples these values were lower than on monometallic catalysts [102]. Interestingly, the H 2 /CO ratio for bimetallic catalysts decreased with increasing Pt content under the same conditions; for 3% Pt1% Ru/TiO 2 this ratio was below 0.2, while for Pt/TiO 2 it was found above 0.8.

Supported Palladium and Iridium
Nakagawa et al. [103] ranked the activity of the supported Ir catalysts as follows: TiO 2 ≥ ZrO 2 ≥ Y 2 O 3 > La 2 O 3 > MgO ≥ Al 2 O 3 > SiO 2 . Carbon deposition was observed only on Al 2 O 3 and SiO 2 supported catalysts and this resulted in the low stability of the catalysts. The effect of the support was attributed to the activation of CO 2 on the metal oxides. Similar order was observed on 5% Ir supported on TiO 2 , MgO, Al 2 O 3 , and SiO 2 when the turnover rate for H 2 formation was compared at 773 K [104]. In these cases, a significant amount of carbon formation on SiO 2 (C/Ir surf = 1.95) and MgO (C/Ir surf = 1.08) supported catalysts was observed after 1 h of reaction at 773 K, on Ir/Al 2 O 3 and Ir/TiO 2 only traces of carbon were detected (C/Ir surf ≤ 0.1).
On Ir/Al 2 O 3 doped with both alkaline (Na and K) and alkaline earth metals (Ba, Ca and Mg) of different concentrations (1, 5 and 10 wt%) was studied the dry reforming of methane [105]. The catalytic performance was affected by acid-base functions of the support that modify the metal-support interaction. It was found that the higher the dopant concentration, the better the performance. The highest activity was obtained for 1% Ir/Al 2 O 3 -10% Mg catalyst.
The turnover rate for CH 4 consumption in the CH 4 + CO 2 reaction increase linearly with increasing CH 4 pressure at 873 K on 0.8 wt% Ir/ZrO 2 , but it were not influenced by CO 2 concentration [106]. These results indicate that Ir surfaces remain essentially uncovered by CH 4 -derived intermediates or by reaction products, because co-reactant or product concentrations would have otherwise influenced the forward rates. CH 4 consumption rates increased with increasing Ir dispersion, as it varied with metal content (0.2-1.6 wt%) or support (ZrO 2 or Al 2 O 3 ). These data contradict with the results of Mark and Maier [17] since they observed that there is no effect of catalyst particle size on the methane conversion at 1073 K in the case of 1% Ir/Al 2 O 3 catalyst.
Long-term experiments show the excellent stability of the Ir/Ce 0.9 Gd 0.1 O 2−x catalyst [107]. Carbon formation was totally inhibited or very limited under the most severe conditions of the study (1073 K, CH 4 /CO 2 = 2).
Ir/Ce 0.9 Pr 0.1 O 2 catalysts prepared by different methods were also stable at 1023 K in the dry reforming reaction [108]. The highest activity of the sample synthesized by deposition-precipitation method was associated with the more enhanced basicity of the support, the more accessible Ir nanoparticles to transform CH 4 and the Ir-support boundary to remove the coke precursors.
The support significantly influenced the rate of the CO 2 + CH 4 reaction on the palladium catalyst. The specific activities of Pd/TiO 2 exceeded by almost one order of magnitude the TOF value for CO formation of the less active Pd/Al 2 O 3 and Pd/SiO 2 , and it was more than twenty times higher than on Pd/MgO [109]. It was suggested that the activity order of the Pd catalysts may be related to their efficiency in the dissociation of CO 2 , where the same sequence of activity was found. It could mean that on supported palladium, the activation of carbon dioxide should play an important role in the CH 4 + CO 2 reaction. The high activity of Pd/TiO 2 may be associated with the extended electronic interaction between Pd and n-type TiO 2 . The oxygen vacancy present on Pd titania interface could also promote the dissociation of carbon dioxide [109].
Pd/Al 2 O 3 shows high initial activity but poor stability in the CO 2 + CH 4 reaction at 973 K due to Pd sintering and to a large amount of carbon deposition. Of the various promoters added to the catalyst (K, Ca, Y, Mn, and Cu), Y or Ca slightly increased the initial conversions, which can be attributed to the moderate reducibility of Pd species and to the increased Pd dispersion. In contrast, while the addition of K, Mn and Cu similarly reduced the reducibility of Pd species, the metal dispersions decreased owing to decoration effect or alloy formation [110]. Y-modified Pd/Al 2 O 3 showed the best stability without activity loss in 20 h of reaction. This is probably because the addition of Y to the catalyst suppresses carbon formation and metal sintering and maintains Pd particle size below 10 nm. This particle size is crucial, below which carbon formation can be completely avoided under the reforming conditions. A 0.86% Pd/CeO 2 catalyst showed activity in the dry reforming already at 623 K due to the presence of highly dispersed and very small Pd-nanoparticles (~2.9 nm), to metal-support interactions, and to the redox property of the support CeO 2 [111]. The high reducibility of CeO 2 (~32.8% Ce 3+ ) influenced methane activation at as low as 623 K. Only a slight decrease in conversion was observed due to sintering, re-oxidation and hydroxylation of the active Pd-nanoparticles at 1073 K during 12 h of reaction. The promoting effect of Ce 3+ was also observed when the activity of 1% Pd/MgO 1−x Ce 3+ x O was found to be greater than that of 1% Pd/MgO 1−x Ce 4+ x O in the reaction at 1173 K [112]. This effect was attributed to the smaller size of Ce 4+ .

Adsorption, Dissociation, and Activation of CO 2 on Noble Metals and on Supported Noble Metal Catalysts
It is known that CO 2 adsorbs on the surface of oxides used as catalyst support with the exception of some acidic oxides such as SiO 2 . Spectroscopic analyses show that a variety of surface species can be formed, depending on the basicity of the surface, forming mixture of bidentate, monodentate, bridged carbonates, and bicarbonates [26]. In some cases, formate species were also detected after CO 2 adsorption. These observations indicate that hydrogen remained on the catalyst surface after reduction [26,113].
Any reaction of CO 2 requires its activation on the surface of the catalyst. There are different possibilities and therefore there are different theories for CO 2 activation on noble metals and on supported metal catalysts. CO 2 could dissociate on metal surfaces to produce CO and adsorbed oxygen [114,115]. The reduction reaction could proceed via the formation of carbonate or formate species on the basic catalytic sites of the support [55].
The activation of CO 2 could occur on the interfacial sites of the catalysts, on the oxygen vacancies formed on the metal-oxide interface [49,88].
Another possibility for the activation of CO 2 is that the hydrogen formed in the decomposition of methane could promote the dissociation of CO 2 , the formation of CO.
The adsorption and activation of CO 2 on clean noble metals has also been widely investigated [116,117]. It is generally accepted that CO 2 adsorbs weakly and molecularly on carefully cleaned noble metals at 100-300 K. In the case of Rh, there was a controversy concerning the nature of the interaction and particularly the dissociation of CO 2 on clean Rh surfaces. In the early studies, it was found that the adsorption of CO 2 is weak and associative on Rh metal [118]. The Somorjai group [119,120] reported that CO 2 dissociate on a Rh foil and on Rh single crystals, but Weinberg [121] and Goodman [122], on the basis of thermodynamic and kinetic results, indicated that the CO 2 sticking probability under the experimental conditions used would be by orders of magnitude smaller than needed to observe dissociation. It was demonstrated by field electron microscopy that the crystal structure of the Rh tip influenced the adsorption and dissociation of CO 2 [115], it was the fastest on {012} facets [114]. On Pt powder already at 293 K adsorbed CO was detected by DRIFTS indicating that CO 2 dissociation occurs [123].
Completely different results were obtained, when potassium or sodium adatoms, an electron donating promoter was deposited on Rh [124], Pt [125], Ru [126], and Pd [127]. After the low temperature (~100 K) CO 2 adsorption CO − 2 formed and it dissociates even at 150-200 K or can be transformed into carbonate-like species and CO, at a higher temperature.
The CO band appeared on the infrared spectra during CO 2 adsorption, or the metal is oxidized indicating the dissociation of CO 2 on supported noble metals on Rh [128], Pt [86], Ir [104], Ru [72,79], and Pd [109]. The preparation, in some cases the reduction temperature of the catalysts, the dispersity or the particle size of metal, the nature of the support, the adsorption temperature, and the CO 2 pressure all have an influence on this process.
The high temperature dissociation of CO 2 occurred on reduced alumina supported noble metals. The extent of CO 2 dissociation (the amount of CO formed related to the number of surface metal atoms) at 773 K was the largest for Ru and Rh, and the smallest for Pt and Ir [18]. On oxidized surfaces, dissociation of CO 2 was not observed. Not everyone has detected CO bands after or during CO 2 adsorption, that is, the dissociation of CO 2 on supported noble metals [129]. Adsorption of CO 2 on Ru/TiO 2 formation of formate occurs and carbonate species associated with the support but CO not up to 723 K whereas in the presence of H 2 part of these species is converted to Ru-bonded carbonyls via the reverse water-gas shift reaction at the metal-support interface [113].
The high temperature dissociation of CO2 occurred on reduced alumina supporte noble metals. The extent of CO2 dissociation (the amount of CO formed related to th number of surface metal atoms) at 773 K was the largest for Ru and Rh, and the smalle for Pt and Ir [18]. On oxidized surfaces, dissociation of CO2 was not observed. Not ev ryone has detected CO bands after or during CO2 adsorption, that is, the dissociation CO2 on supported noble metals [129]. Adsorption of CO2 on Ru/TiO2 formation of fo mate occurs and carbonate species associated with the support but CO not up to 723 whereas in the presence of H2 part of these species is converted to Ru-bonded carbony via the reverse water-gas shift reaction at the metal-support interface [113].
On Ru/SiO2 linearly (2039 cm −1 ) and bridge (1941 cm −1 ) bonded CO, on Ru/Al2O3 on Ru-CO (2029 cm −1 ) were detected by IR spectroscopy after CO2 adsorption at 823 K [72 On the IR spectra registered after CO2 adsorption at 723 K on Pt/TiO2, Pt/ZrO2, Pt/Cr2O and Pt/SiO2 bands characteristic for adsorbed CO (2060-2070 cm −1 and 1860-1894 cm were detectable indicating the CO2 dissociation [86], but the nondissociative adsorptio of CO2 was also observed. (1 The dissociation of CO2 on Rh/TiO2 depends on the reduction temperature, th higher the reduction temperature the greater the CO formation [130]. These observation were explained by the speculation that oxygen vacancies produced by the reduction the catalysts promote the dissociation of CO2. A similar explanation was given for th dissociation of CO2 in the case of vanadia promoted Rh/SiO2 [48], vanadia or titan promoted Rh/Al2O3 [59] and Pt/ZrO2 [96]. The active participation of the support in CO dissociation was suggested in the case of CeO2 supported noble metals [131]. It w found that by increasing the reduction temperature, the reduction of the CeO2 increase and the amount of CO desorbed after CO2 adsorption also increased. The CO2 could b activated on Ce 3+ sites with the formation of CO and Ce 4+ [73,131]. (10) The dissociation of CO 2 on Rh/TiO 2 depends on the reduction temperature, the higher the reduction temperature the greater the CO formation [130]. These observations were explained by the speculation that oxygen vacancies produced by the reduction of the catalysts promote the dissociation of CO 2 . A similar explanation was given for the dissociation of CO 2 in the case of vanadia promoted Rh/SiO 2 [48], vanadia or titania promoted Rh/Al 2 O 3 [59] and Pt/ZrO 2 [96]. The active participation of the support in CO 2 dissociation was suggested in the case of CeO 2 supported noble metals [131]. It was found that by increasing the reduction temperature, the reduction of the CeO 2 increased, and the amount of CO desorbed after CO 2 adsorption also increased. The CO 2 could be activated on Ce 3+ sites with the formation of CO and Ce 4+ [73,131].
M-CeO 2−x + x CO 2 → M-CeO 2 + x CO (11) CO 2 activation was related to the carbonate type species formed on the support [49,87]. On Pt/Al 2 O 3 bicarbonates while on Pt/ZrO 2 bidentate carbonates were detected and the formation of latter species leads to easier activation of CO 2 than that of bicarbonates [95]. On Pt/SiO 2 did not form carbonate and on this sample CO was not detected after adsorption of CO 2 at 775 K [87].
At room temperature, CO 2 adsorbs on 1% Pt/10%ZrO 2 /Al 2 O 3 , the peaks at 1646, 1444, and 1228 cm −1 can be assigned to bidentate carbonate. While temperature increases, these species are activated and migrate to metal particles, being adsorbed as CO absorbing at about 2052 cm −1 (Figure 4). Thus, CO 2 activation on zirconia catalysts involves support and interfacial sites [90]. This conclusion support the finding that CO 2 dissociate on PtZrO 2 at 775 K but adsorbed oxygen or Pt-O was not detectable by EXAFS. It was concluded that the oxygen is consumed by the support at the metal-support interface [49].
The presence of oxygen vacancies in the bulk will be an additional driving force for the reduction of CO 2 .
XPS measurements show that the CO 2 oxidized Rh at 398 K on Rh/Al 2 O 3 during the dissociative adsorption [132] At room temperature, CO2 adsorbs on 1% Pt/10%ZrO2/Al2O3, the peaks at 1646, 1444, and 1228 cm −1 can be assigned to bidentate carbonate. While temperature increases, these species are activated and migrate to metal particles, being adsorbed as CO absorbing at about 2052 cm −1 (Figure 4). Thus, CO2 activation on zirconia catalysts involves support and interfacial sites [90]. This conclusion support the finding that CO2 dissociate on PtZrO2 at 775 K but adsorbed oxygen or Pt-O was not detectable by EXAFS. It was concluded that the oxygen is consumed by the support at the metal-support interface [49]. The presence of oxygen vacancies in the bulk will be an additional driving force for the reduction of CO2.
XPS measurements show that the CO2 oxidized Rh at 398 K on Rh/Al2O3 during the dissociative adsorption [132] After the catalytic reaction at 903 K on Ru/La2O3 Ru 0 and Ru 3+ species were observed by XPS [79]. The oxidation of Ru supported on La2O3 by CO2 was also found by Matsui et al. [68]. Pulsed reaction with 13 CO2 indicates that Ru could be oxidized to give 13 CO; they proposed explaining this process by the following reaction step: Ru/La2O3 + 13 CO2(ad) → 13 CO(gas) + Ru-Ox/La2O3.
The direct participation of La2O3 was suggested in the CO formation from CO2 in the case of La2O3 supported noble metals or La2O3 doped catalysts. The carbon dioxide rapidly reacts with La2O3 to generate oxycarbonate, which reacts slowly with carbon formed in the methane decomposition (Equation (3)) to generate CO [38,79,133,134].
La2O2CO3 + C-S → La2O3 + 2 CO + S (slow step) (14) After the catalytic reaction at 903 K on Ru/La 2 O 3 Ru 0 and Ru 3+ species were observed by XPS [79]. The oxidation of Ru supported on La 2 O 3 by CO 2 was also found by Matsui et al. [68]. Pulsed reaction with 13 CO 2 indicates that Ru could be oxidized to give 13 CO; they proposed explaining this process by the following reaction step: The direct participation of La 2 O 3 was suggested in the CO formation from CO 2 in the case of La 2 O 3 supported noble metals or La 2 O 3 doped catalysts. The carbon dioxide rapidly reacts with La 2 O 3 to generate oxycarbonate, which reacts slowly with carbon formed in the methane decomposition (Equation (3)) to generate CO [38,79,133,134].
La 2 O 2 CO 3 + C-S → La 2 O 3 + 2 CO + S (slow step) (14) In addition to the CO 2 dissociation induced by an oxygen vacancy on Pt/ZrO 2 , O'Connor et al. [96] did not rule out the CO 2 dissociation through carbonate formation.
HCO 3 | Zr + Zr + * → CO* + OH Zr + O Zr (17) Zr means an oxygen vacancy at the zirconia surface and O Zr is an surface oxygen species. It is most likely that these species are located at the Pt/zirconia interface.
Non dissociative adsorption of CO 2 during reaction occurs on the support, in the form of carbonates, as proved by FTIR of CO 2 . Souza et al. [90] used in situ DRIFT experiments to prove that very weakly adsorbed carbonate species are formed, once the lattice oxygen of zirconia has been replenished by CO 2 which could decompose to CO.
The role of CO 2 is frequently assumed to be only a source of oxygen to the support.

Interaction of Methane with the Noble Metals and Supported Noble Metal Catalysts
The earlier results of the adsorption and dissociation of CH 4 on metal films were summarized by Frennet [135]. The theoretical calculations also show that the noble metals are active in methane dissociation [136]. It was found that the dissociation enthalpy of CH 4 is an important factor determining the catalytic activity of the metal. According to the trend in the calculated enthalpies, the most efficient catalysts for methane dissociation were Rh, Ru, and Ir are more active than Pd and Pt. These results are in agreement with experimental observations. Methane activation studies on single crystal surfaces have shown that the activation of C-H bonds depends sensitively not only on the metals but also on surface structure; specifically, surface steps and kinks are much more active than atoms on close-packed surfaces [137].
The dissociation, the activation of methane and its interaction with the catalyst have been studied separately on various supported noble metal catalysts. Treating Pt/SiO 2 [138] or different noble metals [39] with methane the dissociation of it was observed above 473 K, hydrogen and surprisingly ethane were formed. However, the activity of the catalysts quickly dropped to a low value, and after 15-25 min only traces of H 2 and C 2 H 6 were detected. This suggests an accumulation of carbon deposit on the catalysts during this process poisoning the reaction. The rate of the ethane evolution passed through a maximum. The amount of hydrogen greatly exceeded that of ethane on all catalyst samples. The most active sample was Rh, but the largest amount of ethane was measured on Pt/SiO 2 [139]. The formation of ethane has been explained by the dissociation of methane on the surface and the resulting methyl group could dimerize to ethane and could decompose further to produce carbonaceous deposit.
2 CH 3 (a) C 2 H 6 (g) The support influenced the hydrogen and ethane formation in the decomposition of methane. The most effective catalyst in the hydrogen formation was Pd/TiO 2 [140], but among the Rh catalysts, the alumina supported sample was the best followed by Rh/TiO 2 , Rh/SiO 2 , and Rh/MgO [46]. The effect of the supports on the CH 4 dissociation was explained by the different crystal size of the catalysts and/or with the ease of the carbon migration from the metal to the support [46,140]. The highest C 2 H 6 formation was observed in all cases on silica supported samples.
It is generally assumed that the CH 4 dissociation occurs on metallic sites but on Rh/Al 2 O 3 and Rh/ZrO 2 it was found that the CH 4 activation on Rh 0 would be better conducted when RhO x sites are also on the surface, probably via oxygen assisted dissociation [141].
As the Equation (21) shows CH 4 adsorbs reversibly on the surface of noble metals arriving at the equilibrium. This conclusion is supported by the results of steady state isotopic tracing kinetic analysis (SSITKA) which detected methane on the surface of Pd/ZrO 2 and Pd/γ-Al 2 O 3 under reaction conditions [142]. It was found that the rate constant did not depend on the dispersity of Pd, but others reported that the CH 4 activation is structure sensitive [72,143], and influenced by metal dispersion of different noble metals [17,58,[144][145][146]. A direct correlation was found between the Rh dispersion and the turnover rate of the reaction [142,143,146].
The question is whether the surface carbon produced at high temperature from methane decomposition contains any hydrogen or not? By in situ IR spectroscopy on supported Rh could not detect CH x intermediates [46], and when the carbon was reacted with D 2 only CD 4 was formed [147]. Similar was observed on Ru/Al 2 O 3 but on Ru/La 2 O 3 CH 2 D 2 , on Ru/Y 2 O 3 CH 3 D, CH 2 D 2 , and CHD 3 were also detected [148].
Several manuscripts have concluded from the kinetic parameters of the reaction, from the dependence of the reaction rate on the particle size that methane dissociates on the supported noble metals. The activation and dissociation of methane can take place not only directly on the metals but also on the adsorbed O species [136,141,149], which may be formed from the dissociation of CO 2 [18], furthermore the oxygen in the support oxide [8,134], or the surface OH groups [71] can also facilitate this process.
Pakhare et al. [8,134] found by means of CH 4 TPR that the lattice oxygen on the surface of Ru or Pt containing pyrochlors (La 2 Zr 2 O 7 ) help react with CH 4 . The oxygen or OH group can promote the dissociation of CH 4 by reacting with the CH x groups formed in the first step.
Bradford and Vannice [26] detected by IR spectroscopy H 2 C = O (at 1690 cm −1 ) species formation after CH 4 adsorption on TiO 2 supported noble metals. In addition, change in the intensity of OH groups was observed and on Rh and on Pt adsorbed CO were identified.

Surface Species Formed in the Interaction and in the Reaction of CO 2 with CH 4 on Supported Noble Metals
Examination of the interaction of reactants and products formed in the reaction on the surface helps to understand the whole process.
DRIFT spectra registered during co-adsorption of CO 2 + CH 4 gas mixture on Pt powder at 293 K show a marked increase in the intensity of the absorption band characteristic for adsorbed CO compared to CO 2 adsorption alone, implying that CH 4 promotes the dissociation of CO 2 [123], as it was observed on supported Rh [25,46,48], Pd [109] and Ir [25,104]. When the temperature was raised the intensity of the CO band increased, and its position shifted to higher wavenumbers. The spectral feature of CO formed in the co-adsorption differed from that observed during the adsorption of gaseous CO. For example, in the case of supported Rh the doublet due to the twin structure (Rh(CO) 2 ) at 2096 and 2029 cm −1 [150] was missing and the other CO bands, characteristic for linear and bridge bonded CO, absorbed at lower frequencies. The promotion of the dissociation of carbon dioxide is attributed to the effect of hydrogen-as it was found in the interaction of CO 2 + H 2 mixture [128,151]-formed in the decomposition of methane.
After the adsorption of CO 2 + CH 4 gas mixture at room temperature on Al 2 O 3 supported noble metals mainly hydrogen carbonate were found on the IR spectra bands (1650, 1443, 1348, and 1302 cm −1 ) [59]. With increasing the adsorption temperature the intensities of these bands decreased and above 548 K a band characteristic for linearly bonded CO was detected ( Figure 5) [59]. Above 673 K a band was observed at about 1589 cm −1 characteristic for the O-C-O vibration (ν ass ) of formate groups adsorbed on alumina [150]. On V 2 O 5 or TiO 2 doped alumina supported Rh the CO band appeared at lower temperature ( Figure 5).
Similar spectra were recorded during the reaction of CO 2 + CH 4 at 723 K [147] or at 773 K [152]. It was observed that the intensity of formate band increased continuously during the reaction. It is surprising to observe adsorbed CO and formate species on the surface well above their desorption temperature [57,152]. It means that the formation rate of these form is higher than their desorption, decomposition or further reaction rate. This is supported by the observation that flushing the DRIFT cell with He after the catalytic reaction all bands disappeared immediately from the spectra [55].
The linearly bonded CO formed in the reaction of CO 2 + CH 4 similarly as was shown in the low temperature interaction of CO 2 + CH 4 absorbs at lower frequencies (~2030-2020 cm −1 ) than the CO formed after gaseous CO adsorption. This effect was attributed to Rh carbonyl hydride formation [128][129][130].
On Rh/La 2 O 3 CO bands (at 2036 and 1825 cm −1 ) and absorptions characteristic for different La 2 O 2 CO 3 (at 720, 842, 864, 1060 and 1550 cm −1 ) were observed during the exposure of the reaction mixture onto the catalyst at and above 773 K [34]. La 2 O 2 CO 3 formation was detected in all cases when La 2 O 3 supported catalysts were used [153].
In the presentation of the activity of the catalysts, it was often mentioned that the significant decrease in activity in time is caused by carbon deposition. There are several theoretical possibilities for the formation of carbon: Decomposition of methane (3), the Boudouard reaction (6), and the hydrogenation of CO (the reverse reaction of 7). Similar spectra were recorded during the reaction of CO2 + CH4 at 723 K [147] or at 773 K [152]. It was observed that the intensity of formate band increased continuously during the reaction. It is surprising to observe adsorbed CO and formate species on the surface well above their desorption temperature [57,152]. It means that the formation rate of these form is higher than their desorption, decomposition or further reaction rate. This is supported by the observation that flushing the DRIFT cell with He after the catalytic reaction all bands disappeared immediately from the spectra [55].
The linearly bonded CO formed in the reaction of CO2 + CH4 similarly as was shown in the low temperature interaction of CO2 + CH4 absorbs at lower frequencies (~2030-2020 cm −1 ) than the CO formed after gaseous CO adsorption. This effect was attributed to Rh carbonyl hydride formation [128][129][130].
On Rh/La2O3 CO bands (at 2036 and 1825 cm −1 ) and absorptions characteristic for different La2O2CO3 (at 720, 842, 864, 1060 and 1550 cm −1 ) were observed during the exposure of the reaction mixture onto the catalyst at and above 773 K [34]. La2O2CO3 formation was detected in all cases when La2O3 supported catalysts were used [153].
In the presentation of the activity of the catalysts, it was often mentioned that the significant decrease in activity in time is caused by carbon deposition. There are several theoretical possibilities for the formation of carbon: Decomposition of methane (3), the Boudouard reaction (6), and the hydrogenation of CO (the reverse reaction of 7).
On theoretical calculation, it was found that with increasing the particle size increased the carbon formation. At low temperature CO was identified as the precursor for carbon, but CH4 was found to be the main source of carbon at high temperature [154]. Transient isotopic experiments indicate [43,55,152,155] that on Rh/Al2O3 during the reforming reaction accumulated carbon is mainly derived from the CO2 molecule, since after the 13 CH4 + 12 CO2 reaction on Rh/Al2O3 in the temperature programmed oxidation (TPO) mainly 12 CO2 was formed [55,44]. On theoretical calculation, it was found that with increasing the particle size increased the carbon formation. At low temperature CO was identified as the precursor for carbon, but CH 4 was found to be the main source of carbon at high temperature [154]. Transient isotopic experiments indicate [43,55,152,155] that on Rh/Al 2 O 3 during the reforming reaction accumulated carbon is mainly derived from the CO 2 molecule, since after the 13 CH 4 + 12 CO 2 reaction on Rh/Al 2 O 3 in the temperature programmed oxidation (TPO) mainly 12 CO 2 was formed [44,55].
CO-s + s C-s + O-s (24) where (s) is a site on the Rh surface or on the metal support interface.
In the case of Pt/ZrO 2 the main fraction of carbonaceous deposit was formed from the dissociation of CH 4 the preferred species was CH 2 * [156].
Graphitic carbon was observed on the Pt/TiO 2 and Ru/TiO 2 [102] after the reaction by TEM but on Pt/MgO-Al 2 O 3 [80], on Ru supported modified SiO 2 [73], on Rh/α-Al 2 O 3 -La 2 O 3 [38] filamentous carbon was found. According to experience the extent of carbon deposition on noble metals is usually lower than on Ni because carbon atoms can dissolve in Ni crystal and carbon fibers could form. This mechanism does not work on noble metals [157]. Raman spectra of the used Pt/CeO 2 -Al 2 O 3 and Ru/MgO-Al 2 O 3 catalysts [80] especially after the reaction at 923 K, on Rh/La 2 O 3 [153], on different Pd/Al 2 O 3 [110], on Pt/ZrO 2 /SiO 2 [91] show intense bands at 1580-1595 and 1339-1350 cm −1 which could assignation to the so called G and D band, respectively. The G band is related to the graphitic carbon and the D band to the disordered structure [80]. The detection of graphitic carbon by Raman spectroscopy was successful even when with TGA no carbon formation was detected [153].
The formation of graphitic carbon was detected on different Pd/Al 2 O 3 [110], on Ru/SiO 2 [73] by using XRD spectroscopy.
Examination of the reactivity of the surface deposited carbon by temperature programmed hydrogenation (TPH) or oxidation as well as TEM images showed that carbon can occur in different forms on the surface, which can be transformed into each other depending on the temperature and the time elapsed since their formation. The reactivity of the surface carbon was almost the same regardless of whether it was formed either from the decomposition of CH 4 or from the disproportionation of CO or from the reaction of CO 2 + CH 4 . The gasification of surface carbon deposit is an important step enhancing the stability of the catalysts.
The reactivity of the surface carbon formed from CH 4 decomposition at 523 K, studied by TPH [46,63], exhibited practically the same features as those established for supported Rh catalysts following the dissociation of CO [57]. Three different carbon forms were distinguished: (i) The highly reactive form (α), which can be hydrogenated even below 350 K; (ii) a less reactive deposit (β), with T Max = 500 K; and (iii) the relatively inactive form (γ), which reacts with hydrogen only above 650 K. Similar TPH spectra were registered on different RhCu/Al 2 O 3 after decomposition of CH 4 at 623 K [158]. The reactivity of surface carbon, that is, the relative ratio of the three forms, is very sensitive to the temperature of its formation and also to the duration of its thermal treatment. Above 623 K, a significant aging was observed, with transformation of the more reactive form into less reactive ones. When decomposition of CH 4 occurs at 923 K a relatively inactive form of carbon was obtained (T Max = 973 K), suggesting that aging of the carbon produced occurs [155].
These results supported by the observation that in the case of Ru/Al 2 O 3 and Ru/La 2 O 3 the TPH peak shifted to higher temperature as a function of the CH 4 decomposition temperature [148]. On Ru/Al 2 O 3 there is a carbon form which did not react with H 2 up to 1073 K.
TPH for Pt/Al 2 O 3 after exposure to CH 4 at 1070 K showed a peak above 873 K, while on Pt/ZrO 2 the CH 4 formation was observed above 1073 K. These indicate that some of the surface carbon on Pt/Al 2 O 3 is more reactive with H 2 than on Pt/ZrO 2 [95], but some of the coke could not remove by H 2 even at 1273 K.
The TPH profiles of the used Ru/SiO 2 [73], or that of different noble metals supported on spinell [30] catalysts clearly showed the existence of two types of carbonaceous species on the catalysts. A first peak is observed at temperatures between 620 and 680 K on different catalysts depending on the metal. This peak is related to amorphous carbon located at the interior of the active metal particles. A second peak is observed at temperatures above 873 K, identified as whisker-like filamentous carbon. The THP of 0.5% Rh/Al 2 O 3 used in the CO 2 + CH 4 reaction at 923 K for 10 min showed three well resolved peaks. The most reactive hydrogenated at low temperature (373 K), the most abundant reacted at 423-593 K, and the least active carbon form above this temperature [43,155].
The TPO profile of used noble metal catalysts supported on MgAl 2 O 4 depends on the metals. Pt and Ir showed one peak in the temperature range of 573-700 K which was related to the oxidation of amorphous carbon. In addition to this form for the other samples a second peak was also detected; in the case of Pd it was found at higher temperature above 873 K, due to the formation of whisker carbon. On Ru and Rh supported on spinel a TPO peak was observed at about 373 K which could be related to (C α ) carbide like structure [30]. In contrast on Ru/SiO 2 TPO peak characteristic for whisker was observed and the carbide like species was missing [73].
After CO 2 + CH 4 reaction at 923 K four TPO peak were registered on 0.5% Rh/Al 2 O 3 , the T Max were at 373, 453, 543, and 603 K [43] similarly to on 0.5% Rh/La 2 O 3 [30]. Ghelamallah and Grangers found that on Rh/La 2 O 3 the first TPO peak appeared below 523 K and it was assigned to carbon deposited on metal, the second (635-661 K) type of coke is on the metal support interface, and the third one (693-798 K) accumulated on the support, and the most inactive carbon (above 973 K) was identified as graphite [159].
TPO profiles of carbon deposited on Pt during the reaction at 1123 K showed a main peak at 893 K, which is typical for the oxidation of carbon with a nanotube structure. Only a small peak was observed between 613 and 623 K, attributed to oligomeric and polymeric carbon. This indicates that, regardless of the source of carbon (CH 4 or CO 2 ), the majority of the carbon species is present on the catalyst surface in the form of carbon nanotubes [156]. Similar TPO curves were found on different Pd/Al 2 O 3 [110].
With increasing the reaction temperature on Ru substituted La 2 Zr 2 O 7 the amount of carbon deposit decreased although the TPO peaks shifted to higher temperature [8]. This means that the reactivity of deposited carbon decreases with increasing the reaction temperature.
On Rh/Al 2 O 3 the carbon produced in CH 4 decomposition at 773 K react with oxygen above 400 K but up to 570 K small amount of CO 2 was detected. The main CO 2 peak was observed at 745 K [46].
The reactivity of surface carbon was investigated by temperature programmed reaction with CO 2 (the reverse Boudouard reaction (6)). On Rh/Al 2 O 3 the carbon-produced at 773 K in the decomposition of CH 4 -reacted with CO 2 above 550 K and the reaction proceeded at a higher rate above 700 K [46]. Similar results were obtained on Pt/Al 2 O 3 [89,95] peak was observed below and above the reaction temperature and these were assigned to carbon on Pt and on Al 2 O 3 , respectively. For Pt/ZrO 2 also two peaks were registered but both react below reaction temperature and these were attributed to coke on Pt [89,95].

The Proposed Mechanism for the CO 2 + CH 4 Reaction on Supported Noble Metals Catalysts
In the previous chapters the adsorption, dissociation, and activation of CO 2 and CH 4 on supported noble metals were discussed, we presented the different experimental results and the processes proposed based on them. It means there is still some debate on the reaction mechanism of this reaction. Regarding the reaction mechanism for CO 2 + CH 4 reaction basically three different, but same in a few steps' reaction mechanisms were proposed. The first involves the adsorption of CH 4 , the second the activation or dissociation of CO 2 and the third one when both component dissociate in the first step. Further differences between the reaction mechanisms are that the dissociation of CH 4 or CO 2 takes place directly or in some reaction, and that this process takes place on the metal, on the support, or at the interface.
The mechanisms of the reaction could be divided into two groups: Mono-and bifunctional mechanism. According to the monofunctional mechanism the activation of both CO 2 and CH 4 occurs on the metal sites, but in the latter cases the CO 2 is activated on the support or on the metal-support interface and the CH 4 on the metal.
A dissociative methane and carbon dioxide adsorption is proposed by Rostrup-Nielsen and Bak-Hansen [19] on different alumina stabilized magnesia supported noble metals CH 4 + * = CH 3 * → CH 2 * → CH* = C (26) CO 2 + * = O* + CO (27) CH x * + O* = CO + x/2 H 2 + 2* In the case of supported Rh and Pd catalyst, although the dissociation of CO 2 has been shown on these samples, it was suggested that during the reaction, the main route for CO formation is the hydrogen assisted CO 2 dissociation [46,109].

OH (a) = H O + O (a)
In the case of Rh/Al 2 O 3 the reverse Boudouard reaction (6) and the C + H 2 O = CO + H 2 (38) reaction was also confirmed [46]. Raskó and Solymosi showed that the adsorbed CH 3 radical and CO 2 react with each other at the metal-substrate interface on Rh/SiO 2 . CH 3 groups adsorbed on the support migrate from the support to the metal [160].
Direct dissociation of CO 2 [55,71,149] and CO [55,149] and the further reaction of adsorbed oxygen, OH groups were not considered in former mechanism [55,58,149] and therefore it was completed by the following reactions: Mark and Maier [17] proposed a mechanism for the reaction on Rh/Al 2 O 3 where the dissociative CH 4 adsorption is rate limiting step, followed by the direct reaction of CO 2 with adsorbed carbon. This reaction mechanism was also confirmed on Ru [147,161] and Ir catalysts [147].
The following bifunctional mechanism (where p is a platinum site and z a support site) was proposed by Souza et al. [90] for CH 4 + CO 2 reforming: The occurrence of the bifunctional mechanism is supported by the observation that when TiO 2 , Al 2 O 3 , or MgO was mixed with the Rh/SiO 2 catalyst, the conversion increased significantly, but when mixed with SiO 2 it did not change [46]. This was explained by the fact that neither carbonates nor formate are formed on SiO 2 which forms may be responsible for the increase of reaction rate.
Bitter et al. found [49] that the activity of Pt/ZrO 2 increased linearly with increasing the Pt-ZrO 2 perimeter, since the carbonate on the interface is reduced by the CH x species on the metal to form CO and formate. The formate species then decompose to CO and OH and subsequently the OH groups can either be desorbed as water or react with CH 4 to form CO and H 2 .
We have seen that in recent years, many ideas have been developed for the CO 2 + CH 4 reaction, but to date there is no clear explanation for what and why influences CO 2 dissociation, what kind of secondary reactions take place on the surface of noble metal catalysts.

Conclusions
This review summarizes the studies on the CO 2 + CH 4 reaction, the dry reforming of CH 4 on supported noble metal catalysts. Comparing the activity and stability of different supported metals, it was found that the best catalysts are among the noble metals, Rh and Ru. However, these metals are expensive and so to make the process uneconomical, but noble metals can be used to promote the cheaper Ni or Co catalyst which could help in removal of carbon deposit, thus increasing the stability of the catalyst.
Comparing the activity of noble metals and the effect of carriers, there are significant differences in the literature. The acidity, basicity and the reducibility of the supports basically influence the efficiency of the catalyst. The oxygen vacancies formed on the metal-support interface improve the activity as these sites promote CO 2 activation but the SMSI interaction reduces it. Alkaline or alkaline earth metal additive increases CO 2 dissociation and thus the efficiency of the catalysts The possibility of the activation and dissociation of CO 2 and CH 4 on clean metals and on supported noble metals were discussed separately. There are different possibilities and therefore there are different theories for CO 2 dissociation or activation. CO 2 could dissociate on metal surfaces, this reaction could proceed via the formation of carbonate or formate species on the acidic or basic sites of the support, or on the metal-support interface. In the reaction the hydrogen assisted dissociation of CO 2 was also suggested. It is generally accepted that CH 4 decomposes primarily on the metal thus having appropriate dispersion of the metal is important. The activation and dissociation of methane can take place also on the surface O species. The main problem of the reaction is the carbon deposition and the reactivity of the surface coke. Surface carbon can be formed from methane while others suggest that the source of the surface carbon is CO 2 . Carbon can occur in different forms on the surface, which can be transformed into each other depending on the temperature and the time elapsed since their formation. These processes are influenced by, among others, the metal, the metal particle size, the support and therefore significant differences have been found in the order of activity of the catalysts, the carbon deposition, and in the reactivity of surface carbon.
Basically, two reaction mechanisms were proposed, according to the mono-functional mechanism the activation of both CO 2 and CH 4 occurs on the metal sites because the support is completely inert. In the bi-functional mechanism the CO 2 is activated on the support or on the metal-support interface and the CH 4 on the metal and the reaction intermediates react on the interface.