Prospects and Technical Challenges in Hydrogen Production through Dry Reforming of Methane

: Environmental issues related to greenhouse gases (GHG) emissions have pushed the de ‐ velopment of new technologies that will allow the economic production of low ‐ carbon energy vec ‐ tors, such as hydrogen (H 2 ), methane (CH 4 ) and liquid fuels. Dry reforming of methane (DRM) has gained increased attention since it uses CH 4 and carbon dioxide (CO 2 ), which are two main green ‐ house gases (GHG), as feedstock for the production of syngas, which is a mixture of H 2 and carbon monoxide (CO) and can be used as a building block for the production of fuels. Since H 2 has been identified as a key enabler of the energy transition, a lot of studies have aimed to benefit from the environmental advantages of DRM and to use it as a pathway for a sustainable H 2 production. How ‐ ever, there are several challenges related to this process and to its use for H 2 production, such as catalyst deactivation and the low H 2 /CO ratio of the syngas produced, which is usually below 1.0. This paper presents the recent advances in the catalyst development for H 2 production via DRM, the processes that could be combined with DRM to overcome these challenges and the current in ‐ dustrial processes using DRM. The objective is to assess in which conditions DRM could be used for H 2 production and the gaps in literature data preventing better evaluation of the environmental and economic potential of this process.


Introduction
The energy transition is currently one of the major global challenges and will require the development of various new technologies allowing the economical production of lowcarbon energy vectors.Hydrogen (H2) is an energy vector that can be used as a fuel or as raw material in the chemical industry and in the production of synthetic fuels (ammonia, synthetic CH4, synthetic liquid fuels and methanol) [1].Hence, due to its large use in highemitting sectors such as transport and industry, H2 (when produced in a sustainable way) can be a key enabler of the energy transition.
H2 is currently produced from fossil fuels, biomass or water.Currently, steam reforming of methane (SRM, Equation (1)) is the main process used for the dedicated H2 production [1].However, the production of H2 from fossil resources, such as natural gas or coal, is responsible for significant CO2 emissions.According to the International Energy Agency (IEA), 10 tons of CO2 are generated per ton of H2 when it is produced from natural gas.
In order for H2 to have a significant impact on the energy transition, it has to be produced in a sustainable way.Water electrolysis using renewable electricity is the main technology currently investigated since it produces high-purity H2 from renewable energy and water.However, there are still some drawbacks related to high costs, technology readiness level, renewable electricity supply and the coupling of electrolyzers with variable renewable energy depending on the type of electrolyzer used [2].There are already a large number of reviews available in the open literature about H2 production through water electrolysis [3,4].Hence, this technology will not be discussed in the present paper.Another alternative technology that has been considered in the literature for H2 production is dry reforming of methane (DRM, Equation ( 2)).This process allows the conversion of two major greenhouse gases (GHG) into a mixture of H2 and carbon monoxide (CO) with a H2/CO ratio of 1.

SRM:
CH4 + H2O ⇌ 3H2 + CO ΔH298K = 206 kJ mol −1 (1) However, there are issues preventing the use of this process at larger scale, especially for H2 production.DRM is a highly endothermic reaction and both CH4 and CO2 conversion are favored at high temperatures, especially above 600 °C. Figure 1 presents the equilibrium concentrations as a function of temperature for all the species when DRM is operated at atmospheric pressure [5].Several side reactions (Equations ( 3)-( 6)) producing solid carbon and water are also possible and they not only lead to catalyst deactivation, but they also decrease the final H2/CO of the syngas produced, which is a major issue when DRM is considered for H2 production.For temperatures above 800 °C, reverse water gas shift (RWGS) reaction (Equation ( 6)) is especially favored, which reduces the H2 production [6].[5] in the terms of CC-BY license.Copyright 2022 [5].

Methane cracking:
CH 4 ⇌ C (s) +2H Most of the recent DRM papers published in the literature are focused on catalyst development, especially on overcoming the constraints that lead to catalyst deactivation [7][8][9][10][11][12].However, very few of them actually analyze the viability of this process for H2 production [13].The present paper aims to assess and present in which conditions DRM could be used as a viable pathway for H2 production.The paper will first present the performance of the most recent DRM catalysts developed for producing syngas with high H2/CO.Then, it will present other reforming technologies currently investigated that could eventually be combined with DRM to increase the syngas' H2/CO ratio.This part will be followed by pilot and commercial scale DRM process reported in the literature as well as the goals of these processes.Finally, based on these findings, an outlook will be presented highlighting the viability of this process for H2 production based on the current findings and what it would take for its economic viability.

Recent Advances on Catalysts for Syngas Production
There are already numerous reports in the literature on the development of new catalysts for DRM to overcome catalyst deactivation via coking and sintering of support and active phase as well as to decrease the cost effectiveness of the overall catalytic system.Hence, these aspects will not be discussed in this work.The focus of the present section will be to identify the catalysts able to produce syngas with high H2/CO, aiming at H2 production as well as the process conditions required to achieve this ratio.

Noble Metal-Based Catalysts
The catalytic activity of noble metals from groups VIII to X (Ru, Rh, Ir, Pd, Pt) has been thoroughly investigated towards DRM when supported over different materials.These noble metals present high catalytic activity for CH4 and CO2 conversion (X CH 4 and X CO 2 , respectively) and distinct stability against thermal and oxidative sintering and, especially, coke deposition [14,15].The observed activity of noble metals for DRM follow the order Rh > Ru > Ir > Pt > Pd [16].Nonetheless, the use of noble metals for such purpose still present, as major drawbacks, the low abundance, high costs and usual toxicity [17].Recently, research has focused not only on highly active catalysts using noble metals at low concentrations (Table 1), which has been proven effective, but also on the promoting effects of noble metals for transition metal-based catalysts, aiming to enhance their resistance against coke deposition [18,19].
Ruthenium is one of the cheapest noble metals (still almost 500 times more expensive than nickel) that presents a good catalytic performance for DRM [16].Andraos et al. [20] explored the effect of different supports (Al2O3, MgAl2O4 and YSZ) on the DRM activity of mono-(Ru) and bimetallic (Ru, Ni) catalysts for hydrogen production.Despite the high catalyst activity of monometallic Ru/Al2O3 catalyst (T = 750 °C, X CH 4 = 90%, X CO 2 = 93%, H2/CO = 0.94), the highest hydrogen production was obtained when 5 wt% Ni/Al2O3 catalyst was promoted with 1 wt% Ru (T = 750 °C, X = 94%, X = 97%, H2/CO = 0.97).Ru functionalization also increased the overall catalyst stability by preventing catalyst sintering and the formation of poorly active phases, such as NiAl2O4 for the Ru-Ni/Al2O3 catalyst.On the other hand, poor metal dispersion after Ru promotion (maximum dispersion of 6%) was responsible for an increase in carbon deposition over the Ni-based catalyst (from 0.18% to 0.32% for Ni/Al2O3 and Ru-Ni/Al2O3, respectively) [20].A high metal dispersion was found to promote a stronger interaction between Ru particles and the catalyst support and lead to lower carbon deposition and increased catalyst stability [21].
Anil et al. [22] investigated the performance of partially substituted perovskites LaAlO3 by 2 wt% of noble metals Ru, Pt and Pd for the DRM reaction at 800 °C.The authors reported that Ru-substituted LaAl0.98Ru0.02O3−δpresented the highest hydrogen production, reaching a H2/CO ratio 0.85 with CH4 and CO2 conversions of 86% and 100%, respectively.Strong occurrence of RWGS reaction along with DRM was responsible for straying the H2/CO from unity in all cases, especially on the Pt-substituted LaAl0.98Pt0.02O3−δ(H2/CO = 0.60).Moreover, on the stability study at 650 °C for 50 h, no significant efficiency reduction was observed on the LaAl0.98Ru0.02O3−δ.
Rhodium has also been evaluated as an effective noble metal with distinct catalytic activity towards DRM [16].In the study of Moreira et al. [23], the performance of the 0.1 wt% Rh/Al2O3 catalyst was compared with 10 wt% Ni/Al2O3.The authors reported that, when incipient wetness impregnation method was used for catalyst preparation, the Rhbased catalyst presented a higher SBET (191.8 m 2 g −1 ) and a much higher metal dispersion (45.1%) when compared to the 10 wt% Ni/Al2O3 catalyst (154.3 m 2 g −1 and 1.9%, respectively).The catalytic tests at 700 °C showed that the hydrogen selectivity over the 0.1 wt% Rh/Al2O3 was slightly higher, leading to a H2/CO ratio of 0.91, while the 10 wt% Ni/Al2O3 presented a H2/CO ratio of 0.89 under the same test conditions.However, stability tests at 700 °C showed that the 0.1 wt% Rh/Al2O3 catalyst performed up to 184 h of time on stream (TOS) with H2/CO ratio higher than 0.95, CH4 conversion of 65% and CO2 conversion of 58% with no signs of deactivation.On the other hand, the 10 wt% Ni/Al2O3 catalyst showed a strong deactivation trend due to sintering and coke deposition at 24-70 h, with significant decrease in the H2/CO ratio to 0.80.
Platinum is a specially studied noble metal for DRM [24][25][26].Among the optimization goals on Pt-based catalysts, metal dispersion, size and, especially, phase of Pt particles (with ionic Pt 2+ presenting higher DRM activity than metallic Pt 0 ) have demonstrated a great deal of influence on the catalytic performance [27].Carvalho et al. [28] evaluated the deposition of Pt over mesoporous alumina promoted with metal oxides (MgO, ZrO2, CeO2 and La2O3) and reported good stability under DRM conditions.On this study, the authors reported that the Pt/CeO2-Al2O3 catalysts presented the highest activity for DRM at 700 °C with CH4 and CO2 conversions of 90% and 78%, respectively, and a high H2/CO of 0.90.The authors hypothesize that a combination of CH4 cracking and reverse Boudouard reaction may have compromised the H2 production over the other oxide-modified catalysts.Moreover, this catalytic performance was maintained for over 24 h even though coke deposition had taken place.
The resistance of Pt against coke deposition as a main trigger for catalyst deactivation was also reported by Xie et al. [29].The authors evaluated the addition of Pt to CeO2supported cobalt catalysts in order to describe a possible synergistic effect on the bimetallic catalyst responsible for an activity boost.Kinetic and density-functional theory (DFT) calculations revealed that the CO2 is much more easily activated over the PtCo surface (69 kJ.mol −1 ) when compared to the monometallic surfaces.In addition, the authors also reported that the enhanced presence of oxygen radicals on the support surface due to the oxygen storage capacity (OSC) of CeO2 also favored the activation of CH4 via oxygenassisted activation pathway (CH 4 * + O * → CH 3 * + OH * ).Furthermore, the authors reported that the Co/CeO2 catalyst presented low activity on higher temperatures (>600 °C), which yielded the lowest H2/CO ratio of 0.3.On the other hand, the Pt/CeO2 and PtCo/CeO2 catalysts presented around 95% conversion of both CH4 and CO2 at 750 °C with H2/CO higher than 0.95.Wet impregnation and precipitation 800 °C, CH4:CO2 = 1:1, Flow rate = 400 mL min −1 39-100 48-85 0.5-1.00.1-0.42wt% [61] nr-not reported; WHSV-weight hourly space velocity; GHSV-gas hourly space velocity; DMSdendritic mesoporous silica; MSC-mesoporous silica-carbon; MA-mesoporous alumina; SBA-15-mesoporous silica.

Transition Metals-Based Catalysts
Transition metals from the groups VII to X, especially Co and Ni, have demonstrated good catalytic performance for DRM (Table 1).Due to their high catalytic performance and low cost when compared to noble metals, they have been considered as the way to go on improving DRM towards industrial hydrogen production [15,16].However, the resistance of Ni-and Co-based catalysts against sintering and deactivation by coke deposition is much lower when compared to noble metals [18].In this sense, several studies have been focused on improving synthesis methods, catalyst support and metal-support interactions in order to increase both the activity, hydrogen selectivity and the stability of transition metal-based catalysts [62].

Nickel Catalysts
Nickel-based catalysts have been extensively reported as highly active for DRM, with a catalytic activity comparable to iridium (a noble metal with higher DRM than Pt and Pd) [63].Comprehensive studies have shown that the general bi-functional mechanism of DRM goes through a CH4 dehydrogenation over non-valent metal sites, while basic sites on supports and promoters are responsible for CO2 dissociation [64].It has been reported that Ni active sites may present a favorable electron structure for favoring the activation and cleavage of the C-H bonds, which is an important mechanism step for CH4 activation and H2 production [65,66].
Chein et al. [67] investigated hydrogen production over 10 wt% Ni/Al2O3 with CeO2 modification to better understand the role of feed composition and CeO2-Al2O3 interaction on the activity and stability of Ni-based catalysts.The authors reported around 75% conversion of CH4 and CO2 at 800 °C, over 10 wt% Ni/Al2O3 catalysts with a H2/CO ratio of 0.90.Adding up to 5 wt% CeO2 to the alumina support led to an increase in both CH4 and CO2 conversion to 85% and 90%, respectively.Moreover, H2 selectivity also increased, leading to a syngas with a H2/CO of 0.93.Increasing CeO2 loading from 5 to 15 wt% led to the reduction in the surface area from 104.5 to 90.5 m 2 g −1 , increased the probability of Ni particles' agglomeration and decreased the catalyst activity.Regarding feed composition, the authors reported that CH4 conversion increased when feed composition changed from CH4:CO2 = 1:1 to 1:3.However, this increase was not followed by a higher H2 production, and the overall best H2/CO ratio (0.90) was achieved at CH4:CO2 = 1:1.This observation points to the favoring of parallel reactions on higher CH4:CO2 ratios.
Ceria (CeO2) promotion was also found favorable for H2 production in a recent study by Farooqi et al. [11].The authors evaluated the effects of La2O3 and CeO2 promotions on 10 wt% Ni/ Al2O3 catalysts synthesized by the sol-gel method.The authors reported that La-promotion may present a dual role on DRM catalysts by increasing their resistance to coke as well as decreasing the acidic character of the support.However, CeO2 promotion was more advantageous when compared to La2O3 promotion when H2/CO was considered.CeO2 promotion led to a syngas with a H2/CO ratio of 0.95, while a H2/CO ratio of 0.89 was obtained when La2O3 was used as a promoter in the same test conditions.The authors highlighted that the presence of the CeO2 improved the active phase dispersion and reducibility, which led to higher H2/CO on the produced syngas.
Cruz-Flores et al. [68] evaluated the impact of a direct synthesis method via Ni-phyllosilicates for the production of Ni-SiO2 catalysts in comparison to the incipient wetness impregnation method on the catalytic activity and stability of Ni-based catalysts on DRM.The authors reported that the Ni-SiO2 catalysts synthesized via Ni-phyllosilicates presented a higher metal dispersion even for high Ni loadings (15 wt% Ni), which led to a reduced carbon formation on short reaction times.However, for longer reaction times (TOS > 180 min), significant carbon formation was observed in the temperature range of 600-900 °C.The results indicate that for T < 700 °C, the Boudouard reaction was favored, leading to a high carbon deposition.For T = 800-900 °C, the high H2 yields (above 90%) were followed by H2/CO ratio > 1.0 and high carbon formation, indicating the occurrence of CH4 cracking and CO disproportionation reactions.
Mahfouz et al. [69] used KIT-6 mesoporous silica as support for Ni catalysts synthesis via wet impregnation and reported a high surface area of SBET = 422 m 2 g −1 for the 15 wt% Ni/KIT-6 catalyst.The catalytic tests at T = 800 °C showed CH4 and CO2 conversions of 88% and 90%, respectively, and the production of syngas with a H2/CO ratio of 0.92.Moreover, the authors evaluated that, although cerium promotion (ceria/KIT-6 ratio = 3/2) on the 15 wt% Ni/KIT-6 showed a slight increase on CH4 and CO2 conversion, the H2/CO ratio at higher temperatures (>700 °C) was not altered.Furthermore, Mourhly et al. [70] have recently reported the synthesis of Ni-based catalysts over nano-sized mesoporous silica (MSN) as an alternative support.The MSN was prepared by alkaline leaching of silicarich pumice rock.Different Ni loadings varying between 5 and 20 wt% were added to the MSN support by wet impregnation and tested in DRM at 550-750 °C.This study showed that increasing the amount of Ni on the catalysts did not improve CH4 conversion due to the formation of Ni particles clusters with non-reactive aspects for hydrogen production.On the other hand, low Ni loading (5-10 wt%) led to high metal dispersion and strong metal-support interaction, favoring H2 yields close to 70%.
One-pot microwave-assisted combustion synthesis was reported to be a fast and costeffective method for producing high H2-yielding 10 wt% Ni/Al2O3 catalysts by Medeiros et al. [71].The authors showed that a low fuel/oxidant ratio for the combustion-based synthesis resulted in catalysts with high surface area (SBET = 266 m 2 g −1 ) and high reducibility (degree of reduction = 93%).Moreover, the microwave-synthesized 10 wt% Ni/Al2O3 catalysts presented an H2 yield of 58% with low coke deposition (1.2 wt%) and H2/CO ratio close to 1.0 over 20 h of TOS.The same research group has also reported microwave-assisted synthesis of alumina supports for DRM [36].The synthesized alumina presented a high surface area (221.2 m 2 /g) and was used as support for the deposition of 10 wt% Ni via incipient wetness impregnation for DRM evaluation.The catalyst presented significant DRM activity, with over 87% CO2 conversion (T = 700 °C, GHSV = 72 L g −1 h −1 ) at a 20 h test.The authors also report the production of a syngas with H2/CO = 1.09 and 13.7 wt% carbon deposition.
Miri et al. [72] recently investigated the influence of several promoters (Fe, La, Zr, Ce and Ca) on the catalytic performance of 10 wt% Ni/MgAl2O4 for DRM and reported that La, Zr, Ce and Ca were able to improve Ni dispersion over the catalyst, which could increase catalytic activity.Furthermore, Ce was found to be the promoter which better increased CH4 conversion (XCH4 = 48%), at an optimal loading of 3 wt% Ce.Finally, the authors evaluated that the overall H2/CO ratio could be tuned as a function of CH4/CO2 feed composition, and the highest H2/CO = 1.4 was obtained at CH4/CO2 = 2.0.

Cobalt Catalysts
Along with nickel and its applications as DRM catalysts, cobalt has also been investigated as an alternative, inexpensive and earth-abundant metal with distinct catalytic activities for CH4 reforming [73,74].However, cobalt-based catalysts have similar deactivation issues as those observed with nickel catalysts.Studies have shown that coke deposition is also a major trigger for Co-based catalysts deactivation, along with active sites oxidation [47,75,76].Mechanistic studies via DFT showed that the metal-support interaction, metal distribution (related to the metal particle size) and active site oxidation stage play a significant role on the reaction rates for H2 production and on the deactivation of cobalt sites during DRM.[77].
The influence of cobalt loading on sol-gel synthesized Co/MgO catalysts was investigated by Sukri et al. [47] with focus on the carbon deposition over the cobalt sites.The authors evaluated Co-loadings from 10 wt% to 25 wt% and reported an increase in the number of active sites at higher Co loads (20-25 wt%).On the other hand, the active phase at higher Co loads presented a weaker interaction with the catalyst support which led to a higher rate of carbon deposition.The similar catalytic activity shown by all evaluated catalysts showed a saturation behavior on the loading increase in Co.Furthermore, the authors evaluated the stability of 10 wt% Co/MgO catalyst for 50 h at 750 °C and reported the production of syngas with a H2/CO ratio of 0.94 along with high CH4 and CO2 conversions of 82% and 88%, respectively, and a low carbon formation rate (0.0072 g gcat −1 h −1 ).Al Abdulghani et al. [78] investigated the promotion of ZrO2-supported Co catalysts with boron (B) for increasing reactivity towards CH4 and CO2 conversion.The authors showed that B impurities interacted with Co particles on the catalyst and increased hydrogen production as a result of increased reactivity of the Co/ZrO2 catalyst by three times towards CH4 decomposition.
Ayodele et al. [79] investigated the influence of CH4:CO2 feed ratio from 0.1 to 1.0 on the DRM activity of CeO2-supported Co catalysts synthesized with 20 wt% Co by incipient wetness impregnation.The authors also reported that the highest CH4 (75-80%) and CO2 (80-90%) conversions were obtained for a temperature of 750 °C.The final H2/CO ratio reached was 0.98-1.0when the feed ratio (CH4:CO2) varied between 0.8 and 0.9.Moreover, although no activity loss was observed during the tests, TEM images of spent catalysts showed that the particle size of active sites increased, which could, in turn, lead to sintering and irreversible deactivation.
Tran et al. [80] evaluated a novel filament-shaped mesoporous γ-Al2O3 as catalyst support for 10 wt%Co nanoparticles and reported an increase in H2 production when compared to commercial γ-Al2O3 support.The authors reported that the highest CH4 (76%) and CO2 (82%) conversion was obtained at 800 °C and the H2/CO ratio increase from 0.83 to 0.89 when comparing the commercial as synthesized γ-Al2O3 supports, respectively.On the other hand, the authors highlight that such H2/CO increase was a result of CH4 cracking reaction (Equation ( 3)), which resulted in a higher carbon deposition (47 wt%) for the synthesized alumina support.
Tungsten carbide (WC) has been referred to in literature as presenting activity comparable to noble metals and a "Pt-like" behavior for CO2 conversion [81], which has motivated recent studies on its application [82,83].Li et al. [84] have evaluated the DRM activity of 13 wt% Co-based catalysts supported on activated carbon-modified WC (WC-AC) and reported high stability of the prepared catalysts.According to the authors, at 800 °C, both CH4 and CO2 conversions were maintained over 90% for a 50 h test with minor activity drop, while H2/CO ranged from 0.90 to 0.92 during the whole test.The authors evaluated that an optimal 6.5 wt% W loading on the support was responsible for decreasing the activation energy of the reactants and boosted the catalytic activity.Furthermore, Wang et al. [85] also evaluated the activity of the Co/WC-AC catalyst and reported the effects of La-promotion.Although no significant improvement on H2/CO ratio was observed, the authors showed that a 10-20 wt% La addition to the catalyst could prevent catalyst sintering and the formation of poorly active CoWO4 phase on the support, which could lead to catalyst deactivation.

Other Transition Metals
Ni and Co are the most investigated metals for their known and vastly reported catalytic activities towards DRM.However, recent studies have also investigated other transition metals and their role on the catalyst development process for a successful scaling up of DRM process.Dehimi et al. [86] evaluated the effect of temperature on the catalytic activity of Mo-based catalysts doped with Ni on DRM.The catalysts were prepared by incipient wetness impregnation with a nominal load of 20 wt% Mo and a variable load (2-10 wt%) of Ni.The authors reported that the increase in Ni loading to 10 wt% was responsible for a higher Mo dispersion over the catalyst support (Al2O3), which positively affected CH4 and CO2 conversion.However, the authors highlighted that although the high Mo loading may have been responsible for a low conversion at T = 800 °C (X = 30%, X = 48%, H2/CO = 0.40), the Mo-based catalysts presented no coke deposition after the catalytic tests, which may lead to a higher catalyst stability.
Vroulias et al. [87] has recently reported the investigation on the effect of W loading on Ni/Al2O3 catalysts for DRM.The catalysts prepared by wet co-impregnation had a Wloading ranging from 0 to 26.2 wt% of W and were tested for DRM at 700 °C.The highest concentrations of W on the catalysts (19.8 wt% and 26.2 wt%) inhibited DRM activity (X < 20%) and the best results were achieved with intermediate W loadings (6.3 wt% and 11.9 wt%; T = 700 °C, X = 55-60%, X = 60-70%).These loadings, however, led to the production of syngas with a low H2/CO ratio of 0.5-0.8.Finally, the authors reported that the presence of W was responsible for suppressing coke formation due to a double mechanism: the restrained carbon diffusion on the lattice of Ni-W alloys, and the carbon formation-gasification mechanism that took place on the surface of the catalysts and yielded the formation of tungsten carbides.

Bi-and Trimetallic Catalysts
Bi-and trimetallic catalysts have been widely explored for DRM (Table 1).Studies have shown that the presence of more than one metal on the catalyst may result in a synergistic effect, with all metals acting as active phases or, by a promotion effect, when the active metal has its intrinsic activity enhanced by the presence of a second/third doping metal.Such synergistic effects could be explained by the electronic modifications on the catalyst surface as a result of the presence of a second/third metal, which could translate as increasing reactants chemisorption, for example [88].Those effects can range from the stabilization of metal particles to improvements in metal-support interactions, metal dispersion and reducibility [89,90].Increased metal dispersion and strong metal-support interactions, for example, have already been linked to performance improvements on DRM, such as higher H2 yields and suppressing of coke formation [91].Furthermore, recent DFT modeling and calculations have considered that the appropriate fine-tuning active phase composition can lead to the suppressing of DRM side reactions, such as RWGS, which would contribute to higher H2/CO ratio on the final syngas [92].
Al-Fatesh et al. [93] evaluated the effect of calcination temperature and reduction protocols on the DRM activity of Ni-Co bimetallic catalysts supported on Al2O3-ZrO2 mixed oxide.The catalysts were prepared by co-precipitation and subjected to different in situ and ex situ calcination and reduction protocols before being tested on DRM reaction at 500-800 °C.The authors demonstrated that the catalysts calcined at 800 °C and reduced in situ prior to catalytic evaluation at 700 °C yielded the best performance, with CH4 and CO2 conversions of 67% and 76%, respectively, and a H2/CO ratio of 0.91.The authors reported that the calcination at a high temperature triggered sintering and particle growth to a degree that increased both catalytic activity and stability.Bimetallic Ni-Co catalysts were also investigated by Movasati et al. [60].The catalysts were prepared by coimpregnation using CeO2-ZnAl2O4 mixed oxide as support.In this study, the authors reported that the presence of cobalt increased the catalyst's surface area when compared to Ni monometallic catalysts from 39.87 m 2 g −1 to 46.27 m 2 g −1 .The number of basic sites also increased from 180.3 μmol g −1 to 190.1 μmol g −1 when Co was added as a promoter.The metal-support interaction was also favored and prevented the sintering of the active phase.Finally, Ni-Co/CeO2-ZnAl2O4 catalyst yielded a CH4 and CO2 conversion of 76% and 88% at T = 700 °C, respectively, with a high H2/CO ratio of 0.99.
Turap et al. [94] investigated different Co/Ni ratios on the preparation of CeO2-supported Co-Ni catalysts for DRM application.The authors prepared a series of bimetallic catalysts by co-impregnation with Co/Ni ratios of 0.3-1.0 with a fixed 10 wt% Ni content and evaluated the catalytic activity for DRM between 600 and 850 °C.Reducibility studies showed that at the ratio of 0.8 Co/Ni, the Co addition improved catalyst reduction capacity, increased the metal-support interaction, when compared to Co-and Ni-based monometallic catalysts, and changed the configuration of bulk oxygen from CeO2 support.In addition, the 0.8Co-Ni/CeO2 catalyst presented the best catalytic performance at 850 °C, reaching CH4 and CO2 conversions of 99% and 100%, respectively, and a H2/CO ratio of 0.98.
Trimetallic NiFeCu alloys with a Ni/Fe = 3.0 and different amounts of Cu were prepared by co-precipitation and investigated as a catalyst for hydrogen production via DRM in a recent work by Jin et al. [95].The catalytic performance tests showed that the excess addition of Cu (Cu/Fe = 1.5) results in a decreased initial activity for CH4 and CO2 conversion and severe deactivation by carbon deposition (12.7 wt%), due to possible coverage of Ni active sites by Cu.Overall, the Cu addition in the ratio of Cu/Fe = 0.5 delivered the highest H2/CO = 0.60 at T = 750 °C with a CH4 conversion of 50%.H2 production of the trimetallic catalyst was comparable to the bimetallic NiFe catalysts, but the presence of Cu increased the amount of oxygen species on the catalyst surface and decreased carbon deposition by 50%.

Role of Catalyst Support
Although studies have stated that the catalyst support itself would not take part in the DRM reaction [16], it has been demonstrated that several support's features, such as the surface basicity, oxygen mobility and storage capacity, as well as the ability of interacting with active phases, influence catalytic performance [64,90].
The support's surface acidic-basic equilibrium may influence the reaction performance in several different aspects.While acidic sites favor CH4 cracking, which leads to carbon formation [96], the increase in the support's basicity is related to an enhanced removal of carbon deposits, which may lead to higher catalyst stability [97].Gao et al. [98] indicate that such acidic-basic equilibrium on the catalyst support may also influence the electronic environment of the active sites and further favor/inhibit side reactions such as CH4 cracking and CO disproportionation.
The addition of MgO to Al2O3 support is a well-described method of increasing support basicity for DRM.Song et al. [99] have described the role of MgO as two-fold.First, the basic properties of MgO increase the adsorption of CO2, which leads to a higher release of oxygen on the support's surface and, in turn, increases the coke resistance via the gasification of carbon deposits (reverse Boudouard reaction).Second, the presence of MgO leads to the formation of a layered double hydroxide, in which the Mg 2+ and Al 3+ ions are uniformly dispersed and help decrease the agglomeration of active metal particles and, in turn, increase sintering resistance.On the other hand, excessive basicity may also lead to carbon deposition, decrease metal dispersion, lead to lower H2/CO ratios and compromise the overall catalyst performance [98,100].Bagabas et al. [97] investigated the optimization of the MgO content on Al2O3-supported Ni catalysts for DRM and reported that the highest H2/CO = 0.95 (T = 800 °C, X = 86%, X = 91%) was obtained with 2.0 wt% MgO, while higher contents were actually detrimental for the process.
Al-Fatesh et al. [101] evaluated hydrogen production over a series of transition metalmodified (Ti, Mo, Si, W) alumina supports with Ni active phase and reported that adding Si and W increased catalyst thermal stability and yielded 60% and 70% H2 production, respectively, at T = 700 °C.In this study, the authors highlighted the importance of metalsupport interaction on the catalyst activity and showed that harmful interactions between the active phase and the oxide support may significantly compromise the catalyst performance.The formation of poorly active NiTiO3 and NiMoO species, for example, on Tiand Mo-modified alumina was responsible for a drop in H2 yields to 30% and 45%, respectively, at T = 700 °C.
Ceria and ceria-modified supports have been investigated for their oxygen storage abilities due to the capacity of cerium to easily change oxidation states from Ce 4+ to Ce 3+ .Studies have shown that high oxygen retention accounts for a higher catalyst reducibility, which may lead to increased activity, as well as higher oxygen mobility and carbon gasification effectiveness [102,103].The oxygen mobility on the catalyst support bulk structure may influence not only the reaction mechanisms, for example, under DRM conditions, but also the metal-support interaction and the overall catalyst stability [104].On DRM process, the role of oxygen vacancies on the reaction mechanism is two-fold.First, the oxygen vacancies in the lattice of the support act as activation sites for the CO2 to promote the cleavage of the C-O bonds, which increases the overall amount of oxygen in the support surface.Second, the presence of oxygen vacancies is responsible for oxygen mobility on the bulk catalyst support, which can react with carbon deposits to produce CO and avoid catalyst deactivation by coke deposition [14].
The effect of CeO2 modification on catalyst supports was investigated by Faria et al. [105] using a ceria-zirconia mixed oxide promotion on alumina-supported Ni catalysts.The use of ZrO2 in a solid solution with CeO2 has been proved to increase and stabilize the oxygen vacancies, and, consequently, the oxygen storage capacity of the mixed oxide, via the atomic mobility inside the lattice to accommodate the two components with different atomic radii [106].The ceria-zirconia promotion over the alumina support enhanced the oxygen mobility of the catalyst and was found to also increase metal-support interaction, but the probable occurrence of RWGS reaction was responsible for a syngas composition below 1.0 (0.75-0.85) [105].
Overall, catalyst development for hydrogen production via DRM is a major challenge, since the reaction also produces CO in the same amount of H2 (H2/CO = 1) and since side reactions (Equations ( 3)-( 6)) further decrease the H2/CO ratio.Low hydrogen production resulting from competing reactions, catalyst deactivation due to carbon build-up and particle agglomeration can be overcome by the incorporation of promoters and advances on catalyst synthesis methods.However, there is a lack of literature data on the development of catalysts with high H2 selectivity in DRM, even in papers dedicated to H2 via DRM.Further mechanism and kinetic studies are required to better understand how to increase the H2/CO in this process while maintaining the catalytic performance of the catalysts.
Furthermore, different operating strategies, such as combining DRM with other reforming technologies, can also be used to increase the H2/CO ratio of the syngas produced.The catalytic aspects of different reforming processes and their combinations are discussed in the next sessions.

Alternative Processes for Increased H2/CO Ratio
CH4 is a major chemical feedstock and the global primary source for hydrogen, accounting for more than 90% of the annual hydrogen production, via different catalytic reforming processes [107].While SRM is the most mature and most used technology, several other processes, such as DRM, chemical looping methane dry reforming (CL-DRM), methane partial oxidation (POM), combined steam and dry-reforming (CSDRM), for example, have been investigated with a focus on reducing the costs and overcoming the environmental and technical challenges of SRM [12].The H2-production potential of each of those technologies can be evaluated by their resulting H2/CO ratio of produced syngas, which is a function of different catalytic and reaction conditions.Different processes can produce syngas with different H2/CO ratios [108] and, in the case of DRM, this ratio is close to the unity [109], which is suitable for hydrocarbons and methanol production [110,111].When it comes to hydrogen production, the importance of securing a H2/CO ratio as close as possible to the unity is even greater [112], for obvious reasons, but when it comes to syngas production, this is also essential for the direct utilization of the syngas with no need for further hydrogen addition.

Steam Reforming of Methane (SRM)
Steam reforming of methane (SRM) is the most mature and currently applied technology for hydrogen production.This highly endothermic process (Equation ( 1)) is operated at high temperature (700-1000 °C) at around 3-40 bar to produce a hydrogen-rich syngas with H2/CO = 3.0 [113].Figure 2 presents the thermodynamic equilibrium of the SRM process when operated at two different H2O/CH4 ratios.When an equimolar H2O/CH4 is considered (Figure 2A), H2 is produced above 180 °C, and this production increases with the temperature increase.Solid carbon is usually obtained in the range between 450 and 800 °C, which can compromise the operation and lead to catalyst deactivation.For H2O/CH4 = 3.0 (Figure 2B), the carbon deposits are potentially eliminated and CH4 can be completely consumed above 700 °C, which justifies the usual operation with excess steam.For hydrogen production purposes, the SRM is usually combined with water-gas shift reaction (WGS; Equation (7)) operated simultaneously to reduce the CO content and boost the hydrogen concentration [114].Alumina-supported Ni-based catalysts are also commonly used in SRM.For this reaction, similar problems with coke deposition and catalyst sintering are also observed, with the aggravating factor that the carbon deposits can not only reduce the catalytic performance but contribute to an increased pressure drop on the reforming reactors.The operation of SRM under excess steam conditions (H2O:CH4 = 3-5) has been known to help in controlling the carbon deposition over the catalysts [116].
Noh et al. [117] evaluated the performance of Ni-based catalysts supported over calcium aluminate modified SiC for SRM.In this study, the authors aimed to stabilize the H2 production via SRM by taking advantage of the Lewis basicity of calcium aluminate support to avoid coke deposition.Results revealed that the structured catalysts produced delivered a CH4 conversion above 80% during 20 h long tests, with H2 yields above 95% and negligible carbon deposition (T = 850 °C, H2O:CH4 = 3).Controlling the coke deposition as a means of enhancing hydrogen production was also the goal of Cho et al. [118] when investigating eggshell-type Ni/MgAl2O4 pellets as a catalyst for SRM.The authors showed that homo-type catalyst pellets were more prone to carbon deposition and particle agglomeration due to low metal dispersion.On the other hand, superior metal distribution on eggshell-type catalysts led to lower carbon accumulation (2.5% and 39.5%, for eggshelland homo-type catalyst pellets, respectively) and more stable SRM operation.In this case, CH4 conversion remained around 50% for TOS = 6 h with high H2 selectivity (>85%) and overall H2/CO = 0.80-0.85.
Membrane-type reactors and catalysts have recently been investigated for SRM applications towards hydrogen production.In membrane-based configurations, the produced hydrogen can be removed from the reformed gas while the reaction is still taking place.This sort of purification process can produce a high purity H2 stream without the requirements of a second downstream stage and alter the reaction equilibrium to promote higher CH4 conversions.Kim et al. [119] investigated the use of a Pd composite membrane reactor packed with Ru/Al2O3 catalyst for CH4 reforming at mild conditions (T = 500 °C, H2O:CH4 = 3).Despite the low operating temperature, CH4 conversion reached 79.5% when high transmembrane pressure difference (5 bar) was applied to the system.Moreover, the purity of the H2 permeate stream remained above 97% on 145 h long tests and accounted for 99% hydrogen recovery.Wang et al. [120] reported using Ni-based hollow fiber membrane catalysts for SRM and reaching 99% CH4 conversion at 800 °C with hydrogen purity above 95%.In both cases, the authors stated that the transmembrane pressure difference proved to be a very important parameter influencing CH4 conversion for promoting solution and diffusion mechanisms of H2 through the membrane structure.

Chemical Looping Dry Reforming of Methane (CL-DRM)
Chemical looping (CL) is a relatively recent process that has been investigated mainly for CO2 capture and utilization (CCU) [121,122].In the CL process, a reaction is "broken down" to a series of multiple sub-reactions that take place separately.In this process, materials with a high oxygen storage capacity, here called oxygen carriers (OCs), are used as reaction intermediates that can be recycled along the process and regenerated by oxygen and energy transfer [123,124].Several CH4 processing strategies such as partial oxidation and dry reforming can benefit from the stepwise CL strategy.For DRM, chemical looping dry reforming of methane (CL-DRM; Figure 3) consists of two steps (Equations ( 8) and ( 9)).First, CH4 is reacted with an OC and is oxidized to produce a hydrogen-rich syngas mixture (H2/CO = 2.0).This step is usually called the "reduction step" since the OCs are reduced in the process of syngas production.Second, carbon dioxide is used as an oxidant to recycle the OCs in the usually called "oxidant step", producing CO in the process, which can be recovered separately or mixed with the syngas from the first step [125].
The CL-DRM operation offers numerous advantages when compared to the classical co-feed strategy.Overall, the global process corresponds to a classically operated DRM reaction and still presents an endothermic character.However, the equilibrium of the reaction is no longer dependent solely on the thermodynamics and the reactivity of the reactants and the catalysts, but rather on the redox properties of the OCs, which are the major focus of research and optimization [126].Chein and Hsu [126] evaluated the thermodynamic equilibrium of CL-DRM as a function of the OC:CH4 ratio, using Fe2O3 as OC.For OC:CH4 < 1.0, POM is the main observed reaction (Equation ( 10)), yielding H2 and CO as the main products (H2/CO ≈ 2.0).For OC:CH4 > 1.0, H2 and CO production decrease as temperature increases, and CH4 combustion is favored (yielding CO2 and H2O) as the OC:CH4 also increases, reaching total methane combustion for OC:CH4 > 12.0.Overall, the authors evaluate that an optimal OC:CH4 = 1.0 could be used to obtain syngas with H2/CO = 1-2.5 according to the CH4/CO2 feed ratio.
Furthermore, two problems observed on DRM can be avoided by the stepwise operation of CL-DRM.The accumulation of carbon on the catalysts (which is now an OC) due to CH4 dissociation on the reduction step is overcome by the re-oxidation cycle.In this scenario, the deposited coke is converted to CO during the oxidation step.Moreover, since the two reactants, CH4 and CO2, are not in contact with CL-DRM operation, the produced H2 on the reduction step is never in contact with the CO2 injected on the oxidation step.Hence, the occurrence of RWGS reaction (Equation ( 6)), which decreases the H2 selectivity, is also avoided [127,128].Guerrero-Caballero et al. [129] investigated the performance of a series of CeO2-based oxygen carriers with Ni, Co and Fe for CL-DRM application.The authors evaluated the influence of preparation method and metal active phase on the catalytic activity of the obtained powders.The catalytic activity of all samples was tested over 12 cycles of CL and presented a relatively stable conversion of both CH4 and CO2 over the reduction/oxidation cycles.Ni/CeO2 presented the best catalytic performance (T = 800 °C, X CH 4 = 90%, X CO 2 = 85%) and maintained a H2/CO ratio slightly above 2.0, which is the ratio expected for CL-DRM (Equations ( 8) and ( 9)).Despite a lower reactant conversion observed for CeNi0.3Oy(T = 800 °C, X = 62%, X = 62%), the catalysts prepared by co-precipitation presented no coke formation, a steady H2/CO = 2.0 over the 12 CL cycles and no particle sintering, which was related to a better dispersion of the Ni particles on the CeO2 structure.The influence of the active metal (Ni, Co and Fe) was investigated over wet impregnationprepared catalysts.The CL-DRM experiments showed that Co/CeO2 (T = 800 °C, X = 80%, X = 85%) catalysts presented a slightly lower CH4 conversion when compared to Ni/CeO2, while maintaining a similar H2/CO between 2.0 and 2.1.Moreover, similar results were obtained at long-term stability tests that comprised 60 CL cycles, with stable H2/CO ratio between 2.0 and 2.1.As for the Fe/CeO2 catalyst, very low CH4 conversion of less than 5% and rapid complete deactivation (after 6 CL cycles) were obtained.These results were related to the formation of poorly active CeFeO3 mixed oxide.Interestingly, the analysis of spent catalysts showed that Ni and Co were not able to follow the oxidation/reduction cycles, i.e., once they were reduced on CL-DRM conditions, the metallic species were not oxidized again, which highlighted the importance of the oxygen storage capacity of CeO2 on the operation of CL-DRM.
Kim et al. [130] described an efficient CL-DRM process using Ni-Fe-Al mixed oxides as oxygen carriers.In this study, the group showed the superior activity of the trimetallic mixed oxide over binary metallic mixtures, which were submitted to significant deactivation over 12 CL cycles, either by activity loss towards CH4 decomposition or inefficient coke removal on the oxidation step.Overall, Ni-Fe-Al mixed oxides were able to maintain the catalysts' activity over 20 CL cycles, with no signs of deactivation, to avoid particle agglomeration and activity suppression and to deliver a stable H2/CO of 2.1.Interestingly, the authors also demonstrated the potential of Ni-Fe-Al mixed oxide for classical co-fed DRM.In this case, results showed that a H2/CO ratio slightly below 1.0 was achieved for CH4:CO2 = 1:1 feed at T = 650 °C, with over 80% conversion of both reactants.
Sastre et al. [131] evaluated the perovskite-type La0.9Sr0.1FeO3supported on yttriastabilized-zirconia (YSZ) for DRM and CL-DRM and reported a high H2 production (H2/CO = 6.5) during the reforming step for the looping operation.The authors evaluated the DRM process at 850 °C for 24 h and obtained an H2/CO between 0.95 and 0.98 during the tests.For the CL operation, 20 min looping cycles were performed up to 7 h.The authors reported that for the reforming step (CH4 injection), a predominant CH4 cracking (Equation ( 3)) reaction took place and yielded high H2/CO ratios (up to 6.5) with high coke formation.On the other hand, coke was successfully removed during the oxidation step (CO2 injection) and the catalysts did not present significant deactivation during CL operation.

Combined Steam and Dry Reforming of Methane (CSDRM)
A combination of steam and dry reforming of methane (CSDRM) has been investigated as an alternative for modulating the H2/CO ratio with a focus on increasing H2 production and allowing for a higher versatility on the chemical and fuels production steps [132].The use of steam on the feed for DRM may account for a reduced coke formation by oxidation of adsorbed CHX that could otherwise lead to carbon deposits [133].In addition, studies have shown that the convenient adjustment of the CH4/H2O ratio on feed composition leads to the desired modulation of H2/CO ratio [132,134].
Figure 4 presents the thermodynamic equilibrium of CSDRM at different pressures, considering a CH4:CO2:H2O feed ratio of 3:1:2.As both SRM and DRM have an endothermic aspect, the conversion of all reagents is favored at higher temperatures.Lower pressures are related to a decrease in both CH4 and CO2 conversion and lower H2 selectivity.H2/CO ratio decreases with the temperature increase and stabilizes around 2.0 in the range of 800-1000 °C [135].Dan et al. [136] investigated the performance of 10 wt% Ni/Al2O3 catalyst, which is a classical reforming catalyst, on CSDRM conditions.The study has shown that a higher hydrogen production can be achieved by the CSDRM operation at 700 °C with 100% conversion of CH4 and a high H2O/CO2 ratio on the inlet feed.For H2O/CO2 ratios of 2.5, 7.3 and 12.7, the H2/CO ratios achieved on this study were 2.2, 5.0 and 8.0, respectively.In addition, the stability of the 10 wt% Ni/Al2O3 catalyst was evaluated for 24 h at CSDRM conditions and 100% CH4 conversion was maintained for the whole test when H2O/CO2 = 7.3 and 12.7 were used at 700 °C.Moreover, analysis of spent catalyst showed no oxidation or size changes of Ni sites, indicating that deactivation by catalyst sintering or oxidation was not triggered on the reaction conditions, and no carbon deposits were detected.
Batebi et al. [137] reported the influence of feed composition on the CSDRM activity and final hydrogen yields over Ni-and Pd-based catalysts supported on Al2O3.Monoand bimetallic catalysts were prepared by sol-gel method and tested for CSDRM at 500-1000 °C with various (CO2 + H2O)/CH4 ratios (1-3) and CO2/H2O ratios (1-3) under atmospheric pressure.A central composite experimental design was used to evaluate the statistical significance of the influence of each reaction condition on the final CH4 conversion and H2 yield.Based on the ANOVA results, the authors showed that higher temperatures (T > 700 °C) were crucial to achieve higher CH4 conversions (>75%) due to the endothermic character of both SRM and DRM reactions.As for the influence of feed composition, the authors confirmed that the CO2/H2O ratio had no significant impact on the final CH4 conversion and H2 yields.However, when the amount of CO2 was higher in the inlet feed, lower H2/CO ratios were obtained.Finally, higher H2 yields were favored by lower (CO2 + H2O)/CH4 ratios at temperatures lower than 1000 °C, when competitive RWGS reaction takes place and reduces the H2 final production.

Partial Oxidation of Methane (POM)
The catalytic partial oxidation of methane (POM) has attracted attention for being a more energy-efficient process when compared to commercial SRM.The fast kinetics of POM lead to a good response time and system compactness, while its exothermic character is less energy demanding and avoids the requirements of high temperature steam [138].In the partial oxidation process, CH4 (or other hydrocarbons) is reacted with stoichiometric amounts of pure oxygen and yields a H2/CO ratio of 2.0 for the produced syngas (Equation ( 10)), which is suitable for Fischer-Tropsch synthesis [139].However, controlling the reaction selectivity to avoid total combustion can be challenging [140].Figure 5 presents the thermodynamic equilibrium for POM as a function of temperature for different operating pressures for a stoichiometric feed (CH4:O2 = 2.0, Equation ( 10)).Enger et al. [141] highlight that, at higher pressures, higher temperatures are required for obtaining a high selectivity towards H2 and CO (Figure 5b,c).In addition, the authors also comment that high H2 selectivity (>80%) can be obtained for temperatures as low as 550 °C if CH4:O2 ranges between 2.0 and 5.0.Classical noble-and transition-metal-based catalysts are the most reported for POM, and carbon deposition is also a major trigger for catalyst deactivation.Moreover, the temperature gradients resulting from the exothermic POM reaction increase the probability of catalyst deactivation due to particle sintering, which leads to more challenges on catalyst development [139].
Alvarez-Galvan et al. [139] evaluated the performance of different Ni-based catalysts for hydrogen production via POM.Catalysts containing 5 wt% of Ni doped on Al2O3, CeO2, La2O3, MgO and ZrO2 supports were tested for POM activity under stoichiometric feed conditions at T = 750 °C and atmospheric pressure.Among the prepared catalysts, 5 wt% Ni/MgO and 5 wt% Ni/La2O3 presented low activity and rapid deactivation, due to the formation of non-catalytic phases between the metal and the oxide support.The alumina-supported 5 wt% Ni/Al2O3 revealed to be the most efficient and most stable catalytic system, with >80% CH4 conversion and H2 yields throughout the catalytic tests.In addition, the authors showed that Rh-promotion over the Ni/Al2O3 catalyst was responsible for a slight increase in CH4 conversion and H2 yield up to 90%.Finally, the H2/CO ratio for the obtained syngas was stable around 2.5 for the Ni/Al2O3 catalyst with or without Rh-promotion.Ding et al. [142] also investigated the hydrogen production via POM over Ni-based catalysts and reported the use of mesoporous oxides La2O3, Yb2O3, ZrO2 and CeO2 for the modification of SiO2 support.The catalysts were prepared with a 10 wt% Ni loading and tested for their POM activity under stoichiometric feed conditions, atmospheric pressure and T = 800 °C.The authors demonstrated that the modifications with mesoporous solid oxides were able to increase Ni dispersion on the support, which increases catalytic activity and CH4 conversion >75% (reaching 90% for 10 wt% Ni/ZrO2-SiO2).Moreover, the authors reported that hydrogen production via POM can profit from oxygen mobility of oxide supports such as ZrO2, which promotes the combustion-reforming mechanism.In this mechanism, CH4 and O2 first react to form CO2 and H2O, followed by a combined steam and dry reforming promoted by the combustion products and the excess CH4 [139].In this case, H2 selectivity increased almost 20% and the final H2/CO ratio of produced syngas stabilized at 3.0.

Methane Tri-Reforming (TRM)
The methane tri-reforming (TRM) process refers to a combination of the SRM, DRM and POM processes, in which methane is reacted with a mixture of steam, CO2 and oxygen to produce syngas [143].According to its thermodynamic aspects (Figure 6), TRM is favored at higher temperatures (T ≥ 700 °C) due to the already discussed endothermic aspects of the individual reforming reactions (SRM and DRM) and the stability of the reactants.Moreover, lower pressures (1 bar) also favor the TRM equilibrium, although typical reforming reactors usually operate at higher pressures (3-30 bar) to decrease reactor size and meet high-pressure requirements for syngas downstream processes [115,144].Under stoichiometric conditions (2 mol of CH4, 1 mol of H2O, 1 mol of CO2, 0.1 mol of O2), CH4 is almost completely converted (>98%) above 850 °C, oxygen is completely consumed via POM and high syngas selectivity of 1.65 is reached above 800 °C [115].However, the main advantage of the TRM process is the adjustable H2/CO ratio of the produced syngas according to the feed composition, which makes TRM a versatile CH4 reforming process that can be controlled for various downstream applications, such as methanol or Fischer-Tropsch synthesis [145].As a combination of different catalytic reforming processes, TRM also profits from the low cost and known catalytic activity of Ni-based catalysts for reaction with C-H bonds.Maciel et al. [146] evaluated the performance of 5.75 wt% Ni/Al2O3 catalyst for TRM under 1 bar and feed composition CH4:CO2:H2O:O2 = 1.0:0.49:0.3:0.04.The authors reported that the highest hydrogen yield of 37% was achieved at temperatures higher than 850 °C.The competing RWGS reaction was predominant for T < 850 °C and decreased the H2 yield to 4.4%.Majewski and Wood [147] prepared core-shell-type 11 wt% Ni/SiO2 catalysts for TRM and reported high hydrogen production and low coke deposition for T = 750 °C.At this temperature, SRM and DRM reactions were favored, which increased H2 production, while for T ≥ 800 °C, RWGS took place and decreased the overall H2/CO ratio to 1.5-1.7.In this study, the authors reached a H2/CO ratio of 2.6 with 73% and 56% of CH4 and CO2 conversion, respectively, and low carbon formation (5.0 mg g −1 catalyst) for a feed composition of CH4:CO2:H2O:O2:He = 1.0:0.5:0.5:0.1:0.4.
Pino et al. [148] investigated hydrogen production by TRM over 1.76 wt% Ni/CeO2 catalyst at T = 800 °C and described the impact of La-doping and different feed compositions on the final H2/CO ratio.The authors reported that La modifications on the catalysts were able to increase the presence of oxygen vacancies on the ceria support and enhanced Ni dispersion, which account for high CH4 (96%) and CO2 (86%) conversions.However, the La-doping did not show any significant impact on hydrogen production.The modifications on feed composition, on the other hand, were able to modulate the final H2/CO ratio of the syngas.The authors demonstrated that the H2/CO ratio could drop from 2.8 to 1.3 when the inlet molar ratio of CO2/H2O varied from 0.0 (in the absence of CO2) to 2.6.According to Pham Minh et al. [145], the modulation on the H2/CO ratio as a function of the feed composition is due to competitive CH4 reforming reactions that take place as a result of the presence of multiple oxidants (CO2, H2O and O2) on the TRM process.

Large-Scale Applications of DRM
The potential benefits of industrial implementation of DRM have driven several research groups into investigating not only the catalyst optimization aspects but also the process scaling up to commercial level [150].Over the years, different groups have evaluated several aspects of the DRM process on a semi-pilot and pilot scale, aiming at bringing it into commercial application.Kahle et al. [151] have demonstrated the use of Pt-based catalysts for DRM at 850-950 °C and 20 bar and have considered different process aspects such as catalyst position, temperature gradient and feed composition aiming at reducing coking formation at pilot-scale conditions.The authors reported reaching a H2/CO = 0.78 at T = 850 °C, 20 bar, CH4:CO2 = 1:1.In addition, studies have also addressed other aspects, such as the type of reactor [152] and catalyst structure (in the case of different shapes of pelletized catalysts) [150], aiming at maximizing DRM activity and reducing deactivation rates at pilot scale.
While CH4 reforming technologies, such as autothermal reforming and steam reforming, are already used in industrial scale, the commercial application of DRM has been limited as a part of some industrial processes, such as the CALCOR™, SPARG™ and MIDREX™ processes [153].Although such processes do not target H2 production, understanding the commercial-level applications of DRM is a valuable way of assessing the advantages and limitations of the technology.Table 2 presents a summary of the main aspects of the pilot-and commercial-scale applications of DRM.TRL-Technology readiness level; LPG-Liquified petroleum gas.
The CALCOR™ process (Caloric GmbH) aims at the production of highly CO-concentrated syngas (Figure 7).In the first stage of this process, syngas with a low H2/CO ratio (0.42) is produced from the dry reforming of liquefied petroleum gas (LPG) or natu-ral gas (NG) with excess CO2, over a Ni-based catalyst [164].In this process, the coke deposition is minimized by using a catalyst bed with different stages of catalyst reactivity and different catalyst geometries [165].As DRM is a highly endothermic process [166], part of the fuel is burnt to provide heat for the reaction.The process outlet stream is a mixture of CO, CO2, H2, H2O and traces of unreacted CH4.The stream is cooled for H2O removal, then it is scrubbed through packed towers with monoethanolamine (MEA) or methyldiethanolamine (MDEA) for CO2 removal and, finally, is purified by a pressure swing adsorption unit (PSA) for H2 separation that would operate according to the CO purification requirements (can reach up to 99.95% CO) [155].The unreacted CO2 is recycled to the reformer and the H2 produced during the reaction is either burnt as fuel or sold as a separate product.Despite the toxicity, CO is an important chemical building block for the production of several carbon-based chemicals such as acetic acid and phosgene [155,164,167].The SPARG™ (sulfur passivated reforming) process, which was initially designed by Haldor-Topsoe and commercialized by Sterling Chemical Inc. in 1987, is another example of an industrial application of DRM for the production of syngas with lower H2/CO ratios.This process takes advantage of the already existing infrastructure for SRM, combining characteristics of both SRM and DRM, and it is able to deliver syngas with a H2/CO ratio ranging from 1.8 to 3.0 [158,165,168].The process can operate with different CO2 and H2O mixtures and can be considered a type of CSDRM [165].In this process (Figure 8), sulfur is selectively used to poison the catalyst sites that are most active for coke formation in order to minimize it during the reaction, while keeping part of the reforming activity [168].The SPARG™ process is operated between 915 °C and 945 °C, and the coke formation over the Ni catalyst is minimized by the sulfur pretreatment that passivates the main cokeactive sites on the catalyst [165].
Both the CALCOR™ and the SPARG™ processes are designed to prevent the formation of coke under DRM conditions.Unfortunately, these processes require specific conditions for their full applicability.Therefore, in order to reduce these barriers and increase the range of applicability of these processes, a better understanding of the mechanisms of coking and measures related to the development of catalysts are necessary, as well as the choice of the reactor material [158].The MIDREX™ process (Figure 9) was developed in the 1960s and remains, to this day, a leading process in the field of the direct reduction technology [169].This process takes advantage of a combined DRM and SRM process to produce syngas which is later used as a reducing agent for the production of metallic iron [161].In the MIDREX™ process, the reformer is fed with a mixture of CO2:CH4:H2O = 1:1:1.The MIDREX reforming process is performed over a Ni-based catalyst supported on α-alumina, MgO or a combination thereof, and operated between 850 and 1000 °C at 1.9-3.0bar.Basic promoters such as Zr, Ce and La are also frequently used on commercial catalysts for this process, which can increase the overall basicity of the catalysts, prevent carbon deposition and stabilize Ni particles [160,161].Although detailed information on the syngas produced via the MIDREX™ process is not available, since the syngas is not actually the final product, modeling has indicated that the H2/CO ratio of the syngas should be in the range between 2.0 and 2.5 [160]  Linde and BASF have recently collaborated to the advancement of DRM industrial implementation.The DRYREF™ technology (Figure 10) has been developed by Linde with the aims of not only contributing to the efforts towards anthropogenic CO2 utilization, but also for reducing the capital (CAPEX) and operational (OPEX) costs of CH4 reforming technologies [170].The process is based on an innovative catalyst developed by BASF, the SYNSPIRE™ G1-110, which is a Ni-based oxide-supported catalyst, specially designed to prevent coke formation and to operate at pressurized reforming conditions (20-40 bar) [162].The DRYREF™ technology is an application of CSDRM, in which the use of CO2 is designed to reduce the energy requirements of the full SRM process, due to the energy-demanding steam production step.In the process developed by Linde, a lower steam-to-carbon ratio (S/C < 1.8) is combined with CO2 capture and recycling in order to reduce the overall carbon footprint of the process and the high steam requirements of SRM (S/C ≥ 2.5).The result is an overall 5% and 3% reduction in OPEX and CAPEX, respectively, and a variable syngas H2/CO ratio (dependent on the initial S/C ratio) in the range of 1.0-3.0,which can be suitable for several downstream processes, such as aldehydes, methanol, acetic and formic acid synthesis [163].The project started in 2010 and is currently on commercial demonstration scale since 2019, after the commissioning of the Linde Pilot Reformer, in Pullach, Germany [170].The highly endothermic aspect of DRM requires high operating temperatures for high syngas yields, which promotes undesired parallel reactions and deactivation of conventional catalysts by sintering mechanisms.In addition, the limited stability of conventional reactor materials at the required high temperatures further hinders the application of this process in industrial scale.Technological advancements are therefore required to obtain both reactors and catalysts that can withstand reaction temperatures above 1000 °C [171].
None of the processes presented above target the production of H2 as the final product.Hence, large-scale data on DRM aiming at a high H2/CO ratio, as well as detailed information on sizing and overall energy efficiency of the process and specific facilities, are not available in the open literature.Besides catalytic development and process optimization at laboratory scale, life-cycle assessment and techno-economic analysis of this process are also required to evaluate the real environmental advantages of producing H2 through this pathway and to assess its economic viability.

Conclusions and Perspectives
DRM has been extensively reported in the recent literature as a possible alternative pathway for sustainable H2 production since it uses CH4 and CO2 (two main GHGs) as feedstock.However, this process still suffers from catalyst deactivation in the harsh reaction conditions, mainly due to coking and sintering of the active phase.Moreover, economic issues must also be taken into account, especially when H2 production is considered, since this reaction also produces 1 mol of CO per mol of H2 and side reactions (Equations (3)-( 6)) may also occur, further decreasing the H2/CO ratio of the syngas produced.However, as presented in Section 2, the focus of catalyst development has been on improving the stability and activity of catalysts and the H2/CO ratio of the syngas produced is often secondary, even in papers that target DRM as a pathway for H2 production.Some of the works cited in Section 2 suggested that low metal loadings could lead to small metal particles that would be more available to the reactants and could increase the H2 production.High surface area of catalysts and high reducibility of the active phase would increase its activity and increase H2 production.Finally, tuning process conditions, such as CH4/CO2 feed ratio, can also improve CH4 decomposition and increase the H2/CO ratio of the syngas produced.However, there is still a lack of detailed data on the literature dedicated to improving the catalyst design to favor higher H2/CO ratios via this reaction.Some of the technical and economic challenges in DRM for H2 production could be partially overcome by either using a chemical looping process or by combining DRM with other technologies, such as steam reforming and partial oxidation.DRM operated in a chemical looping is an interesting process, since only CH4 and CO2 are used as feedstock, but the process produces syngas with H2/CO generally around 2.0, which is higher than the H2/CO obtained in classical DRM reactions.This is achieved by using oxidation and reduction cycles so the H2 produced from CH4 oxidation with the catalyst is not in contact with the CO2, which prevents the RWGS reaction and assures a high H2/CO ratio.However, catalyst development is still required to improve their oxygen storage capacity before utilization at larger scales.Combined steam and dry reforming of methane (CSDRM) have also been considered as an alternative process to the classical DRM to improve the H2/CO ratio.Steam is added to the feed allowing the production of syngas with a flexible H2/CO ratio up to 8.0 depending on the amount of steam used.The addition of steam also contributes to preventing carbon depositions and catalyst deactivation.However, issues related to the endothermicity of this reaction must also be considered.Methane tri-reforming, which adds steams and oxygen to the feed mixture, has also been investigated, since it can produce syngas with a higher H2/CO of around 3.0.This process can also cope for the endothermicity of the reactions, since part of the CH4 is combusted to generate heat for the reactions.However, there are issues related to catalyst deactivation via sintering and vapor-solid reactions that occur on the surface of catalysts.
Due to these issues related to catalyst performance and thermodynamics, only very few industrial processes use DRM at large scale.However, none of them aim at the production of H2 or of syngas with a high H2/CO ratio.The CALCOR™ process produces a CO-rich syngas while the SPARG™ process produces syngas with a H2/CO ratio lower than 1.Finally, the MIDREX™ process does not target H2 or syngas as the final product.The syngas produced in this process is used as a reducing agent for metallic iron production.
DRM is definitely a very promising process for the production of energy vectors, especially considering that GHGs are used as feedstock.However, the role that it will play in the future large scale of H2 production is still debatable.DRM still faces technical and economic issues related to catalyst deactivation and the endothermicity of the reaction.Moreover, there are still a lot of data missing on how to improve the performance of the catalysts and of the process to increase the selectivity to H2. Mechanistic and kinetic studies for better understanding the performance of the catalysts for H2 production and optimizing reactor design and process conditions are still required before the use of this process at larger scale.Finally, there is also a lack of data on life-cycle and techno-economic analysis of DRM for H2 production.Estimating the costs and the carbon intensity of H2 produced via this process are crucial to better understand the environmental and economic potential of using DRM for H2 production.

Figure 3 .
Figure 3. Schematic representation of classic co-fed DRM operation (A) and stepwise chemical looping DRM (B).
. The MIDREX™ process operates commercially since the 1970s and has currently over 100 plants installed in 21 countries, having decommissioned only the pilotand demonstration-scale plants built pre-1975 [159].

Figure 10 .
Figure 10.Simplified block diagram of the DRYREF™ process for CO and H2 production [162].

Table 1 .
Summary of catalytic systems recently reported for H2 production via DRM.

Table 2 .
Summary of large-scale DRM applications.