Catalytic Performance and Characterization of Ni-Co Bi-Metallic Catalysts in n -Decane Steam Reforming: Effects of Co Addition

: Co-Ni bi-metallic catalysts supported on Ce-Al 2 O 3 (CA) were prepared with different Co ratios, and the catalytic behaviors were assessed in the n -decane steam reforming reaction with the purpose of obtaining high H 2 yield with lower inactivation by carbon deposition. Physicochemical characteristics studies, involving N 2 adsorption-desorption, X-ray diffraction (XRD), H 2 -temperature-programmed reduction (H 2 -TPR), NH 3 -temperature-programmed desorption (NH 3 -TPD), SEM-energy dispersive spectrometer (EDS), and transmission electron microscope (TEM)/HRTEM, were performed to reveal the textural, structural and morphological properties of the catalysts. Activity test indicated that the addition of moderate Co can improve the hydrogen selectivity and anti-coking ability compared with the mono-Ni/Ce-Al 2 O 3 contrast catalyst. In addition, 12% Co showed the best catalytic activity in the series Co-Ni/Ce-Al 2 O 3 catalysts. The results of catalysts characterizations of XRD and N 2 adsorption-desorption manifesting the metal-support interactions were signiﬁcantly enhanced, and there was obvious synergistic effect between Ni and Co. Moreover, the introduction of 12% Co and 6% Ni did not exceed the monolayer saturation capacity of the Ce-Al 2 O 3 support. Finally, 6 h stability test for the optimal catalyst 12%Co-Ni/Ce-Al 2 O 3 demonstrated that the catalyst has good long-term stability to provide high hydrogen selectivity, as well as good resistance to coke deposition. in higher selectivity of hydrogen compared with NCA contrast catalyst. The results indicated that 12% Co showed better hydrogen selectivity (74%), n -decane conversion (100%), and anti-carbon ability (3.90 mg/g-fuel). Moreover, thermal stability and regeneration performance were also tested, and the results showed that reforming activity over C12-NCA catalyst was stable in 6 h, and the hydrogen selectivity and n -decane conversion over regenerated catalysts were almost close to the fresh ones. The results were mainly interpreted on the basis of characterization studies, which revealed the significant role of textural and structural properties, microscopic appearance and dispersion state on reforming performance. Obviously, the dispersion state and the BET-specific surface area decrease caused by the formation of the carbon deposition are the main reasons for the activity difference among these catalysts.


Introduction
Nowadays, hydrogen is recognized as a clean fuel and energy carrier since its combustion produces only water as product [1,2].However, how to produce hydrogen from primary energy sources (such as hydrocarbons) in an efficient and economic way should be further researched and developed [3][4][5][6].In the past few decades, the most effective approach was catalytic reforming of hydrocarbons.Currently, over 50% of the world's hydrogen supply is from steam reforming of hydrocarbons [7].
Nowadays, H 2 is mainly produced by steam reforming of CH 4 and other high-energy density liquid fuels, including ethanol, gasoline, diesel, or jet fuel [8][9][10].An interesting option is hydrogen production from diesel steam reforming.n-decane, one of the main components of diesel, is considered as an ideal source of hydrogen since its availability, easy handling and storage and, relatively high H/C ratio (produce 31 mol of H 2 per mole of reacted n-decane) [11,12].However, the n-decane steam reforming reaction is different with CH 4 , CH 3 OH, C 2 H 5 OH etc., and always accompanied by other side effects (cracking, isomerization, hydrogen transfer reaction).More so, the catalysts used in n-decane steam reforming reaction are easily to lose activity caused by carbon deposition, especially at higher temperatures [13][14][15].Therefore, the catalysts use in n-decane steam reforming reaction are put forward higher requirements.
In order to improve the anti-coking ability of Ni-based catalyst, and have a slight influence on catalytic activity, many scholars introduced another active metal into Ni-based catalyst to form bi-metallic catalysts [37][38][39][40][41][42].Wang et al. [37] introduced Pd into Ni-alumina catalysts, the catalytic activity and stability was obviously improved.Vizcaino et al. [43] found that Cu modified Ni-based catalyst showed better anti-coking ability.The addition of Cu is helpful for the process of eliminating the deposited carbon.In our previous work [44], we have added M (Fe, Co, Cu, Zn) as a promoter into the Ni/Ce-Al 2 O 3 catalyst in order to improve the anti-coking ability.Clearly, Co doped Ni/Ce-Al 2 O 3 showed an excellent coking-resistant effect.But, the catalytic activity have a slightly reduction at high temperature (650~800 • C).In another study [45], we added Co as another active species into Ni/Ce-Al 2 O 3 to form Ni-Co bi-metallic catalyst and investigated the catalytic activity, stability and coking inhibition effect during n-decane reforming.The results showed that the introduction of Ni and Co synchronously can effectively suppress carbon deposition and obviously improve catalytic activity.There was obvious synergistic effect between Ni and Co.However, the difference among different Co content on the Co-Ni/Ce-Al 2 O 3 bi-metallic catalyst has not been discussed.Consequently, it would be valuable to investigate the influence of the content of Ni on n-decane steam reforming.
In this paper, the steam reforming experiments of n-decane over x%Co-Ni/Ce-Al 2 O 3 catalysts with different Co loading were carried out.The effect of different Co loading on catalytic activity and the amount of deposited carbon were discussed.The purpose of this work is screening the suitable catalysts for steam reforming process in order to maximize n-decane conversion and H 2 yield, and minimize the formation of byproducts and carbon deposition.This work provided some positive suggestions for catalysts preparation and optimization by studying the structure-activity correlations.

n-Decane Conversion and H 2 Selectivity
n-decane steam reforming is used as the probe reaction.The initial activity tests over the series catalysts were performed from 650 to 800 • C in order to examine the influence of temperature and different promoters on catalytic performance.n-decane conversion and H 2 selectivity are considered the main parameters to check the advantages and disadvantages of the catalysts, and the results are shown in Figure 1a-d

Thermal Stability and Regeneration of C12-NCA
To better understand the effect of the ordered co-modification in n-decane steam reforming, the 12%Co-Ni/Ce-Al2O3 (C12-NCA) catalyst was screened out with a 6 h stability test at 750 °C and 800 °C, and the results are displayed in Figure 2a.It can be seen in Figure 2a that the H2 selectivity and n-decane conversion have a slightly change within 6 h.The C12-NCA catalyst has a good thermal stability.The C12-NCA catalyst also screened out for regeneration experiment.Carbon deposition on used C12-NCA catalyst was removed by oxygen enriched calcinations at 650 °C.The regenerative C12-NCA catalyst was carried out again in the same reactor as the fresh ones, and the contrast results are presented in Figure 2b.It is found that the n-decane conversions and H2 selectivity over the reused-1(-2) C12-NCA catalyst are approximately equal to the results of fresh one.Therefore, the C12-NCA catalyst is renewable.

Thermal Stability and Regeneration of C12-NCA
To better understand the effect of the ordered co-modification in n-decane steam reforming, the 12%Co-Ni/Ce-Al 2 O 3 (C12-NCA) catalyst was screened out with a 6 h stability test at 750 • C and 800 • C, and the results are displayed in Figure 2a.It can be seen in Figure 2a that the H 2 selectivity and n-decane conversion have a slightly change within 6 h.The C12-NCA catalyst has a good thermal stability.The C12-NCA catalyst also screened out for regeneration experiment.Carbon deposition on used C12-NCA catalyst was removed by oxygen enriched calcinations at 650 • C. The regenerative C12-NCA catalyst was carried out again in the same reactor as the fresh ones, and the contrast results are presented in Figure 2b.It is found that the n-decane conversions and H 2 selectivity over the reused-1(-2) C12-NCA catalyst are approximately equal to the results of fresh one.Therefore, the C12-NCA catalyst is renewable.

N2 Adsorption-Desorption Measurements
Table 1 shows the results of N2 adsorption-desorption results of the fresh and used catalysts.The surface areas of different samples in this work are in the range of similar CA support, even if the introduction of Ni and Co species by impregnation method.The value has a slightly decrease with the addition of Co and Ni, and gradually drops with the increase of Co.The loss can be attributed to the fact that the internal surface area of the CA pore system is progressively covered by Ni, Co species forming a layer [45][46][47].On the other hand, the surface areas of all catalysts used decrease with different levels after n-decane reforming reactions.CA and NCA catalysts decreased by 56% and 44% respectively compared with the fresh ones.Fortunately, the falling range gradually reduced with the addition of Co. Co as the active species showed a better carbon-resistant ability.The results are consistent with the results of the catalyst characterization.

X-ray Diffraction (XRD) Analysis
Figure 3 depicts the X-ray diffraction analysis of the fresh CA, NCA and bi-metallic CNCA catalysts.All the samples present similar characteristic features of γ-Al2O3 at 2θ = 45.7°,66.8°; cubic fluorite structural CeO2 at 2θ value of 28.5°, 56.3°; and the Ce crystallite at 2θ = 34.7°,49.8° and 59.2° by Bragg's refections [44,48].For all the catalysts, there was no CoOx, CoAl2O4, NiO, or NiAl2O4 diffraction peaks detected.This was probably due to the highly dispersion of CoOx and NiO particles are not easy to be detected by XRD [49].Moreover, the synchronous addition of Co and Ni did not form Ni-Co alloy phase.This indicated that active species were strongly interacted with CA support, and all of the as-prepared have a good thermal stability.It also can be seen that the degree of crystallization of all the fresh catalysts are smaller, suggesting that these catalysts are stable at

N 2 Adsorption-Desorption Measurements
Table 1 shows the results of N 2 adsorption-desorption results of the fresh and used catalysts.The surface areas of different samples in this work are in the range of similar CA support, even if the introduction of Ni and Co species by impregnation method.The value has a slightly decrease with the addition of Co and Ni, and gradually drops with the increase of Co.The loss can be attributed to the fact that the internal surface area of the CA pore system is progressively covered by Ni, Co species forming a layer [45][46][47].On the other hand, the surface areas of all catalysts used decrease with different levels after n-decane reforming reactions.CA and NCA catalysts decreased by 56% and 44% respectively compared with the fresh ones.Fortunately, the falling range gradually reduced with the addition of Co. Co as the active species showed a better carbon-resistant ability.The results are consistent with the results of the catalyst characterization.

X-ray Diffraction (XRD) Analysis
Figure 3 depicts the X-ray diffraction analysis of the fresh CA, NCA and bi-metallic CNCA catalysts.All the samples present similar characteristic features of γ-Al 2 O 3 at 2θ = 45.7  [44,48].For all the catalysts, there was no CoO x , CoAl 2 O 4 , NiO, or NiAl 2 O 4 diffraction peaks detected.This was probably due to the highly dispersion of CoO x and NiO particles are not easy to be detected by XRD [49].Moreover, the synchronous addition of Co and Ni did not form Ni-Co alloy phase.This indicated that active species were strongly interacted with CA support, and all of the as-prepared have a good thermal stability.It also can be seen that the degree of crystallization of all the fresh catalysts are smaller, suggesting that these catalysts are stable at high temperatures, which is coincides with the surface area analysis [50,51].This is in agreement with the XRD analysis previously shown and with the observations made in other studies.
Catalysts 2018, 8, x FOR PEER REVIEW 5 of 14 high temperatures, which is coincides with the surface area analysis [50,51].This is in agreement with the XRD analysis previously shown and with the observations made in other studies.Figure 4 shows the reduction profiles of the CA, and Co, Ni modified CA.It can be seen that there is one or two H2 consumption peaks for CA support at the region of 250~350 °C, which could be assigned to the reduction of a small amount of CeO2 to CeOx [28,29].For NCA catalyst, reduction peaks of ~460 °C and ~823 °C which are attributed to the reduction of NiO and NiAl2O4 [40].The highest reduction temperature is between 780 and 900 °C indicate the existence of species of NiO with strong interaction with Al2O3, resulting from the formation of the NiAl2O4 [42].For CNCA catalysts with different Co loading, two reduction peaks around ~300 °C and ~610 °C are ascribed to the reduction of Co2O3 and CoO, respectively [41].Significantly, the reduction temperatures differences of NiO and Co2O3 between these CNCA catalysts can be attributed to the existence of different interaction between Co and Ni.It is noteworthy that Co addition can obviously promote the reduction of NiO, and reach the optimal effect at 12% Co loading.There was obvious synergistic effect between Ni and Co, which is consistent with the results of the work reported by Jiao et al. [48,[52][53][54][55][56].[42].For CNCA catalysts with different Co loading, two reduction peaks around ~300 • C and ~610 • C are ascribed to the reduction of Co 2 O 3 and CoO, respectively [41].Significantly, the reduction temperatures differences of NiO and Co 2 O 3 between these CNCA catalysts can be attributed to the existence of different interaction between Co and Ni.It is noteworthy that Co addition can obviously promote the reduction of NiO, and reach the optimal effect at 12% Co loading.There was obvious synergistic effect between Ni and Co, which is consistent with the results of the work reported by Jiao et al. [48,[52][53][54][55][56].
NH 3 -TPD technology was used to investigate the acidities of the series catalysts, such as total amount, nature, and strength distribution, in order to look for the possible interpretation for the above experimental results, and the profiles are shown in Figure 5.The area of desorption peak goes hand in hand with the total amount of surface acid sites, while the peak temperature is closely related to the strength of individual acid site.The peak temperature in the range of 80 to 200 • C is regarded as weak acid sites, and the desorption temperature between 200 to 400 • C is considered the medium acid sites, while the peak temperature locates at 400 to 700 • C corresponds to strong acid sites.Figure 5 shows that CA and NCA catalysts have three desorption peaks at the region of 100~500 • C, which is regarded as the desorption peak of the weak and medium acid.Moreover, all the Co doped NCA catalysts have one strong desorption peak at 100~400 • C. Obviously, the desorption peak temperature moves to a low temperature area by adding Co, suggesting that the amount of acid sites decrease with the introduction of Co.In our previous studies [12,18], we found that the larger acidity and active strong acid centers are easy to give rise to rapid deactivation of the catalyst due to carbon deposition.It is noteworthy that the addition of Co modifier increases the basicity of the NCA catalyst.This process will result in preventing alkenes further reacting into aromatic or heavier products, which is beneficial to reduce the carbon deposition over catalysts and prolong the work life of catalysts.
Catalysts 2018, 8, x FOR PEER REVIEW 6 of 14 NH3-TPD technology was used to investigate the acidities of the series catalysts, such as total amount, nature, and strength distribution, in order to look for the possible interpretation for the above experimental results, and the profiles are shown in Figure 5.The area of desorption peak goes hand in hand with the total amount of surface acid sites, while the peak temperature is closely related to the strength of individual acid site.The peak temperature in the range of 80 to 200 °C is regarded as weak acid sites, and the desorption temperature between 200 to 400 °C is considered the medium acid sites, while the peak temperature locates at 400 to 700 °C corresponds to strong acid sites.Figure 5 shows that CA and NCA catalysts have three desorption peaks at the region of 100~500 °C, which is regarded as the desorption peak of the weak and medium acid.Moreover, all the Co doped NCA catalysts have one strong desorption peak at 100~400 °C.Obviously, the desorption peak temperature moves to a low temperature area by adding Co, suggesting that the amount of acid sites decrease with the introduction of Co.In our previous studies [12,18], we found that the larger acidity and active strong acid centers are easy to give rise to rapid deactivation of the catalyst due to carbon deposition.It is noteworthy that the addition of Co modifier increases the basicity of the NCA catalyst.This process will result in preventing alkenes further reacting into aromatic or heavier products, which is beneficial to reduce the carbon deposition over catalysts and prolong the work life of catalysts.

Transmission Electron Microscope (TEM) Analysis
In this work, the catalytic activity over the C12-NCA catalyst is better than C15-NCA.However, there is somewhat different texture and structural properties between the two.In order to further study the difference between C12-NCA and C15-NCA, the TEM analysis was used to find the influence of microscopic appearance and dispersion on catalytic activity.Figure 6

Transmission Electron Microscope (TEM) Analysis
In this work, the catalytic activity over the C12-NCA catalyst is better than C15-NCA.However, there is somewhat different texture and structural properties between the two.In order to further study the difference between C12-NCA and C15-NCA, the TEM analysis was used to find the influence of microscopic appearance and dispersion on catalytic activity.Figure 6    In order to further understand the Ni dispersion, the energy dispersive spectrometer (EDS) mappings of C12-NCA and C15-NCA catalysts were examined.As shown in Figure 7, the red sections correspond to Co, the blue sections correspond to Ce, the green sections correspond to Ni, In order to further understand the Ni dispersion, the energy dispersive spectrometer (EDS) mappings of C12-NCA and C15-NCA catalysts were examined.As shown in Figure 7, the red sections correspond to Co, the blue sections correspond to Ce, the green sections correspond to Ni, while the yellow sections correspond to Al. Obviously, Ni and Co particles evenly dispersed on the surface of the CA support over C12-NCA.However, some regions overlap each other for C15-NCA.This may be caused by the partial sintering of 6% Ni and 15% Co loading, and form bigger crystallite and lower metal dispersion.Poor nickel dispersion not only notably impact the physiochemical properties of the catalyst, but also impact the catalytic performance in steam reforming of reactions.Therefore, the dispersion state of active species is one of the main reasons of the activity difference among these catalysts.
Catalysts 2018, 8, x FOR PEER REVIEW 8 of 14 while the yellow sections correspond to Al. Obviously, Ni and Co particles evenly dispersed on the surface of the CA support over C12-NCA.However, some regions overlap each other for C15-NCA.This may be caused by the partial sintering of 6% Ni and 15% Co loading, and form bigger crystallite and lower metal dispersion.Poor nickel dispersion not only notably impact the physiochemical properties of the catalyst, but also impact the catalytic performance in steam reforming of reactions.Therefore, the dispersion state of active species is one of the main reasons of the activity difference among these catalysts.

Carbon Deposition
Table 2 presented the amount of carbon deposition over the series catalysts determined by the temperature programmed oxidation (TPO)-infrared spectrum (IR) analysis.It can be seen that the amount of carbon deposition is varied with the introduction of Ni and Co.Clearly, the amount of carbon deposition over CA catalyst is larger than others, and the values drop off with the Co content.For the bi-metallic C12(C15)-NCA catalyst, the amount of carbon deposition (<4.0 mg/g-fuel) is

Carbon Deposition
Table 2 presented the amount of carbon deposition over the series catalysts determined by the temperature programmed oxidation (TPO)-infrared spectrum (IR) analysis.It can be seen that the amount of carbon deposition is varied with the introduction of Ni and Co.Clearly, the amount of carbon deposition over CA catalyst is larger than others, and the values drop off with the Co content increased.For the bi-metallic C12(C15)-NCA catalyst, the amount of carbon deposition (<4.0 mg/g-fuel) is much less than conventional Ni-alumina catalysts.The possible reasons are that the addition of Co reduces the acidity of the CA catalyst and carbon dioxide adsorption, and decreases carbon deposition generation during n-decane steam reforming process [48].According to the literature [57], Co addition can increase the dispersion state of Ni, prevent metal sintering, and inhibit carbon deposition, which is consistent with our results in this work.

Used Catalyst Characterization
The formation of carbon deposition on the used catalyst was also evidenced by SEM observations.In Figure 8a-g, the micrographs of used catalysts are presented.Carbon deposition grows on the surface of catalysts in different range.It is observed from Figure 8h that the Ni species is occupied at the tip of the filamentous carbon and the Ni metal particle size is found to be ca.30 nm.This can demonstrate that active metal has removed from the surface of catalyst with the generation of filamentous carbon.The results are consistent with the mechanisms of filamentous carbon growth in literature [58].In addition, the C content over the used CA, NCA, C3-NCA, C6-NCA, C9-NCA, C12-NCA, C15-NCA catalysts are 91.14%,63.90%, 56.46%, 49.89%, 34.79%, 14.94% and 8.31%, respectively.It clearly indicates that the C content over the used catalysts has a close relationship with the surface area.Obviously, the Brunauer-Emmett-Teller (BET) specific surface area decrease caused by the formation of the carbon deposition is another main reason of the activity difference among these catalysts.
Catalysts 2018, 8, x FOR PEER REVIEW 9 of 14 much less than conventional Ni-alumina catalysts.The possible reasons are that the addition of Co reduces the acidity of the CA catalyst and carbon dioxide adsorption, and decreases carbon deposition generation during n-decane steam reforming process [48].According to the literature [57], Co addition can increase the dispersion state of Ni, prevent metal sintering, and inhibit carbon deposition, which is consistent with our results in this work.

Used Catalyst Characterization
The formation of carbon deposition on the used catalyst was also evidenced by SEM observations.In Figure 8a-g, the micrographs of used catalysts are presented.Carbon deposition grows on the surface of catalysts in different range.It is observed from Figure 8h that the Ni species is occupied at the tip of the filamentous carbon and the Ni metal particle size is found to be ca.30 nm.This can demonstrate that active metal has removed from the surface of catalyst with the generation of filamentous carbon.The results are consistent with the mechanisms of filamentous carbon growth in literature [58].In addition, the C content over the used CA, NCA, C3-NCA, C6-NCA, C9-NCA, C12-NCA, C15-NCA catalysts are 91.14%,63.90%, 56.46%, 49.89%, 34.79%, 14.94% and 8.31%, respectively.It clearly indicates that the C content over the used catalysts has a close relationship with the surface area.Obviously, the Brunauer-Emmett-Teller (BET) specific surface area decrease caused by the formation of the carbon deposition is another main reason of the activity difference among these catalysts.The reaction effluents were on-line analyzed by Gas Chromatography with FID detector (GC-2010, SHIMADZU Co., Ltd., Kyoto, Japan) equipped with HP-Al/S separation capillary column (CH4, C2H4, C2H6, C3H8, C3H6, C4); and TCD detector with TDX-01 packed column (H2, CO and CO2).Solid products deposited covering the catalysts were investigated by temperature programmed oxidation (TPO), and the characteristics signals of CO and CO2 were tested by IR analyzer.
Activity data were reported as n-decane conversion XA (%), and H2, CO and CO2 selectivity (Si), which are defined as follows: , , where i represents the different gas products detected.

Conclusions
Series Co-Ni bi-metallic catalysts supported on Ce-Al2O3 (CA) with different Co ratios are prepared for n-decane steam reforming towards H2 preparation, so as to screen appropriate Co content.N2 adsorption-desorption, XRD, NH3-TPD, SEM-EDS, and TEM were used to explain the relationship between catalytic activity and textural-structure properties, microscopic appearance and dispersion state of the catalysts.The activity test indicated that the addition of Co resulted in higher selectivity of hydrogen compared with NCA contrast catalyst.The results indicated that 12% Co showed better hydrogen selectivity (74%), n-decane conversion (100%), and anti-carbon ability (3.90 mg/g-fuel).Moreover, thermal stability and regeneration performance were also tested, and the results showed that reforming activity over C12-NCA catalyst was stable in 6 h, and the hydrogen selectivity and n-decane conversion over regenerated catalysts were almost close to the fresh ones.
The results were mainly interpreted on the basis of characterization studies, which revealed the significant role of textural and structural properties, microscopic appearance and dispersion state on reforming performance.Obviously, the dispersion state and the BET-specific surface area decrease caused by the formation of the carbon deposition are the main reasons for the activity difference among these catalysts.Activity data were reported as n-decane conversion X A (%), and H 2 , CO and CO 2 selectivity (S i ), which are defined as follows: where i represents the different gas products detected.

Conclusions
Series Co-Ni bi-metallic catalysts supported on Ce-Al 2 O 3 (CA) with different Co ratios are prepared for n-decane steam reforming towards H 2 preparation, so as to screen appropriate Co content.N 2 adsorption-desorption, XRD, NH 3 -TPD, SEM-EDS, and TEM were used to explain the relationship between catalytic activity and textural-structure properties, microscopic appearance and dispersion state of the catalysts.The activity test indicated that the addition of Co resulted in higher selectivity of hydrogen compared with NCA contrast catalyst.The results indicated that 12% Co showed better hydrogen selectivity (74%), n-decane conversion (100%), and anti-carbon ability (3.90 mg/g-fuel).Moreover, thermal stability and regeneration performance were also tested, and the results showed that reforming activity over C12-NCA catalyst was stable in 6 h, and the hydrogen selectivity and n-decane conversion over regenerated catalysts were almost close to the fresh ones.
The results were mainly interpreted on the basis of characterization studies, which revealed the significant role of textural and structural properties, microscopic appearance and dispersion state on reforming performance.Obviously, the dispersion state and the BET-specific surface area decrease caused by the formation of the carbon deposition are the main reasons for the activity difference among these catalysts.
. The catalytic activities over these catalysts gradually increase with the temperature.Obviously, the presence of Ni or/and Co can effectively promote the rate of the steam reforming reaction, as well as the selectivity of H 2 and n-decane conversion.Moreover, the synchronous introduction of Co and Ni further enhanced the catalytic activity compared with the 6%Ni/Ce-Al 2 O 3 (NCA) catalyst.This demonstrates that the addition of Co could provide sufficient Ni active sites for the reactants.In addition, the catalytic activity of x%Co-Ni/Ce-Al 2 O 3 (CNCA) bi-metallic catalysts with different Co content increases firstly and then decreases with Co addition.The catalytic activity reaches the best when the Co content is 12%.This indicated that moderate Co is favor for promoting the activity.There is a synergistic effect between Co and Ni.Catalysts 2018, 8, x FOR PEER REVIEW 3 of 14 different promoters on catalytic performance.n-decane conversion and H2 selectivity are considered the main parameters to check the advantages and disadvantages of the catalysts, and the results are shown in Figure 1a-d.The catalytic activities over these catalysts gradually increase with the temperature.Obviously, the presence of Ni or/and Co can effectively promote the rate of the steam reforming reaction, as well as the selectivity of H2 and n-decane conversion.Moreover, the synchronous introduction of Co and Ni further enhanced the catalytic activity compared with the 6%Ni/Ce-Al2O3(NCA) catalyst.This demonstrates that the addition of Co could provide sufficient Ni active sites for the reactants.In addition, the catalytic activity of x%Co-Ni/Ce-Al2O3 (CNCA) bi-metallic catalysts with different Co content increases firstly and then decreases with Co addition.The catalytic activity reaches the best when the Co content is 12%.This indicated that moderate Co is favor for promoting the activity.There is a synergistic effect between Co and Ni.

2. 2
Figure 4 shows the reduction profiles of the CA, and Co, Ni modified CA.It can be seen that there is one or two H 2 consumption peaks for CA support at the region of 250~350 • C, which could be assigned to the reduction of a small amount of CeO 2 to CeO x [28,29].For NCA catalyst, reduction peaks of ~460 • C and ~823 • C which are attributed to the reduction of NiO and NiAl 2 O 4 [40].The highest reduction temperature is between 780 and 900 • C indicate the existence of species of NiO with strong interaction with Al 2 O 3 , resulting from the formation of the NiAl 2 O 4[42].For CNCA catalysts with different Co loading, two reduction peaks around ~300 • C and ~610 • C are ascribed to the reduction of Co 2 O 3 and CoO, respectively[41].Significantly, the reduction temperatures differences of NiO and Co 2 O 3 between these CNCA catalysts can be attributed to the existence of different interaction between Co and Ni.It is noteworthy that Co addition can obviously promote the reduction of NiO, and reach the optimal effect at 12% Co loading.There was obvious synergistic effect between Ni and Co, which is consistent with the results of the work reported by Jiao et al.[48,[52][53][54][55][56].NH 3 -TPD technology was used to investigate the acidities of the series catalysts, such as total amount, nature, and strength distribution, in order to look for the possible interpretation for the above experimental results, and the profiles are shown in Figure5.The area of desorption peak goes hand in hand with the total amount of surface acid sites, while the peak temperature is closely related to the strength of individual acid site.The peak temperature in the range of 80 to 200 • C is regarded as weak acid sites, and the desorption temperature between 200 to 400 • C is considered the medium acid sites, while the peak temperature locates at 400 to 700 • C corresponds to strong acid sites.Figure5shows that CA and NCA catalysts have three desorption peaks at the region of 100~500 • C, which is regarded as the desorption peak of the weak and medium acid.Moreover, all the Co doped NCA catalysts have one strong desorption peak at 100~400 • C. Obviously, the desorption peak temperature moves to a low temperature area by adding Co, suggesting that the amount of acid sites decrease with the introduction of Co.In our previous studies[12,18], we found that the larger acidity and active strong acid centers are easy to give rise to rapid deactivation of the catalyst due to carbon deposition.It is noteworthy that the addition of Co modifier increases the basicity of the NCA catalyst.This process

Figure 4 . 14 Figure 5 .
Figure 4. H 2 -temperature-programmed reduction (H 2 -TPR) results of the series catalysts.Catalysts 2018, 8, x FOR PEER REVIEW 7 of 14 shows TEM images and Ni or/and Co particle size distributions of C12-NCA and C15-NCA catalysts.It is found that the Ni or/and Co particle size over C12-NCA is mainly focused on 11-20 nm, while the value for C15-NCA is about 16-25 nm.The average particle size of Ni or/and Co of C15-NCA is significantly larger than C12-NCA.Obviously, poor Ni or/and Co distribution over C15-NCA are observed.It may be the reason of the weaker catalytic activity of C15-NCA catalyst.The results indicate that 6% Ni and 15% Co loading is easy to aggregate after 800 °C calcination, which may be due to redundant Co enriching on the catalyst surface and exceed the monolayer saturation capacity of the CA support.
shows TEM images and Ni or/and Co particle size distributions of C12-NCA and C15-NCA catalysts.It is found that the Ni or/and Co particle size over C12-NCA is mainly focused on 11-20 nm, while the value for C15-NCA is about 16-25 nm.The average particle size of Ni or/and Co of C15-NCA is significantly larger than C12-NCA.Obviously, poor Ni or/and Co distribution over C15-NCA are observed.It may be the reason of the weaker catalytic activity of C15-NCA catalyst.The results indicate that 6% Ni and 15% Co loading is easy to aggregate after 800 • C calcination, which may be due to redundant Co enriching on the catalyst surface and exceed the monolayer saturation capacity of the CA support.
images and Ni or/and Co particle size distributions of C12-NCA and C15-NCA catalysts.It is found that the Ni or/and Co particle size over C12-NCA is mainly focused on 11-20 nm, while the value for C15-NCA is about 16-25 nm.The average particle size of Ni or/and Co of C15-NCA is significantly larger than C12-NCA.Obviously, poor Ni or/and Co distribution over C15-NCA are observed.It may be the reason of the weaker catalytic activity of C15-NCA catalyst.The results indicate that 6% Ni and 15% Co loading is easy to aggregate after 800 °C calcination, which may be due to redundant Co enriching on the catalyst surface and exceed the monolayer saturation capacity of the CA support.

Table 1 .
The textural properties of the fresh and used catalysts.

/g) Pore Volume (mL/g) Mean Pore Diameter (nm)
* The numbers in the parentheses represent the surface area of used catalysts.

Table 1 .
The textural properties of the fresh and used catalysts.
* The numbers in the parentheses represent the surface area of used catalysts.

Table 2 .
The amount of carbon deposition over used catalysts.

Table 2 .
The amount of carbon deposition over used catalysts.