Characterization of Ca-promoted Co/AC catalyst for CO2-CH4 reforming to syngas production

Abstract Ca-promoted Co-based/AC catalysts prepared with various Ca/Co ratios by using impregnation method were applied to synthesis gas production via CO2-CH4 reforming reaction. The influences of preparation methods on the physico-chemical characteristics, catalytic activities and stability of the catalysts were examined in a fixed-bed reactor. The basic physico-chemical characters of prepared catalysts were studied by the H2-TPR, XRD and XPS and BET methods. H2-TPR, XRD and XPS results confirmed that cobalt dispersion improving, the interaction between cobalt and calcium intensify and the sintering of cobalt mitigation is due to calcium addition. Results of XRD and H2-TPR also show that there was a new crystal substance Ca3Co2O6 formed, which has most strong influence on the catalytic activity enhancing and the anti-coking ability of CO2-CH4 reforming catalyst. Through the SEM analysis of the used catalyst after CO2-CH4 reforming, the Co-Ca/AC catalysts were also confirmed to show better sinter-stability. In addition, the addition of calcium can improve the carbon dioxide adsorption properties, so that it is easy to eliminate carbon deposited on the surface of the catalyst by gasification. In particular, the catalyst with 14 wt.% of calcium addition revealed a good activity and anti-sintering ability in CO2-CH4 reforming. On the basis of experimental observations and literature reviewed, a simple carbon dioxide reforming of methane into syngas mechanism over a cobalt/AC catalyst promoted by Ca is proposed.


Introduction
Since the industrial revolution, increasing amount of green gases like CO 2 has been discharged to the atmosphere because of the direct burning of fossil fuels [1]. Consequently, the greenhouse effect has been worsened, leading to a series of severe problems such as global warming and sea level rise. With the advance of technology and people's environmental awareness becoming stronger, how to utilize and fix CO 2 has been attached great importance to governments and researchers all over the world [2][3][4]. Syngas can be produced by catalyzing CH 4 -CO 2 reforming, which is a way of great potential to utilize CO 2 [5][6][7]. The ratio of hydrogen to carbon in the syngas is low, so it can be used as the raw material of oxo synthesis and F-T (Fischer-Tropsch) synthesis oil and it can make full use of the resource available [8][9][10][11][12][13]. Besides, since CH 4 -CO 2 reforming is a strong endothermic reaction, it can be applied in the storage and transport of solar energy and nuclear energy [14,15].
Studies on catalyst for CH 4 -CO 2 reforming fall in two types: precious metal and non-precious metal [16][17][18][19]. The former has good activity and strong resistance to carbon deposition, but it is expensive, so it is not economical to apply in large-scale industry. Among the non-precious metal catalysts, the activity of Ni catalyst can compete with that of precious metal catalyst. Ni-based catalyst is easy to agglomerate and produce carbon deposition, which will speed up the catalyst deactivation [20][21][22]. Besides, Spinel-type NiAl 2 O 4 compound formed by the solid state reaction is not conducive to the catalyst activity, but also is difficult to regeneration. Among the non-precious metal catalysts, although the activity of Co-based catalyst is only inferior to that of Ni-based catalyst, Co-based catalyst appears better catalytic properties; especially a catalyst carrier is a carbon material. Therefore, Co-based catalyst would be a promising industrial catalyst of great potential in carbon dioxide reforming of methane.
Cobalt-based catalysts researches are relatively few as compared to nickel catalysts. Cobalt -based catalysts used for the carbon dioxide reforming of methane are attractive catalysts due to their low cost and relatively high activity. However, cobalt based catalysts used for the carbon dioxide reforming of methane have also some shortcomings, such as susceptibility to carbon deposition and catalyst pellet sintering at high temperatures, cause catalyst deactivation [20][21][22][23][24][25]. One method of improvement in coke resistance and catalytic stability of cobalt based catalysts is the adding additives. The additives are selected from transition metals, the alkaline and alkali earth metals [26][27][28]. When calcium as an additive is added to Co-based catalyst, the CO 2 adsorption by calcium reacts with surface carbon to form CO, decreasing the rate of coke formation, since Ca is a metal with basic properties. That is, the Ca addition has an important role in catalytic characteristics in CH 4 -CO 2 reforming due to the synergistic effect between active components to additives [29]. Souza et al. [30] found that a small amount of cobalt partial substituted by alkali metal, such as Ca, La, Li, or Mg not only can alter the reduction of the cobalt species and the crystal structure of oxides in the structure, but also can improve the specific surface area and the basic properties of the catalyst. The results indicated the textural characters; reducibility and acid/base characters changes in the catalyst materials by the alkali-promoted have great effect on the catalytic characters for CH 4 -CO 2 reforming catalysts and against deposited carbon. In the literature [31], Hou et al. showed that the different catalytic properties and structures catalysts can be obtained by addition of different amount calcium on different supports. When the Ca/Ni ratio is 0.2, Ni-based catalysts prepared by Ca-promoted showed a good catalytic characteristic and anti-carbon deposition, which is attribution to strengthened interaction between Ni and support by Ca adding. The research by Koo et al. [29] showed that Ca-promoted catalysts have a good anti-coking performance and anti-sintering ability in CO 2 reforming of COG. It is due to increase nickel dispersion and strong interaction between Ni and support by Ca addition. Furthermore, the CO 2 adsorption of catalyst increase by Ca addition, and then the CO 2 adsorption react with coke deposited to form CO to retard coke formation. Song et al. [32] have found that oxygen vacancies can be created by doped calcium in the Capromoted Co-based catalysts. The creation of oxygen vacancies has great effect on the catalytic and anti-deposited carbon characters of catalysts.
In recent years, activated carbon as an effective carrier material in the field of catalytic dehydrogenation of cyclohexanol, organic contaminants oxidation, steam-methane reformation, F-T synthesis and CH 4 -CO 2 reforming has been well developed [33][34][35][36][37][38][39][40][41][42]. Their high surface area and thermal stability contributes to improve the dispersion of active components, which is required in CO 2 -CH 4 reforming. The previous research by Shukla et al. [43] has found that the Co/AC catalyst has high activity for phenol oxidative decomposition, which can be attributed to Co 2 O 3 crystallite form and homogeneously distributed on the activated carbon surface. The research of Wang and Lu [44] showed that acid treatment can change the physicochemical properties of activated carbons support. After acid treatment, impurities present in the activated carbon are removal, so the specific surface area, pore volume and adsorption capacity of Ni 2+ are improved. Based on our previous research on activated carbon supported cobalt catalysts for carbon dioxide reforming methane [37,38], it has been found that cobaltbased carbon material catalyst showed good catalytic activity and anti-coke performance.
This paper discusses the influence of Ca-promoted cobaltbased/activated carbon catalysts on the physicochemical property, reforming reactivity and stability for syngas (H 2 /CO) production via CO 2 -CH 4 reforming. Ca was added in various amounts to the Co-based/AC catalysts to form Co-Ca/AC catalysts which were characterized using various methods (XRD, XPS, H 2 -TPR, BET and SEM) in order to elaborate influence of Ca addition on the catalytic nature in reforming process as well as from the mechanistic point of view. In this study, a small addition of Ca has been found to improve some desirable catalytic properties, such as Ca-Co interactions and basicity of the catalyst. These properties are important for CO 2 -CH 4 reforming catalysts with good catalytic performance and anti-coking ability.

Catalyst preparation
Various Ca/Co ratios of Co-Ca/AC catalysts were prepared by adding different Ca with co-impregnation method. The activated carbon carrier (20-40 mesh) was precalcined at 800 C for 1 h in a stream of nitrogen. The precursors of Co and Ca are cobalt nitrate hexahydrate and calcium nitrate tetrahydrate, respectively. First, the prepared activated carbon carrier was immersed in the required amount of nitrate salts nitrate solution for 24 h. Then, the prepared catalyst was dried for 6 h at 110 C. Finally the prepared catalyst was calcined at the desired temperature for 24 h. Before the experimental investigations, the catalysts were reduced at 550 C in flowing hydrogen for 100 min in situ. After that the catalyst was heated to 1000 C and was kept at 1000 C under nitrogen for 15 min, and then cooled to the desired reaction temperature.

Characterization
The Co-Ca/AC crystalline structure of catalysts were identified by Shimadzu XRD-6000 using a Cu target K-ray. Surface morphology of the catalyst both before and after testing was performed with a SEM (JSM-4800, Japan). The TG-DSC data was collected in an Instruments DSC-1150 B with 30 mg samples. The surface analysis and metal oxidation state were identified by X-ray photoelectron spectroscopy, using a monochromatized Al K X-ray source. The BET surface area of the used catalysts was performed with a physisorption analyzer (3H-2000PS1). The reducibility data of Co-Ca/AC catalyst precursors was collected using an AutoChem II System 2920. About 0.05 g of sample was first outgassed at 300 C flowing N 2 for 0.5 h. Then, the temperature was ramped from the room temperature to 900 C at 15 C/min under a 10% H 2 in flowing N 2 (10 mL/min).

Catalytic test
The catalytic activity test for Co-Ca/AC was investigated in a fixed-bed reactor. The mass flow controllers were used to control the flow rate of gas (CH 4 , CO 2 and N 2 ). A thermocouple was used to control the reaction bed temperature. Approximately 3.0 g of the sample was put in the 2.0 cm quartz tube for each run. The mole ratio of CH 4 and CO 2 in the feed gas is 1:1, and the GHSV is 7200 mL g À1 h À1 . A trap (0 C) was used to remove reaction water in product gas. Then the gas was analyzed by two chromatographs (GC-950 and GC9890). The reproductively of the experimental results was more than 97%, and all experiments with large errors were rejected. The conversion of CH 4 and CO 2 were calculated by means of the following equation: Where C is the conversion of CH 4 or CO 2, %; F is the feed or product gas flow, ml min À1 ; X is the content of CH 4 or CO 2 , %; Superscript in and out are inlet and outlet, respectively.

XRD characterization
The Co-Ca/AC catalysts XRD patterns in different calcination temperature were shown in Fig. 1. Comparison with the standard spectra, it was found that hat graphite diffraction peak appears at 26.6 , CaO diffraction peak at 35.6 (PDF-#17-0912), CoO/Co 3 O 4 diffraction peak at 44.8 (PDF-#02-1217and PDF-#02-1079) and Ca 3 Co 2 O 6 diffraction peak at 56.9 (PDF-#23-0111). For calcination at 300 C,400 C and 500 Ccatalyst, CaO diffraction peak is intense and narrow, the characterization diffraction of CoO/Co 3 O 4 was not found. While that sharp CoO/Co 3 O 4 diffraction peak was found in the catalyst prepared at the calcination temperature of 600 C and 700 C, weak diffraction peaks of CaO was observed. These indicate that the calcination temperature exerts an impact on the formation of CoO/Co 3 O 4 and CaO grains and that high temperature helps the crystals of Co x O y grow into bigger Co 2 O 3 grains while lower calcination temperature avails the growth of CaO grains. In addition, a new diffractions peaks centred at 56.6 were also observed on the catalyst prepared at the calcination temperature of 500 C, which were the characteristic diffraction of Ca 3 Co 2 O 6 diffraction peak. The formation of Ca 3 Co 2 O 6 new substances clearly indicates that the metal species of Co and Ca takes place thermal reaction. The formation of Ca 3 Co 2 O 6 changes the catalytic performance of the Co-Ca/AC catalyst. In the reforming reaction, Ca 3 Co 2 O 6 can provide new active sites to make up for the loss of active sites, safeguarding the activity and stability of the catalyst. Similar conclusion has been made by Dang et al. [45] while producing hydrogen from Glycerin water vapor over Co-CaO-Ca 12 Al 14 O 33 catalyst. Fig. 2 presents the TG and DSC result in the used catalysts. It was found that TG curve of Co/AC catalyst has three times obvious quality change with the rise of temperature: 110 C, 300 C and 500 C, respectively. Then TG curve of Co/AC catalyst tends to smooth at the temperature over 600 C. On the DSC curve, a noticeable endothermic peak appears at the temperature lower than 100 C, which results from the volatilization of moisture in the catalyst and volatile substances. Fluctuations can be observed on this curve at 300 C and 500 C, which may either result from the decomposition of Co(NO 3 ) 2 or the interaction between active substances and the carrier. The TG curve of active carbon loading Ca shows that its weight-loss ratio reaches 31.0%, mainly due to the production of Ca(NO 2 ) 2 from the decomposition of Ca(NO 3 ) 2 at 130 C. However, as the temperature rises to 480 C, frit reaction will take place. Consequently, calcium oxide is produced with the rise of temperature.

TG/DSC characterization
Form the TG curve of used Co-Ca/AC catalyst (Fig. 2(c)), it can be found that two clear weight losses take place during the rise of temperature and the quality of sample tends to be stable at the temperature above 500 C, the weight-loss ratio ends up with 30.3%. Its DSC curve indicates has three endothermic peaks. The first endothermic peak near 100 C is caused mainly by the evaporation of free water in the sample. The second endothermic peak near 100-200 C is caused mainly by the volatilization of internal water absorbed in pores insides of the sample. The third weight loss appears at 200-500 C, which is due to the thermal decomposition of Co(NO 3 ) 2 and Ca(NO 3 ) 2 . Compared with Fig. 2(ac), we can found that the temperature of Co-Ca/AC catalyst weight loss tending to be stable is lower than that of Co/AC and Ca/AC catalyst, which implies the formation synergistic effect of Co and Ca in prepared Co-Ca/AC catalyst. The synergistic effect of Co and Ca can accelerates the decomposition of Co(NO 3 ) 2 and Ca(NO 3 ) 2 , and generate active components.

H 2 -TPR results
The H 2 -TPR result of Ca-promoted Co/AC catalysts is shown in Fig. 3. From Fig. 3, it can be found that the catalyst has four redox peaks in the process of hydrogen reduction. The first is in the domain from 250 C to 300 C, a free peak of CoO, which is to be  deoxidized to metal substances with the presence of hydrogen. The second one is in the domain from 300 C to 400 C, belonging to Co 2 O 3 or Co 3 O 4 peak, which will be deoxidized to CoO or Co 2 O 3 . The third is between 500 C and 600 C, it is Co 2 O 3 or CoO peak that will be deoxidized to Co with the presence of hydrogen. The last one is between 700 Cand 850 C, as Co-Ca alloy crystal. Thanks to the interaction between Co and Ca, the redox temperature of this alloy crystal is higher [46,47]. In this paper, the used activated carbon is inert carbon, so the interaction between Co and Ca is stronger than metals and support. The strong interaction can inhibit the sintering of the active metal in the carbon dioxide reforming methane and then maintain its good catalytic activity. Fig. 4 shows the X-ray photoelectron spectra of Co-Ca/AC catalysts. It can be seen from this spectrum that carbon exist mainly in the form of C, CÀ ÀOR and C¼O, and that the latter two can accelerate CH 4 to split into CHx*. Co species displays its presence mainly on CoO peak at 779.5 eV and on Co 3 O 4 peak at 77.12 eV, respectively. Co 2+ peak is stronger than the Co 3+ peak, which is probably due to the changes of chemical environment of Co with the addition of Ca, leaving Co electron rich. This will help prevent CH 4 splitting into coking deposit in CO 2 -CH 4 reforming process and improve the adsorption activity of CO 2 so as to prolong the service life of catalyst [48]. It can also be seen that Ca shows its existence mainly on Ca 2p peak at 346 eV and CaO peak at 347 eV. The transformation of Ca in the catalyst from Ca 0 to Ca 2+ facilitates the transformation of Co from Co 3+ to Co 2+ , which is in the favor of deoxidizing the active substances of catalyst. It reveals that the addition of Ca changes the performance of catalyst by changing the distribution of Co on the surface of active carbon. Fig. 5 presents the SEM mapping images of used Co-Ca/AC catalysts before (a, b) and after (c, d) reaction. It can be seen from Fig. 5(a, b) that active material with a diameter of about 30-50 nm are distributed uniformly on the surface of catalyst and in the gaps of carriers. Comparison with our previous study, it reveals that the addition of Ca can obviously improve the dispersity of Co the active material on the active carbon carrier. That is because of the weak synergistic effect between Co and active carbon which is inert. However, after adding Ca, the stronger synergistic effect between Ca and Co prevents Co grains growing and aggregation. So that, the active ingredients are uniformly distributed on the surface of support. From Fig. 5(c), it can be seen that there appear some flocculent substances on the surface of catalyst after reaction, which is due to formation active filiform carbon in the reforming reaction(CH 4 = C + 2H 2 ) [37][38][39]. Consequently, the carbon deposition and carbon consumption (C + CO 2 = 2CO) reach a dynamic equilibrium. Meanwhile, it can be seen from Fig. 5(d) that many honeycomb structures emerge on the catalyst after the reaction, which is due to possible active substances on the activated carbon surface involved in reforming reactions, resulting in carrier eroded. In addition, it is evident that the active component sizes of used catalysts after reaction have become bigger. Compared with Co/AC catalyst [37,38], it is noteworthy that Co particles in the Co-Ca/AC catalyst changed slower than that in Co/AC catalyst for CH 4 reforming of CO 2 reaction. The Co/AC catalysts with added Ca exhibit good sinter-stability. This is due to the strong interaction between cobalt and calcium, which can suppress Co particles sintering. As XRD and H 2 -TPR characterization showed that Ca 3 Co 2 O 6 is formed after adding Ca to Co/AC catalyst, which identified that calcium addition strengthened the interaction between Co and Ca.

Influences of impregnation sequence
To study the effects of impregnation sequence on the catalytic performance, the different impregnation sequence catalysts were prepared using impregnation. When Ca was impregnated first followed by Co, it is denoted as CaCo catalyst. When Co was  impregnated first followed by Ca, it is denoted as CoCa catalyst. When Ca and was simultaneous impregnated, it is denoted as Co-Ca catalyst. Fig. 6 presents the influence of impregnation sequence on the reforming conversion. It can be seen that the activity of CaCo catalyst and CoCa catalyst is quite similar, while the activity of Co-Ca catalyst is improved obviously. Table 1 showed the characteristic parameters of the pore structure of these catalysts. It can be seen from Table 1, the specific surface area, pore diameter and pore volume of the prepared catalyst become apparently smaller after loading metal, mainly resulting from the blocking of some pores of catalyst. Table 1 reveals that the difference impregnation sequences lead to change characteristic parameters and structure of catalysts. The Co-Ca catalyst has the largest specific surface area and smallest pore diameter. This implies that co-impregnation can increase the specific surface area of catalyst, which may due to the sintering of some active components or pore collapse caused by the twice high-temperature calcinations and the competitive adsorption between Co and Ca [49]. In addition, co-impregnation can improve the active agent penetration, so increases actives sites on the carrier surface. Furthermore, when the catalyst was prepared using the co-impregnation method, catalyst surface may be covered by calcium additive, which has strong effect on the activated carbon surface acid sites [50]. In the step-by-step impregnation, the voids can be filled by another substance; it is difficult for the second substance to enter the pore, so that the pores are blocked. On the other hand, when the catalyst was prepared using step impregnation, calcium additives cannot be evenly distributed in the catalyst, the effect of that is not so   obvious. The catalytic performance of catalyst prepared with coimpregnation was better than that of prepared with step impregnation.

Influences of Ca content
Catalysts with different proportions of Ca (content of Ca ranging from 2% to 30%) were prepared to investigate influences of Ca adding content on catalysts performance. The catalysts was calcined at 500 C for 4 h. Fig. 7 presents the effect of different Ca contents on activities of Co-Ca/AC catalysts. The results show that Ca adding content has an important influence on the activity of catalyst. The catalyst with 14 wt.% of calcium added shows good activities and anti-coking ability for carbon dioxide reforming of methane. This may because that adsorptive property of Ca and Co on active carbon is different. With the increasing of Ca addition, the adsorption balance will break and competitive adsorption will occur. While the Ca content goes higher than 14 wt%, the dynamic equilibrium will be partial to Ca, so that more adsorption sites on the active carbon will be occupied by Ca. The weak catalytic activity of Ca will weaken the CH 4 -CO 2 reforming conversion. On the other hand, as an alkalinous metal, Ca is prone to produce CaCO 3 with CO 2 , which avails decomposing CH 4 while activating CO 2 . Optimization content of Ca co-impregnation Co-Ca/AC catalyst is14 wt.%. Fig. 8 presents the influences of calcination temperature on activity of catalyst. It can be seen from Fig. 8, the calcination temperature has an important influence on the performance of catalyst. It also can be seen that the rise of calcination temperature remarkably raises the activity of catalyst. The activity of catalyst reaches the highest at the calcination temperature of 500 C. However, along with the further increase of calcination temperature, activities of catalysts become gradually decreased. This is probably because of the sintering of active metal at excessive calcination temperature, so that the activity of Co-Ca/AC catalyst is weakened by the growth and agglomeration of metal microliters.

Influences of calcination temperature and time
The influence of calcination time on CH 4 -CO 2 conversion is displayed in Fig. 9. From Figure, with the calcinations time going on, the catalyst becomes more active and reaches the highest at 4 h.   This is due that the active grain formation in the catalyst is a slow process. Within shorter calcination time, stable Co-Ca metal crystal cannot be formed in the catalyst. However, along with the further increase of calcination time (>4 h), the sintering of active metal at excessive calcination time, so that the activity of Co-Ca/AC catalyst is weakened by the sintering and agglomeration of metal microliters.

Stability of Co-Ca/AC
The stability test of Co-Ca/AC catalyst was carried out at 800 C. As shown in Fig. 10, along with the increase of reaction time to 150 min, CH 4 -CO 2 conversion appears a sharp increase as the reaction processes, and afterwards, the reforming conversion tends to stable, and that there is no obvious inactivation within 2000 min. CH 4 -CO 2 reforming performance slowly increases with time, it is because that the active site of catalyst exit induction period. At the beginning of reaction, the active grains formed on the catalyst are not stable. After some time induction, the active site of catalyst gets stable. As a result, this highly active and stable Co-Ca/AC catalyst is a potential catalyst for CH 4 -CO 2 reforming reaction. Similar conclusions have been drawn by Kim et al. [51] in study on the influence of Ni-Ce catalyst on the reforming reaction.

Proposed mechanism
The catalytic properties of carbon-catalyzed reforming of CO 2 -CH 4 have been discussed at domestic and foreign [52]. Classically, the CO 2 -CH 4 reforming reaction is considered as a combination reaction of methane decomposition and carbon dioxide gasification coke deposition on catalyst surface. Beside this, the study also confirmed that the oxygen functional groups on the surface of carbon materials are also involved in the reforming reaction. According to the characterization analysis results above, the proposed mechanism of CH 4 -CO 2 reforming reaction catalyzed by Co-Ca/AC catalyst is shown in Fig. 11. The reduced Co-Ca/AC catalyst results in the formation of dispersed CaO and Co particles on the catalyst support. CO 2 is preferentially adsorbed on the CaO particles to form CaCO 3 . CH 4 is adsorbed on the Co particles and dissociates to form hydrogen and surface carbon species [23,24,53,54]. Besides, the moisture dissociate to produce adsorbed state ÀH and ÀOH. Surface carbon species and produced adsorbed state À ÀOH reaction generated CO, adsorbed state À ÀH generated H 2 . Finally, activated CO 2 from calcium carbonate and surface carbon species dissociated from CH 4 generated CO, and then part calcium carbonate and Co atom generated Ca 3 Co 2 O 6 . The formation of Ca 3 Co 2 O 6 can not only prevent Co active material agglomerate and sintering, but also provide new Co active sites for reforming. Consequently, the activity and stability of the catalyst are ensured. The reaction process is as follows: CaO * + CO 2 ! CaCO 3 * H 2 O(g)+ 2Co * ! Co À OH * + Co À O * (5) Fig. 10. CH 4 and CO 2 conversion with time on stream over Ca-promoted Co/AC catalysts in CO 2 reforming of methane. Fig. 11. Possible mechanism pathways of Ca-promoted Co/AC catalyst over CH 4 -CO 2 reforming.

Conclusions
The good catalytic properties and stability Co-based/AC catalysts for CO 2 -CH 4 reforming have been prepared by Ca doping. The properties of catalyst prepared with co-impregnation were better than that of with step impregnation. It is due that the catalyst surface prepared with the co-impregnation may be covered by calcium additive, which has strong effect on the activated carbon surface acid sites. In particular, the catalyst with 14 wt.% of calcium addition revealed a good activity and antisintering ability in carbon dioxide reforming of methane. Characterization results confirmed that the calcium addition can improve cobalt dispersion, intensify the interaction between cobalt and Ca and restrain the sintering of cobalt. In addition, the addition of calcium can improve the carbon dioxide adsorption properties, so that it is easy to eliminate carbon deposited on the surface of the catalyst by gasification. It also show that there was a new crystal substance Ca 3 Co 2 O 6 formed, which has most important effect on the catalytic performance and the anti-coking ability of CO 2 -CH 4 reforming catalyst. On the basis of experimental observations and previous studies, a simple CO 2 -CH 4 reforming mechanism over a cobalt/AC catalyst promoted by Ca is proposed. It suggested that the removal of adsorbed reactive carbon was the rate-determining step.