γ-Alumina-Supported NiMo Carbides as Promising Catalysts for CO 2 Methanation

CO2 methanation with Hydrogen to form CH4 offers a solution for off-peak renewable energy storage. γ-alumina-supported Mo and Ni-Mo catalysts were used in CO2 methanation, either in their reduced or in their carburized form. The presence of Ni improved the carburization extent of Mo-species, resulting in increased catalytic activity and selectivity for the catalytic CO2 methanation reaction. Carburization generally enhances the basicity of the materials and thus CO2 absorption on their surface. At 300 ̊C, the conversions of CO2 for the reduced Ni-Mo/Al2O3 catalyst and Ni-Mo2C/Al2O3 catalysts were 5.3% and 13.8% respectively with a corresponding selectivity in CH4 of 10.0% and 98.1%, respectively.


Introduction
In view of the potential impact of CO 2 on climate change, its atmospheric concentration needs to be controlled and stabilized.Carbon capture and utilization (CCU) technologies, including the catalytic valorization of CO 2 , can highly contribute to achieving this goal [1].Among the different processes, CO 2 methanation, i.e. its hydrogenation to form CH 4 , stands as a very promising technology, which also offers a solution for off-peak renewable energy storage [2].Though thermodynamically feasible even at ambient temperature, CO 2 methanation is considerably hindered by its extremely slow reaction kinetics.The use of active catalysts is imposed, moreover, undesirable byproducts such as CO start to be L. Yao  Noble metal-based catalysts (Ru, Rh, and Pd) are well known to be active in CO 2 methanation [3] [4].Ni-based catalysts display somehow lower but still acceptable activity being substantially cheaper than noble-metal based ones [5] [6].
Moreover, Ni-containing catalysts suffer from deactivation by: 1) sulfur poisoning; 2) carbon deposition; and 3) Ni-phase sintering [7].Besides, it was reported that the stability of the nickel on the Al 2 O 3 carrier is much higher than on the other carriers [8], and the Al 2 O 3 has a strong interaction with NiO, which may promote the formation of NiAl 2 O 4 spinel phase [9].
Since Levy et al. reported that tungsten carbides displayed similar activity as Pt in neo-pentane isomerization [10].These materials have been subject of growing interest, since they can be employed in many other catalytic reactions [11].Indeed, the activity of molybdenum and tungsten carbides (Mo 2 C and WC) in dry reforming of methane, partial oxidation and stream reforming of methane to synthesis gas was found to be higher than

Catalytic Activity Experiments
The CO 2 methanation activity test were carried out in a tubular quartz reactor at atmospheric pressure using a H 2 /CO 2 /Ar = 12/3/5 reactant mixture (total flow 100 ml/min).The gas hourly space velocity (GHSV) was 20,000 h The conversions of CO 2 and the selectivity of CH 4 during the methanation reaction were calculated using the following equations, respectively:

S
is the selectivity for CH 4 (%).

Physico-Chemical Characterization
CO 2 temperature programmed desorption (CO 2 -TPD) was performed in a BELCAT-M apparatus (BEL Japan).The reduced and carburized catalysts after CO 2 methanation were first degassed at 500˚C for 2 h, then cooled to 80˚C.10% CO 2 -He was fed for 1 h in order to saturate the catalyst's surface.After flushing He for 15 min, the materials were heated up from 80˚C to 800˚C under He, at the rate of 10˚C/min, while the evolution of CO 2 followed with the aid of a TCD detector.H 2 temperature programmed reduction (H 2 -TPR) carried out in the same device as the CO 2 -TPD, for both the calcined and the carburized catalysts.
The materials were first pretreated at 100˚C for 2 h, then reduced from 100˚C to 900˚C at a rate of 7.5˚C/min in 5% H 2 in Ar flow.X-ray photoelectron spectroscopy (XPS) experiment was performed on an AXIS Ultra DLD (KRATOS) spectrometer.

Results and Discussion
The relative sensitivity factor (RSF)-corrected Mo/C area ratios calculated from XPS peak integration for both calcined and carburized catalysts are presented in  1.
Though the presence of carbon-containing compounds, and thus the C 1s peak area, may be dependent on sample/device contamination, the Mo/C ratios are somehow smaller for the two carburized catalyst, in comparison to the calcined ones.This points already to a higher carbon content in the former, as a consequence of effective carburization.The results of the deconvolution of the C 1s, Mo 3d and Ni 2p orbitals can be also found in Table 1.The experimental peaks were decomposed into mixed Gaussian-Lorentzian contributions.The deconvolution of the C 1s orbital was performed as described elsewhere [18] [19], considering different species: carbides (238.[20].In agreement to the deconvolution of the C 1s orbital, the presence of Mo-carburized species increases after the carburization treatment. However, higher amount of oxycarbides seems to be as well produced as a consequence of this treatment.This can be due either to the partial oxidation of molybdenum species during passivation upon or to their incomplete carburization [13] [21].In any case, carburization seems to be more effective when Ni is present in the formulation of the catalyst [13]  The spin-orbit splitting value between Ni 2p 3/2 and Ni 2p 1/2 was found to be around 17.5 eV, pointing to the presence of NiAl 2 O 4 [23].A certain amount of reduced Ni 0 species are formed upon the carburization treatment.Let us note here that the X-ray diffraction patterns acquired for this series of catalysts (not shown) did not evidence the presence of Mo 2 C species, presumably due to its relative low concentration and high dispersion [24] [25].
Table 2 shows the results of the integration of the CO 2 -TPD profiles for this series of catalysts, upon either reduction or carburization, whereas the CO    The results of the methanation experiments are presented in Table 3.It is worth to note that the Mo/Al 2 O 3 catalyst (reduced) was found to be completely   Though the results are quite promising and prove that Ni-Mo carbide catalysts can be successfully used for CO 2 methanation, the carburization treatment needs to be consequently optimized.

Conclusion
−1 .Both Mo/Al 2 O 3 and Ni-Mo/Al 2 O 3 catalysts were either carburized or reduced prior to activity runs.For the reduced catalyst, prior the activity tests, the Mo/Al 2 O 3 and Ni-Mo/ Al 2 O 3 catalysts were reduced in-situ at 900˚C in 5%H 2 in Ar for 1 h, and then cooled down to 250˚C.The catalytic activity experiments were carried out from 250˚C to 500˚C.Steady-state conversions were reached at each temperature (30 min isothermal step).For the carbide catalyst, the methanation experiments were carried out after the in-situ carburization of the sample and its cooling in Ar to 250˚C.The reactants and products were analyzed by a micro gas chromatograph (Varian CPi 4900), equipped with a TCD detector.

3 are detailed in Figure 1 .
2 -TPD profiles acquired for the carburized catalysts Mo 2 C/Al 2 O 3 and Ni-Mo 2 C/Al 2 O Total basicity extremely increases for the catalysts submitted to the carburization treatment.While both the reduced Mo and the Ni-Mo containing catalysts show very small ability to absorb CO 2 , the carburized are able to absorb more than 40 times more CO 2 than the reduced ones.

Figure 2 Figure 2 .
Figure 2 showed that both CO 2 -TPD profiles (acquired for Mo 2 C/Al 2 O 3 and Ni-Mo 2 C) covered almost all the temperature window, which pointing to the presence of basic sites with different strength.The most important part for the CO 2 -TPD was the desorption occurred at low and moderate temperatures, which reflects a major presence of weak and medium-strength basic sites in these carburized catalysts.Note here moreover that the presence of Ni in the bimetallicTable 2. Total basicity, i.e. integration of the CO 2 -TPD profiles, for the different catalysts before and after carburization.Catalyst Amount of CO 2 desorbed (μmol/g) Mo/Al 2 O 3 2.6 Ni-Mo/Al 2 O 3 114.8Mo 2 C/Al 2 O 3 3.3 Ni-Mo 2 C/Al 2 O 3 121.7

Figure 3
Figure 3 presents H 2 -TPR profiles for the carburized catalysts.The Mo 2 C/ Al 2 O 3 catalyst exhibited two main H 2 consumption peaks respectively centered at about 437˚C and 910˚C.The low temperature reduction peak (437˚C) can be assigned either to the reduction of MoO 3 to MoO 2 or to the reduction of some high valent Mo species (MoO x ), The reduction peak occurring observed around 910˚C results from the reduction of MoO 2 to metallic Mo, but can also be ascribed to the reduction of Mo 2 C species[26].If this high temperature peak could be ascribed to the further reduction of MoO 2 to metallic Mo, the first low temperature peak corresponding to the reduction of MoO 3 to MoO 2 should have very similar intensity, i.e. similar H 2 consumption, than the high temperature one, which, indeed, is not the case.Therefore, the high temperature peak can be directly linked to the presence of molybdenum carbide and oxycarbide species.In the case of the Ni-Mo 2 C/Al 2 O 3 catalyst, the peak corresponding to the reduction of high valence Mo-species at low temperature appears shifted to lower temperatures, i.e. around 410˚C, and becomes much weaker.The high temperature peak, corresponding to the reduction of molybdenum carbide and oxycarbide species, also shifts to lower temperature, i.e. around 820˚C.This latter result points out that the presence of Ni affects the carburization process and the type of carburized species formed.In the presence of Ni, the carburization treatment leads most probably to favored carburization of molybdenum and to a lower extent of formation of oxycarbides, which confirming the results obtained through XPS analysis.The H 2 -TPR profiles acquired for the calcined Mo/Al 2 O 3 and Ni-Mo/Al 2 O 3 catalysts can be found elsewhere[27], but they only evidenced the H 2 -consumption peaks typical of Mo and Ni oxide and mixed oxides species.

[ 3 ]
. According to the above results of the physico-chemical characterization, first of all, carburization results in a 40-fold increase the basicity, i.e. the CO 2 absorption ability of these catalysts.As a consequence, and even if the amount of oxycarbide species formed was found to be important, the carburized Mo 2 C/Al 2 O 3 catalyst showed already a better activity vis-à-vis the reduced Mo/Al 2 O 3 one.Additionally, the presence of Ni enhanced the carburization of the molybdenum species in the bimetallic Ni-Mo catalysts.The amount of oxycarbides was reduced at the same when Ni and Mo was coexist.All this facts resulted therefore in the further promotion of the activity and selectivity observed on the Ni-Mo 2 C/Al 2 O 3 catalyst.The formation of a mixed NiAl 2 O 4 phase was also observed through the analysis of the XPS results.The presence of Ni in strong interaction with the alumina support may have also affected the carburization process in the case of the Ni-Mo 2 C/Al 2 O 3 catalyst.
γ-alumina-supported Mo and Ni-Mo catalysts were prepared and submitted ei-L.Yao et al.DOI: 10.4236/mrc.2017.64010143 Modern Research in Catalysis ther to reduction or to a carburization treatment, prior to evaluating their catalytic activity in CO 2 methanation.The presence of Ni facilitated the formation of the Mo 2 C species, considerably reducing the formation of oxycarbides.The CO 2 absorption substantially increased upon carburization, leading to improved catalytic activity in CO 2 methanation.Moreover, the presence of Ni and thus as a consequence of favored carburization, resulted in further enhanced catalytic activity and selectivity.Nevertheless, the carburized catalysts still contained an important amount of oxycarbide species, pointing to incomplete carburization.
et al.
DOI: 10.4236/mrc.2017.64010136 Modern Research in Catalysis produced at temperatures higher than 350˚C.Unfortunately, most of the commercially existing catalytic systems start to be active at this temperature.

2. Experimental 2.1. Catalysts Preparation 2.1.1. Mo/Al2O3 and Ni-Mo/Al2O3 Catalysts
[15]nd Pd-based catalysts, though still lower than the activity measured for Ru and Rh catalysts[12].Shi et al. reported high catalytic activity and stability for a Ni-Mo 2 C catalyst in dry reforming of methane[13].The activity of Mo 2 C and WC was linked to their facility to activate the extremely stable CO 2 molecule.They can be also used as hydrogenation catalysts and, in fact, Huo and co-workers recently reported interesting activity, selectivity and stability in CO methanation for Co-supported on Mo carbide[14].Mo 2 C-and/or WC-based catalysts can be therefore promising materials, able to boost the CO 2 methanation reaction.However, to the best of our knowledge, there are no studies considering the use of Mo 2 C-supported catalysts for this particular application.In our previous work, we found carbide Ni-Mo/Al 2 O 3 catalyst (Ni-Mo 2 C/ Al 2 O 3 ) was a better catalyst for dry reforming of methane than that of reduced Ni-Mo/Al 2 O 3 catalyst[15].It was investigated the influence of preparation condition (the ratio of H 2 /CH 4 ) for the carburization process in detail in the previous work[15].

.2. Mo2C/Al2O3 and Ni-Mo2C/Al2O3 Catalysts
quently kept at 700˚C for 2 h.The gas flow was then switched from CH 4 /H 2 to Ar for cooling down (overnight).

Table 1 (
a), & Table 1(b).The XPS spectra corresponding to Ni-Mo 2 C/Al 2 O 3 carbide catalysts are presented in Figure

Table 1 .
(a) Deconvolution of the C 1s, orbital for the different catalysts before and after carburization (BE in eV in italics); (b) Deconvolution of the Mo 3d and Ni 2p orbitals for the different catalysts before and after carburization (BE in eV in italics).

Table 3 .
Catalytic activity and selectivity of the different catalysts: CO 2 conversion and CH 4 selectivity at temperatures from 250˚C to 500˚C.

Table 3 .
Both the reduced Ni-Mo/Al 2 O 3 and the carburized Mo 2 C/Al 2 O 3 showed very low catalytic activity and very poor CH 4 selectivity in CO 2 methanation reaction.As previously observed for this thermodynamically feasible but strongly kinetically hindered reaction, the CO 2 conversion generally increases with increasing reaction temperatures, i.e. in the case of the Mo 2 C/Al 2 O 3 catalyst it increases from 4.1% at 350˚C to 26.5% at 500˚C.Similar CO 2 conversions were obtained over the reduced Ni-Mo/Al 2 O 3 catalyst.The activity towards CO 2 methanation substantially was improved in the presence of the carburized