(621al) Selective CO Methanation over Ceria-Supported Ni, Co, Fe Catalysts

1. Introduction

CO+3H₂ = CH₄ + H₂O;         ΔH^o= -206 kJ/mol                 (1)

CO₂+4H₂ = CH₄ + 2H₂O;      ΔH^o= -165 kJ/mol                 (2)

CO₂ + H₂ = CO + H₂O;         ΔH^o= 41 kJ/mol                     (3)

In spite of extensive research efforts Ru- and Ni-based systems remain the most active catalysts for CO and CO₂methanation [1]. At the same time, metals such as Co and Fe are known to be active in carbon oxides hydrogenation reactions, including Fischer-Tropsch synthesis and RWGS, but the properties of Fe and Co-based systems in the selective CO methanation are not studied.

This work reports the results of comparative study of Ni-, Co- and Fe/CeO₂catalysts, prepared from nitrate and chloride precursors, in the selective CO methanation.

2. Experimental

Catalysts with metal loading of 10 wt.% were prepared by incipient wetness impregnation of CeO₂ by the water solutions of metal's nitrate (Ni/CeO₂, Co/CeO₂, Fe/CeO₂) and chloride (Ni(Cl)/CeO₂, Co(Cl)/CeO₂, Fe(Cl)/CeO₂) salts. They were characterized by BET, XRD, TEM, EDX, XPS, FTIR and CO chemisorption techniques. Selective CO methanation was studied in a flow reactor at atmospheric pressure in the temperature interval 180 − 360 °C, at WHSV = 29 000 cm³g^-1h^-1 and feed gas composition (vol.%): 1.0 CO, 20 CO₂, 10 H₂O, 65 H₂and He-balance.

3. Results and Discussion

By the temperature dependencies of the CO and CH₄ outlet concentrations in selective CO methanation for all studied catalysts, it is seen, that Fe-based and Co(Cl)/CeO₂ catalysts were inactive in CO and CO₂ methanation reactions. Ni/CeO₂ and Co/CeO₂ catalysts were active in both CO and CO₂ methanation, but showed low selectivity. Ni(Cl)/CeO₂ catalyst showed the best performance in selective CO methanation, being less active than Ni/CeO₂ and Co/CeO₂, but considerably more selective.

XRD, TEM and EDX analysis showed the presence of chlorine on the surface of Ni(Cl)/CeO₂, even the CeOCl particles (Table 1). Thus the observed differences in catalytic behavior of Ni-based catalysts could be associated with chlorine influence.

The TOFs calculated for Ni/CeO₂ and Ni(Cl)/CeO₂ catalysts at 210 °C (at this temperature CO conversion was < 15 % and CO methatation selectivity was 100 %) were similar (Table 1). These data mean, that Ni/CeO₂ and Ni(Cl)/CeO₂ catalysts exhibited similar activity in CO methanation. In other words, the presence of chlorine does not significantly influence on the CO methanation activity, but it dramatically inhibits the CO₂hydrogenation activity, thereby promoting catalyst selectivity.

Table 1.Ni-based catalyst's characterization.

^aPhase mass fraction according to the XRD analysis; ^bAverage particle size determined by the XRD analysis; ^cAverage size of metal particles according to the pulse CO chemisorption data.

 Based on the data obtained, the nature of the active centers, origin of the observed activity order and probable mechanisms of CO and CO₂ methanation over Ni/CeO₂ and Ni(Cl)/CeO₂catalysts are discussed.

4. Conclusions

Catalytic properties of Ni-, Co- and Fe/CeO₂ catalysts, prepared from nitrate and chloride precursors, in the selective CO methanation were studied. The Ni(Cl)/CeO₂ catalyst, prepared from NiCl₂ precursor, was the most efficient. This phenomenon could be explained by ceria surface blocking by chlorine species and appropriate inhibition of CO₂hydrogenation activity.

Acknowledgements

This work was partially supported by the RFBR Grant 14-03-00457-a and MES (Russia).

References

[1] М.М. Zyryanova, P.V. Snytnikov, R.V. Gulyaev, Yu.I. Amosov, A.I. Boronin, V.A. Sobyanin, Chem. Eng. J. 238 (2014) 189.