Comparative studies about CO methanation over Ni(211) and Zr-modified Ni(211) surfaces: Qualitative insight into the effect of surface structure and composition

In order to reveal the underlying mechanism of coke-deposition resistance of the promoter Zr, the comparative studies about the reaction pathways of CO methanation have been carried out over the stepped Ni(211) and Zr-modified Ni(211) surfaces using the density functional theory. DFT results show that CO > COH -> C -> CH -> CH2 -> CH3 -> CH4 and CO -> COH -> HCOH -> CH2OH -> CH2 -> CH3 -> CH4 are mainly responsible for CH4 formation on Ni(211). Thus, in the former pathway, the main contributor to carbon deposition is the dissociation of COH; while in the latter pathway, carbon formation ascribes to the decomposition of CH2 due to the competition of CH2 between hydrogenation and dehydrogenation. Conclusively, Ni(211) is much susceptible to carbon formation. However, ZrNi(211) surface exhibits high resistance to carbons with the energetically favorable pathway of CO -> HCO -> CH2O -> CH2OH -> CH2 -> CH3 -> CH4 since CH2 prefers to be hydrogenated to CH3 rather than being dehydrogenated into CH. On the other hand, for the effects of surface structure and composition on the selectivity, Ni(211) displays a remarkable increase in selectivity to CH4 compared to Ni(111), which ascribes to much difference in the activity of CH2OH between the dissociation to CH2 and the hydrogenation to CH3OH, the former is superior to the latter. On ZrNi(211), even no CH3OH yield is expected. Further, the presence of promoter Zr greatly decreases the overall activation energy of CH4 formation on ZrNi(211) surface, this results in a significant increase of the reaction activity compared to Ni(111) and Ni(211). To sum up, ZrNi(211) is highly activity and selectivity of CH4 formation in CO methanation, and particularly resistant to carbon formation, which will provide a way of fabricating a more effective Ni-based catalyst. (C) 2017 Elsevier B.V. All rights reserved.