A DFT investigation of the mechanisms of CO2 and CO methanation on Fe (111)

Caroline R Kwawu,Evans Adei,Richard Tia,Albert Aniagyei

doi:10.1007/s40243-020-0164-x

Abstract

Insight into the detailed mechanism of the Sabatier reaction on iron is essential for the design of cheap, environmentally benign, efficient and selective catalytic surfaces for CO2 reduction. Earlier attempts to unravel the mechanism of CO2 reduction on pure metals including inexpensive metals focused on Ni and Cu; however, the detailed mechanism of CO2 reduction on iron is not yet known. We have, thus, explored with spin-polarized density functional theory calculations the relative stabilities of intermediates and kinetic barriers associated with methanation of CO2 via the CO and non-CO pathways on the Fe (111) surface. Through the non-CO (formate) pathway, a dihydride CO2 species (H2CO2), which decomposes to aldehyde (CHO), is further hydrogenated into methoxy, methanol and then methane. Through the CO pathway, it is observed that the CO species formed from dihydroxycarbene is not favorably decomposed into carbide (both thermodynamically and kinetically challenging) but CO undergoes associative hydrogenation to form CH2OH which decomposes into CH2, leading to methane formation. Our results show that the transformation of CO2 to methane proceeds via the CO pathway, since the barriers leading to alkoxy transformation into methane are high via the non-CO pathway. Methanol formation is more favored via the non-CO pathway. Iron (111) shows selectivity towards CO methanation over CO2 methanation due to differences in the rate-determining steps, i.e., 91.6 kJ mol−1 and 146.2 kJ mol−1, respectively.

Highlights

To explore the intermediates involved in the hydrogenassisted CO2 and carbon monoxide (CO) methanation on the Fe (111) surface, several starting geometries were optimized by the stepwise hydrogen addition to C O2 and CO, which were optimized to obtain stable conformations
Our results show that CO2 methanation on Fe (111) occurs via the CO pathway through the following transformations: carboxylate protonation to dihydroxycarbene (HOCOH), decomposition to carbon monoxide (CO), protonation of CO to HCO, protonation to H 2CO to alkoxy, protonation on alkoxy ( CH2–OH) and water production, leading to CH2 and C H3 formation
Our spin polarized-D2-GGA-DFT calculations reveal that CO2 methanation will occur via the CO pathway and not the non-CO pathway

Summary

Introduction

Carbon dioxide (CO2) is a cheap, non-toxic and abundant carbon-one (C1) source for chemical processes [1,2,3,4,5,6,7] and its transformations into fuel offer solutions to the problem of global warming as well as helping to meet the world’s increasing energy needs [8]. Despite the simplicity of the reaction, CO2 methanation mechanism appears quite difficult to establish as several different opinions have been expressed on the nature of intermediate compounds involved and the rate-determining step [7]. Late 3d metals, i.e., Fe, Ni, Cu, Co and Zn are metals thought to be responsible for CO2 reduction in nature [15,16,17,18,19]. These transition metals in iron sulfide clusters are thought to provide the needed electrons for the reduction

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Results and discussion

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Conclusion