Recent interest in selective CO2 hydrogenation to useful fuels and chemicals has motivated the development of new catalytic materials that exhibit high selectivity in desired transformations. However, many fundamental details of CO2 hydrogenation on transition metal surfaces are still missing. Of particular interest is uncovering descriptors that can be related to previously observed dependence of metal composition to reaction selectivity. Most notably, the trend between CO and CH4 selectivity in CO2 hydrogenation on transition metals has not yet been explained. In this work we utilized DFT calculations coupled with mean-field microkinetic modeling to develop molecular level insights into the mechanisms that control performance and selectivity of Ru catalysts in CO2 reduction by H2 at atmospheric pressure. The initial CO2 hydrogenation step leading to CH4 production was shown to occur through CO2 dissociation and subsequent hydrogenation of CO* to CHO*. The dissociation of CHO* to form CH* and O* was identified as the rate limiting step for CH4 formation, while the rate limiting step for CO production through the reverse water gas shift (rWGS) reaction was identified as CO* desorption. Based on a scaling relations analysis of competing CHO* dissociation and CO* desorption, O* adsorption energy was found to be an effective descriptor of differences in selectivity between CO and CH4 production previously observed on late-transition metal catalysts. These mechanistic insights provide critical information to guide the design of catalysts with tunable selectivity for CO2 reduction by H2 at atmospheric pressure.

Mechanistic insights into CO2 hydrogenation on transition metal surfaces: A DFT-based microkinetic analysis

251st National Meeting (2016)

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