Role of CO2 in ethylbenzene dehydrogenation over Fe2O3(0 0 0 1) from first principles

First-principles calculations based on density functional theory have been performed to elucidate the reaction mechanism for ethylbenzene dehydrogenation and the role of CO2 in H removal. On the basis of the experimental information and theoretical prediction, three model surfaces with Fe-, ferryl- and O-termination are constructed to represent the active Fe2O3(0 0 0 1) surface. The calculated results indicate that on all of the three surfaces the C–H activation in the methylene group followed by the dehydrogenation of the methyl group is kinetically more favorable. The energy barriers for ethylbenzene dehydrogenation are lowest on the O-terminated surface, but the generated styrene is adsorbed too strongly to be released. As CO2 decomposition and the formation of HCOO are hindered by the relatively high activation energies, CO2 cannot serve as the oxidant to recover the O- and ferryl-terminated surfaces to keep the redox cycle. At the steady state of the reaction the coupling mechanism dominates on the Fe-terminated surface, with the synergistic effect between ethylbenzene dehydrogenation and the reverse water–gas shift reaction. Since the energy barrier for the formation of COOH is comparable to that for H2 formation, both the one-step and two-step pathways are predicted to contribute to the coupling mechanism, although the former is more probable.

Graphical abstractThe comprehensive mechanism for ethylbenzene dehydrogenation in the presence of CO2 on Fe2O3(0 0 0 1) with three terminations have been investigated by DFT calculations. The coupling and redox cycle mechanisms have been considered. On the basis of the calculated results, the most likely reaction pathway and the role of CO2 have been elucidated.Figure optionsDownload full-size imageDownload high-quality image (118 K)Download as PowerPoint slideHighlights► The mechanism for ethylbenzene dehydrogenation in the presence of CO2 is explored. ► Styrene is hard to escape from the most active O-terminated Fe2O3(0 0 0 1). ► The Fe-terminated surface dominates the reaction, with the coupling mechanism. ► Both the one-step and two-step pathways are probable while the former is dominant.