A First Principles Mechanistic Study of Higher Alcohol Synthesis from Syngas on a Stepped Rhodium Surface

The mechanism of higher alcohol synthesis (HAS) from syngas on a stepped Rhodium surface was explored using first principles calculations based on density functional theory. Results showed that the activation of CO proceeds most energetically feasible via a sequential hydrogenation towards CH₂OH, followed by the C–OH bond cleavage yielding CH_x species. Because the initial CO hydrogenation step is highly activated, the cascade of elementary steps toward methane formation is highly favored. The formation of C₂ oxygenates toward ethanol production is kinetically favored by CO insertion to CH₂, or alternatively, by a lower activation barrier CHO insertion to CH₃. On the other hand, the C₃ species is formed more preferably by CO rather than CHO insertion to a CH₃CH₂ fragment, indicating the effect of a more extended carbon structure on the reaction mechanism. The overall reaction mechanism for HAS points to a cycle of CO insertion, hydrogenation, and OH elimination steps.