The new role of surface adsorbed CH_x(x = 1–3) intermediates as a co-adsorbed promoter in self-promoting syngas conversion to form CH_x intermediates and C₂ oxygenates on the Rh-doped Cu catalyst

Syngas conversion to C₂ oxygenates includes two crucial steps that CO activation to form CH_x(x = 1–3) intermediates, followed by its reaction with CO/CHO to form the C–C chain, in which the CH_x(x = 1–3) intermediates were widely recognized as the reactive intermediates to yield C₂ oxygenates. Inspired by the reported studies about the role of co-adsorbed OH intermediates that promote CO activation and the C–C chain formation in syngas conversion, an idea about the new role of surface adsorbed CH_x intermediates in syngas conversion is proposed, that is, whether the surface adsorbed CH_x intermediates itself also act as a co-adsorbed promoter to self-promote CO activation and conversion. This study revealed for the first time that in syngas conversion to C₂ oxygenates over Rh-doped Cu catalysts, the surface adsorbed CH_x(x = 1–3) intermediates not only act as the reactive intermediates to participate into the reactions with CO/CHO, but also itself act as a self-promoter to facilitate CO activation to form CH_x(x = 1–3) intermediates and promote CHO reaction with CH_x(x = 1–3) to form C₂ oxygenates. Especially, the dominant CH₂ intermediate acted as a co-adsorbed promoter exhibits higher activity and selectivity towards itself formation and C₂ oxygenates CH₂CHO instead of methanol and hydrocarbons, respectively. Moreover, the internal mechanism of surface adsorbed CH_x intermediate is explained from the electronic property aspect. This newly so-called CH_x self-promoting syngas conversion mechanism offers new insights into the fundamental role of surface adsorbed CH_x(x = 1–3) intermediates in CO activation and conversion to form C₂ oxygenates, and open a new mechanism that is likely general and involved in the reactions related to CH_x(x = 1–3) intermediates formation.