Experimental and mechanistic insights on the CO2 hydrogenation to ethanol over KRhFe supported catalysts: a study of support effect

The effect of the support on the CO 2 hydrogenation to ethanol reaction over KRhFe catalysts supported on different oxides (SiO 2 , Al 2 O 3 , TiO 2 , TiO 2 -ZrO 2 , and SiO 2 -Al 2 O 3 ) was investigated through a multi-technique approach combining catalytic tests with extensive physicochemical characterisation, including XAS and operando DRIFTS-MS. H 2 -TPR, TEM and XAS analyses revealed that non-reducible supports, such as SiO 2 and Al 2 O 3 , stabilise partially oxidised, well-dispersed Rh and Fe species, while maintaining moderate Rh–Fe interactions. In contrast, reducible supports like TiO 2 and TiO 2 -ZrO 2 facilitate the reduction of RhO x and FeO x species. This translates into markedly different catalytic behaviours: SiO 2 and Al 2 O 3 -supported catalysts exhibited enhanced selectivity towards ethanol, whereas TiO 2 and TiO 2 -ZrO 2 -supported catalysts showed higher CO 2 conversion, predominantly yielding CO and CH 4 . Operando DRIFTS-MS results further revealed plausible support-dependent surface intermediates and reaction pathways. The SiO 2 -supported catalyst appears to mainly follow a CO-mediated pathway, while the Al 2 O 3 -supported catalyst exhibited intense formate-related features, suggesting the participation of formate/methoxy intermediates in oxygenate formation. In contrast, TiO 2 promoted RWGS and methanation reactions, limiting C–C coupling. Finally, the incorporation of ZrO 2 into the support (TiO 2 -ZrO 2 ) improved the formation of oxygenates compared to TiO 2 alone, likely due to enhanced stabilisation of adsorbed CO species and their coupling with methoxy intermediates. Overall, these results underscore the critical role of metal–support interactions in governing the selectivity between C1 products and oxygenates during high-pressure CO 2 hydrogenation.