The direct synthesis of ethanol from syngas is a promising way in the field of clean and efficient use of fossil fuel, but it still suffers from insufficient ethanol selectivity and understanding of active sites. In this work, the Rh-based catalyst with Ce as promoter and ZrO 2 as carrier exhibits excellent catalytic performance, showing the selectivity of ethanol toward 24.3 %. Moreover, the yield of methane decreases to 4.8 %, and the formation of CO 2 is inhibited to a certain degree by tuning the active sites. The carrier is changed by doping Ce into the ZrO 2 lattice, which assists to form the Rh δ+ active sites. The synergetic effect of Ce and Zr is important to influence the state of Rh species in the catalysts, leading to the differences in the CO hydrogenation process. On the other hand, the Rh δ+ active sites are also stabilized by promoted surface oxygen vacancies in the 2.0Rh5.0Ce-ZrO 2 catalyst. The high-activity catalyst with Rh δ+ active sites will be helpful to understand the real active sites of Rh-based catalysts for ethanol synthesis from syngas, thereby guiding further design of the related catalyst with high performance to meet the industrial demand in syngas converting to ethanol.
Secondary-metals promotion is key to tune the CO 2 hydrogenation performance of Rh-based catalyst. We describe the addition of Cu or Co in TiO 2 nanorods supported Rh catalyst (viz., RhM x /TiO 2 ), showing significant impact on the CO 2 hydrogenation activity and productivity. Cu addition can promote the partial hydrogenation to alcohols (methanol and ethanol), whereas Co led to the deep hydrogenation to alkanes (CH 4 , C 2 H 6 ). Comparative time-resolved in situ DRIFTS studies with H 2 /D 2 isotopic exchange further reveal that both surface adsorbed CO* and formates could be the key reaction intermediates during the CO 2 hydrogenation over RhM x /TiO 2 (especially the RhCu 2.5 /TiO 2 ), in which the interaction strength of CO* regulated by secondary-metals addition was key to tune the reaction pathways and productivity. The findings suggested that rational design of active metal sites, for example the synergy between Rh and secondary-metals, is highly needed to promote the CO insertion and C C coupling to produce ethanol.
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