Study On Direct Ethanol Synthesis From Syngas On Rh-based Catalyst Obtained By Reducing Perovskite-type Oxides

In China,the reserves of coal are abundant,but those of oil and gas are poor.Syngas can be obtained by the gasification of coal and biomass,meanwhile,ethanol,which is one part of higher alcohols,can be used as ethanol gasoline,solvent and basic chemical material.Therefore,direct ethanol synthesis（DES）from syngas has important significance.Research works have showed that the main by-products of unpromoted Rh-based catalysts are methane and methanol.Further efforts are still necessary to improve the stability of the catalysts.Adding proper promoters can solve these two problems.In this paper,some different kinds of metal ions have been doped into the A or B sites of perovskite-type oxide for La Rh_1-xCo_xO₃/Si O₂,La Fe_1-xRh_xO₃/Si O₂and La_1-xCe_xFe_0.5Rh_0.5O₃/Si O₂catalyst preparation and the effect of additives is studied.The reason that this is possible is because the composition of perovskite oxides is adjustable.Supported perovskite-type oxides La Rh_1-xCo_xO₃/Si O₂（x=0-0.4,1）catalysts with different cobalt doping amount was prepared by using citrate complexation method.By reducing the catalyst precursors,nanoparticles（NPs）of Rh-Co supported on La2O3 doped Si O2 were generated（Rh-Co-La₂O₃/Si O₂）.The interaction between rhodium and cobalt is an important factor for DES,but the preparation of Rh-Co NPs is a challenge.In this work,perovskite-type oxide was used to intensify the interaction between rhodium,cobalt and La₂O₃in order to restrain the phase transition and the sintering process,intensify the dilution effect of Rh,Co NPs and the interaction of cobalt denoting electron to rhodium.Therefore,the activity and the stability of Rh-Co-La₂O₃/Si O₂were good and the selectivity to ethanol was high with0.2 wt.%Co added amount.La Fe_1-xRh_xO₃/Si O₂catalysts was prepared by using citrate complexation method.During reduction,La Fe_1-xRh_xO₃/Si O₂catalyst precursor was transferred to Rh/La₂O₃-La Fe O₃/Si O₂.The activity tests indicated that the catalysts showed excellent catalytic performance with the addition of La₂O₃and La Fe O₃,which was due to the sites of oxygen vacancies on the surface of La Fe O₃can effectively help the sites of Rh⁰-Rh⁺to adsorb/activate CO for ethanol.The catalyst also exhibited excellent resistance to sintering of Rh NPs and to carbon deposition,attributing to confinement of Rh NPs by La₂O₃and La Fe O₃and the eliminating effect of La₂O₃and oxygen vacancy on La Fe O₃.Moreover,supported La_1-xCe_xFe_0.5Rh_0.5O₃/Si O₂catalysts was prepared and the effect of loading amount of Ce O₂on the catalytic performance was studied.It further indicated that the existence of oxygen vacancy can improve the activity and selectivity to ethanol.In addition,a new strategy for preparing nanocomposites with metal NPs as a core and oxide as a shell has been provided in this work,which was based on the photothermal effect of localized surface plasmon resonance（LSPR）.LSPR can convert the energy of the light into heat through a process called photothermal effect,during which the surface of the metal NPs is heated to a higher temperature than the temperature of surrounding area.In another word,the heat was confined to the interface between the Ag NPs and the aqueoussol medium.This allowed us to generate Ce O₂shell only on the interface and not in the bulk aqueous medium,resulting in cerium gel（changed to Ce O₂after calcination）formation only on the surface of the Ag NPs.The preparation method proposed is easily operated.