Design of Dilute Palladium-Indium Alloy Catalysts for the Selective Hydrogenation of CO2 to Methanol

Adding small amounts of a more reactive dopant metalto a moreselective host metal is a well-known strategy for promoting the catalyticreactivity of the latter without sacrificing its selectivity. Whilesubstantial attention has been given to coinage metal hosts for activatingindividual C-H or O-H bonds, less attention has beengiven to more exotic metals, such as indium, which is highly selectivein the complex hydrogenation of CO2 to methanol. Herein,we describe the synthesis and properties of Pd-In alloys containingPd in various states of aggregation (isolated atom, small clusters,and extended clusters) assessed through various characterization methods.These materials exhibit unique catalytic behaviors, with Pd promotingboth the reaction rate and methanol selectivity in all cases, thoughits efficacy is highest when Pd is present in small clusters. Theinefficiency of high Pd loadings is due in part to losses in Pd accessibility,while the importance of aggregated vs isolated Pd is theorized toresult from accelerated H-2 activation. Methods used tosynthesize more conventional dilute-limit and single-atom alloy catalystscan, therefore, be extended to non-coinage metal hosts to promoteand control more complex chemistries, though the roles of ensemblesize may greatly differ.