Catalysis at the nanoscale may change selectivity

Abstract

Among the many virtues ascribed to catalytic nanoparticles, the prospect that the passage from the macro-to the nanoscale may change product selectivity attracts increasing attention. To date, why such effects may exist lacks explanation. Guided by recent experimental reports, we propose that the effects may result from the coupling between the chemical steps in which the reactant, intermediates, and products are involved and transport of these species toward the catalytic surface. Considering as a thought experiment the competitive formation of hydrogen and formate upon reduction of hydrogenocarbonate ions on metals like palladium or platinum, a model is developed that allows one to identify the governing parameters and predict the effect of nanoscaling on selectivity. The model leads to a master equation relating product selectivity and thickness of the diffusion layer. The latter parameter varies considerably upon passing from the macro-to the nanoscale, thus predicting considerable variations of product selectivity. These are subtle effects in the sense that the same mechanism might exhibit a reverse variation of the selectivity if the set of parameter values were different. An expression is given that allows one to predict the direction of the effect. There has been a tendency to assign the catalytic effects of nanoscaling to chemical reactivity changes of the active surface. Such factors might be important in some circumstances. We, however, insist on the likely role of short-distance transport on product selectivity, which could have been thought, at first sight, as the exclusive domain of chemical factors.energy | nanoparticles | electrocatalysis N anoparticles work wonders in many fields of science and technology. This is particularly true in the area of heterogeneous catalysis (1-10). The advantages of ensembles of nanoparticles over foils of the same catalytic material, as well as the role of the particle size, have been frequently recognized with regard to rates. The answer is more elusive as to the possible effect of nanoscaling on product selectivity. It is tempting to attribute such effects to the creation of surface defects that may be triggered by the deposition of the nanoparticles. We, however, explore another type of rationale.The systems where some relevant information is available are few but concern important processes in the area of contemporary energy challenges (11)(12)(13)(14), namely the electrochemical hydridation of CO 2 into HCO 2 − versus H 2 evolution at a metal electrode in aqueous media (15)(16)(17)(18)(19). This is the case when the metal is palladium with which little formate, if any, is generated upon electrolysis on a Pd foil (18,20), whereas the formate faradaic yield reaches 90-95% with ∼5-nm-diameter Pd nanoparticles dispersed on ∼100-nm carbon particles (18). Very high faradaic yields of formate are similarly found on carbon supported Pt-Pd nanoparticle electrodes in a pH 6.7 phosphate buffer (21,22).How can such major differences in product selectivity be explained whe...

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