The design of optimal active sites in supported metal catalysts often focuses on engineering the composition or geometry of metal nanoparticles. This focus has resulted in the development of powerful predictive theories and synthesis strategies that together have enabled the identification of novel catalytic materials with improved performance. Much less is known about how metal-support interactions can be exploited to control the reactivity of heterogeneous metal catalysts, although the effect of supports on catalytic reactivity or selectivity can be as significant as characteristics of the metal. Demonstrated mechanisms of support effects on metal nanoparticle reactivity include charge transfer, direct participation in catalysis and metal encapsulation, which are primarily caused by variations in the reactivity of surface oxygen on reducible and irreducible supports. Among these support effects, metal encapsulation is the only mechanism that allows intimate interactions between the support and all metal active sites on nanoparticles (greater than ~1 nanometer in diameter) due to migration of reducible supports on top of the metal. Here we will describe a unique metal-support interaction where treatment of TiO2 and Nb2O5 supported Rh catalysts with CO2 rich CO2/H2 atmospheres at elevated temperature (200-300 oC) induces the formation of incommensurate adsorbate (HCO2- and HCO3-) stabilized oxide overlayers on Rh. The overlayer formation transfers significant charge to all exposed Rh active sites, rendering them anionic, and significantly modifying Rh reactivity. Compared to the traditional encapsulation of metal nanoparticles by reducible oxide supports demonstrated originally by Tauster et al., which forms due high temperature reduction in H2, the anionic encapsulation state is more stable against oxidation by H2O. Furthermore, the anionic encapsulation state modifies the reactivity of Rh in the reduction of CO2 by H2 inducing a switch in the main product selectivity from CH4 on bare Rh nanoparticles to CO in the anionic encapsulation state, which does not occur in the traditional encapsulation state. In addition, there is a switch in the selectivity of minor products. The switch in selectivity induced by the anionic encapsulation state is hypothesized to be a result of increased back donation of electrons from Rh to adsorbed CO, which stretches the internal C-O bond and destabilizes CO on the Rh surface.

Manipulating the reactivity of Rh catalysts via anionic strong metal support interactions

251st National Meeting (2016)

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