“What happens when you add water?” is possibly the most frequently asked question after presentations in heterogeneous catalysis. This question is indeed paramount, and I will report on our group’s recent studies of the promotional and inhibiting role of water for CO and H2 oxidation at Au/TiO2 interfaces, as well as for the reduction of phenolic alcohols over Ru/TiO2.
Preferential oxidation (PrOx) of CO is a promising energy efficient alternative to CO methanation for purifying H2 streams from steam reforming processes. With high-purity H2 being the desired product, the obvious challenge is to find a catalyst that readily oxidizes CO, but does not burn H2. Through integrated experimental and computational studies we have produced evidence suggesting that O2 and H2 activation over Au/TiO2 catalysts occurs at the metal-support interface (MSI).[1,2] The activation of O2 on Au is assisted by support protons originating from hydroxyl groups or weakly adsorbed water molecules. Meanwhile, H2 dissociation across the MSI occurs heterolytically resulting in a Au-hydride and a proton on the oxide support. Notably, H2 activation is inhibited by reduced charge transfer from Au to the proton acceptor site located on a basic support hydroxyl.
The concept of water-modulated acid/base strength of sites at the MSI is rather general. For example, we show through first principles kinetic Monte Carlo simulations that the selectivity for direct deoxygenation over hydrogenation of phenol and m-cresol during reductive treatment with H2 can be tuned by adjusting the water partial pressure. In this reaction, Brønsted acidic protons co-catalyze C–OH bond cleavage and are recreated by heterolytic activation of H2 across the Ru/TiO2 interface.[4,5] Thus, rather than invoking the traditional redox terminology such as hydrogen spillover, support reduction or vacancy defects, we provide a new interpretation of the support effects in terms of acid/base chemistry.
Overall, these examples demonstrate the sensitivity of oxide chemistry to the presence of various amounts of moisture, which in turn opens up interesting opportunities to improve catalytic activity and selectivity without the need for time-consuming catalyst design.