Active Site Determination for CO Oxidation on Al2O3 Supported Pt Nanoparticle Catalysts By in-Situ Quantitative FTIR Measurements

CO Oxidation on noble metal surfaces has been widely studied since the 1920’s and is of fundamental importance to the field of surface science and heterogeneous catalysis in addition to many important industrial applications. Detailed kinetics studies suggest that at low temperatures (<250°C), the reaction proceeds via Langmuir-Hinshelwood kinetics, where surface is nearly covered with CO and the reaction rate is limited by the availability of vacant Pt sites for O₂ dissociation. Interestingly, CO oxidation on supported Pt nanoparticles under CO-poisoned conditions has been shown to be a structure insensitive reaction, where turnover frequencies do not change significantly with variations in nanoparticle size and as a result of surface structure. However, theoretical analyses of the proposed CO oxidation mechanism on Pt surfaces have predicted the reaction should exhibit structure sensitivity, where lower index facets of Pt are predicted to be more active.

Recently it has been shown through in-situ Transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) that the presence of CO can induce significant Pt nanoparticle restructuring. However, the potential role of restructuring in explaining the anomalous structure insensitivity for CO oxidation on Pt, has not been identified, due to the lack of a quantitative correlation between the concentrations of various types of active sites measured under reaction conditions to the areal reaction rate. In this work, we utilize in-situ quantitative DRIFTS measurements to measure the concentration of CO bound to under-coordinated (UC) and well-coordinated (WC) Pt sites for various Pt nanoparticle catalysts with sizes ranging from 2-20 nm on oxide supports at room temperature and under typical reaction conditions. This allows for a correlated analysis relating the turnover frequency of CO oxidation over Pt catalysts and the concentration of WC and UC Pt sites. The results demonstrate that CO oxidation on Pt nanoparticle surfaces is structure sensitive, where WC Pt atoms are the active site, but that this effect is masked by adsorbate-induced restructuring of Pt nanoparticle surfaces under CO oxidation conditions. We also investigate the kinetics of the temperature-dependent and transient evolution of CO-induced catalyst reconstruction to gain insights into the activation of catalyst restructuring. Our results provide a complete description of the observed trends in the structure sensitivity of CO oxidation on Pt nanoparticles of various sizes, in agreement with theoretical predictions.