Beyond upper bound estimates of active site densities in heterogeneous catalysis: A note on the critical role of titrant pressure

Abstract

In-situ titrations employed in the estimation of active site densities oftentimes only provide upper bound estimates, not true active site densities. Discounting unselective contributions that result in a disparity between the two requires a treatment of transient titration data more nuanced than merely integrating under the titrant breakthrough curve. Three methods for estimating site densities (and turnover frequencies) are presented in the context of ceria-catalyzed ethene hydrogenation, and their strengths and limitations discussed. Readily accessible supra-monolayer coverages of CO prevalent on ceria surfaces help bring into relief considerations relating to unselective adsorption and active site heterogeneity common, yet infrequently addressed, in the characterization of heterogeneous catalysts using in-situ titration methods.