A Perspective on Counting Catalytic Active Sites and Rates of Reaction Using X-Ray Spectroscopy

Abstract

Identification of active sites and phases in heterogeneous catalysts and the understanding of the reaction mechanism remain highly challenging. In most catalysts, the existence of a multitude of surface species, which are dynamic in relation to reaction conditions, presents a challenge of distinguishing those that are involved in the catalytic cycle from those which are spectators. The emergence of the field of single-site catalysts potentially eliminates these issues, although it can be argued that these systems remain dynamic and that multiple speciation, each a candidate for the active site, often remains a consideration. A perspective on how X-ray spectroscopy and characterization tools in general, can be used to correlate the number of active sites and the rate of their formation, in single-site and redox catalyst systems, is presented. The importance of observing proportionality between spectra features and the reaction rate, to differentiate between active sites and spectator species is discussed. Performing characterisation under catalyticly relevant conditions on structures that are demonstrably representative of actual catalysts is essential.

Oxidation State and Surface Reconstruction of Cu under CO2 Reduction Conditions from In Situ X-ray Characterization

“…In addition to the Cu valence state, the surface can dynamically reconstruct through its interaction with the local environment. 13,14 Since Cu has low cohesive energy and high surface mobility, the Cu atoms at the surface can easily potential range relevant to the CO 2 RR, we observed only metallic Cu at the surface. The corresponding extended X-ray absorption fine structure (EXAFS) spectra displayed a prominent Cu−Cu scattering peak at 2.2 Å, which matches with the metallic Cu reference (Figure 1b).…”

Aggregation‐Induced Improvement of Catalytic Activity by Inner‐Aggregate Electronic Communication of Metal‐Fullerene‐Based Surfactants

“…This result is very astonishing considering that, in general, the mobility and accessibility of the catalytic centers should be lower in the aggregates, and, accordingly, activity should rather decrease or at the most remain constant at c> cac . Aqueous solution of fullerene based vesicles do not behave like nanoparticle systems since a vesicle is a rather dynamic and permeable system. During the reaction, solvent molecules, reactant and product molecules can diffuse in and out the vesicular aggregate therefore, the number of accessible molecules for the reaction is not lowered but there is obviously an effect on the catalyst molecule upon aggregation.…”

Modification of the Acyl Chloride Quench-Labeling Method for Counting Active Sites in Catalytic Olefin Polymerization

“…In fundamental studies on catalytic reactions, speciation and counting of the catalytic active sites play key roles, especially when the reaction mechanism is to be disclosed through systematical analysis on the reaction kinetics [1][2][3][4][5][6]. In studies on heterogeneous Ziegler-Natta (Z-N) catalysts that are widely used in the industry production of polyolefins, there is a long history of pursuing reliable and efficient methods of counting the number of polymerization active sites.…”