Evaluating Differences in the Active-Site Electronics of Supported Au Nanoparticle Catalysts Using Hammett and DFT Studies

Supported metal catalysts, which are composed of metal nanoparticles dispersed on metal oxides or other high-surface-area materials, are ubiquitous in industrially catalysed reactions. Identifying and characterizing the catalytic active sites on these materials still remains a substantial challenge, even though it is required to guide rational design of practical heterogeneous catalysts. Metal-support interactions have an enormous impact on the chemistry of the catalytic active site and can determine the optimum support for a reaction; however, few direct probes of these interactions are available. Here we show how benzyl alcohol oxidation Hammett studies can be used to characterize differences in the catalytic activity of Au nanoparticles hosted on various metal-oxide supports. We combine reactivity analysis with density functional theory calculations to demonstrate that the slope of experimental Hammett plots is affected by electron donation from the underlying oxide support to the Au particles

Using Thiol Adsorption on Supported Au Nanoparticle Catalysts To Evaluate Au Dispersion and the Number of Active Sites for Benzyl Alcohol Oxidation

Two techniques to study the surface chemistry of supported gold nanoparticles were developed. First, phenylethyl mercaptan (PEM) adsorption from hexane solution was followed with UV–vis spectroscopy to evaluate the total amount of surface Au available. Two catalysts, Au/Al₂O₃ and Au/TiO₂, were found to have Au:S surface stoichiometries of ∼2:1, whereas a Au/SiO₂ catalyst had a Au:S surface stoichiometry of ∼1:1. The room temperature equilibrium binding constants for PEM adsorption on the Au/Al₂O₃ and Au/TiO₂ catalysts were similar (∼3 × 10⁵ M^–1; Δ<i>G</i> ≈ −31 kJ/mol); the PEM–Au/SiO₂ binding constant was somewhat larger (∼2 × 10⁶ M^–1; Δ<i>G</i> ≈ −36 kJ/mol). XPS data for all of the catalysts showed no observable changes in the Au oxidation state upon adsorption of the thiol. Implications of these experiments regarding self-assembled monolayers and thiol-stabilized Au nanoparticles are discussed. Second, kinetic titrations (i.e., controlled thiol-poisoning experiments) were developed as a method for evaluating the number of active sites for selective 4-methoxybenzyl alcohol oxidation. These experiments suggested only a fraction of the surface Au (∼10–15% of the total Au) was active for the reaction. When thiol was added with the 4-methoxybenzyl alcohol substrate, more thiol was required to poison the catalyst, suggesting that the thiol and substrate compete for initial adsorption sites, possibly at the metal–support interface. These two methods were combined to evaluate the magnitude of the support effect on selective 4-methoxybenzyl alcohol oxidation. Correcting the catalytic activity of the catalysts to the number of sites determined by thiol titration provided clear evidence that the support has a strong influence on the catalytic activity of Au in benzyl alcohol oxidation

Ecological insights from three decades of animal movement tracking across a changing Arctic

The Arctic is entering a new ecological state, with alarming consequences for humanity. Animal-borne sensors offer a window into these changes. Although substantial animal tracking data from the Arctic and subarctic exist, most are difficult to discover and access. Here, we present the new Arctic Animal Movement Archive (AAMA), a growing collection of more than 200 standardized terrestrial and marine animal tracking studies from 1991 to the present. The AAMA supports public data discovery, preserves fundamental baseline data for the future, and facilitates efficient, collaborative data analysis. With AAMA-based case studies, we document climatic influences on the migration phenology of eagles, geographic differences in the adaptive response of caribou reproductive phenology to climate change, and species-specific changes in terrestrial mammal movement rates in response to increasing temperature.</p