CO Oxidation Over Au/TiO\u3csub\u3e2\u3c/sub\u3e Catalyst: Pretreatment Effects, Catalyst Deactivation, and Carbonates Production

A commercially available Au/TiO2 catalyst was subjected to a variety of thermal treatments in order to understand how variations in catalyst pretreatment procedures might affect CO oxidation catalysis. Catalytic activity was found to be inversely correlated to the temperature of the pretreatment. Infrared spectroscopy of adsorbed CO experiments, followed by a Temkin analysis of the data, indicated that the thermal treatments caused essentially no changes to the electronics of the Au particles; this, and a series of catalysis control experiments, and previous transmission electron microscopy (TEM) studies ruled out particle growth as a contributing factor to the activity loss. Fourier transform infrared (FTIR) spectroscopy showed that pretreating the catalyst results in water desorption from the surface, but the observable water loss was similar for all the treatments and could not be correlated with catalytic activity. A Michaelis–Menten kinetic treatment indicated that the main reason for deactivation is a loss in the number of active sites with little changes in their intrinsic activity. In situ FTIR experiments during CO oxidation showed extensive buildup of carbonate-like surface species when the pretreated catalysts were contacted with the feed gas. A semi-quantitative infrared spectroscopy method was developed for comparing the amount of carbonates present on each catalyst; results from these experiments showed a strong correlation between the steady-state catalytic activity and amount of surface carbonates generated during the initial moments of catalysis. Further, this experimental protocol was used to show that the carbonates reside on the titania support rather than on the Au, as there was no evidence that they poison Au–CO binding sites. The role of the carbonates in the reaction scheme, their potential role in catalyst deactivation, and the role of surface hydroxyls and water are discussed

[1,3-Bis(diphenyl­phosphino)propane-κ2 P,P′]diiodido(perfluoro­propyl)rhodium(III) dichloro­methane solvate

The structure of the title compound, [RhI2(C3F7)(C27H26P2)]·CH2Cl2, at 110 (2) K is an unusual example of a structurally characterized square-based pyramidal alkyl complex of rhodium(III). The Rh—C bond is relatively short at 1.996 (6) Å. This short metal–carbon bond length is typical of perfluoro complexes of transition metals and illustrates the enhanced bond strength in these compounds

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

Decarbonylation, reductive electrochemistry and x-ray crystal structures of some rhodium diphosphine acyl complexes.

Includes bibliographical references (p. ).Acyl complexes of rhodium(III) with chelating diphosphine ligands (P-P = 1,3- bis(diphenylphosphino)propane and others) are well known for their stability toward decarbonylation. Various rhodium diphosphine acyl complexes were synthesized and characterized by IR, NMR (¹H, ¹⁹F, and ³¹P), cyclic voltammetry, elemental analysis and X-ray crystallography. The chemical and electrochemical reduction of the rhodium diphosphine acyl complexes Rh(P-P)(COR)I₂ involves a net two-electron transfer yielding Rh(P-P)(CO)I, alkyl anion and iodide. The mechanism involves an initial one-electron transfer followed by the liberation of one of the iodides. Then a second electron transfer with the migration of alkyl group takes place yielding the 18-electron complex [Rh(P-P)(CO)(R)I]⁻. This 18-electron complex loses the alkyl group as the anion, producing Rh(P-P)(CO)I as the final product. We observed a difference in thermal stability between the acetyl and trifluoroacetyl complexes. Others have found that the acetyl complex is very stable in terms of alkyl migration while the monodentate phosphine analog of this complex undergo alkyl migration followed by the loss of alkyl halide. We discovered that when the acetyl group is replaced by a trifluoroacetyl group the resulting complex is unstable in terms of alkyl migration. It slowly changes from the acyl complex to the alkyl complex in solution at room temperature. If the resulting solution is allowed to stand for a long period of time, ca. 20 days or more, it gives the decarbonylated product Rh(P-P)(CF₃)I₂. When the trifluoroacetyl group is replaced by a difluoroacetyl group the complex does not undergo alkyl migration while replacing it with a chlorodifluoroacetyl group increases the rate of alkyl migration. The pentafluoropropionyl complex also undergoes alkyl migration. X-ray crystal structures of 19 rhodium diphosphine complexes were measured and their geometric parameters are compared with related structures. All five-coordinate complexes have square pyramidal geometries with the acyl group occupying the apical position.by Basu Dev Panthi.Ph.D

Electronic effects in 1,3-diaryl-1,3-diketone reduction potentials correlate with σ

The electrochemical reduction of aryl-substituted dibenzoylmethanes was studied in acetonitrile. They display one-electron couples that are chemically and electrochemically reversible at low concentrations. The formal potentials correlate with only one (σ) of the various Hammett parameters

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