Kinetic and isotopic methods show that NO oxidation on supported Pt clusters involves kinetically relevant reaction of O 2 with vacancy sites on surfaces nearly saturated with oxygen adatoms (O*). The oxygen chemical potential at Pt surfaces that determines the O* coverage is rigorously described by an O 2 virtual pressure and determined by the thermodynamics of NO 2 -NO interconversion reactions. NO oxidation and oxygen isotopic exchange processes are described by the same rate constant, consistent with similar kinetically relevant O 2 dissociation steps for both reactions. NO oxidation, NO 2 decomposition, andO 2 exchange rates increased markedly with increasing Pt cluster size (1-8 nm); these clusters remain metallic at all O 2 virtual pressures prevalent during NO oxidation. These effects of cluster size reflect the higher vacancy concentrations and more facile oxygen desorption on larger Pt clusters, which bind oxygen adatoms weaker than more coordinatively unsaturated surface Pt atoms on smaller clusters. These trends are similar to those found for methane and dimethyl ether combustion on Pt and Pd catalysts, which also require vacancy sites on O*-saturated cluster surfaces in their respective kinetically relevant steps. Inhibition of NO oxidation by NO 2 persists to undetectable NO 2 concentrations; thus, NO oxidation turnover rates increase significantly when NO 2 adsorption sites present on BaCO 3 /Al 2 O 3 are placed within diffusion distances of Pt clusters. NO oxidation rates on intrapellet catalyst-adsorbent mixtures are described accurately by a simple reaction-adsorption model in which NO 2 adsorbs via displacement of CO 2 on BaCO 3 surfaces.

While it is well known that chromium contamination in groundwater represents a considerable threat to the environment, little is known about the heterogeneous processes that govern chromium interaction with solid materials in soil. Using the nonlinear optical laser spectroscopy surface second harmonic generation (SHG), we have studied chromate adsorption and desorption at the fused quartz/liquid water interface in real time, at room temperature and at chromate concentrations between 1 × 10 -6 and 2 × 10 -4 M. Adsorbed chromate is spectroscopically identified via a two-photon resonance of one of its ligand-to-metal charge-transfer bands with the fundamental probe light. Adsorption isotherm measurements at 300 K result in a free chromate adsorption energy ∆G ads of 38 ( 1 kJ/mol at pH 7. Real-time kinetic measurements of chromate adsorption and desorption show reversible chromate binding to the fused quartz/water interface, consistent with the high mobility of Cr(VI) in soils and the ∆G ads determined from our adsorption isotherm measurements. The pH dependence of chromate binding to the fused quartz/water interface is discussed.

Microspectroscopic methods were explored to investigate binder effects occurring in ZSM‐5‐containing SiO2‐ and Al2O3‐bound millimetre‐sized extrudates. Using thiophene as a selective probe for Brønsted acidity, coupled with time‐resolved in situ UV/Vis and confocal fluorescence microspectroscopy, variations in reactivity and selectivity between the two distinct binder types were established. It was found that aluminium migration occurs in ZSM‐5‐containing Al2O3‐bound extrudates, forming additional Brønsted acid sites. These sites strongly influence the oligomer selectivity, favouring the formation of thiol‐like species (i.e., ring‐opened species) in contrast to higher oligomers, predominantly formed on SiO2‐bound ZSM‐5‐containing extrudates. Not only were the location and distribution of these oligomers visualised by 3 D analysis, it was also observed that more conjugated species appeared to grow off the surface of the zeolite ZSM‐5 crystals (containing less conjugated species) into the surrounding binder material. Furthermore, a higher binder content resulted in an increasing overall reactivity owing to the greater number of stored thiophene monomers available per Brønsted acid site.

No abstract

No abstract