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020ChemInform Abstract The surface chemistry of water adsorbed on HZSM5 and a series of alkali-metal ZSM5 is studied at pressures between 10-5 and 1 mbar by using mainly time-resolved transmission absorption IR spectroscopy for analysis. At low pressures (< 10-4 mbar) the most important adsorption sites are Lewis acid sites (octahedrally coordinated Al), whereas at higher pressures strong Bronsted acid sites (bridging hydroxyl) are more important sites for adsorption. Up to a gravimetrically determined number of three adsorbed H2O per strong Bronsted site, bands attributed to hydroxonium ions can be observed. On Li-, Na-, and KZSM5, adsorption of isolated H2O molecules on alkali-metal cations dominates at pressures < 10-2 mbar. At higher equilibrium pressures a second shell of H2O molecules is built up. With Rb-and CsZSM5 interactions between H2O molecules are of high importance for adsorption at H2O equilibrium pressures < 10-2 mbar.

The open circuit potential (OCP) established by the quasi-equilibrated electrode reaction of H2 and H3O+(hydr.), complicates catalytic reactions significantly. The hydrogenolysis rate of benzylic alcohol on Pd/C increases 2-3 orders of magnitude with the pH decreasing from 7 to 0.6. The reaction follows a pathway of protonated benzyl alcohol dehydration to a benzylic carbenium ion, followed by a hydride addition to form toluene. The dehydration of protonated benzyl alcohol is kinetic relevent, thus, being enhanced at lower pH. The OCP stabilizes all cationic species in the elementary steps. Particularly, the initial state (benzyl alcohol oxonium ion) is less stabilized than the dehydration transition state and the product (benzylic carbenium), thus, lowering the free energy barrier of the rate-determining step. In accordance, the rate increased with increasingly negative OCP. Beside OCP, an external negative electric potential in an electrocatlaytic system was also demonstrated to enhance the rate in the same way.

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The high chemical stability of aliphatic carboxylic acid makes catalytic decarboxylation at low temperatures challenging. We show that arylaliphatic acids (Ar-C n H 2n -COOH, n ≥ 1) decarboxylate on carbon-supported Pd nanoparticles (Pd/C) at 90 °C with 100% selectivity. In situ XANES during decarboxylation of preadsorbed substrates indicates that the active phase is α-phase palladium hydride (α-PdH x ). The reaction rate is enhanced by one order of magnitude when hydrogen is preadsorbed. Tracing deuterium labeling positions, it is concluded that carboxylic acid (Ar-C n H 2n -COOH) undergoes an α-C−H bond dissociation on the Pd surface to the Ar-(CH 2 ) n−1 -CH*-COO* intermediate in the first step, followed by the C−COO scission, and finally, Ar-(CH 2 ) n−1 -CH* reacts with two sorbed H to produce Ar-(CH 2 ) n−1 -CH 3 . The high rates are related to the concentration of hydride present on the catalyst particles to complete the catalytic cycle in a Mars−van Krevelen-type mechanism and the rate of H/D exchange at the α-C− H position.