The effects of adsorbate coverage on catalytic surface reactions are not well understood. Here, we contrast the rates of O2 and NO2 dissociations, two competing reactions in NO oxidation catalysis, versus oxygen coverage at a Pt(111) surface. In situ x-ray photoelectron spectroscopy experiments show that the NO2 dissociation rate is less sensitive to O coverage than is O2. Density-functional theory simulations reveal an NO2 reaction pathway that is more adaptable to an increasingly crowded surface than is O2 dissociation. While the rates are comparable at low coverage, NO2 dissociation is many orders of magnitude faster at O coverages typical of NO oxidation catalysis.

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A lab-scale, high-pressure, continuous-flow fast-hydropyrolysis and vapor-phase catalytic hydrodeoxygenation (HDO) reactor has been successfully designed, built and tested with cellulose as a model biomass feedstock. We investigated the effects of pyrolysis temperature on high-pressure cellulose fastpyrolysis, hydrogen on high-pressure cellulose fast-hydropyrolysis, reaction pressure (27 bar and 54 bar) on our reactor performance and candidate catalysts for downstream catalytic HDO of cellulose fasthydropyrolysis vapors. In this work, a liquid chromatography-mass spectrometry (LC-MS) method has been developed and utilized for quantitative characterization of the liquid products. The major compounds in the liquid from cellulose fast-pyrolysis (27 bar, 520 °C) are levoglucosan and its isomers, formic acid, glycolaldehyde, and water, constituting 51 wt%, 11 wt%, 8 wt% and 24 wt% of liquid respectively.Our results show that high pressures of hydrogen do not have a significant effect on the fast-hydropyrolysis of cellulose at 480 °C but suppress the formation of reactive light oxygenate species like glycolaldehyde and formic acid at 580 °C. The formation of permanent gases (CO, CO 2 , CH 4 ) and glycolaldehyde and formic acid increased with increasing pyrolysis temperature in the range of 480 °C-580 °C in highpressure cellulose fast-pyrolysis, in the absence of hydrogen. Candidate HDO catalysts Al 2 O 3 , 2% Ru/Al 2 O 3 and 2% Pt/Al 2 O 3 resulted in extents of deoxygenation of 20%, 22% and 27%, respectively, but led to carbon loss to gas phase as CO and CH 4 . These catalysts provide useful insights for other candidate HDO catalysts for improving the extent of deoxygenation with higher carbon recovery in the liquid product. † Electronic supplementary information (ESI) available. See

The kinetics of the oxidation of NO by O(2) was studied on 1 cm diameter single crystals, Pt(111) and Pt(321), at atmospheric pressure. The surface of the (321) crystal is composed of 20% kink, 20% step, and 60% terrace atoms and simulates small 1-3 nm size Pt particles on supported catalysts, while the (111) surface simulates the most stable plane found on large, >5 nm, particles. The turnover rates (TORs), that is, rate normalized by the exposed platinum, on the two single crystals differ by less than a factor of 2 over the range of conditions studied and are also similar to the TOR on a supported catalyst with an average particle size of 9 nm. Both surfaces show a dynamic kinetic behavior as evidenced by a change in the apparent activation energy and reaction orders as a function of reaction conditions. The oxygen coverage after initial rate experiments on Pt(111) was 0.6 monolayer (ML) on average which is similar to that measured previously by in situ X-ray photoelectron spectroscopy (XPS) under similar conditions. This oxygen overlayer, which is likely controlled by the relative presence of NO and NO(2), inhibits O(2) dissociation but lowers the binding energy of reactants enough to allow the catalysis. Long-term stability studies on Pt(111) correlate catalyst deactivation with irreversibly bound oxygen on the surface at coverages over 1 ML, as measured after reaction. Ex situ Auger electron spectroscopy (AES) and XPS results suggest that the surface defect sites on Pt(321) begin to oxidize relative to atoms on the (111) plane at lower NO(2) to NO ratios.

Palladium oxidation was studied by ambient-pressure X-ray photoelectron spectroscopy in the mbar pressure range on the Pd(111) and Pd(110) surfaces. The oxidation kinetics on both surfaces show an induction period when the oxidation rate was low at the beginning and then accelerated. The slow initial oxidation is governed by (a) the rate of nucleus formation, and (b) the rate of oxide nucleus growth. Depth profiling varying photon energy/kinetic photoelectron energy pointed to a 3D oxidation. It is remarkable that the oxidation of Pd(110) proceeds at ~100 K lower temperatures than on Pd(111). We suggest that at the high temperature required on Pd(111) nucleation is thermodynamically controlled, and therefore, the nucleation rate decreases with temperature. On Pd(110), nucleation is predominantly kinetically controlled and thus the oxidation rate increases with temperature