The beneficial effect of hydrogen on CO oxidation over Au catalysts. A computational study.

Density functional theory calculations have been carried out to explore the effect of hydrogen on the oxidation of CO in relation to the preferential oxidation of CO in the presence of excess hydrogen (PROX). A range of gold surfaces have been selected including the (100), stepped (310) surfaces and diatomic rows on the (100) surface. These diatomic rows on Au(100) are very efficient in H-H bond scission. O(2) hydrogenation strongly enhances the surface-oxygen interaction and assists in scission of the O-O bond. The activation energy required to make the reaction intermediate hydroperoxy (OOH) from O(2) and H is small. However, we postulate its presence on our Au models as the result of diffusion from oxide supports to the gold surfaces. The OOH on Au in turn opens many low energy cost channels to produce H(2)O and CO(2). CO is selectively oxidized in a H(2) atmosphere due to the more favorable reaction barriers while the formation of adsorbed hydroperoxy enhances the reaction rate.

H(2) and CO(2) coadsorption effects in CO adsorption over nanosized Au/gamma-Al(2)O(3) catalysts.

The present study is focused on the kinetic investigation of the effects of H(2) and CO(2) on the rates related to the elementary steps of CO sorption over Au/gamma-Al(2)O(3). The kinetic study was carried out in a wide temperature range (50-300 degrees C) by the novel methodology of reversed flow gas chromatography (RF-GC). The findings of preliminary coadsorption studies of CO with H(2), O(2) and O(2)+H(2) indicate that a reductive pre-treatment of the Au catalyst with a mixture of CO in excess of H(2) can be more beneficial concerning CO oxidation activity at low temperatures, compared to the usual reduction in a diluted hydrogen atmosphere, most probably due to the easier activation of oxygen molecules. At high temperatures the rate of reversed water gas shift reaction becomes significant resulting in H(2) and CO(2) consumption. The kinetic findings indicate that hydrogen strongly influences the adsorption of CO over Au/gamma-Al(2)O(3), by enhancing CO adsorption at lower temperatures and weakening the strength CO binding. On the other hand, CO(2) adsorption competes that of CO under hydrogen-rich conditions. However, the strength of CO(2) bonding is higher compared to that of CO and it further increases at higher temperatures, in agreement with the observed deactivation of the selective CO oxidation in the presence of CO(2).

The role of Oad in the decomposition of NH3 adsorbed on Ir(110): a combined TPD and high-energy resolution fast XPS study.

High energy resolution fast XPS combined with TPD experiments were used to study the effect of chemisorbed oxygen on the adsorption and dissociation of NH(3) on Ir(110). Below 250 K the presence of O(ad) does not influence NH(3) decomposition. Above 250 K O(ad) enhances NH(3) dissociation, which results in three times as much N(2) formation and less molecular NH(3) desorption compared to the experiments without O(ad). The effect of O(ad) can be attributed to destabilization of NH(ad) on the surface, resulting in a further dehydrogenation towards N(ad). The presence of O(ad) on the surface lowers the temperature at which the N(ad) combination reaction takes place by as much as 200 K, due to repulsive interaction between N(ad) and O(ad). NO is formed above 450 K if both N(ad) and O(ad) are present on the surface.

Lateral interactions and multi-isotherms: nitrogen recombination from Rh(111).

Lateral adsorbate-adsorbate interactions result in variation of the desorption rate constants with coverage. This effect can be studied in great detail from the shape of a multi-isotherm. To produce the multi-isotherm, the temperature is increased in a (semi)stepwise fashion to some temperature, followed by maintaining this temperature for a prolonged time. Then, the temperature is stepped to a higher value and held constant at this new temperature. This cycle is continued until all of the adsorbates have desorbed. Using a detailed kinetic Monte Carlo model and an optimization algorithm based on Evolutionary Strategy, we are able to reproduce the shape of the experimentally measured multi-isotherm of nitrogen on Rh(111) and obtain the lateral interactions between the nitrogen atoms.

NH3 adsorption and decomposition on Ir(110): a combined temperature programmed desorption and high resolution fast x-ray photoelectron spectroscopy study.

The adsorption and decomposition of NH3 on Ir(110) has been studied in the temperature range from 80 K to 700 K. By using high-energy resolution x-ray photoelectron spectroscopy it is possible to distinguish chemically different surface species. At low temperature a NH3 multilayer, which desorbs at approximately 110 K, was observed. The second layer of NH3 molecules desorbs around 140 K, in a separate desorption peak. Chemisorbed NH3 desorbs in steps from the surface and several desorption peaks are observed between 200 and 400 K. A part of the NH3ad decomposes into NH(ad) between 225 and 300 K. NH(ad) decomposes into N(ad) between 400 K and 500 K and the hydrogen released in this process immediately desorbs. N2 desorption takes place between 500 and 700 K via N(ad) combination. The steady state decomposition reaction of NH3 starts at 500 K. The maximum reaction rate is observed between 540 K and 610 K. A model is presented to explain the occurrence of a maximum in the reaction rate. Hydrogenation of N(ad) below 400 K results in NH(ad). No NH2ad or NH3ad/NH3 were observed. The hydrogenation of NH(ad) only takes place above 400 K. On the basis of the experimental findings an energy scheme is presented to account for the observations.

Decomposition of methanol on Au(310).

Experimental results on the interaction of methanol with a Au(310) surface studied using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) are reported. At 80 K methanol forms a physisorbed condensed layer above 5 L of exposure. Interestingly, O-H bond scission takes place above 150 K on the surface resulting in an adsorbed methoxy species which is stable till 500 K. This is in contrast with other gold surfaces like Au(111) and Au(110) which showed no evidence for decomposition. The difference in reactivity of Au(310) surface towards methanol is interpreted in terms of the presence of special sites formed by (110) steps and (100) terraces on this surface.