The oxidation of a Cu/SiO2 catalyst with NO or O2 at temperatures varying from 313 to 513 K has been investigated using XPS. Upon admission of NO or O2 to a previously reduced catalyst, a rapid transformation of Cu(0) to Cu(I) is observed. This first stage is followed by slow oxidation of Cu(I) to Cu(II). After exposure to NO the presence of a surface nitride at the catalyst surface is evident. Exposure at low temperatures also leads to the formation of nitrito and nitrato like surface species. These species decompose at 523 K. Desorption of the surface nitride is observed after heating the catalyst at 723 K. The XPS measurements performed on the supported catalyst are in good agreement with experimental data obtained on single crystals.@en

The IR absorbance of adsorbed CO is a suitable probe for evaluating the extent of oxidation of small supported copper particles. The residual absorbance of the main band at about 2120 cm-1 after evacuation of the gas phase and the presence of additional CO absorbance bands indicate the extent of oxidation. Exposure of a reduced catalyst to NO brings about the oxidation of the copper particles. With increasing extent of oxidation the intensity of bands due to adsorbed NO and to adsorbed nitrito and nitrato like species increases. Adsorbed NO already desorbs upon room-temperature evacuation, the nitrito and nitrato like species decompose at temperatures between 423 and 473 K. The formation of N2O (during exposure of NO to a catalyst which is not completely oxidized) and the formation of isocyanate (during subsequent exposure to CO) indicate the removal of weakly adsorbed nitrogen, originating from dissociated NO, from the surface of the copper particles. Upon simultaneous exposure of the reduced catalyst to equimolar amounts of NO and CO at room temperature, a surface reaction proceeds: the copper particles are not severely oxidized and the formation of CO2, N2O and isocyanate is observed. The agreement of the results from IR spectroscopy with those from XPS and anticipated from single-crystal results is good.@en

The kinetics of the adsorption of NO and O2, and of the subsequent reduction with CO have been studied on the stepped copper surfaces Cu(711) and Cu(710). The surfaces exhibit terraces with the (100) structure, and steps with the (111) and the (110) structure, respectively. Gases were admitted at pressures ranging from 6 × 10 6 to 10 1 Pa, temperatures ranged from 370 to 670 K. Upon reaction, the adsorption sites and geometry are similar to those on Cu(100). The reactivity of the entire Cu(100) surface is increased by about an order of magnitude due to the presence of steps. The orientation of the steps only slightly affects the kinetics.@en

Exposure of NO or O2 to Cu(100) or Cu(110) (at temperatures between 370 and 670 K, pressures up to 0.1 Pa) leads to dissociative adsorption. Dissociation on Cu(110) is about an order of magnitude faster than on Cu(100). Saturation of the surface coverage (for both surfaces at about 0.42 monolayer) is followed by the structure-intensitive penetration of oxygen atoms into the crystal. Reduction proceeds via reaction of adsorped CO and adsorbed oxygen, the effect of nitrogen can be explained by site-blocking. Again, the reaction is about an order of magnitude faster on Cu(110) than on Cu(100). In the case of Cu(100) a transition from a disordered to an ordered overlayer is observed at a fractional surface coverage of about 0.3. Nitrogen desorption is more difficult from Cu(100) than from Cu(110). The desorption temperature depends on the presence of oxygen (adsorbed as well as penetrated). At high CO pressures the formation of isocyanate facilitates the desorption of nitrogen.@en

The rate of methane formation by decomposition of methanol is studied at temperatures between 473 and 583 K and methanol pressures between 0.2 and 10.4 kPa. Coverages of adsorbed species are measured using temperature-programmed hydrogenation (TPH), temperature-programmed desorption (TPD), and magnetic measurements. The reaction system can be adequately described assuming the decomposition of methanol to proceed in two steps, viz. a decomposition into CO and H2 followed by methanation of CO. The former reaction is much more rapid than the latter. Assuming CH hydrogenation to be rate-determining, the rate of methane formation could be described by the LHHW rate equation TOF = 1.2 × 105e- -68 RTp1.5 (1 + 1.8 × 10-1e 15 RTp0.5 + 3.1 × 10-16 e 144 RTp)2 TOF is the turnover frequency in s-1, p is the methanol pressure in kPa, T is the temperature in K, and R = 8.314 × 10-3kJ mol-1 K-1. Coverages of adsorbed species predicted by this equation differ strongly from results obtained by measurements of total surface coverages using TPH, TPD, and magnetic measurements. This has been interpreted as an indication that the number of surface sites actively participating in the methanation reaction composes only a small fraction of the total number of sites.@en

A model describing adsorption and desorption in a bed of porous catalyst pellets is presented. The model has been used to simulate temperature-programmed desorption. In particular the effect of a coverage-dependent heat of adsorption has been investigated and the consequences thereof for commonly used interpretation methods are shown. It appears that the method in which the heating rate is varied and the method using desorption rate isotherms yield values which roughly approach the heat of adsorption used for the calculation of the desorption profiles in the case of the coveragedependent heat of adsorption. Adsorption experiments, either in a flow of adsorbate or under static conditions, have also been simulated. If the net rate of adsorption is high, the catalyst surface is covered frontwise. When the amount of adsorbate supplied from the gas phase is insufficient to bring about complete surface coverage, nonuniform coverage results.@en

NO interaction with Cu(111) is shown to be dissociative for temperatures varying from 320 to 570 K and for exposures up to 150 Pa·s. Upon adsorption a saturation surface coverage (0.5 monolayer) of adsorbed oxygen and nitrogen is obtained. After an incubation period penetration of oxygen in subsurface layers is observed, leaving the surface coverage almost unchanged. Both the adsorption and the penetration are first order in the NO pressure, the reactions are not activated. The reduction of the adsorbate at 570 K proceeds in two stages, i.e. the net reduction of subsurface oxygen and the subsequent reduction of adsorbed oxygen. The rate of reduction strongly depends on the amount of adsorbed nitrogen and on the presence of subsurface oxygen. At a CO pressure exceeding 0.1 Pa reduction is followed by the desorption of nitrogen, probably via an isocyanate intermediate. From the reduction experiments and from experiments in which the isothermal desorption rate of nitrogen was measured it is concluded that adsorbed nitrogen interacts with adsorbed oxygen, resulting in a lower reactivity of oxygen and, possibly, nitrogen. This interaction is considerably weakened when also penetrated oxygen is present.@en

The kinetics of the formation of methane from methanol have been studied on a 50 wt% Ni/SiO₂, a 20 wt% Ni/SiO₂, a 10 wt% Ni/Al₂O₃ and a 5 wt% Ni/TiO₂ catalyst. Temperatures were typically between 475 and 585 K with methanol partial pressures between 0.2 and 10.4 kPa. The results were compared with data obtained using a CO/H₂ = 0.5-mixtures as the reactant gas. The apparent activation energy for methane formation was independent of the support. Specific rates (TOF's) increased in the order 20 wt%Ni/SiO₂, 50 wt% Ni/SiO₂, 10 wt% Ni/Al₂O₃, 5 wt% Ni/TiO₂. Both deactivation rates and selectivity towards higher hydrocarbons increased in the same order. Contrary to the results obtained on the other catalysts, methane formation rates are much larger on Ni/TiO₂ when CO/H₂ was used as the reactant gas instead of CH₃OH. The observations were explained assuming a mechanism in which methapnol first decomposes into CO and H₂ followed by the hydrogenation of CO. In case of silica and alumina supported catalysts the first reaction is much faster than the latter and the rate of methane formation from methanol equals the rate of methane formation from CO/H₂. Due to the presence of chemically altered sites on TiO₂-supported nickel the rate of the first reaction (a dehydrogenation) is decreased, whereas the rate of CO-hydrogenation is known to increase strongly. This causes the rate of methane formation from methanol to be lower than the rate of methane formation from CO/H₂. The higher selectivity towards higher hydrocarbons as well as the higher deactivation rates are also explained assuming the presence of chemically altered sites.

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