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The evolution of the dispersed surface molybdate phase on high-surface-area magnesium oxide is studied as a function of weight loading of MoO3 and calcination temperature using in situ Mo L3-edge X-ray absorption near edge spectroscopy, in situ laser Raman spectroscopy, and thermogravimetry−mass spectroscopy. Under ambient, hydrated conditions, the magnesium support is present as magnesium hydroxide, rather than MgO, and the molybdate species is present on the surface as an isolated tetrahedral species for weight loadings of 5−20 wt % MoO3. Under these hydrated conditions the form of the molybdate species is controlled by the net surface pH at the point of zero charge and is the same as that observed in aqueous solution. As the catalyst is calcined in air, the molybdate species transforms from a tetrahedral species, MoO4, to an octahedral one, MoO6. This transformation occurs at the same temperature, ~450 °C, as that at which the support changes from hydroxide to oxide. This transformation temperature is independent of the MoO3 loading. It is suggested that it is this support-induced structural change that drives the change in the form of the surface molybdate species and not the desorption of adsorbed water.

The bonding and reactivity of allyl groups (CH2CHCH2) on a Cu(100) surface are studied by temperature-programmed desorption and near edge X-ray absorption fine structure measurements. Surface allyl groups are generated by dissociative adsorption of allyl chloride at 110 K and/or a coupling reaction of surface methylene (CH2) and vinyl (C2H5) groups. Allyl groups on a Cu(100) surface coordinate as a nearly flat-lying π-allyl species. Allyl radical ejection is the dominant reaction pathway upon heating the Cu(100) surface, which is drastically different from the behavior of allyl on Ag(111) and Al(100) surfaces, where allyl groups couple to form 1,5-hexadiene and disproportionate to produce propylene and an unidentified polymeric species, respectively. On a Cu(100) surface precovered with hydrogen atoms, allyl groups are scavenged by hydrogen atoms to produce propylene at ~ 250 K.

Dechlorination and hydrodechlorination of multiply-chlorinated ethanes and propanes on a clean Cu(100) surface have been studied by Auger electron spectroscopy, temperature-programmed desorption, and chemical displacement measurements. The rate-limiting step in degradation is dissociation of the first C−Cl bond in the molecule, and this process is more facile in CCl2 groups than in CCl groups. The activation energies for C−Cl bond scission on Cu(100) are 12−20% of the gas phase bond dissociation energies, and the extent of dissociation by the physisorbed molecules is a sensitive function of the relative rates of C−Cl bond scission and molecular desorption. The chlorinated hydrocarbon fragments generated on the surface by C−Cl bond cleavage undergo facile α- or β-chlorine elimination while all C−H bonds remain intact. β-Chlorine elimination dominates in cases where chlorine is present at both the α- and β-positions, and as a result of β-chlorine elimination, alkenes are generated and evolved to the gas phase. In cases where no β-Cl is present, α-chlorine elimination yields surface carbene intermediates, which readily couple to form longer chain alkenes. Surface hydrogen atoms readily scavenge these carbene intermediates to form alkyl groups. Upon thermal activation, these alkyls are converted to alkenes via β-hydride elimination or to alkanes via coupling with surface hydrogen. All processes subsequent to the initial dissociative adsorption of the chlorinated hydrocarbon occur with 100% selectivity, and all hydrocarbon products are evolved from the surface below 300 K. No carbon is detected on the surface after reaction, but Cl remains adsorbed up to 750 K where it is evolved as CuCl.