No abstract

The pathways for the reaction of ethanol on model catalysts consisting of Co and CoO films and particles supported on single crystal ZnO(0001) surfaces were studied using X-ray Photoelectron Spectroscopy (XPS) and Temperature Programmed Desorption (TPD). On supported metallic Co films and particles ethanol was found to primarily undergo decarbonylation forming CO, H(2), and adsorbed methyl groups. In contrast, supported CoO particles were found to be largely unreactive toward ethanol. High selectivity to the dehydrogenation product, acetaldehyde, was only observed when the supported Co was partially oxidized and contained both Co(0) and Co(2+). Since acetaldehyde is thought to be a critical intermediate during steam reforming of ethanol (SRE) to produce H(2) and CO(2), the results of this study suggest that partially oxidized Co species provide the active sites for this reaction. This result is consistent with studies of high surface area Co/ZnO catalysts which also suggest that both Co(0) and Co(2+) species are present under typical SRE reaction conditions.

The thermal and chemical stability of vapor-deposited cobalt films on single crystal ZnO(0001) surfaces were investigated. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and atomic force microscopy (AFM) were used to probe the structure and oxidation state of deposited Co layers as a function of coverage and annealing temperature. The Co was found to approximate layer by layer growth at 300 K but agglomerate into particles upon heating to temperatures up to 700 K. Between 700 and 800 K, the Co particles were found to redisperse over the ZnO(0001) surface. XPS analysis revealed that this spreading is accompanied by oxidation of Co to form CoO as a result of reaction with the ZnO support. Additionally, XPS showed that heating above 800 K resulted in the incorporation of Co into the ZnO support. The oxidation of Co via reaction with the ZnO is not expected based on bulk thermodynamics and this result therefore demonstrates that interactions at the interface play a dominate role in directing the structure and oxidation state of the Co for this system. † Part of the "D. Wayne Goodman Festschrift".

Structure–activity relationships for the reaction of ethanol on model Co/YSZ(100) (YSZ = yttria-stabilized ZrO2) steam reforming catalysts were investigated using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy and compared with those obtained previously for the reaction of ethanol on Co/ZnO(0001) and Co foils. Oxygen-free, metallic Co sites were found to be active for ethanol decarbonylation to form CO, H2, and adsorbed CH3 groups, whereas oxygen adatoms on metallic Co promoted ethoxide dehydrogenation to produce acetaldehyde at 330 K during TPD. In contrast, oxidized Co surfaces containing only Co2+ sites were found to be less reactive for ethanol dehydrogenation producing acetaldehyde at 480 K. These results, along with comparisons with those from Co/ZnO(0001) and Co foils, provide new insights into the active sites for ethanol steam reforming on Co-based catalysts and the influence of the support on both catalyst reactivity and stability.

Temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) were used to characterize the adsorption and reaction of acetaldehyde and glycolaldehyde on Zn-modified Pt(111) surfaces. The barriers for both C–H and C–C bond cleavage in adsorbed aldehydes were found to be higher on Pt(111) decorated with Zn adatoms or containing a PtZn near-surface alloy, compared to Zn-free Pt(111). This results in stabilization of aldehydes adsorbed in an η2(C,O) bonding configuration and hinders formation of acyl intermediates as typically occurs on group 10 metals. Adsorbed η2(C,O) acetaldehyde and glycolaldehyde on Zn-modified Pt(111) were found to undergo selective C–O bond scission to produce adsorbed hydrocarbon intermediates and hydroxyl groups, while decarbonylation of acyl species to produce CO and hydrocarbon fragments was the primary pathway on the Zn-free surface. These results provide mechanistic insights that are useful for the design of hydrodeoxygenation catalysts for the upgrading of oxygenates derived from biomass, such as glucose and furfural.