Abstract
We studied the initial stages of Ga interaction with the Cu(001) surface and environment-induced surface transformations in an attempt to elucidate the surface chemistry of the Cu–Ga catalysts recently proposed for CO2 hydrogenation to methanol. The results show that Ga readily intermixes with Cu upon deposition in vacuum. However, even traces of oxygen in the gas ambient cause Ga oxidation and the formation of two-dimensional (“monolayer”) Ga oxide islands uniformly covering the Cu surface. The surface morphology and the oxidized state of Ga remain in H2 as well as in a CO2 + H2 reaction mixture at elevated pressures and temperatures (0.2 mbar, 700 K). The results indicate that the Ga-doped Cu surface under reaction conditions exposes a variety of structures including GaOx/Cu interfacial sites, which must be taken into account for elucidating the reaction mechanism.
Highlights
Gallium-containing alloys and intermetallic compounds have recently received significant attention in catalysis, in particular, for industrially important processes such as selective hydrogenation of alkynes[1] and CO2 hydrogenation to methanol.[2,3] The methanol synthesis was found to proceed with relatively high activity and selectivity onNi5Ga3/SiO2 catalysts even at an ambient pressure
The surface was cleaned by several cycles of Ar+ sputtering and ultrahigh vacuum (UHV) annealing at 800 K, until the lowenergy electron diffraction (LEED) shows sharp diffraction spots and no impurities were detected by Auger electron spectroscopy (AES)
At low Ga coverages, that is, when the ratio of the Ga and Cu peaks in AES spectra is below 0.1, the scanning tunneling microscopy (STM) images revealed squareshaped islands randomly distributed on the terraces (Figure 1a−c)
Summary
Gallium-containing alloys and intermetallic compounds have recently received significant attention in catalysis, in particular, for industrially important processes such as selective hydrogenation of alkynes (on Pd−Ga)[1] and CO2 hydrogenation to methanol (on Ni−Ga).[2,3] The methanol synthesis was found to proceed with relatively high activity and selectivity on. Ni5Ga3/SiO2 catalysts even at an ambient pressure The latter is important for the development of small-scale devices operating at low pressures using solar-generated hydrogen. A strong promotional effect of Ga was observed on SiO2-supported Cu NPs. The addition of Ga increased the methanol formation rate without a significant change in the reverse water−gas shift reaction. The results suggested that Ga promotes Cu by increasing its methanol selectivity, likely by creating new active sites for methanol formation without modifying its oxidation state, which remains mostly metallic under the reaction conditions
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