Surface characterization study of a 1 wt% K-promoted ZnO, higher alcohol synthesis catalyst

The surface of a 1 wt% K-promoted ZnO catalyst has been characterized using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). This catalyst has proven to be active for higher alcohol synthesis (HAS) using a syngas feedstream of equimolar H-2 and CO to produce mixtures of methanol, isobutanol and hydrocarbons. The near-surface region of the fresh catalyst contains primarily ZnO and some adsorbed hydrocarbons, alcohol and water contamination as well. The predominant form of oxygen in the near-surface region of the as-prepared catalyst is hydroxyl groups. According to ISS, the outermost atomic layer of the fresh catalyst consists of Zn, K, C and O with Zn being the predominant element. Exposing the catalyst to 10(-7) Torr of H-2 for 4 h at 300 degrees C, similar to the pretreatment performed before reaction, results in elimination of the carbon contamination and removal of most of the hydroxyl groups. This exposes the underlying, K-enriched ZnO phase which is active for HAS. The 1 wt% K/ZnO catalyst also was examined after aging in the reactor for 1 week under reaction conditions. Some alcohol products are adsorbed on the aged surface as expected. Significant amounts of Na and Cl contaminate the surface of the aged catalyst, but the amount of K present is significantly reduced possibly through sublimation of K2O during reaction. Although K is not observed in the XPS spectra, a small amount is present in the outermost atomic layer according to ISS. An ISS depth profile indicates that the Na, Cl and K are located throughout the near-surface region. Reaction studies indicate that the catalyst activity remains stable even though the K promotor concentration is decreased during reaction. Apparently, the Na initially present in the subsurface region of the catalyst migrates to the surface and acts as a promotor along with the remaining K. (C) 1998 Elsevier Science B.V. All rights reserved.