Surface characteristics and activity of chromia/alumina catalysts prepared by atomic layer epitaxy

A series of CrOx/¿-Al2O3catalysts was prepared by atomic layer epitaxy (ALE) from the vapor phase, with use of sequential saturating reactions of Cr(acac)3vapor and air. The loading was varied from 1.3 to 8.8 wt% of Cr. Lower loadings were achieved by partially blocking the alumina surface with acetylacetone or dipivaloylmethane before introducing Cr(acac)3. The catalytically active material was found to be evenly distributed through the catalyst particles. Low energy ion scattering measurements showed that, up to 7.4 wt% of Cr, the Cr species was dispersed in a monolayer. XPS and UV-vis spectrophotometry revealed that, even at the low loadings, both Cr3+and Cr6+species were present. This was attributed to a stabilization of Cr3+on the alumina support during the ALE process. Loading beyond monolayer coverage was not reflected in the dehydrogenation ofi-butane toi-butene: the activity continued to increase

Sequential saturating reactions of ZrCl4 and H2O vapors in the modifications of silica and g-Alumina with ZrO2

Zirconia-modified silica and alumina samples were prepared by the atomic layer epitaxy technique using successive saturating reactions of ZrCl4 and H2O vapors. The growth of ZrO2 was followed by X-ray diffraction, diffuse reflectance IR Fourier transform spectroscopy, and 1H magic angle spinning NMR measurements. In addition, coverage of the silica surface was estimated by low-energy ion scattering (LEIS). At the reaction temperature of 300 °C the main reaction of ZrCl4, on both supports, occurred with surface OH groups, leading to amorphous ZrClx species. In addition, part of the molecules reacted to form ZrO2 agglomerates while simultaneously chlorinatingOHgroups of the supports. Water treatment removed Cl from the ZrClx surface species, leaving behind Zr-OH groups, and released the directly chlorinated OH groups. Both the reappearing OH groups of the support and the Zr-OH groups were then again available during the next ZrCl4 treatment for both main reactions of ZrCl4. As the reaction cycles were repeated the concentration of amorphous Zr in the samples increased together with the amount of crystalline ZrO2 and surface coverage. On silica, after five cycles of repeated reactions when an average surface density of 5 Zr/nm2 had been achieved, 25% of the original accessible OH groups were still present on the surface. At that point, LEIS measurements showed 50% coverage. On the surface of both silica and alumina, ZrOx was present in the form of tet

Influence of compaction and surface roughness on low-energy ion scattering signals

Investigation of the surface composition of powders often requires compaction. To study the effect of compaction on surface analysis, samples have been compacted at various pressures ranging from 0 Pa (i.e. no compaction) up to 2000 MPa (2 × 104 kg cm-2) Low-energy ion scattering (LEIS) was used to determine the composition of the outermost atomic surface layer. Using scanning electron microscopy, changes in the morphology due to compaction have been detected in the SiO2 test samples. The LEIS yield of a compacted silica powder is found to be independent of the applied pressure during compaction between 2 MPa and 2000 MPa (2 × 104 kg cm-2). Analysis of a submonolayer of Ta2O5 on a silica support shows that the composition of the outermost atomic layer is not changed after compaction up to a pressure of at least 300 MPa. When compaction is applied, the absolute LEIS yield appears to be independent of the specific surface area of silica supports in the range 50–380 m2 g-1. A minor difference in LEIS signals is observed between compacted silica supports and flat quartz samples. In order to determine the surface roughness factor independently, and to study the material dependence of the surface roughness factor, angle-dependent LEIS measurements have been carried out on oxidized silicon, gallium and gold surfaces. The results on the oxidized silicon confirm the small influence of surface roughness for silica particles, whereas measurements on the more closely packed metallic gallium and gold surfaces indicate a significant surface roughness effect. Copyright © 2004 John Wiley & Sons, Ltd