Dehydrogenation of Isobutane over Zinc Titanate Thin Film Catalysts

Abstract

Thin films (80–100 nm) of zinc titanate with Zn/Ti ratios between 0.5 and 2.7 were prepared on 2′′Si(100) wafers via the metallo-organic decomposition (MOD) technique. Their morphology, composition, and structure were extensively characterized by high resolution scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy, energy dispersive X-ray analysis, secondary ion mass spectrometry (SIMS), X-ray diffraction (XRD), and glancing incidence XRD (GIXRD). The catalytic performance of these zinc titanate thin films in isobutane dehydrogenation was studied in a newly designed and built wafer reactor in the pulse mode. The zinc titanate films prepared via MOD consisted of irregularly shaped grains. The Zn/Ti ratio of the film surface as measured with XPS was slightly higher than that of the bulk, indicating some zinc enrichment on the surface. However, the overall composition through the film was reasonably homogeneous as revealed by a SIMS depth profiling study. GIXRD showed the existence of the same zinc titanate phases at all film depths, again indicating a homogeneous film. The zinc titanate phases found in the films depend primarily on the film stoichiometry. For films with a Zn/Ti ratio lower than 1, the phases found were zinc metatitanate (ZnTiO3) with a hexagonal structure and titanium dioxide. In the films with a higher Zn/Ti ratio, the zinc titanate phase was ZnTiO3or Zn2TiO4, both possessing a cubic structure. Catalytic testing of these films in isobutane dehydrogenation showed a clear correlation between the structure and the catalytic performance. Zinc titanate phases with a cubic crystal structure were active for dehydrogenation, but the other phases were not. The most active catalyst has a Zn/Ti ratio close to 2, the stoichiometry of which corresponds to the Zn2TiO4phase. The highest selectivity to isobutene was ca. 90 mol%, both at 823 and 923 K. The isobutane conversion was 2 and 8 mol%, respectively, at these two temperatures. Compared with the catalytic performance of a zinc titanate pellet pressed from powder material, the thin film catalyst exhibited a higher activity and a remarkably better stability.