Effect of Thermal Treatment on the Nickel State and CO Hydrogenation Activity of Titania-Supported Nickel Catalysts

Abstract

A series of 6 wt% Ni/TiO2catalysts were prepared by pore volume impregnation under various calcination conditions (225–600°C, 1–16 h), and were characterized by XRD, XPS, TG-MS, and chemisorption (H2and O2). The results of XRD, XPS, and TG-MS show the presence of incompletely decomposed nickel nitrates and NiO for catalysts calcined at 225–250°C, and the presence of NiO for catalysts calcined at 300–500°C. NiTiO3becomes the dominant nickel species in the catalyst calcined at 600°C. Reducibility of nickel to nickel metal depends on the chemical states of nickel in the calcined catalysts. For catalysts calcined below 600°C, where NiO is the main nickel species, 100% reducibility can be obtained at a reduction temperature of 400°C. However, for catalysts calcined at 600°C, where NiTiO3is the dominant nickel species, complete reduction of nickel species can be achieved at 600°C. Lower turnover frequencies (TOF(H2)) for CO hydrogenation were observed for the catalysts with unreduced nickel phase. When nickel is completely reduced, TOF(H2) is independent of the chemical states of nickel in the calcined catalysts or reduction temperature. The TOF(H2)'s are an order of magnitude higher than that reported for unsupported nickel indicating the promotion effect of titania on CO hydrogenation even after high reduction temperature. However, an increase in ethene formation rate, methane and C2-C4 percentages, and the olefin to paraffin ratio (C2-C4) was observed, along with a significant decrease in the H/O adsorption ratio as reduction temperature was increased to 600°C for all catalysts. This suggests further interaction of TiOxmoieties with nickel surface in addition to blocking.