The effect of the preparation method on the activity of water gas shift catalysts

Abstract

The water gas shift reaction (WGSR) is recognized as an important step in the production of high pure hydrogen for several applications in the chemical and petroleum industries, as well as in fuel cells and mobile vehicles. For all these applications, the WGSR increases the yield of hydrogen produced by the hydrocarbon reforming and decreases the amount of carbon monoxide, which can poison most metallic catalysts. In large-scale hydrogen production processes, the WGSR is carried out in two steps to achieve rates for commercial purposes. The first one involves a high temperature shift (HTS) catalyst based on iron and chromium oxides that operates at 593-723 K and is commercialized as hematite that is converted in situ to magnetite (active phase). This exothermic reduction is performed with large amounts of steam to avoid the overreduction of magnetite to metallic iron, which can catalyze the hydrocarbon production. However, this procedure increases the operational costs and thus catalysts that can be prepared in the active phase are much needed. In addition, it is well-known that the properties of these catalysts largely depend on their preparation method. By considering these aspects, the preparation of chromium-doped magnetite, by different procedures, was described in this work. Samples were prepared by adding: (i) an ammonium hydroxide solution to iron and chromium nitrate solutions, (ii) the metallic salts solutions (previously mixed) to the ammonium hydroxide solution, and (iii) both the metallic solution and the ammonium hydroxide solution to water. The solids were characterized by differential thermal analysis and thermogravimetry and calcined under nitrogen flow at 500 ºC to produce magnetite. The catalysts produced were characterized by Fourier transform infrared spectroscopy, chemical analysis, specific surface area measurements, X-ray diffraction temperature programmed reduction, X-ray photoelectron spectroscopy and Mössbauer spectroscopy and evaluated in WGSR at 370 o C and 1 atm. It was found that the preparation method strongly affects the textural and catalytic properties of chromium-doped magnetite but does not affect pure magnetite. These differences were explained by the different distribution of chromium in solids, which acts as a textural promoter being able to delay the metallic iron production. In addition, chromium decreases the activity per area. The most active catalyst for the water gas shift reaction was obtained by adding the iron and chromium nitrate solutions to the ammonium hydroxide one.