Alkali promoted zinc-aluminum oxide as high temperature water gas shift catalyst

Abstract

A new kind of high-temperature water gas shift catalyst based on K-promoted ZnO/ZnAl2O4, introduced by Haldor Topsøe as SK-501 Flex™, has begun to replace the well-known Fe/Cr oxide catalysts in the industry. In this research article, we focus on optimization of the catalyst system, especially on promotion with alkali metals and on structural characterization using X-ray diffraction. The promotional effect of the alkali metals increases in the order Li < < Na < < Cs < Rb < K. With K, a 20-fold increase of the mass-based intrinsic rate constant compared to an un-promoted sample is observed. The optimum in the K-content, 2.1 wt%, corresponds to a surface concentration of 0.0057 mmol/m2 or 0.0059 mmol/m2 if K only decorates the ZnAl2O4 spinel surface. This coverage coincides with the surface concentration of Al vacancies on the dominating spinel (111) facet, the so-called kagome layer, with a coverage of 0.00586 mmol/m2. Potassium decoration of the spinel surface is corroborated by FTIR studies of promoted and un-promoted samples in a CO atmosphere. The promotion by potassium is both structural, inhibiting growth of the spinel crystals as evidenced by XRD and BET surface area measurements, but also electronic since the activity per surface area strongly increases on K-promotion. Finally, our studies include long term tests of thermal stability and tolerance towards H2S.