The State of Cu Promoter Atoms in High-Temperature Shift Catalysts—An in Situ Fluorescence XAFS Study

Abstract

The state of the copper promoter (0.17–1.5 wt%) was studied in Fe–Cr-based high-temperature shift (HTS) catalysts by in situ fluorescence XAFS combined with on-line gas analysis. The XANES spectra acquired in the fluorescence mode showed that Cu is present as copper metal under working conditions of the high temperature shift catalyst but easily reoxidizes upon air exposure at room temperature. In contrast, the active chromium substituted magnetite phase is stable under these conditions as shown by X-ray diffraction (XRD). Both as-prepared and used (i.e., after HTS run) catalysts were studied during reduction in 2%H2/3%H2O/N2. For the used catalyst, an intermediate Cu(I) phase is detected by in situ fluorescence XANES. The reduction of Cu(II) to Cu(I) occurred at 120–150°C, nearly independent of the Cu concentration, whereas the stability of the Cu(I) phase toward further reduction was significantly dependent on the Cu concentration, being especially high at low copper concentration. For the as-prepared HTS catalysts the initial reduction started at around 250°C, during which Fe(III) and Cr(III)/Cr(VI) containing phases were transformed to a chromium substituted magnetite phase. The copper reduction occurred simultaneously with the reduction of the iron matrix. Thus, the reduction temperature of Cu(II) was significantly shifted to lower temperatures for catalysts which were previously reduced in the HTS reaction. It is proposed that Cu(II) is incorporated into iron and chromium containing phases in the as-prepared catalysts and segregates out forming small metallic clusters on the surface of the oxide phases during activation/reaction.