The state of Cu promoter atoms in high-temperature shift catalysts - An insitu fluorescence XAFS study

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 combine d 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 exposur e at room temperature. In contrast, the active chromium substituted magneti te phase is stable under these conditions as shown by X-ray diffraction (XR D). Both as-prepared and used (i.e., after HTS run) catalysts were studied during reduction in 2%H-2/3%H2O/N-2. 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 degreesC, nearly independent of the Cu co ncentration, whereas the stability of the Cu(I) phase toward further reduct ion was significantly dependent on the Cu concentration, being especially h igh at low copper concentration. For the as-prepared HTS catalysts the init ial reduction started at around 250 degreesC, during which Fe(III) and Cr(I II)/Cr(VI) containing phases were transformed to a chromium substituted mag netite phase. The copper reduction occurred simultaneously with the reducti on of the iron matrix. Thus, the reduction temperature of Cu(II) was signif icantly shifted to lower temperatures for catalysts which were previously r educed in the HTS reaction. It is proposed that Cu(II) is incorporated into iron and chromium containing phases in the as-prepared catalysts and segre gates out forming small metallic clusters on the surface of the oxide phase s during activation/reaction. (C) 2001 Academic Press.