Microstructural studies of the copper promoted iron oxide/chromia water-gas shift catalyst

Abstract

The microstructure of an iron oxide based high temperature water-gas shift (WGS) catalyst has been studied by X-ray diffraction (XRD), high resolution electron microscopy (HREM), high spatial resolution energy dispersive X-ray analysis (STEM-EDX) and X-ray photoelectron spectroscopy (XPS). The iron oxide contains a chromia additive that structurally stabilises the catalyst. Chemical microanalysis indicates that the chromia forms a solid solution within the magnetite Fe 3 O 4 lattice and that no discrete chromia phases are formed. The level of Cr dissolution varies greatly at the intergranular level. XPS and STEM-EDX studies suggest that the activated catalyst is surface enriched in chromia. On the basis of these results a model for the stabilising effect is proposed in which the enriched surface ' shell ' encapsulates each catalyst grain and, being more thermodynamically stable than the iron-rich core, reduces ion diffusion and sintering effects. A Cu dopant is often added to the Fe 3 O 4 /Cr 2 O 3 catalyst to promote its activity. Chemical microanalysis shows that this dopant also exists in solid solution. Furthermore, STEM-EDX and XPS analyses indicate that the catalyst grains also exhibit a strong surface enrichment in the Cu species. Analysis of aged ex-reactor specimens suggest that deactivation occurs as a result of eventual sintering, with a substantial increase in grain-size relative to the fresh catalyst. The Cu dopant is also demonstrated to have a propensity to segregate as CuO on the surface of the catalyst grains in aged specimens.

Natural gas to synthesis gas – Catalysts and catalytic processes

“…However, the evolution of modern techniques has shed a new light on this matter. Using a combination of XPS, XRD and highresolution electron microscopy (Edwards et al, 2002) it was recently shown that Cr forms a solid solution in magnetite. In a freshly reduced Fe/Cr catalyst containing 8% Cr, only a single spinel phase was detected by XRD and by high-resolution electron microscopy (HREM).…”

“…In the paper of Edwards et al (2002) it is described how an Fe/ Cr/Cu catalyst sample was aged for 1000 h at 370 C with a foreseeable loss of surface area from 50 to 17 m 2 /g. Nonetheless the sample occasionally displayed 7e12 nm particles of CuO (the sample must have become oxidised) in contact with magnetite particles as evidenced by HREM.…”

“…The experimentally determined Cr-enrichment of the surface was found in the freshly reduced catalyst. Although aged samples were prepared in the study of Edwards et al (2002) they were not subject to analysis of element distribution within the separate particles. Therefore it cannot be excluded that Cr migrates to the interior of the Fe 3 O 4 particles during WGS operation.…”

Development of chromium-free iron-based catalysts for high-temperature water-gas shift reaction

“…The WGS reactor currently represents the largest volume of any catalyst in role of the Cr 2 O 3 addition on the stabilization of the catalyst structure is still unclear. Edwards et al [15] proposed a model for the high stabilizing effect of chromium whereby a shell of chromium-enriched material is formed on each catalyst grain, but the mechanism by which promotion occurs is still uncertain.…”

In Situ UV-Visible Assessment of Iron-Based High-Temperature Water-Gas Shift Catalysts Promoted with Lanthana: An Extent of Reduction Study

“…Industria WGS catalysts are typically 92 wt % Fe3O4/8 wt % Cr2O3 that serve as activ reaction temperatures close to 400 °C [2]. Numerous research studies h enhancing the stability and activity of these catalysts through the addition o Ce, Hg, Ag and Rh) that can stabilize the iron-chromium structure and provi for the chemical reaction [3][4][5][6][7][8][9][10][11][12][13]. A previous study has shown that the Fe/Cr/Cu high-temperature (HT) water-gas shift (WGS) catalysts chan programmed reduction profiles, generally causing more consumption of Previous studies also showed that lanthana increases the thermal stabil perovskite-like unsupported chromium-free iron-based catalysts for the wat Co-based catalysts for the Fischer-Tropsch synthesis reaction [11][12][13].…”