Unraveling the Chemical State of Cobalt in Co-Based Catalysts during Ethanol Steam Reforming: An in Situ Study by Near Ambient Pressure XPS and XANES

Abstract

The steam reforming of ethanol (ESR) has been studied by near ambient pressure XPS (NAP-XPS), extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES) under in situ conditions in the ALBA synchrotron facility at 200-580 °C and S/C = 3 over different cobalt-based catalysts that showed different catalytic performances: Co3[Si2O5]2(OH)2 (Co-talc), [Co2Mg4Al2(OH)16]CO3·4H2O (Co-hydrotalcite shortened to Co-HT) calcined at 550 °C, and Co3O4 (Co-spinel). Both Co-spinel and Co-talc yield to a greater or lesser degree metallic cobalt under ESR conditions. While Co-spinel shows a complete reduction to metallic cobalt under the conditions used for the XANES measurements, the more bulk-sensitive Co-talc sample exhibits only a partial reduction. On the other hand, under the ESR conditions used with the NAP-XPS, a more surface sensitive technique, the results indicate a higher reduction degree for the Co-talc sample in comparison to the Co-spinel sample. In contrast, the catalyst prepared from the Co-HT does not show metallic cobalt traces under the experimental conditions used with both techniques. On comparison of these three cobalt-based catalysts, the stable operation exhibited by Co-HT under ESR reaction conditions is justified by the absence of metallic cobalt formation under in situ conditions, which is identified as being responsible for the carbon deposition phenomenon that triggers the deactivation suffered by most cobalt-based catalysts during ESR.