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Conditioning of Rh/α-Al2O3 catalysts for H2 production via CH4 partial oxidation at high space velocity
Authors:Alessandra Beretta  Tiziana Bruno  Gianpiero Groppi  Ivan Tavazzi  Pio Forzatti
Affiliation:aNEMAS, Centro di Eccellenza per l’Ingegneria dei Materiali e delle Superfici Nanostrutturate, Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta” Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
Abstract:Experimental evidence and literature indications suggest that the process of methane partial oxidation over Rh catalysts is structure sensitive. Crystal phases and Rh cluster size are thus expected to affect the final catalytic performance. In this work, it is observed that outstanding performances are obtained when the as-prepared catalysts are conditioned through repeated runs at increasing temperature and O2/CH4 = 0.56. Catalysts slowly activate, that is CH4 conversion and synthesis gas selectivity progressively grow with time on stream. On the basis of TPO and CH4 decomposition measurements, this phenomenon is herein explained as the result of a surface reconstruction driven by the repeated exposition to the reaction at high temperature; it is thought that such reconstruction tends to eliminate defect sites and disfavors C-deposition reactions (extremely fast over steps and kinks). Conditioning with O2-enriched feed streams makes conditioning faster, since the accumulation of surface C-species is suppressed; however, the catalyst is eventually less active than a catalyst conditioned with standard feed mixtures. As an alternative, accumulation of carbon can be suppressed and surface reconstruction proceeds faster if the catalyst is directly exposed to the reaction at high temperature for several hours.
Keywords:CH4 partial oxidation  Rh/α  -Al2O3  Rh reconstruction  Structure sensitivity of C–  H bond activation
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