Selective catalytic reduction of NOx by methane over Co-H-MFI and Co-H-FER zeolite catalysts: characterisation and catalytic activity

Co-H-MFI and Co-H-FER were studied as catalysts for NO reduction with methane, under diluted conditions. Catalyst bulk and surface structure was characterised by chemical analysis, XRD, TG-DTA, SEM micrographs, IR studies, UV–vis spectroscopy, ammonia TPD, and TPR. Only the features due to zeolitic frameworks were observed. The UV–vis spectra showed that, after outgassing, Co ions are mainly present as Co2+. FT-IR detected residual bridging hydroxyl and silanol groups in spite of the apparent over-exchange, deduced from the chemical analysis. Co ions gave rise to Lewis acid sites of medium strength, evidenced through in situ analysis of the adsorption of a basic probe molecule. NH3-TPD measurements fully confirmed these results. TPR measurements detected easily reducible and oxidizable Co species, possibly related to nanosized Co oxide phases, together with hardly reducible Co2+ ions. Catalytic tests highlighted the Co-H-FER and Co-H-MFI activity, both in NO-selective catalytic reduction by methane and in NO oxidation to NO2, the latter being the predominant reaction at low temperatures. Some mechanistic features of the CH4-SCR reaction are discussed.