The SCR of NO with methane over In,H- and Co,In,H-ZSM-5 catalysts: The promotional effect of cobalt

Abstract

Zeolite In,H-, Co,H-, and Co,In,H-ZSM-5 were characterized by operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), temperature-programmed reduction by hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS), and activity in the selective catalytic reduction of NO (NO-SCR) by methane. The catalysts were shown to contain indium as [InO]+/[InOH]2+ cations, whereas cobalt was in the form of Co2+/[Co-OH]+ cations or Co-oxide clusters in amounts controlled by the applied preparation method. The NO-SCR by methane was shown to proceed in two coupled processes on distinctly different catalytic sites. One of the processes is the oxidation of NO to NO2 by oxygen over Brønsted acid sites and/or cobalt-oxide species giving NO/NO2 gas mixture (NO-COX reaction). The other process is the N2 formation, which is the result of the reaction of methane and the NO/NO2 mixture (CH4/NO-SCR reaction). Molecules of NO and NO2 were shown to become activated together as NO+/NO3− ion pair in reaction with [InO]+/[InOH]2+ or Co2+/[Co-OH]+ sites. Operando DRIFTS results suggested that the reaction of methane and NO3− generates an intermediate that rapidly reacts with the NO+ to give N2. The promoting effect of the cobalt was related to the significantly higher NO-COX activity of Co-oxide clusters than that of the Brønsted acid sites. The accelerated NO-COX reaction speeds up the formation of NO+/NO3− species and the rate of the methane activation, being the rate-determining step of the NO-SCR reaction. It was also shown that the NO-SCR reaction can proceed if NO-COX and CH4/NO-SCR active sites are separated in space. In order to avoid rate controlling NO2 transport between the sites close proximity of the sites is favorable.