Synthesis and catalytic property of a Co2+-exchanged Beta/Y composite for the selective catalytic reduction of NO by CH4 in the presence of excess oxygen

A new composite of zeolite Beta and zeolite Y (denoted as Beta/Y) has been synthesized by overgrowing and/or epitaxially growing a layer of zeolite Beta on the pseudo-crystals of zeolite Y. The prepared sample was different from the mechanical mixture of these two phases, as shown by NH3-TPD and FE-SEM measurements. One type of remarkably strong acid site was observed for the H-type and the (Co, H)-type composite samples, whereas it is absent in the corresponding analogues of zeolite Beta, Y and their mechanical mixtures. This provides one type of new cationic site for Co2+ in the composites. With comparable contents of zeolite Beta and zeolite Y phases, (Co, H)-Beta/Y catalysts showed much higher catalytic activity than the physical mixture of (Co, H)-Y and (Co, H)-Beta and also the pure phases of these two materials for the reduction of NO by methane (CH4-SCR) in the presence of excess oxygen. This resulted from the formation of a large number of more stable NOy (with y = 2, 3) species enhanced by the presence of such type of strong acid site, and hence one type of new Co2+ ion as well, and more highly dispersed CoxOy species, as revealed by NH3-TPD, NO-TPD, NO2-TPD, (NO + O2)-TPD and DR UV–vis spectroscopy. It was shown that the Co2+ cations in the ion-exchanged sites, strong Brösted acid sites and probably Co oxide-like species collaboratively contribute to the CH4-SCR of NO at high reaction temperature. In contrast, when the reaction was carried out below 700 K, it seems that Co2+ cations play a major role.