A Computational Mechanistic Study on Copper Autoreduction in Cu-CHA Zeolite Catalysts.

Abstract

The activation of Cu-zeolite catalysts is accompanied by an autoreduction reaction, in which a part of Cu(II) species is spontaneously reduced to Cu(I) species. The stoichiometry of autoreduction in which the release of one O₂ is accompanied by the reduction of four Cu(II) to Cu(I) has been proposed, but the detailed mechanism of this autoreduction remains unclear. In this work, we used DFT calculations to study the autoreduction mechanism in Cu-CHA zeolites. The two reduction mechanisms of [CuOH]⁺ to Cu⁺ in CHA-type zeolite were systematically studied. In Mechanism I, two [CuOH]⁺ react via dehydration to form [Cu-O-Cu]²⁺, and the further reaction of two [Cu-O-Cu]²⁺ to produce O₂ is the most critical step, which requires four charge-compensating framework Al in close proximity. In Mechanism II, the production of O₂ occurs via [CuO]⁺ intermediates, and the generation of possible [CuO]⁺ is the most critical step. The exploration of autoreduction reactions in a variety of Cu-CHA models with different Al sittings shows that the O-O distances between two intermediate precursors, i. e., two [Cu-O-Cu]²⁺ in Mechanism I, or two [CuO]⁺ in Mechanism II, are key factors determining the activation barriers of O₂ production during autoreduction.