Correlation Between Catalytic Activity and Metal Cation Coordination: NO Decomposition Over Cu/Zeolites

Abstract

Since Iwamoto’s discovery of an unusually high activity of Cu/zeolites in NO decomposition, there has been an ongoing experimental and theoretical quest to understand the details of the mechanism of this process. Despite this tremendous effort, there are still a number of unsettled questions, including the uncertainty about the nature of the reaction intermediates and about the structure of the active site; either the involvement of only a single extra-framework Cu cation or the concerted effect of two nearby Cu cations, the Cu pair, was proposed. From the vast amount of relevant experimental data available today, it is clear that the activity of the Cu/zeolite catalysts depends on 1) zeolite topology, 2) zeolite composition (Si/Al ratio and Cu loading), and 3) Cu-exchange procedure, including the sample pretreatment. 6,8, 12, 13,15] However, the understanding of the relationship between active site structure (the metal coordination and localization) and catalytic activity is far from complete; it is our goal to increase our knowledge in this respect. The lack of direct experimental evidence about the transition-state structures and about the details of the reaction mechanism justifies the use of quantum chemistry to provide the missing details. However, a reliable description of the reactions catalyzed by transition metals in a complex environment (such as zeolites) represents a major challenge for contemporary computational chemistry since it requires the use of a model that realistically represents the active site (including its environment) and the use of a method that can consistently describe the electronic structure of the system along the reaction path (see the Methods section). Many reaction steps possibly involved in the direct NO decomposition on Cu/zeolites have been investigated previously by employing various models and methods. Several reaction paths proposed in the literature have been compiled (Scheme 1) along with the reaction energies and activation barriers (where available) reported for the elementary reaction steps. Since these energies were obtained at various levels of theory using different types of models for the active site, a direct comparison should be taken with some caution. For example, the reaction energies of 107 and 173 kJmol 1 were reported for reaction step D based on the BP86 functional (using a 1-T cluster model) and based on the B3LYP functional (using a 3-T cluster model), respectively. A majority of the proposed reaction mechanisms start with the interaction of two NO molecules with Cu/zeolite (ZCu) producing ZCuO and N2O (A!B!C). Different routes were proposed for the subsequent conversion of N2O: 1) it can interact with another NO molecule to produce N2 and NO2. One of the G!I!J, A!K!L, or M!L routes can be followed; the rate-determining steps of these reaction routes are characterized with activation barriers of 105, 122, and 126 kJmol 1[20] , respectively. 2) N2O can be reattached to ZCu to release N2 while forming another ZCuO species (process G!H). Note however, that all these reaction paths lead to the production of NO2 and/or ZCuO species. To close the catalytic cycle, the ZCuO needs to be reduced back to ZCu and the NO2 must further react. However, NO2 can readily interact with ZCuO forming ZCuNO3 (process O; DE= 252 kJmol ). It is not clear how to proceed from a stable ZCuNO3 complex towards the products. Another route was proposed following the A!B!C path: a simple reaction mechanism based on the readsorption of N2O on ZCuO (through the O-end) that leads to the formation of N2, O2, and ZCu (D!E!F). An activation barrier for the rate-determining step on this path (the formation of ZCuO2N2) of 152 and 145 kJmol 1 has been reported based on BP86/(1-T cluster) and B3LYP/(3-T cluster) calculations, respectively. Whereas the activation barrier of the D!E!F Scheme 1. A schematic representation of the possible processes taking place during NO removal in zeolites. The reaction energies and activation barriers (denoted by #) are taken from the literature: a) Ref [16] , b) Ref. [17] , c) Ref. [18] , d) Ref. [19] , and e) Ref. [20] . Superscript numbers refer to the spin multiplicity of the adsorption complex.