First-Principles Interrogation of H₂ Reduction Mechanisms on Copper Active Sites in Cu-SSZ-13 Zeolite

<p>Zeolite is a fascinating porous material made up of many chemical building blocks. The use of zeolite is prevalent in modern days applications. Its usage can be widely seen in agriculture, cement making, wastewater treatment, environment protection, and numerous small-scale and micro-scale applications.^1–5 Zeolite is built up by its tetrahedral sites (T-site) such as silicon (Si⁴⁺) and aluminum (Al³⁺) T-sites that are bridged together by oxygen atoms.⁶ An incumbent Si⁴⁺ type tetrahedra which is replaced by Al³⁺ leads to the creation of a -1 charge on its connected oxygen. Within that Al³⁺ inclusive zeolite ring, such as the six-membered-ring (6MR) and the eight-membered-ring (8MR), there can be another Al³⁺ substituted site that gives another -1 charge in the ring. These two Al³⁺ form a -2 charge Al pair in the ring. Altogether, these negative charges can be compensated by many types of cations, such as Bronsted acids (H⁺), metal cations (Cu²⁺, Fe³⁺), or metal combinations with oxygen and hydrogen (CuOH⁺, Fe(OH)₂⁺).^7,8 Copper is a common species in SSZ-13 due to its excellent catalytic properties in SCR and partial methane oxidation (PMO).^10,18,19 In copper exchange, copper prefers to be located either in the 6MR as Cu²⁺ or the 8MR as CuOH⁺.^20,21 The siting of copper species strongly depends on the availability of the ‘low energy’ sites present in these membered-rings. Thus, researchers have formulated a way to estimate the number of these preferred sites using composition parameters such as Si/Al and Cu/Al ratios.^22–24 </p><p>In fact, quantification of copper in zeolite has long been an interest to the catalyst community. In literature, several spectroscopic and molecular adsorption methods have been proposed and studied. In one of those works, Paolucci et al. measured the proportion of Cu²⁺ and CuOH⁺ by reducing the copper species with NO+NH₃ reduction and ammonia temperature programmed desorption (NH₃-TPD).^28–30 The presence of NO and NH₃ reduces Cu²⁺ into Cu⁺ and H⁺ while it reduces CuOH⁺ into Cu⁺.^31,32 When NH₃ is introduced to the species after reduction, it adsorbs on both Cu⁺ and H⁺ sites.^33,34 When temperature ramps up, the temperature which NH₃ desorbs will tell the number of Cu⁺ and H⁺ sites available in the zeolite. Comparing with a NH₃-TPD conducted before the reduction step, the quantity of Cu²⁺ and CuOH⁺ can then be counted. Though NH₃-TPD works satisfactorily in the study, it rarely serves as a final step to quantify Cu²⁺ and CuOH⁺. Thus, several interrogation methods are used to complement each other.^24,33,35,36</p><p>Hydrogen temperature programmed reduction (H₂-TPR) is an easy and direct method to quantify Cu²⁺ and CuOH⁺. H₂ is a simple molecule compared to NH₃. It adsorbs directly onto Cu²⁺ and CuOH⁺ instead of using Cu⁺ and H⁺ as surrogates as in NH₃-TPD. However, it also shares a similar shortcoming with NH₃-TPD, among others. In its case, the type of intermediary species and their formation steps in H₂ reduction of Cu²⁺ and CuOH⁺ have yet been understood. In a big picture, the elementary steps of getting from the reactants (H₂, Cu²⁺, and CuOH⁺) to the products (H₂, H₂O, Cu⁺, and Cu⁺/H⁺) are not yet understood. Therefore, this work will investigate their reduction mechanisms in SSZ-13 zeolite through making and validating hypothesis on their reaction mechanisms. The understanding of this copper reduction mechanism has an impact in the understanding of copper reduction pathway, the types of intermediate species, and possibly the effects of water solvation on copper reduction rate.</p>