Influence of Solvent on Selective Catalytic Reduction of Nitrogen Oxides with Ammonia over Cu-CHA Zeolite

Abstract

The copper-exchanged zeolite Cu-CHA has received considerable attention in recent years, owing to its application in the selective catalytic reduction (SCR) of NO x species. Here, we study the NH3-SCR reaction mechanism on Cu-CHA using the hybrid quantum mechanical/molecular mechanical (QM/MM) technique and investigate the effects of solvent on the reactivity of active Cu species. To this end, a comparison is made between water- and ammonia-solvated and bare Cu species. The results show the promoting effect of solvent on the oxidation component of the NH3-SCR cycle since the formation of important nitrate species is found to be energetically more favorable on the solvated Cu sites than in the absence of solvent molecules. Conversely, both solvent molecules are predicted to inhibit the reduction component of the NH3-SCR cycle. Diffuse reflectance infrared fourier-transform spectroscopy (DRIFTS) experiments exploiting (concentration) modulation excitation spectroscopy (MES) and phase-sensitive detection (PSD) identified spectroscopic signatures of Cu-nitrate and Cu-nitrosamine (H2NNO), important species which had not been previously observed experimentally. This is further supported by the QM/MM-calculated harmonic vibrational analysis. Additional insights are provided into the reactivity of solvated active sites and the formation of key intermediates including their formation energies and vibrational spectroscopic signatures, allowing the development of a detailed understanding of the reaction mechanism. We demonstrate the role of solvated active sites and their influence on the energetics of important species that must be explicitly considered for an accurate understanding of NH3-SCR kinetics.

The Development of the Regenerable Catalytic System in Selective Catalytic Oxidation of Ammonia with High N2 Selectivity

“…It was reported that double peaks at 2237 and 2210 cm –1 appeared when N 2 O adsorbed on the catalyst surface or NH 3 oxidation occurred at high temperatures. Therefore, they were denoted as M-NNO species adsorbed on surface cation and further desorbed to gaseous N 2 O. ,,, The peak of 2237 cm –1 was ascribed to the vibration of the NN bond of adsorbed intermediate, possibly NH 2 NO ads , originating from the reaction of adsorbed NO + and NH 3 . , …”

Computational infrared and Raman spectra by hybrid QM/MM techniques: a study on molecular and catalytic material systems

“…The first two test cases concern molecules in solution, namely, the amino acid L-histidine in a spherical water droplet, and a more complex haem protein in a water shell solvation model. The third test case is the heterogeneous catalyst chabazite (CHA), a typical zeolitic material with excellent performance in selective catalytic reduction of nitrogen oxides with ammonia [100][101][102], which has recently been studied in our group [103,104]. Finally, hydrogen absorption on an ionic metal oxide ZnO polar surface supporting the hydrogenation of carbon dioxide to methanol is investigated.…”

Multiscale QM/MM modelling of catalytic systems with ChemShell

“…Abdul Nasir et al investigated the effect of water and ammonia solvent molecules on the selective catalytic reduction of NO x species by the copper-exchanged zeolite Cu-CHA. 209 The results showed that solvation of the active sites has a strong influence on the energy profile of the catalytic cycle, with differing effects on the oxidative and reductive parts. Liu et al used QM/MM methods to investigate the structure and stability of vanadium-doped active sites in Al-MCM-41, and their potential use as heterogeneous catalysts for the oxidation of furans.…”