Nature of ammonia storage sites in H-SSZ-13 and Cu-SSZ-13

Abstract

In this study, the nature of ammonia storage sites in H-SSZ-13 and Cu-SSZ-13 with different Si/Al and Cu/Al molar ratios is characterized by NH₃ sorption experiments using in situ DRIFT spectroscopy, and by NH₃-TPD flow reactor experiments. In addition, the binding energy and vibrational properties of different surface species are obtained by DFT calculations to aid the interpretation of the experimental results. The NH₃ sorption DRIFTS experiments over Cu-free samples show that NH₃, NH₄⁺ and NH₄⁺·NH₃ ions form during the adsorption of NH₃ over the Brønsted acid sites at 70 °C. DRIFT results show that peaks at around 3735 and 3650 cm⁻¹ become increasingly more negative with increasing Cu loading, which is likely related to solvation of Cu by NH₃. DFT calculations show two possible ammonia-solvated Cu species, i.e. Cu^I(NH₃)₂ and Cu^II(NH₃)₃OH. By comparing the simulated and measured IR spectra, we conclude that the experiments match the simulated infrared spectra of Cu^II(OH)(NH₃)₃, Cu^I(NH₃)₂, NH₄⁺ and NH₄⁺·NH₃. Both DRIFTS experiments and calculations show that the amounts of Cu^I(NH₃)₂ and Cu^II(OH)(NH₃)₃ increase with decreasing Si/Al molar ratio and increasing Cu loading. Furthermore, the TPD results show that the NH₄⁺·NH₃ peak decomposes at relatively low temperatures, i.e. in the low-temperature region. The remaining NH₄⁺ species is stable until decomposition in the high-temperature region.