Reduction and Oxidation of Cu Species in Cu-Faujasites Studied by IR Spectroscopy

Abstract

The process of reduction (by hydrogen and ethanol) and oxidation (by oxygen and NO) of Cu sites in dealuminated faujasite-type zeolites (of Si/Al = 31) was studied by infrared (IR) spectroscopy with CO (for Cu+) and NO (for Cu2+) as probe molecules. Two zeolites were studied: one of them contained mostly Cu+exch., whereas another one contained mostly Cu2+ and Cu+ox. The susceptibility of various forms of Cu for reduction were investigated. IR experiments of CO sorption evidenced that Cu+ox. was more prone for the reduction than Cu+exch. According to NO sorption studies, Cu2+exch. was reduced in the first order before Cu2+ox. Ethanol reduced mostly Cu2+ and, also, some amounts of Cu+. The treatment with oxygen caused the oxidation of Cu+ (both Cu+exch. and Cu+ox.) to Cu2+. The adsorption of NO at 190K produced Cu+(NO)2 dinitrosyls, but heating to room temperature transformed dinitrosyls to mononitrosyls and increased the Cu2+ content.

Oxidation of Ethanol in Cu-Faujasites Studied by IR Spectroscopy

“…Sorption of ethanol at room temperature was followed by the sorption of CO on the studied catalyst at the same temperature. The spectrum of CO interacting with Cu + exch showed the band at 2158 cm −1 , which was also observed in our previous studies [ 39 , 40 ] and was reported by other authors [ 3 , 41 , 42 , 43 ]. When ethanol was preadsorbed, the Cu + exch –CO band was shifted to lower frequencies and the two shifted maxima appeared at 2118 and 2134 cm −1 ( Figure 1 ).…”

“…We also reported the process of reduction and oxidation of the Cu species in CuFAU [ 40 ]. In our study, reducers were hydrogen and ethanol, while the role oxidizers played were oxygen and NO.…”

On the Role of Protonic Acid Sites in Cu Loaded FAU31 Zeolite as a Catalyst for the Catalytic Transformation of Furfural to Furan

“…The contribution of copper redox active centers in the decarbonylation of furfural may be also explained by the production of Brønsted acid sites as a result the reduction of copper species with hydrogen according to the following reactions: 2 Cu + + H 2 = 2 Cu 0 + 2H + and Cu 2+ + H 2 = Cu 0 + 2H + , which was reported in our previous work [ 69 ]. Protonic acid sites generated in this way could participate in the formation of furan.…”

“…In our previous work [ 69 ], we reported the FT-IR analysis of Cu containing FAU31 zeolites in order to investigate the process of reduction of Cu sites with hydrogen and the properties of reduced zeolites. IR spectra of OH groups for two types of Cu-zeolites: in protonic (Cu x H-Z) and sodium form (Cu x Na-Z) were recorded.…”

“…Furthermore, based on FT-IR studies of CO and NO sorption over Cu-FAU31 zeolites, it was indicated that the susceptibility of Cu sites for the reduction with hydrogen depended strictly on the type of Cu species. Order of reduction Cu sites formed the following sequence: Cu + oxide > Cu 2+ > Cu + exch [ 69 ].…”