IR Studies of the Cu Ions in Cu-Faujasites

Abstract

The properties of Cu ions in dealuminated faujasite-type zeolites (Si/Al = 31) containing 1, 2, and 5 wt.% of Cu were investigated by IR spectroscopy with CO and NO as probe molecules. Cu was introduced by impregnation into zeolites in both protonic (HFAU) and sodium (NaFAU) forms of zeolite. Four kinds of Cu species were found: Cu+exch., Cu+oxide, Cu2+exch. (square, planar, and square pyramidal), and Cu2+oxide (CuO). The proportions between these four kinds of Cu depended on the amount of Cu and on the form of zeolite to which Cu was introduced (HFAU or NaFAU). Zeolites with 1 wt.% of Cu introduced to HFAU (denoted as Cu(1)HFAU) contained only Cu+exch., whereas other forms of Cu were present in zeolites of higher Cu contents. The concentration of Cu+exch. was determined by quantitative IR studies of CO adsorption. According to the IR results, some Cu ions were situated inside hexagonal prisms and/or cuboctahedra, and were inaccessible to adsorbed molecules. IR studies also evidenced that Cu ions in oxide forms—Cu+oxide and Cu2+oxide (CuO)—were better electron donors than Cu in exchange positions (Cu+exch. and Cu2+exch).

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

“…The CO bands are significantly smaller for Cu(5)NaFAU, indicating that the amounts of Cu + accessible to CO molecules is also lower, despite a higher Cu content (5 wt.% vs. 2 wt.% in Cu(2)HFAU). This difference was explained in our previous paper [ 27 ] as the result of a different procedure of introduction of Cu to both zeolites. Zeolite Cu(2)HFAU was obtained by the impregnation of HFAU (Si/Al = 31) with Cu(NO 3 ) 2 that caused the production of CuHFAU and HNO 3 , which was removed during calcination, which was done upon the impregnation procedure.…”

“…(NO) 2 dinitrosyls (1730 and 1825 cm −1 ) ( Figure 1 C). At higher temperatures, dinitrosyls decompose, forming mononitrosyls Cu + -NO (1815 cm −1 [ 27 ].…”

“…Furthermore, we showed the difference of the spectrum recorded upon the reduction at 530 K minus the spectrum before treatment in Figure 2 B (bottom line). Direct comparison between the spectra obtained for the samples reduced by hydrogen at various temperatures and the appearance of the differential spectrum (bottom line) led to the conclusion that the 1885 cm −1 band of Cu 2+ -NO is composed of two submaxima: 1880 and 1895 cm −1 assigned [ 27 ] to Cu 2+ ox. (CuO) and to Cu 2+ exch.…”

“…The present study is the continuation of our earlier investigation [ 27 ] in which the status and properties of Cu ions in zeolites of the faujasite (FAU) type of Si/Al = 31 were followed by IR spectroscopy, with CO and NO as the probe molecules. Cu was introduced [ 27 ] by the impregnation method to zeolites in the protonic (HFAU) or sodium form (NaFAU). Cu was in the form of Cu + in the exchange form (Cu + exch.…”

“…The choice of CO as a probe molecule for Cu ions is implied from the existence of a characteristic carbonyl frequency shift, which depends strictly on the copper coordination environment, as well as the oxidation state [ 14 , 15 , 16 ]. In the case of Cu 2+ cations, the CO-IR analysis is insufficient, thus complementary techniques such as Electron Paramagnetic Resonance (EPR) [ 17 , 18 , 19 ], X-ray photoelectron spectroscopy (XPS) [ 20 , 21 , 22 ] and temperature-programmed reduction (TPR) [ 20 , 23 , 24 , 25 , 26 ], as well as IR studies of NO sorption, are needed to receive a detailed characterization of the oxidation state and properties of Cu in zeolites under various red-ox conditions, e.g., [ 27 , 28 , 29 ].…”