IR study of CO adsorption on Cu-ZSM-5 and CuO/SiO₂catalysts: σ and π components of the Cu⁺—CO bond

Abstract

Adsorption of carbon monoxide on CuO/SiO, (1 wt.% CuO) and Cu-ZSM-5 (11 wt.% CuO) catalysts has been studied by IR spectroscopy. CO adsorption on CuO/SiO, leads to formation of: (i) three kinds of unstable Cu2+-C0 species detected only under equilibrium CO pressure and characterized by v(C0) at 2216, 2199 and 2180 cm-', respectively, and (ii) one kind of Cu+-CO carbonyl manifesting an IR band at 2126.5 cm-'. The latter carbonyls possess moderate stability, and some of them are removed upon evacuation. Water replaces CO preadsorbed on the Cu+ ions. Testing the surface of Cu-ZSM-5 with CO reveals the existence of two types of sites: (i) associated Cu+ cations, monitored by a CO band at 2137 cm-' whose intensity is reduced during evacuation, and (ii) isolated Cu+ sites, which form, at high CO equilibrium pressures, dicarbonyls (bands at 2177.5 and 2151 cm-'). Decrease in CO pressure leads to destruction of these species according to the reaction Cu+(CO), -+ Cu'-CO + CO and after evacuation only monocarbonyls are detected by a band at 2158.5 cm-'. These monocarbonyls are stable and resistant towards evacuation. Water is coadsorbed with CO on the isolated Cu' sites, which is accompanied by a ca. 30 cm-' red shift of the 2158.5 cm-' band. This shift is reversible and the original band position is restored after subsequent evacuation.The results show that the state of Cu+ is quite different in Cu-ZSM-5 and CuO/Si02 catalysts. It is assumed that the Cu' sites on CuO/SiO, have one coordinative vacancy each, which leads to formation, primarily, of Cu+-CO monocarbonyls after CO adsorption. On the contrary, the isolated Cu' ions on Cu-ZSM-5 each possess two vacancies, which determine their ability to form dicarbonyls or to coordinate water and CO simultaneously. On the basis of the results obtained it is concluded that the participation (underestimated up to now) of the CJ component in the Cu+-CO bond plays a decisive role with respect to the frequency of CO adsorbed on Cu+ ions and the stability of the corresponding carbonyls.

Spectroscopic Investigation of Species Arising from CO Chemisorption on Titania-Supported Manganese

“…Hence, the following localization of the cus Mn 3+ ions can be proposed: ions characterized by the carbonyl band at 2144 cm −1 are envisaged as products of the CO adsorption. The difference of 30 cm −1 in the stretching frequencies of the corresponding carbonyls is probably due to a difference in the σ component of the bond between Mn 2+ and CO (38). The carbonyl band at 2114 cm −1 present in the spectrum after short evacuation (i.e., the corresponding carbonyls are more stable) can be attributed to Mn 2+ ions which form a bond with CO with a considerable π contribution.…”

Characterization of the Oxidation States of Supported Gold Species by IR Spectroscopy of Adsorbed CO

“…Thus, the most stable surface carbonyls of copper, silver, and gold are expected to be those of univalent cations. Indeed, it is well established that CO is strongly adsorbed on Cu + sites [26,32]. Surface carbonyls of metallic copper are unstable and usually observed only under CO equilibrium pressures [26,33].…”

“…In the former case, CO is bonded to metal atoms, and in the latter it is bonded to metal cations. Typical examples of the latter are carbonyls of elements of group 11 of the periodic table of the elements (copper, silver, and gold) [31,32]. The oxidation state of each of these elements in its stable carbonyl species is +1.…”

FTIR study of low-temperature CO adsorption on Mn-ZSM-5 and MnY zeolites. Effect of the zeolite matrix on the formation of Mn2+(CO) geminal species

“…The most typical examples are the formation of Ag þ ðCOÞ 2 species with Ag-ZSM-5 [12]; Cu 2þ ðCOÞ 2 and Cu þ ðCOÞ 3 species with Cu-ZSM-5 [13][14][15]; and Ni 2þ ðCOÞ 2 and Ni þ ðCOÞ 3 species with Ni-ZSM-5 [16]. The latter two cases indicate, once again, that the process is favoured by a big cationic radius: decreasing oxidation state of the cation is accompanied by an increase of its ionic radius so that even three molecules can be coordinated simultaneously to one cation provided its radius is sufficiently large.…”

“…In these cases, however, the polycarbonyls are normally decomposed without passing through linear species. During the past years a new class of geminal species (the so-called site-specified geminal species [1]) was discovered [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. All these species are formed on metal-exchanged zeolites or on related materials and are decomposed losing their ligands stepwise.…”