Margarita Kantcheva

IR study of CO adsorption on Cu-ZSM-5 and CuO/SiO2 catalysts: σ and π components of the Cu+—CO bond

Adsorption of carbon monoxide on CuO/SiO2(1 wt.% CuO) and Cu-ZSM-5 (11 wt.% CuO) catalysts has been studied by IR spectroscopy. CO adsorption on CuO/SiO2 leads to formation of: (i) three kinds of unstable Cu2+—CO species detected only under equilibrium CO pressure and characterized by ν(CO) at 2216, 2199 and 2180 cm–1, respectively, and (ii) one kind of Cu+—CO carbonyl manifesting an IR band at 2126.5 cm–1. 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–1 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–1). Decrease in CO pressure leads to destruction of these species according to the reaction Cu+(CO)2→ Cu+—CO + CO and after evacuation only monocarbonyls are detected by a band at 2158.5 cm–1. 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–1 red shift of the 2158.5 cm–1 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/SiO2 catalysts. It is assumed that the Cu+ sites on CuO/SiO2 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 σ 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.

State and localization of cobalt, nickel and copper ions adsorbed on titania (anatase)

Spectroscopic methods and chemical analysis are used to investigate Co2+/TiO2, Ni2+/TiO2 and Cu2+/TiO2 samples prepared by adsorption of the corresponding ions from alkali solutions on anatase. Titania is characterized by the presence of two kinds of surface hydroxyl groups and both strong and weak Lewis acid sites. The adsorption of copper ions results in blocking of both types of Lewis acid sites whereas after adsorption of cobalt and nickel ions the strong sites only are blocked. In all three cases the hydroxyl groups of anatase do not participate in the process.The three cations are stabilized in a divalent state on the surface. Cobalt, nickel and part of the copper ions are found to have octahedral surroundings. Introduction of CO to the activated Co2+/TiO2 and Ni2+/TiO2 samples (450 °C, in vacuo) causes the formation of carbonyls of the types Ti4+—CO, Co2+—CO and Ni2+—CO. In the case of sample Cu2+/TiO2, Cu2+ is reduced to Cu+ and carboxylates are observed together with two kinds of Cu+—CO carbonyls. The structures of the supported phases and the possible sites of cation adsorption are discussed.

On the mechanism of CO adsorption on a silica‐supported ruthenium catalyst

The adsorption of CO at room temperature on a Ru/SiO2 catalyst has been studied by means of FTIR spectroscopy. Spectral evidence for formation of water molecules and a quantity of very dispersed ruthenium on the catalyst surface during CO adsorption was found. On the basis of these experimental results a new reaction scheme for the interaction of CO with a silica‐supported ruthenium catalyst is proposed.

Nitrogen dioxide adsorption on deuteroxylated titania (anatase)

The adsorption of No2 on anatase and deuteroxylated TiO2 is investigated by IR spectroscopy. It is established that NO2 reacts with the surface hydroxyl (deuteroxyl) groups of anatase to give nitrates and water molecules (H2O, HOD and D2O) weakly bonded to the nitrates, while on the surface Lewis acidic–basic pairs of sites, NO2 disproportionates to NO+ and nitrates. The stability of the different adsorption forms is studied and their structures are discussed.

Study of peroxide-modified titanium dioxide (anatase)

The compounds formed on titania (anatase) after its treatment with hydrogen peroxide have been investigated by spectroscopic methods, thermal desorption measurements and chemical analysis. Physically adsorbed hydrogen peroxide, hydroperoxide groups, and O–2 anion radicals were found. The possible active sites for adsorption and the structure of the hydroperoxide groups are discussed.

FTIR Spectroscopic Study on Nickel(II)-Exchanged Sulfated Alumina: Nature of the Active Sites in the Catalytic Oligomerization of Ethene

The nature of the active sites in nickel(II)-exchanged sulfated alumina in the reaction of ethene oligomerization has been studied by means of FTIR spectroscopy of adsorbed CO. It has been established that isolated nickel(I) species are the active sites in this process. These sites are formed by a reduction process, in which protonic centers are involved. The latter are due to the presence of covalently-bonded sulfate ions on the catalyst surface.

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)x geminal species

Abstract Adsorption of CO on Mn-ZSM-5 zeolite at 85 K results in formation of physically adsorbed CO, several kinds of H-bonded CO and Mn2+(CO)x geminal species (2202 cm −1 ) . Decreasing the coverage during evacuation results in disappearance of the physically adsorbed CO and the H-bonded forms and in conversion of the dicarbonyls to linear Mn2+–CO species (2214 cm −1 ) . The latter are quite stable at 85 K. Coadsorption 12CO and 13CO reveals that the CO molecules in the geminal polycarbonyls behave as independent oscillators. In contrast, CO adsorption at 85 K on MnNaY zeolite only leads to formation of linear Mn2+–CO species ( 2210 cm −1 ) and mono- and di-carbonyls associated with residual sodium cations. The results are interpreted as evidence that site-specified geminal carbonyls are formed with cations possessing an ionic radius bigger than a critical value. This value is different for different positions in various zeolites and is bigger for cations in SII positions in Y zeolites than is the case of cations in a ZSM-5 matrix.

XPS and IR characterization of manganese ions deposited on alumina

By application of XPS and FTIR spectroscopy of adsorbed CO the effect of preparation conditions on the state and localization of manganese ions deposited on h -Al2O3 is studied. Both Mn 21 and Mn 31 ions are observed on the impregnated sample. The sample obtained by ion exchange contains only Mn 31 ions. The adsorbed CO species are identified. q 1999

Spectroscopic characterization of vanadium(V) oxo species deposited on zirconia

A method for deposition of vanadium(V) oxo species from acidic solutions of ammonium metavanadate on zirconia is described. The samples are synthesized by suspension of the support (powder) in solutions containing three different vanadium(V) precursor ions: the dioxovanadium(V) ion, VO2+, the yellow diperoxo anion [VO(O–O)2]− and the red monoperoxo cation [VO(O–O)]+. Application of vanadium(V) peroxo complexes increases significantly the uptake of vanadium by zirconia. The state and localization of the VOx species on the surface of zirconia were studied by FTIR, UV/VIS and XP spectroscopies. The materials prepared from VO2+ ions contain isolated VO4 groups and domains of orthovanadate species, (VO4)n. The FTIR spectra of adsorbed CO revealed that the number of exposed Zr4+ ions decreased markedly when vanadium(V) peroxo ions were used as precursors. The predominant surface structure in this case was identified as pyrovanadates, V2O7. On all of the samples studied no separate phase of V2O5 was detected.

Spectroscopic characterization of VOx/ZrO2 catalysts prepared using vanadium(V) oxo complexes

A method for deposition of vanadium(V) oxo species on zirconia using different vanadium(V) precursor ions is described. The samples are synthesized by suspension of the support (powder) in solutions containing: (i) the dioxovanadium(V) ion, VO2+; (ii) the yellow diperoxo anion, [VO(OO)2]− and (iii) the red monoperoxo cation, [VO(OO)]+. The application of vanadium(V) peroxo complexes significantly increases the uptake of vanadium by zirconia. The state and localization of the VOx species on the surface of zirconia have been studied by FTIR, UV–Vis and XP spectroscopies.