D. Klissurski

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.

FOURIER TRANSFORM IR STUDY OF NOX ADSORPTION ON A CUZSM-5 DENOX CATALYST

The adsorption and coadsorption of selective catalytic reduction (SCR) reactants and reaction products on CuZSM-5-37 containing 11 wt.-% CuO have been studied by FTIR spectroscopy. The catalyst surface is characterized by both weak acidity and weak basicity as revealed by testing with probe molecules (CO2, NH3, H2O). NO2 adsorption results in formation of different kinds of nitrates. The same species are formed when NO is coadsorbed with oxygen at 180°C. NO adsorption at ambient temperature also leads to formation of nitrates as well as of Cu2+NO species. In the presence of oxygen the latter are converted according to the scheme: NO → N2O3 → N2O4 → NO2 → NO3. It is concluded that the surface nitrates are important intermediates in the SCR process. They are thermally stable and resistant towards interaction with CO2, N2, O2, and are only slightly affected by H2O and NO. However, they posses a high oxidation ability and are fully reduced by propane at 180°C. It is concluded that one of the most important roles of oxygen in SCR by hydrocarbons is to convert NOx into highly active surface nitrates.

Glass formation and structure of glasses in the V2O5MoO3Bi2O3 system

The glass formation region in the V2O5MoO3Bi2O3 system was investigated by the roller-quenching method. Low melting glasses were obtained in the MoO3- and V2O5-rich compositions. Characterization of the glasses was made by X-ray diffraction, differential thermal analysis (DTA) and IR spectroscopy. According to the DTA data, the glass transformation temperature, Tg, for the different compositions varied between 200 and 290°C and the cystallization temperature, TX, was within the interval of 225–330°C. Structural models for glasses of th MoO3Bi2O3 and V2O5Bi2O3 systems were suggested on the basis of IR spectral investigations, by comparing with known crystalline structures. It was shown that the glasses possess [MoO4], [MoO6], [VO4], and [BiO6] groups as basic structural units.

IR spectroscopic study of NOxadsorption and NOx–O2coadsorption on Co2+/SiO2catalysts

Adsorption of nitrogen oxides (NO, NO2) and their coadsorption with oxygen on Co2+/SiO2 samples has been investigated by IR spectroscopy with a view to elucidating the mechanism of selective catalytic reduction (SCR) of NOx with hydrocarbons. A Co2+/SiO2 sample synthesized by ion exchange is characterized by a highly dispersed cobalt and a very weak surface acidity: CO is adsorbed only at low temperature (100 K) forming Co2+–CO carbonyls [ν(CO) = 2180 cm−1]. Adsorption of NO on Co2+/SiO2 leads to the formation of Co2+(NO)2 dinitrosyl complexes (1872 and 1804 cm−1) which are decomposed upon evacuation. Adsorption of NO2, as well as coadsorption of NO and O2, produce NO2 species weakly bound to the support (a band at 1681 cm−1) and N2O4 (a band at 1744 cm−1 with a shoulder at 1710 cm−1), the latter being adsorbed reversibly on both the support and the Co2+ ions. In the second case N2O4 is transformed into surface monodentate nitrates of Co2+ (a band at 1550–1526 cm−1) and partly into bridged nitrates (a band at ca. 1640 cm−1). The monodentate nitrates are stable with respect to evacuation up to 125 °C and act as strong oxidising agents: they are reduced by NO, even at room temperature, and by methane at 100 °C. In the latter case, organic nitro-compounds and isocyanate groups are registered as reaction products (probably intermediate compounds in SCR). The surface species obtained after NO and NO2 adsorption on Co2+/SiO2 prepared from cobalt acetate (active SCR catalyst) are essentially the same as those observed with the ion-exchanged sample. No monodentate nitrates, however, are formed during NO2 adsorption on a Co2+/SiO2 sample synthesized by impregnation with cobalt nitrate, which accounts for the lack of activity of this sample in the SCR.

Glass formation and structure in the V2O5Bi2O3Fe2O3 glasses

Abstract Glass formation V2O5Bi2O3Fe2O3 system was investigated by the roller-quenching method and low melting V2O5- and Bi2O3-rich glasses were obtained. Glasses were characterized by X-ray diffraction, differential thermal analysis (DTA), IR and Mossbauer spectroscopy. The glass transformation temperature, Tg, varied between 270 and 442°C, and the crystallization temperature, Tx, was in the range of 342–550°C. Structural models for glasses of the V2O5Bi2O3Fe2O3 system are suggested on the basis of IR and Mossbauer spectral investigations. It was shown that the glasses posses [VO5], [VO4], [BiO6], [FeO6] and [FeO4] groups as basic structural units.

FTIR study of species arising after NO adsorption and NO + O2 co-adsorption on CoY : comparison with Co-ZSM-5

Abstract CoY, with its low activity in selective catalytic reduction (SCR) of nitrogen oxides, differs from Co-exchanged pentasil zeolites (e.g. Co-ZSM-5). To obtain more information on the SCR mechanism, the NO x species formed after NO adsorption and NO+O 2 co-adsorption on CoY were studied by means of IR spectroscopy and the results were compared with those obtained for Co-ZSM-5. NO adsorption on CoY leads to the formation of Co 2+ (NO) 2 species ( ν s at 1900 and ν as at 1819 cm −1 ) which are characterised by a stability similar to the stability of the dinitrosyls formed on Co-ZSM-5 (1894 and 1812 cm −1 ). This suggests that the Co 2+ (NO) 2 species are not involved in the SCR. The stable species produced upon NO+O 2 co-adsorption on the two samples are very different. The principal compounds formed on Co-ZSM-5 are surface monodentate nitrates characterised by an IR band at ≈1540 cm −1 . These nitrates easily interact with hydrocarbons, which confirms that they are key species in SCR. No monodentate nitrates are formed on CoY. Stable symmetric nitrates (1488 and 1473 cm −1 ) and less stable species, probably bidentate nitrates (1620 and 1320 cm −1 ) appear instead. The symmetric nitrates are converted, during evacuation, into nitro-compounds (1563 and 1383 cm −1 ) that are not removed even by evacuation at 743 K. Interaction of methane with the nitrates on CoY only leads to their partial reduction to nitro-compounds. These results account for the low SCR activity of CoY.

Study of phosphate-modified TiO2 (anatase)

Pure and phosphate-modified titanium dioxide (anatase) have been studied by IR spectroscopy of probe molecules, X-ray photoelectron spectroscopy, X-ray phase analysis, electron microscopy, etc. It is established that the phosphate anions are strongly bonded to the anatase surface and are stable even at the temperature of anatase-rutile transition. The phosphates cannot be removed by washing with water or diluted acids, but are extracted by basic solutions. The active sites for phosphate adsorption are hydroxyl groups and Lewis acid sites on the anatase surface, whereas part of the c.u.s. Ti4+ ions, which exhibit a weak acidity, remains free. Thermal treatment of samples modified by phosphoric acid leads to their dehydroxylation due to the recombination of hydrogen-bonded hydroxyl groups. For samples modified by NaH2PO4 part of the hydrogen ions are substituted by Na+, which leads to “dilution” of the hydroxyls and hinders their recombination. As a result, isolated OH groups, stable at 723 K, exist on the surface. The disappearance of the Lewis acidity on anatase after modification with phosphates allows the assumption that the role of phosphorus as a promoter in some titania-supported catalysts is to block the Ti4+ Lewis acid sites.

Glass formation and structure in the system MoO3–Bi2O3–Fe2O3

Abstract The glass formation region in the MoO 3 –Bi 2 O 3 –Fe 2 O 3 system was investigated by the roller-quenching method. Glasses melted at temperatures 3 -rich compositions. Analysis of the glasses was made by X-ray diffraction, differential thermal analysis (DTA), infra-red spectroscopy (IR), and Mossbauer spectroscopy. According to the DTA data, the glass transformation temperature, T g , for the different compositions varies between 360°C and 440°C and the crystallization temperature, T x , is in the range 390–470°C. Structural model for the glasses are suggested on the basis of IR and Mossbauer spectral data. In the region of MoO 3 rich compositions, the network forming units MoO 6 are connected by Mo–O–Mo bridging bonds. The presence of Me 2 O 3 (Me=Bi, Fe) leads to transformation of MoO 6 to MoO 4 . Thus, in a wide region of compositions the glass network has a scheelite-like structure containing isolated MoO 4 structural units which are surrounded by MeO 6 groups.

An IR spectroscopy study of the state and localization of vanadium-oxo species adsorbed on TiO2 (anatase)

The state and localization of vanadium-oxo species adsorbed on anatase have been studied by IR spectroscopy of probe molecules, UV-vis spectroscopy, XPS, and chemical analysis. The adsorption was carried out from acidic solutions of ammonium vanadate. Two samples were used as adsorbents: (i) pure anatase, characterized by the presence of two kinds (strong and weak) of Lewis acid sites and two kinds of surface hydroxyl groups, and (ii) anatase treated with hydrogen peroxide where physically and chemisorbed peroxides exist. It was established that the active sites for adsorption were, in the first case, the surface hydroxyl groups and the strong Lewis sites only. The catalyst thus obtained (VT1) was characterized by a Lewis acidity, mainly due to the weak acid sites of the support, as well as by a small amount of Bronsted acid sites representing surface VOH groups. When the adsorption proceeded on the peroxide-treated sample, the vanadium coverage was higher, since all Lewis acid sites of anatase participated in the process. A protonic acidity was also observed on this sample (VT2), but, contrary to VT1, the Lewis acidity was associated with the existence of V5+. On the basis of the results obtained, structures of the surface species are proposed. Some possibilities of using the samples as catalysts are discussed.

IR spectroscopy study of NO adsorption and NO + O2 co-adsorption on Al2O3

Adsorption of NO on Al2O3 leads to the formation of small amounts of surface nitro–nitrito (1461, 1318, 1228, 1220 and 1074 cm−1) and nitrato (1652 cm−1) complexes. Introduction of oxygen into the system causes oxidation of the NO2− compounds to nitrates (1628, 1591, 1565, 1298, 1259 and 1042 cm−1) whose concentration increases with time and with the oxygen amount. The nitrates obtained in this way are stable up to 723 K. They increase the Lewis acidity of Al3+ sites in the vicinity, as a result of which mononitrosyls of the type Al3+(NO3−)-NO (1938 cm−1) are formed. In addition, weakly bound N2O3 (1895, 1565, 1298 cm−1) and N2O4 (1752 cm−1) are obtained. Analysis of the spectra in the 2900–2400 cm−1 region shows the presence of combination bands at 2883, 2624 and 2546–2526 cm−1, which can be useful for the spectral identification of the surface anionic nitrogen-oxo species.