Konstantin Hadjiivanov

Identification of Neutral and Charged N x O y Surface Species by IR Spectroscopy

The infrared spectral performance of the N x O y species observed on oxide surfaces [N2O, NO−, NO, (NO)2, N2O3, NO+, NO2 − (different nitro and nitrito anions), NO2, N2O4, N2O5, NO2, and NO3 − (bridged, bidentate, and monodentate nitrates)] is considered. The spectra of related compounds (N2, H-, and C-containing nitrogen oxo species, C─N species, NH x species) are also briefly discussed. Some guidelines for spectral identification of N x O y adspecies are proposed and the transformation of the nitrogen oxo species on catalyst surfaces are regarded.

Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule

Abstract The review is a summary and analysis of the data characterizing CO adsorption on surface cationic sites of oxides including supported materials and microporous and mesoporous materials. The contributions of various types of CO bonding to the IR frequency shifts of carbon-bonded molecules are analyzed, namely, the increase of the CO stretching frequency in cases of electrostatic and σ bonding and the decrease of the frequency with π bonding. Polycarbonyls, bridging CO, oxygen-bonded CO, and tilted CO are also considered. The main part of the review is a collection of the experimental results characterizing carbonyls of individual metal ions. The spectral behavior of CO bonded to metal atoms is also assessed in the cases when the metal ions are easily reduced to metal (Cu, Ag, Au, Pd, or Pt) or cationic carbonyls are produced after CO adsorption on supported metals (Ru, Rh, Ir, and Os). The interaction of CO with surface OH groups is also considered. It is demonstrated that IR spectroscopy of adsorbed CO is an efficient methodology to characterize cationic surface sites in terms of their nature, oxidation states, coordination environment and coordinative unsaturation, and location at faces, edges or corners of microcrystallites. When applied to materials with surface hydroxyl groups CO undergoes hydrogen bonding and information can be collected on the proton acid strength.

FT-IR study of NO + O2 co-adsorption on H-ZSM-5: re-assignment of the 2133 cm-1 band to NO+ species

Whereas NO adsorption at room temperature on activated H-ZSM-5 (Si/Al = 29) caused only negligible changes in its IR spectrum, addition on O2 to NO led to the appearance of bands at 2133 and 977 cm-1. Concomitantly, the number of acidic zeolite OH groups decreased while H2O hydrogen-bonded to zeolite OH groups developed. Introduction of small amounts of 18O2 did not change the 2133 cm-1 band wavenumber, nor the use of a partly deuteroxylated D–H-ZSM-5 sample. In such a case, HOD formation was detected. The results obtained evidence that the 2133 cm-1 band, generally considered as characterizing NO+2 species, is, in fact, due to NO+ species occupying cationic positions in the zeolite. The 977 cm-1 band is attributed to the Olattice–NO+ vibration. A scheme of the NO+ formation, involving NO2 molecules as NO oxidizing agent, is proposed.

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.

Species formed after NO adsorption and NO+O2 co-adsorption on TiO2: an FTIR spectroscopic study

Adsorption of NO and its co-adsorption with oxygen on TiO2 (Degussa P-25) were studied by FTIR spectroscopy. It was found that NO adsorption results in its disproportionation to NO− (1170 cm−1), N2O22− (1335 cm−1) and nitrates (1650–1550 and 1240–1220 cm−1). The nitrate bands develop with time and coordinated NO (ca. 1900 cm−1) is formed. Addition of oxygen to NO results in a strong increase in concentration of the nitrates and formation of NO+ (2206 cm−1). In addition, species assigned to nitrocomplexes (1520 and 1284 cm−1) are found. The stability and reactivity of the different surface compounds as well as their interconversion are studied and discussed.

FTIR study of CO and NH3 co-adsorption on TiO2 (rutile)

Abstract FTIR spectroscopy has been used to study CO and ammonia adsorption and co-adsorption on rutile. Ammonia is adsorbed coordinatively, but with time, different dissociative ammonia forms appear. At room temperature CO is adsorbed forming one type of Ti4+–CO carbonyls alone (absorption band at 2191 cm−1) and is oxidized to bicarbonates. At 100 K CO occupies, at first, the sites detected at room temperature, whereas the increase in amount of adsorbed CO results in new adsorption forms: (i) CO adsorbed on titanium cations inert at room temperature (monitored by an intense band with a maximum at 2183 cm−1), (ii) CO adsorbed on hydroxyl groups (band at 2150 cm−1), and (iii) physically adsorbed CO (band at 2140 cm−1). Pre-adsorbed ammonia decreases the acidity of both, the Lewis acid sites and the surface Ti4+–OH groups. Low-temperature CO adsorption on rutile partly poisoned by ammonia allows to detect only the sites inert towards CO adsorption at room temperature. On the basis of the results obtained, the active sites for adsorption of ammonia and CO are discussed and comparison between the surface properties of anatase and rutile is made.

Effect of water on the reduction of NOx with propane on Fe‐ZSM‐5. An FTIR mechanistic study

Adsorption of NO on Fe‐ZSM‐5 leads to formation of Fen+–NO (n = 2 or 3) species (1880 cm-1), Fe2+(NO)2 complexes (1920 and 1835 cm-1) and NO+ (2133 cm-1). Water strongly suppresses the formation of NO+ and Fen+(NO)2 and more slightly the formation of Fen+ –NO. Introduction of oxygen to NO converts the nitrosyls into surface nitrates (1620 and 1575 cm-1) and this process is almost unaffected by water. The nitrates are thermally stable up to ca. 300°C, but readily interact with propane at 200°C, thus forming surface C–H–N–O deposit (bands in the 1700–1300 cm-1 region). Here again, water does not hinder the process. The C–H–N–O deposit is relatively inert (it does not interact with NO or NO + O2 at ambient temperature) but, at temperatures higher than 250 °C, it is decomposed to NCO- species (bands at 2215 (Fe–NCO) and 2256 cm-1 (Al–NCO)). In the presence of water, however, the Fe–NCO species only are formed. At ambient temperature the NCO- species are inert towards NO and O2, but easily react with a NO + O2 mixture. The mechanism of the selective catalytic reduction of nitrogen oxides on Fe‐ZSM‐5 and the effect of water on the process are discussed.

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.

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.