Nature of the sites of dissociative adsorption of dihydrogen and light paraffins in ZnHZSM-5 zeolite prepared by incipient wetness impregnation

A DRIFT study of ZnHZSM-5 zeolites with Si/Al ratios of 15 or 41 and a Zn loading of 0.8 wt% revealed a high thermal stability of bridging OH groups that was practically the same as in the pure hydrogen forms. It was concluded that the incipient wetness impregnation of NH₄ZSM-5 zeolite with zinc nitrate and the subsequent high-temperature treatment results only in a minor amount of ion exchange. A considerable part of the modifying zinc forms nanometric ZnO clusters inside the channels of the zeolite. The use of the low-temperature adsorption of dihydrogen as a probe indicated the appearance, after high-temperature vacuum pretreatment, of three different Lewis acid sites connected with coordinatively-unsaturated Zn²⁺ ions. The strongest Lewis sites, with an H–H stretching frequency of adsorbed molecular hydrogen of 3940 cm⁻¹, dissociatively adsorbed hydrogen, methane and propane at both room and elevated temperatures. These sites are represented either by Zn²⁺ ions on the walls of the main channels of the zeolite (α sites according to Mole et al.) or by Lewis-base pairs on the surface of nanometric clusters of zinc oxide. This revised version was published online in July 2006 with corrections to the Cover Date.     

In situ DRIFTS study of the effect of structure (CeO2–La2O3) and surface (Na) modifiers on the catalytic and surface behaviour of Pt/γ-Al2O3 catalyst under simulated exhaust conditions

The nature and relative populations of adsorbed species formed on the surface of un-promoted and sodium-promoted Pt catalysts supported either on bare Al₂O₃ or CeO₂/La₂O₃-modified Al₂O₃, were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under simulated automobile exhaust conditions (CO + NO + C₃H₆ + O₂) at the stoichiometric point. The DRIFT spectra indicate that interaction of the reaction mixture with the Pt/Al₂O₃ catalyst leads mainly to formation of formates and acetates on the support and carbonyl species on partially positively charged Pt atoms (Pt^δ+). Although enrichment of Al₂O₃ with lanthanide elements (CeO₂ and La₂O₃) does not significantly modify the carboxylate species formed on the support, it causes significant modification of the oxidation state of Pt, as indicated by the appearance of a substantial population of carbonyl species on reduced Pt sites (Pt⁰–CO). This modification of the Pt component is enhanced when Na-promotion is used, leading to formation of carbonyl species only on electron enriched Pt (i.e., fully reduced Pt⁰ sites) and to the formation of NCO on these Pt entities (2180 cm⁻¹). The latter are thought to result from enhanced NO dissociation at Na-modified Pt sites. These results correlate well with observed differences in the catalytic performance of the three different systems.     

Adsorption of benzothiophene on Y zeolites investigated by infrared spectroscopy and flow calorimetry

Diffuse reflectance infrared spectra of benzothiophene adsorbed on different Y zeolites reveal that the cations and protons in the zeolites are the sites responsible for the adsorption of benzothiophene. On NaY, benzothiophene was molecularly adsorbed on the cations through the electrophilic interaction between the cations and the thiophenic rings. On the transition metal ion exchanged NiY and CuY zeolites, because of the presence of the d-electrons in the cations, the thiophenic rings interact with the cations to form the π-complexes through the σ–π electron donations. In the presence of hydroxyl species in the zeolites, the adsorbed sulfur compounds attach to the protons molecularly via the electrophilic interaction and undergo the opening of the thiophenic rings depending on the acidity of the zeolites and the adsorption amount. The apparent heat of adsorption of benzothiophene in normal octane on the Y zeolites determined by flow calorimetry shows that the adsorption strength based on the measured heat for each mole sulfur adsorbed on the Y zeolite is in the order of CuY > NiY > NaY  USY. For USY, due to the endothermic breakage of the thiophenic ring of benzothiophene induced by the acid sites of the zeolite, the apparent heat of adsorption is similar to that obtained from the adsorption on NaY. This work demonstrates that the transition metal ion exchanged zeolites exhibit excellent properties for sulfur adsorption because of the formation of the π-complexes and that the acidity of the zeolites is not advantageous for sulfur removal due to the strong adsorption and decomposition of the adsorbed species.     

DRIFT,XPS and XAS Investigation of Au–Ni/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Synergetic Catalyst for Allylbenzene Isomerization

Au/Al₂O₃ and Ni/Al₂O₃ prepared by deposition-precipitation and impregnation methods, respectively, and Au–Ni/Al₂O₃ prepared by impregnation of the Au/Al₂O₃ system with nickel have been studied by physicochemical methods. The catalysts were tested in allylbenzene isomerization at 443 K. The isomerization on Au/Al₂O₃ proceeds with a rather low rate, while monometallic Ni/Al₂O₃ is inactive. Unlike monometallic catalysts, Au–Ni/Al₂O₃ demonstrates a strong synergetic effect: the isomerization rate on the bimetallic sample exceeds the sum of the rates over the individual metals by an order of magnitude. The catalysts structure was estimated by XPS, DRIFT, and XAS techniques. It was revealed that nickel in Au–Ni/Al₂O₃ and Ni/Al₂O₃ exist as Ni²⁺ cations with the coordination state close to that in the precursor, gold in Au/Al₂O₃ exist as Au⁰ nanoclusters, whereas the Au⁰ nanoclusters co-exist with Au³⁺ cations in the bimetallic Au–Ni/Al₂O₃ sample. Reasons of the synergetic catalytic effect for Au–Ni/Al₂O₃ are discussed in terms of the formation of new positively charged Au catalytic centers.     

Molecular mechanisms in partial oxidation of methane on Ir/α-Al2O3: reactivity dependence on catalyst properties and transport phenomena limitations

An in situ DRIFT and mass spectrometric study of catalytic partial oxidation of methane with Ir/-Al₂O₃ has enlightened relationships between the formation of surface metal carbonyl clusters and residence time and temperature conditions. Some cluster species produced during catalytic partial oxidation were also originated during CO₂ hydrogenation and CO₂ reforming experiments described in previous literature. An EXAFS analysis of the catalyst precursor, prepared through a solid-liquid reaction between Ir₄(CO)₁₂ clusters and the reactive surface sites of-Al₂O₃, is also included to discuss clusters structure produced at the surfaces.     

Basic investigation of the chemical deactivation of V2O5/WO3-TiO2 SCR catalysts by potassium,calcium, and phosphate

Topics in Catalysis - The influence of the combustion products of different lubrication oil additives and impurities in fuel or urea solution on the activity and selectivity of V2O5/WO3-TiO2...     

Influence of Hydrocarbon Chemisorption on the NOx Adspecies over Supported Noble-Metal Catalysts

In situ and time-resolved DRIFT methods were used to monitor the change in NO _x adspecies on Pt(1%)–TiO₂ and Rh(1%)–TiO₂ catalysts during interaction with propene with the aim to determine whether or not propene chemisorption and interaction with the catalyst induces a change in the nature of the NO _x adspecies prior to their reduction. The nature of NO _x adspecies produced by interaction of the NO + O₂/He feed with the catalyst is different on Pt- and Rh–TiO₂ (in the Pt–TiO₂ catalyst the IR more intense adspecies are nitrate, while in the Rh–TiO₂ catalyst nitrosyl species are the IR more intense), but modification of the nature of the adspecies prior to their conversion is observed in both cases. The interpretation of the data provides indication about the nature of the reactive NO _x species and the presence of multiple pathways in the mechanism of their conversion.     

DME synthesis from carbon dioxide and hydrogen over Cu–Mo/HZSM-5

HZSM-5 supported copper–molybdenum oxide prepared by an impregnation method has been applied for the first time in the hydrogenation of CO₂. The effect of the loading of Mo, the reaction temperature and the stability of hydrogenation of CO₂ were investigated. The results indicate that adding a small amount of Mo (Mo/Cu = 1/2 wt) markedly enhances the catalytic activity. The highest DME selectivity was more than 70%. The change of species on the catalyst surface due to the addition of Mo was also investigated by in situ DRIFT.     

Study on the mechanism of NH3-selective catalytic reduction over CuCexZr1--<?Pub Caret?>x/TiO2

Copper--cerium--zirconium catalysts loaded on TiO₂ prepared by a wet impregnation method were investigated for NH₃-selective catalytic reduction (SCR) of NO_x. The reaction mechanism was proposed on the basis of results from in situ diffuse reflectance infrared transform spectroscopy (DRIFT). When NH₃ is introduced, ammonia bonded to Lewis acid sites is more stable over CuCe_0.25Zr_0.75/TiO₂ at high temperature, while Br?nsted acid sites are more important than Lewis acid sites at low temperature. For the NH₃+NO+O₂ co-adsorption, NH₃ species occupy most of activity sites on CuCe_0.25Zr_0.75/TiO₂ catalyst, and mainly exist in the forms of NH₄⁺ (at low temperature) and NH₃ coordinated (at high temperature), playing a crucial role in the NH₃-SCR process. Two different reaction routes, the L-H mechanism at low temperature (<200°C) and the E-R mechanism at high temperature (>200°C), are presented for the SCR reaction over CuCe_0.25Zr_0.75/TiO₂ catalyst.     

The Auxin-like Activity of Humic Substances is Related to Membrane Interactions in Carrot Cell Cultures

A detailed characterization of two humic fractions was performed: One with low relative molecular mass (LMr < 3,500 Da) and one with high relative molecular mass (HMr > 3,500 Da). Distinct ¹H NMR spectroscopic patterns were observed for the two fractions. HMr showed an aromatic proton region, an intense and broad region (3.0–5.0 ppm) attributed to sugar-like and polyether components, and an intense doublet at 1.33 ppm (identified as protons of the β-CH₃ in lactate). In contrast, LMr did not show resonances due to aromatic protons and was characterized by a broad unresolved region, assigned to sugar-like components. The ¹³C NMR spectra showed that the LMr humic fraction was richer in carboxylic and aliphatic C groups compared to HMr fraction. These substances were fluorescein-labeled [fluorescein isothiocyanate (FITC)], and their interaction with carrot cells in culture was monitored for 10 d, and compared to FITC–indole-3-acetic acid (IAA) to clarify their mechanisms of biological activity. After different incubation times, fluorescein staining of carrot cells and decrease of fluorescein concentration in the culture medium were evaluated. Fluorescent membrane staining was only present in IAA and the LMr humic fraction treated cell cultures. A consequential decrease of fluorescein concentration in the culture media was also observed. Pretreatment of carrot cells with unconjugated IAA or LMr humic fraction markedly reduced fluorescein staining of both FITC–IAA and FITC–LMr humic fraction. Blocking tests gave indirect evidence of possible binding of the LMr humic fraction to IAA cell membrane receptors. These results indicate that the two humic fractions behave differently. Only LMr humic fraction, like IAA, interacts with cellular membranes in carrot cell cultures.