FTIR evidence for surface azides formed upon ammonia adsorption on copper catalysts

The adsorption of ammonia on an alumina supported copper catalyst and on unsupported copper powder was studied by diffuse reflectance FTIR spectroscopy in the temperature range 423–643 K. Bands in the region between 1950 and 2350 cm^–1 were identified as being due to surface-bound azides. These species are suggested to be possible precursors in the copper nitride formation observed earlier.     

Zirconia aerogels and xerogels: Influence of solvent and acid on structural properties

High-surface-area zirconia aerogels with meso- to macroporosity have been prepared by an acid-catalyzed alkoxide-sol-gel route with tetrabutoxyzirconium(IV) and subsequent high-temperature supercritical drying at 578 K. The effect of solvent (ethanol, propanol, butanol, t-amylalcohol), amount of nitric acid, calcination temperature, and drying method was studied by nitrogen physisorption, X-ray diffraction, Fourier transform Raman and diffuse reflectance infrared Fourier transform spectrosopy, scanning electron microscopy, thermal analysis, and temperature-programmed desorption of NH₃. After calcination in air at 573 or 773 K, the aerogels possess specific surface areas of up to 270 or 180 m² · g^–1, respectively. The use of ethanol as solvent resulted in the highest specific surface areas and pore volumes (up to 1.5 cm³ · g^–1) among all samples studied, whereas bulky t-amylalcohol caused a shift of the maxima of the broad pore size distributions from 30 to 70 nm. With the corresponding xerogels, prepared via the same wet-chemical procedure but evaporatively dried at ambient temperature, butanol resulted in a maximum at 3 nm and t-amylalcohol in a bimodal pore size distribution with maxima at 3 and 15 nm. The variation of the acid-to-alkoxide ratio in the range 0.08–0.12 at a hydrolysis level of 4 did not significantly influence the structural properties of aerogels and related xerogels. In contrast to the aerogels, the xerogels had significantly lower specific surface areas and prominent microporosity. All uncalcined aerogels contained crystalline ZrO₂, whereas the corresponding uncalcined xerogels were X-ray amorphous and crystallized only during calcination at 573 K. Both aerogels and xerogels possessed Brønsted-type and Lewis-type acid sites. With the xerogels, the density of acid sites on the surface was significantly lower. This behaviour is attributed to the higher amounts of organic residues which persisted in and on the xerogels up to 773 K and thus blocked the acid sites partially.     

Axial variation of the oxidation state of Pt-Rh/Al2O3 during partial methane oxidation in a fixed-bed reactor: An in situ X-ray absorption spectroscopy study

Probing the structure of materials in situ is of central importance in heterogeneous catalysis. Mostly, this is done in an integral manner, that is without spatial resolution. However, at high conversion in a catalyst bed prominent concentration and/or temperature profiles may exist which can result in significant spatial variation of the catalyst structure. In the present study, X-ray absorption spectroscopy combined with on-line mass spectrometry was used to monitor the structural changes of a Pt-Rh/Al₂O₃ catalyst in a fixed-bed reactor during partial oxidation of methane. The reaction ignited at 310 °C and integral X-ray absorption spectroscopy showed that the Rh-Pt-particles were reduced at the same time. However, monitoring with a beam of 1 mm × 0.6 mm size along the axial position of the catalyst bed uncovered that Rh and Pt were still in oxidized state in the entrance region, whereas they were in reduced state in the zone at the end of the catalyst bed. The gradual transition from the reduced to the oxidized state was found to shift towards the bed entrance if the temperature was slightly increased.An erratum to this article can be found at .     

Catalytic Synthesis of Higher Aliphatic Amines from the Corresponding Alcohols

Aliphatic amines are of considerable industrial importance and find application in almost every field of modern technology, agriculture, and medicine [1], Lower aliphatic amines (C₁ to C₆) are important intermediates for the chemical and pharmaceutical industries. A large number of drugs, herbicides, pesticides, dyes, and other chemicals contain amino pups which originate from reactions with such intermediates, Many important applications of higher aliphatic amines (fatty amines) and their derivatives (most important derivatives are quarternary ammonium compounds) are based on their cationic surface activity. Relatively small amounts of such compounds are usually required to achieve the desired changes in surface and colloidal properties. Thus, not surprising, one of the first applications of fatty amines was in the flotation separation of nonmetallic materials such as potash, feldspar, phosphate, and mica. Today, probably the biggest demand for fatty amines lies in the production of fabric softeners. There are other important applications for aliphatic amines in the plastics and protective coat industries as emulsion stabilizers, mold release agents, pigment dispersers, and flushing agents. They are used as catalysts for polyurethane production. For granular products, alkylamines are used as anticaking and antidusting agents. In the rubber industry they are used as oxidation inhibitors and catalysts for accelerating vulcanization. Aliphatic amines find also many applications in the petroleum industry, especially as corrosion inhibitors and as components of lubricating oils, greases, and fuel oil where they act as sludge dispersants and stabilizers. They are added to gasoline as corrosion inhibitors.     

Selective catalytic reduction of nitric oxide over vanadia grafted on titania. Influence of vanadia loading on structural and catalytic properties of catalysts

The selective catalytic reduction (SCR) of NO by NH₃ has been studied over vanadia/ titania catalysts prepared by selective immobilization of vanadyl alkoxide species on two structurally different titania supports. The loading of vanadia was varied from 1.8 to 7.5 ,mol V⁵⁺ per m² surface area. Comparative kinetic measurements at 150 °C show that the NO turnover frequencies increase by more than an order of magnitude when the vanadia loading is increased from 1.8 to 3 mol V⁵⁺/m². In the region of lower SCR activity, i.e. at lower coverages ( 2 mol V⁵⁺/m²), small clusters and ribbons of vanadia are detected in the Raman spectra, whereas at loadings where maximum NO turnovers are achieved ( 3 mol V⁵⁺/m²) the prevalent vanadia species are well-developed two-dimensional vanadia layers bound to titania.     

Enantioselective hydrogenation of ethyl pyruvate. Influence of oxidative treatment of cinchonidine-modified platinum catalyst and hemiketal formation in alcoholic solvents

The reasons for the increase in the rate and enantiomeric excess after oxidative (aerobic) treatment of Pt/alumina in ethanol have been investigated. It is demonstrated that this treatment results in the formation of acetic acid and consequently in the protonation of the quinuclidine n₁ of cinchonidine. This favours the cinchonidine-pyruvate interaction and improves enantioselectivity. In addition, the reaction rate is enhanced due to acid catalysis of the carbonyl reduction. NMR and UV measurements indicate the rapid transformation of ethyl pyruvate to the corresponding hemiketal in primary alcohols as solvents. It is shown that the possible involvement of this hemiketal (and that formed between cinchonidine and ethyl pyruvate) as an intermediate in the pyruvate hydrogenation mechanism can be excluded.     

Thermal ageing phenomena and strategies towards reactivation of NO x - storage catalysts

The thermal ageing and reactivation of Ba/CeO₂ and Ba/Al₂O₃ based NO _x -storage/ reduction (NSR) catalysts was studied on model catalysts and catalyst systems at the engine. The mixed oxides BaAl₂O₄ and BaCeO₃, which lower the storage activity, are formed during ageing above 850 °C and 900 °C, respectively. Interestingly, the decomposition of BaCeO₃ in an atmosphere containing H₂O/NO₂ leads again to NO _x -storage active species, as evidenced by comparison of fresh, aged and reactivated Pt-Ba/CeO₂ based model catalysts. This can be technically exploited, particularly for the Ba/CeO₂ catalysts, as reactivation studies on thermally aged Ba/CeO₂ and Ba/Al₂O₃ based NSR catalysts on an engine bench showed. An on-board reactivation procedure is presented, that improved the performance of a thermally aged catalyst significantly.     

Harmful by-products in selective catalytic reduction of nitrogen oxides by olefins over alumina

The selective reduction of nitrogen dioxide and nitrogen monoxide by olefins (ethene, propene) has been studied over two different -aluminium oxides in the temperature range 473–873 K. Nitrogen dioxide was reduced more effectively than nitrogen monoxide with both, ethene and propene, as a reductant. At temperatures exceeding 700 K, ammonia was formed as a by-product over one type of alumina. Concentrations in the range 30–40 ppm were determined for propene in combination with both, NO and NO₂, while no ammonia was produced with ethene as a reductant. In addition, significant formation of hydrogen cyanide up to 70 ppm was observed with propene over both aluminium oxides starting from either NO or NO₂. In contrast, hydrogen cyanide formation remained below 10 ppm with ethene as a reductant. Nitrous oxide formation did not exceed 10 ppm for all investigations. The results show that for alumina catalysts ethene is a more suitable reductant than propene due to its lower tendency to form undesired by-products.     

Titania aerogels prepared by low-temperature supercritical drying. Influence of extraction conditions

Titania aerogels with meso- to macroporosity and high specific surface area were prepared by varying the conditions of semicontinuous extraction of methanolic titania gels with CO₂. The conditions varied were extraction temperature, extraction duration, and CO₂ in liquid or supercritical state. The resulting titania aerogels were characterised by means of nitrogen physisorption, X-ray diffraction, thermal analysis and transmission electron microscopy. All uncalcined aerogels contained significant amounts of organic residues (12–14 wt% elemental carbon), and remained X-ray amorphous during calcination in air up to 673 K. Thermoanalytical studies showed that crystallization generally occurred in the range 730–745 K. The variation of the extraction temperature at either constant density or pressure of CO₂, the use of either liquid or supercritical CO₂, and the duration of extraction greatly influenced surface area, pore size distribution, and pore volume. The highest specific surface area (623 m² g^–1) and nitrogen pore volume (4.0 cm³ g^–1) were obtained, if the density of supercritical CO₂ corresponded to that of methanol at the lowest temperature applied (313 K). The studies indicate that textural properties can be varied over a wide range by choosing appropriate extraction conditions.     

Probing Active Sites During Palladium-Catalyzed Alcohol Oxidation in “Supercritical” Carbon Dioxide

Structural information has been gained during aerobic benzyl alcohol oxidation in “supercritical” carbon dioxide at 150 bar on alumina-supported palladium by X-ray absorption spectroscopy while monitoring simultaneously the performance of the catalyst. The reduction of the catalyst by benzyl alcohol could be monitored by the analysis of the near-edge region of the Pd K-edge. The palladium constituent was mainly in metallic state under operating conditions. Partial reoxidation was observed when only oxygen in “supercritical” carbon dioxide in the absence of alcohol was fed. The catalytic activity of the PdO_x/Al₂O₃ catalyst during benzyl alcohol oxidation was comparable to that in a conventional continuous fixed-bed reactor and depended on the oxygen concentration in the feed. The rate of alcohol conversion went through a maximum when the oxygen concentration was increased. At maximum rate, part of the palladium was in the oxidized state. Upon further increase of the oxygen concentration, the activity decreased because of the formation of surface palladium oxide. The reaction rate in “supercritical” carbon dioxide was strikingly higher than that observed for the corresponding liquid-phase oxidation.