Untersuchungen an oxidischen Katalysatoren. XXXIV. Redoxverhalten des Nickels in NiNaY-Zeolithen 1. Reduzierbarkeit und Reoxydierbarkeit von Nickel in NiNaY-Zeolithen

Studies on Oxide Catalysts. XXXIV. Redoxbehaviour of Nickel in Zeolite NiNaY. 1. Reducibility and Reoxidizability of Nickel in Zeolites NiNaY The properties of metallic nickel in reduced (470–870 K) and reoxidized (470, 670 K) samples were studied by chemical analysis (reaction with K₂Cr₂O₇) and spectroscopic methods (FMR, IR after CO adsorption, UV/VIS). The reduction of Ni²⁺ cations from oxidic clusters proceeds in an onestep reaction. Contrary to this, isolated Ni²⁺ cations are reduced stepwise to Ni⁺ cations and subsequently to metallic nickel. The reduction degree depends in characteristic manner on the reduction temperature. Metallic nickel which was reduced at temperatures < 620 K, can be completely reoxidized at 470 K. Higher temperatures result in metallic aggregations which are not completely reoxidized even at 670 K.     

Study of the state of nickel in ion-exchanged NiNaY zeolite by IR spectroscopy using molecular hydrogen adsorbed at low temperatures

The state of nickel cations in NiNaY zeolite subjected to thermal vacuum and oxidative-reductive treatment was analyzed using diffuse reflectance IR spectra of molecular hydrogen, CO, and hydroxy groups. In a sample dehydrated at 570 K, nickel forms oxide particles, which are presumably located in big cavities of a zeolite. Heating of the sample at 670 K results in decomposition of the oxide with the participation of hydroxy groups followed by the formation of stabilized Ni²⁺ cations in large cavities of the zeolite. Calcination of NiNaY zeolite in a hydrogen atmosphere at 730 K leads to Ni²⁺ cation reduction to the metal state and to the formation of acidic hydroxy groups. Nevertheless, Ni²⁺ cations can be regenerated by reoxidation of the reduced sample with oxygen at 670 K and further evacuation at 870 K. Thermal destruction of NiO in the reoxidized sample also proceeds on acidic hydroxy groups, but the temperature of this process is higher than the temperature of oxide decomposition in the initial NiNaY zeolite. The IR spectra of molecular hydrogen complexes with Ni²⁺ cations were observed for the first time. A considerable decrease in the H-H stretching vibration frequency for the coordinated molecules (up to 210 cm’1) can be explained by electron density redistribution between a hydrogen molecule and the cation in the complex     

Charakterisierung und katalytische Aktivität von Ni2+-ausgetauschten X- und Y-Zeolithen. I. TPR-Untersuchungen an NiNaX- und NiNaY-Zeolithen

Charaterization and Catalytic Activity of Ni²⁺ Exchanged X and Y Zeolites. I. TPR Studies on NiNaX and NiNaY Zeolites . The structure of TPR spectra of NiNaX and NaNiY zeolites variously exchanged is determined by the location of the cations. In case of X zeolites a peak appears with a maximum at 750–800 K (reduction on S_II and S_I, positions) and for higher exchange degrees an additional one at about 1000 K (reduction on S_I positions). Three ranges of reduction may be separated in case of Y zeolites (reduction on S_II, S_I′, and S_I). With increasing Si/Al ratios the maximum of the hightemperature peak is shifted to higher temperatures. The reduction at temperatures up to 800 K resulted in higher reduction degrees for X reolites while the overall reduction up to high temperatures led to higher reduction degrees for Y zeolites. The kinetic analysis by means of two different methods yielded the following activation energies: (85 ± 10) or (86 ± 2) kJ/mole, respectively, for the low-temperature peak, and (223 ± 12) or (214 ± 2) kJ/mole, respectively, for the high-temperature peak.     

Untersuchungen an Metallkatalysatoren. XXXII. Zur Legierungsbildung und Dispersität von Nickel-Rhenium-Katalysatoren

Investigations on Metal Catalysts. XXXII. On Alloying and Dispersion of Nickel-Rhenium Catalysts Unsupported Ni? Re catalysts were prepared by reduction of mixtures from NiO and NH₄ReO₄ at 400°C with hydrogen (1^st series), followed by a heat treatment at 650°C in flowing hydrogen (2^nd series). The bimetallic powders were characterized by DTA investigations, X-ray measurements, N₂ adsorption, and CO chemisorption. The degree of alloying and the changes in dispersion as a result of adding a second metal to a basic one is discussed.     

Untersuchungen an oxidischen Katalysatoren. XLIII. Thermoanalytische und katalytische Untersuchungen an platinhaltigen H-ZSM-5-Zeolithen

Studies on Oxide Catalysts. XLIII. Thermoanalytic and Catalytic Investigations of Platinum-Containing Zeolites H-ZSM-5 By impregnation with H₂PtCl₆ from ethanolic solution, after reduction a higher metal dispersion in zeolites Pt/H-ZSM-5 is achieved than by impregnation from aqueous solution. Over such samples, bifunctional catalyzed reactions are more favoured than over zeolites Pt/H-ZSM-5 obtained by impregnation with H₂PtCl₆ from aqueous solution.     

Untersuchungen an oxidischen Katalysatoren. XXVIII. Einfluß der Vorbehandlungsbedingungen auf Die katalytischen Eigenschaften von NiNaY- und NiCoNaY-Zeolithen

Studies on Oxide Catalysts. XXVIII. Influence of Pretreatment on Catalytic Properties of Zeolites NiNaY and NiCoNaY The influence of the pretreatment conditions on the acidic properties, the reduction degree of nickel, and the catalytic activity and selectivity of zeolites NiNaY and NiCoNaY in the alkylation of benzene with ethylene has been investigated. From the experimental results can be concluded that sec. buthylbenzene from benzene and ethylene is only formed on zeolite samples which contain both together BRÖNSTED acidic centres and Ni²⁺ cations or NiO. Catalysts which contain only metallic nickel are inactive for this reaction. Catalysts with nickel in different forms are active in the formation of ethylbenzene, diethylbenzene, butane and sec. buthylbenzene, the yields depending on the relation of Ni°, Ni²⁺, and NiO. Changing this relation makes possible to regulate the selectivity in the alkylation of benzene with ethylene.     

Temperature-programmed reduction study on carbon-supported platinum-gold alloy catalysts

Alloy catalysts of Pt-Au/C with different Pt/Au ratios were prepared by the precipitation-deposition of metal chlorides and reduced by H(2) at 470 K. The surface composition of alloy crystallites deposited on the prepared catalysts was characterized by a technique of temperature-programmed reduction (TPR). In the characterization, O(2) was chemisorbed on the reduced catalysts and the chemisorbed O(2) was reduced by TPR. A low-temperature routine (LT) in the temperature range between 120 and 430 K was used for the TPR characterization. Monometallic catalysts of Au/C and Pt/C showed a reduction peak in the LT-TPR at reduction temperature (T(r))=145 and 240 K, respectively. T(r) from alloyed catalysts fell in the range and increased monotonously with their Pt/Au ratios. Interior Pt atoms in deposited alloy particles tended to segregate toward their surface during oxidation treatment at elevated temperatures.     

Hydroxidgruppen an Zeolithen. III. Eigenschaften von Hydroxidgruppen an ZnNaY-, CuNaY-, NiNaY- und CrNaY-Zeolithen

Hydroxide Groups on Zeolites. III. Properties of Hydroxide Groups on ZnNaY, CuNaY, NiNaY, and CrNaY Zeolites The properties of hydroxide groups in dependence on the degree of exchange were studied by IR spectroscopy of ZnNaY, CuNaY, NiNaY, and CrNaY zeolites. Five kinds of hydroxide groups occur on these zeolites: Hydroxid groups limiting the lattice, 3 kinds of structural hydroxide groups, and MeOH⁺ groups. These are the same kinds of hydroxide groups as exist on alkaline earths-Y-zeolites. Some of the OH groups act as acid Brönsted centers. The number of acid Brönsted centers reaches maximum values at degrees of exchange of 40–50% after a pretreatment at 300–400°C. The zeolitic structure is partially destroyed in ZnNaY, CuNaY, and especially CrNaY, at high degree of exchange.     

Some observations on the liquid immiscible nature of the Iron-Indium and Aluminium-Indium Systems

Hydroxide Groups on Zeolites. IV. Formation of Hydroxide Groups by Reduction of Y-Zeolites Containing Transition Metal Ions The formation of hydroxide groups as result of the reduction is studied by IR spectroscopy on NiNaY, CuNaY, and ZnNaY zeolites. After transformation of the OH groups into OD groups it was found that D₂ reduces the transition metal ions at 350°C and that simultaneously the concentrations of two types of the hydroxides groups which are already present before this treatment are increased markedly, viz. the concentration of an acid species and that of a weakly acid species. In NiNaY and CuNaY zeolites, the reduction at 350°C proceeds much more slowly than in ZnNaY zeolites. The reduction of ZnNaY zeolites is connected with their transformation to ultrastable zeolites.     

Untersuchungen an Nickel-Zeolith-Trägerkatalysatoren. I. Herstellung und Charakterisierung von Nickel-Zeolith-Trägerkatalysatoren

By means of cation exchange nickel zeolites of the types A, X and Y were prepared with different (0.5–12.5% of weight) nickel, contents. Nickel zeolites of the A type up to 14% exchange and zeolites of the X type up to 27% exchange do not show changes in structure. Nickel zeolite catalysts were prepared by reduction of the nickel zeolites with hydrogen. Analyses of nickel metal in these catalysts showed that only part of the nickel cations is reduced. By means of hydrogen chemisorption it could be proved that the metallic nickel does not exist in atomic dispersion and is deposited on the external surface as well as inside the pores.     

制备条件对Ni在Al_2O_3表面上的分配形态及Ni/Al_2O_3体系催化加氢性能的影响

用比表面和孔体积测定、XRD和程序升温还原(TPR)方法研究了一系列不同制备条件对Ni在Al_2O_3表面上的分配形态及其对CO加氢反应催化性能的影响.实验发现,本文所用制备条件对催化剂的比表面、孔体积及孔径分布无显著的影响.浸渍时间愈长,催化剂上能在较低温度下还原的物种愈少,催化剂上CO的加氢活性愈低;在浸渍前对载体作室温抽空处理后制得的催化剂上,Ni在Al_2O_3表面是高度分散的,这一高度分散物种易于在低温下还原,使该催化剂具有很高的CO加氢活性和生成CH_4选择性;载体的抽空温度及对载体加热均对Ni物种形态及其分配、催化剂性能有明显影响.     

Study of reduction kinetics of mixed oxides NiO-V2O5 with hydrogen

The reduction with hydrogen of mixed oxides NiO-V₂O₅ of various composition was studied at 350–470? by means of thermogravimetry. With increasing content of V₂O₅ the reduction rate and the oxidation state of vanadium in the reduction product decrease. A correlation has been found between some physico-chemical properties of the system and reduction kinetics which can be quantitatively described by an Erofeev equation. The mutual interaction of the two components is demonstrated by the non-monotonous dependences of the reoxidation, secondary reduction kinetics and rate of reduction of thermally treated samples on the composition.     

Redox Treatment Effects on the Magnetic Properties of Copper Chromite

Magnetic susceptibility and low-temperature neutron diffraction measurements were performed to study the effect of redox treatments of copper chromite on its magnetic properties. When copper chromite is subjected to reduction at 453-643 K, magnetic ordering with T _c = 130 K disappears. Reoxidation does not completely restore the magnetic properties. The changes in the magnetic properties of copper chromite induced by sequential reduction and reoxidation correlate with its structural changes.     

Effect of the Phase of Titanium Oxide on Metal-Support Interaction

The metal-support interaction in the system of platinum supported on titanium oxides of different crystalline structures was investigated. Titanium oxide powders used as supports were prepared by neutralization of TiOCl₂-analogue solution with either NaOH or NH₃ base at varied pH. Pt was introduced onto the supports by impregnation. The phenomenon of strong metal-support interaction (SMSI) was determined by the suppression of chemisorption of H₂ over samples reduced at 773 K, in comparison with the behavior of samples reduced at 473 K. The loss of adsorption capacity was proved not due to metal sintering by examining the recovery of adsorption capacity after reduction, reoxidation and reduction cycle, accompanied with examination of the size of Pt particle with an x-ray diffractiometer and a transmission-electron microscope. The results showed that SMSI phenomenon was common to Pt supported on titanium oxides of all the varied crystalline structures. However, the chemisorption capacity of Pt reduced at 473 K varied greatly with the sodium content in the supports, which in turns affects the crystalline structure of titanium oxide. These results are attributed mainly to electronic effect endowed by the support.     

Untersuchungen an oxidischen Katalysatoren. XLI. Redoxverhalten des Nickels in NiNa–Y-Zeolithen 3. Reduzierbarkeit von Ni2+ -Ionen und Eigenschaften des reduzierten Nickels in aciden NiNa–Y-Zeolithen

Studies on Oxide Catalysts. XLI. Redox Behavior of Nickel in Zeolites NiNa? Y. 3. Reducibility of Ni²⁺ Ions and Properties of the Reduced Nickel in Acidic Zeolites NiNa? Y The reducibility of nickel ions in zeolites NiNa? Y and the properties of metallic nickel were evaluated by tpr measurements, oxygen chemisorption and conversion of cyclohexane. In NiNa? Y samples which contain NH₄⁺(H⁺) and/or Al³⁺(H⁺) ions the reducibility of Ni²⁺ ions is decreased caused by increasing acidity and the metal dispersion is improved. The electronic interaction between the acidic support and the metallic nickel leads to a decrease of both dehydrogenation and hydrogenolysis activity whereas the dehydrogenation selectivity increases.     

Relationship between structure and CO oxidation activity of ceria-supported gold catalysts

Gold catalysts supported on cerium oxide were prepared by solvated metal atom dispersion (SMAD), by deposition-precipitation (DP), and by coprecipitation (CP) methods and were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). The catalytic activity was tested in the CO oxidation reaction. The structural and surface analyses evidenced the presence of a modified ceria phase in the case of the DP sample and the presence of pure ceria and gold metal crystallites in the case of the SMAD and CP samples. The DP sample, after a mild treatment in air at 393 K, exhibited only ionic gold, and it was very active below 273 K. By comparing the activities of the different catalysts, it is suggested that the presence of small gold particles, as obtained by the SMAD technique, is not the main requisite for the achievement of the highest CO conversion. The strong interaction between ionic gold and ceria, by enhancing the ceria surface oxygen reducibility, may determine the particularly high activity.     

Surface characterization of silica-supported cobalt oxide catalysts

Silica supported cobalt oxides were prepared by the impregnation method, using an aqueous solution of cobalt nitrate hexahydrate (Co(NO₃)₂ · 6H₂O), then calcined at different temperatures (510, 620 and 870 K). Characterization of the samples was carried out by X-ray diffraction, N₂-adsorption at −196°C, UV–Vis diffuse reflectance spectroscopy and KBr-IR spectroscopy of the calcination products. The surface acidity was studied by IR spectroscopy of adsorbed pyridine at different temperatures (300, 370, 470 and 570 K). Results indicated that Co₃O₄ is the stable phase on silica, however, dispersion of minor amount of cobalt oxide could not be ruled out. Results also indicated that the crystallinity of the formed Co₃O₄ increased by increasing the loading level and/or the calcination temperature. Furthermore, the surface area of the support was decreased by increasing the loading level and the calcination temperatures. It has been also found that the surface of the supported catalysts exposed strong different Lewis acid sites.     

Na3OsH7 – Synthese,Struktur und magnetische Eigenschaften,sowie Untersuchungen zur Existenz einer analogen Rutheniumverbindung

Na₃OsH₇ – Synthesis, Structure, and Magnetic Properties as well as Investigations on the Existence of the Analogous Ruthenium Compound Na₃OsH₇ was synthesized by the reaction of sodium hydride with osmium powder under a hydrogen pressure of more than 1500 bar at 870 K. X‐ray investigations on powdered samples and elastic neutron diffraction experiments on the deuterated compound led to the atomic arrangement (space group: P4₂/mnm), which is characterized by isolated [OsD₇]‐anions. The coordination polyhedron formed by the seven deuterium ligands can be described as a distorted pentagonal bipyramide. Magnetic susceptibility measurements in the temperature range between 3.5 K and room temperature revealed a weak temperature independent paramagnetism. Quantum mechanical calculations confirm these facts and show in detail that the large value of the spin‐orbit coupling constant is responsible for the magnetic behaviour of the osmium (IV) compound. To synthesize the analogous ruthenium hydride it was necessary to increase the hydrogen pressure during the reaction up to 5000 bar. X‐ray investigations showed that Na₃RuH₇ crystallizes in an atomic arrangement isotypic to that of the osmium compound.     

Metal-Support interaction in Silica-Supported Cobalt Catalyst

The variations in catalytic properties for CO hydrogenation and adsorptive properties for H₂ and CO of silica-supported cobalt catalyst as a function of reduction temperature (400 to 700 °C) were investigated. A mild sintering of cobalt metal from 130 Å to 145 Å occurred only when the reduction temperature was further increased from 600 to 700 °C. However, a monotone decrease in the activity (per gram Co) for CO hydrogenation was observed with increasing reduction temperature. This is accompanied with a decrease in methane and an increase in olefin formation. The decrease in activity can not be accounted for by the variation of crystallite size of cobalt metal. A concurrent suppression in the amount of hydrogen and CO adsorptions was observed after reduction temperature was raised to 700 °C. The turnover frequencies, based on cobalt dispersion estimated from H₂ and CO chemisorption, remained, more or less, similar with increasing reduction temperature from 400 to 600 °C, but increased ca. 3 and 1.5 times, respectively, at 700 °C. The relative amount of subcarbonyl species (2062 cm^?1) with respect to the linear adsorbed CO (2010 cm^?1) was found to increase as well. Furthermore, reduction of silica surface at 700 °C was indicated by the presence of ?Si-H, which was detected by infrared spectroscopy at 2294 cm^?1. This shows the presence of metal-support interaction for silica-supported cobalt systems after reduction treatment at 700 °C.     

Charakterisierung und katalytische Aktivität von Ni2+-ausgetauschten X- und Y-Zeolithen. II. Die Reduzierbarkeit des Ni2+ durch niedere Olefine und die Dimerisierungsaktivität der Ni2+-ausgetauschten Zeolithe

Characterization and Catalytic Activity of Ni²⁺ -X and -Y Zeolites. II. Reducibility of Ni²⁺ by Low Olefines and the Dimerization Activity of the Ni²⁺ -Zeolites The reducibility of Ni²⁺ in X and Y zeolites by hydrogen, but-1-ene, propene, and ethene is compared. The degree of reduction was determined after isothermal reduction and reoxidation by the TPR method. At 673 K on X zeolites the reducibility decreases in the order: H₂ > but-1-ene, propene > ethene. On Y zeolites an inversion takes place: but-1-ene, propene > H₂, ethene. The mechanism of reduction by olefins should be determined by an intermediate splitting off of a hydride ion as a reducing species. Such a mechanism explains the higher degree of reduction in the more acid Y zeolites. Assuming low valent nickel as an active center in ethen dimerization the induction period results from the reduction of Ni²⁺ ions.