Technical Catalysts Research Articles

On the characterization of surface VOx-species by XPS: layered oxidic systems as model catalysts

The influence of the support on vanadia, immobilized on Au, SiO2, Al2O3 and TiO2, suitable for the selective catalytic reduction (SCR) of nitric oxide by ammonia has been investigated. The layered oxidic substrates, which are deposited onto gold coated silicon wafers, represent model systems for the surfaces of technical SCR catalysts, that are suitable for surface science investigations. The oxidation states of the surface vanadia species immobilized on these samples have been studied by XPS after well-defined sample treatment in both oxygen and hydrogen atmospheres at 573 K and 723 K. From the observed shifts of the V 2p3/2 binding energies, and from peak deconvolution, vanadia layers on gold, silica and alumina are found to exhibit characteristics similar to those of bulk vanadia. This behavior is characterized by the relative stability against a reductive atmosphere at 573 K. Changes in the oxidation states of the surface vanadia species are prominent only after treatment at 723 K. Titania exhibits a unique support interaction with vanadia which allows the reduction of vanadia at temperatures as low as 573 K. This fact may contribute to the outstanding performance of VOx/TiO2 in the selective reduction of nitric oxide by ammonia in waste gases.

Preparation and surface characterization of silica-supported ZrO2 catalysts; comparison of layered model systems with powder catalysts

Model supports consisting of a thin layer of SiO 2 on a silicon single crystal have been used to study ZrO 2 /SiO 2 /Si model systems made by the same wet chemical preparation methods as used in the preparation of technical catalysts. The results obtained with the models are in good agreement with those on ZrO 2 catalysts on porous silica supports

Nuclear quadrupole interaction of 99Mo(β −) 99Tc in various molybdenum sulfur cluster compounds and thermal decomposition products of (NH 4) 2 [Mo 3S(S 2) 6]· nH 2O ( n=0–2)

The nuclear quadrupole interaction (NQI) of 99Mo(β −) 99Tc was measured by time differential perturbed angular correlations (TDPAC) in the following MoS-cluster compounds and ions: (NH 4) 2 [Mo IV 3S(S 2) 6· nH 2O ( n = 0–2), [Mo IV 3SS 3 (CN) 9] 5−, (NH 4) 2[Mo V 2(S 2) 6]·2H 2O and [Mo III 2S 2 (CN) 8] 6−. All compounds exhibit low NQIs with the exception of (NH 4) 2 − [Mo IV 3S(S 2) 6]· nH 2O which shows in addition to the low NQI a large fraction of a high NQI, possibly a consequence of the nuclear transmutation. In addition, we studied the thermal decomposition products of the [Mo 3S(S 2) 6] 2− cluster towards MoS 2.15, which may serve as a model for technical hydrodesulfurization catalysts.

Materials from controlled exothermic reactions

Technical ceramics, shape‐memory alloys and catalysts are among teh materials which can be produced using self‐propagating high‐temperature synthesis (SHS). The high‐temperatures and synthesis rates result in self‐purification, self annealing and self‐purfication phenomena which lead to materials which differ considerably in structure from those conventionally produced. The method and its applications are described by one of its discoverers.

The nature of the iron oxide-based catalyst for dehydrogenation of ethylbenzene to styrene: I. Solid-state chemistry and bulk characterization

The active catalyst for the dehydrogenation of ethylbenzene is generated from a precursor material consisting of hematite and potassium hydroxide (with additional promotors) during the initial phase of catalyst operation at 873 K in a steam atmosphere. The active phase is a thin layer of KFeO 2 supported on a solid solution of K 2Fe 22O 34 in Fe 3O 4. The ternary K 2Fe 22O 34 phase acts as storage medium from which the active surface is continuously supplied with a near-monolayer coverage of potassium ions in an environment of Fe 3+ ions. The catalyst undergoes a continuous solid-state transformation caused by the migration of potassium ions. This requires a certain degree of imperfection in the matrix lattice which originates from the catalyst preparation and from the addition of promotors which act on the iron oxide lattice rather than on the surface chemistry. The identity of the active phase with KFeO 2 was confirmed by independent synthesis of this phase and comparison of its catalytic activity with that of the technical catalyst.

Automotive exhaust gas catalysts: Surface structure and activity

To elucidate the influence of ceria promotion with three-way catalysts for automotive emission control, a standard set of samples with and without precious metals (platinum, rhodium) and with different alumina-to-ceria ratios were characterized by surface analytical methods and performance and activity measurements. The results permit some insight into the chemical and structural composition of these technical catalysts, but the currently available methods do not allow a surface characterization sufficient to be related to elementary catalytic steps. Under reaction conditions, the catalyst surface, especially ceria and rhodium, turns out to be extremely variable, thus admitting a clear surface definition only in steady-state modes and not in real operation.

In-situ studies of catalysts by XAFS and Mössbauer spectroscopy

The unique advantages of XAFS and Mossbauer spectioscopy applied to studies of catalysts are discussed. These advantages include the importantin situ capability, as well as the possibility to provide information on X-ray amorphous materials. The successful use of the two techniques is demonstrated by examples of two important technical catalyst systems, hydrotreating and ammonia synthesis catalysts, representing supported and unsupported catalysts.

Zeolites: Catalysts for Organic Syntheses

AbstractZeolites have proved to be valuable technical catalysts in petrochemistry and in oil processing. The characteristic properties of zeolites, such as acidity, shape‐selectivity and thermal stability also enable them to be used for highly selective synthesis in the fields of chemical intermediates and fine chemicals. This interesting area of application has grown continuously in recent years. The present article summarizes the various standard types of reaction involved in organic syntheses which can be catalyzed by zeolites; these include, inter alia, electrophilic and nucleophilic substitution reactions, isomerization of double bonds and carbon skeletons, as well as addition, elimination and hydrogenation reactions.

Disperse Festkörper als Katalysatoren

AbstractThe activity of a catalyst increases as the degree of dispersion of the active component increases. In the case of supported catalysts it is possible to achieve dispersion right down to single active atoms. On the other hand, the smallest individual units of pure substances which can be obtained are generally 1 — 10 nm crystals. These finely dispersed systems can be stabilized by admixture with just small quantities of foreign substances. At temperatures above 600°C certain metal catalysts are capable of undergoing spontaneous dispersion under reaction conditions. — The degree of dispersion and the specificity of a catalyst are interrelated in a complex manner. Certain so‐called demanding reactions proceed most readily on finely dispersed systems (with low‐indexed crystallographic faces), others on coarsely dispersed active components (with predominantly highly‐indexed faces). — Although the selectivity of reactions taking place on model catalysts frequently correlates with the degree of dispersion, such a relationship has only seldom been observed in the case of technical catalysts.

L-2-hydroxyisocaproate dehydrogenase?A new enzyme from Lactobacillus confusus for the stereospecific reduction of 2-ketocarboxylic acids

A new dehydrogenase from Lactobacillus confusus has been purified. The following reactions are catalyzed by this enzyme: $$\vec F_\alpha$$ Various 2-ketocarboxylic acids are stereospecifically reduced to the corresponding l-2-hydroxycarboxylic acids. In the reverse reaction the NAD dependent dehydrogenation of l-2-hydroxycarboxylic acids is observed. l-2-hydroxyisocaproate seems to be the best substrate for this enzyme which we therefore call l-2-hydroxyisocaproate dehydrogenase (l-HicDH). The enzyme requires NAD(H) as a cofactor, which cannot be replaced by NADP(H). The large scale purification of the enzyme is described starting with 24 kg wet cells, it involved homogenization using a continuously operating high speed bead mill, liquid-liquid extraction with aqueous two-phase systems and diafiltration followed by ion-exchange chromatography on DEAE-cellulose. At this stage the enzyme was purified 177-fold resulting in a specific activity of 30 U/mg. This technical catalyst can be used for the continuous production of l-2-hydroxycarboxylic acids in an enzyme membrane reactor. Further purification up to 450–480 U/mg can be carried out by interfacial salting out chromatography or affinity chromatography, respectively. Properties of the purified enzyme were investigated in detail especially the parameters which are important for an industrial application.

A combined instrument for the investigation of catalytic reactions by means of gas chromatography, secondary ion and gas phase mass spectrometry, Auger and photoelectron spectroscopy, and ion scattering spectroscopy

A combined instrument has been developed which couples a microreactor operated with technical catalysts with a combined surface analytical instrument. The microreactor is equipped with gas chromatography and gas phase mass spectrometry for continuous control of the reaction parameters. The surface analytical part of the instrument incorporates secondary ion and thermal desorption mass spectrometry, Auger and photoelectron spectroscopy, and ion backscattering spectroscopy, all of which have been developed to their full versatility and efficiency. The transfer of the catalyst from the reactor to the surface analysis station is performed by a valveless lock within about one minute in a vacuum free of organic contaminations. The whole system can be operated fully under computer control. First applications of this instrument have been devoted to the oxidation of methanol to formaldehyde on silver catalysts.

Effects of γ radiation on the surface and catalytic properties of NiO, Co 3O 4 and Fe 2O 3-Cr 2O 3 solids

The effects of different doses of γ radiation between 5 and 20 Mrad on the surface and catalytic properties of NiO, Co 3O 4 and a technical catalyst (denoted by Fe 2O 3-Cr 2O 3) were investigated. NiO was prepared by thermal decomposition of Ni(OH) 2 in air at 300 °C; Co 3O 4 specimens were obtained by the thermal dissociation of basic cobalt carbonate at 500 and 700 °C. Fe 2O 3-Cr 2O 3 was a technical catalyst employed in the CO-steam reaction. The catalytic activity of NiO in the oxidation of CO at room temperature decreased drastically after irradiation with the smallest dose in spite of the observed increase (19.5%) in its surface area. An increase (about 25%) in the catalytic activity of Co 3O 4 was found when the samples were irradiated with a dose of 5 Mrad, and the catalysis of CO oxidation was carried out at 145 °C and 220 °C on the Co 3O 4 calcined at 500 °C and 700 °C respectively. When the dose attained 20 Mrad the activity of the Co 3O 4 samples decreased, reaching values slightly higher, however, than those of the unirradiated samples. In contrast, γ irradiation led to a decrease in the surface area of the Co 3O 4 samples to an extent proportional to the dose employed. No appreciable change in the catalytic activity of Fe 2O 3-Cr 2O 3 in CO oxidation at 200 °C was observed after irradiation with doses up to 20 Mrad. These doses, however, effected a decrease (13%) in the surface area of the solid.

Calorimetric determination of the heat of adsorption of nitrogen on a technical iron catalyst for ammonia synthesis at 400°C

The heat of adsorption of nitrogen on a technical iron catalyst at 400°C has been found to be 209 kJ/mol at small coverages. The measurements have been carried out in a Calvet high temperature microcalorimeter.

Cobalt- and nickel-molybdenum catalysts: Their properties and applications for hydrodesulphurisation

Sulphur compounds in crude oil cuts can be divided into those which are easy and those which are difficult to desulphurise owing to steric effects. When the degree of desulphurisation is plotted against the reciprocal volume hourly space velocity we obtain a curve which may be regarded as being composed of several straight lines with different gradients. Their slopes correspond to the different rates with which the two groups of sulphur compounds are removed. For technical catalyst tests, such conversion-throughput graphs are used to determine the activities (by volume or by weight) of different catalysts. The industrial manufacture of hydrodesulphurisation catalysts can be divided into the following steps: preparation of the carrier, addition of the active compounds, calcination and sulphiding. Each step influences the activity. The usual starting material for the carrier is gelatinous boehmite which is converted to γ-Al 2O 3 by heat treatment. η-Al 2O 3 is less suitable as a carrier. The chemical and physical properties of the carrier are important in determining the way in which the metal oxides are incorporated and hence the chemical activity. The pore size distribution also has to be adjusted to the particular hydrodesulphurisation problem. A precise prediction of the most favourable Co(Ni):Mo ratio is not possible. The ratio depends mainly on the properties of the carrier and the way in which the metal oxides are added. The most active catalysts are those prepared by impregnation of alumina with solutions of molybdate and cobalt (or nickel) salts. The influence of promoters has been investigated but efficient promoters for the Co-Mo catalyst have not been found. The activity of the Ni-Mo catalyst can, however, be increased by phosphoric acid. The oxide catalyst is activated by pre-heating in an atmosphere of hydrogen and hydrogen sulphide. The sulphiding must be started at low temperatures (about 250 °C) in order to avoid reduction of the oxides before conversion to the sulphides or more likely to oxysulphides. A partial reduction to the metals before sulphiding results in a considerable loss of activity. Compared with Co-Mo catalysts, Ni-Mo catalysts possess. at a given level of hydrodesulphurisation activity, a higher activity for the reduction of nitrogen compounds and the hydrogenation of unsaturated compounds; they are preferred for hydrogenation of aromatics and removal of nitrogen.

Very Small Metallic and Bimetallic Clusters: The Metal Cluster-Metal Surface Analogy in Catalysis and Chemisorption Processes

Abstract Why very small metal clusters? From a practical standpoint, industry often wants its metallic catalysts in a finely divided form so as to maximize the surface area available for a given mass of metal. Obviously, stability toward agglomeration and reactant accessibility are also necessary prerequisites for the successful performance of highly dispersed metal particle catalysts. Solid supported catalysts usually achieve this end. Moreover, such catalysts offer the advantage of convenient handling in a process system; however, technical metallic catalysts are usually ill defined and poorly understood, and better defined systems are necessary for fundamental experimental work.

The catalytic oxidation of 1-butene over bismuth molybdates: Promoters for the Bi 2O 3·3MoO 3 catalyst

The effect of the promoters BiPO 4, Fe 2O 3, and Cr 2O 3 on the catalytic activity of the moderately active Bi 2O 3·3MoO 3 catalyst ( 2 3 ) were investigated for the oxidation of butene to butadiene. All compounds if present in quantities equivalent to 10% (mole ratio's) show a pronounced promoting effect. For two of the three promoters, BiPO 4 and Fe 2O 3, it could be shown that for these concentrations the promoter action is caused by the formation of the far more active Bi 2O 3·MoO 3 (Koechlinite) phase ( 2 1 ). This conclusion is consistent with the observations that the 2 1 lines appear in the X-ray diagrams. The kinetics of the reaction shift to those earlier observed for the 2 1 phase and finally that the adsorptive properties for butene and butadiene become more similar to those earlier reported for 2 1 than for 2 3 . The catalytic properties of the system Bi 2O 3·3MoO 3 + Fe 2O 3·3MoO 3 were studied in the entire range from Bi to Fe. It was found that a new compound (X) is formed possessing an X-ray diagram not earlier found by us but occasionally reported on technical BiMo catalysts by other authors. The compound X exists along the entire range from Bi to Fe = 8 2 to 1 9 . It possesses a fairly strong activity for the butene-butadiene conversion and is also a selective catalyst.

The preparation and use of ultrathin metal films as model systems for highly dispersed supported catalysts

A technique is described for the preparation, under ultrahigh vacuum conditions, of ultrathin metal films for use as catalysts. These films have an average “thickness” of the order of a “monolayer” of metal on the substrate. It is suggested that film catalysts of this type may be used as models for highly dispersed technical metal catalysts. Their use is illustrated by the reactions of n-hexane on ultrathin platinum and nickel film catalysts.

The Function of the promotors in the Technical Ammonia Catalyst

AbstractAn attempt is made to give a phenomenological description of the relationships between the catalytic activity and the distribution of the promotors in the reduced ammonia catalyst. Investigations were carried out on series of catalysts whose compositions were gradually brought closer to that of technical ammonia catalysts by the addition to iron of one new activator (Al2O3, K2O, CaO, MgO, SiO2) at a time.

The Oxidation of Olefins with Palladium Chloride Catalysts

AbstractThe oxidation of olefins to carbonyl compounds with palladium compounds, especially the oxidation of ethylene to acetaldehyde, is at present carried out on a technical scale. The reaction takes place via a palladium‐olefin complex, the formation of which is inhibited by halide ions. Hydrolysis to the carbonyl compound is inhibited by hydrogen ions. The knowledge gained by studying the reaction of olefins with pure solutions of palladium salts allows important conclusions to be drawn concerning the action of technical catalyst solutions containing copper and palladium chloride.

ADSORPTION OF OXYGEN BY A SILVER CATALYST

An apparatus is described to measure volumetrically the amount of gas adsorbed by a catalyst at constant pressure and at exposure times as short as 0.1 min. The volumes of oxygen adsorbed by a technical silver catalyst at pressures of 200, 400, and 700 mm. and in the temperature range of 180° to 300°C. have been measured. Surface area determinations of 0.33 to 0.38 sq. meter per gm. of catalyst by physical adsorption and chemisorption methods show that each silver atom of the surface is associated with approximately one oxygen adatom at a monolayer coverage. The initial rate of adsorption of oxygen is directly proportional to its pressure. Observed transitions in the oxygen adsorption rates indicate that more than one type of oxygen complex occurs on the catalyst surface. The Elovich equation provides the best approximation of the adsorption rate data but is of limited applicability in interpretation of the mechanism of adsorption. Values of 22–29 kcal./mole and 17–25 kcal./mole have been calculated for the apparent activation energy and isosteric heat of adsorption respectively.