Industrial Oxide Catalyst Research Articles

MoVW mixed metal oxides catalysts for acrylic acid production: from industrial catalysts to model studies

(MoVW)5O14-type oxides were identified as the active and selective components in industrial acrylic acid catalysts. Tungsten is suggested to play an important role as a structural promoter in the formation and stabilization of this oxide. Vanadium is responsible for high catalytic activities but is detrimental for the stability of this oxide at the necessary high concentrations for optimum catalytic performance. The activity of mixed MoVW oxide catalysts for methanol, propene, and acrolein partial oxidation could be considerably improved, when the amount of the (MoVW)5O14-type oxide was increased by thermal annealing. A model is proposed on the basis of the correlation between Raman wavenumber and bond order and degree of reduction, which explains the observed different selectivities of MoO3−x and the (MoVW)5O14-type oxides in terms of metal–oxygen bond strengths, i.e. oxygen basicity and oxygen lability, respectively. According to this model, the (MoVW)5O14 mixed oxide catalyses partial oxidation because of its intermediate C–H activation and oxygen releasing oxygen functionalities. However, these (MoVW)5O14-type industrial oxidation catalysts are heterogeneous and highly complex systems. Their physicochemical characterization also revealed that their chemical bulk and surface compositions vary with thermal activation and oxygen potential. A core-shell model is suggested to describe the active catalyst state, the shell providing a high number of active centers, the core high electronic conductivity and ion mobility. The fact that the surface composition of such catalysts is considerably different from their bulk compositions, most probably implies that the “molecular structure” at their surface differs too considerably from their bulk crystal structure. Hence, the posed question about the active catalyst structure and its relation to its catalytic performance cannot unambiguously be explained by the crystallographic structure, but still remains unsolved.

Removal of nitrogen oxides from exhaust by selective reduction with hydrocarbons on industrial oxide catalysts

Under strong wind conditions, snowflakes break upon collision with a snow surface, and their fragments are blown away; this leads to the development of drifting snow. If these fragments accumulate, the density of the deposited snow would exceed that of new snow deposited under calm conditions. In this study, experiments on the processes of fracture and accumulation of snowflakes were conducted in a cold wind tunnel and field observations were also carried out.The experimental results obtained by using artificial snowflakes are as follows: snowflakes do not break if U1 < 2 m s− 1, where U1 is the wind speed at 1 m height. Within a certain wind speed range, 2 ≤ U1 < 3 m s− 1, snowflakes partially break and accumulate at the collision point. If U1 ≥ 5 m s− 1, snowflakes are completely decomposed into snow crystals, which are blown away. These wind speed values vary slightly with the hardness of the snow surface. The critical wind speed for both fracture and accumulation, U1 = 5 m s− 1, is close to the impact threshold of drifting snow.The observational results for natural snowflake collisions almost agreed with the experimental results. These findings can be used in a model of drifting snow to explicitly describe the effect of snowfall on drifting snow and they will be useful in understanding the wind speed dependence of the new snow density.

Oxide catalysts for ammonia oxidation in nitric acid production: properties and perspectives

This paper generalizes the results of long-term efforts aimed at research and development of industrial oxide catalysts for ammonia oxidation in the nitric acid production within two-bed (Pt gauzes+monolithic oxide layers) technology of the high pressure process. Main factors determining performance of precious metals and oxides in the high-temperature ammonia oxidation are considered. The surface oxygen bonding strength determined by the surface atomic structure appears to be the most important. From this point of view, existing approaches to synthesis of mixed oxide systems including perovskites with controlled nitric oxide selectivity and good stability in the high-temperature process of ammonia oxidation are analyzed. Main features of the bulk oxide monolithic catalysts production technology and principles of a two-bed system design based upon the process mathematical modeling are briefly outlined. Proven economic benefits of this technology recently commercialized in Russia at nitric acid plants are debated.

Investigation of H2S oxidation by oxygen on oxide catalysts in sulfur condensation conditions

H2S oxidation with oxygen has been studied on three industrial oxide catalysts (Fe−Cr−Zn, Cu−Cr−Al, V−Ti−Al). Thermodynamically possible changes in the composition of the catalysts have been evaluated. Regularities determining deep or partial oxidation of H2S have been found. Deep oxidation is connected to the presence of active oxygen on the catalyst surface; its removal results in a decrease of activity and increase of the sulfur selectivity. Oscillations caused by periodic adsorption-desorption of sulfur on the catalyst surface have been observed on the most active V−Ti−Al catalysts in oxygen excess.

ChemInform Abstract: Oxygen Spillover for the Design of Industrial Oxidation Catalysts

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Struktur‐Aktivitäts/Selektivitäts‐Beziehungen von Oxidationskatalysatoren

Structure‐Activity/Selectivity Correlation of Oxidation CatalystsVarious reactive forms of oxygen are active in the selective oxygen functionalization of olefins and aromatic compounds.These forms are different on different crystal faces of transition metal oxides as shown by their different cation‐oxygen bond lengths. Therefore, face specifity of transition metal oxides as catalysts can be expected for the selective oxidation of hydrocarbons.Furthermore, the reactivity of the framework oxygen in the transition metal oxides for selective oxygen functionalization of hydrocarbons is dependent on the nature of the catalytic cycle (one‐ or two‐electron‐processes).Therefore, structure sensitivity of oxidation catalysts on the selectivity of transition metal oxides as oxidation catalysts is possible. Starting from this concept the following phenomena of the activity/selectivity relations are discussed for the most important industrial oxidation catalysts: —ensemble effects on the selectivity of supported Ag catalysts for the oxidation of ethylene to ethylenoxide. —face specifity of multicomponent Bi/Mo‐oxide catalysts in the selective oxidation of propylene to acrolein and in the ammoxidation of propylene. —structure sensitivity of V2O5‐containing catalysts in the oxidation of aromatic compounds.

The role of mobile oxygen species in the selective oxidation of isobutene

A simple experiment can introduce this contribution: SB{sub 2}O{sub 4} particles on the one hand, and SnO{sub 2} particles on the other hand, are prepared independently, and then mixed at room temperature. A comparison of the methacrolein yields in isobutene oxidation over identical weights of mixtures of different compositions indicates a dramatic synergy: for adequate proportions (Sb{sub 2}O{sub 4}/(Sb{sub 2}O{sub 4} + Sno{sub 2}) = 0.3-0.6) the yield is multiplied by a factor of ten, compared to the added yields which would be reached with the corresponding weights of the simple oxides, if isolated. There is no indication of formation of any mixed phase during the catalytic experiments. All industrial oxide catalysts used for selective catalytic oxidation are composed of more than one phase. The above mentioned results suggest that there is a fundamental reason for that. But the majority of the fundamental studies have dealt with single phases and all the published mechanisms consider phenomena taking place in a single phase. The factors influencing activity and selectivity which are considered are those corresponding to the characteristics of this single phase. Our objective was to investigate the role of external phases.

Preparation and properties of raney copper-zinc catalyst pellets for methanol synthesis

Raney catalysts were produced by alkali leaching cylindrical pellets of a Cu-39 wt.pct.Al-17.8 wt.pct. Zn alloy to various depths using a range of leachant concentrations and temperatures. The resultant catalysts were then evaluated by measuring their activity to methanol synthesis from H 2-5 vol.pct. CO-4 vol.pct. CO 2 at 493 K and 4500 kPa. It was found that specific surface areas decreased with increasing leach depth, the decline being more pronounced at higher alkali concentrations and at higher leach temperatures. Specific activities, measured as pellet averages, increased with leach depth until they reached a maximum and then declined. The depths at which maximal activities were achieved increased with increasing alkali concentrations and decreasing temperatures. The overall result was to yield pellets for which catalytic activities were optimal for particular leach depths. Activities comparable to industrial oxide catalyst pellets were achieved under some leach conditions

Electron microscopy of industrial oxidation catalysts

Abstract A combination of in situ high-voltage electron microscopy (HVEM), X-ray/ electron diffraction methods, X-ray and electron probe microanalysis as well as high-resolution phase contrast lattice imaging (<2 AS resolution) microscopy techniques has been used to investigate the reaction properties of an industrial tellurium-molybdenum oxide catalyst system used, for example, in the ammoxidation of propylene to produce organic chemicals such as acrylonitrile, acrolein and other hydrocarbon conversion reactions. In situ electron microscopy experiments conducted on the catalysts in reducing/oxidizing gaseous environments with pressures comparable to those used in the commercial reactions have revealed degradation products which have been analysed by high-resolution microscopy. In this industrial system at typical catalyst operating temperatures no evidence for crystallographic shear planes has been obtained and the implications concerning the role of such extended defects in industrial catalytic reaction...