Wolfgang Grünert

Methanol synthesis over ZnO: A structure-sensitive reaction?

In order to identify active sites on ZnO powdered catalysts in methanol synthesis a total of five ZnO samples with different degrees of crystallinity were characterized by means of N2 physisorption, XRD, TEM, and EXAFS. With respect to catalysis, high-pressure methanol synthesis was performed as a test reaction. A linear area-activity relationship for the highly crystalline materials was obtained, but at high BET surface areas a strong deviation from linearity was found. The observed phenomena provide evidence for a structure-sensitivity, suggesting that a specific active site is favored for methanol formation. Based on earlier work with polycrystalline ZnO powder and with respect to the current theoretical and experimental work with ZnO single crystals, the polar ZnO faces are assumed to be highly relevant for the catalytic activity under methanol synthesis conditions.

Reduction and metathesis activity of MoO3/Al2O3 catalysts: I. An XPS investigation of MoO3/AI2O3 catalysts

Abstract MoO 3 /Al 2 O 3 catalysts (2–13 wt% MoO 3 ) were investigated by XPS in the oxidized form, after thermal treatment in flowing Ar (973 K), and after reduction in H 2 (673–973 K), which are conditions typically employed in the activation of these catalysts for the metathesis reaction. A new assignment of Mo 3d binding energies to Mo oxidation states was applied in the analysis of the reduced samples. During the thermal treatment in flowing Ar, part of the hexavalent Mo present in the initial samples underwent reduction to Mo(V), which could also be detected by EPR. The reduction of alumina-supported Mo(VI) in H 2 was found to produce surfaces, on which Mo(VI), Mo(V), Mo(IV), Mo(II), and, at reduction temperatures above 900 K, Mo(O), coexist. For reduction temperatures of about 800 K, distributions of these Mo states, which differ from those reported in the literature by a Mo(V) contribution not exceeding 10% of the total Mo and by the presence of Mo(II), were derived. When reduced MoO 3 /Al 2 O 3 catalysts were subsequently treated in flowing inert gas at 973 K a partial reoxidation of the surface was observed.

Methane partial oxidation to synthesis gas using nickel on calcium aluminate catalysts

Abstract Nickel was supported on calcium aluminate carriers that were obtained with varying CaO to Al 2 O 3 molar ratios and calcination temperatures. The variations of the supports lead to catalysts of different surface properties and catalytic performance. Metallic nickel (Ni 0 ) was proven to be the active species for the methane partial oxidation reaction. The presence of filamentous carbon on used catalysts was also suggested. The differences in the catalytic activity and selectivity for the methane partial oxidation reaction was ascribed to a varying degree of reducibility of the surface nickel species.

Reduction and aromatization activity of chromia-alumina catalysts: I. Reduction and break-in behavior of a potassium-promoted chromia-alumina catalyst

The reduction of a potassium-promoted Cr2O3Al2O3 catalyst and the transient behavior of its aromatization activity were studied in gradientless and pulse flow reactors and by temperature-programmed reduction. Above 773 K, the oxidized catalyst is reduced in two stages. In the first, very rapid stage, essentially the transition of Cr6+ to Cr3+, a considerable retention of hydrogen and carbon from the reducing agent (H2, CH4, or n-hexane) is observed. Aromatization activity is fully developed after this stage. Delayed recoverage of activity after this rapid reduction step as found in flow and gradientless reactors is due to the poisoning action of H2O released in the second, slow stage of catalyst reduction, which leads to (a) species not determining aromatization activity.

Synthesis and characterization of silica MCM-48 as carrier of size-confined nanocrystalline metal oxides particles inside the pore system

Abstract The interpenetrating 3-dimensional channel system of silica MCM-48 has been selected for the deposition of Cu/Zn/O mixed metal oxide particles. With the wet impregnation technique aqueous solutions of metal acetates have been used to load the calcined form of the mesoporous silica. Successive impregnation yielded metal contents of ca. 9 wt.%. Calcination of the composite transformed the acetates to the metal oxides. X-ray powder diffraction and solid-state MAS NMR showed the uptake of the metal salt inside the pore system. N 2 -adsorption, X-ray diffraction and TEM confirmed the mesoporous structure. XPS measurements and EXAFS analysis (Cu K- and Zn K-edges) confirmed the metal uptake. Whereas nano-disperse CuO particles have been obtained ZnO shows no regular structure and seems to have reacted with the silicate channel surface by coating the channel wall.

Selective catalytic reduction of NO under lean conditions by methane and propane over indium/cerium-promoted zeolites

Abstract The potential of zeolites MFI and MOR promoted by In and Ce as catalysts for the SCR of NOx by methane and propane was studied, particularly considering their stability in the presence of water vapour. A preparation route for a zeolite-based Ce/In composite catalyst was developed and the influence of several reaction conditions was studied to optimise the catalyst performance. TPR and FTIRS were used to check the preparation steps and to investigate the catalyst structure. These data were used together with results from characterisation by X-ray-based techniques to describe the generation and catalytic function of different indium species ((InO)+, In2O3 cluster) and their co-operation with ceria in the composite catalyst. The primary role of the CeOx promoter is to catalyse the oxidation of NO to NO2. Probably, NO2 reacts with methane on Lewis-acidic and redoxactive (InO)+ species, but the further reaction of the intermediate formed needs the presence of residual Bronsted-acid sites. SCR of NOx by methane or propane is still partially inhibited by adsorption competition between water and the hydrocarbons on the (InO)+ species. Higher SCR activity could be achieved by increased hydrocarbon partial pressure in the feed, shifting presumably the competitive adsorption to enhanced adsorption of methane and propane, respectively. Furthermore, combining of methane and propane as reductants opens an opportunity to broaden the ‘temperature window’ for the SCR of NOx. Different influence of hydrothermal ageing was obtained with methane and propane, respectively, as reducing agent.

A colloidal ZnO/Cu nanocatalyst for methanol synthesis

Free-standing, ZnO surface decorated Cu nanoparticles of 1-3 nm size were obtained by sequential co-pyrolysis of [Cu(OCHMeCH2NMe2)2] and ZnEt2 in squalane in the absence of additional surfactants and proved to be highly active quasi homogeneous catalysts for methanol synthesis from CO and H2.

Active sites for NO reduction over Fe-ZSM-5 catalysts

A study of Fe-ZSM-5 catalysts with variable amounts of isolated, oligomeric and heavily aggregated Fe3+ oxo sites (as evidenced by UV-Vis and EPR spectroscopic data) and their catalytic properties in the selective catalytic reduction of NO by isobutane or by NH3 is presented, which allows development of a unified concept of the active Fe sites in these reactions, according to which isolated Fe sites catalyse both SCR reactions while oligomeric sites, though also involved in the selective reduction path, limit the catalyst performance by causing the total oxidation of the reductant.

Sulfide catalysis without coordinatively unsaturated sites: hydrogenation, cis-trans isomerization, and H2/D2 scrambling over MoS2 and WS2.

Simple test reactions as ethene hydrogenation, 2-butene cis-trans isomerization and H(2)/D(2) scrambling were shown to be catalyzed by MoS(2) and WS(2) in surface states which did not chemisorb oxygen and were, according to XPS analysis, saturated by sulfide species. This is a clear experimental disproof of classical concepts that require coordinative unsaturation for catalytic reactions to occur on such surfaces. It supports new concepts developed on model catalysts and by theoretical calculations so far, which have been in need of confirmation from real catalysis.

Nano-brass colloids: synthesis by co-hydrogenolysis of [CpCu(PMe3)] with [ZnCp*2] and investigation of the oxidation behaviour of α/β-CuZn nanoparticles

A novel, non-aqueous organometallic access to colloidal copper and copper/zinc (brass) nanoparticles is described. Hydrogenolysis of the precursor [CpCu(PMe3)] (1) in mesitylene at 150 °C and 3 bar H2 quantitatively gives elemental Cu. Analogously, a solution of [ZnCp*2] (2) reacts with H2 to give elemental Zn in 100% yield. Co-hydrogenolysis of 1 and 2 in exactly equimolar quantities selectively yields the intermetallic phase β-CuZn characterised by powder X-ray diffraction (PXRD). Deep red colloidal solutions of nano-Cu as well as red to violet colloids of nano-brass alloys (α/β-CuZn) are obtained by co-hydrogenolysis of 1 and 2 in the presence of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) as surfactant. All samples of the general formula Cu1−xZnx (0.09 ≤ x ≤ 0.50) were characterised by means of elemental analysis, PXRD, transmission electron microscopy (TEM, EDX and SAED) and UV-Vis absorption spectroscopy. The presence and alloying of metallic Cu and Zn in the β-CuZn sample as a representative example of the series was confirmed by extended X-ray absorption fine structure spectroscopy (EXAFS). The oxidation behaviour of the nanoparticles was investigated by EXAFS, PXRD and UV-Vis spectroscopy indicating, that CuOx@Cu core–shell type particles were formed for pure copper particles, while in the case of brass particles preferential oxidation of the Zn component takes place, which results in core–shell particles of the type (ZnO)δ@Cu1−xZnx−δ.