Helmut Knözinger

Preparation of solid catalysts

Developing Industrial Catalysts. Bulk Catalysts and Supports. Supported Catalysts. Zeolites and Related Molecular Sieves. Solid Superacids. Catalyst Forming. Computer-Aided Catalyst Design.

IR spectroscopy of small and weakly interacting molecular probes for acidic and basic zeolites

The application of small and weakly interacting probe molecules for the characterization of acidic and basic properties by FTIR spectroscopy is exemplified by using H- and alkali cation-exchanged zeolites as typical solid Bronsted and Lewis acids and Lewis bases. Criteria for the selection of probe molecules are given. Bronsted acidity can be characterized by the H-bonding method when CO and N2 are used as molecular probes. Quantum chemical calculations are shown to provide important additional information on the electronic nature of the adsorption interaction and the vibrational behaviour of the probe molecule. Lewis acidity dominates in cation-exchanged zeolites for small cations (Li+, Na+) whereas basic properties develop with increasing cation radius. CO, CO2, N2 and CH4 interact with cation centers, the interaction energy decreasing with increasing cation radius. CO at very low equilibrium pressures permits a siting of Na+, and the Al distribution in six-rings (SII-sites) can be probed. CH4 interacts with cations in the M+···H3CH configuration having C3v symmetry. CH-acids such as Cl3CH(D), acetylene and methylacetylene, are shown to be potentially suitable probe molecules for basic properties using the H-bonding method. All three molecules undergo Oz2−···H–C H-bonding and the induced red-shift of the C–H stretching frequency permits a ranking of the base strength of a given series of materials.

Characterization of mixed copper-manganese oxides supported on titania catalysts for selective oxidation of ammonia

Titania-supported copper-manganese mixed oxides were investigated in view of the selective oxidation of ammonia to dinitrogen and characterized by X-ray diffraction, photoelectron spectroscopy and temperature programmed reduction. The mixed oxides proved to be more active than the corresponding individual oxides. The TPR-results show that the reduction occurs at lower temperatures than that of the unsupported oxides. It is concluded that titania facilitates the change of oxidation states. Most active and selective for N2-formation is a catalyst with a molar ratio of Cu: Mn = 20:80. It contains a crystalline Cu1+xMn2−xO4 spinel phase (determined by XRD) and an additional X-ray amorphous Mn2+ -containing species (determined by XPS), possibly Mn3O4 or MnO. Copper is present in the mono- and divalent oxidation state (shown by XPS). All catalysts hardly loose activity with time on stream, above all the copper-rich catalysts are very stable.

The dehydration of alcohols on alumina: XIV. Reactivity and mechanism

Abstract The product distributions and Arrhenius parameters for the zero-order range have been determined for the dehydration on alumina of a series of primary, secondary, and tertiary aliphatic alcohols and for some alicyclic alcohols. The E 2 -like transition state structure is thought to be influenced by inductive, hyperconjugative, and steric effects. The elimination of the β-proton from a trans -position is assumed to proceed via an “inclination” of the adsorbed structure towards the catalyst surface, which bears the basic center. Thus, the cis -preference is easily explained by steric restrictions between certain groups of the adsorbed species and the catalyst surface. The sequence of the activation energies can be explained in the transition state model by the simultaneous influence of ionic, hypcrconjugative, and steric effects, while activation entropies probably demonstrate a certain tunnel contribution in the β-proton abstraction.

Carbon monoxide — A low temperature infrared probe for the characterization of hydroxyl group properties on metal oxide surfaces

Carbon monoxide adsorption at 80 K on γ-Al2O3, α-Cr2O3, TiO2 (rutile), SiO2-Al2O3, MgO and CeO2 was studied by transmission IR spectroscopy. It is shown that individual frequency shifts for the various types of surface OH groups when they are engaged in H-bonding interaction with CO, can be resolved. Heats of adsorption can be estimated from these shifts and a relative acidity scale for the different OH groups on a given oxide can thus be established. It is demonstrated that the high-frequency OH groups on γ-Al2O3 and α-Cr2O3 do not interact with CO, while the low-frequency groups do form H-bonds. The surface OH groups on MgO and CeO2 are too basic and remain unperturbed by CO. SiO2-Al2O3 behaved similar to SiO2 and all OH groups on rutile appear to possess sufficient proton donor strength for H-bond formation with CO.

XPS study of copper aluminate catalysts

The oxidation states and compositions of the surfaces of CuAl2O4 and NiO promoted CuAl2O4 were investigated by X-ray photoelectron spectroscopy (XPS). It is shown that an appreciable enrichment of the copper content within the probing depth of CuAl2O4 occurs as compared to the bulk concentration. This trend is still enhanced by the addition of small amounts of NiO. The aluminates are being reduced in vacuo at temperatures ⩾ 200°C to form Cu2O and Al2O3. Reduction in CO and H2 at 500°C leads to the formation of metallic Cu within the probing depth, while the nickel appears to be only partially reduced. Prereduced catalysts can be reoxidized by NO and O2 at temperatures ⩾ 200°C. The original spinel structure can be reformed under fairly mild conditions (approximately 450°C), which is explained by the assumption of very small and highly reactive particles being formed during reduction-reoxidation cycles. Treatment of the catalysts in NO + CO reaction mixtures produces Cu species in low oxidation states and Cu2+. The low oxidation states seem to be essential for the catalytic activity of the system.

Solid-solid wetting and formation of monolayers in supported oxide systems

Abstract The spreading of active oxides MoO 3 , WO 3 and V 2 O 3 on the surface of support oxides such as λ -Al 5 O 3 , TiO 2 (anatase) and SiO 2 has been studied by laser Raman spectroscopyy and ion scattering spectroccopy. It is shown that MoO 3 spreads over the surface of λ -Al 2 O 3 and TiO 2 at 720 K and tends to form “monolayers” on these supports while this phenomenon does not seem to occur on SiO 2 . Spreading occurs in the absence and presence of H 2 O vapour, whereas a chemical transformation of MoO 3 into a surface polymolybdate is only observed in the presence of H 2 O vapour, its rate being strongly influenced by the H 2 O partial pressure. WO 3 also seems to spread on λ -Al 2 O 3 , although presumably to a lesser extent than MoO 3 , and only under more cever conditionsV 2 O 5 undoubtedly spreads on TiO 2 and The spreading process is phenomenologically discussed in terms of solid-solid wetting, the microscopic mechanism, however, remains unknown. It can only be speculated that the different crystallographic structures which result in significantly different cleavage behaviour of the active oxides might be related to the spreading mechanism. On the other hand, there is good evidence for

The effect of CeO2 structure on the activity of supported Pd catalysts used for methane steam reforming

Palladium (Pd) supported on CeO2-promoted γ-Al2O3 with various CeO2 (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO2 structure and dispersion on the γ-Al2O3 support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO2 structure. Pd is present on the oxidized CeO2-promoted catalysts as Pd0, Pd+ and Pd2+, at ratios strongly dependent on CeO2 structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO2 and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO2. FTIR spectra of adsorbed CO revealed that after pre-treatment under H2 or in the presence of amorphous CeO2, partial encapsulation of Pd particles occurs. CeO2 structure influences the CH4 steam reforming reaction rates. Crystalline CeO2 and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO2 as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al2O3 or on amorphous (reduced) CeO2/Al2O3 catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO2/γ-Al2O3 than on Pd/γ-Al2O3, which is attributed to a catalytic synergism between Pd and CeO2. The low rates on the reduced Pd/CeO2/Al2O3 catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration.

Species formed after NO adsorption and NO+O2 co-adsorption on TiO2: an FTIR spectroscopic study

Adsorption of NO and its co-adsorption with oxygen on TiO2 (Degussa P-25) were studied by FTIR spectroscopy. It was found that NO adsorption results in its disproportionation to NO− (1170 cm−1), N2O22− (1335 cm−1) and nitrates (1650–1550 and 1240–1220 cm−1). The nitrate bands develop with time and coordinated NO (ca. 1900 cm−1) is formed. Addition of oxygen to NO results in a strong increase in concentration of the nitrates and formation of NO+ (2206 cm−1). In addition, species assigned to nitrocomplexes (1520 and 1284 cm−1) are found. The stability and reactivity of the different surface compounds as well as their interconversion are studied and discussed.

Spectroscopic characterization of Tio2/SiO2 catalysts

Abstract TiO2/SiO2 (7 wt% TiO2) has been prepared via modification of the SiO2 surface by reaction with Ti[OCH(CH3)2]4. Physical characterization of the material was performed by means of XRD, diffuse reflectance spectroscopy (DRS), XPS, and low-temperature IR spectroscopy of adsorbed CO. TiO2 exists in highly dispersed form on the SiO2 surface. Interestingly, a quantum size effect leads to a shift of the absorption edge of TiO2 toward higher energy. XPS difference spectra indicate the formation of Ti3+ during reduction. This is confirmed by CO adsorption in the temperature range 80