K. Otto

Characterization of Pd/γ-alumina catalysts containing ceria

The effects of adding CeO2 to PdAl2O3 as a catalyst modifier were investigated by X-ray photo-electron spectroscopy and X-ray diffraction. Catalytic effects were demonstrated by using propane oxidation as a model reaction. It was found that CeO2 promotes oxidation of Pd to PdO both with and without alumina. High-temperature reduction (⋍920 °C) and the presence of Pd are required for the total conversion to bulk CeAlO3 from CeO2Al2O3. Pd also assists in the oxidation of bulk CeAlO3 to CeO2 at elevated temperatures. In ambient air, Pd facilitates surface oxidation of CeAlO3 to CeO2, whereas surface Pd acquires an oxidation state between Pd0 and PdO. On PdAl2O3 the propane oxidation rate is found to be lowered by CeO2 if the oxygen concentration exceeds that of the Stoichiometric ratio.

The oxidation of CO by O2 and by NO on supported chromium oxide and other metal oxide catalysts

Abstract The oxidation of CO by NO and by O 2 has been studied on a number of supported transition metal oxide catalysts. On supported chromia the oxidation of CO by NO is faster than by O 2 ; however, with mixtures of NO and O 2 the reaction is selective towards O 2 . The application of continuous mass-spectrometer monitoring to the study of surface state changes in chromia catalysts is outlined. The method was extended to determine the oxidation state of the surface in situ during reaction. In the oxidation-reduction cycle involving alternate passage of HeCO and HeO 2 mixtures at 500 °, the majority of the surface atoms undergo a change of oxidation state from Cr 6+ to Cr 2+ . The extent of this change diminishes with decreasing temperature. Mixtures of He and NO oxidize the surface of supported chromia to a lesser extent than HeO 2 . During the CONO reaction the average surface oxidation state is lower than in the presence of oxygen. A tentative explanation is offered for the selectivity for oxygen over nitric oxide in the oxidation-reduction reactions on commonly employed catalysts.

Dispersion studies on the system La2O3γ-Al2O3

Abstract According to the literature and earlier studies in this laboratory, lanthanum oxide appears to be the best of the known additives for inhibiting the sintering of high-surface-area aluminas. The substrates of the present studies were largely γ-alumina, with minor components of transition aluminas, σ-alumina, and possibly θ-alumina. Alumina samples with different lanthanum concentrations, produced by impregnation with aqueous lanthanum nitrate, followed by calcination at various temperatures, were studied by chemisorption of carbon dioxide, Auger electron spectroscopy, Raman spectroscopy, and by X-ray powder diffraction (XRD). All results are consistent with the following description: Up to a concentration of 8.5 μmol La m 2 , the lanthana is in the form of a two-dimensional overlayer, invisible by XRD or Raman spectroscopy. For greater lanthana concentrations, the excess lanthana forms crystalline oxides, detectable by XRD. In samples calcined to 650 °C, this crystalline phase is cubic lanthanum oxide. After calcination at 800 °C, the lanthana reacts to form the lanthanum aluminate LaAlO3.

The adsorption of nitric oxide on iron oxides

Abstract Adsorption isotherms and rates were measured for NO chemisorption on supported and unsupported samples of Fe 2 O 3 and Fe 3 O 4 in the 26–150 °C temperature range. The adsorption behavior is well described by Freundlich isotherms in the pressure range from 1 to 200 Torr. Monolayer coverage is attained at 500 Torr for the reduced samples and at 25000 (by extrapolation) on the oxidized samples. Kinetic measurements were evaluated using the Elovich equation. These plots were monotonic in the case of supported Fe 2 O 3 and showed a sharp discontinuity in the case of supported Fe 3 O 4 at θ = 0.5. A close parallelism is noted with previously studied chemisorption behavior on reduced and oxidized supported chromium oxide.

NO adsorption on Cu-ZSM-5: assignment of IR band at 2133 cm−1

The adsorption of nitrogen oxides on Cu-ZSM-5 was studied by infrared spectroscopy to elucidate the species associated with the band at 2133 cm−1. The band was found for both NO and NO2 adsorption. Labeling experiments with15NO revealed that the associated surface species contained nitrogen and, most likely, an N-O bond. Co-adsorption experiments of NO and oxygen produced adsorbed nitronium, NO2+, as the principal, associated species. Adsorption of nitrogen oxides on dispersed CuO and the HZSM-5 support demonstrated that the 2133 cm−1 band was not necessarily associated with copper ions. A relatively strong correlation between the bands at 2133 and 3615 cm−1 indicates that the primary adsorption sites of NO2+ are the strongly protic, bridging Si(OH)Al framework hydroxyls. Once these were filled, other, weaker acid sites began to adsorb NO2O.

Effects of the surface structure of Rhγ-Al2O3 on the hydrogenolysis of n-pentane, on the oxidation of n-butane, and on the reduction of nitric oxide

Abstract In low-concentration Rh catalysts, supported on γ-Al 2 O 3 , the metal is present in a dispersed phase with every Rh atom or ion exposed on the surface. This form persists up to a surface concentration of 2.5 μmol/m 2 (BET). At higher concentrations three-dimensional particles are formed. Three model reactions were investigated on Rh catalysts of both kinds: hydrogenolysis of n -pentane under strongly reducing conditions, oxidation of n -butane under strongly oxidizing conditions, and reduction of nitric oxide by hydrogen under mildly oxidizing conditions. The n -butane oxidation proceeded in the same fashion on both types of Rh catalysts, while the two other reactions were found to be structure sensitive. On the dispersed phase, n -pentane cleaves predominantly into ethane and propane, while on the three-dimensional particles this activity is sharply diminished. Similarly, the product selectivity and the kinetic parameters of the NO-H 2 reaction differ distinctly on the two phases of supported Rh.

The influence of platinum concentration and particle size on the kinetics of propane oxidation over Pt/γ-alumina

For the regulation of automotive emissions it is of interest to understand the parameters that control the oxidation rates of different hydrocarbon species. Propane oxidation was selected as a model reaction for this investigation. Catalysts were prepared by multiple impregnation of γ-alumina with chloroplatinic acid. Concentrations ranged from 0.03 to 30 wt% Pt. The reaction took place in a recirculation batch reactor with an initial mixture of 10 Torr C3H8, 100 Torr O2, and 650 Torr Ar. The oxidation rate increased sixfold with increasing Pt concentration. When calculated per Pt surface atom, based on CO chemisorption, the rate increased by two orders of magnitude in the same Pt concentration range. The apparent activation energy remained unchanged at 22.1 kcal/mol within a standard deviation of ±3.4 kcal/mol. The increase in specific reaction rate with particle size was confirmed by sintering experiments. The rate increase reflects an increase in the preexponential factor. The results suggest that propane oxidation is expedited by a favorable ensemble of active Pt sites, which are more likely to form on larger crystallites. Kinetic parameters of propane and methane oxidation are compared. Theoretical site densities, derived from fundamental kinetics for common reaction mechanisms, are compared with the number of Pt surface atoms as measured by chemisorption.

Catalysis of carbon-steam gasification by ash components from two lignites

Abstract Ash transfer from a reactive to a less reactive coal is an interesting possibility for improving and equalizing gasification characteristics of coals. To assess the catalytic action of coal impurities in the steam gasification of carbon, three approaches were used. In the first series, the effects of different coal ashes on the gasification kinetics of graphite were compared. A parallel study was made by adding lignite ash to a coal of low reactivity. Finally, gasification rates of chars prepared from demineralized coals were measured. While it was found that ash from reactive coals can significantly enhance the gasification rates of chars derived from coals of lesser reactivity, it was not possible to distinguish clearly between a catalytic lowering of the activation energy and an increase in the number of gasification sites. The gasification enhancement by lignite ash may open practical possibilities for blending coals of different reactivity, and warrants further study to identify the constituents associated with this effect.

The adsorption of nitric oxide on chromia supported on alumina

Abstract Adsorptive properties of chromia supported on alumina have been studied with an electrobalance for NO as an adsorbate. Isotherms are presented in the range from −78 ° to 150 °C for both an oxidized and a reduced surface. The isotherms are of the Freundlich type at pressures between 0.1 and 300 torr. Maximum coverage corresponds to approximately one molecule of NO per atom of chromium in the surface. The adsorption of NO can therefore be used to assess the actual chromia surface in the presence of a support material. Changes in pressure and temperature influence the coverage on an oxidized surface more strongly than on a reduced surface. Kinetic measurements at constant temperature and pressure show a significant difference between an oxidized and a reduced chromia surface. In the first case a uniform decrease of the adsorption rate with time is observed, while in the second case a fast initial adsorption is followed by a much slower process, indicated by a sharp break in the corresponding Elovich curves. The kinetic parameters as well are influenced by temperature more strongly for the oxidized than for the reduced state. An attempt is made to relate these chemisorption results to the catalytic behavior of supported chromia for the reduction of NO.

Catalytic steam gasification of graphite: Effects of calcium, strontium, and barium with and without sulfur

Abstract Alkaline earths strongly catalyze steam gasification of graphite. The catalytic effect increases in the order Ca