G.W. Graham

Studies of the water-gas-shift reaction on ceria-supported Pt, Pd, and Rh: Implications for oxygen-storage properties

Abstract Steady-state, water-gas-shift kinetics were measured on model, ceria-supported, Pd, Pt, and Rh catalysts and compared to rates obtained on alumina-supported catalysts. When ceria was calcined at low temperatures prior to addition of the precious metal, the specific rates were found to be identical for each of the metals, with an activation energy of 11 ± 1 kcal/mol and reaction orders of zero and one for CO and H 2 O respectively. For comparison, specific rates on Rh/alumina were at least two orders of magnitude lower. However, ceria structure strongly affected the results. When ceria was calcined to high temperatures to increase crystallite size, prior to the addition of Pd, specific rates were a factor of 50 lower at 515 K and the activation energy was found to be much higher, 21 ± 1 kcal/mol. By comparison with results from an earlier study of CO oxidation [17], we propose that water-gas shift on ceria-supported metals occurs primarily through a bifunctional mechanism in which CO adsorbed on the precious metal is oxidized by ceria, which in turn is oxidized by water. Deactivation of the catalyst following growth in the ceria crystallite size is due to the decreased reducibility of large ceria crystallites. The implications of these results for automotive, emission-control catalysts is discussed.

Automotive exhaust catalysis

Abstract Research in the field of automotive exhaust catalysis has paralleled the broader growth in heterogeneous catalysis research—beginning in the 1960s, progressing through commercialization in the mid-1970s, and continuing today. The general trend has been one of increasingly complex catalyst formulations in response to increasingly stringent emission standards. Nowhere is this more evident than in the various means that have been employed to most effectively utilize the noble metal components. These efforts will continue, but with greater emphasis on optimizing catalyst formulations for lean-burn applications and reducing catalyst cost and complexity without sacrificing performance.

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.

Characterization of model automotive exhaust catalysts: Pd on ceria and ceria–zirconia supports

Model Pd automotive three‐way catalysts were prepared with high‐surface‐area Zr‐rich ceria–zirconia powders as support materials, aged for 12 h at 1050°C under redox conditions simulating automotive exhaust gases, and characterized by a combination of techniques including oxygen storage capacity (OSC) measurements. Differences in OSC amongst aged catalysts made with materials of similar ceria–zirconia compositions, but produced by different processes, were weakly correlated with differences in support surface area. Evidence of encapsulation of Pd particles was found in most of the aged catalysts. The promotional effect of zirconia on ceria reducibility, well known from previous temperature‐programmed H2 reduction studies, was apparent in Ce 3d core‐level spectra from X‐ray photoelectron spectroscopy measurements, which showed that both X‐ray‐induced and H2 reduction become increasingly facile with increasing zirconia content. A slight Ce enrichment at the surface of the fresh catalysts made with the more Zr‐rich powders was found to increase upon aging.

Empirical method for determining CeO2-particle size in catalysts by raman spectroscopy

The addition of ceria, CeO{sub 2}, to automotive exhaust-gas catalysts has been linked to greater thermal stability of the alumina support, higher precious-metal dispersion, and wider range of parameters for three-way operation. Some, if not all, of these beneficial effects are undoubtedly influenced by the dispersion of the ceria, which is typically added at the 5-10 wt% (including the monolith) level. Methods of measuring its dispersion, or particle size, are thus important tools for studying how ceria functions. Traditionally, either transmission electron microscopy (TEM) or X-ray diffraction (XRD) have been applied to the problem of particle size determination in catalysts. In this report, the authors show how Raman spectroscopy may be used to determine the size of CeO{sub 2} particles in a way that is easier than TEM and faster than XRD.

CO oxidation for the characterization of reducibility in oxygen storage components of three-way automotive catalysts

Abstract Steady-state, CO-oxidation kinetics at 515 K have been measured on model, Pd catalysts, prepared by vapor deposition of Pd onto either zirconia, praseodymia, ceria, or a ceria-zirconia mixed oxide. A second rate process (RE2), associated with both the metal and the oxide support and observed previously on ceria-supported catalysts in excess CO [7], was also found for Pd supported on ceria-zirconia, but neither zirconia nor praseodymia had any effect on CO oxidation under the conditions of our study. For ceria and ceria-zirconia, deactivation, through the loss of RE2, caused by high-temperature calcination, was examined, with the Pd added after calcination so that the metal particle size was not a factor in deactivation. For ceria, there was a strong dependence on calcination temperature, with almost complete loss of RE2 above 1170 K. XRD showed that the loss was accompanied by a large increase in the crystallite size. Results for ceria-zirconia showed that the loss in this case was more gradual, with CO oxidation activity due to RE2 maintained to much higher calcination temperatures. Taking the importance of RE2 as a measure of the ability of the catalyst to use oxygen from the oxide, the implications of these results with respect to oxide structure and the effect of aging on oxygen-storage properties of reducible oxides are discussed.

Growth and characterization of reactively sputtered thin‐film platinum oxides

Thin‐film oxides of platinum have been prepared by reactive sputtering and characterized primarily by x‐ray diffraction (XRD) and Raman scattering. Different phases of the Pt‐O system were obtained by adjusting sputtering parameters such as gas composition, deposition rate, and substrate temperature. Optimum conditions for producing crystalline α‐PtO2 and PtO were identified. Raman spectra of α‐PtO2 films show two sharp lines at 514 and 560 cm−1, in agreement with those from commercially available powders. XRD and high‐resolution scanning electron microscopy (HRSEM) analyses indicate that these samples have a poorly crystallized structure with lack of order along the c axis. The Raman spectra from PtO show two broad peaks at 438 and 657 cm−1, close to the lines seen in PdO, which has the same structure as PtO. Estimates from XRD line broadening and HRSEM photographs indicate mean crystallite sizes on the order of 300 A. Infrared reflectivity spectra yield two of the three ir‐active phonons, and along with...

The preparation of high-surface area, thermally-stable, metal-oxide catalysts and supports by a cellulose templating approach

Abstract A simple and convenient templating approach for the preparation of metal-oxide catalysts and supports has been studied. The method, based on the absorption of an aqueous solution of metal salts by cellulose material, followed by drying and combustion of the organic matrix, leads to the formation of high-surface area mesoporous materials with unusually high thermal stability. Examples include Ce–Zr mixed oxides (BET surface areas of 90–130xa0m 2 /g after calcination at 800°C for 2xa0h; 21–30xa0m 2 /g after 12xa0h at 1050°C) and La-stabilized alumina (BET surface areas of 275–320xa0m 2 /g after calcination at 800°C for 2xa0h, 88–141xa0m 2 /g after 12xa0h at 1050°C). The pore-size distribution, morphology, and the effect of preparation parameters on surface area are discussed.

In situ ellipsometric study of a palladium catalyst during the oxidation of methane

Ellipsometry is used to follow the growth of a PdO layer on the surface of a thick Pd-film catalyst during methane oxidation at ∼ 500°C. The oxide layer that develops under rich conditions (excess CH4) is quite porous and roughens with time. Little CO is formed during this period, but the CO2 formation rate increases until spontaneous oscillations develop, which correlate with changes in the ellipsometric data. These changes indicate that the porous oxide rapidly converts to a metal-rich state, which has decreased catalytic activity, and then slowly reoxidizes.

Incorporation of La3+ into a Pt/CeO2/Al2O3 catalyst

The effects of La3+ incorporation into a Pt/CeO2/Al2O3 catalyst were investigated by a combination of activity, temperature-programmed reduction (TPR), oxygen storage capacity (OSC), noble-metal surface area, and X-ray diffraction (XRD) measurements. Incorporation of La3+ ions into the Al2O3, before CeO2 is added, promoted higher Pt and CeO2 dispersions. The oxygen storage capacity was also higher in the presence of La3+. This is attributed to a combination of Pt and CeO2 particle-size effects and possible blockage of the reaction between Al2O3 and CeO2. The XRD data show that La3+ forms LaAlO3 with Al2O3 and prevents α-Al2O3 formation after various heat treatments.