Robert F. Hicks

An XPS study of metal-support interactions on Pd/SiO2 and Pd/La2O3

Abstract The electronic properties of palladium supported on SiO 2 and La 2 O 3 were investigated by X-ray photoelectron spectroscopy. Spectra were collected after each stage of catalyst preparation: deposition of the palladium chloride precursor, oxidation in air at 623 K, and reduction in H 2 at 573 K. The Pd 3d 5 2 binding energies recorded following precursor deposition and oxidation were the same on both catalysts. However, after reduction the Pd 3d 5 2 binding energies of the Pd La 2 O 3 samples shifted below the corresponding values for metallic Pd, while the Pd 3d 5 2 binding energies of the Pd SiO 2 samples did not. Furthermore, the binding energies for the Pd La 2 O 3 samples decreased with increasing metal loading, the largest Pd particles apparently exhibiting the greatest interaction. A maximum shift of −0.7 eV relative to the Pd foil value was observed for 8.8% Pd La 2 O 3 . This binding energy shift is interpreted as arising from a change in the chemical state of Pd, i.e., that Pd supported on La 2 O 3 is more electronegative than zero-valent Pd alone. A model is proposed which suggests that a thin covering of the La 2 O 3 support lies on a portion of the Pd surface. This covering is partially reduced during H 2 treatment at 573 K and, because of the electropositive nature of La, the additional charge is distributed amongst the surface Pd atoms. It should be noted that the La 2 O 3 support surface after H 2 reduction is a complex mixture of La(OH) 3 , LaO(OH), La 2 O 3 , and La 2 (CO 3 ) 3 . The exact composition of the support depended strongly on the reduction temperature and the amount of Pd deposited.

Effects of metal-support interactions on the chemisorption of H2 and CO on Pd/SiO2 and Pd/La2O3

Abstract The adsorption of H 2 and CO was investigated on SiO 2 - and La 2 O 3 -supported Pd catalysts, and the structure of adsorbed CO was characterized by infrared spectroscopy. For each of the Pd SiO 2 catalysts, the ratio of adsorbed H atoms, or adsorbed CO, to surface Pd atoms is unity. The stoichiometry for atomic adsorption of H 2 on Pd La 2 O 3 is also unity, independent of Pd dispersion. By contrast, the adsorption stoichiometry for CO decreases linearly from 0.6 to 0 as the Pd dispersion decreased from 30 to 8%. The suppression of CO adsorption is attributed to patches of partially reduced support material, LaO x , transferred to the surface of the Pd crystallites during catalyst preparation. The fraction of the Pd crystallite surface covered by LaO x increases with Pd dispersion, in agreement with conclusions based on earlier XPS studies. Infrared studies indicate that the structures of CO adsorbed on Pd La 2 O 3 and Pd SiO 2 are similar, but that the strength of adsorption is weaker for Pd La 2 O 3 than for Pd SiO 2 . This is attributed to a weakening in the σ-bond component of the Pdue5f8CO bond due to charge transfer from the LaO x patches to the Pd crystallites. The absence of any suppression of H 2 adsorption of Pd La 2 O 3 indicates that H 2 adsorption occurs both on the exposed Pd surface atoms as well as on the LaO x patches covering the balance of the surface Pd atoms.

Effects of metal-support interactions on the hydrogenation of CO over Pd/SiO2 and Pd/La2O3

A study of CO hydrogenation over PdSiO2 and PdLa2O3 has been carried out for the purpose of identifying the effects of Pd dispersion, Pd morphology, and support composition on the catalytic activity of supported Pd. The specific activity of each catalyst for methanol and methane synthesis was determined from microreactor studies carried out at a fixed set of reaction conditions. Palladium dispersion was measured by H2ue5f8O2 titration, and the morphology of the Pd crystallites, as expressed by the distribution of Pd(100) and Pd(111) planes, was determined from in situ infrared spectra of adsorbed CO. The crystallite morphology of the PdSiO2 catalysts is the same, independent of Pd weight loading: 90% of the surface is comprised of Pd(100) planes and 10% of the surface is comprised of Pd(111) planes. By contrast, the crystallite morphology of the PdLa2O3 catalysts changes with Pd loading. Primarily Pd(100) planes are exposed at low-weight loadings while Pd(111) planes are exposed at high-weight loadings. The Pd dispersion has little effect on the methanol turnover frequency over both PdSiO2 and PdLa2O3, for dispersions between 10 and 20%. On the other hand, the methane turnover frequency is independent of Pd dispersion over PdSiO2, but increases with decreasing dispersion over PdLa2O3. It is further observed that the Pd morphology influences the specific activity of PdLa2O3 for methanol synthesis: Pd(100) is nearly threefold more active than Pd(111). For a fixed morphology, the specific methanol synthesis activity of PdLa2O3 is a factor of 7.5 greater than that of PdSiO2.

Design and construction of a reactor for in situ infrared studies of catalytic reactions

Transmission infrared spectroscopy is a very useful technique for characterizing the structure of adsorbed species present on a catalyst under reaction conditions. Reactors for conducting such in situ studies have been described by a number of authors. The details for a very simple reactor which can be used to study catalytic reactions at moderate temperatures (<573 K) and elevated pressures are described. The reactor has a very low dead volume and a short optical path length, and is built in such a fashion that infrared windows and catalyst samples can be replaced quite easily. The reactor described here has been used to study the hydrogenation of CO at temperatures up to 573 K and pressures as high as 2.4 MPa. No problems have been encountered with either extended operation or cycling of the temperature between 373 and 573 K. The upper limit on the operating temperature is 590 K and is set by the Kalrez O-rings. The upper limit on pressure is not as clearly established.

Kinetics of methanol and methane synthesis over Pd/SiO2 and Pd/La2O3

Abstract A study of the kinetics of methanol and methane synthesis over Pd/SiO2 and Pd/La2O3 has been carried out. The activation energy and the orders with respect to H2 and CO partial pressures are found to be essentially the same for methanol synthesis over both catalysts. This suggests that the methanol reaction mechanism is unaffected by support composition. The higher specific activity of Pd/La2O3 relative to Pd/SiO2 for methanol synthesis is believed to be due to small differences in the relative strengths of H2 and CO adsorption. The rate expressions for methane synthesis over Pd/ La2O3 and Pd/SiO2 differ significantly. This strongly suggests that the methanol reaction mechanism for the two catalysts is different, or alternatively that the mechanism is the same but the rate-limiting Step is different.

The influence of metal-support interactions on the catalytic properties of Pd/La2O3

Abstract Evidence is presented for the existence of metal-support interactions between Pd and La2O3. XPS spectra indicate that the Pd 3d 5 2 binding energy is shifted below the value of bulk Pd by as much as 0.7 eV. The negative binding energy shift increases with decreasing Pd dispersion. The chemisorption of CO at 298K is suppressed on the Pd microcrystallites, and the amount of suppression also increases with decreasing Pd dispersion. This latter effect exerts a strong influence on the morphology of the Pd surface during methanol synthesis. The turnover frequency for CH3OH synthesis from CO and H2 correlates with the Pd morphology: Pd(100) planes are 2.8 times more active than Pd(111) planes. The division of the Pd surface into a mixture of these two planes is inferred from the infrared spectrum of adsorbed CO. For a fixed Pd morphology, the CH3OH turnover frequently is independent of Pd dispersion. By contrast, the CH4 turnover frequency decreases with increasing Pd dispersion, and it is insensitive to the distribution of Pd(100) and Pd(111) planes.