I. Stará

XPS, ISS and TPD study of Pd–Sn interactions on Pd–SnOx systems

Abstract The sensitivity and selectivity of SnO2 based gas sensors could be improved by doping of small amount of transition metals. In this work we used X-ray photoelectron spectroscopy, ion scattering spectroscopy, and thermal desorption techniques to investigate Pd evaporated on SnOx thin layer substrate, prepared by spray pyrolysis. The evolution of Pd/SnOx layer morphology with increasing amount of Pd deposits was studied using the XPS inelastic background shape analysis. The observations are compared to the results obtained from natural SnO2 crystal and metallic Sn substrates. A strong Pd–Sn bimetallic interaction was observed, resulting in the formation of PdSn alloy of noble metal-like electronic structure. This feature also corresponds to the presence of two CO desorption states with low energy peaks at approximately 390 K. The relation of our results with the operation mechanism of gas detection are discussed.

Influence of substrate structure on activity of alumina supported Pd particles : CO adsorption and oxidation

Abstract Recently the unexpected effects of partial CO dissociation were reported for small Pd particles deposited on γ-alumina, prepared by thermal oxidation of aluminium whilst this behaviour was not observed on Pd α- alumina model catalysts. In this study we compared the CO adsorption and oxidation properties of Pd particles deposited on α-alumina substrates of different surface stoichiometry with those of Pd on γ-alumina. It was shown that the oxygen adsorption capacity as well as the reactivity of α-alumina supported catalysts was lower than those of γ-alumina supported catalysts. The CO and oxygen sticking probability measurements indicated the CO and O diffusion over the support that in the case of CO decreased with T. The temperature dependent CO oxidation rate was explained in terms of its limitation by the CO diffusion process which is less important in the case of the aluminium rich alumina surface.

CO adsorption on palladium model catalysts: XPS Pd–Al2O3 interaction study

The metal–substrate interaction (MSI) represents one of the most important effects determining the properties of supported catalysts. It is expected to be one of the driving forces of the size effect in catalysis. In this work we investigated the MSI by X-ray photoelectron spectroscopy (XPS) in the case of small Pd particles deposited on γ- and α-alumina substrates. We compared the binding energy and initial state variations as a function of Pd coverage. The initial state has been found to be shifted to higher positive values for a more strongly interacting substrate (γ-Al2O3), whilst for the inert-like sapphire substrate the MSI was less important. The initial state shift value was associated with the electron transfer in the substrate clusters direction. It was shown that the substrate–metal charge transfer could be responsible for partial CO dissociation on γ-Al2O3 supported Pd particles.

Study of CO desorption and dissociation on Rh surfaces

Abstract The adsorption of CO on Rh foil and small Rh particles supported by Al 2 O 3 polycrystalline surfaces was studied using temperature programmed desorption (TPD). The supported Rh particles were prepared by evaporation, using the principles of electron bombardment. The activation energy of desorption was calculated. The results show the dependence on the CO exposure of the sample as well as the size effect. Desorption and recombination peaks were measured. During CO desorption the CO 2 production was followed. The dependence of CO oxidation rate on the kind of sample is discussed.

XPS and TDS study of CO interaction with Pd–AlOx systems

Abstract The metal–substrate and metal–metal interactions (MSI, MMI) represent important effects determining the properties of supported catalysts, gas sensors and gettering alloys. We investigated the MSI and MMI effects by the X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) in the case of Pd films deposited on Al 2 O 3 and Al substrates. The study shows that the particle-size dependent metal–substrate interaction plays an important role in CO–Pd chemisorption, namely, in the case of “aluminium rich” Pd–aluminium oxide interface. CO chemisorption exhibits a low-temperature desorption feature at 360 K characteristic for Pd–Al and very small Pd particles. The MSI is explained by the formation of a Pd–Al intermetallic interface exhibiting a strong bimetallic Pd–Al interaction.

Size effect study of carbon monoxide oxidation by Rh surfaces

Abstract The catalytic oxidation of CO on Rh foil and small Rh particles supported by Al 2 O 3 polycrystalline surfaces was studied by the transient experiment of molecular beam surface scattering technique. The supported Rh particles were prepared by evaporation, using the principle of electron bombardment. The results show the dependence of the CO 2 production rate on surface morphology and particle size. The defect rich Rh foil surface and smaller particles were found to be more active. The high oxygen adsorption capacity of 2.5 nm particles as well as their time dependent form of the CO 2 production rate is explained by the oxygen diffusion into the subsurface region.

Role of Pd–Al bimetallic interaction in CO adsorption and catalytic properties of bulk PdAl alloy: XPS, ISS, TDS, and SIMS study

Abstract CO adsorption on the bulk PdAl bimetallic systems has been studied using XPS, ISS, TDS, and SIMS under UHV conditions. Pd exhibits strong interaction with Al atoms resulting in the formation of noble metal-like electronic structure of PdAl alloy. The shifts of both Pd core levels and of Pd valence d-band centroid towards higher binding energies compared to bulk Pd were observed. The surface structure of the alloy changes with temperature (Al surface segregation), ion bombardment (preferential Al sputtering), and is also strongly affected by the presence of ambient CO. The Pd–Al bond is weakened upon the interaction with CO, which tends to dissociate on the surface even at room temperature, with carbon fraction bonding to Pd atoms and oxygen to Al. The TDS of CO spectra consisted of one to three desorption peaks, all lying lower than those from Pd foil, indicating a distinct weakening of the Pd–CO chemisorption bond. Moreover, the partial CO dissociation on PdAl was indicated both as adsorption capacity decay and CO2 and H2O production.

CO diffusion over the alumina support of Pd particle model catalysts

Abstract The CO adsorption rate of alumina-supported Pd particles increases with CO diffusion over the support surface. The diffusion of admolecules is interrupted by desorption or by trapping by catalyst particles. The mean diffusion length determines the width of the so-called “capture zones” around the particle. In molecular beam scattering studies, an increase in diffusion length and consequently in capture-zone width is observed as the sticking probability S i of the impinging molecule increases. In this study, the parameter S i is determined for CO interaction with Pd particles supported on different types of alumina substrates as a function of substrate temperature. The results allow an evaluation of the adsorption probability of CO on alumina as well as the mean diffusion length versus substrate temperature.

Study of CO interaction with alumina-supported Pd particles

Abstract Anomalous effects such as the enhanced sticking probability of CO and O 2 and the partial CO dissociation were measured on small Pd particles deposited on γ-alumina. Recently, a dependence of the sticking coefficient on the substrate has also been reported on Pd (0001) α-alumina but no dissociation has been observed in this case. We present a detail study of CO adsorption/desorption and catalytic reaction properties for a series of alumina substrates. Particularly, we used α-alumina of surface orientations (0001) and (1102). Before the Pd deposition, the substrates were exposed to annealing at different temperatures in vacuum and in the air. The adsorption and reaction parameters of Pd particles change with crystallographical structure and stoichiometry of the substrate surface, essentially due to the effect of reactants diffusion over the substrate.

CO adsorption on Al2O3-supported Pd clusters: XPS study

Abstract The adsorption of carbon monoxide on small alumina-supported Pd particles have been studied by X-ray photoelectron spectroscopy (XPS). The results showed clearly different surface properties of bulk metal and supported clusters. The effect of partial CO dissociation was observed on Pd deposited on γ-alumina and on clean aluminium, but not on small Pd particles prepared on thin amorphous oxide film on Al substrate. CO–Pd interaction was determined from C 1s photoelectron spectra that exhibited two CO-related components. The dissociation activity was monitored as a rise of C 1s signal at 285 eV, while the molecularly adsorbed CO exhibited the intensity at 287 eV. The study showed that beside the particle size, the metal–substrate interaction (MSI) plays an important role in CO–Pd adsorption process.