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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.

The metal–substrate interaction (MSI) represents an important effect determining the properties of supported heterogeneous catalysts. We investigated the MSI effects by X-ray photoelectron spectroscopy, ion scattering spectroscopy and thermal desorption spectroscopy in the case of Pd films vapor deposited on non-stoichiometric aluminum oxide substrates. We compared the obtained results with these of bulk Pd and Pd/Al reference alloy. The observed MSI is explained by the formation of an intermetallic interface exhibiting a strong bimetallic transition-metal (Pd)/s, p-metal (Al) interaction. The fundamental studies of Pd/Al bimetallic alloys are of an importance due to the role that they play in the modification of chemisorption and catalytic properties of real Pd/Al-oxide systems as well as in the realization of novel Pd-alloy catalysts. The reported behavior represents a new way in explanation of former unexpected CO adsorption properties of alumina supported Pd particles.

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.

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.

The metal–substrate and metal–metal interactions represent important effects determining the properties of supported catalysts. We investigate the CO adsorption mechanism on Pd particles supported on Al 2 O 3 and Al by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). CO–metal interaction is investigated by TP desorption of CO and by XPS of C 1s intensities that exhibit three CO-related components. The dissociation activity is monitored as a rise of C 1s signal at 285 eV while the molecularly adsorbed CO exhibits the intensity at 287 eV. Pd–substrate interaction is determined from the Pd core- and valence-level binding energy variations. The influence of the Pd–Al alloying process on the Pd MNN Auger transition is shown. The studies show that the metal–substrate and metal–metal interactions play an important role in CO–Pd adsorption process.

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.

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.

Reflection electron energy loss spectroscopy (EELS) operated at low primary electron energy Ep (~ 500 eV) can be used as a coverage-sensitive probe in the case of supported noncontinuous layers. Quantitative analysis by EELS is substantially complicated by the fact that EEL intensity depends on two material factors: K(E), differential cross section for energy loss of E, and electron backscattering factor η(Ep). Both factors were determined experimentally using deposit and substrate reference samples. The contribution of pure substrate and pure deposit EEL curves to composed EEL spectra of the investigated deposit/substrate system has been found by fitting it with combination of reference K(E)λ curves (λ stands for IMFP). The fit results corrected using η(Ep) factors permitted one to evaluate the deposit coverage. The method was tested using the reference Au fractional deposit on the sapphire substrate.

A model of a coverage-dependent desorption energy, E d ( Θ ), and a preexponential factor, v ( Θ ), is proposed in which lateral forces and a compensation effect are taken into account. Two parameters are applied: a coverage-dependent energy E lat ( Θ ) of lateral forces and a constant T c which describes the relation between v and E d . Together with the Polanyi-Wigner equation, formulas for E lat ( Θ ) and v ( E d ) as taken from literature allow to obtain a modified Arrhenius plot. With values of E lat and T c properly chosen, the plot makes again a straight line with parameters which correspond to zero-coverage values of desorption energy and preexponential factor. As the analysis is based on the application of two mutually independent parameters ( E lat , T c ) the ambiguity of results can occur. It can be reduced if a set of TDS spectra with different initial values of coverage are analyzed. The model has been applied to a set of CO desorption spectra from small Pd supported particles as well as from a Pd single crystal surface.

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.

Stoichiometry of alumina surface can influence the catalytic properties of alumina supported catalysts. Different types of the (0001) α-alumina surface were prepared by thermal treatment under atmosphere or vacuum and by ion sputtering. Heating under vacuum and ion sputtering lead to the formation of an aluminium rich surface layer. This is observed via an excitation of the Al surface plasmon and an increase of the Al/O Auger intensity ratio. Heating of the alumina crystal under air gives well stoichiometric oxide surface. But γ-alumina layer prepared by thermal heating of aluminium under air exhibits a lack of oxygen without formation of an Al rich surface film.

The adsorption of CO and O 2 on Rh foil and Rh small particles supported by an Al 2 O 3 polycrystalline substrate were investigated by means of a molecular beam technique. The supported Rh particles of different sizes were prepared by evaporation using the principle of electron bombardment. The Rh foil used was plane annealed and Ar + ion bombarded. The effect of Rh surface morphology on the results is discussed. All the sticking coefficients obtained on small particles show that the part of adsorbed molecules reach the Rh particles by diffusion across the Al 2 O 3 substrate. The diameters of the diffusion zones are calculated.

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.

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.

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.

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.

The adsorption of CO on Pd(111) and small Pd particles deposited on an alumina support has been studied by means of temperature programmed desorption (TPD) using a molecular beam doser. The supported Pd particles with an average size of 27 and 2.5 nm were prepared by electron bombardment induced thermal evaporation. The CO desorption data were obtained for the Pd particles as well as for a Pd(111) single crystal. Major differences in the Pd particle and single crystal data were observed. On the 2.5 nm Pd particle surface different CO adsorption states were found. The decay in CO adsorption capacity during repeated adsorption/desorption cycles caused by simultaneous accumulation of surface carbon from partial CO dissociation was monitored. It is shown that surface carbon formation reduces the value of desorption activation energy for the adsorption state with higher binding energy. The calculated activation energy of desorption shows that CO desorption kinetics is particle size dependent.

The adsorption of carbon monoxide on small alumina-supported Pd particles and on a Pd(111) single crystal has been studied with a molecular beam techniques. The measured sticking coefficient shows that CO adsorbs on palladium surface according to precursor states. It is noted that a temperature dependent diffusion of gas molecules on the support influences the sticking probability of CO.

Using multi-component catalysts offers the possibility to optimize the individual reaction steps. In the last decades, great attention has been focused on bimetallic systems, representing the next step from monometallic catalysts to multi-component ones. However, this approach requires detailed knowledge of both the influence of individual components and their mutual cooperation. In the study of these factors it is advantageous to use ultra-high vacuum conditions where well-defined-metal surfaces, so-called model catalysts, can be prepared and characterized by modern surface-science techniques. The Pd/Nb model catalysts were prepared by depositing a small amount of palladium on polycrystalline niobium substrates. The film growth was investigated via variations of intensity ratio of excited Pd and Nb plasmons. It has been found that Pd-Nb interface was formed by a continuous Pd layer growing on Nb substrate.

The adsorption and oxidation of carbon monoxide on small alumina-supported Pd particles have been studied by temperature-programmed desorption (TPD) using molecular beam techniques. The results of size effect studies show clearly different surface properties of bulk metal and supported clusters. Surprising effect of partical CO dissociation was observed on small Pd particles deposited on γ-alumina, prepared by thermal oxidation of aluminum, and not on Pd/α-alumina model catalysts. The activation energy of CO desorption, Ed, was determined as a function of particle size. The Ed decreased with decreasing particle size. The CO and oxygen sticking probability measurements indicated the CO and oxygen diffusion over the support. It was shown that the oxygen adsorption capacity and the reactivity of alumina-supported catalysts were higher than those of Pd(111).

CO adsorption properties of the bimetallic Pd/Al systems have been studied using thermally programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under UHV conditions. The samples were prepared by vacuum evaporation of Pd onto pure Al substrates. Measured TPD and XPS spectra of CO from bimetallic surfaces were compared to the ones of clean bulk metals. The deposited Pd ad-layers exhibited strong interaction with Al support, which determined the chemical activity of Pd towards interaction with CO. The formation of PdAl alloy of noble metal-like electronic structure with the d-band centered at 4.5 eV below Fermi level was observed. The Pd 3d 5/2 core level exhibited binding energy 1.8 eV higher than that of the surface atoms of bulk Pd with further slight increase finishing at 2.1 eV. The thermal desorption spectra of CO from the Pd/Al system consists of two peaks, both lower than those from Pd foil, indicating a distinct weakening in the strength of the CO adsorption bond. Heating induced alloying, an appearance of surface oxygen and a carbon contamination on bimetallic surfaces were observed. The correspondence of these effects with the character of electronic interaction between Pd and Al is discussed.

Rhodium particles were vapor-deposited on (-1012) α-alumina and polycrystalline γ-alumina substrates. Variations of morphological parameters during the growth and/or thermal annealing in vacuum, oxygen and CO atmosphere have been investigated by means of static SIMS and XPS. The ratios of Rh/substrate ion species and the Rh 3d electron binding energy variations indicated changes in metal coverage given by particle size and dispersion variations. The heating of as-deposited particles in vacuum led to the particle coalescence. On the other hand the oxygen interactions with particles gave an increase of particle dispersion.