SMSI State Research Articles

Strong Metal\u2013Support Interaction and Reactivity of Ultrathin Oxide Films

Noble metal particles supported on transition metal oxides (TMO) may undergo a so-called strong metal–support interaction via encapsulation. This perspective addresses catalytic properties of the metal catalysts in the SMSI state which can be explained on the basis of complementary studies performed on well-defined, metal-supported TMO films. In particular, the results of low temperature CO oxidation revealed the key role of weakly bound oxygen species provided by a two-dimensional (“monolayer”) oxide film. The binding energy of such oxygen atoms can be used as a descriptor for oxidation activity. Reaction rate enhancement often observed for TMO films partially covering the metal surface is rationalized within a mechanism in which the metal acts as a promoter to create the most active “oxidered/oxideox” interface formed by reduced and oxidized phases in the film. Although only low temperature CO oxidation is considered, it is tempting to generalize these ideas to other oxidation reactions following the Mars–van Krevelen type mechanism. In addition, metal-supported ultrathin TMO films may serve as well-defined model systems to examine different aspects of the “electron theory of catalysis” which was proposed long ago and which is based on electron transfer mechanisms.Graphical

CO oxidation as a test reaction for strong metal–support interaction in nanostructured Pd/FeOx powder catalysts

A series of differently loaded palladium–iron catalysts was prepared by a controlled co-precipitation method of the nitrate precursors, in order to ensure homogeneous Pd particle size-distribution. After characterization of the pre-catalysts by various techniques, different controlled reduction conditions were applied to investigate the interactions within the Pd–iron system, containing reversible and irreversible processes like phase transformations, SMSI, sintering and alloying. Strong indications for the reversible surface decoration of the Pd nanoparticles with iron oxide species via strong metal–support interaction were found by the combined results of DRIFTS, CO-chemisorption, TEM and XPS measurements. This SMSI state was found to be unstable. It was observed independent of bulk phase or palladium particle size. Catalytic CO-oxidation was found to be a suitable test reaction for the study of the phenomenon: higher activity as well as oxidative deactivation of the SMSI state was observed by investigating the light-off behavior in repeated, temperature-programmed cycles as well as by isothermal measurements. The instability was found to be higher in case of higher Pd dispersion. In addition, bulk properties of the Pd–Fe system, like alloying, were investigated by detailed XRD measurements.

Hydrogenolysis of carbon–halogen and carbon–carbon bonds over Pd/Nb2O5–Al2O3 catalysts

Pd/Nb2O5–Al2O3 catalysts exhibited interesting properties in the hydrodechlorination of dichlorodifluoromethane. Introduction of niobia to alumina increases the selectivity for CH2F2, from ∼40% (for Pd/ Al2O3) up to ∼75%, for similar metal dispersions. High-temperature reduction (773 K, HTR) of Pd/Nb2O5–Al2O3 generates the SMSI state, understood as blocking the Pd surface by (partly) reduced niobia. The extent of SMSI effect is judged from a direct comparison between the performance of differently pretreated Pd/Nb2O5–Al2O3 catalysts tested in n-hexane conversion (hydrogenolysis/isomerisation). After HTR of the niobia-containing Pd catalysts, the hydrodechlorination activity disappeared, however the SMSI state became reversed during hydrodechlorination. It is speculated that CCl2F2 and liberated HCl, interact with NbOx decorants, converting them into volatile niobium oxychloride species.

Effect of Sn addition to Pt/CeO 2–Al 2O 3 and Pt/Al 2O 3 catalysts: An XPS, 119Sn Mössbauer and microcalorimetry study

The effect of adding Sn to Pt/CeO 2–Al 2O 3 and Pt/Al 2O 3 catalysts was studied with X-ray photoelectron spectroscopy (XPS), 119Sn Mössbauer spectroscopy, and adsorption microcalorimetry of CO at room temperature. Catalysts were reduced in situ at 473 (non-SMSI state) and 773 K (SMSI state). 119Sn Mössbauer and XPS results indicated that the presence of cerium in bimetallic catalysts inhibited reduction of tin, and that tin facilitated the reduction of cerium(IV) to cerium(III). Microcalorimetric analysis indicated that adding cerium caused the appearance of a more heterogeneous distribution of active sites, whereas adding tin led to a higher homogeneity of these sites. Reduction at 773 K decreased the Pt surface area as measured by CO chemisorption for all catalysts used in this study. Adding tin to Pt/Al 2O 3 and Pt/CeO 2–Al 2O 3 also decreased the Pt surface area due to formation of PtSn and possibly Pt–SnO x species. Adding cerium to Pt/Al 2O 3 caused a loss of Pt surface area only when the catalyst was reduced at 773 K, presumably due to migration of the reduced cerium onto Pt particles. Adding cerium to Pt/Al 2O 3 caused an increase in the catalytic activity for crotonaldehyde hydrogenation, whereas adding Sn to Pt/Al 2O 3 decreased the activity of Pt/Al 2O 3 catalysts. Higher reduction temperatures caused an increase in the initial catalytic activity for crotonaldehyde hydrogenation for all catalysts studied. Selectivity enhancements for crotyl alcohol formation in crotonaldehyde hydrogenation were observed for the Ce- and Sn-promoted catalysts after reduction at 773 K.

Remarkable support effect for liquid phase methanol reforming with water over supported Pt–Ru catalysts

Support effect for the catalytic activity of liquid phase reforming of methanol with water was investigated over various supported Pt–Ru bimetallic catalysts. The SiO 2, TiO 2, Al 2O 3, MgO, CeO 2 and ZrO 2 were used as support materials by conventional impregnation method. Basic oxide supports improved the selectivity to CO 2, whereas acidic supports suppressed the catalytic activity and selectivity. The Pt–Ru/TiO 2 catalyst exhibited highest activity and selectivity for CO 2, which were improved by higher temperature reduction. The improved activity and selectivity were attributable to the occurrence of SMSI state and Pt–Ru alloy, respectively. The reforming reaction over Pt–Ru/TiO 2 catalyst proceeded through partially dehydrogenated HCOOCH 3 and HCOOH intermediate with the similar mechanism as that over SiO 2 supported Pt–Ru catalyst.

Pt/CeO 2 catalysts in crotonaldehyde hydrogenation: Selectivity, metal particle size and SMSI states

Pt/CeO 2 catalysts with different metal particle sizes were prepared by impregnation of chlorine-free precursor, Pt(NH 3) 4(NO 3) 2, with different Pt loadings (1, 3, 5, 10 wt%), on a commercial ceria. These catalysts and a CePt 5 powder, which was regarded as a reference catalyst, were tested in the hydrogenation of crotonaldehyde as a function of the reduction treatment (from 473–973 K). A dramatic difference in the influence of the reduction temperature on the reactivity was observed between the high and low loaded metal catalysts: after reduction at 973 K, high selectivity to crotyl alcohol (83%) was observed on the high loaded catalyst while on the 1% Pt/CeO 2, crotyl alcohol selectivity was below 35%. Characterization by TEM, XPS and XRD showed differences in the particle size distributions and the presence of various nanostructural modifications during the increase of the reduction temperature. On the other hand, the reactivity of CePt 5 powder indicates no ability of this compound for the carbonyl bond hydrogenation. The different SMSI states which could influence the reactivity are discussed: formation of epitaxial Pt (1 1 1) layer on CeO 2 instead of Pt–CeO x interfacial sites or CePt 5 sites, has been retained to be responsible for the increase in crotyl alcohol selectivity.

Effect of the nature of the support on the enantioselective hydrogenation of 1-phenyl-1,2-propanedione over supported iridium catalysts

The enantioselective hydrogenation of 1-phenyl-1,2-propanedione at 298 K and 40 bar over modified supported iridium catalysts has been studied. Cinchonidine has been used as chiral inducer. The catalysts were obtained by impregnation of Ir(acac)3 on three different supports: SiO2, TiO2 and MoO3, followed by calcination in air and reduction under hydrogen at 773 K. All the solids were characterized by nitrogen adsorption-desorption isotherms at 77 K, H2 chemisorption, XRD, TEM, TPR and XPS. It was found that Ir/SiO2 and Ir/TiO2 catalysts reduced at high temperatures, 773 K, possess similar metal particle size, close to 2.0 nm, eventhough the H/Ir ratio obtained from H2 chemisorption showed larger differences, with the H/Ir ratio being lower for titania- and molybdenum-supported iridium catalysts. In these samples, migration of the partially reduced supports, TiOx and MoOx moieties, on the metal crystals induce the creation of Irδ+ species. TPR and XPS results confirmed that the metal component was not completely reduced. The activity was influenced by the nature of the support, being more active those in the SMSI state such as Ir/TiO2 and Ir/MoO3 being more active. This has been attributed to the presence of electron deficient metal species, Irδ+, which are responsible for the polarization of the carbonyl bond of the substrates, thus favoring the activity and enantioselectivity of the reaction. The effect of different solvents on the activity and the enantioselectivity of the reaction was also studied. The highest enantiomeric excess (ee) of (R)-1-phenyl-1-hydroxy-2-propanone (20%) was obtained with the Ir/TiO2 catalyst using acetic acid as solvent.

Sulfur tolerance of Pd/Al 2O 3 and Pd/TiO 2 in naphthalene hydrogenation in the presence of dimethyldisulfide

Hydrogenation of naphthalene over Pd/TiO 2 and Pd/Al 2O 3 catalysts in the presence of dimethyldisulfide (DMDS) was performed in a batch reactor at 473 K and 2.45 MPa; the two catalysts were compared in detail in terms of sulfur tolerance. As compared with Pd/Al 2O 3, Pd/TiO 2 showed high turnover frequency (TOF) and sulfur tolerance, in both the presence of dimethyldisulfide (DMDS) and the absence of DMDS. PdS bond strength was determined by temperature-programmed reduction of sulfide catalysts (H 2S-TPR) profiles and CO uptake after H 2 reduction of sulfided catalyst at various temperatures. The results show that desorption of H 2S from sulfided Pd/TiO 2 occurs at 553 K, whereas desorption of H 2S from sulfided Pd/Al 2O 3 requires a higher temperature (773 K), indicating that the PdS bond in Pd/TiO 2 is significantly weaker than that in Pd/Al 2O 3. Furthermore, the H 2S-TPR profiles of Pd/TiO 2 reduced at 573–773 K clearly indicate that the PdS bond in Pd/TiO 2 reduced at 573 K is weak, whereas reduction at high temperature (SMSI state) strengthened the PdS bond. The high temperature reduction may result in formation of Ti 3+ surrounding the Pd metals and spoil the catalytic activity. Therefore, the presence of Ti 4+, which has a high electronegativity, is concluded to weaken the PdS bond and to be an important factor for high sulfur tolerance.

Infrared study of CO and 2-butenal co-adsorption on Zn modified Pt/CeO2–SiO2catalysts

Infrared spectra of CO alone and CO followed by the introduction of 2-butenal (crotonaldehyde) have been recorded for Pt/CeO2–SiO2 and Zn doped Pt/CeO2–SiO2 catalysts which had been pre-reduced at 473, 623 and 773 K. Samples treated at the highest reduction temperature showed the greatest selectivity toward the unsaturated alcohol although no clear spectroscopic evidence for the formation of an SMSI state was obtained. Although 773 K reduced Zn containing catalysts showed the greatest yield, there was no IR spectroscopic evidence to support the formation of significant quantities of a Pt–Zn alloyed phase or that the addition of Zn enhanced Pt-support interactions beyond increasing the amount of surface reduced ceria. Results show that, contrary to popular opinion, the development of an SMSI state which enhances the density of support-metal interface sites is not essential in order to increase the yield of unsaturated alcohol and the presence of reduced cationic sites at the interface zone may be the crucial factor. Although IR spectra in the νCO region were dominated by dipole coupling effects, enhanced intensity of carbonyl bands in the 2030–1950 cm−1 region for 773 K reduced samples and for Zn containing samples reduced at lower temperatures suggests that sites responsible for these carbonyl bands may be the active sites in the selective hydrogenation reaction.

Support effects in Pt/TiO2–ZrO2 catalysts for NO reduction with CH4

ZrO2–TiO2 mixed oxides, prepared using the sol–gel method, were used as supports for platinum catalysts. The effects of catalyst pre-reduction and surface acidity on the performance of Pt/ZT catalysts for the reduction of NO with CH4 were studied. The diffuse reflectance infrared Fourier transformed (DRIFT) spectra of CO adsorbed on the Pt/ZT catalysts, and also on the Pt/T and Pt/Z references, pre-reduced at 773K in hydrogen, revealed that an SMSI state is developed in the Ti-rich oxide-supported platinum catalysts. However, no shift in the binding energy of Pt 4f7/2 level for Pt/T and Pt deposited on Ti-rich support counterparts pre-reduced at 773K was found by photoelectron spectroscopy. The DRIFT spectra of the catalysts under the NO+O2 co-adsorption revealed the appearance of nitrite/nitrate species on the surface of the Zr-containing catalysts, which displayed acidic properties, but were almost absent in the Pt/T catalyst. The intensity of these bands reached a maximum for the Pt/ZT(1:1) catalyst, which in turn exhibited a larger specific area. In the absence of oxygen in the feed stream, the NO+CH4 reaction showed DRIFT spectra assigned to surface isocyano species. Since the intensity of this band is higher for the Pt/ZT (9:1) catalyst, it seems that such species are developed at the Pt–support interface.

Transformation of methane formation sites into methanol formation ones during COH 2 reaction over Pd/CeO 2 in its SMSI state

The phenomena of induction period of methanol formation, observed in COH 2 reaction over Pd/CeO 2 catalysts under SMSI state, was investigated in depth. The magnitude of the induction period was dependent on the extent of SMSI, and higher temperature H 2 reduction lengthened it accompanied with the increase of the number of active sites for methane formation. On the contrary, by the pretreatment of SMSI surface with water vapor, this induction period almost disappeared with the drastic decrease of methane formation rate. These results indicate that methane formation sites would be transformed into methanol formation sites by the oxidation of water vapor formed during COH 2 reaction. Infrared spectroscopic investigation of adsorbed CO after various pretreatments indicated that during the induction period thin layers of reduced ceria, which preferentially covered Pd(1 1 1) plane under SMSI state, were removed from the Pd(1 1 1) plane by formed water vapor during COH 2 reaction. It was concluded that Pd(1 1 1) plane adjacent to ceria would be the efficient active sites for methanol formation.

New evidence for the electronic nature of the strong metal-support interaction effect over a Pt/TiO 2 hydrogenation catalyst

Analysis of the Pt 4 f line asymmetry evidenced that the suppression of the hydrogenation activity of Pt/TiO 2 in the SMSI state is caused by a decrease in the d-electron density at the Fermi level of platinum particles, while the net charge of the metal particles remains unaltered.

Crotonaldehyde Hydrogenation on Pt/TiO2and Ni/TiO2SMSI Catalysts

A kinetic and DRIFTS (diffuse reflectance FTIR) investigation of crotonaldehyde adsorption and hydrogenation was conducted over TiO2-supported Pt and Ni with the intent of gaining insight into the adsorption modes of molecules with carbonyl groups on these catalysts in the SMSI and non-SMSI states. Significant enhancement in selectivity toward crotyl alcohol was observed with each catalyst after reduction at 773 K. DRIFT spectra under reaction conditions identified crotonaldehyde species strongly adsorbed through the C=C bond and weakly coordinated through both the C=C and the C=O bonds on these catalysts after reduction at 573 K, which gave a peak at 1693 cm−1. After reduction at 773 K, an additional adsorbed species with a strong band at 1660 cm−1, indicating a significant interaction between the carbonyl group and the surface, was observed, which is presumed to be stabilized at interfacial Pt–TiOxand Ni–TiOxsites. A decrease in the surface coverage of this species paralleled a drop in selectivity to crotyl alcohol with time on stream. After reduction at 573 K, decarbonylation occurred during the initial few minutes on stream to create adsorbed CO on Pt/TiO2in addition to carbon deposition, but these reactions were significantly suppressed after reduction at 773 K, presumably due to a TiOxoverlayer which covers part of the Pt surface and breaks up the ensembles of Pt atoms required for these reactions.

98/00286 Modeling of the combustion of coal-derived fuel as using oxygen and air

Rh catalysts have been prepared by impregnation of silica or titania and their activities and selectivities determined for the reaction of n-hexane. Titania-supported Rh catalysts are more active than silica-supported and have a different selectivity for isomerization being much less selective for the formation of 2- methylpentane. The results provide evidence of a real support effect even in catalysts reduced at low temperatures. When the Rh/titania catalysts are reduced at high temperatures to induce “Strong Metal-Support Interactions” the turnover number is increased but the selectivity and product distribution are unaffected. When the SMSI Rh/titania catalysts are re-oxidized at 298 K to partially reverse the SMSI state the turnover number decreases by more than two orders of magnitude, but the selectivity does not change. It is concluded that although SMSI catalysts behave differently from “normal” Rh/titania catalysts the creation of the SMSI state is not a prerequisite for obtaining high activity or unusual selectivity. A model is presented in which the active centres for the n-hexane reaction are considered to be Rh atoms in flat surfaces. The reforming of hydrocarbons is a process of major industrial importance. The results obtained demonstrate that metal-support interactions can affect the selectivity of a metal in reforming reactions and provide information on the nature of the active centre required for the skeletal rearrangement of hydrocarbons.

98/00291 Pressurized fluidized bed gasification of coal using recirculated flue gas and oxygen and combustion of the resulting fuel gas

Rh catalysts have been prepared by impregnation of silica or titania and their activities and selectivities determined for the reaction of n-hexane. Titania-supported Rh catalysts are more active than silica-supported and have a different selectivity for isomerization being much less selective for the formation of 2- methylpentane. The results provide evidence of a real support effect even in catalysts reduced at low temperatures. When the Rh/titania catalysts are reduced at high temperatures to induce “Strong Metal-Support Interactions” the turnover number is increased but the selectivity and product distribution are unaffected. When the SMSI Rh/titania catalysts are re-oxidized at 298 K to partially reverse the SMSI state the turnover number decreases by more than two orders of magnitude, but the selectivity does not change. It is concluded that although SMSI catalysts behave differently from “normal” Rh/titania catalysts the creation of the SMSI state is not a prerequisite for obtaining high activity or unusual selectivity. A model is presented in which the active centres for the n-hexane reaction are considered to be Rh atoms in flat surfaces. The reforming of hydrocarbons is a process of major industrial importance. The results obtained demonstrate that metal-support interactions can affect the selectivity of a metal in reforming reactions and provide information on the nature of the active centre required for the skeletal rearrangement of hydrocarbons.

98/00289 Nickel-based catalyst compositions and process or manufacture of synthesis gas by methane reforming

Rh catalysts have been prepared by impregnation of silica or titania and their activities and selectivities determined for the reaction of n-hexane. Titania-supported Rh catalysts are more active than silica-supported and have a different selectivity for isomerization being much less selective for the formation of 2- methylpentane. The results provide evidence of a real support effect even in catalysts reduced at low temperatures. When the Rh/titania catalysts are reduced at high temperatures to induce “Strong Metal-Support Interactions” the turnover number is increased but the selectivity and product distribution are unaffected. When the SMSI Rh/titania catalysts are re-oxidized at 298 K to partially reverse the SMSI state the turnover number decreases by more than two orders of magnitude, but the selectivity does not change. It is concluded that although SMSI catalysts behave differently from “normal” Rh/titania catalysts the creation of the SMSI state is not a prerequisite for obtaining high activity or unusual selectivity. A model is presented in which the active centres for the n-hexane reaction are considered to be Rh atoms in flat surfaces. The reforming of hydrocarbons is a process of major industrial importance. The results obtained demonstrate that metal-support interactions can affect the selectivity of a metal in reforming reactions and provide information on the nature of the active centre required for the skeletal rearrangement of hydrocarbons.

The State of Zirconia Supported Platinum Catalysts for CO2/CH4Reforming

In Pt/ZrO2catalysts used for CO2/CH4reforming to syngas, not all accessible Pt contributes equally to the activity of the catalyst. The catalytic activity is determined by the accessibilityof Pt on the Pt-ZrO2perimeter. This is explained in terms of theCO2activation via carbonate species on the support which must bein the proximity of the Pt particles to react with the methaneactivated on the metal. The significance of the support is alsoemphasized by the low activity of Pt black and Pt/SiO2, which wereincapable of forming carbonates on the support. The perimeterconcentration of Pt/ZrO2can be changed by changing the Ptconcentration in the catalyst or by increasing the calcinationtemperature which causes sintering of the Pt particles. In contrastto the calcination temperature, the reduction temperature did notinfluence the activity although the hydrogen chemisorption capacitywas markedly decreased by increasing the reduction temperature. Assintering was excluded on basis of particle size determination fromEXAFS results a SMSI state on Pt/ZrO2during high temperaturereduction is concluded to exist. This state does not persist underreaction conditions due to the presence of adsorbed oxygen fromCO2dissociation.

The influence of MSI (metal-support interactions) on phenylacetaldehyde hydrogenation over Pt catalysts

TiO 2-supported Pt, after a high-temperature reduction (HTR) at 773 K to induce the SMSI state, had a turnover frequency (TOF) for phenylacetylaldehyde hydrogenation that was 15–20 times higher than TOF values for Pt dispersed on either SiO 2 or η-Al 2O 3 and, since all supported catalysts had Pt dispersions of unity (1 nm crystallites), the TOF for Pt TiO 2 (HTR) based on complete dispersion was still 2–5 times greater than the other supported catalysts. More importantly, selectivity to 2-phenylethanol was markedly enhanced over Pt TiO 2 (HTR), comprising 70% of the product at conversions as high as 60%. Pt TiO 2 after a low temperature reduction (LTR) at 473 K gave neither the high TOFs nor the enhanced selectivity. Among these catalysts the highest activation energy of 12.4 kcal/mol was attained with Pt TiO 2 (HTR) and the reaction order on phenylacetaldehyde was between 0 and 1 2 while that on H 2 was around 1 2 or higher. At higher conversions (over 50%) significant hydrogenolysis activity to form benzene and toluene occurred on all Pt catalysts except Pt TiO 2 (HTR). The higher hydrogenation activity and suppressed hydrogenolysis capability with the Pt TiO 2 (HTR) catalyst is attributed to TiO x species migrating onto the Pt surface to create active sites at the Pt-titania interface while simultaneously destroying large ensembles of Pt atoms required for hydrogenolysis reactions; however, competitive desorption/readsorption processes must also play a significant role in enhancing selectivity to the intermediate phenylethanol product.

FTIR Spectroscopic Study and CO Hydrogenation on V, Nb, and Ta Oxide Promoted Rh/SiO2Catalysts

The CO hydrogenation over Rh/SiO2catalysts promoted with oxides of Vb transition metals was studied. Two different kinds of promoter effects occurred which were established by high temperature reduction (HT reduction, Tred≥ 673 K) and HT calcination (Tcalc≥ 973 K), followed by reduction at 673 K, respectively. While HT reduction favors formation of ethanol, HT calcination, followed by reduction at 673 K enhances the selectivity toward methanol. The overall activity increased in the order V < Nb < Ta. The impact of promotion on the metal state was characterized by FTIR spectroscopy of adsorbed CO at 85 K. Modification of Rh/SiO2with promoter oxide causes a slight high frequency shift for linearly bonded CO on rhodium. Reduction at 673 K leads to a decrease in CO chemisorption capacity. In this state the rhodium metal surface is thought to be partially covered by promoter oxide and hence compares to the conventional SMSI state. This effect is enhanced if the reduction is preceded by HT calcination. The capability of the promoter oxide to spread over the metal surface increases in the order Ta < Nb < V. While the total CO chemisorption capacity is lowered in the SMSI state, the relative amount of linearly to bridge-bonded CO increases and characteristic changes in the band shape of bridge-bonded CO occur. A high frequency component at around 1920 cm−1characteristic of bridge-bonded CO which is present after LT reduction (Tred=573 K) disappears after HT reduction.

Palladium-lanthanum interaction phenomena in Pd-LaCl 3 /SiO 2 and Pd-La 2 O 3 /SiO 2 catalysts

The effect of La addition and the nature of the precursors on the surface properties of Pd/SiO2 are studied. Samples containing 0.5 wt% Pd were prepared by incipient wetness impregnation and characterized by H2 and CO chemisorption, TEM, TPR, EDX, XPS, Ar+-sputtering and CO/FTIR. The use of nitrate precursors improves both reducibility and dispersion of Pd. Lanthanum addition makes the metal reduction more difficult, indicating that a Pd-La interaction takes place. When starting from Cl- precursors, Pdn+ species are formed at the surface, whereas only Pd0 is found when nitrate precursors are used. The addition of LaCl3 increases the Pd dispersion and hinders the formation of Pdn+ species through a dilution effect. In a sample prepared from Pd(NO3)2+La(NO3)3, the La2O3 formed upon calcination originates a SMSI-like effect on the palladium. This effect is suggested by the strong reduction of the H2 and CO chemisorption capacity of Pd in this sample, in spite of the fact that the dispersion of palladium calculated from TEM increases in the presence of La. From XPS results this SMSI state appears to be due to a physical decoration of Pd by La2O3 rather than to an electronic Pd-La2O3 interaction. The ``decoration model'' is further confirmed by the progressive increase of the Pd/La atomic ratio observed upon XPS/Ar+-sputtering at different times the sample containing La. The hypotheses of both dilution and decoration effects caused by LaCl3 and La2O3, respectively, are strengthened by the CO/FTIR results.