FTIR study of bimetallic Pt-Sn/Al2O3 catalysts (original) (raw)
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A chemisorption and XPS study of bimetallic Pt-Sn/Al2O3 catalysts
Journal of Catalysis, 1991
Bimetallic Pt-Sn/AlzO3 catalysts with nominally 1 wt% Pt and varying tin contents (0-3.25 wt%) prepared by coimpregnation of nonporous Degussa alumina were characterized by chemisorption of H2, 02, and CO at room temperature. The surface compositions and oxidation states of the reduced catalysts were tracked by XPS. Addition of tin to Pt resulted in significant differences in the gas uptake characteristics of the three adsorbates. Both H2 and CO showed an initial increase in gas uptake with addition of small amounts of tin, and then the chemisorbed amount tended to drop off with further addition of tin. In the case of 02 adsorption, there was a steady increase in gas uptake with increasing tin content. XPS of the reduced catalysts showed that in aluminasupported samples most of the tin was in a valence state of either Sn(II) or Sn(IV). On the other hand, large amounts of zero-valent tin were found in a SiO2-supported Pt-Sn catalyst which had been prepared from the same precursors and reduced under identical conditions. This supports the notion that interactions between the alumina support and tin prevent the complete reduction of tin.
Journal of the Chemical Society, Faraday Transactions, 1997
CO chemisorption on Pt catalysts supported on non-acidic alumina prepared from platinum acetylacetonate has been studied by FTIR spectroscopy. The stretching frequency for isolated CO species is observed at 2032 cm~1 for highly dispersed Pt samples (dispersion 0.99) and at 2067 cm~1 for poorly dispersed samples (dispersion 0.09). This frequency shift is in agreement with single-crystal and on supported Pt catalyst data, and may be ascribed to the change of coordination at the Pt chemisorption site. Large shifts, reaching 180 cm~1 can be observed upon coadsorption of ammonia suggesting the possibility of long-range interactions involving the collective electronic properties. CO adsorption has also been studied on well dispersed bimetallic samples obtained by modifying the well dispersed catalyst with Sn or Ge. Quite di †erent e †ects of the two additives were Pt/Al 2 O 3 observed : whereas Ge decreased the dipoleÈdipole coupling in the adsorbed layer, Sn did not. This suggests that Sn segregates at sites of low coordination at the surface of the Pt particles, while Ge is more homogeneously distributed. The coadsorption of ammonia and CO on the bimetallic particles also shows that the electronic properties of Pt were dramatically modiÐed by Ge and una †ected by Sn.
Applied Surface Science, 1998
Four catalysts containing 1% w/w Pt deposited on various sepiolite supports were prepared. Two natural sepiolites and another two obtained by acid treatment of one of them were used. Both the sepiolites used as metal supports and the catalysts were characterized for structure and surface properties, using 1 H and 29 Si MAS NMR spectroscopy, and 29 Si CP/MAS NMR spectroscopy, in addition to hydrogen, pyridine, and CO 2 chemisorption measurements. The activity of the catalysts in the hydrogenation of benzene to cyclohexane and the dehydrogenation of cyclohexane to benzene was examined and the catalyst supported on the natural sepiolite called Pangel was found to be the most active of all.
Characterization of Pt-Sn bimetallic catalysts supported on alumina and niobia
Applied Catalysis A: General, 1993
Alumina and niobia Pt-Sn supported bimetallic catalysts were characterized by TPR, hydrogen chemisorption and cyclohexane dehydrogenation. The TPR profdes of niobia supported Pt-Sn catalysts showed the presence of different precursors from the ones obtained on alumina. Furthermore, the hydrogen uptakes indicated that the amount of metallic tin formed in the niobia supported catalysts was higher than in the alumina supported catalysts. After reduction at 773 K, the platinum/niobia catalyst displayed a strong metal-support interaction (SMSI) effect, with the creation of new interfacial active sites. The addition of a small tin content led to a suppression of the SMSI. Increasing the amount of tin, however, induced a marked poisoning of the platinum.
Journal of Catalysis, 1999
Pt-Sn/Al 2 O 3 catalysts containing 0.30 wt% Pt and 0, 0.15, 0.30, and 0.45 wt% Sn and prepared from Cl-free precursors have been studied by CO chemisorption and FTIR of adsorbed CO after each cycle in a series of six oxychlorination-reduction cycles or six oxidation-reduction cycles followed by oxychlorination-reduction. Spectra of CO on each catalyst directly after oxychlorination are also reported. After oxychlorination catalysts contained exposed Pt sites present as Pt • (covered with O-adatoms), Pt(II), Pt(IV) oxide, PtCl 2 (forming PtCl 2 CO and PtCl 2 (CO) 2 with CO), and PtO x Cl y , the relative proportions of these species varying with Sn content. Tin hindered both catalyst sintering during oxidation and redispersion during oxychlorination. The IR results for oxidised catalyst after subsequent reduction were compatible with Pt • dispersed over a Sn(II)-modified alumina surface. However, addition of chlorine promoted greater intimacy between Pt and Sn with the latter blocking low coordination Pt sites and reducing the size of exposed ensembles of Pt atoms. Two Pt(0.3%)-Sn(0.3%)/Al 2 O 3 catalysts prepared from tin(II) oxalate and tin(II) tartrate gave different results emphasising the sensitivity of catalyst character to preparation procedure.
Mössbauer spectroscopy studies of Sn-Pt/Al2O3 catalysts prepared by controlled surface reactions
Applied Catalysis, 1991
Mossbauer spectroscopy was used to study the chemical state of tin in a new type of Sn-Pt/Al,Oa catalysts. Controlled surface reaction of tin tetraethyl with hydrogen preadsorbed on platinum resulted in the exclusive formation of a Pt-SnEt, surface complex, which decomposed in hydrogen at 773 K to Pt,Sn alloy (3 <x< 4). Contamination of the catalyst by air at room temperature resulted in partial oxidation of tin in the alloy to SnO,. Upon oxidation at 673 K tin segregated to SnO,. Both SnO, and Pt,Sn alloy were detected in the rereduced catalyst. Anchoring SnCl, onto lithiated Pt/Al,Oa and subsequent reduction of the catalyst in hydrogen did not result in the formation of tin-platinum alloy, indicating an exclusive tin-alumina interaction.
Pt–Sn/Al2O3 catalysts: effect of catalyst preparation and chemisorption methods on H2 and O2 uptake
Chemical Engineering Journal, 2004
Pulse and static hydrogen and oxygen chemisorption techniques for determining the platinum dispersion in bimetallic Pt-Sn catalysts were carried out and compared for a monometallic Pt/Al 2 O 3 catalyst and a series of coimpregnated Pt-Sn catalysts containing 1 wt.% of Pt. The pulse chemisorption method gave lower uptakes of hydrogen and oxygen compared to the static volumetric chemisorption method. The differences in hydrogen and oxygen gas uptake behavior can be attributed to the different equilibration times provided by the two chemisorption methods. In the coimpregnated catalyst series, the O/Pt ratio increased with increasing Sn content. The H/Pt ratio, on the other hand, reached the maximum value at 0.1 wt.% Sn.
In situ XPS investigation of Pt(Sn)/Mg(Al)O catalysts during ethane dehydrogenation experiments
Surface Science, 2007
Calcined hydrotalcite with or without added metal (Mg(Al)O, Pt/Mg(Al)O and Pt,Sn/Mg(Al)O) have been investigated with in situ Xray photoelectron spectroscopy (XPS) during ethane dehydrogenation experiments. The temperature in the analysis chamber was 450°C and the gas pressure was in the range 0.3-1 mbar. Depth profiling of calcined hydrotalcite and platinum catalysts under reaction, oxidation and in hydrogen-water mixture was performed by varying the photon energy, covering an analysis depth of 10-21 Å . It was observed that the Mg/Al ratio in the Mg(Al)O crystallites does not vary significantly in the analysis depth range studied. This result indicates that Mg and Al are homogeneously distributed in the Mg(Al)O crystallites. Catalytic tests have shown that the initial activity of a Pt,Sn/Mg(Al)O catalyst increases during an activation period consisting of several cycles of reduction-dehydrogenation-oxidation. The Sn/Mg ratio in a Pt,Sn/Mg(Al)O catalyst was followed during several such cycles, and was found to increase during the activation period, probably due to a process where tin spreads over the carrier material and covers an increasing fraction of the Mg(Al)O surface. The results further indicate that spreading of tin occurs under reduction conditions. A PtSn 2 alloy was studied separately. The surface of the alloy was enriched in Sn during reduction and reaction conditions at 450°C. Binding energies were determined and indicated that Sn on the particle surface is predominantly in an oxidised state under reaction conditions, while Pt and a fraction of Sn is present as a reduced Pt-Sn alloy.
Characterization and Catalytic Activity of Bimetallic Pt-In/Al2O3and Pt-Sn/Al2O3Catalysts☆
Journal of Catalysis, 1998
Bimetallic platinum-indium and platinum-tin supported on alumina catalysts were investigated by temperature programmed reduction (TPR), hydrogen chemisorption, and UV-Vis diffuse reflectance spectroscopy (DRS). DRS results indicated that the interaction between Pt and In or Sn takes place during the reduction step. The TPR results showed that, after reduction at 773K, about 50-80% of indium and 25-50% of tin are in a zero-valent state in the bimetallic system, depending on the preparation method. Also, there is a partial shift of the reduction of In 3+ and Sn 4+ to the platinum precursor temperature range. The interaction between indium and platinum was also demonstrated by the decrease of platinum adsorption capacity, when indium was present. The use of model reactions were shown to be adequate to differentiate the effects of Sn and In on Pt/Al 2 O 3 catalysts. The turnover frequency for cyclohexane dehydrogenation, a structure insensitive reaction, was not affected by the presence of the promoter. In the case of reactions that require larger platinum ensembles to occur, the presence of the promoter caused a decrease in the turnover frequency, due to the dilution of platinum surface atoms with the promoter atoms. There was a larger decrease in the turnover frequency of the methylcyclopentane hydrogenolysis when In was used as promoter, indicating that In dilutes the Pt atoms more homogeneously than Sn. For the n-heptane conversion, the addition of In or Sn improved the stability of the catalysts, caused a decrease in the selectivity for hydrogenolysis, and an increase in the selectivity for dehydrogenation and aromatization products. The main difference between In and Sn was that Sn promoted a higher selectivity for isomerization products.
Journal of Catalysis, 2006
The effect of Sn addition to Pt/CeO 2-Al 2 O 3 and Pt/Al 2 O 3 catalysts was studied with X-ray photoelectron spectroscopy (XPS), 119 Sn 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"). 119 Sn Mössbauer and XPS results indicated that the presence of cerium in bimetallic catalysts inhibited reduction of tin. Furthermore, it was found that tin facilitated reduction of cerium (IV) to cerium (III). Microcalorimetric analysis indicated that cerium addition caused the appearance of a more heterogeneous distribution of active sites, whereas tin addition 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. Tin addition to Pt/Al 2 O 3 and Pt/CeO 2-Al 2 O 3 also decreased the Pt surface area due to formation of PtSn and possibly Pt-SnO x species. Cerium addition to Pt/Al 2 O 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. Cerium addition to Pt/Al 2 O 3 caused an increase in the catalytic activity for crotonaldehyde hydrogenation, whereas Sn addition to Pt/Al 2 O 3 decreased the activity of Pt/Al 2 O 3 catalysts. Higher reduction temperature caused an increase in the initial catalytic activity for crotonaldehyde hydrogenation for all the catalysts studied. Selectivity enhancements for crotyl alcohol formation in 2 crotonaldehyde hydrogenation were observed for the Ce and Sn promoted catalysts after a reduction at 773 K.