The Pt/Al2O3 System: Infrared Studies

Marchese, Leonardo; Boccuti, M.R.; Coluccia, S.; Lavagnino, S.; Zecchina, Adriano; Bonneviot, Laurent; Che, M.

doi:10.1016/s0167-2991(08)60726-0

Cited by 15 publications

(8 citation statements)

References 22 publications

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“…As observed by other authors,39 when studying bulk platinous compounds (Pt, Pt2+, and PP), white line intensities are a reliable indication of changes in Pt oxidation state. However, monitoring of the white line in this study is not a suitable method to establish quantitatively changes since the intensities of white lines in L-edges reflect two separate processes in the catalyst: variation in the metal particle size and variations in Ptn+/Pt (n = 2, 4) ratio.40 Although the second process determines to a higher degree the intensity of the white line, the fist one can be important in supported metals in highly dispersed state. 41 An extra complication arises from the presence of chemisorbed hydrogen which influences the intensity of the white line.…”

Section: Discussionmentioning

confidence: 90%

Decomposition of the Precursor [Pt(NH3)4](OH)2, Genesis and Structure of the Metal-Support Interface of Alumina Supported Platinum Particles: A Structural Study Using TPR, MS, and XAFS Spectroscopy

Muñoz‐Páez¹,

Koningsberger²

1995

J. Phys. Chem.

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During the preparation of alumina supported platinum catalysts, the precursor [Pt(NH3)4](OH)2 decomposes to a neutral Pt(NH3)zO species during the drying process at 120 "C. Treatment in flowing hydrogen at 180 "C leads to partial reduction of the platinum ammine complex and formation of platinum metal particles. A large increase in metal particle size is observed after a treatment under flowing H2 at 200 "C. The final reduction at 350 "C causes the total disappearance of the platinum precursor with a further increase in platinum particle size. The direct reduction at 350 "C yields the biggest metal particles (35 A) while calcination before reduction produces a much higher dispersion (metal particle diameter 10 A). The beneficial effect of calcination, already observed by many authxs when using [Pt(NH3)4](OH)2 as a precursor for the preparation of highly dispersed WyA1203, can now be explained because this treatment avoids the formation of the mobile neutral Pt(NH3)zO complex. The metal particles produced by treatment in flowing hydrogen at 180 "C present a metal-oxygen contribution at 2.7 A formed at the metal-support interface. This long distance is assumed to be caused by the presence of hydrogen in the metal-support interface based upon our results in combination with other TPD and EXAFS studies. A second metal-oxygen contribution with similar coordination number is detected at 3.86 A. This is a consequence of the presence of the first shell metal-oxygen at 2.7 8, and implies a [ 11 11 epitaxy in the metal-support interface.

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Section: Discussionmentioning

confidence: 90%

Decomposition of the Precursor [Pt(NH3)4](OH)2, Genesis and Structure of the Metal-Support Interface of Alumina Supported Platinum Particles: A Structural Study Using TPR, MS, and XAFS Spectroscopy

Muñoz‐Páez¹,

Koningsberger²

1995

J. Phys. Chem.

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“…Properties of Adsorbed CO. An indirect but efficient way to monitor changes in the electronic structure of a supported metal cluster compared with a free metal moiety is to analyze the adsorption properties of a probe molecule. The most widely used probe molecule is CO because its vibrational stretching frequency ω(CO) is sensitive to small variations of the electronic structure at the adsorption site of a metal cluster or particle , However, analysis of the CO vibrational frequency shifts indicates that it is mainly related to the amount of back-donation from filled d orbitals of the metal particle to the empty antibonding 2π* orbital of CO; Pauli repulsion from the metal substrate also plays a role .…”

Section: Resultsmentioning

confidence: 99%

Faujasite-Supported Ir₄Clusters: A Density Functional Model Study of Metal−Zeolite Interactions

et al. 1999

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The interaction of a metal cluster, Ir 4 , and a zeolite support was investigated computationally with the aid of a density functional method and a cluster model of a zeolite, i.e., a six-ring consisting of six O atoms and six T (Si or Al) atoms facing a supercage of a faujasite framework. Structural parameters are reported for an Ir 4 tetrahedron interacting with the zeolite six-ring. The calculations indicate two Ir-O distances, which match those reported on the basis of EXAFS spectroscopy at about 2.1-2.2 and 2.5-2.7 Å for various transition and noble metal clusters on zeolite (and metal oxide) supports, including Ir 4 in the supercages of zeolite NaY. The calculations indicate an Ir-Ir distance of about 2.5 Å, only a few hundredths of an Ångstrom more than the value calculated for the free Ir 4 cluster, but about 0.2 Å less than the values observed repeatedly by EXAFS spectroscopy for zeolite-supported clusters approximated as Ir 4 . The experimental distances characterizing the zeolite-supported clusters are in close agreement with the crystallographic and calculated value reported for the coordinatively saturated cluster Ir 4 (CO) 12 and favor the suggestion that the supported clusters investigated with EXAFS spectroscopy were not entirely ligand free (i.e., that their formation by decarbonylation of the parent Ir 4 (CO) 12 did not proceed by simple, complete removal of CO ligands). Consequently, calculations were performed for unsupported model clusters Ir 4 with single H or C atoms as ligands; the results match the EXAFS data characterizing the Ir-Ir distance and favor the suggestion of carbon on the zeolite-supported clusters. The bonding of a single CO molecule to the supported Ir 4 at the on-top site was also modeled to probe changes in the electronic structure of the metal cluster in comparison with an unsupported metal cluster. The results show that the interaction of the metal cluster with the zeolite fragment induces a notable electron donation from the support to the metal cluster; it also causes a moderate charge rearrangement in the bonding region between the Ir and O centers, accompanied by a considerable polarization of the electron density toward the apex of the cluster not interacting directly with the zeolite. Generalizing this result, we suggest that small noble metal clusters interacting mainly with basic oxygen atoms of zeolite and metal oxide supports are nearly zerovalent or slightly negatively charged and that some effects of supports in catalysis may be explained by this charge transfer and by polarization.

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“…Na, Na + , and NH 3 species have been attached to the Pt 4 cluster as ligands occupying a 3-fold position; in these models, the CO adsorbate has been placed in an on-top fashion at the opposite side of the cluster (structures D and E of Figure ). On-top adsorption has been experimentally found to be the most favorable for CO on platinum clusters and surfaces in the low-coverage regime. − Geometry optimizations for all the complexes containing Na, Na + , and NH 3 species, with or without adsorbed CO, were carried out under the C 3 v symmetry constraint. T d symmetry has been imposed to the neutral and charged Pt 4 reference clusters.…”

Section: Computational Methods and Modelsmentioning

confidence: 99%

“…[3][4][5][6][7][8][9][10][11][12][13][14] By use of extended X-ray adsorption fine structure (EXAFS) spectroscopy, very small encaged metal clusters of 4-10 atoms were detected in zeolites. 8,9,[15][16][17][18] CO molecules have been widely used to probe structural and electronic properties of supported metal particles benefiting from the high sensitivity of the C-O stretching frequency ω(CO) to the oxidation states of metal particles, 19,20 their size, 11,14,21 and the strength of metal-support interactions. 22 Correlations of the electronic state and size of the clusters with the C-O vibrational frequency shift ∆ω(CO) have been put forward.…”

Section: Introductionmentioning

confidence: 99%

“…Owing to the importance of transition metals and especially platinum as catalysts in many industrial processes, much work has been devoted during the past decade to the characterization of small metal particles entrapped in zeolite cages. − By use of extended X-ray adsorption fine structure (EXAFS) spectroscopy, very small encaged metal clusters of 4−10 atoms were detected in zeolites. ,,− CO molecules have been widely used to probe structural and electronic properties of supported metal particles benefiting from the high sensitivity of the C−O stretching frequency ω(CO) to the oxidation states of metal particles, , their size, ,, and the strength of metal−support interactions . Correlations of the electronic state and size of the clusters with the C−O vibrational frequency shift Δω(CO) have been put forward. ,, In addition to infrared (IR) spectroscopy, several other methods including X-ray photoelectron spectroscopy , (XPS) and catalytic tests, such as competitive hydrogenation of benzene and toluene, , were employed to characterize the electronic state of metal species confined to zeolite cavities.…”

Section: Introductionmentioning

confidence: 99%

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Small Platinum Clusters in Zeolites: A Density Functional Study of CO Adsorption on Electronically Modified Models

et al. 1998

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Very small transition metal particles can be stabilized inside zeolite cavities. Both electron-enriched and electron-deficient encapsulated metal species have been proposed on the basis of experimental data. In this work, structure and adsorption properties of the cluster Pt 4 , in both neutral and electronically modified forms, have been studied computationally with the help of a scalar-relativistic density functional method. The species Pt 4 + has been chosen to represent the case of a metal particle interacting with an electron attracting zeolite host; likewise, Pt 4 -has been taken to mimic the effect of an electron-donating host. Adsorption of CO probe molecules at on-top, bridge, and 3-fold hollow sites of the moieties Pt 4 , Pt 4 + , and Pt 4 -has been investigated to determine a relationship between the cluster charge and the C-O vibrational frequency shift ∆ω(CO). The chemical effect of electron-donor and electron-acceptor species on the electronic structure of the Pt 4 clusters and on the properties of adsorbed CO probes has been also explicitly taken into account by employing various models XPt 4 CO (X ) Na, Na + , NH 3 ). Properties of adsorbed CO probe molecules were calculated to be rather sensitive to the electronic state and the adsorption site of the Pt 4 particles, in line with experimental findings. A linear correlation between the effective charge of the metal cluster and the adsorption-induced vibrational frequency shift ∆ω(CO) has been found for CO adsorbed at on-top positions.

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The Pt/Al2O3 System: Infrared Studies

Cited by 15 publications

References 22 publications

Decomposition of the Precursor [Pt(NH3)4](OH)2, Genesis and Structure of the Metal-Support Interface of Alumina Supported Platinum Particles: A Structural Study Using TPR, MS, and XAFS Spectroscopy

Decomposition of the Precursor [Pt(NH3)4](OH)2, Genesis and Structure of the Metal-Support Interface of Alumina Supported Platinum Particles: A Structural Study Using TPR, MS, and XAFS Spectroscopy

Faujasite-Supported Ir₄Clusters: A Density Functional Model Study of Metal−Zeolite Interactions

Small Platinum Clusters in Zeolites: A Density Functional Study of CO Adsorption on Electronically Modified Models

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The Pt/Al2O3 System: Infrared Studies

Cited by 15 publications

References 22 publications

Decomposition of the Precursor [Pt(NH3)4](OH)2, Genesis and Structure of the Metal-Support Interface of Alumina Supported Platinum Particles: A Structural Study Using TPR, MS, and XAFS Spectroscopy

Decomposition of the Precursor [Pt(NH3)4](OH)2, Genesis and Structure of the Metal-Support Interface of Alumina Supported Platinum Particles: A Structural Study Using TPR, MS, and XAFS Spectroscopy

Faujasite-Supported Ir4Clusters: A Density Functional Model Study of Metal−Zeolite Interactions

Small Platinum Clusters in Zeolites: A Density Functional Study of CO Adsorption on Electronically Modified Models

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Faujasite-Supported Ir₄Clusters: A Density Functional Model Study of Metal−Zeolite Interactions