Interaction of Pt clusters with the anatase TiO(2)(101) surface: a first principles study

The adsorption of Pt(n)() (n = 1-3) clusters on the defect-free anatase TiO(2)(101) surface has been studied using total energy pseudopotential calculations based on density functional theory. The defect-free anatase TiO(2)(101) surface has a stepped structure with a step width of two O-Ti bond distances in the (100) plane along the [10] direction and the edge of the step is formed by 2-fold-coordinated oxygen atoms along the [010] direction. For a single Pt adatom, three adsorption sites were found to be stable. Energetically, the Pt adatom prefers the bridge site formed by 2 2-fold-coordinated oxygen atoms with an adsorption energy of 2.84 eV. Electronic structure analysis showed that the Pt-O bonds formed upon Pt adsorption are covalent. Among six stable Pt(2) adsorption configurations examined, Pt(2) was found to energetically favor the O-O bridge sites on the step edge along [010] with the Pt-Pt bond axis perpendicular to [010]. In these configurations, one of the Pt atoms occupies the same O-O bridge site as for a single Pt adatom and the other one either binds a different 2-fold-coordinated oxygen atom on the upper step or a 5-fold-coordinated Ti atom on the lower terrace. Three triangular and three open Pt(3) structures were determined as minima for Pt(3) adsorption on the surface. Platinum trimers adsorbed in triangular structures are more stable than in open structures. In the most stable configuration, Pt(3) occupies the edge O-O site with the Pt(3) plane being upright and almost perpendicular to the [001] terrace. The preference of Pt(n)() to the coordinately unsaturated 2-fold-coordinated oxygen sites indicates that these sites may serve as nucleation centers for the growth of metal clusters on the oxide surface. The increase in clustering energy with increasing size of the adsorbed Pt clusters indicates that the growth of Pt on this surface will lead to the formation of three-dimensional particles.     

Characterizing TiO2(110) surface states by their work function

The unreconstructed TiO(2)(110) surface is prepared in well-defined states having different characteristic stoichiometries, namely reduced (r-TiO(2), 6 to 9% surface vacancies), hydroxylated (h-TiO(2), vacancies filled with OH), oxygen covered (ox-TiO(2), oxygen adatoms on a stoichiometric surface) and quasi-stoichiometric (qs-TiO(2), a stoichiometric surface with very few defects). The electronic structure and work function of these surfaces and transition states between them are investigated by ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES). The character of the surface is associated with a specific value of the work function that varies from 4.9 eV for h-TiO(2), 5.2 eV for r-TiO(2), 5.35 eV for ox-TiO(2) to 5.5 eV for qs-TiO(2). We establish the method for an unambiguous characterization of TiO(2)(110) surface states solely based on the secondary electron emission characteristics. This is facilitated by analysing a weak electron emission below the nominal work function energy. The emission in the low energy cut-off region appears correlated with band gap emission found in UPS spectra and is attributed to localised electron emission through Ti(3+)(3d) states.     

Effect of the TiO2 reduction state on the catalytic CO oxidation on deposited size-selected Pt clusters

The catalytic activity of deposited Pt(7) clusters has been studied as a function of the reduction state of the TiO(2)(110)-(1 × 1) support for the CO oxidation reaction. While a slightly reduced support gives rise to a high catalytic activity of the adparticles, a strongly reduced one quenches the CO oxidation. This quenching is due to thermally activated diffusion of Ti(3+) interstitials from the bulk to the surface where they deplete the oxygen adsorbed onto the clusters by the formation of TiO(x) (x ? 2) structures. This reaction is more rapid than the CO oxidation. The present results are of general relevance to heterogeneous catalysis on TiO(2)-supported metal clusters and for reactions involving oxygen as intermediate.     

Small Au and Pt clusters at the anatase TiO2(101) surface: behavior at terraces, steps, and surface oxygen vacancies

The adsorption properties of Au and Pt metal nanoclusters on TiO2 anatase (101) were calculated using density functional theory. Structures and energetics of adsorbed Au and Pt monomers, dimers, and trimers at clean anatase TiO2(101) terraces and two major step edges, as well as O-vacancies, were systematically determined. The theoretical predictions were tested by vapor-depositing small coverages of Au and Pt on anatase (101) and investigating the resulting clusters with Scanning Tunneling Microscopy. On the clean surface, Au shows a strong tendency to form large clusters that nucleate on step edges. A preference for adsorption at type D-(112) steps is observed, which is probably a result of kinetic effects. For Pt, clusters as small as monomers are observed on the terraces, in agreement with the predicted large binding energy of 2.2 eV. Step edges play a less important role than in the case of Au. Oxygen vacancies, produced by electron irradiation, dramatically influence the growth of Au, while the nucleation behavior of Pt was found to be less affected.     

Influence of Pt nanocrystallinity on electrochromism of TiO2

The Pt nanocrystals as nanonetwork conductors improved electrochromic properties of TiO2 such as increased optical modulation and fast response time, compared to pure TiO2 as well as TiO2 incorporated by amorphous Pt nanophases. It is likely that an enhancement of cathodic electrochromism in TiO2-Pt is due to an improved injection or extraction of electron/proton caused by Pt nanocrystals.     

Kinetics of Benzene and Toluene Hydrogenation on a Pt/TiO2 Catalyst

The kinetics and mechanism of benzene and toluene hydrogenation on a Pt/TiO₂ catalyst were studied in steady-state and non-steady-state regimes in the presence or absence of strong metal–support interactions (SMSI). It was found that the kinetics and mechanism of the test reactions were independent of SMSI. The observed effect of a decrease in the catalytic activity in the SMSI state was due to structural changes in the active centers of the catalyst and to the presence of strongly bound hydrogen species on the surface; this was also supported by thermal-desorption data.     

Variable work function of semiconducting thin-film oxide electrodes: a case study of SnO2 and TiO2

Journal of Solid State Electrochemistry - By atomic layer deposition, we prepared TiO2 thin films, which do not crack upon thermal treatment at 450–500 °C. The calcination...     

Reactivity of sub 1 nm supported clusters: (TiO2)n clusters supported on rutile TiO2 (110)

Metal oxide clusters of sub-nm dimensions dispersed on a metal oxide support are an important class of catalytic materials for a number of key chemical reactions, showing enhanced reactivity over the corresponding bulk oxide. In this paper we present the results of a density functional theory study of small sub-nm TiO(2) clusters, Ti(2)O(4), Ti(3)O(6) and Ti(4)O(8) supported on the rutile (110) surface. We find that all three clusters adsorb strongly with adsorption energies ranging from -3 eV to -4.5 eV. The more stable adsorption structures show a larger number of new Ti-O bonds formed between the cluster and the surface. These new bonds increase the coordination of cluster Ti and O as well as surface oxygen, so that each has more neighbours. The electronic structure shows that the top of the valence band is made up of cluster derived states, while the conduction band is made up of Ti 3d states from the surface, resulting in a reduction of the effective band gap and spatial separation of electrons and holes after photon absorption, which shows their potential utility in photocatalysis. To examine reactivity, we study the formation of oxygen vacancies in the cluster-support system. The most stable oxygen vacancy sites on the cluster show formation energies that are significantly lower than in bulk TiO(2), demonstrating the usefulness of this composite system for redox catalysis.     

Photocatalytic decomposition of gaseous acetone using TiO2 and Pt/TiO2 catalysts

Metallic platinum was photodeposited on TiO₂ particles, and morphological characteristics of the Pt/TiO₂ catalyst were determined. The dispersion of metallic platinum was uniform and did not alter the morphology of the TiO₂ particles. However, absorbance of the Pt/TiO₂ catalyst for light with wavelength more than 400 nm was significantly improved by the addition of metallic platinum. Gaseous acetone was decomposed in an annular photoreactor coated with TiO₂ or Pt/TiO₂ catalysts using a UV or a fluorescent lamp as light source. The decomposition of acetone with the application of a UV lamp was obviously enhanced for experiments conducted with Pt/TiO₂ catalyst. Decomposition of acetone was promoted considerably with increasing oxygen concentration for experiments conducted with oxygen less than 50,000 ppmv, yet the decomposition of acetone was kept relatively constant for experiments conducted with oxygen above 50,000 ppmv. On the basis of the mass balance for carbon species, the amount of organic intermediates formed for experiments conducted under various conditions was found to be minimal. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 209–216, 2008     

Carbon monoxide oxidation on Pt/TiO2 sol-gel catalysts

Comparative results (specific area, metallic dispersion, and activity in the carbon monoxide oxidation) on sol-gel and impregnated Pt/TiO₂ catalysts are presented. In order to explain the important differences between the two preparations, among them high resistivity of sol-gel catalyst to sintering, the formation of anchored and/or partially buried particles into the support is proposed.     

Effect of TiO2 Calcination Pretreatment on the Performance of Pt/TiO2 Catalyst for CO Oxidation

In order to improve the CO catalytic oxidation performance of a Pt/TiO₂ catalyst, a series of Pt/TiO₂ catalysts were prepared via an impregnation method in this study, and various characterization methods were used to explore the effect of TiO₂ calcination pretreatment on the CO catalytic oxidation performance of the catalysts. The results revealed that Pt/TiO₂ (700 °C) prepared by TiO₂ after calcination pretreatment at 700 °C exhibits a superior CO oxidation activity at low temperatures. After calcination pretreatment, the catalyst exhibited a suitable specific surface area and pore structure, which is beneficial to the diffusion of reactants and reaction products. At the same time, the proportion of adsorbed oxygen on the catalyst surface was increased, which promoted the oxidation of CO. After calcination pretreatment, the adsorption capacity of the catalyst for CO and CO₂ decreased, which was beneficial for the simultaneous inhibition of the CO self-poisoning of Pt sites. In addition, the Pt species exhibited a higher degree of dispersion and a smaller particle size, thereby increasing the CO oxidation activity of the Pt/TiO₂ (700 °C) catalyst.     

Photocatalytic reactivity of surface platinized TiO2: substrate specificity and the effect of Pt oxidation state

Surface platinized TiO(2) (Pt/TiO(2)) has been frequently studied, but its photocatalytic reactivities reported in the literature are not consistent in some cases. To understand the discrepancies, the effects of Pt speciation on TiO(2) on the photocatalytic degradation (PCD) of a few chlorinated organic compounds (trichloroethylene (TCE), perchloroethylene (PCE), dichloroacetate, etc.) were investigated with several Pt/TiO(2) samples that were prepared differently. The oxidation state of Pt deposits was analyzed by X-ray photoelectron spectroscopy and was found to be the most important factor in determining the initial PCD rates of chlorinated organic compounds. TiO(2) with oxidized Pt species (Pt(ox)/TiO(2)) was less reactive than TiO(2) with metallic Pt (Pt(0)/TiO(2)) for all substrates tested. In particular, Pt(ox)/TiO(2) strongly inhibited the PCD of TCE and PCE whereas it was more reactive than pure TiO(2) for the PCD of other compounds. The photocurrents obtained with the Pt(ox)/TiO(2) electrode were lower than those with the Pt(0)/TiO(2) electrode, which was ascribed to the role of Pt(ox) species as a recombination center. It is proposed that TCE adsorbed on Pt(ox) chemically mediates the charge recombination through the redox cycle of TCE. The Pt effects in photocatalysis are highly substrate-specific and depend on the Pt-substrate interaction as well as the properties of Pt deposits.     

Pt/TiO2光催化降解甲苯

选择甲苯作为研究对象,探讨了在不同光源照射、不同光催化剂的作用下,气相甲苯的降解情况,结果表明甲苯在紫外杀菌灯照射下有明显的光化学反应发生。在光催化剂作用、转折灯照射时,甲苯发生催化氧化反应;并对催化剂在降解甲苯反应中的失活原因做了初步探讨。     

Synthesis of TiO2 photocatalysts with abundant surface defects using a TiO2@NaCl precursor

TiO2 nanoparticles have been synthesized by using a TiO2@NaCl core-shell structure as the precursor. The surface defects were well preserved by the NaCl shell, and therefore high oxygen adsorption capacity was observed. After the NaCl shell was removed, the resulting pure TiO2 nanoparticles were of anatase phase and uniform size of around 20?24 nm. The presence of an abundance of surface defects contributes to the high photocatalytic activity of the synthesized materials, and the TiO2 materials obtained from the TiO2@NaCl precursor can be used as efficient photocatalysts for degradation of rhodamine B under UV light irradiation.     

Local reactivity of O2 with Pt3 on Co3Pt and related backgrounds

We study the local reactivity of molecular oxygen with bimetallic substrates of a platinum trimer island supported on nanotips of CoPt, Pt, Co, Ni, and Fe. Because of the reduced interatomic distances and varying interaction strengths with the substrates, the supported island interaction with oxygen can be tuned from stronger to weaker relative to the interaction of a freestanding island with oxygen despite that there is no well-behaved trend with the binding energy of the island to the substrates.     

Semiempirical calculations of TiO2 (rutile) clusters

The densities of states for small (TiO₂)_x-clusters, x = 1, 3, 6, 9, and 14, have been calculated by means of the INDO method. The shape of the valence bands' density of states (DOS ) are discussed in terms of the distribution of coordination numbers. A one-slab cluster with uniform distribution of the coordination numbers was used to compare our calculations with experimental spectra. The photoelectric DOS and DOS for a cluster with an oxygen vacancy are in very good agreement with experimental findings for the TiO₂ (001) surface. O1s core level shifts between a surfacelike and a bulklike oxygen atom have been estimated. It is concluded that the observed surface–bulk shift for the TiO₂ (001) surface contains a substantial relaxation contribution. © 1992 John Wiley & Sons, Inc.     

Adsorption and dissociation of H2O2 on Pt and Pt-alloy clusters and surfaces

The adsorption of H(2)O(2) on Pt and Pt-M alloys, where M is Cr, Co, or Ni, is investigated using density functional theory. Binding energies calculated with a hybrid DFT functional (B3PW91) are in the range of -0.71 to -0.88 eV for H(2)O(2) adsorbed with one of the oxygen atoms on top Pt positions of Pt(3), Pt(2)M, and PtM(2), and enhanced values in the range of -0.81 to -1.09 eV are found on top Ni and Co sites of the Pt(2)M clusters. Adsorption on top sites of Pt(10) yields a weaker binding of -0.48 eV, whereas on periodic Pt(111) and Pt(3)Co(111) surfaces, H(2)O(2) generally dissociates into two OH radicals. On the other hand, attempts to attach H(2)O(2) on bridge sites cause spontaneous dissociation of H(2)O(2) into two adsorbed OH radicals, suggesting that stable adsorptions on bridge sites are not possible for any of the clusters or extended surfaces that are being studied. We also found that the water-H(2)O(2) interaction reduces the strength of the adsorption of H(2)O(2) on these clusters and surfaces.     

Dynamics of water confined on a TiO2 (anatase) surface

Inelastic neutron scattering has been employed to probe the vibrational density of states of water confined by an oxide surface, namely, nanoparticles of the anatase polymorph of TiO2. The heat capacity of confined water has been measured by adiabatic calorimetry and compared with values derived from the vibrational density of states. Both inelastic neutron scattering and calorimetry demonstrate restricted mobility and lower heat capacity and entropy of confined water as compared to the bulk.     

DRIFTS investigation and DFT calculation of the adsorption of CO on Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2

Molecular structures and vibrational spectra of the CO species adsorbed on the Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2 have been investigated by means of density functional theory (DFT) calculation and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The geometrical structures and vibrational frequencies were obtained at the MPW1PW91/SDD level. Theoretical calculation shows that the calculated IR spectra were in good agreement with the experimental results. The calculated results clarify the assignment of the adsorbed CO species on the surface of Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2.