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     

Propane oxidation with chemically bound oxygen on Pt/TiO2/Al2O3 and Pt/CeO2/Al2O3 catalysts

Possible mechanisms are suggested for propane oxidation on Pt/TiO₂/Al₂O₃ and Pt/CeO₂/Al₂O₃ catalysts in the cyclic reactant supply mode. As compared to the steady-state process, the process conducted as catalyst oxidation-reduction cycles results in a very different product composition: it is more selective toward partial oxidation products and yields much smaller amounts of complete oxidation products. It is established by isothermal and temperature-programmed oxygen desorption that, under the reaction conditions examined, the oxygen desorbed from the catalyst surface into the gas phase makes a negligible contribution to propane oxidation. It is proved by XPS that propane oxidation is due to the chemically bound oxygen of the catalyst. The hypothetical mechanism of the process includes propane activation on Pt followed by the transfer of the activated species to the oxygen-storing component (TiO₂ or CeO₂), where the intermediates are oxidized by chemically bound oxygen.     

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.     

Magnetic susceptibility of ultradispersed rutile and related Pt/TiO2 catalysts

The magnetic susceptibilities of ultradispersed rutile and catalysts 1% Pt/TiO₂ and 3% Pt/TiO₂ were measured. Rutile-based catalysts are paramagnetic, and their magnetic susceptibilities increase as the platinum content increases. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1356–1359, July, 1998.     

First principles study of CO oxidation on TiO2(110): the role of surface oxygen vacancies

The reactivities of the stoichiometric and partially reduced rutile TiO2(110) surfaces towards oxygen adsorption and carbon monoxide oxidation have been studied by means of periodic density functional theory calculations within the Car-Parrinello approach. O2 adsorption as well as CO oxidation are found to take place only in the presence of surface oxygen vacancies (partially reduced surface). The oxidation of CO by molecularly adsorbed O2 at the O-vacancy site is found to have an activation energy of about 0.4 eV. When the adsorbed O2 is dissociated, the resulting adatoms can oxidize incoming gas-phase CO molecules with no barrier. In all studied cases, once CO is oxidized to form CO2, the resulting surface is defect-free and no catalytic cycle can be established.     

The real role of carbon in Pt/C catalysts for oxygen reduction reaction

A new electrocatalysis of carbon materials for oxygen reduction reaction (ORR) on Pt/C catalysts was discovered. It was found that there exist two kinds of electroactive sites on these supports of carbon materials, which can effectively electrocatalyze the reduction of peroxide intermediated from oxygen reduction on Pt, as this provides continuous driving force to move the equilibrium toward the production of peroxide from ORR.     

Selective photoreduction of nitric oxide to nitrogen by nanostructured TiO2 photocatalysts: role of oxygen vacancies and iron dopant

Conventional TiO(2)-based photocatalysts oxidize NO(x) to nitrate species, which do not spontaneously desorb and therefore deactivate the catalyst. We show that the selectivity of this reaction can be changed by creating a large concentration of oxygen vacancies in TiO(2) nanoparticles through thermal reduction in a reducing atmosphere. This results in the photoreduction of nitric oxide (NO) to N(2) and O(2), species which spontaneously desorb at room temperature. The activity of the photoreduction reaction can be greatly enhanced by doping the TiO(2) nanoparticles with Fe(3+), an acceptor-type dopant that stabilizes the oxygen vacancies. Moreover, the photoinduced reduction of Fe(3+) to Fe(2+) provides a recombination pathway that almost completely suppresses the formation of NO(2) and thus enhances the selectivity of the reaction for N(2) formation. Gas chromatography confirms that N(2) and O(2) are formed in a stoichiometric ratio, and the activity for NO decomposition is found to be limited by the concentration of oxygen vacancies. A series of internally consistent reaction equations are proposed that describe all experimentally observed features of the photocatalytic process. The observed influence of oxygen vacancies on the activity and selectivity of photoinduced reactions may lead to new routes toward the design of highly selective photocatalysts.     

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.     

The photocatalytic and thermal catalytic reduction of CO2 with H2 over Pt/TiO2 catalysts

浸渍法制备了Pt负载量为0.5 to 2%的Pt/TiO2催化剂,考察它们在光照和加热条件下二氧化碳催化加氢性能.结果表明,二氧化碳加氢反应均可在Pt/TiO2的催化下进行,但在不同反应条件下加氢反应通过不同方式进行.在加热条件下,二氧化碳可转化为一氧化碳和甲烷,且在低温加热条件下一氧化碳是主产物(CO选择性为100%,250℃,0.5%Pt/TiO2).在1.5%Pt/TiO2催化剂上,当反应温度从250℃升高到450℃时,CH4的选择性由0增加到60.94%.同时,增加Pt的负载量也会导致CH4的选择性的增加.然而,在光照条件下,产物只有甲烷.CO2-TPD结果表明,二氧化碳通过羰基基团与作为吸附中心的Pt相连接.结合催化活性与表征结果,提出在光照条件下,反应可能以二氧化碳和氢气分别被光生电子活化反应生成甲酸中间体,随后经由甲酸加氢和脱水生成甲烷的机理进行.而在加热条件下,反应可能以二氧化碳首先吸附在催化剂表面形成羰基Pt物种,随后加氢生成一氧化碳,一氧化碳继续加氢生成甲烷的机理进行.     

Characterization and reactivity of copper oxide catalysts supported on TiO2-ZrO2

A series of copper catalysts supported on TiO2-ZrO2 with copper loading varying from 1.0 to 21.6 wt % were prepared by a wet impregnation method. The catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy, electron spin resonance (ESR), temperature programmed reduction (TPR), and Brunauer-Emmett-Teller specific surface area measurements. Copper dispersion and metal area were determined by N2O decomposition by the passivation method. XRD results suggest that the copper oxide is present in a highly dispersed amorphous state at copper loadings <16.8 wt % in the sample and as a crystalline CuO phase at higher Cu loadings. Copper dispersion increases with Cu loading up to 5.1 wt % and levels off at higher loadings. The XPS peak intensity ratios of Cu 2p(3/2)/Ti 2p(3/2) and Cu 2p(3/2)/Zr 3d(5/2) were compared with the copper dispersion calculated from N2O decomposition. ESR results suggest the presence of two types of copper species on the TiO2-ZrO2 support. TPR profiles reveal the presence of highly dispersed copper oxide at lower temperatures and bulk CuO at higher temperatures. The catalytic properties were evaluated for the vapor-phase dehydrogenation of cyclohexanol to cyclohexanone and related to the dispersion of Cu on TiO2-ZrO2.     

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.     

Temperature dependence of oxygen reduction activity at Nafion-coated bulk Pt and Pt/carbon black catalysts

Oxygen reduction reaction (ORR) activity and H(2)O(2) formation at Nafion-coated film electrodes of bulk-Pt and Pt nanoparticles dispersed on carbon black (Pt/CB) were investigated in 0.1 M HClO(4) solution at 30 to 110 degrees C by using a channel flow double electrode method. We have found that the apparent rate constants k(app) (per real Pt active surface area) for the ORR at bulk-Pt (with and without Nafion-coating) and Nafion-coated Pt/CB (19.3 and 46.7 wt % Pt, d(Pt) = 2.6 to 2.7 nm) thin-film electrodes were in beautiful agreement with each other in the operation conditions of polymer electrolyte fuel cells (PEFCs), i.e., 30-110 degrees C and ca. 0.7 to 0.8 V vs RHE. The H(2)O(2) yield was 0.6-1.0% at 0.7-0.8 V on all Nafion-coated Pt/CB and bulk-Pt and irrespective of Pt-loading level and temperature. Nafion coating was pointed out to be a major factor for the H(2)O(2) formation on Pt catalysts modifying the surface property, because H(2)O(2) production was not detected at the bulk-Pt electrode without Nafion coating.     

Precursor chemistry for TiO2: titanium complexes with a mixed nitrogen/oxygen ligand sphere

Novel mixed amido-malonato complexes of titanium are reported. The complexes were synthesized by partially replacing the amido groups from the complexes [Ti(NMe2)4] and [Ti(NEt2)4] via Br?nstedt acid/base reactions, using the malonate-ligands di-isopropylmalonate (Hdpml) and di-tert-butylmalonate (Hdbml). Four representative complexes were synthesized and fully characterised by 1H NMR, 13C NMR, CHN analysis and mass spectrometry. The crystal structures of the six-coordinated complexes [Ti(NMe2)2(dbml)2] (3) and [Ti(NEt2)2(dbml)2] (4) are presented and discussed. The complexes are solids and the chemical and thermal characteristics of the complexes strongly depend on the substitution at the malonate ligand. While dpml containing complexes show a promising behaviour for classical MOCVD, dbml containing complexes seem to be more suitable for liquid injection-metal-organic chemical vapour deposition (LI-MOCVD). Based on its thermal characteristics, the most promising complex for thermal CVD, [Ti(NEt2)2(dpml)2] (2) was selected for preliminary MOCVD experiments, which indicate a good suitability for the deposition of TiO2 thin films.     

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.     

Charge state and activity of Pt/C catalysts in oxygen reduction reaction

Electrochemical experiments with a rotating disk electrode are used to measure specific catalytic activity of Pt/C structures in the oxygen reduction reaction at the density of Pt nanoparticles on the glassy carbon support surface below one monolayer. The specific activity maximum is found at the coverage of about 0.4 monolayer. An explanation of the observed dependence is suggested that is based on consideration of the relationship between the surface density and charge state of the system of metallic catalyst particles. A numeric model is developed that describes charge transfer in the catalyst structure due to the difference in the work functions between the metal nanoparticles and support with account for the discrete nature of the nanoparticle charging and their mutual polarization. Calculations show that the carbon support coverage by Pt particles of about 0.4 monolayer corresponds to the largest amount of charged particles with the maximum energy of electrons, which provides the maximum catalyst activity and explains the dependence observed in the experiment.     

Study of well-characterized Pt?Ru/C catalysts by oxygen electroreduction

A series of well characterized Pt–Ru bimetallic catalysts supported on graphite has been studied by means of the electroreduction of oxygen in a supporting electrolyte of 1 N H₂SO₄. A synergetic effect is observed when the activity per gram of catalyst is considered. However, this effect disappears when the activity values are corrected by the active surface area.

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Photocatalytic decomposition of liquid-water on the Pt-loaded TiO2 catalysts: Effects of the oxidation states of Pt species on the photocatalytic reactivity and the rate of the back reaction

The photocatalytic decomposition of liquid water on Pt-loaded TiO₂ (Pt/TiO₂) catalysts was investigated. The results obtained by XPS and XRD measurements of the catalysts as a function of the calcination temperature as well as the photocatalytic decomposition reactions of H₂O clearly indicate that controlling the oxidation state of Pt as well as the amount of loaded Pt species are both important factors in the design of water-splitting photocatalysts having high efficiency and stoichiometry.     

Au/TiO2催化剂上非催化CO氧化反应中活性氧物种的生成和消除

在定量的瞬时产物分析(TAP)反应器中,于80 oC下采用CO脉冲和O2脉冲补充等方法,研究了高温(400 oC)焙烧的Au/TiO2催化剂上活性氧物种的移除反应活性,特别是活性氧物种的性质。以往的研究大多关注的是CO催化氧化反应中活性氧物种及其性质,在典型的反应条件下该物种的形成和消除是可逆的;而本研究表明,催化剂直接焙烧后就存在额外的氧物种;该物种对CO氧化反应也具有活性,但其在典型的反应条件下不生成或生成很少。基于此,讨论了Au/TiO2催化剂上CO氧化反应的机理,特别是不同活性氧物种的作用。     

Highly improved oxygen reduction performance over Pt/C-dispersed nanowire network catalysts

Electrochemical reduction of oxygen is studied over a novel nanowire network catalyst made of highly-dispersed Pt nanoparticles into electrospun Pt nanowire network architecture, which shows an excellent mass activity increase by 50% or higher per equal Pt mass than the conventional cathode electrocatalysts of Pt/C in polymer electrolyte membrane fuel cells.