The strong metal–support interaction (SMSI) in Pt–TiO2 model catalysts. A new CO adsorption state on Pt–Ti atoms (original) (raw)
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Journal of Molecular Catalysis, 1984
X-ray photoelectron spectroscopy (XPS), electron microscopy and cyclic voltammetry have been used to characterize the behaviour of two differently prepared platinum catalysts supported on TiOz (anatase). One of these two kinds of catalysts, consisting of Pt deposits obtained by thermal hydrolytic decomposition of aqueous H*PtCl, onto Ti-supported TiOz films and subsequently reduced in argon at 550 "C, has been shown to exhibit a series of anomalous properties denoting the existence of strong metalsupport interaction (SMSI).
EPR characterization of Ti3+ ions at the metal-support interface in Pt/TiO2 catalysts
Journal of Catalysis, 1988
Titania-supported Pt catalysts prepared by cation exchange 0.44 and 1.0 wt% loading and reduced at low, medium, and high temperatures (513,573, and 773 K, respectively) were studied by EPR. The spectra of four different Ti3+. tons have been resolved at 17 K; only two of these species are surface Ti'+ which are sensitive to CO or Hr adsorption. One of these surface species, characterized by an axial g tensor at g, = 1.969 and 811 = 1.936, is specific of the presence of platinum since it does not exist in reduced TiOz. Its signal undergoes a temperature-dependent broadening effect which is mostly removed at 17 K; this is related to its proximity to a platinum particle allowing an efficient relaxation mechanism through the conduction electrons of the metal. This ion can be titrated reversibly by hydrogen and irreversibly by carbon monoxide. Its intensity relative to other Ti3+ ions increases when the reduction temperature increases from 373 to 573 K. This trend is probably maintained for the reduction at 773 K, but no accurate conclusions can be drawn since the EPK signal undergoes a strong decrease in intensity and line broadening due to spin pairing and spin-spin relaxation mechanisms. These two effects are related to the increase of Ti3+ concentration in the vicinity of platinum, the metal promoting the reduction of the support. This species, located at the metal-support interface, may either account for the TiO, fragments of the support which migrate on the metal particles in the SMSI state or be a precursor of these fragments.
Journal of Molecular Catalysis A: Chemical, 2015
The dynamics of the CO adsorption on Pt nanoparticles deposited on TiO 2 (101) (pure, N-doped and/or reduced) have been investigated using UV-visible diffuse reflectance spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy and density functional theory. The results point to that N-doping and oxygen vacancies in the Pt/N-TiO 2 system should favour catalytic reactions in which CO conversion into CO 2 takes place mediated by support surface O atoms.
Journal of Catalysis, 2008
The effects of alkali additives on the physicochemical and chemisorptive properties of 0.5% Pt/TiO 2 have been investigated over catalysts promoted with variable amounts of Na (0-0.2 wt%) or Cs (0-0.68 wt%) with the use of diffuse reflectance infrared spectroscopy (DRIFTS) and temperature-programmed (TPD, TPR) techniques. It has been found that addition of alkalis does not affect adsorption of CO and H 2 on the surface of Pt crystallites, indicating the absence of strong electronic-type interactions between these sites and the promoters. However, the presence of alkalis results in the creation and population of new sites with increased electron density, proposed to be located at perimetric sites of Pt crystallites, which are in contact with the support. The adsorption strength of these sites toward CO increases with increasing alkali content, which is evidenced by the development of new, low-frequency IR bands in the ν(CO) region. In contrast, addition of alkali results in weakening of hydrogen adsorption on sites located at the metal/support interface, which is reflected to a significant shift of the corresponding TPD peak toward lower temperatures. Results of CO-TPD experiments indicate that CO adsorbed on Pt interacts with hydroxyl groups associated with the support to yield formate, which decomposes during TPD to CO 2 and H 2. Thermal decomposition of formate is accomplished at lower temperatures in the presence of alkali. Finally, CO-TPR experiments indicate that the reducibility of TiO 2 is enhanced in the presence of alkali, which can be related to the creation of the new sites at the metal/support interface.
Journal of Catalysis, 2005
Model catalysts of Pd nanoparticles and films on TiO 2 (110) were fabricated by metal vapour deposition (MVD). Molecular beam measurements show that the particles are active for CO adsorption, with a global sticking probability of 0.25, but that they are deactivated by annealing above 600 K, an effect indicative of SMSI. The Pd nanoparticles are single crystals oriented with their (111) plane parallel to the surface plane of the titania. Analysis of the surface by atomic resolution STM shows that new structures have formed at the surface of the Pd nanoparticles and films after annealing above 800 K. There are only two structures, a zigzag arrangement and a much more complex "pinwheel" structure. The former has a unit cell containing 7 atoms, and the latter is a bigger unit cell containing 25 atoms. These new structures are due to an overlayer of titania that has appeared on the surface of the Pd nanoparticles after annealing, and it is proposed that the surface layer that causes the SMSI effect is a mixed alloy of Pd and Ti, with only two discrete ratios of atoms: Pd/Ti of 1:1 (pinwheel) and 1:2 (zigzag). We propose that it is these structures that cause the SMSI effect.
The Journal of Physical Chemistry B, 2005
TiO 2 -and γ-Al 2 O 3 -supported Pt catalysts were characterized by HRTEM, XPS, EXAFS, and in situ FTIR spectroscopy after activation at various conditions, and their catalytic properties were examined for the oxidation of CO in the absence and presence of H 2 (PROX). When γ-Al 2 O 3 was used as the support, the catalytic, electronic, and structural properties of the Pt particles formed were not affected substantially by the pretreatment conditions. In contrast, the surface properties and catalytic activity of Pt/TiO 2 were strongly influenced by the pretreatment conditions. In this case, an increase in the reduction temperature led to higher electron density on Pt, altering its chemisorptive properties, weakening the Pt-CO bonds, and increasing its activity for the oxidation of CO. The in situ FTIR data suggest that both the terminal and bridging CO species adsorbed on fully reduced Pt are active for this reaction. The high activity of Pt/TiO 2 for the oxidation of CO can also be attributed to the ability of TiO 2 to provide or stabilize highly reactive oxygen species at the metal-support interface. However, such species appear to be more reactive toward H 2 than CO. Consequently, Pt/TiO 2 shows substantially lower selectivities toward CO oxidation under PROX conditions than Pt/γ-Al 2 O 3 .
Electrochimica Acta, 2017
A facile approach to enhance catalytic activity and durability of TiO 2-supported Pt nanocatalysts by tuning strong metal support interaction (SMSI) is investigated in this work. No need for a high temperature treatment, the strong metal-support interaction (SMSI) in TiO 2-supported Pt can be induced at 200 C by H 2 reduction. Moreover, electrochemical methods (methanol oxidation reaction and cyclic voltammetry) are first reported ever to be effective characterization tools for the coverage state caused by SMSI. In addition, the SMSI has also been confirmed by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and Transmission Electron Microscopy. It is found that the encapsulation of TiO 2-x species on the surface Pt clusters was induced and modified by thermal reduction and fluoric acid treatment. The catalytic activity and durability of the TiO 2-supported Pt nanocatalysts are strongly dependent of the state of SMSI. The proposed SMSI-tunable approach to enhance the ORR activity and stability is also proved applicable to Pt/Ti 0.9 Nb 0.1 O 2 nanocatalysts. We believe that the reported approach paves the way for manipulating the activity and stability of other TiO 2-supported metal nanocatalysts. Furthermore, the suggested electrochemical methods offer facile and effective ways to verify the presence of coverage state before combining with other physical analysis.
Molecular Catalysis, 2017
TiO 2 is a widely used material due to its electronic and catalytic properties, which are of interest for technological applications. In catalysis it is generally used as support for different catalyzers, such as for example Pt subnanoclusters given that they improve the efficiency of the material. In this work we use an ab initio DFT + U modeling method to study the structure and energetic of Pt 4 clusters deposited on rutile TiO 2 (110) stoichiometric and reduced surfaces. For the Pt-titania system we examine the relative stability between the flat versus 3D tetrahedral Pt 4 structures, and characterize the cluster/substrate interaction. We determine their equilibrium geometries, adsorption energies, charge transfer effects and electronic density of states to characterize different aspects of the metal-oxide interaction. For both, the stoichiometric and reduced rutile TiO 2 (110), we find that the flat square configuration is preferred, as experiments indicate. In particular, we are interested in the potential activity of these cluster-supported systems for the oxidation of CO adsorbed on Pt. To examine this behavior we evaluate the structure, electronic DOS properties and charge transfer effects for the adsorption of CO on both the flat and tetrahedral Pt 4 isomers over the stoichiometric and reduced TiO 2 rutile surfaces. The results point to the planar cluster on the stoichiometric surface as the most stable configuration for CO adsorption, while for the CO conversion to CO 2 the tetrahedral Pt 4 cluster on the stoichiometric TiO 2 surface would be the most favorable catalytic substrate.