Author: Yisun ChengPublisher:
ISBN:
Category :
Languages : en
Pages : 286
Book Description
Studies of CO Oxidation Catalysis Using Pt/TiOx Thin Films as Supported Model Catalysts and Sensing Elements
Author: Yisun ChengDissertation Abstracts International
Author: Publisher:
ISBN:
Category : Dissertations, Academic
Languages : en
Pages : 924
Book Description
Thin Film Models of SMSI (Show Strong Metal-Support Interactions) Catalysts: Pt on Oxidized Titanium
Author: D. N. BeltyonPublisher:
ISBN:
Category :
Languages : en
Pages : 29
Book Description
Thin films of Pt, vapor deposited on an oxidized Ti foil, have been utilized as models of high surface area Pt/TiO2 catalysts which show strong metal-support interactions (SMSI). Both fully oxidized (Ti(4+)) and partially reduced (significant amounts of Ti(3+)) surfaces were studied. As a function of Pt coverage, the development of bulk Pt chemisorption characteristics occurs at significantly lower coverages on the oxidized substrate. In particular, at a given Pt coverage there is less chemisorption of both CO and H2 on the reduced, a compared to the oxidized, TiO2 and the desorption spectra peak at lower temperatures. No evidence for significant differences in the chemical state of the Pt is found by XPS and AES. We propose a model, consistent with earlier work on high surface area Pt/TiO2 catalysts, in which the morphology of the Pt on the reduced catalysts is significantly flatter (dominated by (111) terraces) and is slightly modified electronically, as compared to the oxidized sample.
Scientific and Technical Aerospace Reports
Author: First-principles Simulations of the Oxidation of Methane and CO on Platinum Oxide Surfaces and Thin Films
Author: Publisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
The catalytic oxidation activity of platinum particles in automobile catalysts is thought to originate from the presence of highly reactive superficial oxide phases which form under oxygen-rich reaction conditions. The thermodynamic stability of platinum oxide surfaces and thin films was studied, as well as their reactivities towards oxidation of carbon compounds by means of first-principles atomistic thermodynamics calculations and molecular dynamics simulations based on density functional theory. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of alpha-PtO2, which appears not to be reactive towards either methane dissociation or carbon monoxide oxidation. A PtO-like structure is most stable on the Pt(100) surface at oxygen coverages of one monolayer, while the formation of a coherent and stress-free Pt3O4 film is favoured at higher coverages. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The computed net driving force for the dissociation of methane on the Pt3O4(100) surface is much larger than on all other metallic and oxide surfaces investigated. Moreover, the enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO2 is observed to occur without any appreciable energy barrier in molecular dynamics simulations. These results, combined, indicate a high catalytic oxidation activity of Pt3O4 phases that can be relevant in the contexts of Pt-based automobile catalysts and gas sensors.
Catalysis By Gold
Author: Geoffrey C BondPublisher: World Scientific
ISBN: 1908979852
Category : Science
Languages : en
Pages : 383
Book Description
Gold has traditionally been regarded as inactive as a catalytic metal. However, the advent of nanoparticulate gold on high surface area oxide supports has demonstrated its high catalytic activity in many chemical reactions. Gold is active as a heterogeneous catalyst in both gas and liquid phases, and complexes catalyse reactions homogeneously in solution. Many of the reactions being studied will lead to new application areas for catalysis by gold in pollution control, chemical processing, sensors and fuel cell technology. This book describes the properties of gold, the methods for preparing gold catalysts and ways to characterise and use them effectively in reactions. The reaction mechanisms and reasons for the high activities are discussed and the applications for gold catalysis considered./a
Spatiotemporal Dynamics of Heterogeneous Catalysis
Author: Steven Y. YamamotoModification of Titanium Dioxide Support for Carbon Monoxide Oxidation
Author: Publisher:
ISBN:
Category : Carbon monoxide
Languages : en
Pages : 212
Book Description
This research is to study the effect of surface modification of TiO2 support on the catalytic activity in CO oxidation. Two methods were employed for surface modification of TiO2 support: the creation of defective sites on TiO2 surface and the modification with trace element. The results found that the presence of higher amount of Ti3+ on the larger crystallite size TiO2 stabilizes small metal particles during impregnation, calcination, and reduction steps via stronger metal-support interaction; hence higher CO oxidation activities. In addition, the modification of TiO2 support with trace element can improve the CO oxidation activity. All of element dopants (i.e., Al, Si and P), the Si-modified TiO2 showed the highest catalytic performance of Ag/TiO2 catalyst. However, there existed an optimum content of Si/Ti molar ratio at ca. 0.05-0.1 which resulted in an improved catalytic activity of Ag/TiO2 in CO oxidation. There was no improvement in CO oxidation activity of the Ag/TiO2 catalyst when the Si/Ti was further increased to 0.3 due probably to the formation of amorphous SiO2 instead of the Ti-O-Si bond. The P-modified TiO2 supported Ag catalysts using phosphorus precursor in the form of oxide promoted the weak adsorbed oxygen species and resulted in catalytic activity improvement in CO oxidation.
Model Catalyst Studies of the CO Oxidation Reaction on Titania Supported Gold Nanoclusters
Author: James Daniel StiehlPublisher:
ISBN:
Category : Catalysts
Languages : en
Pages :
Book Description
Charge Transfer and Support Effects in Heterogenous Catalysis
Author: Antoine HervierPublisher:
ISBN:
Category :
Languages : en
Pages : 119
Book Description
The kinetic, electronic and spectroscopic properties of two-dimensional oxide-supported catalysts were investigated in order to understand the role of charge transfer in catalysis. Pt/TiO2 nanodiodes were fabricated and used as catalysts for hydrogen oxidation. During the reaction, the current through the diode, as well as its I-V curve, were monitored, while gas chromatography was used to measure the reaction rate. The current and the turnover rate were found to have the same temperature dependence, indicating that hydrogen oxidation leads to the non-adiabatic excitation of electrons in Pt. A fraction of these electrons have enough energy to ballistically transport through Pt and overcome the Schottky barrier at the interface with TiO2. The yield for this phenomenon is on the order of 10−4 electrons per product molecule formed, similar to what has been observed for CO oxidation and for the adsorption of many different molecules. The same Pt/TiO2 system was used to compare currents in hydrogen oxidation and deuterium oxidation. The current through the diode under deuterium oxidation was found to be greater than under hydrogen oxidation by a factor of three. Weighted by the difference in turnover frequencies for the two isotopes, this would imply a chemicurrent yield 5 times greater for D2 compared to H2, contrary to what is expected given the higher mass of D2. Reversible changes in the rectification factor of the diode are observed when switching between D2 and H2. These changes are a likely cause for the differences in current between the two isotopes. In the nanodiode experiments, surface chemistry leads to charge flow, suggesting the possibility of creating charge flow to tune surface chemistry. This was done first by exposing a Pt/Si diode to visible light while using it as a catalyst for H2 oxidation. Absorption of the light in the Si, combined with the band bending at the interface, gives rise to a steady-state flow of hot holes to the surface. This leads to a decrease in turnover on the surface, an effect which is enhanced when a reverse bias is applied to the diode. Similar experiments were carried out for CO oxidation. On Pt/Si diodes, the reaction rate was found to increase when a forward bias was applied. When the diode was exposed to visible light and a reverse bias was applied, the rate was instead decreased. This implies that a flow of negative charges to the surface increases turnover, while positive charges decrease it. Charge flow in an oxide supported metal catalyst can be modified even without designing the catalyst as a solid state electronic device. This was done by doping stoichiometric and nonstoichiometric TiO2 films with F, and using the resulting oxides as supports for Pt films. In the case of stoichiometric TiO2, F was found to act as an n-type dopant, creating a population of filled electronic states just below the conduction band, and dramatically increasing the conductivity of the oxide film. The electrons in those states can transfer to surface O, activating it for reaction with CO, and leading to increased turnover for CO oxidation. This reinforces the hypothesis that CO oxidation is activated by a flow of negative charges to the surface. The same set of catalysts was used for methanol oxidation. The electronic properties of the TiO2 films again correlated with the turnover rates, but also with selectivity. With stoichiometric TiO2 as the support, F-doping caused an increase in selectivity toward the formation of partial oxidation products, formaldehyde and methyl formate, versus the total oxidation product, CO2. With non-stoichiometric TiO2, F-doping had the reverse effect. Ambient Pressure X-Ray Photoelectron Spectroscopy was used to investigate this F-doping effect in reaction conditions. In O2 alone, and in CO oxidation conditions, the O1s spectrum showed a high binding energy peak that correlated in intensity with the activity of the different films: for stoichiometric films, the peak decreased in intensity with F-doping, while for nonstoichiometric films, the opposite was observed. No such changes were visible in the C1s spectrum, confirming the role of O activation in the reaction. This thesis adds to the body of knowledge on the importance of charge transfer at the metal-oxide interface in shaping the reactivity of heterogeneous catalysts, and provides examples of how this can be the basis for new methods to tune reactivity.