Empty semiconductor surface states: core-level photoyield (original) (raw)
Related papers
The effect of acid treatment on the surface chemistry and topography of graphite
Carbon, 2013
Highly oriented pyrolytic graphite (HOPG) samples were investigated as model catalyst supports. The surfaces were treated with dilute HCl and HNO 3 under ambient conditions and examined with atomic force microscopy and scanning tunnelling microscopy (STM) and Xray photoelectron spectroscopy (XPS). Raised features were formed on the HOPG surface after acid treatment. These protrusions were typically 4-6 nm in height and between 10 to 100 nm in width, covering 5% to 20% of the substrate for acid concentrations between 0.01 and 0.2 M. Both width and surface density of the features increases with acid concentration but the heights are not affected. STM images show that the graphite lattice extends over the protrusions indicating that the features are "blisters" on the surface rather than deposited material, a view that is supported by the XPS which shows no other significant adsorbates except for oxygen in the case of the nitric acid. We propose that penetration of the acid at defective sites leads to a decrease in the interplanar van der Waals forces and a local delamination similar to the "bubbles" reported between exfoliated graphene sheets and a substrate. These findings are important in the context of understanding how carbon supports stabilise active components in heterogeneous catalysts.
2003
This review substantiates the molecular approach to the study of the catalytic action of various systems, which consists in the comparative study of the nature and reactivity of adsorbed species and considering the problems of pressure and material gaps. The pressure gap problem can be solved by a continuous increase in the pressure of the reaction mixture, including carrying out in situ studies. The solution to the problem of material gap is possible when one passes from bulk to dispersed samples, which model real supported catalysts. As the last step that can build a bridge between surface science and catalysis, the study of nanoparticle reactivity toward the reactants of a catalytic reaction with varying sizes of nanoparticles is proposed. The scope of such an approach is demonstrated by the study of silver catalysts of ethylene epoxidation. It was found that the catalytic action of silver in the process of ethylene oxide synthesis is determined by the possibility of formation of electrophilic adsorbed atomic oxygen. Its formation is more efficient under the action of reaction mixtures at high pressures and on the surfaces of silver species with sizes smaller than 50 nm. It is shown that the reaction center should also contain the nucleophilic form of O ads , which itself is only active in the complete oxidation of ethylene but creates the Ag 1+ sites for ethylene adsorption. The disappearance of O nucl with a decrease in the size of silver particles below 50 nm leads to a drastic decrease in the rate of ethylene epoxidation. The reaction mechanism made it possible to propose systems with an abnormally high value of selectivity to ethylene oxide (>90%).
Applied Catalysis A: General, 2013
Alumina supported bimetallic overlayer catalysts of platinum on nickel (Ni@Pt) and platinum on cobalt (Co@Pt) were synthesized using the directed deposition technique. First principle computational and single crystal literature studies in overlayer systems have indicated that overlayer formation can have an electronic effect on the surface Pt metal that ultimately will result in decreased binding strength for adsorbed species such as H 2 and CO. In order to determine electronic modifications to the platinum metal surface from overlayer synthesis, several characterization techniques were examined. These techniques include: hydrogen and carbon monoxide chemisorption, an ethylene hydrogenation descriptor reaction, and X-ray absorption spectroscopy studies. Non-structured bimetallic alloy catalysts were also examined (Ni-Pt and Co-Pt) and when compared to overlayer catalysts exhibited notably different behavior. Hydrogen and carbon monoxide chemisorption results demonstrated that overlayer catalysts showed decreased adsorption of both species when compared to both their Pt and non-structured bimetallic counterparts indicating the formation of an overlayer system rather than alloyed or monometallic clusters on the support surface. Ni@Pt and Co@Pt catalysts also demonstrated reduced activity for ethylene hydrogenation when compared to both Pt only and non-structured bimetallic alloy catalysts, further indicating overlayer formation. An observed increase in XAS white line intensity for the overlayer catalysts when compared to pure Pt also indicates a change in the electronic structure of the Pt overlayer consistent with chemisorption, reactivity, and computational predictions. These findings all suggest that the directed deposition technique effectively modified the behavior of the surface Pt in a manner consistent with literature predictions.
A Nexafs Study of Nitric Oxide Layers Adsorbed from a nitrite Solution onto a Pt(111) Surface
Journal of Physical Chemistry C, 2008
NO molecules adsorbed on a Pt(111) surface from dipping in an acidic nitrite solution are studied by near edge X-ray absorption fine structure spectroscopy (NEXAFS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) techniques. LEED patterns and STM images show that no long range ordered structures are formed after NO adsorption on a Pt(111) surface. Although the total NO coverage is very low, spectroscopic features in N K-edge and O K-edge absorption spectra have been singled out and related to the different species induced by this preparation method. From these measurements it is concluded that the NO molecule is adsorbed trough the N atom in an upright conformation. The maximum saturation coverage is about 0.3 monolayers, and although nitric oxide is the major component, nitrite and nitrogen species are slightly co-adsorbed on the surface. The results obtained from this study are compared with those previously reported in the literature for NO adsorbed on Pt(111) under UHV conditions.
Nexafs Study of Nitric Oxide Layers Adsorbed from a Nitrite Solution onto a Pt(111) Surface
The Journal of Physical Chemistry C, 2008
NO molecules adsorbed on a Pt(111) surface from dipping in an acidic nitrite solution are studied by near edge X-ray absorption fine structure spectroscopy (NEXAFS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) techniques. LEED patterns and STM images show that no long range ordered structures are formed after NO adsorption on a Pt(111) surface. Although the total NO coverage is very low, spectroscopic features in N K-edge and O K-edge absorption spectra have been singled out and related to the different species induced by this preparation method. From these measurements it is concluded that the NO molecule is adsorbed trough the N atom in an upright conformation. The maximum saturation coverage is about 0.3 monolayers, and although nitric oxide is the major component, nitrite and nitrogen species are slightly co-adsorbed on the surface. The results obtained from this study are compared with those previously reported in the literature for NO adsorbed on Pt(111) under UHV conditions.
2014
Chemical bonding at solid surfaces and interfaces is influential in a wide range of important technological applications including catalysis, fuel cells, batteries, chemical sensors, and device fabrication for microelectronics, computers, solar cells, and all variety of coatings. Adsorption and adhesion energetics are key elements in understanding interfacial properties, and these properties can be used to develop functional industrial materials. First, the properties of single-crystalline oxide surfaces are reviewed in detail, particularly in regards to the adsorption energetics of these surfaces. This includes the largest collection of experimental adsorption data on single-crystalline oxide surfaces ever presented, from which trends in the thermodynamic properties of adsorbates are revealed which greatly expand our understanding of the physical processes occurring on these surfaces. Among these trends is the discovery that the entropy of adsorbed molecules tracks their gas-phase entropy, retaining ~2/3 of that entropy upon adsorption. This allows for a method of predicting not only entropies of adsorption, but also the kinetic prefactors associated with many classes of elementary surface reactions. These estimations of desorption prefactors are then used to improve calculations of adsorption energies from temperature programmed desorption (TPD) measurements for many systems. Metal adsorption on oxide surfaces and the strength of the binding at metal / oxide interfaces are then discussed. The motivation here is to understand oxide-supported transition metal nanoparticles such as those used in industrial heterogeneous catalysis. For metal atom adsorption, adsorption energetics and adhesion energies are directly related to the energy of the adsorbed atoms, which define their stability, sintering rates, and reactivity, and which are found to vary with both the size of the nanoparticle and the nature of the oxide support. The experimental techniques necessary for obtaining these values, as well as the data analysis involved, is explained, and in several cases improved upon. In particular, a new single crystal adsorption calorimeter capable of making the first direct measurements of adsorption energies for metals with high bulk cohesive energies has recently been completed. These studies greatly expand upon the understanding of and ability to measure the thermodynamic properties associated with adsorption on single-crystalline surfaces.
Material Concepts in Surface Reactivity and Catalysis
Wise, Henry, Date. Material concepts in surface reactivity and catalysis / Henry Wise and Jacques Oudar. p. cm. Includes index. ISBN 0-12-759940-1 (alk. paper) 1. Surface chemistry. 2. Catalysis. 3. Catalysts. I. Oudar, Jacques. II. Title. QD506.W57 1990 541.33-dc20 89-33274 CIP Printed in the United States of America 90 91 92 93 9 8 7 6 5 4 3 2 1 1175 Κ 1425 X 1025 Κ Κ 1220 Κ Ί 8 950 Κ θ 1175 Κ