The surface science of titanium dioxide

Plasmon-induced photoelectrochemistry in the visible region was studied at gold nanoparticle−nanoporous TiO2 composites (Au−TiO2) prepared by photocatalytic deposition of gold in a porous TiO2 film. Photoaction spectra for both the open-circuit potential and short-circuit current were in good agreement with the absorption spectrum of the gold nanoparticles in the TiO2 film. The gold nanoparticles are photoexcited due to plasmon resonance, and charge separation is accomplished by the transfer of photoexcited electrons from the gold particle to the TiO2 conduction band and the simultaneous transfer of compensative electrons from a donor in the solution to the gold particle. Besides its low-cost and facile preparation, a photovoltaic cell with the optimized electron mediator (Fe2+/3+) exhibits an optimum incident photon to current conversion efficiency (IPCE) of 26%. The Au−TiO2 can photocatalytically oxidize ethanol and methanol at the expense of oxygen reduction under visible light; it is potentially appli...

Mechanisms and Applications of Plasmon-Induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles

Although technological practice should minimize environmental impact, this is not always economically feasible. During the past decade, for example, there has been increasing global concern over th...

CO2 Reforming of CH4

Steam Reforming and Graphite Formation on Ni Catalysts

Detailed studies of elementary chemical processes on well-characterized single crystal surfaces have contributed substantially to the understanding of heterogeneous catalysis. Insight into the structure of surface alloys combined with an understanding of the relation between the surface composition and reactivity is shown to lead directly to new ideas for catalyst design. The feasibility of such an approach is illustrated by the synthesis, characterization, and tests of a high-surface area gold-nickel catalyst for steam reforming.

http://science.sciencemag.org/content/sci/279/5358/1913.full.pdf

Design of a Surface Alloy Catalyst for Steam Reforming

The dynamics of the activated dissociative chemisorption of CH4 on Ni(111) are studied by molecular beam techniques coupled with high‐resolution electron energy loss spectroscopy. The probability of the dissociative chemisorption of CH4 increases exponentially with the normal component of the incident molecule’s translational energy and with vibrational excitation. The dissociative chemisorption probability of CD4 exhibits the same trends with a large kinetic isotope effect. High‐resolution electron energy loss spectroscopy identifies the nascent products of the dissociative chemisorption event as an adsorbed methyl radical and a hydrogen atom. These results, which have shown that there is a barrier to the dissociative chemisorption, are interpreted in terms of a deformation model for the role of translational and vibrational energy in promoting dissociative chemisorption. The barrier likely arises largely from the energy required to deform the molecule sufficiently to allow a strong attractive interaction between the carbon and the Ni surface atoms. Tunneling is suggested as the final process in the C–H bond cleavage. The presence of this barrier to dissociative chemisorption presents a plausible explanation for the pressure gap in heterogeneous catalysis.

Dynamics of the activated dissociative chemisorption of CH4 and implication for the pressure gap in catalysis: A molecular beam–high resolution electron energy loss study

Activated dissociative chemisorption of CH4 on Ni(111): Observation of a methyl radical and implication for the pressure gap in catalysis

Studies in heterogeneous catalysis have long speculated on or have provided indirect evidence for the role of hydrogen embedded in the catalyst bulk as a primary reactant. This report describes experiments carried out under single-collision conditions that document the distinctive reactivity of hydrogen embedded in the bulk of the metal catalyst. Specifically, the bulk H atom is shown to be the reactive species in the hydrogenation of CH 3 adsorbed on Ni(111) to form CH 4 , while the H atoms bound to the surface were unreactive. These results unambiguously demonstrate the importance of bulk species to heterogeneous catalytic chemistry.

http://science.sciencemag.org/content/sci/257/5067/223.full.pdf

The Chemistry of Bulk Hydrogen: Reaction of Hydrogen Embedded in Nickel with Adsorbed CH3

It is becoming increasingly apparent that chemistry at surfaces, whether it be heterogeneous catalysis, semiconductors etching, or chemical vapor deposition, is controlled by much more than the nature and structure of the surface. Recent experiments that principally make use of molecular beam techniques have revealed that the energy at which an incident molecule collides with a surface can be the key factor in determining its reactivity with or on the surface. In addition, the collision energy of an incident particle has proven essential to the finding of new mechanisms for reaction or desorption of molecules at surfaces, collision-induced activation and collision-induced desorption. These phenomena are often responsible for the different surface chemistry observed under conditions of high reactant pressure, such as those present during a heterogeneous catalytic reaction, and of low pressure of reactants (

https://science.sciencemag.org/content/sci/249/4965/133.full.pdf

New Mechanisms for Chemistry at Surfaces

The vibrational modes of hydrogen embedded in a Ni crystal are shown to be detectable by high-resolution electron-energy-loss spectroscopy and to be unambiguously distinguishable from the vibrational modes of adsorbed hydrogen on the basis of the dependence of the inelastic electron intensity on electron impact energy. The embedded hydrogen has a vibrational frequency of 800--850 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and is observed to recombine and desorb as ${\mathrm{H}}_{2}$ between 180 and 220 K. The absorption of hydrogen into Ni(111) is achieved under UHV conditions by exposure to atomic hydrogen. As much as an equivalent of 8 monolayers has been absorbed.

S. T. Ceyer

Papers

Dynamics of the activated dissociative chemisorption of CH4 and implication for the pressure gap in catalysis: A molecular beam–high resolution electron energy loss study

Activated dissociative chemisorption of CH4 on Ni(111): Observation of a methyl radical and implication for the pressure gap in catalysis

The Chemistry of Bulk Hydrogen: Reaction of Hydrogen Embedded in Nickel with Adsorbed CH3

New Mechanisms for Chemistry at Surfaces

Hydrogen embedded in Ni: Production by incident atomic hydrogen and detection by high-resolution electron energy loss