Improving the activity of gold nanoparticles for the water-gas shift reaction using TiO2-Y2O3: an example of catalyst design.

In the last ten years, there has been an acceleration in the pace at which new catalysts for the water-gas shift reaction are designed and synthesized. Pt-based catalysts remain the best solution when only activity is considered. However, cost, operation temperature, and deactivation phenomena are important variables when these catalysts are scaled in industry. Here, a new catalyst, Au/TiO2-Y2O3, is presented as an alternative to the less selective Pt/oxide systems. Experimental and theoretical techniques are combined to design, synthesize, characterize and analyze the performance of this system. The mixed oxide demonstrates a synergistic effect, improving the activity of the catalyst not only at large-to-medium temperatures but also at low temperatures. This effect is related to the homogeneous dispersion of the vacancies that act both as nucleation centers for smaller and more active gold nanoparticles and as dissociation sites for water molecules. The calculated reaction path points to carboxyl formation as the rate-limiting step with an activation energy of 6.9 kcal mol-1, which is in quantitative agreement with experimental measurements and, to the best of our knowledge, it is the lowest activation energy reported for the water-gas shift reaction. This discovery demonstrates the importance of combining experimental and theoretical techniques to model and understand catalytic processes and opens the door to new improvements to reduce the operating temperature and the deactivation of the catalyst.

Gold, copper, and platinum nanoparticles dispersed on CeO(x)/TiO(2)(110) surfaces: high water-gas shift activity and the nature of the mixed-metal oxide at the nanometer level.

At small coverages of ceria on TiO(2)(110), the CeO(x) nanoparticles have an unusual coordination mode. Scanning tunneling microscopy and density-functional calculations point to the presence of Ce(2)O(3) dimers, which form diagonal arrays that have specific orientations of 0, 24, and 42 degrees with respect to the [1 -1 0] direction of the titania substrate. At high coverages of ceria on TiO(2)(110), the surface exhibits two types of terraces. In one type, the morphology is not very different from that observed at low ceria coverage. However, in the second type of terrace, there is a compact array of ceria particles with structures that do not match the structures of CeO(2)(111) or CeO(2)(110). The titania substrate imposes on the ceria nanoparticles nontypical coordination modes, enhancing their chemical reactivity. This phenomenon leads to a larger dispersion of supported metal nanoparticles (M = Au, Cu, Pt) and makes possible the direct participation of the oxide in catalytic reactions. The M/CeO(x)/TiO(2)(110) surfaces display an extremely high catalytic activity for the water-gas shift reaction that follows the sequence Au/CeO(x)/TiO(2)(110) < Cu/CeO(x)/TiO(2)(110) < Pt/CeO(x)/TiO(2)(110). For low coverages of Cu and CeO(x), Cu/CeO(x)/TiO(2)(110) is 8-12 times more active than Cu(111) or Cu/ZnO industrial catalysts. In the M/CeO(x)/TiO(2)(110) systems, there is a strong coupling of the chemical properties of the admetal and the mixed-metal oxide: The adsorption and dissociation of water probably take place on the oxide, CO adsorbs on the admetal nanoparticles, and all subsequent reaction steps occur at the oxide-admetal interface. The high catalytic activity of the M/CeO(x)/TiO(2)(110) surfaces reflects the unique properties of the mixed-metal oxide at the nanometer level.

Au <--> N synergy and N-doping of metal oxide-based photocatalysts.

N-doping of titania makes photocatalytic activity possible for the splitting of water, and other reactions, under visible light. Here, we show from both theory and experiment that Au preadsorption on TiO2 surfaces significantly increases the reachable amount of N implanted in the oxide. The stabilization of the embedded N is due to an electron transfer from the Au 6s levels toward the N 2p levels, which also increases the Au-surface adhesion energy. Theoretical calculations predict that Au can also stabilize embedded N in other metal oxides with photocatalytic activity, such as SrTiO3 and ZnO, producing new states above the valence band or below the conduction band of the oxide. In experiments, the Au/TiN(x)O(2-y) system was found to be more active for the dissociation of water than TiO2, Au/TiO2, or TiO(2-y). Furthermore, the Au/TiN(x)O(2-y) surfaces were able to catalyze the production of hydrogen through the water-gas shift reaction (WGS) at elevated temperatures (575-625 K), displaying a catalytic activity superior to that of pure copper (the most active metal catalysts for the WGS) or Cu nanoparticles supported on ZnO.

Atomic layer deposition of hafnium oxide from hafnium chloride and water.

Hafnium oxide (HfO2) is a leading candidate to replace silicon oxide as the gate dielectric for future generation metal-oxide-semiconductor based nanoelectronic devices. Atomic layer deposition (ALD) has recently gained interest because of its suitability for fabrication of conformal films with thicknesses in the nanometer range. This study uses periodic density functional theory (DFT) to investigate the mechanisms of both half-reactions occurring on the growing surface during the ALD of HfO2 using HfCl4 and water as precursors. We find that the adsorption energy and the preferred site of adsorption of the metal precursor are strong functions of the water coverage. As water coverage increases, the metal precursor preferentially interacts with multiple surface adsorption sites. During the water pulse the removal of Cl can be facilitated by either a ligand exchange reaction or the dissociation of Cl upon increase in coordination of the Hf atom of the precursor. Our predicted potential energy surface indicates that a more likely mechanism is hydration of the adsorbed Hf complex up to a coordination number of 7, followed by the dissociation of a chloride ion that is stabilized by solvation. Born-Oppenheimer molecular dynamics (BOMD) simulations of an adsorbed metal precursor in the presence of a multilayer of water shows that Cl dissociation is facile if sufficient water molecules are present to solvate the Cl(-) anions. Hence, solvation plays a crucial role during the water pulse and provides an alternative explanation for why ALD growth rates for this system decrease at high temperatures.

Interaction potentials from periodic density-functional theory calculations: molecular-dynamics simulations of Au clusters deposited on the TiN (001) surface.

Molecular-dynamics simulations of gold particles deposited on a TiN (001) surface have been accounted for through classical pair potentials describing the atom force field. The interaction between Ti-N, Ti-Ti, N-N, Au-Au, Au-Ti, and Au-N pairs was estimated by following a procedure in which the interaction energy between two sets of atoms is estimated from density-functional calculations performed with periodic boundary conditions using plane waves as basis set. The pair potentials were expressed as the sum of two contributions: long range in a Coulomb form and a short-range term, which included the rest of the energy contributions. Simulations of the TiN (001) isolated surface reproduced the already described surface relaxation, with a rippling parameter in agreement with that found from a purely first-principles approach. Simulations of gold deposition on such surfaces showed the formation of metal clusters with well-defined fcc structure and epitaxially grown.