William E. Kaden

Electronic Structure Controls Reactivity of Size-Selected Pd Clusters Adsorbed on TiO2 Surfaces

Cluster Electronics and Catalysis Many practical catalysts consist of small metal clusters on oxide supports, and the activity of these clusters usually varies with their size. In order to sort out some of the competing effects that lead to such variations, Kaden et al. (p. 826) size-selected palladium clusters (from single atoms to clusters up to 25 atoms) and deposited them on a crystal face of the rutile phase of titanium dioxide. X-ray photoemission studies and temperature-programmed reaction measurements showed that the activity of these model catalysts for CO oxidation was related to the electronic energy, which was reflected in the Pd 3d electron binding energy. Ion-scattering studies showed that the clusters formed flat single- or double-layer islands. The activity of these model catalysts for carbon monoxide oxidation reflects changes in cluster electronic structure. The catalytic activity of metal clusters of different sizes adsorbed on oxide surfaces can be explored systematically by using model catalysts. We studied the temperature-programmed reaction of CO with O2 catalyzed by Pd clusters (Pdn, for n = 1, 2, 4, 7, 10, 16, 20, and 25) that were size-selected in the gas phase and deposited on rutile TiO2(110). X-ray photoemission spectroscopy revealed that the Pd 3d binding energy varied nonmonotonically with cluster size and that the changes correlated with strong size variations in CO oxidation activity. Taking final-state effects into account, low activity was correlated with higher-than-expected Pd 3d binding energy, which is attributed to a particularly stable valence electronic structure; electron transfer from the TiO2 support to the Pd clusters also occurs. Ion scattering shows that small clusters form single-layer islands on the surface and that formation of a second layer begins to occur for clusters larger than Pd10.

Size-Dependent Oxygen Activation Efficiency over Pdn/TiO2(110) for the CO Oxidation Reaction

The dissociative binding efficiency of oxygen over Pd(n)/TiO(2)(110) (n = 4, 7, 10, 20) has been measured using temperature programmed reaction (TPR) mass spectrometry and X-ray photoemission spectroscopy (XPS) following exposure to O(2) with varying doses and dose temperatures. Experiments were carried out following two different O(2) exposures at 400 K (10 L and 50 L) and for 10 L of O(2) exposure at varying temperatures (T(surf) = 200, 300, and 400 K). During TPR taken after sequential O(2) and CO (5 L at 180 K) exposures, unreacted CO is found to desorb in three features at T(desorb) ≈ 150, 200, and 430 K, while CO(2) is observed to desorb between 170 and 450 K. We show that Pd(20) has exceptionally high efficiency for oxygen activation, compared to other cluster sizes. As a consequence, its activity becomes limited by competitive CO binding at low O(2) exposures, while other Pd(n) sizes are still limited by inefficient O(2) activation. This difference in mechanism can ultimately be related back to differences in electronic properties, thus making this question one that is interesting from the theoretical perspective. We also demonstrate a correlation between one of the two CO binding sites and CO(2) production, suggesting that only CO in that site is reactive.

Thin silica films on Ru(0001): monolayer, bilayer and three-dimensional networks of [SiO4] tetrahedra

The atomic structure of thin silica films grown over a Ru(0001) substrate was studied by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, low energy electron diffraction, helium ion scattering spectroscopy, CO temperature programmed desorption, and scanning tunneling microscopy in combination with density functional theory calculations. The films were prepared by Si vapor deposition and subsequent oxidation at high temperatures. The silica film first grows as a monolayer of corner-sharing [SiO(4)] tetrahedra strongly bonded to the Ru(0001) surface through the Si-O-Ru linkages. At increasing amounts of Si, the film forms a bilayer of corner-sharing [SiO(4)] tetrahedra which is weakly bonded to Ru(0001). The bilayer film can be grown in either the crystalline or vitreous state, or both coexisting. Further increasing the film thickness leads to the formation of vitreous silica exhibiting a three-dimensional network of [SiO(4)]. The principal structure of the films can be monitored by infrared spectroscopy, as each structure shows a characteristic vibrational band, i.e., ∼1135 cm(-1) for a monolayer film, ∼1300 cm(-1) for the bilayer structures, and ∼1250 cm(-1) for the bulk-like vitreous silica.

Ultrathin Silica Films: The Atomic Structure of Two‐Dimensional Crystals and Glasses

For the last 15 years, we have been studying the preparation and characterization of ordered silica films on metal supports. We review the efforts so far, and then discuss the specific case of a silica bilayer, which exists in a crystalline and a vitreous variety, and puts us into a position to investigate, for the first time, the real space structure (AFM/STM) of a two-dimensional glass and its properties. We show that pair correlation functions determined from the images of this two-dimensional glass are similar to those determined by X-ray and neutron scattering from three-dimensional glasses, if the appropriate sensitivity factors are taken into account. We are in a position, to verify, for the first time, a model of the vitreous silica structure proposed by William Zachariasen in 1932. Beyond this, the possibility to prepare the crystalline and the glassy structure on the same support allows us to study the crystal-glass phase transition in real space. We, finally, discuss possibilities to use silica films to start investigating related systems such as zeolites and clay films. We also mention hydroxylation of the silica films in order to adsorb metal atoms modeling heterogenized homogeneous catalysts.

Cluster size effects on sintering, CO adsorption, and implantation in Ir/SiO2

A series of planar model catalysts have been prepared via deposition of Ir(n) (+) on thermally grown amorphous SiO(2)/Si(100) and ion scattering spectroscopy was used to probe surface structure as a function of cluster size, impact energy, and surface temperature. Deposition of Ir(2) or Ir(10) at low energies and room temperature results in stable clusters forming one- or two-dimensional single layer islands on the oxide surface. Heating the samples to 750 K leads to agglomeration, forming multilayer structures on the surface. Ir(1) deposited under similar conditions sinters into large clusters at room temperature. Deposition at 110 K at least partially stabilizes the Ir atoms with respect to diffusion and sintering. At higher deposition energies, partial implantation into the surface is observed, but this appears to be insufficient to stabilize the clusters against sintering at elevated temperature. At low temperatures, substrate-mediated adsorption of CO is found to be highly efficient, leading to near saturation coverages of CO bound atop the Ir(n) clusters. The CO can be removed by careful He(+) sputtering. The deposition/binding behavior of Ir(n) on SiO(2) is quite different from Ir(n)/TiO(2)(110), for which the clusters bind in three-dimensional morphology, starting at Ir(5). That system also shows substrate-mediated adsorption of CO, but the CO preferentially binds at the periphery of the clusters rather than on top.

The safer and scalable mechanochemical synthesis of edge-chlorinated and fluorinated few-layer graphenes

Abstract Halogen functionalization of the edges of the graphene sheets can improve processability, add new properties, and enhance its interactions with other materials. Through functionalization, improved materials can be realized. Typically, halogenated graphenes are produced from pure or reactive halogen sources. Current approaches present significant safety challenges. By generating reactive dichlorine monoxide (Cl2O) in situ, a chlorinated graphene with the nominal composition C17Cl2OH can be realized safely and scalably. Chlorinated graphene can be used as a precursor for an array of functionalized materials by mechanically driven solid-state metathesis reactions. For example, nearly 75% of the chlorine in chlorinated graphene can be exchanged with fluorine by using the safer fluorine-containing compound ammonium fluoride (NH4F) as a reagent. A material with the composition C34Cl3F(OH)2 is realized. Preliminary work shows that F–graphene has oxygen reduction properties and Cl–graphene can improve existing zinc–air fuel cells. A scalable production of chlorinated and fluorinated graphenes and graphites will accelerate their adoption in fuel cells, batteries, polymer composites, and catalysts.

A bifunctional catalyst for efficient dehydrogenation and electro-oxidation of hydrazine

The chemical energy stored in energetic materials may often be utilized in various ways, which motivates the development of multifunctional catalysts for flexible and efficient utilization of the chemical energy. Hydrazine is a promising energy carrier due to its high energy density and high hydrogen content, which can be utilized as a chemical hydrogen storage medium or a fuel for direct fuel cells. Herein, we propose a bifunctional catalyst for efficient dehydrogenation and electro-oxidation of hydrazine. As a proof-of-concept study, a carbon-black-supported Pt0.2Ni0.8 nanoparticle catalyst has been developed with high activity and durability for both complete dehydrogenation (with a turnover frequency of 673 h−1 and a H2 generation rate of 188 L h−1 gmetal−1) and electro-oxidation (with a mass activity of 132 mA mgmetal−1) of hydrazine under mild conditions, outperforming other catalysts including Pt, Ni, Pd0.2Ni0.8, and Au0.2Ni0.8 nanoparticles. Such a bifunctional catalyst can enable the utilization of hydrazine as a promising energy carrier for both on-demand hydrogen generation and electricity generation via direct hydrazine fuel cells, enhancing its flexibility for future onboard applications.