Showing papers by "Suljo Linic published in 2010"

Enhancing Photochemical Activity of Semiconductor Nanoparticles with Optically Active Ag Nanostructures: Photochemistry Mediated by Ag Surface Plasmons

Abstract: Composite materials composed of optically active Ag nanostructures and TiO2 photocatalysts show enhanced photoactivity compared to the pure TiO2 in the decomposition of methylene blue. The enhanced photochemical activity is attributed to radiative transfer of energy, mediated by surface plasmons, from Ag particles to the semiconductor leading to higher concentrations of charge carriers (e−/h+ pairs) in the semiconductor and therefore to higher photochemical activity. We demonstrate that the performance of the composite photocatalysts is a strong function of the size and shape of Ag nanostructures. This can be explained by the size- and shape-specific optical activity of Ag nanostructures. We show that by rationally changing the size and shape of Ag nanostructures it is possible to maximize photochemical activity of a semiconductor at a given excitation wavelength.

Communications: Exceptions to the d-band model of chemisorption on metal surfaces: The dominant role of repulsion between adsorbate states and metal d-states

Abstract: We show that there is a family of adsorbate-substrate systems that do not follow the trends in adsorption energies predicted by the d-band model. A physically transparent model is used to analyze this phenomenon. We found that these adsorbate-substrate pairs are characterized by the repulsive interaction of the substrate d-band with the renormalized adsorbate states. The exceptions to the d-band model are mainly associated with the adsorbates having almost completely filled valence shell, and the substrates with nearly fully occupied d-band, e.g., OH, F, or Cl adsorption on metals and alloys characterized by d(9) or d(10) substrate surface atoms.

Shape- and Size-Specific Chemistry of Ag Nanostructures in Catalytic Ethylene Epoxidation

Abstract: Catalytic selectivity in the epoxidation of ethylene to form ethylene oxide on alumina-supported silver catalysts is dependent on the geometric structure of catalytically active Ag particles and reaction conditions. Shape and size controlled synthesis of Ag nanoparticles is used to show that silver nanocubes exhibit higher selectivity than nanowires and nanospheres. For a given shape, larger particles offer improved selectivity. The enhanced selectivity toward ethylene oxide is attributed to the nature of the exposed Ag surface facets; Ag nanocubes and nanowires are dominated by (100) surface facet and Ag nanospheres are dominated by (111). Furthermore, the concentration of undercoordinated surface sites is related to diminished selectivity to ethylene oxide. We demonstrate that a simple model can account for the impact of chemical and physical factors on the reaction selectivity. These observations have also been used to design a selective catalyst for the ethylene epoxidation reaction.

Establishing Relationships Between the Geometric Structure and Chemical Reactivity of Alloy Catalysts Based on Their Measured Electronic Structure

Abstract: While it is fairly straightforward to predict the relative chemical reactivity of pure metals, obtaining similar structure-performance relationships for alloys is more challenging. In this contribution we present experimental analysis supported with quantum chemical DFT calculations which allowed us to propose a simple, physically transparent model to predict the impact of alloying on the local electronic structure of different sites in alloys and on the local chemical reactivity. The model was developed through studies of a number of Pt alloys. The central feature of the model is that hybridization of d-orbitals in alloys does not lead to significant charge transfer between the constituent elements in the alloy, and therefore the width of the local density of d-states projected on a site, which is easily calculated from tabulated parameters, is an excellent descriptor of the chemical reactivity of the site.

Overcoming Limitation in the Design of Selective Solid Catalysts by Manipulating Shape and Size of Catalytic Particles: Epoxidation Reactions on Silver

Abstract: Conventional solid metal catalysts, usually synthesized using wet impregnation strategies, are almost always quasi-spherical particles mainly terminated by low-energy surface facets, such as the (111) surface for fcc crystals and the (0001) surface for hcp crystals, and supported on high surface area materials. The diameter of these metal particles is from a few tens to a few hundreds of nanometers. The particle shapes and sizes represent a reasonable compromise between simple and scalable synthesis, relatively high surface to volume ratio, and high resistance to sintering and degradation under reaction conditions. A major negative consequence of the lack of diversity in the size and shape of metallic catalytic particles is that it has been difficult to identify and optimize selective catalysts for the production of reactive, commodity chemicals. These difficulties stem from large differences in the chemical activity of different metal elements. For example, identical surface sites of neighboring metals in the periodic table of elements bind various adsorbates with differences in adsorption energies of up to about 1 eV (ca. 100 kJ mol ), which means that whereas one metal might be chemically inert for a particular chemical transformation, its first neighbor in the periodic table might be overly reactive and equally inefficient. The above-discussed obstacles have prevented the design of selective solid catalysts for direct partial oxidation of propylene to propylene oxide (PO; C3H6+ =2 O2!C3H6O). PO is used as a chemical intermediate in the production of foams, coatings, adhesives, propylene glycol, and glycol ethers. It has a market on the order of several billion dollars per year. PO is currently produced in costly homogeneous processes which require expensive post-reaction separation and use environmentally harmful oxidation reagents, mainly chlorohydrins or hydrogen peroxide, instead of air or oxygen. Direct heterogeneous processes are attractive because of the easy separation, long lifetime, and regenerability of solid catalysts, and the environmentally benign and cheap oxidizing agents (molecular O2). Conventional epoxidation catalysts containing silver particles dispersed on a-alumina support, used in the analogous catalytic epoxidation of ethylene to form ethylene oxide (C2H4+ =2 O2!C2H4O), are not selective since they activate allylic hydrogen in propylene, leading to complete combustion. In contrast, less chemically reactive gold-based catalysts do not activate O2 efficiently without sacrificial agents such as H2 and are therefore limited by low activity. Development of a robust heterogeneous catalyst for direct epoxidation of propylene is one of the most important problems in heterogeneous catalysis and surface chemistry, and it is often referred to as the holy grail of heterogeneous catalysis. Lei et al. recently showed that size-selected clusters of Ag (Ag3 trimers; Figure 1), and small aggregates of the Ag trimers (effectively nanoparticles with a diameter of ca. 3.5 nm) dis-

Communications: Developing relationships between the local chemical reactivity of alloy catalysts and physical characteristics of constituent metal elements

Abstract: We have used X-ray absorption spectroscopy and quantum chemical density functional theory calculations to identify critical features in the electronic structure of different sites in alloys that govern the local chemical reactivity. The measurements led to a simple model relating local geometric features of a site in an alloy to its electronic structure and chemical reactivity. The central feature of the model is that the formation of alloys does not lead to significant charge transfer between the constituent metal elements in the alloys, and that the local electronic structure and chemical reactivity can be predicted based on physical characteristics of constituent metal elements in their unalloyed form.

Photoactive compositions containing plasmon-resonating nanoparticles

Abstract: Disclosed herein are photoactive compositions that include a semiconductor and plasmon-resonating nanoparticles that are capable of concentrating light at a wavelength that is substantially the same as the wavelength of light necessary to promote an electron from a valance band to a conduction band in the semiconductor. As such, the plasmon-resonating nanoparticles direct light to the band gap of the semiconductor at an increased intensity (relative to when such nanoparticles are not present). And because of that increased intensity, the photoactive composition can be more efficiently used to catalyze a photochemical reaction or generate electrical potential in a photovoltaic cell.

From Molecular Insights to Novel Catalysts Formulation

Abstract: First-principles methods can be utilized to obtain elementary step mechanisms for chemical reactions on model systems. In this chapter, we will illustrate how this molecular information can be employed to motivate novel heterogeneous catalyst formulations. We will discuss a few examples where first-principles studies on idealized model systems were utilized, along with various experimental tools, to identify alloy catalysts that exhibit improved performance in a number of catalytic processes. We will emphasize the role of molecular approaches in the formulation of these catalysts.