Randall J. Meyer

Increased Silver Activity for Direct Propylene Epoxidation via Subnanometer Size Effects

Silver Cluster Catalysts for Propylene Oxide The formation of ethylene oxide—in which an oxygen atom bridges the double bond of ethylene—can be made directly and efficiently from ethylene and oxygen with the aid of silver catalysts (typically comprising a small silver cluster on aluminum oxide). Similar approaches are not so successful for making propylene oxide—an important starting material for polyurethane plastics, which are made from chlorinated intermediates. Lei et al. (p. 224) report that silver trimers, Ag3, deposited on alumina are active for direct propylene oxide formation at low temperatures with only a low level of formation of CO2 by-product, unlike larger particles that form from these clusters at higher temperatures. Density functional calculations suggest that the open-shell nature of the clusters accounts for the improved reactivity. Clusters of three silver atoms deposited on alumina are active for the low-temperature direct formation of propylene oxide. Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag3 clusters and ~3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.

Size and support effects for CO adsorption on gold model catalysts

CO adsorption on gold particles deposited on well-ordered alumina and iron oxide films was studied with temperature-programmed desorption. Scanning tunneling microscopy was used to provide correlative structural characterization. The results show that the adsorption of CO on gold exhibits a size effect in that small particles adsorb CO more strongly. For a given particle size (∼3 nm), CO desorption temperature (at ∼170 K) is essentially independent of the supports studied. Therefore, support effects seen in CO oxidation on real catalytic systems must arise from the interaction of oxygen rather than CO with these catalysts.

Determination of CO, H2O and H2 coverage by XANES and EXAFS on Pt and Au during water gas shift reaction

The turn-over-rate (TOR) for the water gas shift (WGS) reaction at 200 degrees C, 7% CO, 9% CO(2), 22% H(2)O, 37% H(2) and balance Ar, of 1.4 nm Au/Al(2)O(3) is approximately 20 times higher than that of 1.6 nm Pt/Al(2)O(3). Operando EXAFS experiments at both the Au and Pt L(3) edges reveal that under reaction conditions, the catalysts are fully metallic. In the absence of adsorbates, the metal-metal bond distances of Pt and Au catalysts are 0.07 A and 0.13 A smaller than those of bulk Pt and Au foils, respectively. Adsorption of H(2) or CO on the Pt catalysts leads to significantly longer Pt-Pt bond distances; while there is little change in Au-Au bond distance with adsorbates. Adsorption of CO, H(2) and H(2)O leads to changes in the XANES spectra that can be used to determine the surface coverage of each adsorbate under reaction conditions. During WGS, the coverage of CO, H(2)O, and H(2) are obtained by the linear combination fitting of the difference XANES, or DeltaXANES, spectra. Pt catalysts adsorb CO, H(2), and H(2)O more strongly than the Au, in agreement with the lower CO reaction order and higher reaction temperatures.

In situ electron energy loss spectroscopy study of metallic Co and Co oxides

Determining the Co valence, particularly in Co-based nanocatalysts is a longstanding experimental challenge. In this paper, we utilize in situ electron energy-loss spectroscopy and first-principles density functional theory calculations to distinguish between metallic Co, Co3O4, as well as CoO. More specifically, differences in the O K- and Co L-edges are utilized to determine the Co valence in different Co-oxide particles. We will further demonstrate that while the metallic Co L3/L2-ratio equals that of partially reduced Co3O4, the near-edge fine-structure of the metallic Co L-edge exhibits additional features not present in any Co-oxide. The origin of these features will be discussed. Based on our experimental and theoretical results, we will propose a fitting method to distinguish metallic Co from Co-oxides.

CO Oxidation on a Pd/Fe3O4(111) Model Catalyst

Abstract Adsorption of CO and O2 on Pd particles deposited on a well-ordered Fe3O4(111) film was studied by temperature programmed desorption. Scanning tunneling microscopy was used to provide structural information. The results show that CO adsorbed on Pd particles reacts with oxygen of the oxide support and forms CO2. The reaction occurs at the particle/oxide interface and exhibits a particle size effect such that the smaller particles produce more CO2. Oxidation of Pd/Fe3O4 alters the oxide structure due to Pd catalyzed oxygen migration into the film and results in formation of an oxygen reservoir, which favors CO oxidation reaction.

Manganese Triazacyclononane Oxidation Catalysts Grafted under Reaction Conditions on Solid Cocatalytic Supports

Manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) are highly active and selective alkene oxidation catalysts with aqueous H(2)O(2). Here, carboxylic acid-functionalized SiO(2) simultaneously immobilizes and activates these complexes under oxidation reaction conditions. H(2)O(2) and the functionalized support are both necessary to transform the inactive [(tmtacn)Mn(IV)(μ-O)(3)Mn(IV)(tmtacn)](2+) into the active, dicarboxylate-bridged [(tmtacn)Mn(III)(μ-O)(μ-RCOO)(2)Mn(III)(tmtacn)](2+). This transformation is assigned on the basis of comparison of diffuse reflectance UV-visible spectra to known soluble models, assignment of oxidation state by Mn K-edge X-ray absorption near-edge spectroscopy, the dependence of rates on the acid/Mn ratios, and comparison of the surface structures derived from density functional theory with extended X-ray absorption fine structure. Productivity in cis-cyclooctene oxidation to epoxide and cis-diol with 2-10 equiv of solid cocatalytic supports is superior to that obtained with analogous soluble valeric acid cocatalysts, which require 1000-fold excess to reach similar levels at comparable times. Cyclooctene oxidation rates are near first order in H(2)O(2) and near zero order in all other species, including H(2)O. These observations are consistent with a mechanism of substrate oxidation following rate-limiting H(2)O(2) activation on the hydrated, supported complex. This general mechanism and the observed alkene oxidation activation energy of 38 ± 6 kJ/mol are comparable to H(2)O(2) activation by related soluble catalysts. Undesired decomposition of H(2)O(2) is not a limiting factor for these solid catalysts, and as such, productivity remains high up to 25 °C and initial H(2)O(2) concentration of 0.5 M, increasing reactor throughput. These results show that immobilized carboxylic acids can be utilized and understood like traditional carboxylic acids to activate non-heme oxidation catalysts while enabling higher throughput and providing the separation and handling benefits of a solid catalyst.

Selective Adsorption of Manganese onto Rhodium for Optimized Mn/Rh/SiO2 Alcohol Synthesis Catalysts

Using supported rhodium‐based catalysts to produce alcohols from syngas provides an alternative route to conventional fermentation methods. If left unpromoted, Rh catalysts have a strong selectivity towards methane. However, promotion with early transition metal elements has been shown to be effective to increase alcohol selectivity. Therefore, a key design objective is to increase the promoter–metal interaction to maximize their effectiveness. This can be achieved by the use of the strong electrostatic adsorption (SEA) method, which utilizes pH control to steer the promoter precursor (in this case MnO4−) onto Rh oxide supported on SiO2. Mn‐promoted catalysts were synthesized by both SEA and traditional incipient wetness impregnation (IWI) and subsequently characterized by STEM and extended X‐ray absorption fine structure methods. Using STEM–electron energy loss spectroscopy mapping, catalysts prepared by SEA were shown to have a higher degree of interaction between the promoter and the active metal. The reduction behavior of the catalysts obtained by X‐ray absorption near‐edge spectroscopy and temperature‐programmed reduction demonstrated a minimal change in Rh if promoted by SEA. However, catalytic results for CO hydrogenation revealed that a significant improvement of ethanol selectivity is achieved if the promoter was prepared by SEA in comparison with the promoter prepared by IWI. These results suggest that intimate interaction between the promoter and the metal is a critical factor for improving selectivity to higher alcohols.

CO+NO versus CO+O2 Reaction on Monolayer FeO(111) Films on Pt(111)

Thin oxide films grown on metal single crystals are used in many “surface science” research groups in attempts to understand the surface chemistry of metal oxides. In addition, these films are employed as suitable supports for modeling highly dispersed metal catalysts (for reviews, see Refs. [1]–[4]). However, in the case of ultrathin films that are only a few angstroms in thickness, the metal substrate underneath the film often affects the properties of metal clusters by charge transfer through the film. These observations can, in principle, be traced back to the so-called “electronic theory of catalysis” developed in the 1950s and 1960s, and are primarily based on a Schottky barrier model, which predicts, in particular, that by varying the thickness of oxide films, the reactivity of heterogeneous catalysts can be controlled. However, these ideas faded away, primarily because of a lack of successful examples of the promotional effects of thin oxide films on catalytic activity and/or selectivity. Recently, it has been demonstrated that a thin oxide film grown on a metal may exhibit higher catalytic activity than the metal substrate under the same reaction conditions. Indeed, a thin FeO(111) film grown on Pt(111) is active for CO oxidation at 450 K, a temperature far below that at which Pt(111) itself is active. Furthermore, the rate enhancement was observed on Fe3O4-supported Pt nanoparticles, [13] which underwent encapsulation by an FeO(111) film as a result of the strong metal– support interaction. It has been suggested that, in the millibar pressure range (1 mbar=100 Pa) of O2, the bilayer Fe–O film on Pt(111) transforms into a trilayer O–Fe–O film that catalyzes CO oxidation according to a Mars–van Krevelen-type mechanism. A density functional theory (DFT) study corroborated this scenario. The DFT results showed that, by overcoming a small energy barrier of about 0.3 eV, O2 is chemisorbed on the Fe atom, which is pulled out of the pristine FeO film. In the chemisorbed state, electrons are transferred from the oxide/metal interface to oxygen, resulting in a O2 2 species, which then dissociates, thus forming a local O–Fe–O trilayer structure. Further DFT studies revealed that the reaction is site-specific within the large Moir unit cell formed due to an approximately 10% mismatch between the FeO(111) and Pt(111) lattices. This mismatch explains scanning tunneling microscopy (STM) results that showed the formation of close-packed O–Fe–O islands rather than a continuous FeO2 film. [15]

Electron energy-loss spectroscopy study of metallic Nb and Nb oxides

We present a series of electron energy-loss spectroscopy (EELS) studies on niobium (Nb) and its oxides (NbO, NbO2, and Nb2O5) to develop a reliable method for quantifying the oxidation state in mixed niobium oxide thin films. Our approach utilizes a combination of transmission electron microscopy and EELS experiments with density functional theory calculations to distinguish between metallic niobium and the different niobium oxides. More specifically, the differences in the near-edge fine-structure of the Nb M-edge and O K-edge provide sufficient information to determine the valence state of niobium. Based on these observed changes in the core-loss edges, we propose a linear relationship that correlates the peak positions in the Nb M- and O K-edges with the Nb valence state. The methods developed in this paper are also applied to ultrathin niobium oxide films to examine the effects of low-temperature baking on the films’ oxidation states.

In situ diffraction of highly dispersed supported platinum nanoparticles

For catalytic metal nanoparticles ( 2 nm) where diffraction patterns of the metallic phase are obtainable in air, we show that on exposure to air the surface is oxidized with a metallic core producing misleading results with respect to particle size and lattice parameter. Results from XRD are cross-correlated with scanning transmission electron microscopy and three other synchrotron X-ray techniques, small angle diffraction (SAXS), pair distribution function (PDF) and X-ray absorption spectroscopy (XAS), to provide detailed characterization of the structure of very small nanoparticles in the metallic phase.