Paul J. Dietrich

Stabilization of Copper Catalysts for Liquid‐Phase Reactions by Atomic Layer Deposition

Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid-phase catalytic reactions (e.g., hydrogenation of biomass-derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid-phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X-ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ-Al2 O3 . Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under-coordinated copper atoms on the nanoparticle surface.

A Reusable Unsupported Rhenium Nanocrystalline Catalyst for Acceptorless Dehydrogenation of Alcohols through γ-C–H Activation†

Rhenium nanocrystalline particles (Re NPs), of 2 nm size, were prepared from NH4ReO4 under mild conditions in neat alcohol. The unsupported Re NPs convert secondary and benzylic alcohols to ketones and aldehydes, respectively, through catalytic acceptorless dehydrogenation (AD). The oxidant- and acceptor-free neat dehydrogenation of alcohols to obtain dihydrogen gas is a green and atom-economical process for making carbonyl compounds. Secondary aliphatic alcohols give quantitative conversion and yield. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Re K-edge X-ray absorption near-edge structure (XANES), and X-ray absorption fine structure (EXAFS) data confirmed the characterization of the Re NPs as metallic rhenium with surface oxidation to rhenium(IV) oxide (ReO2). Isotope labeling experiments revealed a novel γ-CH activation mechanism for AD of alcohols.

Synergistic Effects in Bimetallic Palladium–Copper Catalysts Improve Selectivity in Oxygenate Coupling Reactions

Condensation reactions such as Guerbet and aldol are important since they allow for C-C bond formation and give higher molecular weight oxygenates. An initial study identified Pd-supported on hydrotalcite as an active catalyst for the transformation, although this catalyst showed extensive undesirable decarbonylation. A catalyst containing Pd and Cu in a 3:1 ratio dramatically decreased decarbonylation, while preserving the high catalytic rates seen with Pd-based catalysts. A combination of XRD, EXAFS, TEM, and CO chemisorption and TPD revealed the formation of CuPd bimetallic nanoparticles with a Cu-enriched surface. Finally, density functional theory studies suggest that the surface segregation of Cu atoms in the bimetallic alloy catalyst produces Cu sites with increased reactivity, while the Pd sites responsible for unselective decarbonylation pathways are selectively poisoned by CO.

Operando X‐ray Absorption Spectroscopy Studies of Sintering for Supported Copper Catalysts during Liquid‐phase Reaction

Operando X‐ray absorption spectroscopy is used to measure simultaneous changes in catalyst structure and changes in catalytic activity versus time during the liquid phase hydrogenation of furfural over supported copper catalysts. This approach allows the size of the copper nanoparticles to be monitored continuously versus time‐on‐stream, such that these changes in dispersion can be accounted for in the calculation of turnover frequency. It is shown that sintering of the copper nanoparticles is the predominant mode of catalyst deactivation for a Cu/γ‐Al2O3 catalyst throughout its time‐on‐stream, leading to irreversible loss of catalytic activity. In contrast, this mode of deactivation is eliminated by atomic layer deposition of an alumina overcoat; however, deposition of carbonaceous deposits in the small pores of the overcoat leads to deactivation that is reversible upon calcination of the catalyst.

ABE Condensation over Monometallic Catalysts: Catalyst Characterization and Kinetics

Herein, we present work on the catalyst development and the kinetics of acetone‐butanol‐ethanol (ABE) condensation. After examining multiple combinations of metal and basic catalysts reported in the literature, Cu supported on calcined hydrotalcites (HT) was found to be the optimal catalyst for the ABE condensation. This catalyst gave a six‐fold increase in reaction rates over previously reported catalysts. Kinetic analysis of the reaction over CuHT and HT revealed that the rate‐determining step is the C−H bond activation of alkoxides that are formed from alcohols on the Cu surface. This step is followed by the addition of the resulting aldehydes to an acetone enolate formed by deprotonation of the acetone over basic sites on the HT surface. The presence of alcohols reduces aldol condensation rates, as a result of the coverage of catalytic sites by alkoxides.

Temperature Programmed Reduction of a PdCu Bimetallic Catalyst via Atmospheric Pressure in situ STEM-EDS and in situ X-Ray Adsorption Analysis

Temperature programmed reduction (TPR) is a classical method for the analysis of metal catalysts but provides little detail of what is actually occurring in the system if other methods are not employed to determine which changes are taking place. Coupling TPR with in situ X-Ray absorption analysis allows specific changes in bulk metal oxidation state via analysis of the X-Ray absorption near edge structure (TPR-XANES) or particle size and alloy structure by analysis of the extended X-Ray fine structure.(TPR-EXAFS)[1]. Recently, it has become possible to observe temperature programmed reduction via in situ scanning transmission electron microscope (STEM) energy dispersive X-ray spectroscopy (XEDS) at hydrogen pressures up to one atmosphere [2]. This correlative study uses a combination of: TPR, XANES, XEDS and S/TEM methodologies to study the evolution of particle morphologies on a PdCu/TiO2 catalyst and explain the reduction profile observed in the conventional TPR of this system. This catalyst is representative of a large group of bimetallic catalysts useful for a range of chemical conversions including catalytic reforming, hydrotreating, emissions controls, and biomass conversion.

Linking Performance with Particle Configuration on Bimetallic Pt/Co/MWCNT Catalysts for Aqueous Phase Reforming by Aberration Corrected STEM coupled with EELS

1. School of Materials Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana, USA (Currently at UOP LLC, A Honeywell Company) 2. Forney Hall of Chemical Engineering, Purdue University, West Lafayette, Indiana, USA 3. Argonne National Laboratory, Argonne, Illinois 60439, USA 4. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA