UC Berkeley

Rhodium Catalysts in the Oxidation of CO by O₂ and NO: Shape, Composition, and Hot Electron Generation

by

James Russell Renzas

Doctor of Philosophy in Chemistry

University of California, Berkeley

Professor Gabor A. Somorjai, Chair

Professor Stephen R. Leone

Professor Jeffrey Bokor

It is well known that the activity, selectivity, and deactivation behavior of heterogeneous catalysts are strongly affected by a wide variety of parameters, including but not limited to nanoparticle size, shape, composition, support, pretreatment conditions, oxidation state, and electronic state. Enormous effort has been expended in an attempt to understand the role of these factors on catalytic behavior, but much still remains to be discovered. In this work, we have focused on deepening the present understanding of the role of nanoparticle shape, nanoparticle composition, and hot electrons on heterogeneous catalysis in the oxidation of carbon monoxide by molecular oxygen and nitric oxide. These reactions were chosen because they are important for environmental applications, such as in the catalytic converter, and because there is a wide range of experimental and theoretical insight from previous single crystal work as well as experimental data on nanoparticles obtained using new state-of-the-art techniques that aid greatly in the interpretation of results on complex nanoparticle systems.

In particular, the studies presented in this work involve three types of samples: ~ 6.5 nm Rh nanoparticles of different shapes, ~ 15 nm Rh_1-xPd_x core-shell bimetallic polyhedra nanoparticles, and Rh ultra-thin film (~ 5 nm) catalytic nanodiodes. The colloidal nanoparticle samples were synthesized using a co-reduction of metal salts in alcohol and supported on silicon wafers using the Langmuir-Blodgett technique. This synthetic strategy enables tremendous control of nanoparticle size, shape, and composition. Nanoparticle shape was controlled through the use of different organic polymer capping layers. Bimetallic core-shell nanoparticles were synthesized by careful choice of metal salt precursors. Rh/TiO_x and Rh/GaN catalytic nanodiodes were fabricated using a variety of thin film device fabrication techniques, including reactive DC magnetron sputtering, electron beam evaporation, and rapid thermal annealing. The combination of these techniques enabled control of catalytic nanodiode morphology, geometry, and electrical properties.

The prepared nanocatalysts and nanodiodes were characterized with a wide variety of modern techniques before and after reaction in order to investigate catalyst size, shape, composition, lattice structure, and electrical properties. In particular, the catalysts were investigated using Scanning Electron Microscopy to determine coverage and morphology, Transmission Electron Microscopy to determine size, shape, and morphology, X-Ray Photoelectron Spectroscopy to determine elemental composition and oxidation state, X-Ray Diffraction to determine crystallinity and lattice parameters, and Current-Voltage analysis to determine nanodiode barrier height and electrical properties. The use of this broad array of analytical techniques enabled thorough understanding of the catalysts and the role their properties play in catalysis.

The catalytic behavior of the catalysts was measured in CO oxidation by O₂ and by NO in-situ using Gas Chromatography and, for the catalytic nanodiodes, chemicurrent analysis. Both techniques were performed using the same ultra-high vacuum chamber. Kinetic data gathered using these techniques was analyzed and compared to the body of literature on related catalysts in order to further the understanding of the role of particular catalyst parameters and properties on their behavior during reaction.

Nanoparticle shape dependence in the oxidation of CO by NO was studied on Rhodium nanocubes and nanopolyhedra from 230 - 270°C. The nanoparticles were characterized using Scanning Electron Microscopy, Transmission Electron Microscopy, and X-Ray Diffraction. At 8 Torr of NO and 8 Torr of CO, the nanocubes were found to have increased turnover frequency and decreased activation energy relative to the nanopolyhedra catalysts. The nanopolyhedra were found to behave similarly to Rh (111) single crystal catalysts, whereas the nanocubes were found to have behavior intermediate to that found on Rh (111) and Rh (100) single crystal catalysts.

Bimetallic 15 nm Pd-core Rh-shell Rh_1-xPd_x nanoparticle catalysts with overall compositions of Rh, Rh_0.8Pd_0.2, Rh_0.6Pd_0.4, Rh_0.4Pd_0.6, Rh_0.2Pd_0.8, and Pd were synthesized and supported on p-type Silicon wafers using the Langmuir-Blodgett technique. The nanoparticle catalysts were characterized using Scanning Electron Microscopy, Transmission Electron Microscopy, X-Ray Diffraction, and X-Ray Photoelectron Spectroscopy. In the reaction of 40 Torr CO with 100 Torr O₂, the bimetallic core-shell catalysts were found to exhibit enhanced activity relative to monometallic Rh and Pd nanoparticle catalysts of the same size. This synergetic effect was analyzed in light of the data from characterization, previous work performed by our group (including the present author) using Ambient-Pressure X-Ray Photoelectron Spectroscopy to study the oxidation and surface segregation behavior of identical bimetallic core-shell nanoparticles in-situ during reaction at pressures on the order of hundreds of milliTorr, and previous work on related systems. The observed synergy is postulated to be the result of preferential adsorption of CO on Pd surface sites and preferential dissociative adsorption and oxide-formation by O₂ on Rh surface sites during reaction.

Identical bimetallic 15 nm Pd-core Rh-shell Rh_1-xPd_x nanoparticle catalysts were also synthesized, characterized, and studied in the reaction of CO with NO. Due to the increased complexity of the reaction of CO with NO relative to the reaction of CO with O₂, this reaction was studied in a variety of relative pressure conditions, ranging from 8 Torr NO and 8 Torr CO to 120 Torr NO and 8 Torr CO. In equal pressures of NO and CO, the catalysts were found to have no synergetic enhancement of activity. At these conditions, activity scaled roughly linearly with the relative composition of Rh. At relatively high pressures of NO, however, the catalysts demonstrated very different behavior. Initially, the bimetallic catalysts demonstrated extremely high activity relative to monometallic catalysts in the same conditions. Using results from Ambient-Pressure X-Ray Photoelectron spectroscopy at similar relative pressures of NO and CO, as well as data from related systems, this synergy was deduced to be caused by preferential adsorption of CO on available metallic Pd surface sites on the core-shell catalyst. After many hours of oxidation in these conditions, however, the bimetallic catalysts were found to deactivate such that, as in the case of equal pressures of CO and NO, product formation scaled linearly with Rh molar fraction. This deactivation may be caused by eventual migration of N adatoms onto Pd sites.

In the final study presented in this work, ultra-thin film 5 nm Rh/TiO_x and Rh/GaN catalytic metal-semiconductor Schottky nanodiodes were studied in the reaction of NO with CO and the reaction of CO with O₂. These devices were fabricated using a combination of reactive sputtering, electron beam evaporation, and rapid thermal annealing and characterized using a variety of techniques, including current-voltage analysis for the determination of Schottky barrier height. Barrier heights on the TiO_x-based nanodiodes were found to be very sensitive to local gas composition, whereas barrier heights on GaN-based devices were found to be more stable. The kinetic behavior of the devices was measured using both gas chromatography and chemicurrent analysis. Hot electron chemicurrent was determined through comparison of the measured current in reaction and the measured thermoelectric current at similar barrier height conditions. Similar activation energies were found using both techniques. This indicates that there is a direct correlation between hot electron production and catalytic activity.