Preparation Of Active And Stable High-Surface Area Catalysts By Atomic Layer Deposition

Deactivation of catalytic functional oxides through the loss of surface area is a major concern. The conventional approach to maintain high-surface area of these materials is to incorporate the functional components onto a support which is less susceptible to sintering. Conventional impregnation tends to introduce large crystallites and often does not increase the surface area of the functional component. To address this issue, ALD was used in this work to engineer materials on the surface of interest. ALD has been used to form uniform oxides in a layer-by-layer manner with excellent compositional control. However, since ALD was developed in the semi-conductor industry to produce relatively thick films on a flat surface, the design criteria are very different from what is required for catalytic applications. First, the rapid cycling with high-velocity carrier gases that are commonly used in semiconductor fabrications will create diffusion limitations in porous structures. Second, when carrier gases are used, most reagents pass through the reactor without being incorporated into the sample. This is prohibitively expensive for catalytic applications. In this thesis, a static ALD system which avoids these issues was developed for preparing catalysts in two primary areas: (a) high-surface area active supports with excellent thermal stability, and (b) stabilization of precious metals. The first area involved fabricating thin films of Fe2O3, CeO2, CeZrO4, and LaFeO3 on porous Al2O3. These high-surface area films were shown to be uniform and they exhibited excellent thermal stability up to 1273 K when used as supports for Pd in methane and CO oxidation. With compositional control by ALD, CeZrO4 and LaFeO3, complex oxides would otherwise require complex synthesis or high temperature treatments, were easily fabricated at moderate conditions. The second area involved stabilizing metal particles by thin films of LaFeO3 and ZrO2 prepared by ALD. Pd supported on LaFeO3 is of interests as it is the classical example of a “smart” catalyst capable of redispersing metal particles following redox cycling conditions. The LaFeO3 catalysts were shown to exhibit properties expect for smart catalysts. Overcoating thin films of ZrO2 on Pd to improve its thermal stability was also demonstrated

Atomic Layer Deposition on Porous Materials: Problems with Conventional Approaches to Catalyst and Fuel Cell Electrode Preparation

Atomic layer deposition (ALD) offers exciting possibilities for controlling the structure and composition of surfaces on the atomic scale in heterogeneous catalysts and solid oxide fuel cell (SOFC) electrodes. However, while ALD procedures and equipment are well developed for applications involving flat surfaces, the conditions required for ALD in porous materials with a large surface area need to be very different. The materials (e.g., rare earths and other functional oxides) that are of interest for catalytic applications will also be different. For flat surfaces, rapid cycling, enabled by high carrier-gas flow rates, is necessary in order to rapidly grow thicker films. By contrast, ALD films in porous materials rarely need to be more than 1 nm thick. The elimination of diffusion gradients, efficient use of precursors, and ligand removal with less reactive precursors are the major factors that need to be controlled. In this review, criteria will be outlined for the successful use of ALD in porous materials. Examples of opportunities for using ALD to modify heterogeneous catalysts and SOFC electrodes will be given

Modification of LSF-YSZ Composite Cathodes by Atomic Layer Deposition

composite, Solid-Oxide-Fuel-Cell (SOFC) electrodes of La0.8Sr0.2FeO3 (LSF) and yttria-stabilized zirconia (YSZ) were prepared by infiltration methods and then modified by Atomic Layer Deposition (ALD) of ZrO2, La2O3, Fe2O3, or La2O3-Fe2O3 codeposited films of different thicknesses to determine the effect of surface composition on cathode performance. Film growth rates for ALD performed using vacuum procedures at 573 K for Fe2O3 and 523 K for ZrO2 and La2O3 were determined to be 0.024 nm ZrO2/cycle, 0.019 nm La2O3/cycle, and 0.018 nm Fe2O3/cycle. For ZrO2 and Fe2O3, impedance spectra on symmetric cells at 873 K indicated that polarization resistances increased with coverage in a manner suggesting simple blocking of O2 adsorption sites. With La2O3, the polarization resistance decreased with small numbers of ALD cycles before again increasing at higher coverages. When La2O3 and Fe2O3 were co-deposited, the polarization resistances remained low at high film coverages, implying that O2 adsorption sites were formed on the co-deposited films. The implications fo these results for future SOFC electrode development are discussed

Atomic Layer Deposition on Porous Materials: Problems with Conventional Approaches to Catalyst and Fuel Cell Electrode Preparation

Atomic layer deposition (ALD) offers exciting possibilities for controlling the structure and composition of surfaces on the atomic scale in heterogeneous catalysts and solid oxide fuel cell (SOFC) electrodes. However, while ALD procedures and equipment are well developed for applications involving flat surfaces, the conditions required for ALD in porous materials with a large surface area need to be very different. The materials (e.g., rare earths and other functional oxides) that are of interest for catalytic applications will also be different. For flat surfaces, rapid cycling, enabled by high carrier-gas flow rates, is necessary in order to rapidly grow thicker films. By contrast, ALD films in porous materials rarely need to be more than 1 nm thick. The elimination of diffusion gradients, efficient use of precursors, and ligand removal with less reactive precursors are the major factors that need to be controlled. In this review, criteria will be outlined for the successful use of ALD in porous materials. Examples of opportunities for using ALD to modify heterogeneous catalysts and SOFC electrodes will be given

High-surface-area, iron-oxide films prepared by atomic layer deposition on \u3b3-Al2O3

High-surface-area iron oxides were prepared by Atomic Layer Deposition (ALD) on 130-m(2)/g gamma-Al2O3 for use as a catalyst support. Measurements of the sample mass, surface area, and pore-size distribution as a function of the number of ferrocene-O-2 ALD cycles at 623 K suggested that the iron oxide grew as a dense, conformal film with a growth rate similar to 0.016-nm per cycle. While films with 20 ALD cycles (20Fe(2)O(3)Al(2)O(3), 0.25 g Fe2O3/g Al2O3) were difficult to distinguish by HAADF STEM, EDS mapping indicated the Al2O3 was uniformly coated. Raman Spectroscopy showed the films were in the form of Fe2O3; but XRD measurements on samples with as many as 100 ALD cycles (100Fe(2)O(3)-Al2O3, 0.84g Fe2O3/g Al2O3) showed no evidence for crystalline iron-oxide phases, even after calcination at 1073 K. Specific rates for the water-gas-shift (WGS) reaction on the ALD-coated samples were significantly lower than those on bulk Fe2O3. However, addition of 1 wt.% Pd to Fe2O3/Al2O3 supports prepared by ALD exhibited specific rates that were much higher than that observed when I wt.% Pd was added to Fe2O3/Al2O3 prepared by conventional impregnation of Fe salts, suggesting more uniform contact between the Pd and FeOx phases on samples prepared by ALD

Investigation of the Thermodynamic Properties of Surface Ceria and Ceria–Zirconia Solid Solution Films Prepared by Atomic Layer Deposition on Al2O3

The properties of 20 wt % CeO2 and 21 wt % Ce0.5Zr0.5O2 films, deposited onto a γ-Al2O3 by Atomic Layer Deposition (ALD), were compared to bulk Ce0.5Zr0.5O2 and γ-Al2O3-supported samples on which 20 wt % CeO2 or 21 wt % CeO2–ZrO2 were deposited by impregnation. Following calcination to 1073 K, the ALD-prepared catalysts showed much lower XRD peak intensities, implying that these samples existed as thin films, rather than larger crystallites. Following the addition of 1 wt % Pd to each of the supports, the ALD-prepared samples exhibited much higher rates for CO oxidation due to better interfacial contact between the Pd and ceria-containing phases. The redox properties of the ALD samples and bulk Ce0.5Zr0.5O2 were measured by determining the oxidation state of the ceria as a function of the H2:H2O ratio using flow titration and coulometric titration. The 20 wt % CeO2 ALD film exhibited similar thermodynamics to that measured previously for a sample prepared by impregnation. However, the sample with 21 wt % Ce0.5Zr0.5O2 on γ-Al2O3 reduced at a much higher P O 2 and showed evidence for transition between the Ce0.5Zr0.5O2 and Ce0.5Zr0.5O1.75 phases

Investigation of the Thermodynamic Properties of Surface Ceria and Ceria–Zirconia Solid Solution Films Prepared by Atomic Layer Deposition on Al2O3

The properties of 20 wt % CeO2 and 21 wt % Ce0.5Zr0.5O2 films, deposited onto a γ-Al2O3 by Atomic Layer Deposition (ALD), were compared to bulk Ce0.5Zr0.5O2 and γ-Al2O3-supported samples on which 20 wt % CeO2 or 21 wt % CeO2–ZrO2 were deposited by impregnation. Following calcination to 1073 K, the ALD-prepared catalysts showed much lower XRD peak intensities, implying that these samples existed as thin films, rather than larger crystallites. Following the addition of 1 wt % Pd to each of the supports, the ALD-prepared samples exhibited much higher rates for CO oxidation due to better interfacial contact between the Pd and ceria-containing phases. The redox properties of the ALD samples and bulk Ce0.5Zr0.5O2 were measured by determining the oxidation state of the ceria as a function of the H2:H2O ratio using flow titration and coulometric titration. The 20 wt % CeO2 ALD film exhibited similar thermodynamics to that measured previously for a sample prepared by impregnation. However, the sample with 21 wt % Ce0.5Zr0.5O2 on γ-Al2O3 reduced at a much higher P O 2 and showed evidence for transition between the Ce0.5Zr0.5O2 and Ce0.5Zr0.5O1.75 phases

Stabilization of ZrO2 Powders via ALD of CeO2 and ZrO2

ZrO2 powders were modified by atomic layer deposition (ALD) with CeO2 and ZrO2, using Ce(TMHD)4 and Zr(TMHD)4 as the precursors, in order to determine the effect of ALD films on the structure, surface area, and catalytic properties of the ZrO2. Growth rates were measured gravimetrically and found to be 0.017 nm/cycle for CeO2 and 0.031 nm/cycle for ZrO2. The addition of 20 ALD cycles of either CeO2 or ZrO2 was found to stabilize the surface area of the ZrO2 powder following calcination to 1073 K and to suppress the tetragonal-to-monoclinic transition. Shrinkage of ZrO2 wafers was also suppressed by the ALD films. When used as a support for Pd in CO oxidation, the CeO2-modified materials significantly enhanced rates due to interactions between the Pd and the CeO2. Potential applications for modifying catalyst supports using ALD are discussed

Fabrication of Large Area Metal-on-Carbon Catalytic Condensers for Programmable Catalysis

Catalytic condensers stabilize charge on either side of a high-k dielectric film to modulate the electronic states of a catalytic layer for electronic control of surface reactions. Here, carbon sputtering provided for fast, large-scale fabrication of metal-carbon catalytic condensers required for industrial application. Carbon films were sputtered on HfO2 dielectric/p-type Si with different thickness (1, 3, 6, 10 nm), and the enhancement of conductance and capacitance of carbon films was observed upon increasing carbon thickness following thermal treatment at 400 °C. After Pt deposition on the carbon films, the Pt catalytic condenser exhibited high capacitance of ~210 nF/cm^2 that was maintained at a frequency ~1,000 Hz, satisfying the requirement for a dynamic catalyst to implement catalytic resonance. Temperature programmed desorption of carbon monoxide yielded CO desorption peaks which shifted in temperature with varying potential applied to the condenser (−6 V or 6 +V) indicating a shift in the binding energy of carbon monoxide on the Pt condenser surface. A substantial increase of capacitance (~2,000 nF/cm^2) of the Pt-on-carbon devices was observed at elevated temperatures of 400 °C that can modulate ~10% of charge per metal atom when 10 V potential was applied. A large catalytic condenser of 42 cm^2 area Pt/C/HfO2/Si exhibited high capacitance of 9,393 nF with low leakage current/capacitive current ratio (<0.1), demonstrating the practicality and versatility of the facile, large-scale fabrication method for metal-carbon catalytic condensers

A Characterization Study of Reactive Sites in ALD-Synthesized WOx/ZrO2 Catalysts

A series of ZrO2-supported WOx catalysts were prepared using atomic layer deposition (ALD) with W(CO)6, and were then compared to a WOx/ZrO2 catalyst prepared via conventional impregnation. The types of sites present in these samples were characterized using temperature-programmed desorption/thermogravimetric analysis (TPD-TGA) measurements with 2-propanol and 2-propanamine. Weight changes showed that the WOx catalysts grew at a rate of 8.8 &times; 1017 W atoms/m2 per cycle. Scanning transmission electron microscopy/energy-dispersive spectroscopy (STEM-EDS) indicated that WOx was deposited uniformly, as did the 2-propanol TPD-TGA results, which showed that ZrO2 was completely covered after five ALD cycles. Furthermore, 2-propanamine TPD-TGA demonstrated the presence of three types of catalytic sites, the concentrations of which changed with the number of ALD cycles: dehydrogenation sites associated with ZrO2, Br&oslash;nsted-acid sites associated with monolayer WOx clusters, and oxidation sites associated with higher WOx coverages. The Br&oslash;nsted sites were not formed via ALD of WOx on SiO2. The reaction rates for 2-propanol dehydration were correlated with the concentration of Br&oslash;nsted sites. While TPD-TGA of 2-propanamine did not differentiate the strength of Br&oslash;nsted-acid sites, H&ndash;D exchange between D2O and either toluene or chlorobenzene indicated that the Br&oslash;nsted sites in tungstated zirconia were much weaker than those in H-ZSM-5 zeolites