Two-Dimensional Perovskite Crystals Formed by Atomic Layer Deposition of CaTiO3 on γ-Al2O3.

CaTiO3 films with an average thickness of 0.5 nm were deposited onto γ-Al2O3 by Atomic Layer Deposition (ALD) and then characterized by a range of techniques, including X-ray Diffraction (XRD) and High-Resolution, Transmission Electron Microscopy (HRTEM). The results demonstrate that the films form two-dimensional crystallites over the entire surface. Lattice fringes from HRTEM indicate that the crystallites range in size from 5 to 20 nm and are oriented in various directions. Films of the same thickness on SiO2 remained amorphous, indicating that the support played a role in forming the crystallites.

Metal Exsolution to Enhance the Catalytic Activity of Electrodes in Solid Oxide Fuel Cells.

Exsolution is a novel technology for attaching metal catalyst particles onto ceramic anodes in the solid oxide fuel cells (SOFCs). The exsolved metal particles in the anode exhibit unique properties for reaction and have demonstrated remarkable stabilities under conditions that normally lead to coking. Despite extensive investigations, the underlying principles behind exsolution are still under investigation. In this review, the present status of exsolution materials for SOFC applications is reported, including a description of the fundamental concepts behind metal incorporation in oxide lattices, a listing of proposed mechanisms and thermodynamics of the exsolution process and a discussion on the catalytic properties of the resulting materials. Prospects and opportunities to use materials produced by exsolution for SOFC are discussed.

Highly active dry methane reforming catalysts with boosted in situ grown Ni-Fe nanoparticles on perovskite via atomic layer deposition.

With the need for more stable and active metal catalysts for dry reforming of methane, in situ grown nanoparticles using exsolution are a promising approach. However, in conventional exsolution, most nanoparticles remain underneath the surface because of the sluggish diffusion rate of cations. Here, we report the atomic layer deposition (ALD)-combined topotactic exsolution on La0.6Sr0.2Ti0.85Ni0.15O3-δ toward developing active and durable catalysts. The uniform and quantitatively controlled layer of Fe via ALD facilitates the topotactic exsolution, increasing finely dispersed nanoparticles. The introduction of Fe2O3 yields the formation of Ni-Fe alloy owing to the spontaneous alloy formation energy of -0.43 eV, leading to an enhancement of the catalytic activity for dry methane reforming with a prolonged stability of 410 hours. Overall, the abundant alloy nanocatalysts via ALD mark an important step forward in the evolution of exsolution and its application to the field of energy utilization.

Enhancing Oxygen Exchange Activity by Tailoring Perovskite Surfaces.

A detailed understanding of the effects of surface chemical and geometric composition is essential for understanding the electrochemical performance of the perovskite (ABO3) oxides commonly used as electrocatalysts in the cathodes of ceramic fuel cells. Herein, we report how the addition of submonolayer quantities of A- and B-site cations affects the rate of the oxygen reduction reaction (ORR) of Sr-doped LaFeO3 (LSF), LaMnO3 (LSM), and LaCoO3 (LSCo). Density functional theory calculations were performed to determine the stability of different active sites on a collection of surfaces. With LSF and LSM, rates for the ORR are significantly higher on the A-site terminated surface, while surface termination is less important for LSCo. Our findings highlight the importance of tailoring the surface termination of the perovskite to obtain its ultimate ORR performance.

Evidence and Model for Strain-Driven Release of Metal Nanocatalysts from Perovskites during Exsolution.

The evolution of the surface morphology during exsolution of Ni from the perovskite La0.4Sr0.4Ti0.97Ni0.03O3-δ under reducing conditions was determined using atomic force microscopy. The exsolution process was found to initially induce the formation of a 20-30 nm deep pit on the oxide surface followed by the emergence of a Ni particle at the bottom of the pit. Continued emergence of the particle results in it nearly filling the pit, producing a unique structure in which the Ni particle is socketed into the oxide surface. We also show that this morphological evolution can be explained using a simple energy-based model that accounts for the interplay between the surface free energy and the strain energy induced by the included metal nucleate. The unique socketed structure results in strong anchorage between the exsolved particles and the oxide host lattice, which imparts both high thermal stability and unique catalytic activity.

Nano-socketed nickel particles with enhanced coking resistance grown in situ by redox exsolution.

Metal particles supported on oxide surfaces are used as catalysts for a wide variety of processes in the chemical and energy conversion industries. For catalytic applications, metal particles are generally formed on an oxide support by physical or chemical deposition, or less commonly by exsolution from it. Although fundamentally different, both methods might be assumed to produce morphologically and functionally similar particles. Here we show that unlike nickel particles deposited on perovskite oxides, exsolved analogues are socketed into the parent perovskite, leading to enhanced stability and a significant decrease in the propensity for hydrocarbon coking, indicative of a stronger metal-oxide interface. In addition, we reveal key surface effects and defect interactions critical for future design of exsolution-based perovskite materials for catalytic and other functionalities. This study provides a new dimension for tailoring particle-substrate interactions in the context of increasing interest for emergent interfacial phenomena.

Surface modification of La(0.8)Sr(0.2)CrO(3-δ)-YSZ dual-phase membranes for syngas production.

Oxygen permeation fluxes were studied in Mixed Ionic and Electronic Conducting (MIEC) membranes based on composites of 40 vol% La(0.8)Sr(0.2)CrO3 (LSCr) and 60 vol% yttria-stabilized zirconia (YSZ), using ambient air and flowing CO to establish a P(O2) gradient. The ambipolar conductivity of the dense LSCr-YSZ composite was determined for membranes with dense layers that were 115 µm and 650 µm thick. Other parts of the investigation focused on how modifications to the surface on the CO side affected the fluxes. Using a porous LSCr-YSZ composite on the surface as the base case, oxygen fluxes were shown to increase dramatically upon addition of 5 wt% CeO2 as a catalyst and an additional increase was observed with 1 wt% Pt. Changes in the structure of the porous composite LSCr-YSZ surface to improve connectivity of the YSZ phase also led to large increases in the oxygen fluxes.

Thermal and photochemical reactions of methanol on nanocrystalline anatase TiO2 thin films.

The catalytic and photo-catalytic activity of well-defined anatase TiO2 nanocrystals for the partial oxidation of methanol was investigated using temperature-programmed desorption (TPD) in ultra-high vacuum in order to determine how crystallite size and shape affect reactivity. The TiO2 films used in this study were prepared from well-defined TiO2 nanocrystals synthesized by colloidal methods. These nanocrystals had a truncated bi-pyramidal shape which exposes primarily (101) and to a lesser extent (001) surfaces and ranged in size from 10 to 25 nm. Two distinct regimes of reactivity were investigated, namely in the dark and under UV light illumination. In the dark, methanol adsorbed dissociatively on the (001) planes and only molecularly on the (101) surfaces. Dissociated methoxy groups on the (001) surfaces coupled to produce dimethyl ether, suggesting the presence of fourfold coordinate Ti cations. Under UV light illumination, the nanocrystals were additionally found to be active for the photo-catalytic oxidation of methanol to methyl formate. On the (101) surfaces, this reaction proceeded in a stepwise photocatalytic pathway involving dehydrogenation of methanol to form methoxy groups and then formaldehyde, followed by coupling of these latter two species to form methyl formate. The (001) surfaces were also found to be photo-catalytically active but surface methoxy groups could be produced thermally and the reaction proceeds only to formaldehyde in the absence of molecularly adsorbed methanol. The overall photocatalytic activity of the nanocrystals was also was found to increase with increasing crystallite size. The results of this study show that thin films of well-defined nanocrystals are excellent model systems that can be used to help bridge the materials gap between studies of single crystal surfaces and high surface area polycrystalline catalysts.

Direct in situ probe of electrochemical processes in operating fuel cells.

The function of systems and devices in many technologically important applications depends on dynamic processes in complex environments not accessible by structure and property characterization tools. Fuel cells represent an example in which interactions occur under extreme conditions: high pressure, high temperature, in reactive gas environments. Here, scanning surface potential microscopy is used to quantify local potential at electrode/electrolyte interfaces in operating solid oxide fuel cells at 600 °C. Two types of fuel cells are compared to demonstrate two mechanisms of ionic transport at interfaces. Lanthanum strontium ferrite-yttria-stabilized zirconia (LSF-YSZ) and lanthanum strontium manganite-yttria-stabilized zirconia (LSM-YSZ) cross-sectional electrode assemblies were measured to compare mixed ionic electronic conducting and electronic conducting mechanisms. Direct observation of the active zones in these devices yields characteristic length scales and estimates of activation barrier changes.

Alloy formation and chemisorption at Zn/Pt(111) bimetallic surfaces using alkali ISS, XPD, and TPD.

Alloy formation and chemisorption at bimetallic surfaces formed by vapor-depositing Zn on a Pt(111) single crystal were investigated primarily by using X-ray photoelectron diffraction (XPD), X-ray photoelectron spectroscopy (XPS), low-energy alkali ion scattering spectroscopy (ALISS), low electron energy diffraction (LEED), and temperature programmed desorption (TPD). A wide range of conditions were investigated to explore whether deposition and annealing of Zn films could produce well-defined, ordered alloy surfaces, similar to those encountered for Sn/Pt(111) surface alloys. These attempts were unsuccessful, although weak, diffuse (2 × 2) spots were observed under special conditions. The particular PtZn bimetallic alloy created by annealing one monolayer of Zn on Pt(111) at 600 K, which has a Zn composition in the surface layer of about 5 at. %, was investigated in detail by using XPD and ALISS. Only a diffuse (1 × 1) pattern was observed from this surface by LEED, suggesting that no long-range, ordered alloy structure was formed. Zn atoms were substitutionally incorporated into the Pt(111) crystal to form a near-surface alloy in which Zn atoms were found to reside primarily in the topmost and second layers. The alloyed Zn atoms in the topmost layer are coplanar with the Pt atoms in the surface layer, without any "buckling" of Zn, that is, displacement in the vertical direction. This result is expected because of the similar size of Pt and Zn, based on previous studies of bimetallic Pt alloys. Zn atoms desorb upon heating rather than diffusing deep into the bulk of the Pt crystal. Temperature programmed desorption (TPD) measurements show that both CO and NO have lower desorption energies on the PtZn alloy surface compared to that on the clean Pt(111) surface.

Exceptional thermal stability of Pd@CeO2 core-shell catalyst nanostructures grafted onto an oxide surface.

Monolayer films of highly catalytically active Pd@CeO2 core-shell nanocomposites were grafted onto a planar YSZ(100) (yttria-stabilized zirconia, YSZ) single crystal support that was functionalized with a CVD-deposited layer of triethoxy(octyl)silane (TEOOS). The resulting monolayer films were found to exhibit exceptionally high thermal stability compared to bare Pd nanoparticles with the Pd@CeO2 nanostructures remaining intact and highly dispersed upon calcining in air at temperatures in excess of 1000 K. The CeO2 shells were also shown to be more easily reduced than bulk CeO2, which may partially explain their unique activity as oxidation catalysts. The use of both TEOOS and tetradecylphosphonic acid (TDPA) as coupling agents for dispersing Pd@CeO2 core-shell nanocomposites onto a high surface area γ-Al2O3 support is also demonstrated.

Reaction pathways for ethanol on model Co/ZnO(0001) catalysts.

The pathways for the reaction of ethanol on model catalysts consisting of Co and CoO films and particles supported on single crystal ZnO(0001) surfaces were studied using X-ray Photoelectron Spectroscopy (XPS) and Temperature Programmed Desorption (TPD). On supported metallic Co films and particles ethanol was found to primarily undergo decarbonylation forming CO, H(2), and adsorbed methyl groups. In contrast, supported CoO particles were found to be largely unreactive toward ethanol. High selectivity to the dehydrogenation product, acetaldehyde, was only observed when the supported Co was partially oxidized and contained both Co(0) and Co(2+). Since acetaldehyde is thought to be a critical intermediate during steam reforming of ethanol (SRE) to produce H(2) and CO(2), the results of this study suggest that partially oxidized Co species provide the active sites for this reaction. This result is consistent with studies of high surface area Co/ZnO catalysts which also suggest that both Co(0) and Co(2+) species are present under typical SRE reaction conditions.

Ensemble vs. electronic effects on the reactivity of two-dimensional Pd alloys: a comparison of CO and CH3OH adsorption on Zn/Pd(111) and Cu/Pd(111).

The adsorption of CO and CH(3)OH on two-dimensional PdCu alloys on Pd(111) was studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS), and compared to results previously obtained for analogous PdZn alloys on Pd(111). Cu addition to the Pd(111) surface was found to alter the preferred adsorption sites for CO from threefold to bridge Pd sites and decrease the activity for the dehydrogenation of CH(3)OH. However, the effect of Cu was much less dramatic than that observed for Zn on Zn-modified surfaces. Preliminary DFT calculations also show that Cu causes less perturbation of the electronic structure of nearby Pd sites relative to Zn. The experimental results for the surface PdCu alloys indicate that Cu predominantly has an ensemble effect on reactivity while more significant long-range electronic interactions in addition to ensemble effects appear to be important for PdZn.

Zn modification of the reactivity of Pd(111) toward methanol and formaldehyde.

The adsorption and reaction of methanol and formaldehyde on two-dimensional PdZn alloys on a Pd(111) surface were studied as a function of the Zn content in the alloy in order to understand the role of Zn in Pd/ZnO catalysts for the steam reforming of methanol (SRM). Temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) data show that Zn atoms incorporated into the Pd(111) surface dramatically decrease the dehydrogenation activity and alter the preferred bonding sites for adsorbed CO, CH3O, and CH2O intermediates. The experimental results obtained in this study are consistent with previous theoretical studies of this system and provide new insight into how Zn alters the reactivity of Pd.

Probing the effect of local structure on the thermodynamic redox properties of V(+5): a comparison of V2O5 and Mg3(VO4)2.

Coulometric titration, an electrochemical method for measuring oxidation isotherms, has been used to characterize the redox properties of V2O5 and Mg3(VO4)2 between 823 and 973 K. V2O5 shows distinct regions in the isotherms corresponding to equilibrium with mixtures of V2O3 and V2O4 and of V2O4 and V2O5. From this data, the enthalpies for oxidation of V2O3 to V2O4 and for V2O4 to V2O5 are shown to be -380 +/- 10 and -285 +/- 20 kJ mol-1 O2, respectively. Oxidation isotherms for Mg3(VO4)2 exhibit a single step between the oxidized sample (all V+5) and a completely reduced sample (all V+3). The enthalpy of oxidation is found to increase with the oxidation state of the sample, from -370 +/- 30 kJ mol-1 O2 at an O:V ratio of 1.5 to -460 +/- 10 kJ mol-1 O2 at an O:V ratio of 2.5.