Recent worldwide interest in building a decentralized, hydrogen-based energy economy has refocused attention on the solid oxide fuel cell (SOFC) as a potential source of efficient, environmentally friendly, fuel-versatile electric power. Due to its high operating temperature, the SOFC offers several potential advantages over polymer-based fuel cells, including reversible electrode reactions, low internal resistance, high tolerance to typical catalyst poisons, production of high-quality waste heat for (among other uses) reformation of hydrocarbon fuels, as well as the possibility of burning hydrocarbon fuels directly. Today, SOFCs are much closer to commercial reality than they were 20 years ago, due largely to technological advances in electrode material composition, microstructure control, thin-film ceramic fabrication, and stack and system design. These advances have led to dozens of active SOFC development programs in both stationary and mobile power and contributed to commercialization or development in a number of related technologies, including gas sensors,1 solid-state electrolysis devices,2 and iontransport membranes for gas separation and partial oxidation.3 Many reviews are available which summarize the technological advances made in SOFCs over the last 15-35 yearssreaders who are primarily interested in knowing the state-of-the art in materials, design, and fabrication (including the electrodes) are encouraged to consult these reviews.4-12 This review focuses on the factors governing SOFC cathode performancesadvances we have made over † Dedicated to Brian Steele, 1929-2003. Researcher, Entrepreneur, Consensus Seeker. 4791 Chem. Rev. 2004, 104, 4791−4843

Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes

Gold catalysts have recently been attracting rapidly growing interests due to their potential applicabilities to many reactions of both industrial and environmental importance. This article reviews the latest advances in the catalysis research on Au. For low-temperature CO oxidation mechanistic arguments are summarized, focusing on Au/TiO 2 together with the effect of preparation conditions and pretreatments. The quantum size effect is also discussed in the adsorption and reaction of CO over Au clusters smaller than 2 nm in diameter. In addition, recent developments are introduced in the epoxidation of propylene, water-gas-shift reaction, hydrogenation of unsaturated hydrocarbons, and liquid-phase selective oxidation. The role of perimeter interface between Au particles and the support is emphasized as a unique reaction site for the reactants adsorbed separately, one on Au and another on the support surfaces.

Advances in the catalysis of Au nanoparticles

Hydrogen is a promising energy carrier in future energy systems. However, storage of hydrogen is a substantial challenge, especially for applications in vehicles with fuel cells that use proton-exchange membranes (PEMs). Different methods for hydrogen storage are discussed, including high-pressure and cryogenic-liquid storage, adsorptive storage on high-surface-area adsorbents, chemical storage in metal hydrides and complex hydrides, and storage in boranes. For the latter chemical solutions, reversible options and hydrolytic release of hydrogen with off-board regeneration are both possible. Reforming of liquid hydrogen-containing compounds is also a possible means of hydrogen generation. The advantages and disadvantages of the different systems are compared.

Chemical and Physical Solutions for Hydrogen Storage

Most of the world’s hydrogen supply is currently obtained by reforming hydrocarbons. ‘Reformate’ hydrogen contains significant quantities of CO that poison current hydrogen fuel-cell devices. Catalysts are needed to remove CO from hydrogen through selective oxidation. Here, we report first-principles-guided synthesis of a nanoparticle catalyst comprising a Ru core covered with an approximately 1–2-monolayer-thick shell of Pt atoms. The distinct catalytic properties of these well-characterized core–shell nanoparticles were demonstrated for preferential CO oxidation in hydrogen feeds and subsequent hydrogen light-off. For H2 streams containing 1,000 p.p.m. CO, H2 light-off is complete by 30 ∘C, which is significantly better than for traditional PtRu nano-alloys (85 ∘C), monometallic mixtures of nanoparticles (93 ∘C) and pure Pt particles (170 ∘C). Density functional theory studies suggest that the enhanced catalytic activity for the core–shell nanoparticle originates from a combination of an increased availability of CO-free Pt surface sites on the Ru@Pt nanoparticles and a hydrogen-mediated low-temperature CO oxidation process that is clearly distinct from the traditional bifunctional CO oxidation mechanism. To produce hydrogen by reforming hydrocarbons, efficient catalysts capable of removing carbon monoxide are needed. This can now be achieved via a preferential oxidation mechanism using nanoparticle catalysts consisting of a ruthenium core covered with platinum.

Ru-Pt core-shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen.

CO Oxidation over Supported Gold Catalysts—“Inert” and “Active” Support Materials and Their Role for the Oxygen Supply during Reaction

Kinetics of the Selective CO Oxidation in H2-Rich Gas on Pt/Al2O3☆

Kinetics of the Selective Low-Temperature Oxidation of CO in H2-Rich Gas over Au/α-Fe2O3

Correlation between CO surface coverage and selectivity/kinetics for the preferential CO oxidation over Pt/γ-Al2O3 and Au/α-Fe2O3: an in-situ DRIFTS study

Kinetic measurements on preferential CO oxidation in a H2-rich atmosphere (PROX) over a bimetallic, carbon supported
 PtSn catalyst reveal a high activity and selectivity already at low temperatures (0–80°C), superior to
 a commercial Pt/Al2O3 system The selectivity, though steadily decreasing with temperature, is remarkably
 high, 85% at low temperatures around 0–20°C, and even at 120°C it is, at 45%, still higher than that of standard Pt catalysts
 The observation that CO desorption is not rate limiting and that the selectivity decreases with increasing
 temperature, can be explained in a mechanistic model involving separation of the reactant adsorption
 sites (bifunctional surface), with competing CO and hydrogen adsorption on Pt sites/areas and oxygen adsorption predominantly on Sn sites and SnOx islands on/adjacent to the active PtSn particles The reaction takes place
 in a bifunctional way at the perimeter of these islands or by invoking a spill-over process This model
 is supported by CO temperature-programmed desorption (TPD), in situ
 diffuse reflectance IR Fourier transform
 spectroscopy (DRIFTS), and x-ray photon spectroscopy (XPS) measurements, which indicate that under reaction
 conditions the surface CO coverage on the metallic particles is high, but decreases with temperature, and that only part of the Sn is reduced, included in PtSn alloy particles, while another part is in an oxidic state, forming
 SnOx islands on and presumably also beside the active particles Its excellent performance makes PtSn an
 interesting catalyst for fuel gas purification in
 low temperature
 polymer electrolyte membrane
 fuel cell technology (PEM-FC)

https://pubs.rsc.org/en/content/articlepdf/2001/cp/b008062o

Bimetallic PtSn catalyst for selective CO oxidation in H2-rich gases at low temperatures

The performance of a 0.5 wt.% Rh/MgO catalyst for the oxidative removal of CO from H2-rich methanol reformate (1% CO, 65% H2, 10% H2O, rest CO2) was investigated and compared to that of a conventional 0.5 wt.% Pt/γ-Al2O3 catalyst and a 5 wt.% Ru/γ-Al2O3 catalyst. Temperature screening experiments and 10–30 h activity measurements reveal a high activity and selectivity in the temperature region between 175 and 300 °C, in combination with a relatively low deactivation. Water vapor in the feed gas (≤10%) has little effect on the reaction characteristics. The low activity for the reverse water gas shift (RWGS) reaction, whose rate is more than four orders of magnitude lower than the rate for CO oxidation, and for the methanation reaction at temperatures below 300 °C allow to operate Rh/MgO at higher temperatures, around 250 °C, while for Pt/γ-Al2O3 and Ru/γ-Al2O3 the reaction temperatures are limited to 200 and 150 °C, respectively, due to the decrease in selectivity (Pt/γ-Al2O3) and the onset of the methanation reaction (Ru/γ-Al2O3) at higher temperatures. Model calculations, using kinetic data measured in realistic reformate, predict that the minimum amount of noble metal required for the complete removal of CO from the feed gas (≤10 ppm) is by two orders of magnitude lower for Rh/MgO operated at 250 °C than for Pt/γ-Al2O3 and Ru/γ-Al2O3 catalysts at 200 and 150 °C, respectively, at a comparable O2 excess. Due to the low RWGS activity the CO exit concentration does not increase significantly above the tolerance limit of state-of-the-art Pt–Ru fuel cell anodes of 100 ppm under dynamic load conditions, down to 1% load. For the other two catalysts the CO exit concentration are significantly higher under these conditions, reaching values above 500 ppm for Ru/γ-Al2O3 at 1% load and the WGS equilibrium value of 2000 ppm for Pt/γ-Al2O3. The predictions are verified by measurements in a microreactor under similar reaction conditions. The reaction characteristics make Rh/MgO an ideal PROX catalyst for reaction at higher temperatures (250 °C), in particular for applications requiring highly dynamic operation under variable load conditions.

M.J. Kahlich

Papers

Kinetics of the Selective CO Oxidation in H2-Rich Gas on Pt/Al2O3☆

Kinetics of the Selective Low-Temperature Oxidation of CO in H2-Rich Gas over Au/α-Fe2O3

Correlation between CO surface coverage and selectivity/kinetics for the preferential CO oxidation over Pt/γ-Al2O3 and Au/α-Fe2O3: an in-situ DRIFTS study

Bimetallic PtSn catalyst for selective CO oxidation in H2-rich gases at low temperatures

CO removal from realistic methanol reformate via preferential oxidation—performance of a Rh/MgO catalyst and comparison to Ru/γ-Al2O3, and Pt/γ-Al2O3