Raymond J. Gorte

Direct oxidation of hydrocarbons in a solid-oxide fuel cell

The direct electrochemical oxidation of dry hydrocarbon fuels to generate electrical power has the potential to accelerate substantially the use of fuel cells in transportation and distributed-power applications. Most fuel-cell research has involved the use of hydrogen as the fuel, although the practical generation and storage of hydrogen remains an important technological hurdle. Methane has been successfully oxidized electrochemically, but the susceptibility to carbon formation from other hydrocarbons that may be present or poor power densities have prevented the application of this simple fuel in practical applications. Here we report the direct, electrochemical oxidation of various hydrocarbons (methane, ethane, 1-butene, n-butane and toluene) using a solid-oxide fuel cell at 973 and 1,073 K with a composite anode of copper and ceria (or samaria-doped ceria). We demonstrate that the final products of the oxidation are CO2 and water, and that reasonable power densities can be achieved. The observation that a solid-oxide fuel cell can be operated on dry hydrocarbons, including liquid fuels, without reforming, suggests that this type of fuel cell could provide an alternative to hydrogen-based fuel-cell technologies.

Studies of the water-gas-shift reaction on ceria-supported Pt, Pd, and Rh: Implications for oxygen-storage properties

Abstract Steady-state, water-gas-shift kinetics were measured on model, ceria-supported, Pd, Pt, and Rh catalysts and compared to rates obtained on alumina-supported catalysts. When ceria was calcined at low temperatures prior to addition of the precious metal, the specific rates were found to be identical for each of the metals, with an activation energy of 11 ± 1 kcal/mol and reaction orders of zero and one for CO and H 2 O respectively. For comparison, specific rates on Rh/alumina were at least two orders of magnitude lower. However, ceria structure strongly affected the results. When ceria was calcined to high temperatures to increase crystallite size, prior to the addition of Pd, specific rates were a factor of 50 lower at 515 K and the activation energy was found to be much higher, 21 ± 1 kcal/mol. By comparison with results from an earlier study of CO oxidation [17], we propose that water-gas shift on ceria-supported metals occurs primarily through a bifunctional mechanism in which CO adsorbed on the precious metal is oxidized by ceria, which in turn is oxidized by water. Deactivation of the catalyst following growth in the ceria crystallite size is due to the decreased reducibility of large ceria crystallites. The implications of these results for automotive, emission-control catalysts is discussed.

Control of Metal Nanocrystal Size Reveals Metal-Support Interface Role for Ceria Catalysts

A Measure of Metal-Oxide Interfaces The rate of a catalytic reaction can sometimes be enhanced by using a different metal oxide as the support for adsorbed metal nanoparticles. Such enhancement is often attributed to more active sites at the metal-oxide interface, but it can be difficult to quantify this effect. Cargnello et al. (p. 771, published online 18 July) synthesized monodisperse nanoparticles of nickel, platinum, and palladium and dispersed them on high-surface-area ceria or alumina supports. High-resolution transmission electron microscopy enabled a detailed analysis of interfacial site structure, which showed that the rate of CO oxidation on ceria was indeed enhanced greatly at interface sites. Comparing nanocrystals of different sizes on different oxides shows that ceria-metal interface sites enhance carbon monoxide oxidation. Interactions between ceria (CeO2) and supported metals greatly enhance rates for a number of important reactions. However, direct relationships between structure and function in these catalysts have been difficult to extract because the samples studied either were heterogeneous or were model systems dissimilar to working catalysts. We report rate measurements on samples in which the length of the ceria-metal interface was tailored by the use of monodisperse nickel, palladium, and platinum nanocrystals. We found that carbon monoxide oxidation in ceria-based catalysts is greatly enhanced at the ceria-metal interface sites for a range of group VIII metal catalysts, clarifying the pivotal role played by the support.

Exceptional Activity for Methane Combustion over Modular Pd@CeO2 Subunits on Functionalized Al2O3

Addressing a Burning Issue Complete combustion of methane is required in order to avoid the unproductive emission of this greenhouse gas into the atmosphere. Palladium catalysts can help to promote complete combustion, but high-temperature operating conditions also promote aggregation of catalyst particles (“sintering”) that lowers their surface area and overall activity. Cargnello et al. (p. 713; see the Perspective by Farrauto) report that cerium oxide–coated Pd catalyst particles could be fully dispersed on an alumina surface prepared with a hydrophobic coating. This treatment resisted Pd sintering up to temperatures of 800°C, and also enabled complete combustion of methane to occur at temperatures as low as 400°C. A catalyst allows complete combustion of methane, a more powerful greenhouse gas than carbon dioxide, to occur at lower temperatures. There is a critical need for improved methane-oxidation catalysts to both reduce emissions of methane, a greenhouse gas, and improve the performance of gas turbines. However, materials that are currently available either have low activity below 400°C or are unstable at higher temperatures. Here, we describe a supramolecular approach in which single units composed of a palladium (Pd) core and a ceria (CeO2) shell are preorganized in solution and then homogeneously deposited onto a modified hydrophobic alumina. Electron microscopy and other structural methods revealed that the Pd cores remained isolated even after heating the catalyst to 850°C. Enhanced metal-support interactions led to exceptionally high methane oxidation, with complete conversion below 400°C and outstanding thermal stability under demanding conditions.

Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbons

Recent developments in solid-oxide fuel cells (SOFC) that electrochemically oxidize hydrocarbon fuels to produce electrical power without first reforming them to H2 are described. First, the operating principles of SOFCs are reviewed, along with a description of state-of-the-art SOFC designs. This is followed by a discussion of the concepts and procedures used in the synthesis of direct-oxidation fuel cells with anodes based on composites of Cu, ceria, and yttria-stabilized zirconia. The discussion focuses on how heterogeneous catalysis has an important role to play in the development of SOFCs that directly oxidize hydrocarbon fuels.

Direct Oxidation of Hydrocarbons in a Solid Oxide Fuel Cell: I. Methane Oxidation

The performance of Cu cermets as anodes for the direct oxidation of in solid oxide fuel cells was examined. Mixtures of Cu and yttria‐stabilized zirconia (YSZ) were found to give similar performance to Ni‐YSZ cermets when was used as the fuel, but did not deactivate in dry . While Cu‐YSZ was essentially inert to methane, the addition of ceria to the anode gave rise to reasonable power densities and stable operation over a period of at least 3 days. Proof of direct oxidation of came from chemical analysis of the products leaving the cell. The major carbon‐containing product was , with only traces of CO observed, and there was excellent agreement between the actual cell current and that predicted by the methane conversion. These results demonstrate that direct, electrocatalytic oxidation of dry methane is possible, with reasonable performance.

Design parameters for temperature programmed desorption from porous catalysts

Abstract A model for temperature programmed desorption from porous catalysts is analyzed to determine when concentration gradients are present, when lag time due to diffusion or sample-cell detection is important, and when readsorption affects the spectrum. Dimensionless groups of catalyst parameters were determined which allow a priori calculation of which effects are important and show how to change the experiment to avoid difficulties in interpretation. Concentration gradients are shown to be due mainly to carrier-gas flow rates and may be difficult to avoid. Readsorption can easily change the desorption temperature by several hundred K and cannot be eliminated by increasing the carrier-gas flow rate. Lag times due to pressure build up in the catalyst and to sample-cell detection are shown to be particularly serious since they can affect the heat of adsorption calculated using variable heating rate methods. The analysis shows that great care should be taken in interpreting temperature programmed desorption results and, in many cases, only qualitative features can be obtained.

Direct Oxidation of Liquid Fuels in a Solid Oxide Fuel Cell

We report stable power generation. ∼0.1 W/cm 2 , form the direct electrochemical oxidation, without reforming, of toluene. n-decane, and synthetic diesel fuel at 973 K in a solid oxide fuel cell with a composite anode containing Cu and ceria. The liquid fuels were injected directly into the anode compartment and the performance for each fuel with 50% N 2 dilution was stable for a period of at least 12 h. Photographs of Cu-yttria-stabilized zirconia (YSZ) and Ni-YSZ composites exposed to 50% toluene in N 2 at 973 K for 1.5 h showed little carbon formation on the Cu-YSZ hut demonstrated that the Ni-YSZ was covered with carbon and fractured by this environment.

What do we know about the acidity of solid acids

With a proper understanding of the nature of solid acidity, zeolites and other solid acids have great potential for replacing homogeneous acids in a wide range of catalytic applications. This paper describes results from our laboratory on the characterization and description of the acid sites in high‐silica zeolites, especially H‐MFI. A crucial observation from this work is the identification of stoichiometric adsorption complexes, one molecule per framework Al, for a wide range of adsorbates, including amines, alcohols, nitriles, ketones, and thiols. Examples are given in which temperature‐programmed desorption is used to identify these complexes and characterize their initial chemistries. Calorimetric measurements on the 1:1 complexes have been used to compare the enthalpies of protonation in the zeolite to enthalpies of protonation in the gas phase and in aqueous phase and to demonstrate that a gas‐phase basis provides better predictive capabilities. The issue of carbenium‐ion stabilities is discussed, as well as the unusual catalytic properties of acid sites formed by framework substitution of Fe. The effect of sorption and cavity size on reactions is described. Finally, the problems associated with trying to define or characterize solid acids by using ammonia TPD or 13C NMR isotropic shifts of ketones without proper consideration of the complicated nature of these techniques are discussed.

Design parameters for temperature-programmed desorption from a packed bed☆

A model for temperature-programmed desorption from a packed bed of spherical particles is presented. It is shown that the conclusions and criteria derived in an earlier paper for a slab in a CSTR apply to this experimental configuration as well when the parameters are properly defined (R. J. Gorte, J. Catal. 75, 164, 1982). Dimensionless groups of easily measured catalyst parameters are presented which enable the experimenter to determine when complications due to readsorption, concentration gradients in the particles, lag times due to diffusion and hold-up in the sample cell, and gradients along the length of the catalyst bed are important. It is shown that the activation energy of the desorption process may not be the heat of adsorption when diffusion in the particles is activated, such as is the case with zeolites. Another important result is that concentration gradients along the length of the bed will be difficult to eliminate without using flow rates high enough to cause gradients in the particles. Conditions for minimizing difficulties and for determining the proper interpretation are given.