Canan Karakaya

Highly durable, coking and sulfur tolerant, fuel-flexible protonic ceramic fuel cells

Protonic ceramic fuel cells, like their higher-temperature solid-oxide fuel cell counterparts, can directly use both hydrogen and hydrocarbon fuels to produce electricity at potentially more than 50 per cent efficiency1,2. Most previous direct-hydrocarbon fuel cell research has focused on solid-oxide fuel cells based on oxygen-ion-conducting electrolytes, but carbon deposition (coking) and sulfur poisoning typically occur when such fuel cells are directly operated on hydrocarbon- and/or sulfur-containing fuels, resulting in severe performance degradation over time3–6. Despite studies suggesting good performance and anti-coking resistance in hydrocarbon-fuelled protonic ceramic fuel cells2,7,8, there have been no systematic studies of long-term durability. Here we present results from long-term testing of protonic ceramic fuel cells using a total of 11 different fuels (hydrogen, methane, domestic natural gas (with and without hydrogen sulfide), propane, n-butane, i-butane, iso-octane, methanol, ethanol and ammonia) at temperatures between 500 and 600 degrees Celsius. Several cells have been tested for over 6,000 hours, and we demonstrate excellent performance and exceptional durability (less than 1.5 per cent degradation per 1,000 hours in most cases) across all fuels without any modifications in the cell composition or architecture. Large fluctuations in temperature are tolerated, and coking is not observed even after thousands of hours of continuous operation. Finally, sulfur, a notorious poison for both low-temperature and high-temperature fuel cells, does not seem to affect the performance of protonic ceramic fuel cells when supplied at levels consistent with commercial fuels. The fuel flexibility and long-term durability demonstrated by the protonic ceramic fuel cell devices highlight the promise of this technology and its potential for commercial application.Tests on a versatile protonic ceramic fuel cell resistant to carbon deposition and sulfur poisoning show that its durability and the wide range of fuels it can accept make it suitable for use in industry in the near future.

Mass Transfer Effects in Stagnation Flows on a Porous Catalyst: Water-Gas-Shift Reaction Over Rh/Al 2 O 3

Abstract Water-gas-shift (WGS) and reverse water-gas-shift (RWGS) reactions are numerically investigated in a stagnation-flow on a porous Rh/Al2O3 catalyst. External and internal mass transfer effects are studied using three different models for the mass transport and chemical conversion inside the porous catalyst: the dusty-gas model, a set of reaction-diffusion equations, and the effectiveness factor approach. All three models are coupled with the boundary layer equations to describe the potential flow on the stagnation disc, and a multi-step surface reaction mechanism is implemented. The numerically predicted species profiles in the external boundary layer are compared with recently measured profiles. Internal mass transfer limitations are more significant than external ones in case of the 100 μm thick catalyst layer. The effects of catalyst structure (thickness, mean pore diameter, porosity, tortuosity) as well as flow rate and pressure on chemical conversion are discussed.

Chapter Two - Spatial Resolution of Species and Temperature Profiles in Catalytic Reactors: In Situ Sampling Techniques and CFD Modeling

Spatial resolution of species and temperature profiles can provide valuable information for understanding, design, and optimization of catalytic reactors. The combination of experimental investigation and CFD modeling does not only improve our knowledge but also helps to discover uncertainties and limitations of novel scientific techniques for an adequate interpretation of the observations. Two lab-scale reactor configurations with in situ capillary techniques are investigated experimentally and numerically for the resolution of spatial species and temperature profiles: the stagnation flow on a catalytically coated disc and the flow through a catalytically coated honeycomb monolith, in which CO is totally and CH4 is partially oxidized, respectively, over Rh/Al2O3 catalysts. CFD simulations reveal two significant items for the interpretation of the measured profiles: internal mass transport inside the catalyst in the stagnation flow reactor and the impact of the capillary probe in the honeycomb monolith.

Detailed Reaction Mechanisms for the Oxidative Coupling of Methane over La2O3/CeO2 Nanofiber Fabric Catalysts

We develop and validate detailed reaction mechanisms to represent the oxidative coupling of methane (OCM) over a La2O3/CeO2 nanofabric catalyst. The reaction mechanism includes 39 reversible gas‐phase reactions and 52 irreversible surface reactions between 22 gas‐phase species and 11 surface species. We use a model‐based interpretation of spatially resolved concentration and temperature profiles measured by using a laboratory‐scale packed‐bed reactor. The reaction mechanisms are validated for inlet feed compositions in the range of 7≤CH4/O2≤11. The results are supported by a reaction pathway analysis that provides insight into the relative contributions of the gas‐phase and surface reactions to form the desired C2+ and the undesired COx products. The results provide new quantitative insights into the complex nature of the OCM chemistry, which can assist practical process and reactor development.

Surface reaction kinetics of the oxidation and reforming of propane over Rh/Al2O3 catalysts

A multi‐step surface reaction mechanism for partial oxidation and steam reforming of propane over Rh/Al2O3 catalysts is presented. The mechanism is also applicable to model reactions of the subsystems H2/CO/H2O/CO2/O2/CH4. A stagnation–flow reactor with a catalytically coated disk is used to determine the surface reaction rate and spatial concentration profiles on top of the catalytic plate using a micro‐probe sampling technique. The reactor configuration facilitates one‐dimensional modeling of coupled diffusive and convective transport within the gas‐phase boundary layer coupled with detailed heterogeneous chemistry models of the zero‐dimensional surface. The reaction system is studied at varying inlet concentrations and temperatures. The established reaction kinetics are furthermore tested by simulation of autothermal reforming of propane in an annular reactor previously described by Pagani.

In Situ Formation of Metal Carbide Catalysts

Metal carbide catalysts are essential to many widely used chemical processes. Fischer‐Tropsch synthesis, methane dehydroaromatization and biomass conversion catalysts are typically prepared in situ from a metal oxide precursor with a carbon‐containing gas. The reduction process of the metal oxide affects the final catalyst, as does the carburization gas mixture and metal promoters. By looking at materials that are carburized in situ, new insights can be gained about catalyst activation, fuel processing, and deactivation stages. The main focuses of this Review are iron carbide, molybdenum carbide and nickel carbide; analyzing catalyst synthesis methods, reduction steps, in situ carburization and improvements to the native processes. By combining years of research on these catalysts, trends and similarities are observed that can be used to improve current catalytic studies.