Nanoscale Carbide and Nitride Catalysts
Since the discovery by Boudart and coworkers that early transition metal carbides and nitrides can be produced with surface areas in excess of 100 m2/g (domains <10 nm), there has been substantial interest in their use as catalysts. These nanocrystalline materials possess catalytic properties that resemble those of the Pt-group metals, and can be resistant to poisoning by sulfur, a ubiquitous contaminant in many petroleum and chemical process streams. The goals of our projects in this area are to: (1) develop a better fundamental understanding of the surface and catalytic properties of high surface area carbides and nitrides including their acid/base properties and interactions with sulfur, and (2) identify new catalyst-reaction combinations using combinatorial, high through-put screening and computational methods.
Interactions with Sulfur
Early transition metal carbides and nitrides have attracted considerable interest as potential catalysts and electrocatalysts. These materials possess catalytic properties that resemble those of the Pt-group metals and are among the most active catalysts known for the hydrodenitrogenation (HDN), and hydrodesulfurization (HDS) reactions. However, unlike many of the Pt-group metals, a number of early transition metal carbide catalysts have been reported to be tolerant towards sulfur. The nature of interactions between surfur and these materials is still largely ill-defined. Our initial studies to characterize these interactions involve the adsorption of liquid organo-sulfur compounds.
The interstitial element had a significant effect on performance with Mo2C exhibiting higher adsorption than Mo2N. On the other hand, metal nitrides showed a greater affinity for benzothiophene the adsorption compared to thiophene. Contrary to expectations, adsorption capacity decreased with increasing surface area.
Basic Character of Nitrides
Other current studies focus on assessment of the basic characteristics of early transition metal nitrides. Solid base catalysts are an attractive alternative to the use of liquid bases for various catalytic processes. A major incentive for designing solid-base catalysts is to replace corrosive, homogeneous processes with environmentally friendly and economically cheaper heterogeneous processes. Weak base sites on both Mo2N and W2N have been identified and characterized using a combination of CO2 temperature programmed desorption (TPD) experiments and the thermal decomposition of 2-methyl-3-butyn-2-ol (MBOH) under various conditions. Other reactions used to probe the basic character of these materials were the dehydration of iso-bunatol to 1 and 2-butenes and the isomerization of 2,3-dimethylbut-1-ene to 2,3-dimethylbut-2-ene. The base site density was observed to be a function of reduction temperature and time. For both these catalysts, a reduction temperature of ~ 550oC was found to be optimum for the generation of base sites. In the decomposition of MBOH using Mo2N, the turnover frequencies for acetone formation were compared with those for known bases such as ZnO and MgO. Weak base site density correlates well to the MBOH decomposition rate suggesting that these sites are responsible for the catalysis. In addition, a drastic decrease in activity was observed upon the introduction of CO2, however the activity was restored when CO2 was removed. Currently, the characterization of strong base sites on nitride catalysts as well as the acid-base properties of oxynitrides and bimetallic nitrides are underway.
High Throughput Screening
Synthesis and screening of carbide/nitride materials is time consuming. To address this, we have designed and fabricated a combinatorial system allowing the simultaneous synthesis of up to 20 catalysts and their subsequent screening in a single run. The conventional approach of catalyst synthesis and testing involves passivation and reduction steps which alter the surface properties of the carbide/nitride and thus may not reflect the intrinsic activity of the material when tested. As synthesis and testing in the new reactor does not require passivation or reduction steps, it provides a unique opportunity for in-situ measurement of native catalytic activities. To validate the performance of the combinatorial synthesis system, all 20 quartz reactors were used to synthesize Mo2C. The passivated materials were characterized with respect to their XRD patterns. All 20 samples had similar XRD patterns and matched that of standard Mo2C synthesized in a conventional reactor. These results were highly reproducible and BET surface areas of combinatorially synthesized materials were comparable to that of conventionally synthesized Mo2C.
Water Gas Shift and Steam Reforming Catalysts
The water gas shift and steam reforming reactions are important steps in the conversion of hydrocarbons into H2 for chemicals and petroleum processing. These reactions are also key processes in the production of H2-rich gas for fuel cells. Presently available catalysts are not sufficiently active or durable for portable and vehicle applications. We are developing catalysts based on nanostructured gold and carbide supports. These materials have demonstrated activities that are competitive with those of commercial Cu-Zn shift catalysts. The carbide supported materials are also stable during thermal cycling. The focus of our work is to identify strategies to improve activities for the carbides and stabilize the oxide supported gold catalysts.
Water Gas Shift
We are developing catalysts based on high surface area Mo carbides and oxide supported gold. These materials have demonstrated activities that are competitive with those of commercial Cu-Zn shift catalysts. The carbides were also stable during thermal cycling, however, activities and surface areas for the Fe-oxide supported gold catalysts decreased substantially with use.
The focus of our work is to optimize activities for the carbides and stabilize the oxide support gold catalysts. With improvements these materials could be used as intermediate and low temperature shift catalysts.
Methanol Reforming
Methanol reforming is gaining interest for H2 generation to power portable electronic devices like cellular phones, laptop computers, and personalized digital assistants. Copper based catalysts that are typically used for this reaction have several drawbacks, including, precise pretreatment control, deactivation at high temperatures, and loss of activity when exposed to condensed water.
We are developing noble and base metal supported carbides/nitrides as well as oxide supported noble metal catalysts. High throughput screening of the Mo2C supported metal catalysts helped us to identify promising candidates that exhibit both high activity and CO2 selectivity comparable to that of a commercial Cu/Zn/Al catalyst. Our results showed that the noble metal impregnated catalysts yielded higher H2 production rates compared to the base metal catalysts. In contrast, the base metal catalysts exhibited better CO2 selectivity. Our ultimate goal is to develop and characterize highly active and selective carbide/nitride supported catalysts.
We are also developing oxide supported noble metal based catalysts for both steam (SR) and autothermal reforming (ATR) of methanol. It is well-known that for portable electronic applications, low temperature performance is extremely important. A commercial Cu/Zn/Al catalyst could not light-off ATR below 200oC while some of the noble metal catalysts were active at temperatures as low as 110oC. The Cu based catalysts, however, were very active at higher temperatures (> 200oC). The focus of this work is to develop novel catalysts that can light off the reaction at low temperatures and are more active and selective than the currently existing Cu based formulations. Ultimately, we will also try to define the overall ATR reaction pathway.
Hydrogen Production from Water
The efficient and cost-effective production of hydrogen is one of the key challenges to the commercialization of fuel cells. In this research we are developing materials for phototelectrochemical and thermochemical systems. Photoelectrochemical systems are essentially a combination of photovoltaic (solar cells) and electrolysis components. Our research in this area presently focuses on the fabrication of GaInNAs for high-efficiency multi-junction photovoltaic cells and nanostructured supported gold electrocatalysts for water electrolysis such as TiO2 nanotubes.
Thermochemical processes use heat from high temperature sources including high temperature-gas cooled nuclear reactors to produce hydrogen and oxygen from water. One of the most efficient processes involves the sulfur-iodine thermochemical cycle. For this thermochemical cycle the decomposition of sulfuric acid is one of the least efficient steps. We are developing durable catalytic materials that will selectively decompose sulfuric acid into SO2, the desired product.
Microsystem-Based Fuel Processors for Fuel Cells
Microreactors offer opportunities to carry out reactions under conditions where transport limitations are minimized, and achieve significant process intensification. Our current research in this area focuses on the development of microreactor systems for hydrogen generation from hydrocarbons and alcohols, and reactions involving free radicals including NOx reduction. The development of small and inexpensive systems to produce H2 will address a key challenge to the commercialization of hydrogen fuel cells. These systems will be based on microchannel reactors, new high activity catalysts, novel sulfur adsorbents and microcombustors being developed by a collaborative team at the University of Michigan.
Fuel Processor
Proton exchange membrane (PEM) fuel cells operating with H2 from hydrocarbon liquids have emerged as leading candidates to replace batteries in portable electronic devices and power cleaner, more fuel efficient vehicles. A key challenge to their commercialization is the lack of sufficiently small and inexpensive fuel processors to convert hydrocarbons like methanol, gasoline and diesel into H2. Improvements in the performance and cost will require the development of innovative reactor designs, better performing catalysts, and better integration. The goal of this collaborative project is to develop low-cost fuel processors based on microchannel reactors, novel high activity catalysts, novel sulfur adsorbents and microcombustors. The microchannel system will result in significant improvements in the system efficiency and reductions in the fuel processor size. Methanol, gasoline and natural gas fuel processors will be demonstrated ranging in size from 10W to 10kW.
Micro-Fuel Cells
The deployment of wireless microelectromechanical systems (MEMS) will depend on the availability of micro-power supplies. Fuel cells are excellent candidates for use as micro-power supplies, combining energy densities that are higher and recharge times that are faster than batteries. Our goal is to develop highly efficient, mW-sized hydrogen PEM fuel cells and demonstrate their use in powering a MEMS device. These micro-fuel cells will be fabricated using microfabrication techniques that are similar to those used to manufacture microelectronic devices. The micro-fuel cells will incorporate high activity electrocatalysts and will be integrated into a system that includes a microheater with temperature sensors, and a fuel storage and delivery system.
