Room 3016, Engineering Building
University of Wyoming
College of Engineering and Applied Science
Department of Chemical & Petroleum Engineering
1000 E. University Avenue
Laramie, WY 82071
phone: (307) 766-6772
Fax: (307) 766-2221
My research interest is nanoscale materials design and synthesis for catalytic applications. The design and synthesis is then complemented with reactivity studies and in-situ characterization of the catalytic material. In particular, X-ray absorption spectroscopy (XAS) experiments are periodically conducted at the Advanced Photon Source at Argonne National Laboratory. The combination of design, synthesis, and characterization allows for the direct translation of variations in the material at the atomic/molecular scale to expressed macroscopic properties such as improved reactivity. Our main area of current research involving nanoscale design and synthesis of catalytic materials is pseudomorphic overlayer bimetallic catalysts.
Pseudomorphic Overlayer Catalysts:
Supported metal catalysts have been the mainstay of heterogeneous catalysts for many years. These catalysts are widely used since they combine the desirable catalytic properties of the metal with the enhanced activity resulting from their support on a high surface area material. More complicated reactions often use bimetallic catalysts. The drawback of bimetallic catalysts is that they are often a simple alloy resulting from an equilibrated mixture of the two metals. In particular, the bimetallic catalysts usually have both elements randomly distributed throughout the particle which therefore contains no atomic uniformity in the arrangement of the two elements. Design of supported bimetallic catalysts by synthesizing non-equilibrated arrangements could lead to new compositions of matter and ultimately to more efficient catalysts.
The importance of materials design at the atomic level is demonstrated through the use of computational quantum-chemical techniques. It has been shown theoretically that pseudomorphic overlayers of a metal on top of a different metal may provide lower intrinsic barriers to reaction products than pure metals. Therefore, it is desirable to extend this concept from the theoretical to the practical by synthesizing actual supported metal catalysts that could have industrial applications. This is an important fundamental step in the evolution of bimetallic catalysts for increasingly rigorous catalytic applications. As an initial proof-of-principle example, we examined Pd monolayer on Re particles for the hydrogenation of ethylene. While this reaction is not of industrial importance, it is representative of the significant number of hydrogenation reactions that do occur industrially.Synthesis of metallic catalysts with pseudomorphic overlayers is being carried out using the directed synthesis technique. Using this technique, we have synthesized Pd on Re pseudomorphic overlayer catalysts. These catalysts are then characterized using hydrogen chemisorption to determine heat of adsorption and ethylene hydrogenation to determine reactivity. Finally, hydrogen heat of adsorption and hydrogenation reactivity can be correlated as shown in the figure below:
From this figure, we can see that hydrogenation activity decreases as hydrogen heat of adsorption decreases. The observed slight decrease in activity is understood by considering the reaction mechanism and the consequences of the decreased heat of adsorption for hydrogen. Because ethylene hydrogenation is first order in hydrogen and slightly negative order in ethylene (not shown), the rate depends on the hydrogen surface coverage. The observed decrease in hydrogen adsorption strength will increase the H2 desorption rate, decrease the H2 surface coverage, and decrease the overall rate as observed.
Since the hydrogen heat of adsorption can be correlated with the calculated center of the d-band for these overlayer materials, and the d-band center can also be calculated, we can now predict in advance the rate of reaction for a particular combination of materials. For example for the ethylene hydrogenation reaction, instead of Pd on Re, Au on Re is predicted to have stronger hydrogenation adsorption and thus higher reactivity. Unfortunately, Au will migrate subsurface on Re and thus this system is not realistic.
We have recently extended the application of this type of catalyst towards biofuels. We received funding from NSF to prepare Pt on Ni and Pt on Co catalysts for the aqueous phase reforming reaction of lactose to produce hydrogen. Lactose is a byproduct of cheese production and represents a significant disposal issue. Hydrogen is a valuable resource and is desired for use in fuel cells. Since the lactose is produced from cows which eat grass produced from carbon dioxide in the atmosphere, this represents a carbon neutral source of hydrogen. This work showed, that as above, the Pt on Ni or Pt on Co could decrease the H2 heat of adsorption as predicted. As anticipated, this also resulted in an increase in the turnover frequency for H2 generation.
Using Life Cycle Assessment to Guide Catalysis Research:
We have demonstrated the use of life cycle assessment (LCA) as a tool to drive catalyst development by comparing the environmental impact of acrylic acid production from propylene, the current commercial feedstock, to propane as an alternate feedstock. Acrylic acid is currently produced in a two step process from propylene. Because of its lower cost, propane is an attractive alternative to propylene; however no catalysts are currently available which can compete with the high yield of the propylene process. A comparison of the two feedstocks at the 87% yield of the current commercial propylene process demonstrated that switching to propane would decrease the environmental impact of the process by 20%. Determination of environmental impact as the yield from the potential propane process was varied, predicts that at yields exceeding 6%, the propane process will have a lower environmental impact than the current propylene process. The current catalyst yield of up to 48% for the propane process exceeds these values. If reaction and waste gas heat are converted to electricity instead of steam, yields in excess of 61% will result in a lower total impact for the propane process. Based on raw material costs, the economic break-even point for the propane process is 59% yield. The similar yields of ~60% from propane required by economics and for a lower environmental impact represents a factor of 1.25 increase in yield over the current state-of-the-art propane catalyst compared to a factor of 1.81 increase in yield required to equal the current propylene yield. Thus, the proposed propane process may be much closer to viability than previously realized. This analysis provides an example of how LCA can compare chemical production from two different feedstocks, even if a catalyst for the reaction of interest has not been designed. The LCA analysis can also be used to determine target goals for catalysis research.