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Projects

Methanol Chemistry on Transition Metal and Alloy Surfaces:
Optimizing the Anode of Fuel Cell Catalysts
 

Global demand for methanol in 1999 was nearly nine billion gallons. Among other applications, methanol is used to produce formaldehyde, MTBE, and acetic acid.  A small - but steadily increasing - niche for this chemical is in fuel supply to steam reformers and to direct methanol fuel cells (DMFC's).  DMFC's hold out the promise of direct conversion of methanol to electricity, a process that can be as much as four times more efficient than combustion in internal combustion engines.  Before DMFC's will be commercially viable, however, a number of technological problems must be solved.  Chief among these is the need to develop an electrooxidation catalyst that will both produce a high power density and be impervious to CO poisoning over time.  Platinum and platinum-ruthenium catalysts have been found to be effective in this chemistry, but improved catalysts are still needed.  Theoretical tools can assist in this process both by providing fundamental understanding of methanol behavior on currently existing catalysts and by suggesting new alloys that may lead to improved catalyst performance. 

We are currently performing a systematic study of methanol partial oxidation on Pt(111).  We have fully analyzed the methanol decomposition pathway that proceeds through hydroxyl O-H scission. To see a  movie of the proposed rate-limiting step in this process (O-H bond scission in methanol to produce the methoxy radical), together with the ensuing step in the reaction pathway (C-H scission of methoxy), click here.   

We are in the process of evaluating an alternative pathway for methanol decomposition on Pt(111), the scission of a methanol C-H bond to produce atomic hydrogen and hydroxymethylene.  Preliminary results indicate that this pathway could be energetically competitive with O-H bond scission. 

While methanol usage in fuel cells is our primary area of study, we are also interested in the methanol synthesis process.  Industrially, methanol is often produced over Cu/ZnO/alumina.  Although this process is quite valuable, much debate remains over both the active phase of the catalyst and the exact mechanism of methanol synthesis on these catalysts.  We have studied the partial oxidation of methanol (the microscopic reverse process of methanol synthesis from CO) on Cu(111) in an effort to gain fundamental insight into this reaction. To see a movie of the proposed rate-limiting step (C-H bond scission in the methoxy radical), click here. 

 

For more information on this project, please refer to the following publications: 

A First-Principles Study of Methanol Decomposition on Pt(111), J. Greeley, M. Mavrikakis, Journal of the American Chemical Society 124, 7193 (2002) [DOI]

Methanol Decomposition on Cu(111): A DFT Study, J. Greeley, M. Mavrikakis, Journal of Catalysis, 208, 291 (2002) [DOI]

Competitive Paths for Methanol Decomposition on Pt(111), J. Greeley, M. Mavrikakis, Journal of the American Chemical Society 126, 3910 (2004). [DOI]

Prediction of experimental methanol decomposition rates on platinum from first principles, S. Kandoi, J. Greeley, M. A. Sanchez-Castillo, S. T. Evans, A. A. Gokhale, J. A. Dumesic, M. Mavrikakis Topics in Catalysis, 37(1), 17-28 (2006). [DOI]

Reactivity descriptors for direct methanol fuel cell anode catalysts, with P. Ferrin, A. U. Nilekar, J. Greeley, M. Mavrikakis, J. Rossmeisl, Surface Science, 602, 3424 (2008). [DOI]

Structure sensitivity of methanol electrooxidation on transition metals, P. Ferrin, M. Mavrikakis, Journal of the American Chemical Society, 131, 14381 (2009). [DOI]

 

Jeff Greeley 
Research Assistant 
Department of Chemical Engineering 
University of Wisconsin at Madison 

 

Ab-Initio Study of Methanol Synthesis from CO2 Hydrogenation on Cu(111)

Methanol is a very import chemical compound that is used in the synthesis of other chemicals, such as Formaldehyde, Formic Acid, and MTBE. Although the use of MTBE fades out in the near future there are other applications emerging. Direct Methanol Fuel Cells (DMFC) are an interesting alternative to H2­ powered fuel cells and may become a major energy source.

Currently, Methanol is industrially synthesized from CO/CO2/H2 mixtures (syngas) at high pressures (40-120 bar) on Cu/ZnO/Al­2O3 catalysts and temperatures between 200-300 °C. It has been proposed that the main source of carbon in methanol is CO2 and not CO. However, copper is also a very good catalyst for the water gas shift reaction, which is able to oxidize CO with H2O to CO2. The goal of this project is to determine a feasible reaction mechanism that explains the formation of methanol from CO2. Therefore we use Density Functional Theory (DFT) and investigate the thermodynamic and kinetic properties of intermediate elementary steps on the surface. From this data we can make conclusions about the stability of reaction intermediates and kinetic limitations of possible reaction pathways.

 

Lars Grabow 
Research Assistant
Department of Chemical and Biological Engineering 
University of Wisconsin-Madison 

 

Adsorption and Dissociation of Dioxygen on Transition Metal and Alloy Surfaces: Optimizing the Cathode of Fuel Cell Catalysts  

Oxidation processes catalyzed heterogeneously by transition metals have tremendous importance in the chemical, energy, pharmaceutical, and microelectronics industries. Examples range from the production of such important chemicals as methanol and ethylene epoxide to the removal of volatile organics in industrial emission. Oxygen is usually the preferred oxidant for its availability (air) and environmental friendliness. Therefore, an understanding of the surface behavior and surface chemistry of O2 is essential to improving existing processes and designing more effective catalysts. Furthermore, fuel cell technologies could be a major beneficiary. The reduction of oxygen is the primary reaction that takes place on the cathode of many types of fuel cells, and the activation of O2 is most likely the Rate Limiting Step for the entire fuel cell operation. 

Of the many reaction possibilities open to the O2 molecule, one that happens perhaps most frequently is its dissociation into two oxygen atoms - which are much more reactive than the molecule itself. The dissociation of the oxygen molecule on transition metal surfaces is therefore of considerable research and practical interest. The dissociation of O2 is known to take place via different mechanisms on different metal surfaces. On some surfaces the oxygen molecule directly dissociates upon adsorption. On others, the molecule adsorbs initially intact into a precursor state. Subsequent thermal equilibration with the surface determines whether the O2 precursor desorbs or dissociates. Molecularly adsorbed O2 species have indeed been detected experimentally on various transition metal surfaces at low temperature (e.g., Pt(111), Pd(111), Ni(111), Cu(111), Cu(110), Ag(110), and Ir(111)). Their presence, even though elusive under real reaction conditions, is shown to impart particular characteristics to the dissociation of O2 on these surfaces. 

We have used self-consistent, periodic DFT calculations to study O and O2 adsorption and O2 dissociation on several transition metal surfaces, including Ir(111), Cu(111) and Cu(211), and Au(111) and Au(211). Our goal is to ascertain the geometry and energetics of possible precursors on these surfaces, identify possible dissociation pathways, and determine the dissociation activation energies. We are currently studying the interaction of O and O2 with various alloys of Cu, Au, Pt and other elements. The primary emphasis will be on the optimization of the cathode catalyst for low-temperature PEM fuel cells. 

 

For more information on this project, please refer to the following publications: 

Adsorption and dissociation of O2on Pt-Co and Pt-Fe alloys, Y. Xu, A. Ruban, M. Mavrikakis, Journal of the American Chemical Society 126, 4717 (2004). [DOI]

Adsorption and Dissociation of O2 on Gold Surfaces: Effect of Steps and Strain, Y. Xu and M. Mavrikakis, Journal of Physical Chemistry B 107 (2003) 9298. [DOI]

The adsorption and dissociation of O2 molecular precursors on Cu: The effect of steps, Y. Xu and M. Mavrikakis, Surface Science, 538 (2003) 219. [DOI]

Universality in heterogeneous catalysis, J.K. Nørskov, T. Bligaard, A. Logadottir, S. Bahn, L.B. Hansen, M. Bollinger, H. Bengaard, B. Hammer, Z. Sljivancanin, M. Mavrikakis, Y. Xu, S. Dahl, and C.J.H. Jacobsen, Journal of Catalysis, 209 (2002) 275. [DOI]

Adsorption and dissociation of O2 on Ir(111), Y. Xu and M. Mavrikakis, Journal of Chemical Physics, 116 (2002) 10846. [DOI]

Adsorption and dissociation of O2 on Cu(111): Thermochemistry, reaction barrier and the effect of strain, Y. Xu and M. Mavrikakis, Surface Science, 494 (2001) 131. [DOI]

 

Ye Xu 
Research Assistant 
Department of Chemical Engineering 
University of Wisconsin - Madison 

  

Surface Chemistry of Gallium Nitride (GaN) 

Group III-nitrides possess direct band gaps ranging from 2.0-6.3eV and band gap engineering could lead to devices emitting bright light covering the entire spectrum from green to UV. However, until a decade ago p-type doping of GaN could not be achieved and efficiently working devices, which require p-n junctions, could not be produced. This doping problem was overcome so that the first highly efficient light emitting devices based on GaN could be built and are commercially available today. For example, many traffic lights use GaN.  

The next major problem in the development of high-quality GaN based devices concerns the material quality. Sapphire is the most suitable substrate on which growth of GaN can be performed. However, sapphire has a lattice mismatch of 13% with respect to GaN. Therefore, it is no surprise that attempts to grow GaN directly on sapphire resulted in a huge number of defects, which penetrate the GaN epilayer (threading defects). Various techniques exist to overcome the defects introduced by lattice mismatch. This includes lateral epitaxial overgrowth (LEO) where a masking layer is included between the seeding material and the grown films. Small windows in the masking layer allows the alignment of the grown film with the seeding layer. Dislocations are then confined to the regions above windows, which improve the quality of the film above the masked regions. Control of the lateral:vertical growth ratio is important as a reduction in the window size allows for the production of larger high-quality regions.  

Various dopants and surfactants can be introduced to alter the lateral:vertical growth ratio of LEO grown GaN. Studies using density functional theory is ideal for insight into the mechanism by which these alter the growth ratio as growth rates are directly related to the surface chemistry. In connection with  Dr T. F. Kuech's experimental efforts, we are currently investigating the effect of Sb and Bi on the adsorption and diffusuion of Ga and N on various facets of GaN, so that we can improve the control of defect-free GaN single crystals. 

 

For more information on this project, please refer to the following publications: 

Influence of Bi impurity as a surfactant during the growth of GaN by metalorganic vapor phase epitaxy, L. Zhang, H. F. Tang, J. Schieke, M. Mavrikakis, and T. F. Kuech, Journal of Crystal Growth 242, 302 (2002) [DOI] 

The addition of Sb as a surfactant to GaN growth by metal organic vapor phase epitaxy, with: L. Zhang, H. F. Tang, J. Schieke, M. Mavrikakis, and T. F. Kuech, Journal of Applied Physics 92, 2304 (2002) [DOI]

 

Jaco Schieke 
Research Assistant 
Department of Chemical Engineering 
University of Wisconsin - Madison 

 

Study of Active Sites for CO adsorption on Methanol Synthesis Catalyst 

The synthesis of methanol on Cu/ZnO catalysts is a reaction widely performed in industry, and it has been the subject of numerous research investigations over the past several decades. Research has attempted to discern both the detailed structure of the active site of the methanol synthesis catalysts and the relationship between this structure and the activity of the methanol synthesis reaction.  

Among several unsettled aspects of the methanol synthesis mechanism, the nature of the active intermediate (either CO or CO2) for hydrogenation to methanol has been heavily disputed. We are currently studying several aspects of MeOH synthesis catalyst by using Density Functional Theory (DFT) Calculations. Experimental studies have shown a downward shift of 50cm -1 of CO on the catalyst with respect to highly reduced catalyst. Our aim is to explain this downward shift of frequency and hence determine the active site for CO adsorption. 

Among a variety of models we are exploring the possibilities of strain effects, effect of Cu or and adspecies on the substrate Cu(111) surface. Similarly we are also considering the effect of Cu and Zn being alloyed together in bulk phase or on the surface alone. It is interesting to see the modification of surface properties of the surface by alloying.  Alloying also increases the number of distinct sites possible on the surface of the catalyst. For example for the Cu3Zn alloy which we are studying, there are 11 different sites where CO may be adsorbed. The geometry of Cu3Zn(111) as well as the sites at which adsorption of CO may take place may be viewed below. 

The geometry of Cu3Zn(111) (left) and the sites at which adsorption of CO may take place (right).

  

For more information on this project, please refer to the following publications: 

CO Vibrational Frequencies on Methanol Synthesis Catalysts: a DFT study, J. Greeley, A. Gokhale, J. Kreuser, H. Topsoe, N-Y. Topsoe, J. A. Dumesic, M. Mavrikakis, Journal of Catalysis 213, 63 (2003) [DOI]

 

Jeffrey Greeley and Amit Gokhale 
Research Assistants 
Department of Chemical Engineering 
University of Wisconsin - Madison 

  

Surface Chemistry of Copper Tin alloys 

Copper catalysts are widely used in industry for a variety of Chemical Reactions like Water-Gas Shift reaction, methanol synthesis and hydrogenation reactions. Studies on Pt-Sn alloys have shown a drastic change in the catalytic properties of Pt. Similar observations have also been noted when Sn was added to Ru or Rh based catalyst.  

Carbon monoxide in general does not adsorb on Sn.  Thus the rate of coking, and hence the deactivation of the catalyst, is likely to be drastically reduced by the addition of Sn to it. Sn is also likely to reduce the heat of adsorption of CO on the metal it is alloyed with. Hence an alloy of Cu and tin would seemingly work well for water gas shift reaction as compared to Cu alone. With this motive, we are currently studying the adsorption of different species like O, H, OH, N, CO and common poisons like S and C on Cu6Sn5 using periodic self-consistent Density Functional Theory (DFT) calculations.  

Cu6Sn5 itself has a NiAs type of structure. Hence a slab of CuSn may be either Cu terminated or Sn terminated. These slabs may be viewed below. 

Cu terminated slab (left) and Sn terminated slab (right). 

In collaboration with Dr J. A. Dumesic we are currently pursuing the characterization of of Cu-Sn alloy catalysts with a variety of techniques including Mössbauer and XRD. Our goal is to use experiment and theory to correlate electronic structure with alloy composition, and design the next generation of improved catalysts for a variety of important applications. 

 

For more information on this project, please refer to the following publication: 

Effect of Sn on the Reactivity of Cu Surfaces; A. A. Gokhale, G. W. Huber, J. A. Dumesic, M. Mavrikakis, Journal of Physical Chemistry B, 108, 14062 (2004) [DOI]

 

Amit Gokhale 
Research Assistant 
Department of Chemical Engineering 
University of Wisconsin - Madison 

 

Low Temperature Water Gas Shift Reaction

The water gas shift reaction (WGSR), CO + H2O  CO2 + H2 is an industrially important route to H2 production and plays an important role in many current technologies such as methanol synthesis, methanol steam reforming, ammonia synthesis, coal gasification, as well as fuel cell technology. Using quantum chemical methods such as Density Functional Theory (DFT), I have studied this reaction extensively on a series of ten late transition metals. This study has helped us to determine the characteristics that define a good WGS catalyst, and hence allowed us to screen for potentially active WGS catalysts.

Since Cu-based catalysts are used in industry for WGSR I have calculated detailed minimum energy paths for all the elementary steps on a Cu(111) surface using DFT and built a robust microkinetic model for WGSR on copper catalysts using the DFT-generated parameters. This model conclusively proves that the carboxyl mechanism is the dominant mechanism on this surface and points out that the formate species is essentially a spectator on the catalytic surface under typical WGSR conditions. 

 

Amit Gokhale 
Research Assistant 
Department of Chemical Engineering 
University of Wisconsin - Madison 

 

Low Temperature Water Gas Shift Reaction on Gold 

Water gas shift (WGS) is the reaction of water and carbon monoxide to produce hydrogen and carbon dioxide (CO + H2O   CO2 + H2), and is an important step in the production of H2 via the steam reforming of hydrocarbons. Hydrogen can be used as a fuel in fuel cells for power generation in a variety of applications and in the production of ammonia. WGS also plays a role in the overall chemistry of hydrocarbon oxidation reactions and auto exhaust combustion catalysis. 

WGS is an exothermic reaction (H = -40.6 KJ/mol). Depending on conditions, various catalysts are used to accelerate this reaction. For example, the high-temperature shift reaction is performed at 590-720 K using a catalyst based on iron-oxide while the low-temperature shift reaction is performed at 470-520K using a (Cu, Zn, Al)-based catalyst. Recent work has suggested that supported gold catalysts are very active in the WGS reaction at low temperatures; even more than the CuO/ZnO catalyst currently used industrially. Along with their promising technical performance, the relatively low and stable price of gold compared with the cost of platinum group metals adds to the incentives for exploring gold catalyst technologies. 

Various mechanisms have been proposed for this reaction, the most popular one being the redox mechanism. We are using periodic, self-consistent density functional theory (DFT) calculations to study the reaction mechanism of WGS on Au single crystal surfaces. The thermochemistry of relevant reaction intermediates and the activation barriers of the various elementary steps of the proposed alternative WGS reaction mechanisms are determined and analyzed. Furthermore, the vibrational frequencies of the reaction intermediates and of the transition states are calculated which could then be used to calculate the pre-exponential factors for each elementary step. All this first principles derived information is used to build a micro-kinetic model for bridging the pressure and temperature gap and develop improved WGS catalysts. 

Using state-of-the-art theoretical tools, we hope to gain insights with respect to several issues related to the enhanced activity of WGS reaction over Au catalysts that still remain controversial. Key mechanistic aspects of WGS are examined using appropriate experimental methods. 

 

Shampa Kandoi 
Research Assistant 
Department of Chemical and Biological Engineering
University of Wisconsin-Madison 

 

Preferential Oxidation of CO in the Presence of Hydrogen 

As the energy sector moves towards hydrogen, hydrogen fuelled proton exchange membrane (PEM) fuel cells have been recognized as a promising energy technology for replacement of internal combustion engines in automobiles and other transportation systems. However, an important problem for the usage of PEM fuel cells is their sensitivity to low levels (ppm) of CO because CO easily poisons PEM fuel cell anode. A promising method to remove trace amounts of CO from H2 supplied to the anode is preferential oxidation of CO (PROX) in the presence of excess H2 .  

An effective PROX catalyst should meet several important requirements: (i) show high activity for CO oxidation (ii) oxidize CO much easier than hydrogen (iii) function at temperatures similar to that of PEM fuel cells and (iv) be resistant towards deactivation by carbon di-oxide and water present in the feed. A good PROX catalyst should be able to selectively oxidize 10,000 ppm of CO to concentrations of less than 5 ppm at low temperatures, without decreasing the H2 content of the fuel gas. 

The PROX reaction has been studied experimentally on supported monometallic as well as bimetallic catalysts extensively. Platinum, gold and copper based catalysts, like Pt/-Al2O3 , Pt/A-zeolite, Au/-Fe2O3, Cu mixed with ceria oxide have been shown to be promising PROX catalysts. In this study we have used a combination of DFT and microkinetic modeling to explain the superior low-temperature performance of Au and Cu catalysts, as compared to Pt catalysts. We have employed periodic, self-consistent density functional theory (DFT) calculations to address competitive CO oxidation (CO + O CO2) and hydrogen oxidation (H + O  OH, H + OH   H2O, OH + OH   H2O + O) on Au(111), Cu(111) and Pt(111). The geometry and thermochemistry of various reactive intermediates are studied, as well as reaction pathways and activation energies for the corresponding elementary steps. A rigorous barrier analysis scheme is implemented for the appropriate decomposition of the activation energy barrier of each elementary step into its major components. Finally, using the DFT-derived parameters for surface reaction steps and handbook values of gas phase reaction thermochemistry, we construct a simple micro-kinetic model to predict trends in PROX selectivity on Au, Cu and Pt surfaces as a function of temperature. 

 

For more information on this project, please refer to the following publication: 

Why Au and Cu are More Selective than Pt for Preferential Oxidation of CO at low Temperature; S.Kandoi, A.A. Gokhale, L.C. Grabow, J.A. Dumesic, and M. Mavrikakis, Catalysis Letters 93, 93 (2004). [DOI]

 

Shampa Kandoi, Amit Gokhale and Lars Grabow 
Research Assistants 
Department of Chemical and Biological Engineering 
University of Wisconsin-Madison 

 

Fischer Tropsch Synthesis

Fischer Tropsch Synthesis (FTS) affords a direct route to produce clean hydrocarbon liquid fuel from syngas (a mixture of CO and H2). Traditionally coal or natural gas have been used as feedstocks but there is growing promise that biomass derived H2 could be utilized. Co and Fe are the conventional FTS catalysts but it has been recently found that Pt promoters significantly improve the performance of Fe catalysts. We seek to explain this Pt-promotion using ab initio techniques based on Density Functional Theory (DFT) and microkinetic modeling. A comprehensive model would lead to a better understanding of this important industrial process especially in the light of similar models for pure Co and Fe which have been previously developed in our group.

 

Collaborators: Prof. C. H. Bartholomew (Brigham Young University)

 

Rahul Nabar
Research Assistant 
Department of Chemical and Biological Engineering
University of Wisconsin-Madison