Water-Gas Shift Activity of Platinum Promoted by Alkali Ions
ABSTRACT Alkali additives have been widely used as promoters for Pt based catalysts in industrial... more ABSTRACT Alkali additives have been widely used as promoters for Pt based catalysts in industrial catalysts such as the synthesis of ammonia[1], automotive emission control, CO hydrogenation, and recently PROX and WGSR[ 2,3]. The presence of alkali ions (Na, K) added in small amounts significantly promotes the activity of platinum for the low-temperature WGS reaction. Remarkably, low content (<1 at%) platinum on various oxide supports modified by a small amount (1 9 at%) of alkali metals shows catalytic performance comparable to Pt on ceria, the most active support reported to date[8]. The introduction of alkali metals on the Pt- free or doped support changes the surface of the supports; it provides reducible surface oxygen at low temperatures (<200oC) and active OH groups (~150oC). On the other hand, alkali dopants suppress the sintering of Pt particles and maintain most of the Pt in oxidized state. The CO-Pt bond is weaker when Pt is more electron-deficient, thus CO is more active to involve in the reaction pathway. These findings, supported by DFT calculations, and detailed characterization of the active catalysts will be presented. References [1] Y. Larichev, D. Shlyapin, P. Tsyrul'nikov, V. Bukhtiyarov, Catal. Lett. 120 (2008) 204-209. [2] H. Evin, G. Jacobs, J. Ruiz-Martinez, G. Thomas, B. Davis, Catal. Lett. 120 (2008) 166-178. [3] A. Hagemeyer, et al. ; HONDA GIKEN KOGYO KK (HOND) SYMYX TECHNOLOGIES INC. PCT, 2006 [4] D. Pierre, W. Deng, M. Flytzani-Stephanopoulos, Topics in Catalysis 46 (2007) 363-373.
Dimethyl Ether (DME) is an attractive energy source for on-board hydrogen generation as well as d... more Dimethyl Ether (DME) is an attractive energy source for on-board hydrogen generation as well as direct electro-oxidation in fuel cells. DME has physical properties similar to liquefied petroleum gas (LPG) and unlike methanol, it is non-toxic. Furthermore, DME is the simplest ether molecule, making it an interesting molecule for selectivity studies. Here we present a first principles, density functional theory, analysis of the thermodynamics of DME decomposition over transition metal catalysts (Re, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au). The most stable binding configurations and energies are determined for decomposition intermediates. From this, the most thermodynamically favorable decomposition pathway is determined for each of the transition metals studied. The electro-oxidation of DME at a polymer electrolyte membrane (PEM) fuel cell anode is explored using a simple electrochemical model [1, 2]. From the model we are able to determine efficient catalysts for fuel cell applic...
Methanol Electro-Oxidation: A First Principles Study
Methanol has been viewed as a possible alternative fuel source for low-temperature fuel cell appl... more Methanol has been viewed as a possible alternative fuel source for low-temperature fuel cell applications. In direct methanol fuel cells (DMFCs), methanol is oxidized at the anode of a polymer-electrolyte-membrane (PEM) fuel cell to form CO2, protons and electrons. These fuel cells have many of the same advantages as the traditional H2-PEM, fuel cells, including high power density and theoretical efficiency. Furthermore, the storage issues limiting wide use of H2 fuel cells can be avoided. However, still, the economic costs of Pt-based catalysts are prohibitive to commercial viability, necessitating the development of novel anode materials. In order to engineer new materials for this reaction, it is essential to understand the fundamental challenges in the electro-oxidation reaction mechanism. Using first-principles, density functional theory (DFT) calculations, and a simplified model of the electro-chemical environment,1 we have investigated the mechanism for methanol electro-oxida...
Recently, there has been significant interest in finding alternative feeds to hydrogen for fuel c... more Recently, there has been significant interest in finding alternative feeds to hydrogen for fuel cell applications. Direct methanol fuel cells (DMFCs), while addressing the transportation and storage issues of hydrogen suffers from CO poisoning and difficulties in on-board carbon capture. Ammonia presents a promising alternative feedstock because it is easily liquefied and its products (nitrogen and water) are environmentally benign. Here we present a first principles, density functional theory, analysis of electrochemical oxidation of ammonia. Using a simple electrochemical model [1, 2], the reaction is studied as a function of temperature, applied potential, and pH. Our study first focuses on the transition metals: Ta, Mo, W, Re, Os, Ru, Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au allowing us to predict the onset potential and activity. From these results we determine optimal binding characteristics for a catalyst, which are used to screen for new materials. Promising candidates are...
ABSTRACT Using the binding energy of OH* and CO* on close-packed surfaces as reactivity descripto... more ABSTRACT Using the binding energy of OH* and CO* on close-packed surfaces as reactivity descriptors, we screen bulk and surface alloy catalysts for methanol electro-oxidation activity. Using these two descriptors, we illustrate that a good methanol electro-oxidation catalyst must have three key properties: (1) the ability to activate methanol, (2) the ability to activate water, and (3) the ability to react off surface intermediates (such as CO* and OH*). Based on this analysis, an alloy catalyst made up of Cu and Pt should have a synergistic effect facilitating the activity towards methanol electro-oxidation. Using these two reactivity descriptors, a surface PtCu3 alloy is proposed to have the best catalytic properties of the Pt–Cu model catalysts tested, similar to those of a Pt–Ru bulk alloy. To validate the model, experiments on a Pt(111) surface modified with different amounts of Cu adatoms are performed. Adding Cu to a Pt(111) surface increases the methanol oxidation current by more than a factor of three, supporting our theoretical predictions for improved electrocatalysts.
Electrocatalytic Oxidation of Ammonia on Transition-Metal Surfaces: A First-Principles Study
ABSTRACT We investigate the catalytic electro-oxidation of ammonia on model close-packed surfaces... more ABSTRACT We investigate the catalytic electro-oxidation of ammonia on model close-packed surfaces of Au, Ag, Cu, Pd, Pt, Ni, Ir, Co, Rh, Ru, Os, and Re to derive insights for the reaction mechanism and evaluate the catalysts based on their energy efficiency and activity in the context of their application in fuel cells. Two mechanisms, which are differentiated by their N-N bond formation step, are compared: (1) a mechanism proposed by Gerischer and Mauerer, whereby the N-N bond formation occurs between hydrogenated NHx adsorbed species, and (2) a mechanism in which N-N bond formation occurs between N adatoms. The results of our study show that the mechanism proposed by Gerischer and Mauerer is kinetically preferred and that the formation of N adatoms poisons the surface of the catalyst. On the basis of a simple Sabatier analysis, we predict that Pt is the most active monometallic catalyst followed by Ir and Cu, whereas all other metal surfaces studied here have significantly lower activity. We conclude by outlining some design principles for bimetallic alloy catalysts for NH3 electro-oxidation.
Dimethyl ether is an attractive alternative to petroleum fuels due to its physical properties, co... more Dimethyl ether is an attractive alternative to petroleum fuels due to its physical properties, comparable energy density to methanol and ethanol, and minimal deleterious environmental/toxicological effects. For direct fuel cells, it has a number of advantages over other prominent fuels, including easier storage with respect to hydrogen, lower toxicity and crossover when compared to methanol, and more facile complete oxidation as compared to ethanol (which includes a relatively difficult to break C−C bond). However, the dimethyl ether electro-oxidation reaction is poorly understood, hindering the development of improved electrocatalysts. Using periodic, self-consistent (PW91-GGA) density functional theory calculations, we evaluate the thermochemistry of dimethyl ether (DME) electro-oxidation, at the elementary step level, on 12 model, closed-packed facets of pure transition metals: Au, Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co, Os, Ru, and Re. From the calculated thermochemistry, we determine the most probable reaction paths on each of these surfaces, focusing on Pt as a model system. Our results predict two key electrooxidation peaks. At lower potentials, there is a peak corresponding to partial oxidation of DME to CO (and other surface poisoning species) or complete oxidation to CO 2 via formic acid as a key intermediate. A second, higher-potential peak is due to complete oxidation of adsorbed CO (and other surface poisoning species) to CO 2 . Assuming the catalysts remain in their metallic state during the DME electro-oxidation process, our results suggest that the onset potential of the surfaces increases in the order Cu < Ni < Os < Rh < Ir < Co < Ru < Pt < Ag < Pd < Re < Au. Using our results, we construct a theoretical phase diagram showing predicted catalyst activity based on two key reactivity descriptors, the free energies of adsorbed CO and OH. We compare all results to methanol electro-oxidation to understand key mechanistic differences and their impacts on optimal catalyst design for direct DME fuel cells.
Ethanol is an important industrial chemical, both as a fuel and as a feedstock for other chemical... more Ethanol is an important industrial chemical, both as a fuel and as a feedstock for other chemicals. Additionally, it can be used as a model molecule for selectivity of C-O and C-C bond cleavage, as it is the simplest molecule containing both types of bonds. The relative ease of cleavage of these two bonds determines the overall reaction product distribution. Previous work in our group has studied this reaction on the Pt (111) surface using density functional theory (DFT) [1]. There they found a Brønsted-Evans-Polanyi (BEP) relationship between the final state and the transition state for C-O and C-C cleavage. A second correlation (called the 'scaling relation) has also been recently developed by Abild-Pedersen, et al.
Catalytic Ethanol Decomposition: A Case Study in Reducing Brute-Force DFT Work for Surface Reactivity by Combining BEP and Scaling Relations.’
Peter A. Ferrin 1 , Dante Simonetti 1 , Shampa Kandoi 1 , James A. Dumesic 1 , Jens K. Norskov 2 ... more Peter A. Ferrin 1 , Dante Simonetti 1 , Shampa Kandoi 1 , James A. Dumesic 1 , Jens K. Norskov 2 , and Manos Mavrikakis 1 . (1 ... In this study, we combine first-principles methods with Bronsted-Evans-Polanyi correlations 1-3 and the approach formulated by Abild-Pedersen, et ...
Periodic, self-consistent DFT-GGA(PW91) calculations are used to study the interaction of hydroge... more Periodic, self-consistent DFT-GGA(PW91) calculations are used to study the interaction of hydrogen with different facets of seventeen transition metals-the and facets of face-centered cubic (fcc) metals, the (0001) facet of hexagonal-close packed (hcp) metals, and the and facets of body-centered cubic (bcc) metals. Calculated geometries and binding energies for surface and subsurface hydrogen are reported and are, in general, in good agreement with both previous modeling studies and experimental data. There are significant differences between the binding on the close-packed and more open (100) facets of the same metal. Geometries of subsurface hydrogen on different facets of the same metal are generally similar; however, binding energies of hydrogen in the subsurface of the different facets studied showed significant variation. Formation of surface hydrogen is exothermic with respect to gas-phase H 2 on all metals studied with the exception of Ag and Au. For each metal studied, hydrogen in its preferred subsurface state is always less stable than its preferred surface state. The magnitude of the activation energy for hydrogen diffusion from the surface layer into the first subsurface layer is dominated by the difference in the thermodynamic stability of these two states. Diffusion from the first subsurface layer to one layer further into the bulk does not generally have a large thermodynamic barrier but still has a moderate kinetic barrier. Despite the proximity to the metal surface, the activation energy for hydrogen diffusion from the first to the second subsurface layer is generally similar to experimentally-determined activation energies for bulk diffusion found in the literature. There are also some significant differences in the activation energy for hydrogen diffusion into the bulk through different facets of the same metal.
ABSTRACT Using the binding energy of OH* and CO* on close-packed surfaces as reactivity descripto... more ABSTRACT Using the binding energy of OH* and CO* on close-packed surfaces as reactivity descriptors, we screen bulk and surface alloy catalysts for methanol electro-oxidation activity. Using these two descriptors, we illustrate that a good methanol electro-oxidation catalyst must have three key properties: (1) the ability to activate methanol, (2) the ability to activate water, and (3) the ability to react off surface intermediates (such as CO* and OH*). Based on this analysis, an alloy catalyst made up of Cu and Pt should have a synergistic effect facilitating the activity towards methanol electro-oxidation. Using these two reactivity descriptors, a surface PtCu3 alloy is proposed to have the best catalytic properties of the Pt–Cu model catalysts tested, similar to those of a Pt–Ru bulk alloy. To validate the model, experiments on a Pt(111) surface modified with different amounts of Cu adatoms are performed. Adding Cu to a Pt(111) surface increases the methanol oxidation current by more than a factor of three, supporting our theoretical predictions for improved electrocatalysts.
Structure Sensitivity of Methanol Electrooxidation on Transition Metals
Journal of The American Chemical Society, 2009
We have investigated the structure sensitivity of methanol electrooxidation on eight transition m... more We have investigated the structure sensitivity of methanol electrooxidation on eight transition metals (Au, Ag, Cu, Pt, Pd, Ir, Rh, and Ni) using periodic, self-consistent density functional theory (DFT-GGA). Using the adsorption energies of 16 intermediates on two different facets of these eight face-centered-cubic transition metals, combined with a simple electrochemical model, we address the differences in the reaction mechanism between the (111) and (100) facets of these metals. We investigate two separate mechanisms for methanol electrooxidation: one going through a CO* intermediate (the indirect pathway) and another that oxidizes methanol directly to CO(2) without CO* as an intermediate (the direct pathway). A comparison of our results for the (111) and (100) surfaces explains the origin of methanol electrooxidation&amp;amp;amp;amp;amp;amp;#39;s experimentally-established structure sensitivity on Pt surfaces. For most metals studied, on both the (111) and (100) facets, we predict that the indirect mechanism has a higher onset potential than the direct mechanism. Ni(111), Au(100), and Au(111) are the cases where the direct and indirect mechanisms have the same onset potential. For the direct mechanism, Rh, Ir, and Ni show a lower onset potential on the (111) facet, whereas Pt, Cu, Ag, and Au possess lower onset potential on the (100) facet. Pd(100) and Pd(111) have the same onset potential for the direct mechanism. These results can be rationalized by the stronger binding energy of adsorbates on the (100) facet versus the (111) facet. Using linear scaling relations, we establish reactivity descriptors for the (100) surface similar to those recently developed for the (111) surface; the free energies of adsorbed CO* and OH* can describe methanol electrooxidation trends on various metal surfaces reasonably well.
We report that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbe... more We report that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H2O + CO → H2 + CO2) used for producing H2. The alkali ion-associated surface OH groups are activated by CO at low temperatures (~100°C) in the presence of atomically dispersed platinum. Both experimental evidence and density functional theory calculations suggest that a partially oxidized Pt-alkali-Ox(OH)y species is the active site for the low-temperature Pt-catalyzed WGS reaction. These findings are useful for the design of highly active and stable WGS catalysts that contain only trace amounts of a precious metal without the need for a reducible oxide support such as ceria.
Modeling Ethanol Decomposition on Transition Metals: A Combined Application of Scaling and Brønsted-Evans-Polanyi Relations
Journal of The American Chemical Society, 2009
Applying density functional theory (DFT) calculations to the rational design of catalysts for com... more Applying density functional theory (DFT) calculations to the rational design of catalysts for complex reaction networks has been an ongoing challenge, primarily because of the high computational cost of these calculations. Certain correlations can be used to reduce the number and complexity of DFT calculations necessary to describe trends in activity and selectivity across metal and alloy surfaces, thus extending the reach of DFT to more complex systems. In this work, the well-known family of Brønsted-Evans-Polanyi (BEP) correlations, connecting minima with maxima in the potential energy surface of elementary steps, in tandem with a scaling relation, connecting binding energies of complex adsorbates with those of simpler ones (e.g., C, O), is used to develop a potential-energy surface for ethanol decomposition on 10 transition metal surfaces. Using a simple kinetic model, the selectivity and activity on a subset of these surfaces are calculated. Experiments on supported catalysts verify that this simple model is reasonably accurate in describing reactivity trends across metals, suggesting that the combination of BEP and scaling relations may substantially reduce the cost of DFT calculations required for identifying reactivity descriptors of more complex reactions.
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Papers by Peter Ferrin