Hello, A change in allocation has been requested: Requester: rmeyer (Randall Meyer) Project: intermetallic Title: Fundamental studies of disparate metals in p/d alloys Description: project has moved in two directions simultaneously over the last year. First, we have finally completed a study of water gas shift catalysis over PdZn(111) and NiZn(111). NiZn and PdZn were chosen for examination as candidates for replacement of copper for low temperature water gas shift as their density of states profiles mirror that of copper. At least three distinct catalytic mechanisms are possible: 1) a redox mechanism whereby CO reacts with O on the surface or 2) a mechanism by which formate (HCOO) produced from CO and OH decomposes or 3) a mechanism which involves a carboxyl (OCOH) intermediate formed from hydroxyl and CO. Rodriguez and Liu had previously found that while the reaction went through carboxyl on Au, the reaction was a redox reaction on Cu. In both cases, the formation of formate was thermodynamically favorable, but formate is far too stable dehydrogenate to release CO2. The redox and carboxyl mechanisms can be compared Redox CO + * ↔ CO* H2O + * ↔ H2O* H2O* + * ↔ OH* + H* OH* ↔ O* + H* OH* + OH* ↔ H2O* + O* CO* + O* ↔ CO2* + * CO2* ↔ CO2 + * H* + H* ↔ H2 + 2* Carboxyl CO + * ↔ CO* H2O + * ↔ H2O* H2O* + * ↔ OH* + H* OH* + CO* ↔ COOH* COOH* + OH* ↔ H2O* + CO2* COOH* ↔ CO2* + H* CO2* ↔ CO2 + * H* + H* ↔ H2 + 2* The first three steps and last two steps are the same in both mechanisms. Previously we have shown that a competition exists between CO oxidation and water activation as the rate limiting step regardless of the mechanism11. On both surfaces, the carboxyl mechanism is preferred over the redox mechanism. However, the rate limiting step in the carboxyl mechanism varies between the two surfaces. Over NiZn(111), the rate limiting step is the reaction between CO and OH to form carboxyl (COOH) whereas over PdZn(111) the dissociation of COOH to CO2 and H has the highest barrier. In both cases, the highest barrier was about 1.00 eV so the overall rate of the reaction may be similar. Over Cu(111), Mavrikakis and co-workers has determined that H2O dissociation is the slow step with a barrier of 1.15 eV12 whereas the barriers for water dissociation over PdZn(111) and NiZn(111) are 0.95 and 0.82 eV respectively. It is interesting to note that the reaction enthalpy over Cu(111) is 0.37 eV whereas it reduces to 0.20 eV over PdZn(111) and is actually exothermic over NiZn(111) (ΔE=−0.26 eV). This work will be submitted for publication in the coming weeks. Second we have performed PDOS calculations on Pt clusters in an effort to understand how the PDOS of Pt is related to the XANES spectra. We have examined both size effects as well as the effect of simple adsorbates such as CO and H2. Our calculations indicate Recent work from Schweitzer et al. suggests that the position of the edge is directly related to the density of states at the Fermi level (and not due to the charge transfer effects). For CO, the d-electron density is significantly depleted and the center of the d-band is shifted away from the Fermi level from −2.61 eV for Pt(111) to −4.17 eV for CO/Pt(111). We hypothesize that the shift in the edge is related to the depletion of states at the Fermi level and that the increase in edge intensity is related to the presence of empty (presumably anti-bonding) states above the Fermi level that are created from hybridization of d-states of the metal with adsorbate states as proposed by Hoffmann. Conversely, in the case of OH, antibonding states are partially filled leading to an increase in electron density near the edge and a shift to lower energy in the simulated XANES. In addition to adsorbate effects, we have also probed size effects. Experimentally we observe a decrease in intensity at the edge as the particle size falls below 3 nm but an increase in intensity slightly beyond the edge. In our preliminary investigation, we have found that simulations of the XANES for a Pt6 cluster as a model of an extremely small cluster, reasonable agreement exists with experiment. However, two competing effects emerge. First, as the particle size shrinks, the average metal-metal coordination number shrinks. This leads to a localization of the DOS and increases the DOS near the edge. However, metal particles are also known to undergo lattice contraction as the particle size shrinks to the nanometer scale. The lattice contraction has been linked to changes in the hybridization of metal bonds as the particle seeks to minimize its undercoordination. Compressing the Pt lattice, moves the d-band center away from the Fermi edge and results in an increase in sta tes 2 eV above the edge as the d-band is smeared over a larger energy range. In our simulations, we find that the loss of intensity at the edge is related to undercoordination while the increase in intensity beyond the edge is related to the diffuse states above the edge arising from lattice contraction. Calculations will continue on two fronts. First, we will move our studies of intermetallics to acrolein hydrogenation. . In this case, four possible intermediates have been suggested which give rise to the two products. Hydroxyallyl and allyloxy could be formed if the aldehyde functionality is hydrogenated as intermediates to the unsaturated alcohol. However, if the double bond is hydrogenated, either 1-formylethyl or 2-formylethyl will serve as an intermediate for hydrogenation to the saturated aldehyde. Density functional theory (DFT) calculations from Rosch and co-workers suggest that over silver surfaces, 1-formyl ethyl is the favored intermediate to the dominant (but undesired) product. We will compare our results over Pd alloys with Pd and Ag monometallic surfaces. Second, we will continue our examination of XANES of supported clusters, focusing on PtMo alloys for which interesting results of WGS exist. Current: undetermined amount Justification: Requested: 200000 A specific reason has been given: I had been given 120,000 hours for the past six months. We have used this time plus a bit more during that allocation period. Therefore, I would like to request an additional 80,000 on top of the 120,000 that I hope to receive for the second half of 2011. This needs to be approved and the final allocation amount decided upon. Thank You, The LCRC Accounts System