Hello, A yearly allocation for the LCRC cluster has been requested with the following updated information: Submitter/PI: Randall Meyer Project Name: intermetallic Division: CSE Project title: Fundamental studies of disparate metals in p/d alloys Associated funding: NSF Other Systems: NCSA 100,000 hr/yr Science: The project will use the VASP (Vienna Ab Initio Simulation Package) code to study the formation of unique phases in nanoalloys. Experimentally our group is working with Jeff Miller and Chris Marshall to synthesize alloy nanoparticles involving a d-band transition metal and a "p-electron" metal such as Ga, Ge or Sn. One interesting aspect of these alloys is that they form intermetallics as opposed to homogeneous alloys. However, it is not known how surface segregation may manifest itself in these systems. The primary aim of the proposed theoretical work is to understand the thermodynamics of nanoparticles relative to bulk phases in an effort to understand how to improve catalyst selectivity. Project description: The project has moved in two directions simultaneously over the last year. The first portion of the project focuses upon changes in the XANES spectra upon alloying. Previously we had examined the simulated XANES of PdPt alloys using CASTEP and found that changes in orbital overlap led to an broadening of the PDOS of Pd (now overbonded) and a narrowing of the PDOS of Pt (now underbonded). Alloying Pd with Ni shows the same shift in the absorption edge from that observed for PdPt although one might expect that the shifts should be different. As the shifts in PdNi mirror those of PdPt, this would suggest that the driving force must actually be different (since obviously we would initially expect 3d and 5d metals to have opposite effects on Pd). Simulated Pd/Ni PDOS for Pd and PdNi bulk shows that upon alloying of a 4d metal (Pd) with a smaller 3d metal there is a predicted narrowing of the d-band in the Ni and simultaneous broadening of the Pd in the allo y (again this mirrors the effect in PdPt although if the effect in PdNi and PdPt were the same then the PDOS should reveal opposing behavior). In this case, we attribute the observed behavior to domination by geometric (strain) effects. Ni in a PdNi alloy is effectively in expansion since the lattice constant of PdNi (3.76 Å) is larger than for plain Ni (3.52 Å). This results in the observed narrowing of Ni PDOS. Conversely, Pd is effectively in compression (lattice constant for Pd is 3.89 Å) and its d-band is expanded. Unlike the case for PdPt alloy, PdNi is dominated by strain effects and results in altering the reactivity such that Ni which is more reactive of the two metals becomes even more reactive upon alloying, and Pd the less reactive of the two metals, becomes even less reactive upon alloying. We have attempted to extend the trends for alloying of Pd with 3d metals which all possess a filled d-band (Zn, Ga and Ge). We have found that the d-band center is -2.62, -3.08 and -2.73 eV for PdZn, PdGe and PdGa, respectively. Therefore, there is an increase in d-band center relative to the Fermi level in all three cases compared to plain Pd bulk (-2.01 eV). Further observation of the simulated Pd PDOS shows a substantial depletion of d-states at the Fermi level and a contraction of the d-states for all three alloys. In addition, this trend is confirmed if we look at the Bader charge analysis of the system. The amount of charge transferred from Pd to Ga, Zn and Ge is -0.47, -0.41 and -0.29 e- respectively, showing that the magnitude correlates with the observed trend in the position of the leading edge. Furthermore, the work functions for Ga, Zn and Ge are 4.2, 4.33 and 5.0 eV, respectively. It shows that the trend correlates with the work function of the alloying el ement. Second, we have examined acrolein hydrogenation as a test reaction for an examination of alloy effects in support of experimental work with Chris Marshall and Jeff Miller in CSE. Acrolein hydrogenation can proceed via two paths as either the aldehyde functionality (forming propanal) or the double bond (forming allyl alcohol) may be hydrogenated. Furthermore there are two intermediates along each path as the first hydrogen is added in a different location than the second location (hydroxyallyl, allyloxy, 1-formylethyl and 2-formylethyl). We have examined the full reaction pathway on Ag(111) (the thermodynamically preferred surface of Ag) and compared Ag with AgIn alloy surfaces as AgIn alloys have been shown to exhibit superior selectivity for allyl alcohol. First, we find that the rate limiting step over all surfaces may not be hydrogenation of acrolein or any intermediate but rather hydrogen activation as hydrogen dissociation has a barrier exceeding 1.3 eV on all surfaces. Second, acrolein bonds very weakly to Ag surfaces and can adopt essentially any orientation. Focusing on the differences between the paths on the Ag(111) surface it can be assumed that the most favourable reaction path towards allyl alcohol is that of allyloxy as intermediate and that production of propanal follows the 1-formylethyl path. Comparing the first step of the reaction from adsorption of acrolein to the intermediate, it is clear that the presence of indium in the catalyst in the case of allyloxy does not make a big difference in the activation barrier or the reaction enthalpy. On the other hand, the barrier for hydrogenation of acrolein to 1-Formylethyl greatly increases with increasing In content. Overall the results suggest that the activity of the In containing catalysts should decrease while the selectivity increases. Calculations will continue on two fronts. First, we will continue our analysis of XANES data of alloys with CASTEP. We will now shift our focus to core-shell alloys. In collaboration with Mike Wong from Rice University, we have examined the XANES of PdAu core-shell alloys (Pd is the shell) with varying ratios of the two metals. We are now in the process of building appropriate models to explain the experimental results with the hope that we can develop general rules which can predict the XANES spectra (and reactivity) for any combination of core-shell alloys. Second, we will continue to examine acrolein hydrogenation over Ag surfaces but our focus will now move to dilute alloys using the VASP code. Recently our experiments have shown that the reactivity of Ag0.999X0.001 alloys can be significantly higher than Ag alone and can possess higher selectivity as well. We will examine unit cells with a single surface atom of Pd (or Ru, Rh etc.) as a model for these dilute alloy systems. The working hypothesis is that isolated metal atoms may help activate hydrogen (or act as anchoring points for the C=O bond in acrolein). We will perform calculations on these systems to test how non-contiguous atoms may aid in improving the catalysis. Finally we will begin our examination of neopentane hydrogenolysis/isomerization to support additional experiments in Jeff Miller’s lab. Hydrogenolysis involves a carbon cleavage and hydrogenation and results in butane and methane.Isomerization involves a ring closure-ring opening through a cyclopropyl intermediate to produce iso-pentane. The primary products would be methane and i-butane for hydrogenolysis and iso-pentane for isomerization. In both cases, further reaction steps can occur to produce secondary products, such as propane and additional methane. Isomerization of the primary product, iso-pentane, can lead to formation of n-pentane. Our preliminary experiments indicate that while Pd has a preference for the hydrogenolysis pathway (5% selectivity to n-pentane), Pt produces both n-pentane and isopentane (combined 60% selectivity). In addition, there are both size and alloy effects for this system. We will focus on understanding the reaction pathway for both hydro genolysis and isomerization on Pt(111) and Pd(111) in an effort to understand the selectivity differences. If successful, we will move to examine PdPt alloys which are even more selective than Pt surfaces. Project URL: Current FY Hours Used: undetermined amount New FY Requested allocation: 450000 Q1: 100000 Q2: 100000 Q3: 100000 Q4: 150000 Justification: Thank You, The LCRC Accounts System