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. More recently we have added a second component in an examination of single atom alloys.
Project description: The project has moved in two directions simultaneously over the last year. The project has focused on neopentane conversion as the test reaction for the past year to support experiments in Jeff Miller’s lab. The primary products would be methane and isobutane for hydrogenolysis and isopentane for isomerization. In both cases, further reaction steps can occur to produce secondary products such as propane and additional methane from hydrogenolysis,while isomerization of the primary product, isopentane, 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 isopentane (up to 60% selectivity). In addition, there are both size and alloy effects for this system. From our experiments we have found that the selectivity to isopentane can be correlated to the binding energy of CO. This can be rationalized as an electronic effect as the str
ength of adsorption of CO can be related to the d-band center of the metal. It follows that the binding energy of neopentane will also vary in a linear manner with the location of the d-band center of the metal and therefore the adsorption energy of neopentane (and by analogy, the d-band center of the metal) appears to be a descriptor for the selectivity of the catalyst. The fact that the smallest Pt particles and the largest Pd particles tested have similar CO binding energies and similar isomerization selectivities suggests that geometric effects do not play a role in the selectivity. We have used VASP calculations performed on Fusion to examine the mechanisms of hydrogenolysis and isomerization on Pt(111) and Pd(111) in an effort to understand the selectivity differences. We have found that both mechanisms must pass through a common intermediate: a hydrocarbon fragment whereby a C-H bond has been broken at two locations on neopentane and a (CH2)2C(CH3)2 fragment is bound
to the surface. Mechanisms of C-C bond cleavage or isomerization involving only a single attachment to the surface have much higher barriers and must be ruled out. Most recently we have begun an examination of neopentane hydrogenolysis over PdZn(111). Experimentally, the activity of this alloy is found to be orders of magnitude lower than either Pt or Pd even though its CO binding energy would suggest that it should have a very high selectivity. The low activity can be rationalized by an inability of the PdZn(111) surface to bind neopentane in the desired configuration for reaction (i.e. a double attachment to the surface through a (CH2)2C(CH3)2 intermediate is not favored.
In the new time allotment, we will focus on completing our study of the mechanisms of hydrogenolysis and isomerization for neopentane on Pt(111) and Pd(111). In addition although our initial study of PdZn(111) we found that the adsorption of neopentane on the surface has changed we need to determine if new reaction pathways are available on PdZn(111). Finally our goal for FY 14 is to see how electronic effects and geometric effects can both be exploited to find new catalysts with higher selectivity to the isomerization product.
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. For the past year we have focused upon examination of new alloys for the selective hydrogenation of acrolein. Specifically we have become interested in highly dilute alloys in which one highly active metal is embedded in low concentration in a matrix of another metal. For example, Sykes and co-workers have recently demonstrated that Pd atoms embedded in a Cu(111) surface can act as unique sites (named as Single Atom Alloys) for the activation of hydrogen and thereby boost activity and selectivity in selective hydrogenation1-4. DFT calculations predicted that the barrier of for hydrogen dissociation was reduced from 0.4 eV to 0.02 eV b
ut that the adsorption enthalpy was not strongly affected (-0.35 eV on the alloy but -0.20 eV on Cu(111)).
Based upon this result, we have continued our own studies of selective hydrogenation of acrolein. Acrolein hydrogenation is a simple but challenging reaction as selective hydrogenation of the C=C bond is thermodynamically favored over hydrogenation of the C=O bond of the aldehyde by ~ 35 kJ/mol and therefore, kinetic control is required to obtain the unsaturated alcohol (allyl alcohol), the desired product. Although the reaction is a relatively straightforward selectivity play between hydrogenation of the double bond and hydrogenation of the aldehyde functionality, the reaction can be manipulated with regard to choice of metal, particle size effects, support effects, and total reaction pressure. The primary metal of choice is Ag5-7, alloys have been employed previously for the selective hydrogenation of acrolein. In our calculations we will examine candidate single atom alloys (SAAs) based upon Ag where an active metal (eg. Pd, Rh, Ru) is inserted in the Ag matrix in dilute
amounts so as to prevent the presence of contiguous atoms of the active metal. These calculations are correlated with experimental results on analogous systems.
We have begun to perform DFT calculations on simple models of the SAA surfaces to gain insight into our parallel experimental results. A series of Ag(111) slabs have been constructed with a single atom of the alloying element replacing one Ag atom in the surface layer.
Element C=C C=O
Ag –0.05 –0.03
Co –1.05 –0.87
Ni –0.68 –0.51
Cu –0.05 –0.11
Ru –1.44 –0.91
Rh –1.20 –0.53
Pd –0.46 –0.11
Ir –1.39 –0.60
Pt –0.59 –0.04
Au –0.05 –0.02
Table 1 Acrolein bonding to M@Ag(111) surfaces
Element ΔE Eact
Ag 0.87 1.38
Co –0.88 barrierless
Ni –0.49 barrierless
Cu 0.24 1.17
Ru –1.26 barrierless
Rh –1.03 barrierless
Pd –0.18 0.23
Ir –1.51 barrierless
Pt –0.54 barrierless
Table 2 Hydrogen Dissociation over M@Ag(111) surfaces
Two sets of calculations have been performed. First, the SAA has been tested for their ability to adsorption acrolein. As previously seen by Sautet the adsorption configuration of acrolein can play a large role in its ability to be selectively hydrogenated8. We have examined the adsorption of acrolein in a variety of configurations including C=O bonded to the heteroatom of the SAA vs. the C=C bond directly below the heteroatom in the SAA. Our initial results indicate that the presence of the active metal (eg. Pd) does not result in the formation of a strong Pd-O bond between the aldehyde functionality of acrolein and the surface. From Table 1, we can see that the more oxophillic 3d metals have a stronger tendency to activate the C=O bond (although in every case activation of the C=C bond is still favored). However, before making new recommendations about catalyst compositions, other bonding configurations (i.e. both the C=O and C=C bonds in contact with the surface) still n
eed to be tested. Second, the presence of the SAA should serve to lower the barrier to H2 dissociation and thereby increase the activity. Therefore another important aspect to explore is how the presence of a second heteroatom either directly neighboring or in the second coordination sphere of the SAA site influences both acrolein adsorption and hydrogen activation. We have calculated the reaction enthalpy and barrier for the same group of Ag based SAAs as shown in Table 2. For a acrolein catalyst to be both active and selective, we hypothesize that both criteria (a favorable acrolein adsorption configuration and the ability to readily dissociate hydrogen) will be necessary. Therefore we are currently attempting to expand our investigation of these SAA models with new guest/host combinations.
To complete the objectives of FY13-14, we are requesting 400,000 cpu hours. VASP calculations require significant computational time. For the supercell sizes that we are using, we expect that typical calculations on Fusion take 2-4 hours using 8 cores. We expect that using Blues, the performance will be even better allowing us to move rapidly through the proposed work.
References
[1] G. Kyriakou, M.B. Boucher, A.D. Jewell, E.A. Lewis, T.J. Lawton, A.E. Baber, H.L. Tierney, M. Flytzani-Stephanopoulos, E.C.H. Sykes, Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations, Science 335 (2012) 1209-1212.
[2] H.L. Tierney, A.E. Baber, E.C.H. Sykes, Atomic-Scale Imaging and Electronic Structure Determination of Catalytic Sites on Pd/Cu Near Surface Alloys, J Phys Chem C 113 (2009) 7246-7250.
[3] H.L. Tierney, A.E. Baber, J.R. Kitchin, E.C.H. Sykes, Hydrogen Dissociation and Spillover on Individual Isolated Palladium Atoms, Phys Rev Lett 103 (2009) 246102.
[4] A.E. Baber, H.L. Tierney, T.J. Lawton, E.C.H. Sykes, An Atomic-Scale View of Palladium Alloys and their Ability to Dissociate Molecular Hydrogen, Chemcatchem 3 (2011) 607-614.
[5] M. Bron, D. Teschner, A. Knop-Gericke, F.C. Jentoft, J. Krohnert, J. Hohmeyer, C. Volckmar, B. Steinhauer, R. Schlogl, P. Claus, Silver as acrolein hydrogenation catalyst: intricate effects of catalyst nature and reactant partial pressures, Phys Chem Chem Phys 9 (2007) 3559-3569.
[6] M. Bron, D. Teschner, A. Knop-Gericke, B. Steinhauer, A. Scheybal, M. Havecker, D. Wang, R. Fodisch, D. Honicke, A. Wootsch, R. Schlogl, P. Claus, Bridging the pressure and materials gap: in-depth characterisation and reaction studies of silver-catalysed acrolein hydrogenation, J Catal 234 (2005) 37-47.
[7] H. Wei, C. Gomez, N. Guo, T. Wu, R.J. Lobo-Lapidus, C. Marshall, J.T. Miller, R.J. Meyer, Selective Hydrogenation of Acrolein on Supported Silver Catalysts: A Kinetics Study of Particle Size Effects, J. Catal. 298 (2013) 18-26.
[8] D. Loffreda, F. Delbecq, F. Vigne, P. Sautet, Chemo-regioselectivity in heterogeneous catalysis: Competitive routes for C=O and C=C hydrogenations from a theoretical approach, J Am Chem Soc 128 (2006) 1316-1323.
Project URL:
Current FY Hours Used: undetermined amount
New FY Requested allocation: 400000
Q1: 100000
Q2: 100000
Q3: 100000
Q4: 100000
Justification:
Thank You,
The LCRC Accounts System
