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. Interestingly we can turn off both isomerization and hydrogenolysis reactions by using an alloy like PdZn which cannot activate C-C bonds. 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 (CH)2C(CH3)2 intermediate is not favored. We have now applied this result to another reactio
n: propane dehydrogenation. Pd itself is very active for propane conversion, but selectivity is extremely poor resulting in methane from hydrogenolysis and considerable coke formation which ultimately leads to rapid deactivation. By comparison, although PdZn also deactivates significantly, the conversion stabilizes at a low level (8%) with high selectivity (98%) to propene (with minimal selectivity to methane, ethane and ethene).
In order to explore the performance of the PdZn catalyst and to understand how Zn affects the selectivity, we have performed density functional theory calculations of C3H8 dehydrogenation over both Pd(111) and PdZn(111) surfaces. The PdZn(111) surface is comprised of alternating rows of Pd and Zn so hollow sites of Pd are not available for adsorption and the bridging sites are spaced at a Pd-Pd bond distance of 2.90 Å as compared to Pd(111) where the Pd-Pd distance is 2.75 Å. First we observe that propane adsorption to both surfaces is dominated by van der Waals forces. Calculations performed in the absence of the van der Waals corrections showed that propane does not interact with either surface (not surprising for an alkane). However, once the vdW corrections were applied then adsorption energies were between 0.5-1.0 eV.
The initial dehydrogenation of propane can occur either by removal of hydrogen from the center carbon or a terminal carbon. We find that removal of the hydrogen from the center carbon to form a CH3CHCH3 intermediate is preferred thermodynamically (over formation of CH3CH2CH2) on both surfaces but the energy difference is slightly smaller on Pd(111) (0.13 eV vs. 0.24 eV on PdZn(111)). Comparing the two surfaces, we find that the barrier for CH3CHCH3 formation on PdZn(111) is 1.39 eV while, the barrier drops to only 0.85 eV on Pd(111). Similarly, while the reaction is thermoneutral on Pd(111), the reaction is endothermic by 0.77 eV on PdZn(111). The second dehydrogenation event to form C3H6 follows a similar pattern. On Pd(111), the reaction is again essentially thermoneutral with a barrier of 0.56 eV while on PdZn(111) the barrier is 0.89 eV (but also essentially thermoneutral). Once C3H6 is formed, the molecule may either desorb or undergo further dehydrogenation. On Pd(111
), the barrier for dehydrogenation to CH3CHCH is 1.33 eV but rises to 1.48 eV for the same reaction on PdZn(111). Conversely, propylene can desorb with an energy of 1.02 eV from PdZn(111), but is bound with an energy of 1.76 eV on Pd(111). This demonstrates why coking on the PdZn(111) surface is reduced: propene can desorb before it undergoes further dehydrogenation while on Pd(111), it is too strongly adsorbed and will undergo further reaction. In addition, we find that the hydrogenolysis of propane (which leads to methane formation), seems to follow the previous work of Iglesia et al., requiring removal of 4 Hs before C-C bond cleavage can occur1. This implies that hydrogenolysis is also prevented by the desorption of propene before dehydrogenation can take place.
In the new time allotment in FY14, we will focus on completing our study of the mechanisms of hydrogenolysis, dehydrogenation, and isomerization for alkanes. In addition although our initial study of PdZn(111) we found that the presence of zinc increased the barriers for C-H and C-C activation which made for a less active but more selective catalyst for propane dehydrogenation. The question we face is whether we can shut down hydrogenolysis while maintaining activity for isomerization. Therefore we will search for other similar alloys to determine if the geometry of the surface can be used to prevent C-C cleavage while allowing for a transition state for isomerization.
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 hydrogenation2.
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 Ag3-5, alloys have been employed previously for the selective hydrogenation of acrolein. In our calculations we have examined 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. It appears that Pd may be the optimal dopant because the process maintains a low barrier whereas the use of other metals (Ru, Pt, Ni, Rh) results in barrierless dissociation where no kinetic control can be exerted or high barriers (in the case of Au and Cu) which require significant energy (temperature) to overcome. DFT calculations predict that the barrier of for hydrogen dissociation is reduced from 1.15 eV to 0.31 eV when adding a single atom of Pd to the Ag(111) surface. This dramatic lowering of the barrier may allow us to shift the rate limiting step from hydrogen dissociation to a surface hydrogenation event. This could explain why both the activity and selectivity go up when Pd is present as the kinetic control on the system shifts from hydrogen activation to surface reactions between adsorbed species.
More recently we have discovered two additional results which have provided some exciting leads. First, we have calculated acrolein adsorption on various surfaces of Ag. We have found that acrolein prefers to adsorb with C=O oriented toward the surface on Ag(100) unlike any other surface (111,110,221). This implies that the selectivity on Ag(100) surfaces might be significantly better than on Ag(111). We still need to perform additional calculations to determine if the barriers to hydrogenation of the C=O bond are lower than those for C=C hydrogenation. We are also planning experiments on Ag(100) nanocubes and Ag(111) cuboctahedra to verify if this lead can be verified. Second, we have identified that strong support effects exist in this system. We have found that Ag/CeO2 and Ag/TiO2 catalysts are significantly more selective than Ag/SiO2. At 10% conversion, the selectivity of Ag/CeO2 is nearly 80%, approximately double that of Ag/SiO2. Therefore, we will build structural
models of Ag/MOx to determine how the support participates in the reaction and why the selectivity is increased when using highly reducible supports.
To complete the objectives of FY14-15, 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] D.W. Flaherty, E. Iglesia, Transition-State Enthalpy and Entropy Effects on Reactivity and Selectivity in Hydrogenolysis of n-Alkanes, J. Am. Chem. Soc. 135 (2013) 18586-18599.
[2] 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.
[3] 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.
[4] 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.
[5] 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.
Industry partnership:
Project URL:
Current FY Hours Used: undetermined amount
New FY Requested allocation: 300000
Q1: 75000
Q2: 75000
Q3: 75000
Q4: 75000
Justification:
Storage requirements:
Thank You,
The LCRC Accounts System
