Paul Crawford

Publications (6)16.3 Total impact

• Article: Recent Advances in Understanding CO Oxidation on Gold Nanoparticles Using Density Functional Theory

  Ying Chen, Paul Crawford, P. Hu
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  ABSTRACT: Since the discovery of a series of Au-based catalysts by Haruta etal. considerable progress has been made in understanding Since the discovery of a series of Au-based catalysts by Haruta etal. considerable progress has been made in understanding the active role of Au in CO oxidation catalysis. This review provides a summary of recent theoretical work performed in this the active role of Au in CO oxidation catalysis. This review provides a summary of recent theoretical work performed in this field; in particular it addresses DFT studies of CO oxidation catalysis over free and supported gold nanoparticles. Several field; in particular it addresses DFT studies of CO oxidation catalysis over free and supported gold nanoparticles. Several properties of the Au particles have been found to contribute to their unique catalytic activity. Of these properties, the properties of the Au particles have been found to contribute to their unique catalytic activity. Of these properties, the low-coordination state of the Au atoms is arguably the most pertinent, although other properties of the Au cluster atoms, low-coordination state of the Au atoms is arguably the most pertinent, although other properties of the Au cluster atoms, such as electronic charge, cannot be ignored. The current consensuses regarding the mechanism for CO oxidation over Au-based such as electronic charge, cannot be ignored. The current consensuses regarding the mechanism for CO oxidation over Au-based catalysts is also discussed. Finally, water-enhanced catalysis of CO oxidation on Au clusters is summarized. catalysts is also discussed. Finally, water-enhanced catalysis of CO oxidation on Au clusters is summarized.
  Catalysis Letters 04/2012; 119(1):21-28. · 2.24 Impact Factor
• Article: Insights into the Staggered Nature of Hydrogenation Reactivity over the 4d Transition Metals

  Paul Crawford, Bronagh McAllister, P. Hu
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  ABSTRACT: Hydrogenation reactions at transition metal surfaces comprise a key set of reactions in heterogeneous catalysis. In this paper, density functional theory methods are employed to take an in-depth look at this fundamental reaction type. The energetics of hydrogenation of atomic C, N, and O have been studied in some detail over low index Zr, Nb, Mo, Tc, Ru, Rh, and Pd surfaces. Detailed bonding analysis has also been employed to track carefully the chemical changes taking place during reaction. A number of interesting horizontal and vertical trends have been uncovered relating to reactant valency and metal d-band filling. A general correlation has also been found between the reaction barriers and the reaction potential energies. Moreover, when each reaction is considered independently, correlation has been found to improve with decreasing reactant valency. Bonding analysis has pointed to this being related to the relative position of the transition state along the reaction coordinate and has shown that as reactant valency decreases, the transition states become progressively later.
  03/2009;
• Source

  Available from: upenn.edu

  Article: Trends in C-O and C-N bond formations over transition metal surfaces: an insight into kinetic sensitivity in catalytic reactions.

  Paul Crawford, P Hu
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  ABSTRACT: Transition metal catalyzed bond formation is a fundamental process in catalysis and is of general interest throughout chemistry. To date, however, the knowledge of association reactions is rather limited, relative to what is known about dissociative processes. For example, surprisingly little is known about how the bond-forming ability of a metal, in general, varies across the Periodic Table. In particular, the effect of reactant valency on such trends is poorly understood. Herein, the authors examine these key issues by using density functional theory calculations to study CO and CN formations over the 4d metals. The calculations reveal that the chemistries differ in a fundamental way. In the case of CO formation, the reaction enthalpies span a much greater range than those of CN formation. Moreover, CO formation is found to be kinetically sensitive to the metal; here the reaction barriers (E(a)) are found to be influenced by the reaction enthalpy. CN formation, conversely, is found to be relatively kinetically insensitive to the metal, and there is no correlation found between the reaction barriers and the reaction enthalpy. Analysis has shown that at the final adsorbed state, the interaction between N and the surface is relatively greater than that of O. Furthermore, in comparison with O, relatively less bonding between the surface and N is observed to be lost during transition state formation. These greater interactions between N and the surface, which can be related to the larger valency of N, are found to be responsible for the relatively smaller enthalpy range and limited variation in E(a) for CN formation.
  The Journal of Chemical Physics 06/2007; 126(19):194706. · 3.33 Impact Factor
• Article: Importance of electronegativity differences and surface structure in molecular dissociation reactions at transition metal surfaces.

  Paul Crawford, P Hu
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  ABSTRACT: The dissociative adsorption of N2 has been studied at both monatomic steps and flat regions on the surfaces of the 4d transition metals from Zr to Pd. Using density functional theory (DFT) calculations, we have determined and analyzed the trends in both straight reactivity and structure sensitivity across the periodic table. With regards to reactivity, we find that the trend in activation energy (Ea) is determined mainly by a charge transfer from the surface metal atoms to the N atoms during transition state formation, namely, the degree of ionicity of the N-surface bond at the transition state. Indeed, we find that the strength of the metal-N bond at the transition state (and therefore the trend in Ea) can be predicted by the difference in Mulliken electronegativity between the metal and N. Structure sensitivity is analyzed in terms of geometric and electronic effects. We find that the lowering of Ea due to steps is more pronounced on the right-hand side of the periodic table. It is found that for the early transition metals the geometric and electronic effects work in opposition when going from terrace to step active site. In the case of the late 4d metals, however, these effects work in combination, producing a more marked reduction in Ea.
  The Journal of Physical Chemistry B 01/2007; 110(49):24929-35. · 3.70 Impact Factor
• Article: Reactivity of the 4d transition metals toward N hydrogenation and NH dissociation: a DFT-based HSAB analysis.

  Paul Crawford, P Hu
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  ABSTRACT: The density functional theory (DFT) based hard-soft acid-base (HSAB) reactivity indices, including the electrophilicity index, have been successfully applied to many areas of molecular chemistry. In this work we test the applicability of such an approach to fundamental surface chemistry. We have considered, as prototypical surface reactions, both the hydrogenation of atomic nitrogen and the dissociative adsorption of the NH molecular radical. By use of a DFT methodology, the minimum energy reaction pathways, and corresponding reaction barriers, of the above reactions over Zr(001), Nb(110), Mo(110), Tc(001), Ru(001), Rh(111), and Pd(111) have been determined. By consideration of the chemical potential and chemical hardness of the surface metal atoms, and the principle of electronegativity equalization, it is found that the charge transferred to the NH radical during the process of dissociative adsorption correlates very well with that determined by Mulliken population analysis. Furthermore, it is found that the stability of the NH/surface transition state complex relates directly to this charge transfer and that the trend in transition state stability predicted by a HSAB treatment correlates very strongly with that determined by DFT calculations. With regards to N hydrogenation, we find that during the course of the reaction, H loses cohesion to the surface, as it must migrate from a 3-fold hollow site to either a bridge or top site, to react with N. Partial density of states (PDOS) and Mulliken population analysis reveal that this loss of bonding is accompanied by charge transfer from H to the surface metal atoms. Moreover, by simple modeling, we show that the reaction barriers are directly proportional to this mandatory charge transfer. Indeed, it is found that the reaction barriers correlate very well with the electrophilicity index of the metal atoms.
  The Journal of Physical Chemistry B 04/2006; 110(9):4157-61. · 3.70 Impact Factor
• Article: The importance of hydrogen's potential-energy surface and the strength of the forming R-H bond in surface hydrogenation reactions.

  Paul Crawford, P Hu
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  ABSTRACT: An understanding of surface hydrogenation reactivity is a prevailing issue in chemistry and vital to the rational design of future catalysts. In this density-functional theory study, we address hydrogenation reactivity by examining the reaction pathways for N+H-->NH and NH+H-->NH(2) over the close-packed surfaces of the 4d transition metals from Zr-Pd. It is found that the minimum-energy reaction pathway is dictated by the ease with which H can relocate between hollow-site and top-site adsorption geometries. A transition state where H is close to a top site reduces the instability associated with bond sharing of metal atoms by H and N (NH) (bonding competition). However, if the energy difference between hollow-site and top-site adsorption energies (DeltaE(H)) is large this type of transition state is unfavorable. Thus we have determined that hydrogenation reactivity is primarily controlled by the potential-energy surface of H on the metal, which is approximated by DeltaE(H), and that the strength of N (NH) chemisorption energy is of less importance. DeltaE(H) has also enabled us to make predictions regarding the structure sensitivity of these reactions. Furthermore, we have found that the degree of bonding competition at the transition state is responsible for the trend in reaction barriers (E(a)) across the transition series. When this effect is quantified a very good linear correlation is found with E(a). In addition, we find that when considering a particular type of reaction pathway, a good linear correlation is found between the destabilizing effects of bonding competition at the transition state and the strength of the forming N-H (HN-H) bond.
  The Journal of Chemical Physics 01/2006; 124(4):044705. · 3.33 Impact Factor