We investigate the reactivity of various Pt xPd y combinations (with x + y = 10 and various x: y ... more We investigate the reactivity of various Pt xPd y combinations (with x + y = 10 and various x: y ratios) towards the adsorption of specific intermediates of the oxygen reduction, using the B3PW91 hybrid density functional theory. The reactivity is shown to be not only sensitive to the composition of the cluster, but also to the atomic distribution. The calculations indicate that two different ensembles: one ordered and one randomly mixed, with overall composition Pt 3Pd 7 are thermodynamically more favorable than pure Pt 10 for the oxygen reduction reaction. The reasons for this behavior are clearly explained in terms of the atomic and electronic distribution, which makes the Pd atoms to act as electron donors both to Pt atoms and to the adsorbates, thus the reactivity of the Pd atoms in such environment becomes intermediate between Pt and Pd. Moreover, it is found that in a mixed Pt 3Pd 7 state the electronic distribution makes the average atom more similar to Pt than to Pd, whereas in an ordered Pt 3Pd 7 cluster, the average atom is more similar to Pd than to Pt.
The adsorption of H(2)O(2) on Pt and Pt-M alloys, where M is Cr, Co, or Ni, is investigated using... more The adsorption of H(2)O(2) on Pt and Pt-M alloys, where M is Cr, Co, or Ni, is investigated using density functional theory. Binding energies calculated with a hybrid DFT functional (B3PW91) are in the range of -0.71 to -0.88 eV for H(2)O(2) adsorbed with one of the oxygen atoms on top Pt positions of Pt(3), Pt(2)M, and PtM(2), and enhanced values in the range of -0.81 to -1.09 eV are found on top Ni and Co sites of the Pt(2)M clusters. Adsorption on top sites of Pt(10) yields a weaker binding of -0.48 eV, whereas on periodic Pt(111) and Pt(3)Co(111) surfaces, H(2)O(2) generally dissociates into two OH radicals. On the other hand, attempts to attach H(2)O(2) on bridge sites cause spontaneous dissociation of H(2)O(2) into two adsorbed OH radicals, suggesting that stable adsorptions on bridge sites are not possible for any of the clusters or extended surfaces that are being studied. We also found that the water-H(2)O(2) interaction reduces the strength of the adsorption of H(2)O(2) on these clusters and surfaces.
... E-mail: [email protected] and [email protected] ... Details of the c... more ... E-mail: [email protected] and [email protected] ... Details of the calculated adsorption geometries are compiled in table 1, together with adsorption energies with respect to gas-phase Au (Eads > 0 meaning exothermic adsorption). ...
Achieving a high degree of selectivity in the synthesis of single-walled carbon nanotubes in a sy... more Achieving a high degree of selectivity in the synthesis of single-walled carbon nanotubes in a systematic and efficient way is one major barrier to be overcome before the exceptional properties of these materials can be exploited commercially. Many studies seem to indicate that the state and structure of the catalyst in CCVD play a fundamental role in determining structural features of the nanotubes such as chirality. Selective growth has been reported in both monometallic and carbide nanoparticles; however the contribution that either of them brings to this process has not been completely elucidated. The stability and the influence of the latter in the growing nanotube structure is the focus of this study. We determine the stability and relevant features of carbide nanoparticles and the interactions between carbide facets and the nascent carbon structure using ab initio molecular dynamics simulations. We use 55-metal-atom Ni and Co clusters to model the catalyst and run our simulat...
Cl--water hydrogen bonds decays faster than the coordination number. By analogy to adsorption phe... more Cl--water hydrogen bonds decays faster than the coordination number. By analogy to adsorption phenomena, augmentation in the local solvent density relative to the bulk (clustering) is interpreted in three regions corresponding to gas, near-critical, and liquid-like densities: The lifetime ...
Free energies of solvation for Cl -, OH -, Na + , HCl, and H 2 O are calculated at ambient condit... more Free energies of solvation for Cl -, OH -, Na + , HCl, and H 2 O are calculated at ambient conditions and several supercritical water (SCW) states using molecular dynamics-free energy perturbation computer simulation. From ambient to SCW, the magnitude of reductions in the coordination number, number of hydrogen bonds, and other specific interactions are in the opposite order of the solute-water interaction energies, which are, from strongest to weakest, Na + > OH -> Cl -> H 2 O > HCl. This trend also applies to the magnitude of the solvation free energy, although the changes for Na + are very small, from ambient conditions all the way to 673 K and 0.087 g/cm 3 . Bivalent cations exhibit a well-defined second shell that remains at supercritical conditions. Modest discrepancies are observed between simulation and experiment both for the local density of water about Sr 2+ and for the free energy of solvation of NaCl, illustrating the challenges to both theory and experiment.
Nanoparticles –from a few Angstroms to tens of nanometers- have been used as catalysts well befor... more Nanoparticles –from a few Angstroms to tens of nanometers- have been used as catalysts well before the word nanotechnology became popular. It is not surprising to expect that very small particles, having a large surface/volume ratio and a large proportion of low-coordinated sites may be much more reactive than flat surfaces. However, obtaining a uniform catalyst material, with welldefined particle
We investigate the reactivity of various Pt xPd y combinations (with x + y = 10 and various x: y ... more We investigate the reactivity of various Pt xPd y combinations (with x + y = 10 and various x: y ratios) towards the adsorption of specific intermediates of the oxygen reduction, using the B3PW91 hybrid density functional theory. The reactivity is shown to be not only sensitive to the composition of the cluster, but also to the atomic distribution. The calculations indicate that two different ensembles: one ordered and one randomly mixed, with overall composition Pt 3Pd 7 are thermodynamically more favorable than pure Pt 10 for the oxygen reduction reaction. The reasons for this behavior are clearly explained in terms of the atomic and electronic distribution, which makes the Pd atoms to act as electron donors both to Pt atoms and to the adsorbates, thus the reactivity of the Pd atoms in such environment becomes intermediate between Pt and Pd. Moreover, it is found that in a mixed Pt 3Pd 7 state the electronic distribution makes the average atom more similar to Pt than to Pd, whereas in an ordered Pt 3Pd 7 cluster, the average atom is more similar to Pd than to Pt.
The adsorption of H(2)O(2) on Pt and Pt-M alloys, where M is Cr, Co, or Ni, is investigated using... more The adsorption of H(2)O(2) on Pt and Pt-M alloys, where M is Cr, Co, or Ni, is investigated using density functional theory. Binding energies calculated with a hybrid DFT functional (B3PW91) are in the range of -0.71 to -0.88 eV for H(2)O(2) adsorbed with one of the oxygen atoms on top Pt positions of Pt(3), Pt(2)M, and PtM(2), and enhanced values in the range of -0.81 to -1.09 eV are found on top Ni and Co sites of the Pt(2)M clusters. Adsorption on top sites of Pt(10) yields a weaker binding of -0.48 eV, whereas on periodic Pt(111) and Pt(3)Co(111) surfaces, H(2)O(2) generally dissociates into two OH radicals. On the other hand, attempts to attach H(2)O(2) on bridge sites cause spontaneous dissociation of H(2)O(2) into two adsorbed OH radicals, suggesting that stable adsorptions on bridge sites are not possible for any of the clusters or extended surfaces that are being studied. We also found that the water-H(2)O(2) interaction reduces the strength of the adsorption of H(2)O(2) on these clusters and surfaces.
... E-mail: [email protected] and [email protected] ... Details of the c... more ... E-mail: [email protected] and [email protected] ... Details of the calculated adsorption geometries are compiled in table 1, together with adsorption energies with respect to gas-phase Au (Eads > 0 meaning exothermic adsorption). ...
Achieving a high degree of selectivity in the synthesis of single-walled carbon nanotubes in a sy... more Achieving a high degree of selectivity in the synthesis of single-walled carbon nanotubes in a systematic and efficient way is one major barrier to be overcome before the exceptional properties of these materials can be exploited commercially. Many studies seem to indicate that the state and structure of the catalyst in CCVD play a fundamental role in determining structural features of the nanotubes such as chirality. Selective growth has been reported in both monometallic and carbide nanoparticles; however the contribution that either of them brings to this process has not been completely elucidated. The stability and the influence of the latter in the growing nanotube structure is the focus of this study. We determine the stability and relevant features of carbide nanoparticles and the interactions between carbide facets and the nascent carbon structure using ab initio molecular dynamics simulations. We use 55-metal-atom Ni and Co clusters to model the catalyst and run our simulat...
Cl--water hydrogen bonds decays faster than the coordination number. By analogy to adsorption phe... more Cl--water hydrogen bonds decays faster than the coordination number. By analogy to adsorption phenomena, augmentation in the local solvent density relative to the bulk (clustering) is interpreted in three regions corresponding to gas, near-critical, and liquid-like densities: The lifetime ...
Free energies of solvation for Cl -, OH -, Na + , HCl, and H 2 O are calculated at ambient condit... more Free energies of solvation for Cl -, OH -, Na + , HCl, and H 2 O are calculated at ambient conditions and several supercritical water (SCW) states using molecular dynamics-free energy perturbation computer simulation. From ambient to SCW, the magnitude of reductions in the coordination number, number of hydrogen bonds, and other specific interactions are in the opposite order of the solute-water interaction energies, which are, from strongest to weakest, Na + > OH -> Cl -> H 2 O > HCl. This trend also applies to the magnitude of the solvation free energy, although the changes for Na + are very small, from ambient conditions all the way to 673 K and 0.087 g/cm 3 . Bivalent cations exhibit a well-defined second shell that remains at supercritical conditions. Modest discrepancies are observed between simulation and experiment both for the local density of water about Sr 2+ and for the free energy of solvation of NaCl, illustrating the challenges to both theory and experiment.
Nanoparticles –from a few Angstroms to tens of nanometers- have been used as catalysts well befor... more Nanoparticles –from a few Angstroms to tens of nanometers- have been used as catalysts well before the word nanotechnology became popular. It is not surprising to expect that very small particles, having a large surface/volume ratio and a large proportion of low-coordinated sites may be much more reactive than flat surfaces. However, obtaining a uniform catalyst material, with welldefined particle
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