JOURNAL OF PHYSICAL CHEMISTRY A, cilt.112, ss.9584-9590, 2008 (SCI İndekslerine Giren Dergi)
Photoionization efficiency curves were measured for gas-phase PtC, PtO, and PtO, using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. The molecules were prepared by laser ablation of a platinum tube, followed by reaction with CH4 or NO and supersonic expansion. These measurements provide the first directly measured ionization energy for PtC, IE(PtC) = 9.45 +/- 0.05 eV. The direct measurement also gives greatly improved ionization energies for the platinum oxides, IE(PtO) = 10.0 +/- 0.1 eV and FE(PtO2) = 11.35 +/- 0.05 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to greatly improved 0 K bond dissociation energies for the neutrals: D-0(Pt-C) = 5.95 +/- 0.07 eV, D-0(Pt-O) = 4.30 +/- 0.12 eV, and D-0(OPt-O) = 4.41 +/- 0.13 eV, as well as enthalpies of formation for the gas-phase molecules Delta H-f.0(0)(PtC(g)) = 701 +/- 7 kJ/mol, Delta H-f.0(0)(PtO(g)) = 396 +/- 12 kJ/mol, and Delta H-f.0(0)(PtO2(g)) = 218 +/- 11 kJ/mol. Much of the error in previous Knudsen cell measurements of platinum oxide bond dissociation energies is due to the use of thermodynamic second law extrapolations. Third law values calculated using statistical mechanical thermodynamic functions are in much better agreement with values obtained from ionization energies and ion energetics. These experiments demonstrate that laser ablation production with direct VUV ionization measurements is a versatile tool to measure ionization energies and bond dissociation energies for catalytically interesting species such as metal oxides and carbides.