Island formation during kinetic rate oscillations in the oxidation of CO over Pt/SiO2: A transient Fourier transform infrared spectrometry study

Transient Fourier transform infrared spectroscopy (FTIR) spectroscopy and m ass spectroscopy were used in tandem to study CO adsorption and kinetic rat e oscillations in the oxidation of CO at intermediate pressure (1-10 Torr) on a series of silica gel-supported platinum catalysts of varying dispersio n and preparation. FTIR shows that the stretching frequency of linearly bon ded CO decreases as the dispersion of the platinum particles increases (207 6 cm(-1) for D = 25% to 2058 cm(-1) for D = 99%), but CO bonded to electron -deficient platinum (CO-Ptdelta+) results in a frequency of similar to 2080 cm(-1) for all particle sizes. Isotope-mixing experiments and dipole coupl ing simulations were performed in order to determine the dipole and chemica l components of the coverage-dependent IR peak shift on supported platinum. The results compare favorably to single-crystal studies and show a chemica l shift of 10 cm(-1) and a dipole shift of 25 cm(-1) that are independent o f particle dispersion. Reaction studies at T = 180-300 degreesC and O-2:CO = 0.5-4.0 suggest that the oxidation/reduction model and the carbon deposit ion model cited in the literature are both inadequate to explain the observ ed oscillations over these catalysts. A mechanism is suggested in which den sely packed islands of CO block a portion of the active sites during the re action. Differences in catalyst precursor (chloroplatinic acid or tetraammi ne platinum(II) nitrate) affect the amplitude of the oscillations observed in this study. The feedback mechanism driving the oscillations remains uncl ear but is most likely related to the Pt(100) or Pt(110) surface-phase tran sition or chlorine impurities supporting the formation of islands through e lectrostatic interaction. (C) 2001 Academic Press.