In situ FTIR, EXAFS, and activity studies of the effect of crystallite size on silica-supported Pt oxidation catalysts

The effect of crystallite size on the activity of silica-supported Pt catalysts during CO oxidation has been investigated by kinetic studies and in situ infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. Catalysts containing 2% w/w Pt on silica were prepared by two different methods, rendering catalysts with dispersion values ranging between 0.29 and 0.80. Kinetic results indicate that the turnover frequency (TOF) of CO oxidation increases with increasing particle size under O2-rich conditions, confirming that the CO oxidation reaction over Pt/SiO2 catalysts is structure sensitive under an oxidizing environment. This characteristic is not only related to the gas phase surrounding the supported catalyst but also to the crystallite size range analyzed. EXAFS results show the formation of completely metallic Pt particles upon reduction in H2 at 300 °C for 1 h regardless of the particle size, whereas after a subsequent oxidation pretreatment only the smallest particles are fully oxidized. At room temperature, the oxidized Pt surface does not adsorb CO, but under oxidizing conditions (1% CO, 10% O2 in He) the oxidized catalysts show activity at T>100 °C. Fourier-transform infrared spectroscopy indicates that even this low CO concentration leads to reduction of the oxidized surface under reaction conditions. The results presented here clearly show that the active surface of Pt/SiO2 catalysts during CO oxidation is the metallic Pt, and that different sites on a Pt crystallite have different oxidation rates depending on its size. These results show the sensitivity of CO oxidation activity to the preparation method, pretreatment, and most significantly to the reaction atmosphere.