The particle size dependence of CO oxidation on model planar titania supported gold catalysts measured by parallel thermographic imaging

Abstract

A parallel thermographic screening methodology has been developed which allows the measurements of the particle size and support influences on model planar heterogeneous catalysts. A screening chip was designed and fabricated in order to produce multiple fields of low stress silicon nitride membranes that exhibit low thermal conductivity and heat capacity. The heat generated on supported model catalysts in an exothermic reaction on the membranes was measured using a thermal (infra-red) imaging camera, which in turn provided a measure of the turn over frequency (TOF) for the reaction. The catalytic activity for CO oxidation on titania supported gold model catalysts with varying particle size has been measured on 100 catalysts simultaneously. The reaction has been investigated at 80 °C and 170 °C, and pressures ranging between 0.06 mbar and 1.5 mbar for various O2:CO ratios. Under all conditions investigated, a monotonic increase in the TOF is observed with decreasing particle diameter (d) which is proportional to ca. d−1.8 in the range 6 > d/nm > 1.5. This is in the opposite direction to the number of potentially active perimeter sites which increases linearly with increasing particle size on these catalysts. We show that the surface area specific activity of the gold is increasing even more steeply with reduced particle size, and is proportional to ca. d−4. This rate of increase in activity is significantly higher than one would expect by any increase one may expect as a result of more active low co-ordinate sites on the gold. The steep increase in activity is ascribed to an electronic interaction between the substrate and the particle.