A high-throughput thermographic screen for model heterogeneous catalysts

Abstract

Despite the fact that it is now accepted that the particle size and the support in heterogeneous catalysts strongly influences the activity and selectivity of the catalysts, it has been a challenge to measure such effects systematically, especially on model systems. A high-throughput (parallel) thermographic screening methodology is described here, which allows such measurements to be made. 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 model, supported catalysts in an exothermic reaction deposited on the membranes could then be monitored using a thermal (infra-red) imaging camera. The temperature of the catalyst under steady state reaction conditions was used as a measure of mass or specific catalytic activity. The effectiveness of the screening method to determine catalytic activity was demonstrated for titania supported platinum and gold model catalysts by studying the CO oxidation reaction. For both supported metals we observe strong particle size effects in the activity.

Platinum nanoparticles in the size range of ca. 1 to 8 nm and gold nanoparticles between ca. 1 to 6 nm in size were deposited on an amorphous titania support, as determined from transmission electron microscope image analysis. The reaction of CO and O₂ on these model catalysts was measured using the thermographic screening method. The model, supported catalysts were also characterised by TEM and XPS both before and after reaction. The reaction over titania supported platinum was investigated in the temperature range of 80 ºC to 240 ºC and pressure range 0.072 mbar to 2.4 mbar. The turnover frequency (TOF) and specific mass activity of Pt for CO oxidation increased monotonically with decreasing particle size by a factor of 17. XPS showed that there was no change in the particle size distribution during the reactions. While there is no apparent consensus in the literature concerning a particle size effect for this system, this result is in good agreement with recent findings on high area supported catalysts, finding a seven-fold increase in activity between 10 nm and 1nm particle sizes.

The CO oxidation on titania supported gold nanoparticles was investigated at 80 ºC and 170 ºC and pressure ranging between 0.06 mbar and 1.5 mbar. The TOF and specific mass activity increased monotonically with decreasing particle size by a factor of 19. XPS again showed that there was no significant change in the particle size distribution during the reactions. While there is no apparent consensus in the literature concerning the activity trend with particle size for this system, this result is consistent with most data published in the literature.

The absolute activities measured at low temperature (ca. 80 ºC) and pressure (0.11 mbar) in this study for Au/TiO₂ nanoparticle catalyst are higher than Pt/TiO₂ nanoparticles over the ranges of particle sizes investigated.

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Identifiers

ORCID for Brian E. Hayden: orcid.org/0000-0002-7762-1812

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