(740a) Kinetic Phenomena In Low-Temperature CO Oxidation on Nanostructured Pt Catalysts

Nanostructured materials have found much attention in catalysis over the past decade due to the very high surface area and strongly increased activity observed in the ?nanosize regime?. While the majority of the published work to-date has focused on the effect of ?nano-sizing? the active phase of the catalyst, the low stability of these ? typically metallic - nanoparticles even at low reaction temperatures has so-far limited their applicability and created interest in stabilizing metal nanoparticles through embedding them in nanostructured porous supports. Utilizing such an approach, we have previously demonstrated the synthesis of highly active and sintering-resistant nanocomposite catalysts, in which metal nanoparticles are embedded into a high-temperature stabilized alumina matrix. These materials thus combine the high reactivity of nano-sized noble metal particles with the excellent stability of barium hexa-aluminates (BHA).

Here, we present a study of CO oxidation at low-temperature conditions (T<300K) over such Pt-BHA nanocomposite catalyst. The catalysts were synthesized, characterized via TEM, XRD, BET, and chemisorption, and then tested in low-temperature CO oxidation at low pressure and ambient pressure conditions. For the low-pressure investigations, reaction progress was measured by monitoring the adsorbed reactants via in-situ FTIR, while the ambient-pressure fixed-bed reactor studies monitored concentrations in the effluent stream via mass spectrometry. In excellent agreement between the two approaches we find an activation energy for CO oxidation over Pt-BHA which is significantly lower (~3-fold) than previously published values for conventional supported Pt catalysts. Direct comparison with a commercial Pt-Al2O3 catalyst confirms the strong reduction in activation energy.

Furthermore, we find a surprising CO2 ?hold-up? in the nanostructured BHA, which induces a delay in the appearance of the reaction product in the product stream. The CO2 hold-up is confirmed in fixed-bed reactor experiments comparing Pt-BHA with the commercial Pt-Al2O3 catalyst with and without a pure BHA layer behind the catalyst bed. Interestingly, this hold-up is not caused by pore diffusion limitations, but rather by the interaction between the product and the support material, and hence the transition to a true kinetic regime occurs with increasing temperature, rather than ? as usual ? with decreasing temperature, i.e. it gives rise to a ?wrong-way? behaviour of the transition from a transport-controlled to a true kinetic regime. We expect that such effects will become more prevalent with the increasing utilization of nanostructured support materials, and unawareness of this phenomenon might easily lead to misinterpretation of the kinetic data.

The results from the kinetic investigation, including the impact of the product ?hold-up' on the apparent reaction kinetics will be discussed in detail in the presentation.