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Impacts of Metal-Adsorbate Bonds on Photon Activation Mechanism and Nanoparticle Reconstruction in Heterogeneous Catalysis

  • Author(s): Kale, Matthew James
  • Advisor(s): Christopher, Phillip N
  • et al.
Abstract

In this work, we have developed methods to elucidate the fundamental influence of catalytically relevant adsorbates on industrially-relevant heterogeneous catalytic processes. Specifically, we focused on the strong interactions between carbon monoxide (CO) and supported Pt nanoparticle catalysts by measuring the impact bond formation on the geometric structure of Pt nanoparticles and exploiting the local electronic structure at the Pt-CO interface to activate targeted adsorbate-metal bonds via photoexcitation. Two examples of revealing the impact of adsorbates on heterogeneous metal catalysts and how these interactions can be exploited to control catalytic rates and selectivity are presented.

It is well known that the catalyst structure can play an important role in the observed reactivity of the catalyst (due to the different reactivity of surface metal atoms with varying coordination number), and it has been shown that adsorbates can alter catalyst surface structures under reaction conditions. However, the amount of catalyst restructuring in CO covered Pt surfaces has not been quantitatively measured in-situ and related to CO oxidation activity, a critical reaction system in the ubiquitous automotive catalytic converter. In-situ quantitative FTIR was utilized to measure the change in the fraction of well-coordinated (WC) and under-coordinated (UC) Pt catalytic sites due to the introduction of CO oxidation reaction conditions to Pt nanoparticle catalysts with diameters ranging from 1.5-20 nm. The results show that larger Pt particles undergo a significant and reversible surface reconstruction, increasing the fraction of UC catalytic sites by ~400%, when exposed to CO oxidation reaction conditions. By relating the quantitative IR measurements to reactivity measurements and a structure sensitivity model based on Density Functional Theory (DFT) calculations, it was demonstrated that WC Pt atoms are the active site for CO oxidation, but that CO-induced restructuring of Pt nanoparticle surfaces masks this effect in particle size dependent rate measurements. These results provide a complete picture of the structure sensitivity of CO oxidation on Pt catalysts, bridging previously existing discrepancies between theoretical and experimental reports.

The second example investigates the utilization of solar energy for inducing new pathways to activating targeted metal-adsorbate bonds, enabling control of catalytic reaction rates and selectivity on metal nanoparticle surfaces. It was demonstrated that the use of supported, sub 5-nanometer Pt nanoparticles as photocatalysts for CO oxidation enables the direct photoexcitation of hybridized Pt-CO electronic states with high cross-sections through a resonant process using 450 nm photons. Direct photoexcitation of the Pt-CO states enabled wavelength dependent control over the rate of CO oxidation and due to the adsorbate specificity of the bond activation mechanism, enables control over selectivity in the preferential CO oxidation reaction in H2 rich streams. These reports demonstrate the importance of understanding fundamental geometric and electronic impacts of strongly bound catalytically relevant adsorbates on catalytic metal surfaces, and how they impact reactivity and selectivity in important chemical reactions.

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