Raspberry-colloid-templated catalysts as a model thermocatalytic platform

Abstract

Nanoparticle (NP)-supported catalysts are critical to the industrial production of over 90% of the chemicals and raw materials used today. Their catalytic performance is predicated on a combination of geometric and electronic descriptors associated with the properties of the NPs, support, and the (NP–support) interactions between them. However, existing catalyst preparative methods of nucleating and/or immobilizing NPs on support surfaces do not permit independent variation of NP or support properties as NP nucleation and growth characteristics are dependent on the support chemistry and vice versa. Consequently, such interconnected material properties cannot enable systematic investigations whereby individual NP or support properties are independently tuned to elucidate unambiguous and valuable structure–property relationships to guide future catalyst designs. Separately, this challenge is also exacerbated under thermocatalytic reaction conditions of high temperature, pressure, and mechanical agitation, which accelerates NP sintering and uncontrolled NP size growth, further confounding catalytic analyses. An effective model catalytic platform for fundamental structure–property studies should thus possess two pre-requisites: independent tunability of structural properties (NP and support) and high thermomechanical stability to preserve these as-synthesized structural properties under typical reaction conditions. To address this gap, I adapted the raspberry-colloid-templating (RCT) strategy previously developed by the Aizenberg group. In Chapter 1, I outline the RCT synthetic methodology and highlight two key design features: partial NP entrenchment into the support which confers enhanced catalytic stability against NP sintering, and synthetic modularity for independent combinatorial variations of the catalyst’s building blocks and their spatial organization from the nanoscale to macroscale. These two unique features yield thermomechanically stable RCT catalysts with numerous degrees of freedom to isolate and independently tune potential catalytic descriptors, thereby facilitating unambiguous studies to derive newfound structure–property relationships that guide future catalyst designs. In the rest of this dissertation, I describe how I leveraged on these two key design features to employ the RCT strategy as a well-defined and synthetically robust model thermocatalytic platform to elucidate important structural insights into catalyst design that cannot be easily achieved using traditional catalyst preparation methods. Specifically, I highlight my investigations into three structural features found in practically all NP-supported catalysts: properties of NP ensembles as a collective entity, NP–support interfaces, and individual NP properties. First, I demonstrate how using pre-formed colloidal NPs, in combination with the synthetic decoupling of the NP and support formation steps in the RCT method, disentangle the effects of NP proximity (Chapter 2), a collective NP ensemble property, from the effects of NP size (Chapter 3), to independently tune catalytic activity and selectivity, respectively. Second, I illustrate how the support chemistry and NP embedding effects can be deconvoluted to accentuate catalytic contributions arising from NP–support interfacial sites (Chapter 4), while also revealing nanoscale wetting phenomena at the interface that I subsequently exploited to direct bimetallic catalyst synthesis (Chapter 5). Third, I show how the RCT method can be applied to isolate individual NP properties from (all) other potential structural descriptors to facilitate systematic evaluations into individual NPs properties. This point is exemplified through separate studies into nanoscale effects of the surface Pd ensemble sizes in dilute Pd-in-Au alloyed NPs on competitive reactant adsorption energetics (Chapter 6), and distinguishing the surface- and vapor-mediated sintering pathways of Pt and Pd diesel oxidation catalysts (Chapter 7). Finally, I summarize my work, provide an outlook on the RCT catalyst platform, and discuss future opportunities, challenges, and applications (Chapter 8).