Person:

Lim, Kang Rui Garrick

Nanoparticle-supported Pt and Pd catalysts are employed industrially to convert CO and hydrocarbon residue from incomplete diesel fuel combustion. However, these catalysts deactivate over time due to sintering, especially for Pt nanoparticles which readily generate volatile species under high operating temperatures. Here, we turned the detrimental vapor-mediated sintering of Pt into an advantage by using a physical mixture of Pt and Pd catalysts prepared using a raspberry-colloid-templating (RCT) method. The RCT method produced Pt/Al2O3 and Pd/Al2O3 catalysts with partially embedded NPs to inhibit surface-mediated sintering pathways. As validated using an industry-defined emission control test protocol, aging a physical mixture of Pt/Al2O3 and Pd/Al2O3 at high temperature produced an alloyed PtPd/Al2O3 catalyst that outperformed the fresh catalyst mixture and both individual catalysts for hydrocarbon conversion, while exhibiting high catalytic stability and resistance to sintering and to SO2 poisoning. X-ray photoelectron spectroscopy revealed that in the aged catalyst mixture, half of the Pd content existed in their more active metallic state, compared to the less active oxide forms in the fresh mixture and both individual catalysts, explaining the unusual activity enhancement. Our results represent a practical approach to producing active and stable PtPd/Al2O3 diesel oxidation catalysts for emission control applications.

In the one-pot reaction between nitroarenes, aldehydes, and hydrogen, the desired outcome is the selective hydrogenation of nitroarenes to form aminoarenes that condense with aldehydes to form pharmaceutically relevant imines and N-alkylamines. One approach to facilitate the selective hydrogenation of nitroarenes over aldehydes involves using bimetallic catalysts with near equimolar ratios. However, structural characterization of metallic ensembles on the nanoparticle surface is challenging at such high alloying ratios, which hinders the elucidation of clear structure–property relationships. Here, we prepared a well-controlled series of dilute Pd-in-Au alloy catalysts with a fixed nanoparticle size as a model system to investigate the effects of surface Pd ensemble size, from single atoms to dimers and trimers, in the one-pot hydrogenation reaction between nitrobenzene and benzaldehyde as our probe reaction. The highest (near unity) selectivity to condensation products was achieved using the catalyst with the lowest Pd content prepared (Pd2Au98/SiO2), which predominantly exposed surface Pd single atoms as verified by surface-sensitive spectroscopy. Theoretical calculations reveal that Pd single atoms were inactive for benzaldehyde adsorption and thus enabled selective nitrobenzene hydrogenation. On the contrary, the adsorption of benzaldehyde became stronger than nitrobenzene for Pd trimers and larger ensembles, explaining the enhanced competitive adsorption from benzaldehyde with increasing Pd content. Our results demonstrate that the commonly used (near equimolar) alloying ratio is rather arbitrary and may not necessarily produce the highest selectivity to condensation products. Instead, we illustrate how nanoscale Pd ensemble size control tunes competitive kinetics to steer selectivity towards forming the desired condensation products.

Nanoparticle (NP)-supported catalysts are critical to the production of over 90% of 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 interactions between them. However, existing catalyst preparative methods of nucleating or immobilizing NPs on support surfaces do not permit independent variation of either 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 the catalytic role(s) of each structural descriptor or their combination, especially if their contributions exert orthogonal effects on catalytic performance.

To address this gap, we devised a raspberry-colloid-templating (RCT) strategy. In this Account, we outline the RCT synthetic methodology and highlight two key design features: partial NP embedding into the support which enhances catalytic stability against NP sintering while maintaining high reactant accessibility, and synthetic modularity for independent combinatorial variations of the catalyst’s building blocks and in their spatial organization. These two features yield thermomechanically stable RCT catalysts with multiple degrees of freedom at different length scales to isolate and independently tune potential catalytic descriptors, thereby deriving unambiguous structure–property relationships to guide future catalyst designs.

We describe how we leveraged these two key design features to employ the RCT strategy as a well-defined and synthetically robust model thermocatalytic platform to deconvolute the individual effects of traditionally coupled structural descriptors and elucidate important insights into catalyst design that cannot be easily achieved using conventional catalyst preparative methods. We highlight our recent investigations into three structural features found in all NP-supported catalysts: individual NP properties, properties of NP ensembles as a collective entity, and NP–support interfaces. First, we show how using pre-formed colloidal NPs in the RCT method decouples the NP and support formation steps to facilitate systematic evaluations of individual NPs properties. We exemplify this point through separate studies into the nanoscale effects of Pd ensemble sizes on the surfaces of PdAu alloyed NPs on reactant adsorption energetics, and the sintering behavior of Pt and Pd NP diesel oxidation catalysts. Second, we demonstrate how the colloidal templating steps in the RCT strategy controls the NP spatial localization to tune NP proximity, a collective NP ensemble property, at a fixed NP size, to induce local enrichment of reaction intermediates within the pore structure to direct catalytic selectivity. Third, we illustrate how partial embedding of NPs in RCT catalysts not only accentuates catalytic contributions arising from NP–support interfacial sites, but also reveals nanoscale wetting effects at the interface that we exploited to direct bimetallic catalyst synthesis. Interspersed throughout this Account, we also describe the advanced characterization and modelling tools adapted to probe the RCT catalyst structure and establish structural insights underpinning some of our main results. Finally, we provide an outlook on the RCT catalyst platform and speculate on its future opportunities, challenges, and practical applications.