62nd Annual Report on Research 2017

The research in the Huang Group continues focusing on the design, synthesis, and characterization of well-defined nanostructures for applications in heterogeneous catalysis. Using well-defined nanocatalysts, our long-term goal is to understand the structure-catalytic property relationship, which will be used to guide the future design of catalysts to achieve efficient, economical, and environmentally friendly chemical transformations.

Impact of Proposed Research. Precious metals and metal alloys are important heterogeneous petrochemical catalysts. However, precious metals have low natural abundances that hinder their large-scale application. Using precious metal-containing alloy in catalysis could reduce the usage of the expensive metals, but metal alloys typically have unstable surface structures due to the segregation of elements under reaction conditions, which renders the identification of their surface active sites and the understanding of reaction mechanisms difficult. This research project is designed to address these limitations by using pure phase intermetallic nanoparticles (iNPs) as model catalysts. Intermetallic compounds (IMCs), consisting of two or more metallic/metalloid elements, adopt specific, well-defined crystal structures that are distinctly different from random alloys. The modified geometric and electronic structures of IMCs make them enticing catalytic materials because they will alter the binding configuration and energies of surface adsorbates (reactants, intermediates, and products), and thus their catalytic activity and selectivity. Considering the availability of IMCs with different structures (~100,000 discovered so far), their catalytic properties remain largely unexplored.

Figure 1. Converting mSiO₂-encapsulated Pt NPs to PtM iNPs.

Our efforts started with the first-time discovery of a room temperature active catalyst for carbon monoxide (CO) oxidation, NaAu₂ IMC (J. Am. Chem. Soc. 2013, 135, 9592-9595). However, the NaAu₂ IMC is a bulk material with a very low surface area (< 0.1m²/g) that is not preferred for catalysis. In our last report, we summarized a newly developed “ship-in-a-bottle” method as shown in Figure 1 to prepare iNPs with high surface-to-volume ratios (Nanoscale 2015, 7, 16721-16728). Using this method, we further prepared Pt₃Sn, PtSn, PtPb, Pt₃Zn, and PtZn iNPs. We used furfural hydrogenation to test the activity and selectivity of these iNPs and found that the specific activity per surface Pt site on PtSn@mSiO₂ is about 40 times that of Pt@mSiO₂ (ACS Catal. 2016, 6, 1754-1763). In PtSn iNPs, Pt benefits from an ordered environment, which is deemed responsible for the observed high activity and selectivity in the hydrogenation of furfural to furfuryl alcohol. The mesoporous silica shell not only allows the access of reactant molecules to the encapsulated iNPs but also prevents the aggregation of the iNPs during high-temperature treatments up to 750 °C. This “ship-in-a-bottle” strategy provides a new route to synthesize iNP catalysts with a broad spectrum of compositions and uses.

During this funding year, we extended the application of the iNPs in the selective hydrogenation of nitrostyrene to aminostyrene. In nitrostyrene hydrogenation, we found that Pt, Pt₃Sn, and Pt₃Zn showed high activity in nitrostyrene hydrogenation, but lead to low selectivity to aminostyrene. On the contrary, PtSn and PtZn showed low activity but high selectivity to aminostyrene. These iNPs was studied extensively with many bulk and surface characterization techniques. With these detailed characterizations, we aim to build the correlation between the structure and catalytic properties of these iNPs. This work is currently under review and will be discussed in detail in our next narrative report.

Another significant achievement during this period is the differentiation of the capping agent effect from surface structure of bimetallic nanoparticles in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol. Many contradictory publications attributed the enhanced catalytic selectivity of many bimetallic NPs to either the surface-modifying ligands or the addition of the secondary metals. For example, alkylamine ligands adsorbed on bimetallic Pt₃Co surface could enhance the selective hydrogenation of α,β-unsaturated aldehydes. On the contrary, the enhanced selectivity in the same reaction for oleylamine-capped PtCo NPs was attributed solely to the surface structure changes induced by Co atoms. In many bimetallic catalyst systems, it is hard to differentiate the roles of secondary metals or surface capping agents, thus impeding the rational design of bimetallic catalysts. Using mesoporous silica encapsulated PtFe bimetallic NPs, we could study the bare PtFe surface free of any organic capping agents. The separated study on the effect of the surface structure or capping agent was made possible by the mesoporous silica shell that allows the pretreatment of the PtFe nanoparticles to a high temperature for a complete removal of any organic capping agent used during synthesis. We found that alloy Pt with Fe along enhanced the cinnamyl alcohol selectivity in cinnamaldehyde hydrogenation. Meanwhile, adding fluorinated capping agent could further enhance the activity and selectivity of PtFe in this reaction. This work is also under review, and we will discuss it in detail in our next report.

During the current funding year, we have also made Pd-, Rh-, and Au-based iNPs using the ship-in-a-bottle strategy. We are currently studying the catalytic properties of these iNPs for selective oxidation and reduction reactions. Research in this area will allow us to establish the correlation of catalytic properties of iNPs with their surface geometric and electronic structures. This knowledge will serve as a guideline for the design of superior catalysts for important chemical transformation processes.

Impact of the ACS-PRF Award on Students and the PI. This ACS-PRF award has supported four graduate students during this funding year. We have published three peer-reviewed journal papers and two more are under review currently. These students learned the fundamental colloidal synthesis methods in preparation well-controlled nanostructures and many characterization techniques, such as transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy (FTIR). They also learned the operation of flow reactors to evaluate catalytic properties of nanomaterials. This knowledge will benefit them for their future career.

This DNI award is also a great support for the PI’s early academic career to develop this new research direction on catalysis using iNPs. Through this support, the PI’s group has acquired critical preliminary results for the application of funds from other federal agencies. This research also brought many interesting collaborations between the PI’s group and other research institutes.