THRUST AREA: Catalysis

A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process.  “Supported heterogeneous catalysts” consist of catalytic active sites carried on the surfaces of porous solids, known as “supports”.  Such catalytic materials are especially important for chemical manufacturing because they minimize the loss of valuable catalytic materials and the need for catalyst separation and recovery.

Research Goals

Current research in catalysis nanoscience at USC seeks a deeper understanding of the relationships among synthesis procedures, catalyst nanostructures, and catalyst performance (activity, selectivity, lifetime) for synergistic bimetallic catalysts. Despite the substantial advances in heterogeneous catalysis over the last four decades, bimetallic catalysts (featuring active sites with two or more metallic elements) remain poorly understood and underutilized in practice.  Nonetheless, bimetallic systems have provided examples of some of the most active and selective catalysts yet discovered.  Bimetallic catalysts offer performance advantages due to "synergy":  the two active metals somehow cooperate to enhance activity and/or selectivity in ways not seen in catalysts employing the individual metals.  While many cases of synergy in bimetallic catalysts have been documented, we are just beginning to understand how the atomic and nanoscale structure of the bimetallic active sites results in synergistic performance. To achieve this understanding, we are investigating different synthetic techniques (described below) to create bimetallic nanoparticles with controlled particle size and composition.  Deposition of nanoparticles onto porous inorganic supports with subsequent activation leads to model bimetallic catalysts.  We evaluate the performance of these catalysts with an array of established techniques.  Theoretical modeling tools help us to analyze and rationalize experimental observations at every stage.

This approach has several advantages.  First, separation of the synthesis, deposition, and activation steps enables characterization of the nanostructure of the bimetallic active sites at every step of the catalyst preparation process.  Second, this approach promises unprecedented control over synthetic variables with the possibility of making bimetallic alloys, such as Pt-Au, with atomic proportions not attainable via conventional means.  Third, the approach will enable us to unambiguously relate nanostructural characteristics to catalyst performance, yielding mechanistic insights.  This knowledge will eventually allow us to design catalysts from first principles – that is, knowing the desired reaction and the possible side reactions, we will be able to design a bimetallic nanoparticle and support with optimal activity and selectivity for the target reaction.  We can use this capability to design – figuratively, and perhaps, literally – chemical manufacturing analogs of biological metabolic pathways.

Research Activities

Two well-established groups are working to synthesize new nanoparticle catalyst materials with enhanced activity and selectivity for target reactions.  Specifically, we are developing the means to control the size, shape, composition, and atom-level arrangement of bimetallic nanoparticles that will serve as catalytic active sites.  In addition, we are learning how to deliver the particles to supports and activate them – essentially nanoscale fabrication. 

Fig. 1:  Dendrimer-based catalyst synthesis.

Our synthesis efforts are focused in two main areas:  dendrimer-templated synthesis, and bimetallic cluster complexes.  Research on dendrimer-based routes to bimetallic catalysts is funded by the National Science Foundation via a $2M NIRT grant awarded in 2001.  Figure 1 depicts our catalyst synthesis scheme based on the use of poly(amidoamine) (PAMAM) dendrimers.  Certain metal ions (Cu⁺², Pt⁺², Pd⁺²) form complexes with the dendrimers’ interior chemical groups due to favorable physical interactions or ligand exchange reactions (Figure 1a-b).  Subsequent chemical reduction leads to precipitation of metallic nanoparticles.  The dendrimers stabilize the nanoparticles and serve as the delivery agent for self-assembly and immobilization of nanoparticles on porous inorganic supports (Figure 1b) as well as in 2-D and 3-D structures on surfaces (Figure 1c, d).  Oxidation and reduction treatments produce active catalysts without significant changes in nanoparticle size under optimized conditions. 

Fig. 2:  a bimetallic cluster complex, H₄Pt₃Ru₆(CO)₂₁

Synthesis efforts based on bimetallic cluster complexes share many of the same objectives.  In this approach, considerable effort goes into developing chemical synthesis routes to produce cluster complexes, such as one shown in Figure 2, with precisely defined structures and compositions.  Preparation of catalysts from these complexes begins with deposition of the complexes within porous inorganic supports (silica, alumina, etc.), followed by reduction with H₂ at elevated temperatures.  This produces nanoparticles in situ (Figure 3).  Compared to the dendrimer route, bimetallic complexes offer better control of composition, but they do not enable direct control of the final nanoparticle size.

Fig. 3:  Pt-Ru nanoparticles on MgO derived from a Pt-Ru bimetallic complex.