(169b) Tailoring CO Bond Energies Via Compositional Tailoring in Bimetallic Nanocatalysts

The past two decades have seen an explosive development in the synthesis of ?nanocatalysts?, i.e. catalytic materials which are synthesized with well-defined control over the structure of the materials from the nano-scale up. A growing number of studies have shown that ?nano-sizing? the active component in a catalyst can lead to dramatic increases in catalytic activity, and even ? as in the well-known case of ?nano?-gold ? to entirely new catalytic properties. At the same time, it has been well-known for many decades that the addition of a second metal can alter the catalytic properties (activity and, more significantly, selectivity) of a metal-based catalyst. However, relatively few studies have to-date looked at the combined effect of tailoring both size and composition in catalysis.

We are reporting on the synthesis of bimetallic nanocomposite catalysts, in which size and composition of the metal component as well as the morphology of the support structure are well-controlled. Bimetallic nanoparticles are embedded and confined into the pores of a nanostructured ceramic oxide via a microemulsion-templated sol-gel synthesis approach. These bimetallic nanocomposites are synthesized, characterized, and then evaluated with regard to their CO adsorption/desorption properties. The CO bond energy is chosen as a characteristic parameter since it plays a crucial role in a wide range of energy-related catalytic reactions (ranging from water-gas shift, to preferential CO oxidation and alcohol synthesis from syngas), and at the same time is a fairly easily characterized quantity that allows direct comparison between different catalyst materials. Based on a wide range of studies in the published literature, it is known that both particle size and the addition of promoters have a pronounced effect on CO bond energies, and it can hence be expected that a well-controlled tailoring of size and composition of the active component should allow significant control over this important catalytic parameter.

In the present contribution, we report on the synthesis of Pt-based bimetallic nanocomposites. PtM-BHA (M= Fe, Sn, Cu, Au; BHA= barium hexa-aluminate) are synthesized in a two-step process: First, Pt-BHA is prepared through a ?one-pot' synthesis of metal-ceramic nanocomposites, a route which has been developed in our laboratory in recent years for monometallic nanocomposite. This (monometallic) nanocomposite is then transformed into a bimetallic material via solution-based conversion strategies, either through a polyol-mediated process or a galvanic replacement route. A range of Pt-based nanocomposites were synthesized and then characterized by SEM, TEM(EDAX), XRD, BET/BJH, and CO TPD. Formation of true bimetallic nanoparticles is confirmed via XRD and electron diffraction.

We find that the ?conversion' process does not affect the morphology of the original nanocomposite significantly. The bimetallic particles are well dispersed in the ceramic matrix and the nanocomposites retain high specific surface areas of >300 m2/g. However, introduction of the second metal fundamentally changes the CO chemisorption property of PtM-BHA in comparison to pure Pt-BHA. Significant peak shifts are observed in CO-TPD both for PtSn/BHA and PtFe/BHA catalysts, while PtCu/BHA appears to show a superposition of the (monometallic) Pt/BHA and Cu/BHA CO-TPD patterns. Most significantly, the CO desorption energy can be gradually adjusted via adjusting the Pt:M ratio, as demonstrated for PtSn/BHA catalysts: With increasing Sn content, desorption peaks gradually shift towards lower temperatures, i.e. lower CO bond energies.

Synthesis, characterization, and reactive test of these bimetallic nanocomposites will be discussed in detail in the presentation.