(317d) Alloying and Ligand Effects in Supported Gold Nanoparticle Catalysts for CO Oxidation

Alloying metals are an interesting and important method to modulate the chemistry of active sites in nanoparticles. This effect of alloying is more pronounced as nanoparticles become small enough to be classified as nanoclusters (< 2 nm). In this size range, the dispersion significantly increases and the metal utilization improves as almost all atoms are present on the surface and are available as active sites. Furthermore, small particle sizes result in a greater fraction of undercoordinated sites which are often very active for catalysis.

Introducing alloying metals to gold has a few different effects, namely the ability to compress or to strain the structure of intermetallic bonds, inducing a charge transfer from one metal to another, or to change the composition of the active site surface. These effects all alter the energy of the transition state and introduce a new chemical structure moving from one state to another. In addition to composition, the environment surrounding the surface plays a role in the activity of the catalyst. There are many efforts which aim to generate a “naked” nanoparticle with an entirely ligand-free surface; however, in this work, the aim is to observe changes in catalytic performance with ligands attached. The presence of the ligands is important to alter both the steric demand of reactants that reach a catalytically active surface and the possible ligand electron donation/withdrawal with the metallic surface. Bearing this in mind, we select two preparation methods. Strong Electrostatic Adsorption (SEA) for the “naked” nanoparticles, and a Sterically Demanding Ligand Synthesis (SDLS) for a phosphine capped nanoparticle.

This work demonstrates the effects of alloying Au with Cu, Co, Ni, Pd and Ru metals on the active site and the impact of a sterically demanding ligand such as triphenylphosphine (TPP) on both the selectivity and activity for gas-phase CO oxidation. Differences in metallic composition as well as ligand-metal interactions show observable differences in activity compared to monometallic Au. Ligand-bound nanoparticles show activity for CO oxidation in the range of 340-420 K, while the “naked” nanoparticle synthesis illustrates activity at a temperatures of about 300 K, with the exception of AuRu showing activity at a temperature of 330 K. Interestingly, AuNi bimetallic particles show the lowest activity for SDLS relative to the other alloys but show the highest activity for SEA synthesis relative to the other bimetallic nanoparticle compositions. STEM confirms all alloyed particles are in the size range of 2-5 nm, which have high fractions of undercoordinated bimetallic surface atoms, except AuRu which have larger particle sizes of 6.8±1.7 nm. Alloys show a shift in Enthalpy-Entropy compensation with ligand presence. We also observe trends in the material structure cell parameter with the activation energy of the material.

DRIFTS allows for the elucidation relationship between CO chemisorption properties and the structure of the materials. CO vibrations are present at about 2000 cm^-1 for Au-FCC metals while about 2060 cm^-1 for Au-HCP metals. The lower IR frequency indicates a higher electron density of the material due to greater pi-backbonding. Ligand coverage is a reasonable explanation for decreased activity in SDLS. The trends in the location of the chemisorbed CO in the IR spectra are consistent with the lower electron density that is observed in the “naked” nanoparticles which suggest that the ligand interaction provides an electron-rich surface. In addition, the activation energy trends with both the alloy composition as well as the presence of the ligand.