The Energetics of Supported Metal Nanoparticles: Relationships to Sintering Rates and Catalytic Activity

Abstract

Transition metal nanoparticles on the surfaces of oxide and carbon support materials form the basis for most solid catalysts and electrocatalysts, and have important industrial applications such as fuel production, fuels, and pollution prevention. In this Account, I review my laboratory group's research toward the basic understanding of the effects of particle size and support material on catalytic properties. I focus on studies of well-defined model metal nanoparticle catalysts supported on single-crystalline oxide surfaces. My group structurally characterized such catalysts using a variety of ultrahigh vacuum surface science techniques. We then measured the energies of metal atoms in these supported nanoparticles, using adsorption calorimetry tools that we developed. These metal adsorption energies increase with increasing size of the nanoparticles, until their diameter exceeds about 6 nm. Below 6 nm, the nature of the oxide support surface reaches also greatly affects the metal adsorption energies. Using both adsorption calorimetry and temperature programmed desorption (TPD), we measured the energy of adsorbed catalytic intermediates on metal nanoparticles supported on single crystal oxide surfaces, as a function of particle size. The studies reveal correlations between a number of characteristics. These include the size- and support-dependent energies of metal surface atoms in supported metal nanoparticles, their rates of sintering, how strongly they bind small adsorbates, and their catalytic activity. The data are consistent with the following model: the more weakly the surface metal atom is attached to the nanomaterial, the more strongly it binds small adsorbates. Its strength of attachment to the nanomaterial is dominated by the number of metal-metal bonds which bind it there, but also by the strength of metal/oxide interfacial bonding. This same combination of bond strengths controls sintering rates as well: the less stable a surface metal atom is in the nanomaterial, the greater is the thermodynamic driving force for it to sinter, and the faster is its sintering rate. These correlations provide key insights into how and why specific structural properties of catalyst nanomaterials dictate their catalytic properties. For example, they explain why supported Au catalysts must contain Au nanoparticles smaller than about 6 nm to have high activity for combustion and selective oxidation reactions. Only below about 6 nm are the Au atoms so weakly attached to the catalyst that they bind oxygen sufficiently strongly to enable the activation of O₂. By characterizing this interplay between industrially important rates (of net catalytic reactions, of elementary steps in the catalytic mechanism, and of sintering) and their thermodynamic driving forces, we can achieve a deeper fundamental understanding of supported metal nanoparticle catalysts. This understanding may facilitate development of better catalytic nanomaterials for clean, sustainable energy technologies.