Anchored metal nanoparticles: Effects of support and size on their energy, sintering resistance and reactivity

Abstract

Many catalysts consist of metal nanoparticles anchored to the surfaces of oxide supports. These are key elements in technologies for the clean production and use of fuels and chemicals. We show here that the chemical reactivity of the surface metal atoms on these nanoparticles is closely related to their chemical potential: the higher their chemical potential, the more strongly they bond to small adsorbates. Controlling their chemical potential by tuning the structural details of the material can thus be used to tune their reactivity. As their chemical potential increases, this also makes the metal surface less noble, effectively pushing its behavior upwards and to the left in the periodic table. Also, when the metal atoms are in a nanoparticle with higher chemical potential, they experience a larger thermodynamic driving force to sinter. Calorimetric measurements of metal vapor adsorption energies onto clean oxide surfaces in ultrahigh vacuum show that the chemical potential increases with decreasing particle size below 6 nm, and, for a given size, decreases with the adhesion energy between the metal and its support, E_adh. The structural factors that control the metal/oxide adhesion energy are thus also keys for tuning catalytic performance. For a given oxide, E_adh increases with (ΔH_sub,M − ΔH_f,MOx)/Ω_M^2/3 for the metal, where ΔH_sub,M is its heat of sublimation, ΔH_f,MOx is the standard heat of formation of that metal's most stable oxide (per mole of metal), and Ω_M is the atomic volume of the bulk solid metal. The value ΔH_sub,M − ΔH_f,MOx equals the heat of formation of that metal's oxide from a gaseous metal atom plus O₂(g), so it reflects the strength of the chemical bonds which that metal atom can make to oxygen, and Ω_M^2/3 simply normalizes this energy to the area per metal atom, since E_adh is the adhesion energy per unit area. For a given metal, E_adh to different clean oxide surfaces increases as: MgO(100) ≈ TiO₂(110) ≤ α-Al₂O₃(0001) < CeO_2−x(111) ≤ Fe₃O₄(111). Oxygen vacancies also increase E_adh, but surface hydroxyl groups appear to decrease E_adh, even though they increase the initial heat of metal adsorption.