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, Eadh. The structural factors that control the metal/oxide adhesion energy are thus also keys for tuning catalytic performance. For a given oxide, Eadh increases with (deltaHsub,M--deltaHf,MOx)/OmegaM2/3 for the metal, where deltaHsub,M is its heat of sublimation, deltaHf,MOx is the standard heat of formation of that metal's most stable oxide (per mole of metal), and OmegaM is the atomic volume of the bulk solid metal. The value deltaHsub,M--deltaHf,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 OmegaM2/3 simply normalizes this energy to the area per metal atom, since Eadh is the adhesion energy per unit area. For a given metal, Eadh to different clean oxide surfaces increases as: MgO(100) approximately TiO2(110) < or = alpha-Al2O3(0001) < CeO2-x(111) < or = Fe3O4(111). Oxygen vacancies also increase Eadh, but surface hydroxyl groups appear to decrease Eadh, even though they increase the initial heat of metal adsorption.

Effect of Support Preparation and Nanoparticle Size on Catalyst–Support Interactions between Pt and Amorphous Silica

“…Both the net charge and the adhesion energy are important physicochemical properties, because the reactivity for nucleophilic/electrophilic reactants can be expected to scale with charge, and adhesion energy is directly related to NP sintering resistance. 14 Our results show that charge transfer from Pt to silica can significantly affect the local electronic structure of Pt surface sites at the Pt−silica perimeter. This suggests that this charge transfer may significantly affect the adsorption energies at the NP−support interface and, hence, result in reactivity scaling with NP−support interfacial perimeter purely due to a localized electronic effect resulting from metal−support interactions.…”

Magnetically Separable and Sustainable Nanostructured Catalysts for Heterogeneous Reduction of Nitroaromatics

“…Furthermore, industrial heterogeneous catalysts composed of supported metal particles or NPs were also prepared for practical applications. However, while supporting the catalyst can increase their stability, it decreases their reactivity [18]. Although using these catalysts achieved some important criteria of an ideal catalyst, such as reusability and cost effectiveness, significant drawbacks, including low catalytic activity and selectivity, sever reaction conditions, side reaction production and metal species leaching, remained unresolved [19].…”

Interaction of Zr with CeO2(111) Thin Film and Its Influence on Supported Ag Nanoparticles

“…Regarding the mechanism of metal induced reduction of CeO 2 films, there are several explanations can be found in the literatures: (1) metal to oxide charge transfer; (2) reverse spillover of lattice oxygen from the ceria surface to the metal; (3) metal-catalyzed desorption of surface oxygen atoms; (4) metal-catalyzed reactions of surface O atoms with background CO gas to make CO 2 ; and (5) metal-induced accumulation of subsurface oxygen vacancies from deep in ceria to the bottom of metal clusters 25,28,[49][50][51][52]. In the present case, it is evident that the reduction of CeO 2 surface is attributed to the oxidation of deposited Zr.…”