Metal-Support Interactions
Metal nanoparticles dispersed on oxide supports are the working form of many catalysts and sensors, and their size, shape and stability depend on how strongly the metal bonds to the oxide. Weakly interacting metals tend to sinter (particles grow and their number falls) on heating, while strongly interacting metals adhere better, can be partially oxidized at the interface, and may reduce the support. Adhesion energy is commonly linked to the oxygen affinity of the metal, for example its heat of oxide formation or heat of oxygen adsorption. See also Scanning tunneling microscopy and Transition metal oxide thermochemistry.
Gold versus molybdenum nanoparticles on TiO2(110) (2003)
Kitchin, Barteau and Chen used STM to compare Au and Mo nanoparticles deposited at room temperature on TiO2(110)-(1x2). Both formed 1 to 3 nm particles, but a flash anneal to 773 K grew Au from 2.7 to 5.5 nm diameter while Mo only went from under 2 nm to 2.7 nm. Longer anneals at 783 K did coarsen Mo, producing particles elongated along [001] that sat on the Ti rows with widths near one row spacing, which Au never showed. Using adhesion energies on alumina, heats of oxygen adsorption (about 86.5 kcal/mol O for Mo versus about 20 for Au) and NIST-JANAF heats of formation, the authors argue that partial oxidation of Mo at the interface by oxygen from titania is thermodynamically feasible while full oxidation to MoO2 or MoO3 is not, and that this interfacial bonding explains both the sintering resistance and the row-templated shape. They propose the heat of oxygen adsorption as an indicator of nanoparticle thermal stability on oxides (Kitchin et al. 2003). Summary: kitchin2003a.
Pd particle size and oxygen sensing on TiO2(001) (2003)
McCormick and coworkers deposited Pd on TiO2(001) and annealed it for 2 h to sinter the particles, finding by STM typical sizes of 1 to 6 nm at 300 K, 5 to 12 nm after 573 K and 10 to 23 nm after 773 K. Four-point probe measurements showed that only the 573 K surface substantially increased the resistivity response to O2 compared with bare TiO2(001). The authors propose that Pd particles adsorb O2 and spill oxygen over to the oxide, and that sintering at higher temperature reduces the number of effective sites, making the promotional effect of the supported metal strongly size dependent (McCormick et al. 2003). Summary: mccormick2003.
Pt and Fe on Mo2N supports (2024)
On Mo2N, Pt forms a well-dispersed overlayer whose geometry (Pt−Pt 2.74 Å) and electronic structure resemble Pt(111), in contrast to the 3D particles Pt forms on γ-Al2O3; powder Pt/Mo2N reached 65.4% dispersion by CO chemisorption versus 2.6% for Fe. In-situ XANES showed Pt mostly metallic but with a stronger white line than Pt foil, attributed to Pt−Mo2N electronic interaction, and the Pt/Mo2N interface shifted isopropanol chemistry from dehydration toward dehydrogenation (Porter et al. 2024). Summary: porter2024.