Stability and Temperature-Induced Agglomeration of Rh Nanoparticles Supported by CeO₂

Abstract

The effects of reduction by H2 and by heat treatment in vacuum and in O2 flow on Rh particle size changes of Rh/CeO2 samples were studied by X-ray photoelectron spectroscopy (XPS), high-resolution electron microscopy (HRTEM), and CO adsorption followed by diffuse reflectance infrared spectroscopy (DRIFTS). Low-temperature (373-423 K) reduction of Rh without agglomeration is demonstrated. An average particle size of 2.3 ± 1.1 nm was measured by HRTEM regardless of the metal loading (1-5%). On Rh/CeO2, a significant particle size increase of the Rh particles was detected on heating (773 K). In this work, we suggest that the temperature-induced surface decrease resulting from the sintering of Rh is favored only for well-dispersed particles. XP spectra revealed that the mobile oxygens of CeO2 fundamentally determine the oxidation state of the supported metals. At elevated temperature, the oxidation of the reduced support surface as well as the metal component takes place because of the segregation of ceria oxygens. When the aggregated particles were reoxidized, the redispersion of Rh was observed probably because of the formation of Rh-O-Ce bonds.

A new paradigm on the chemical potentials of components in multi-component nano-phases within multi-phase systems

“…According to eqn (4g), the standard chemical potential of a one-component nano-phase is proportional to the specic surface area of the phase. Comparing eqn (2) and (4g), parameter z for the free standing one-component nano-phase is identical to the specic surface area of the phase:…”

“…Eqn (2) and (6e) form the basis of the Kelvin paradigm. Using these equations, the classical Kelvin, Ostwald-Freundlich and Gibbs-Thomson equations are derived for spherical nanophases (see ESI A †).…”

“…Comparing eqn (2) and (11f), the following expression is obtained for parameter z for the nano-sized sessile droplet:…”

“…Let us substitute eqn (12a) into eqn (12d) and (e). Further, let us substitute the resulting equations, together with eqn (12b) and (c) into eqn (10) From the comparison of eqn (2) and (12f) the expression for parameter z is obtained as:…”

“…The steady development of nano-technologies [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] requires a solid theoretical basis and clear techniques to calculate the equilibrium of nano-systems. Nano-systems contain at least one phase with at least one of its dimensions below 100 nm.…”