Thermodynamics and Phase Diagrams of Ln-Rh-O Systems for Regenerative Catalysts

Finely dispersed rhodium metal supported on ceria is an important constituent of current automobile exhaust emission control systems. It is the catalyst element of choice for NO_x reduction. Low-temperature reduction of LnRhO₃ is a convenient method for generating Rh nanocrystals on oxide substrates. Composition and size of nanoparticles and their interparticle distance can be controlled by changing the composition of the oxide precursor and reduction conditions. Catalyst degradation occurs by two main mechanisms; catalyst agglomeration and soot deposition. Oxidation in air removes hydrocarbons and converts the metal to a rhodite. Subsequent low-temperature reduction restores the catalyst.

To understand the oxidation-reduction chemistry of Rh in catalyst systems, systematic studies were undertaken on thermodynamic properties of LnRhO₃ compounds exhibiting orthorhombic perovskite structure and phase diagrams of systems Ln-Rh-O. Solid-state electrochemical cells were used to measure the Gibbs energy of formation of several LnRhO₃ compounds over a range of temperature. From the results enthalpies and entropies of formation are derived. The results show interesting systematic trends. These studies provide the main thermodynamic information in the literature on lanthanide rhodites. Using the data for rhodites and auxiliary information from the literature on the binary systems, phase diagrams of systems Ln-Rh-O were computed using several different variables to obtain a panoramic view of phase relations in these systems. The diagrams suggest methods for improving catalyst performance by enhancing oxygen buffer capacity and reducing cost.