Surface interactions in the system RhAl2O3

Under oxidizing conditions Rh is easily dispersed on γ-Al₂O₃ up to a saturation concentration of > 10% of the support area. The average oxidation state of the dispersed phase is Rh³⁺. Excess Rh is present in the three-dimensional Rh₂O₃ particles. The interaction with the support in air is weak at <600 °C and both the dispersed and three-dimensional phases are easily reduced. Multiple chemisorption of CO, NO, and H is noted on the dispersed phase. In the limit two adsorbate molecules reside on the surface Rh in very dilute samples. The multiplicity is a strong function of Rh concentration and invalidates the measurement of accessible metal area in dilute samples. H₂-O₂ titration, with proper precautions, gives more reliable results, due to a constant surface oxide stoichiometry. In heat treatment (>600 °C), Rh-oxide interacts with both γ-Al₂O₃ and γ-Al₂O₃, diffusing into the subsurface region and the bulk. This process can only be partially reversed by reduction in H₂ (>550 °C). Thus, the measured value of surface Rh will depend also on the prereduction procedure. Exposure of $\text{Rh}\text{Al}{}_2\text{O}{}_3$ catalysts to high temperature under oxidizing conditions will cause loss of active area by both particle growth and by diffusion into the bulk of the support. In concentrated samples one can distinguish among the three Rh states: dispersed on surface, particulate, and dissolved in the support.