Rhodium supported on tetragonal or monoclinic ZrO2 as catalyst for the partial oxidation of methane

The catalytic partial oxidation (CPO) of methane on Rh/ZrO2 catalysts was investigated at different Rh loading (0.04-3.3 wt%), by using both monoclinic (m-ZrO2) and tetragonal (t-ZrO2) zirconia as supports. m-ZRhx and t-ZRhx catalysts, were prepared by dry impregnation of zirconia supports with a solution of Rh(NO3)(3), and characterized by XRD, BET analysis, XPS, FTIR spectroscopy (using CO as probe molecule) and H-2/O-2 titration. CPO of methane was studied in a flow apparatus fed by a reactant mixture of CH4:O-2 = 2:1% (v/v) in N-2 (contact time tau similar or equal to 2 ms). Characterization results show that the nature and dispersion of supported Rh species strongly depends on the support. The H-2/O-2 titration showed that Rh species are more dispersed on t-ZrO2 than on m-ZrO2. The XPS results indicated the presence of Rh in the metallic state both in large and small Rh delta+ clusters. Consistently with XPS, FTIR with CO probe molecule revealed the coexistence of Rh-0-CO carbonyls on Rh-0 large particles and Rh+-(CO)(2) dicarbonyls arising from very small clusters. Rh species in the metallic state are more homogeneous (i.e. Rh-0 species with similar near-neighbor atoms) and Rh species with a lower metal character (i.e. those interacting with the support) are more abundant in the t-ZRhx samples than in the m-ZRhx ones. For methane CPO, Rh supported on the tetragonal zirconia is far more active and selective than Rh supported on the monoclinic zirconia. The catalytic results suggest that the different Rh dispersion alone cannot account for the different catalytic performances of the t-ZRhx and m-ZRhx samples. (c) 2013 Elsevier B.V. All rights reserved.