Catalytic partial oxidation of n-tetradecane using pyrochlores: Effect of Rh and Sr substitution

Abstract

The catalytic partial oxidation (CPOX) of transportation fuels into synthesis gas (H 2 + CO) for fuel cells is complicated by the large quantities of aromatics and sulfur-containing compounds commonly found in these fuels. Traditional supported metal catalysts are easily poisoned by these species which adsorb strongly onto the electron-rich metal clusters. The use of noble metal and/or oxide based catalyst systems may offer higher activity and stability, but only if the metal can be bound into a thermally stable structure. To that end, Rh metal was substituted into the structure of a lanthanum zirconate (LZ) pyrochlore to give La 2Rh y Zr (2− y) O (7- ξ,) (LRZ) to produce a strongly bound, well-dispersed metal which is active for CPOX. A second catalyst was prepared in which Sr was substituted for a portion of La in the LRZ structure, producing La (2− x) Sr x Rh y Zr (2− y) O (7- ξ) (LSRZ). Each of these pyrochlore catalysts, including the unsubstituted LZ, were characterized and screened for activity in the CPOX of n-tetradecane (TD), which is a surrogate for linear paraffins typical of diesel fuel. Results were compared to a commercial Rh/γ-Al 2O 3 catalyst. X-ray diffraction patterns of both the LZ and LRZ showed that each had the cubic unit-cell pyrochlore structure. However, substitution of Sr resulted in a binary perovskite-pyrochlore phase with a defect SrZrO 3 phase. Hydrogen pulse chemisorption and temperature programmed reduction studies confirmed that Rh metal was substituted into the structure of the LRZ and LSRZ, and was reducible. Activity screening with the CPOX of TD showed that the Rh substituted in both LRZ and LSRZ is able to retain activity-producing essentially equilibrium synthesis gas yields, as was the Rh/γ-Al 2O 3. Temperature programmed oxidation experiments performed after the CPOX of TD demonstrated that the amount of carbon was quantitatively similar for each catalyst (roughly 0.3 g carbon/g catalyst after each run), with the exception of LSRZ, which had significantly less carbon (0.17 g carbon/g catalyst). It is speculated that improved oxygen ion mobility in the LSRZ material, which resulted from Sr substitution, was responsible for the reduction in carbon formation on the surface.