Pure and Applied Chemistry

Abstract: Carbon deposition from catalytic methane decomposition has drawn increasing interest recently. Previously, we have found the carbon formation depends on the crystalline structure of the support, following the trend of Ni/CeO₂ > Ni/CaO > Ni/MgO, because Ni supported on MgO is uniformly dispersed and can stabilize high-x CH x intermediates. We have also found that the addition of Pt can inhibit the carbon deposition on Co/Al₂O₃ because the alloying between Pt and Co results in the better dispersion of Co on the support. Furthermore, it was revealed that by judging the Ni/Mg molar ratio from 1 to 0.25 we could reduce the diameter of deposited carbon nanotubes from 20 to 12 nm, with substantially smaller production rate. All of these previous studies indicated that better dispersion of the supported metal would benefit the decreasing of carbon deposition. Here we present our recent investigation of the effect of support particle size on the carbon deposition. Three different types of 10 wt% Co/Al₂O₃ catalysts were prepared: Co on commercial Al₂O₃ (Cat 1), Co on sol-gel-processed Al₂O₃ (Cat 2), and sol-gel-made homogeneous Co-in-Al₂O₃ (Cat 3). TEM showed that the diameter of the Co₃O₄ particles in sol-gel Al₂O₃ is only around 6 nm, while it is 20-40 nm in the commercial catalyst. By using XRD and FTIR, Co was identified as crystalline Co₃O₄ in the as-prepared Cat 1 sample, CoAl₂O₄ in Cat 2, and amorphous Al₂O₃ in Cat 3, indicating the best dispersion in Cat 3. Methane CO₂ reforming was studied on the three catalysts. Longer lifetime was measured for Cat 3 as compared to those on Cat 1 and Cat 2 (>20 h vs. 1 h). The support size effect is discussed.