Effects of H 2O and CO 2 addition in catalytic partial oxidation of methane on Rh

Abstract

The autothermal catalytic partial oxidation (CPO) of methane was performed at short contact times (∼8 ms) over three Rh-based catalysts: 5 wt% Rh/ α-Al 2O 3, 5 wt% Rh/2 wt% γ-Al 2O 3/ α-Al 2O 3, and 5 wt% Rh/2 wt% Ce/2 wt% γ-Al 2O 3/ α-Al 2O 3. The effects of H 2O addition (10%, 20%, and 40% of the total feed) and CO 2 addition (20% and 35%) were studied over a wide range of inlet C/O ratios (0.75–1.2) by means of the capillary sampling technique. Over Rh/ α-Al 2O 3 samples, spatially resolved concentration profiles revealed that the rate of CH 4 reforming is independent of the concentration of H 2O and CO 2. Differences in the product distribution followed the behavior expected from water gas shift (WGS) chemistry: in H 2O-rich tests, the production of H 2 and CO 2 increased at the expense of H 2O and CO, while the opposite was observed in CO 2-rich tests. CPO experiments with a simultaneous feed of H 2O and CO 2 (40% H 2O, 20% CO 2, 20% CH 4, C/O = 1) provided direct evidence that H 2O is the preferential co-reactant of CH 4 in reforming and that CO 2 reforming is absent. Addition of a γ-Al 2O 3 washcoat to the catalyst significantly enhanced the rate of steam reforming while revealing the limits of Rh WGS activity. Products were equilibrated for nearly all cases examined over the Ce-promoted catalyst, indicating that WGS kinetics, either forward or reverse, have an important role in the CPO mechanism, particularly with H 2O and CO 2 co-feed. The experimental results clearly illustrate the flexible nature of CPO process. They show that the H 2/CO can be varied within a wide range of values in a one-step process, and syngas production is sustainable under highly diluted conditions.