Abstract
The performance of a 0.5wt.% Rh/MgO catalyst for the oxidative removal of CO from H2-rich methanol reformate (1% CO, 65% H2, 10% H2O, rest CO2) was investigated and compared to that of a conventional 0.5wt.% Pt/γ-Al2O3 catalyst and a 5wt.% Ru/γ-Al2O3 catalyst. Temperature screening experiments and 10–30h activity measurements reveal a high activity and selectivity in the temperature region between 175 and 300°C, in combination with a relatively low deactivation. Water vapor in the feed gas (≤10%) has little effect on the reaction characteristics. The low activity for the reverse water gas shift (RWGS) reaction, whose rate is more than four orders of magnitude lower than the rate for CO oxidation, and for the methanation reaction at temperatures below 300°C allow to operate Rh/MgO at higher temperatures, around 250°C, while for Pt/γ-Al2O3 and Ru/γ-Al2O3 the reaction temperatures are limited to 200 and 150°C, respectively, due to the decrease in selectivity (Pt/γ-Al2O3) and the onset of the methanation reaction (Ru/γ-Al2O3) at higher temperatures.Model calculations, using kinetic data measured in realistic reformate, predict that the minimum amount of noble metal required for the complete removal of CO from the feed gas (≤10ppm) is by two orders of magnitude lower for Rh/MgO operated at 250°C than for Pt/γ-Al2O3 and Ru/γ-Al2O3 catalysts at 200 and 150°C, respectively, at a comparable O2 excess. Due to the low RWGS activity the CO exit concentration does not increase significantly above the tolerance limit of state-of-the-art Pt–Ru fuel cell anodes of 100ppm under dynamic load conditions, down to 1% load. For the other two catalysts the CO exit concentration are significantly higher under these conditions, reaching values above 500ppm for Ru/γ-Al2O3 at 1% load and the WGS equilibrium value of 2000ppm for Pt/γ-Al2O3. The predictions are verified by measurements in a microreactor under similar reaction conditions.The reaction characteristics make Rh/MgO an ideal PROX catalyst for reaction at higher temperatures (250°C), in particular for applications requiring highly dynamic operation under variable load conditions.
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