Catalytic performance of rhodium-based catalysts for CO preferential oxidation in H2-rich gases

CO preferential oxidation (CO-PROX) can lead to a reduction of the CO content in the hydrogen-rich gas derived from hydrocarbon re-forming down to at least 10 ppmv or below, so as to enable its direct feed to standard polymer electrolyte membrane fuel cells (PEM FCs). Rh-based catalysts supported on A zeolites (3A, 4A, and 5A), alumina, titania, and ceria were prepared and tested for potential application in CO-PROX operating over a temperature range compatible with PEM FCs (80-100 degrees C). Among the prepared catalysts, the 1 % Rh-zeolite 3A catalyst, tested with a weight space velocity (WSV) of 0.66 N.L.min(-1).g(cat.)(-1), was found to be the most suitable one for the CO-PROX at low temperature: it reduced the inlet CO concentration below 10 ppmv within a temperature range of at least 80-120 degrees C without the appearance of undesirable side reactions. Tests at progressively lower O-to-CO feed ratio and the same WSV value were carried out for the sake of reducing H-2 consumption and improving CO-PROX selectivity. For 1% Rh-3A zeolite the minimum lambda value, ensuring a sufficiently wide temperature range of a nearly complete CO conversion at temperatures compatible with PEM FCs operation, was found to be equal to 3. Finally, to decrease the catalyst cost, the Rh load on the catalyst was tentatively reduced from 1 to 0.5%. A better distribution of the active element crystallites over the support surface was even obtained for this last catalyst. When operating at lambda = 3 and at WSV = 0.66 N.L.min(-1).g(cat.)(-1), the 0.5% Rh-3A catalyst could effectively reduce the inlet CO concentration below 10 ppmv within a temperature range of 100-140 degrees C, without the appearance of undesired side reactions.