Preferential oxidation of CO over Pt/γ-Al₂O₃ and Au/α-Fe₂O₃: Reactor design calculations and experimental results

We present calculations of the required oxygen excess and noble metal mass together with integral flow experiments for the preferential oxidation (PROX) of CO in simulated reformer gas (1% CO, low concentrations of O₂, 75% H₂, balance N₂) over Pt/γ-Al₂O₃ at 200°C and Au/α-Fe₂O₃ at 80°C under different load conditions, i.e., at different contact times. The calculations are based on kinetic data of both catalysts determined in differential flow experiments. It is demonstrated that these calculations give realistic values for the minimum noble metal mass and the oyxgen excess required for a desired CO conversion. At optimum contact-time, the minimum CO exit concentration on a Pt/γ-Al₂O₃ catalyst at 200°C was found to be 60 ppm, increasing to 200 ppm at five-fold higher contact-time. This is attributed to the reverse water-gas shift reaction taking place as a competing reaction to the selective CO oxidation over Pt/γ-Al₂O₃. On a Au/α-Fe₂O₃ catalyst, the minimum CO exit concentration of <3 ppm (detection limit) increases to 30 ppm by increasing the contact-time by a factor of five. A two-step PROX reactor using Pt/γ-Al₂O₃ in a first stage at 200°C to oxidize the majority of CO down to ∼1000 ppm and Au/α-Fe₂O₃ in a second stage at 80°C is proposed for the complete oxidation of CO under dynamic load conditions.