High-Pressure Kinetic Interactions between CO and H₂ during Syngas Catalytic Combustion on PdO

The catalytic combustion of H ₂ /CO/O ₂ /N ₂ mixtures over PdO was investigated at pressures 3 to 10 bar, H ₂ :CO volumetric ratios 1:5 to 3:1, and global equivalence ratios ψ = 0.13 and 0.23. The catalyst surface temperatures were controlled to 540-690 K, a range especially important for hybrid hetero-/homogeneous combustion approaches with large gas turbines at idle or part-load operation and for microreactors with recuperative small-scale turbines. In situ Raman measurements determined the major gas-phase species concentrations over the catalyst boundary layers in a channel-flow reactor, thermocouples monitored the surface temperatures, and surface characterization identified the catalyst oxidation state (PdO) and surface morphology. A 2-D CFD code with a detailed catalytic reaction mechanism simulated the experiments. Simulations and measurements of the combustion of the individual fuel components revealed pressure dependencies ~p ^0.74 and ~p ^0.10 for the CO and H ₂ reactivities, respectively, at the investigated equivalence ratios. In the combustion of H ₂ /CO blends, transition temperatures (TTRAN) were identified, below (above) which H ₂ inhibited (promoted) chemically the oxidation of CO. The transition temperatures decreased with increasing H ₂ :CO volumetric ratio, pressure, and equivalence ratio. Sensitivity analysis indicated that the H ₂ and O ₂ adsorption reactions had the larger inhibiting effect on CO oxidation, particularly at lower pressures. Comparisons with other noble metals showed that the PdO transition temperatures were higher than those on Pt and Rh. Even though this behavior favored Pt and Rh for the ignition of syngas in practical catalytic burners, the H ₂ and CO kinetic coupling (H ₂ inhibition) was considerably weaker on PdO at T < TTRAN, thus rendering PdO also potentially suitable for low temperature syngas ignition.