Natural gas (methane/ CH
4) emissions are known to significantly accelerate global warming. Platinum (Pt) and palladium (Pd) catalysts with gaseous oxygen (O
2) are commonly used to convert CH
4 into less environmentally harmful CO
2. Despite their effectiveness, competitive adsorption between O
2 and CH
4 along with their differing site requirements may induce isothermal multiplicity as illustrated in Fig. 1A. Experimentally measured and model estimated CH
4 conversions at 525°C increase with O
2 concentration forming an “ignited” branch reaching a maximum of 60% at 3500 (experimental) ppm O
2. After which, the rate is extinguished to a lower branch with conversion of 8%. Decreasing the O
2 concentration starting on the extinguished branch at 525°C (Fig. 1A) leads to an ignition in conversion at 2100 ppm O
2 back onto the ignited branch. Unique solutions at equivalent experimental conditions beg the question as to which catalytic steps govern the individual branches. Mathematical formalisms including the degree of rate control can be computed along experimentally observed stable and unobserved unstable branches to elucidate their corresponding rate determining steps (RDSs). Fig. 1B shows that O
2 adsorption has a degree of rate control near unity and is thus the RDS along the excited branch. Conversely, CH
4 adsorption has the highest degree of rate control along the extinguished branch thereby denoting it the RDS. Degrees of rate control computed in Fig. 1B via a modified pseudo-arclength continuation algorithm demonstrate consistency as they sum to unity across all branches and coverages computed via site balances are equal to those estimated via thermodynamic degrees of rate control. This work not only identifies key RDSs along unique solutions of multivalued rates during catalytic methane oxidation but also demonstrates that degrees of rate control (and thus RDSs) are functions of both operating and initial conditions (path dependent).
