## 3 Results and Discussion
### CO Reactivity Analysis
This selectivity pattern may seem counterintuitive since the zones with the most pronounced improvements in conversion efficiency, namely points B and C, upon H\({}_{2}\) introduction, appeared least affected by the CO with -OH reaction pathway. Moreover, the selectivity for this reaction was more significant with 500 ppm in the H\({}_{2}\) molar fraction than with 10,000 ppm. This seemingly contradictory behavior can be elucidated by examining the interplay between oxidation reactions and inhibition dynamics, alongside the -OH coverage, as guided by Figure 8. This figure illustrates -OH coverage at both 500 and 10,000 ppm, focusing on the inlet and middle cross-sections of the monolith under cold high-altitude conditions. At points D and E, the CO conversion efficiency was initially high even under CDC operation. Therefore, the presence of H\({}_{2}\) in the exhaust gases did not alter significantly the reactivity with O\({}_{2}\). Consequently, as observed in Figure 8a, the relatively abundance of H\({}_{2}\) allowed for significant -OH coverage. However, at points B and C, the introduction of 500 ppm in the H\({}_{2}\) molar fraction enhanced the CO reactivity via the -OH pathway. Yet, this H\({}_{2}\) concentration was insufficient  
Figure 7: Axial distribution of CO selectivity to -OH along the cross-section of the monolith at 2500 m & \(-7\,^{\circ}\)C as a function of the exhaust mass flow and catalyst inlet temperature for 500 and 10,000 ppm in H\({}_{2}\) molar fraction: (**a**) 500 ppm—Inlet cross-section; (**b**) 500 ppm—Middle cross-section; (**c**) 10,000 ppm—Inlet cross-section; and (**d**) 10,000 ppm—Middle cross-section. to sustain high -OH coverages, limiting the reaction rate, particularly in the inlet cross-section, as shown in Figure 8a. With 10,000 ppm in H2 molar fraction, the ability to generate and maintain -OH coverage improved significantly in the inlet section (Figure 8c), leading to a substantial increase in CO concentration and decreased inhibition, thus facilitating the oxidation via the O2 pathway towards the rear end of the monolith. This resulted in high activity but low selectivity for the -OH reaction, as shown in Figure 7d. After analyzing all cases, it was found that there is a significant correlation between the -OH coverage and selectivity to this species, confirming that in operating points where conversion efficiency improved, most notably under cold high-altitude conditions, the reaction rate was limited by the capability to stabilize -OH coverage. This explains why no saturation in conversion efficiency enhancement was observed with higher H2 molar fractions at altitude. With 10,000 ppm of H2, the CO selectivity to -OH was not saturated, indicating a higher selectivity for reactions with O2. However, the reaction with -OH effectively reduced CO enough to decrease inhibition, thereby enabling reactivity with O2 and achieving high CO conversion efficiency. Moreover, at high temperatures, -OH coverage neared saturation at the inlet. These areas did not benefit from further H2 introduction. Conversely, low-temperature regions also reached -OH saturation, but the prevailing temperatures were too low for significant reactivity, hence not benefiting from increased H2 concentration. Therefore, it is evident that certain operating points do not gain from H2, whereas those that do require higher H2 molar fractions for cold high-altitude conditions compared to warm sea-level conditions. This is mainly due to the higher CO molar fractions in these conditions, which necessitated higher H2 molar fractions to stabilize sufficient -OH coverage to guarantee the abatement of CO.