## 3 Results and Discussion
### CO Reactivity Analysis  
For the CO case, Figure 4 shows the percentage increase in CO conversion efficiency relative to the baseline results shown in Figure 2a under warm sea-level conditions, as a function of the exhaust mass flow and catalyst inlet temperature. Each plot is devoted to 500, 1000, 5000, and 10,000 ppm of H\({}_{2}\) molar fraction into the catalyst inlet gas, as previously detailed in Section 2. The findings indicated an enhancement in CO reactivity with increasing molar fractions of H\({}_{2}\), particularly evident within the low-temperature range under low flow condition. Figure 4: Increase in CO conversion efficiency at 0 m & 20 \({}^{\circ}\)C as a function of the exhaust mass flow and catalyst inlet temperature for different H\({}_{2}\) inlet molar fractions: (**a**) 500 ppm; (**b**) 1000 ppm; (**c**) 5000 ppm; and, (**d**) 10,000 ppm. Figure 3: CO and HC selectivity to O\({}_{2}\) and -OH in OC calibrated and OC reference: (**a**) CO oxidation by O\({}_{2}\); (**b**) HC oxidation by O\({}_{2}\); (**c**) CO oxidation by -OH; and, (**d**) HC oxidation by -OH. tions (around 100 \({}^{\circ}\)C and 35-50 kg/h). This effect extended to higher temperatures as the operating conditions transitioned to higher flow rates (around 150 \({}^{\circ}\)C with 60-70 kg/h). The improvement in conversion efficiency was most pronounced in the operating points with low conversion efficiency in the absence of H\({}_{2}\). As shown in Table 2, these points were characterized by elevated CO concentrations and low temperatures, avoiding the oxidation by O\({}_{2}\). The improvement in CO conversion efficiency consistently increased up to 5000 ppm in the H\({}_{2}\) molar fraction, resulting in a significant improvement above 60% at approximately 150 \({}^{\circ}\)C and an exhaust mass flow of 65 kg/h. This substantial increase differed from the 20% improvement observed at 1000 ppm and less than 10% at 500 ppm in H\({}_{2}\) molar fraction, accompanied by an expansion of the enhanced region to higher temperatures. However, the addition of 10,000 ppm of H\({}_{2}\) did not lead to a discernible improvement in reactivity beyond that achieved with 5000 ppm, being the enhancement stabilized at 60%. This trend indicated a saturation point beyond which further increases in H\({}_{2}\) concentration did not significantly impact reactivity. The effect of introducing H\({}_{2}\) on improving CO conversion efficiency was also observed under cold high-altitude conditions. Figure 5 shows the percentage increase in CO conversion efficiency relative to baseline results shown in Figure 2c, considering cold high-altitude scenarios influenced by exhaust mass flow and catalyst inlet temperature. Similar to the warm sea-level counterpart, the same molar fractions of H\({}_{2}\) were introduced into the catalyst inlet gas. An enhancement band was observed at 150 \({}^{\circ}\)C in the low-to-mid mass flow range (30-60 kg/h) for the cases of 1000, 5000, and 10,000 ppm of H\({}_{2}\) molar fraction. In contrast to the warm sea-level case, introducing 10,000 ppm further extends this band to the highest mass flow rates (70-80 kg/h). The difference in the increase in CO conversion efficiency profiles with respect to the warm sea-level case was primarily attributed to high CO raw concentration in cold high-altitude conditions (see Table 3). This limited the effectiveness of H\({}_{2}\) in enhancing CO reactivity. At lower molar fractions of H\({}_{2}\), such as 500 ppm, the enhancement in conversion efficiency was negligible and limited to regions with low exhaust mass flow and temperature, where the concentration of CO and inhibition were reduced. However, as the H\({}_{2}\) molar fraction increased, the enhancement region shifted towards higher exhaust mass  
Figure 5: Increase in CO conversion efficiency at 2500 m & \(-7\)\({}^{\circ}\)C as a function of the exhaust mass flow and catalyst inlet temperature for different H\({}_{2}\) inlet molar fractions: (**a**) 500 ppm; (**b**) 1000 ppm; (**c**) 5000 ppm; and (**d**) 10,000 ppm.