## 3 Results and Discussion
### HC Reactivity Analysis  
As regards the impact of H2 on HC reactivity, Figure 9 shows an improvement in HC conversion efficiency under warm sea-level and cold high-altitude scenarios. This figure illustrates the percentage increase in HC conversion efficiency as a function of the mass flow rate of exhaust gas and the inlet temperature, particularly at high molar fractions  
Figure 8: Axial distribution of -OH coverage 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 ppm and 10,000 ppm in H2 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.  
of H\({}_{2}\) (5000 and 10,000 ppm). It is observed that the most significant enhancements in HC conversion efficiency induced by H\({}_{2}\) corresponded with the conditions that were also the most affected by H\({}_{2}\) in terms of CO conversion efficiency, specifically at temperatures around 140 \({}^{\circ}\)C-150 \({}^{\circ}\)C and an exhaust mass flow ranging between 60 and 65 kg/h. However, in the case of HC, this improvement was less significant, as it aligned with zones where adsorption was dominant in the absence of H\({}_{2}\), leading to moderate HC conversion efficiencies, as shown in Figure 2b,d.  
Moreover, introducing H\({}_{2}\) gradually increased the HC conversion under warm sea levels. This was manifested by the fact that introducing 5000 ppm in the H\({}_{2}\) molar fraction (Figure 9a) led to maximum improvements of around 10%, while introducing 10,000 ppm resulted in a maximum improvement exceeding 20%. On the other hand, in cold high-altitude conditions, the enhancement was only relevant with the introduction of 10,000 ppm of H\({}_{2}\), as shown in Figure 9d, with conversion efficiency improvements exceeding 30% at point B, whereas introducing 5000 ppm (Figure 9c) yields only negligible impact (maximum improvement of around 5%).  
These results indicate that the impact of H\({}_{2}\) on HC required a higher H\({}_{2}\) molar fraction than in the CO case. It can be attributed to two complementary factors. Firstly, increased reactivity was facilitated by the reaction pathway between HC and -OH, according to reaction \(R_{12}\) detailed in Table 4. Given the inherently lower reactivity of HC compared to CO, significant coverage of -OH was required, requiring higher molar fractions of H\({}_{2}\). Secondly, greater efficiency in the CO conversion reaction reduced the inhibition caused by this species, facilitating the oxidation of HC with O\({}_{2}\)[16].  
Finally, the fact that the improvement in the efficiency of the HC reaction was more moderate compared to that of CO was because H\({}_{2}\) promoted the HC oxidation but in parallel to the adsorption process, which still played a relevant role. This replacement of one reaction mechanism by another was reflected in the selectivity to the HC adsorption, which is depicted in Figure 10 for warm sea-level conditions with 0 ppm of H\({}_{2}\) (plot a) and with 10,000 ppm of H\({}_{2}\) (plot b), and for cold high-altitude conditions with 0 ppm of H\({}_{2}\) (plot c) and with 10,000 ppm of H\({}_{2}\) (plot d).  
The analysis of Figure 10 showed a decrease in the selectivity for adsorption in HC upon the introduction of H\({}_{2}\). When 10,000 ppm of H\({}_{2}\) were introduced, the selectivity adsorption significantly decreased in the area with temperatures around 140-150 \({}^{\circ}\)C and exhaust mass flow of 60 to 65 kg/h, falling from 100% with 0 ppm of H\({}_{2}\) in both environmental conditions to 2% with 10,000 ppm of H\({}_{2}\) for the warm sea-level case and to 39% for the cold high-altitude case. This replacement of adsorption by \(\cdot\)OH oxidation was the most significant phenomenon related to the effect of H\({}_{2}\) on the HC conversion, not implying a particularly relevant change in the overall HC conversion efficiency.