## 3 Results and Discussion
### CO Reactivity Analysis
flows, highlighting the role of the -OH reaction pathway and the decrease in inhibition caused by CO consumption due to this carboxylic pathway. Higher H2 molar fractions made this effect increasingly apparent. Unlike warm sea-level conditions, high molar fractions of H2 did not lead to saturation effects in cold high-altitude conditions at every operating point due to the inherently lower initial reactivity of the catalyst in the absence of H2. Therefore, significantly greater reactivity was observed when the H2 molar fraction was increased from 5000 to 10,000 ppm, especially in areas of high mass flow, which were highly affected by inhibition under CDC operation. To elucidate the improvement in CO reactivity observed in the regions depicted in Figures 4 and 5 and to understand the variability in the response to H2 introduction across different operating points, the analysis of the axial distribution of conversion efficiency throughout the catalyst was assessed. This involved identifying the regions of the OC that exhibited the highest reactivity at each operating point, as well as those areas most affected by the introduction of H2. For this purpose, three sections along the length of the monolith were selected. First, the catalyst inlet cross-section corresponds to 4.66 mm from the inlet of the monolith. Then, the middle cross-section was 69.99 mm, and the outlet cross-section was 135.34 mm. The study prioritizes the cold high-altitude case for a more detailed examination, given its more notable increase in reactivity attributed to higher levels of inhibition under CDC operation. Table 5 presents points A-E from Figure 5 as representative of operating points corresponding to the region where a significant increase in reactivity was observed upon introducing H2. These points are arranged in increasing order regarding inlet gas temperature and exhaust mass flow to the OC. Thus, Figure 6 presents the axial distribution of CO conversion efficiency along the monolith length for points A-E. Each subplot within Figure 6 corresponds to a different H2 molar fraction at the catalyst inlet gas. The results for the CDC operation are shown in Figure 6a revealed three different trends across the five operational points considered. At operating point A, no reactivity was observed across any section of the monolith due to the low inlet gas temperature (93 \({}^{\circ}\)C) and very high CO molar fraction (1194 ppm), resulting in significant inhibition. Points B and C exhibited higher inlet gas temperatures but also high CO molar fractions (616 ppm and 461 ppm, respectively). In these cases, some reactivity was observed, increasing along the monolith. As CO started to react at the inlet region of the monolith, its concentration decreased, thereby reducing inhibition in subsequent regions and enhancing overall reactivity along the monolith. In these scenarios, the entire length of the monolith contributed to the reaction, and an expanded catalyst volume would enhance CO conversion efficiency. Finally, points D and E, characterized by high inlet gas temperatures and low CO concentrations (low inhibition), demonstrated conversion efficiencies near 100%, with most conversion occurring at the inlet cross-section of the monolith. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Point**} & **Exhaust Mass** & **OC Inlet** & **CO Inlet** & **HC Inlet** & **Molar** \\  & **Flow [kg/h]** & **Temperature** & **Fraction [ppm]** & **Fraction [ppm]** \\ \hline A & 54 & 92.5 & 1194 & 838 \\ B & 59 & 147.6 & 616 & 257 \\ C & 60 & 158.8 & 461 & 268 \\ D & 66 & 207.2 & 308 & 175 \\ E & 68 & 253.0 & 222 & 97 \\ \hline \end{tabular}
\end{table} 
Table 5: Selected points for 2500 m & \(-7\)\({}^{\circ}\)C conditions. Upon introducing 500 ppm in the H2 molar fraction, as shown in Figure 6b, the conversion efficiency resulted largely consistent with the CDC scenario.