## 3 Results and Discussion
### CO Reactivity Analysis
Only minor differences at points B and C, with only a marginal increase in conversion efficiency in the initial cross-section of the monolith, contributed to increasing the reactivity along the whole monolith due to the early inhibition decrease. This enhanced conversion efficiency in the inlet region is attributed to the reaction pathways enabled by the presence of H2, particularly through the formation of -OH radicals on the catalytic surface. At this relatively low H2 concentration, the generation of these surface groups was limited, resulting in only a modest reactivity increase at the monolith inlet. Points D and E also experienced a boost in reactivity from the H2 presence at the inlet, with point D showing slightly higher reactivity. Compared to point E, it resulted in greater conversion efficiency due to its lower inlet mass flow and, hence, longer residence time. However, since both points already achieved near 100% conversion efficiency, no significant improvement was noted in the overall conversion efficiency increase map (Figure 5a). The enhancement effects were more pronounced when the H2 molar fraction was increased to 5000 and 10,000 ppm, as illustrated in Figure 6c,d. At points B and C, the increase in reactivity resulting from the high molar fraction of H2 led to greater availability of -OH radicals at the monolith inlet, consequently increasing the conversion efficiency to near 100% in these inlet regions. For operating points D and E, the enhanced reactivity improved conversion efficiency in the inlet region, but only marginally, as these points already face limitations due to mass transfer rather than reactivity. Furthermore, the elevated availability of -OH radicals with this high H2 concentration promoted certain reactivity even at operating point A, despite its low temperature. The findings depicted in Figure 6 demonstrated that H2 predominantly influences the inlet cross-section of the monolith, with the observed variations in reactivity in subsequent sections resulting from alterations in CO concentration initiated in these inlet cross-sections. To understand why the impact of H2 presence was primarily taking place at the inlet of the monolith, it is crucial to examine the selectivity of CO reactions with O2 and -OH across different operating points and cross-sections of the monolith. Figure 7 represents the selectivity of CO to -OH in the context of cold high-altitude conditions with 500 ppm  
Figure 6: Axial distribution of CO conversion efficiency with different H2 molar fractions for 2500 m & –7 °C and points A, B, C, D, and E from Table 5: (**a**) 0 ppm; (**b**) 500 ppm; (**c**) 5000 ppm; and (**d**) 10,000 ppm. (Figure 7a,b) and 10,000 ppm (Figure 7c,d) in H\({}_{2}\) molar fraction, at the inlet and medium cross-sections of the monolith. The reactivity of CO with -OH varied significantly between the inlet and middle cross-sections. In particular, the selectivity to CO with -OH was considerably higher in the inlet cross-section of the monolith for 500 ppm and 10,000 ppm in the H\({}_{2}\) molar fraction, approaching a value of 1 for most operating points, except in areas showing the most significant increases in conversion efficiency. This selectivity sharply declined along the monolith length, with much lower values in the middle cross-sections, reaching its peak at approximately 60% for 500 ppm and 80% for 10,000 ppm. These peaks in CO selectivity to -OH occurred in regions with minimal increases in conversion efficiency. The areas of high selectivity in the middle cross-section aligned with areas of low temperature where there was no reactivity (point A) and, to a lesser degree, with areas of high temperature where conversion efficiency was already elevated without H\({}_{2}\) (points D and E). In both scenarios, reactivity in these areas remained low due to unfavourable conditions that avoided oxidation (point A) or oxidation that had already occurred in the inlet cross-section, leaving no residual CO to react (points D and E).