## 4 Conclusions  
In this work, the reactivity of an OC was investigated in the presence of H\({}_{2}\) under high-altitude and low-temperature conditions. Combining experimental and modeling techniques, the research extended the conventional reaction mechanism to include alternative CO and HC oxidation pathways based on the formation of \(\cdot\)OH at the active sites of the catalyst. This reaction mechanism was incorporated into a dual-layer washcoat catalyst model sensitive to varying H\({}_{2}\) molar fractions on the CO and HC reactivity. The robustness of the model was validated against experimental data, accurately reflecting CO and HC conversion efficiencies across various environmental conditions. Introducing H\({}_{2}\) highly enhanced CO conversion efficiencies at specific temperature ranges and mass flow rates. At warm sea level, an optimal H\({}_{2}\) molar fraction of 5000 ppm was identified, beyond which no further benefits were observed. However, in cold high-altitude conditions, improvements were evident even with 10,000 ppm in H\({}_{2}\) molar fraction. A detailed analysis revealed that the greatest enhancement in reactivity due to H\({}_{2}\) occurred in the inlet regions of the monolith, primarily attributed to reactions with \(\cdot\)OH radicals. Additionally, a significant portion of the reactivity comes from the O\({}_{2}\) pathway, promoted by the reduction in CO molar fraction resulting from the \(\cdot\)OH pathway. In the case of HC, the lower selectivity of \(\cdot\)OH for this species, combined with its higher conversion at low temperatures due to the adsorption mechanism, results in a less pronounced improvement in HC conversion efficiency upon introducing H\({}_{2}\). This improvement was observed but required higher H\({}_{2}\) molar fractions at the inlet. Figure 10: HC selectivity to adsorption as a function of the exhaust mass flow and catalyst inlet temperature for 0 and 10,000 ppm in H\({}_{2}\) molar fraction at different ambient conditions: (**a**) 0 ppm—0 m & 20 \({}^{\circ}\)C; (**b**) 0 ppm—2500 m & –7 \({}^{\circ}\)C; (**c**) 10,000 ppm—0 m & 20 \({}^{\circ}\)C; and, (**d**) 10,000 ppm—2500 m & –7 \({}^{\circ}\)C.  
The impact of H\({}_{2}\) concentration on conversion efficiency was closely related to operational and environmental factors. Extremely low exhaust temperature conditions, below 100 \({}^{\circ}\)C, did not benefit from H\({}_{2}\)-induced reactivity enhancement. However, points with higher temperatures and moderate CO concentrations did benefit, with warm sea-level conditions requiring lower H\({}_{2}\) molar fractions than cold high-altitude conditions. Thus, it's advisable to adjust H\({}_{2}\) levels in the exhaust as a function of the engine operating conditions and ambient boundaries, avoiding its presence at low engine demand where it's ineffective and increasing it as ambient temperature decreases and altitude increases. Conceptualization, J.R.S. and P.P. ; Methodology, P.P. and C.C. ; Software, E.J.S. and C.C. ; Formal analysis, P.P. and E.J.S. ; Investigation, J.R.S., P.P., E.J.S., and C.C. ; Resources, J.R.S. ; Writing--original draft preparation, P.P., E.J.S., and C.C. ; Writing--review and editing, J.R.S., P.P., E.J.S., and C.C. ; Supervision, J.R.S. and P.P. ; Project administration, J.R.S. and P.P. ; Funding acquisition, J.R.S. and P.P. All authors have read and agreed to the published version of the manuscript. This research has been supported by Grant PID2020-114289 RB-I00 funded by the Spanish Ministerio de Ciencia e Innovacion, Agencia Estatal de Investigacion (MCIN/AEI/10.13039/ 501100011033). Additionally, [PERSON] thanks PhD funding from Universitat Politecnica de Valencia through grant FPI-2022-S2-49099, which is supported by project CIPROM/2021/061 funded by Generalitat Valenciana. Dataset available on request from the authors  
The authors declare no conflicts of interest.