## 4 Model
### Trap Simulation  
The effect of the time length of the oxidation phase is fundamental for an effective regeneration of the catalyst. To this end, we simulated the complete (isothermal) trap, by setting \(CO^{in}\) = 5000 ppm and \(\tau_{1}\) = 95 s. In Figure 13, CO\({}_{2}\) (a and CO (b), are shown as a function of time as obtained by simulating both the adsorption (Phase I) and the oxidation (Phase II) step. Figure 12: Plane residence time/CO inlet concentration, the bubble dimension is proportional to the time at which CO outlet concentration is equal to 100 ppm. When the CO outlet concentration reaches 100 ppm, the feed is switched to \(CO^{in}=0\) and O\({}_{2}\) is fed (5 vol %). The corresponding evolution of the catalyst sites is shown in Figure 14 (c and d) as well. During the adsorption phase, we observe that the active sites (sites Cu\({}^{2+}\)-O\({}^{2-}\)-Ce\({}^{4+}\)) start to be covered by CO (\(\theta_{1}\) decreases, \(\theta_{2}\) increases). The activation of oxygen transfer from the Ce sites to the Cu sites (step 4 of the reaction mechanism) should also be noted, since \(\theta_{6}\) slightly increases and \(\theta_{5}\) decreases. CO is present as adsorbed over the catalyst (\(\theta_{2}\)) and only partially oxidized to CO\({}_{2}\) (\(\theta_{3}\)). During the oxidation phase, both the Cu- (\(\theta_{1}\)) and Ce- (\(\theta_{5}\)) oxidized sites are restored. However, it is worth noting that the time required for reaching the initial values is quite high (\(\sim\)400 s). Figure 13: CO\({}_{2}\) (**a**) and CO (**b**) outlet concentration and site concentrations (**c**) as functions of time, complete trap simulation. \(\tau_{1}=95\) s. Adsorption phase: \(CO^{in}=5000\) ppm and \(O_{2}{}^{in}=0\). Oxidation phase: \(CO^{in}=0\); O\({}_{2}{}^{in}=5\) vol.%. In order to optimize the trap operation, we performed simulations of several subsequent cycles. In Figure 15, the adsorption/oxidation cycles are shown as obtained by setting the duration of the oxidation step at 100 s, whereas time length of the adsorption phase is adjusted by the time at which CO outlet concentration reaches 100 ppm, corresponding to about 85 s for the first adsorption phase, in agreement with above results. From the plot it appears that the amount of CO trapped/converted increases cycle by cycle, as suggested by the increasing length of the adsorption phase (84.6 s; 107.15 s; 462.15 s). The site concentrations as functions of time during the cycles are shown in Figure 14d as well. Excluding the first adsorption phase, \(\theta_{6}\) (i.e., reduced ceria sites) steadily decreases cycle by cycle, while oxidized ceria sites (\(\theta_{5}\)) and surface carbonates (\(\theta_{3}\)) increase on average, thus suggesting that CO  
Figure 14: CO\({}_{2}\) (**a**) and CO (**b**) concentrations and site concentrations (Cu-containing (**c**) and ceria (**d**) sites) as functions of time, complete trap simulation. \(\tau_{1}\) = 95 s. Adsorption phase: CO\({}^{in}\) = 5000 ppm and \(O_{2}{}^{in}\) = 0. Oxidation phase: CO\({}^{in}\) = 0; O\({}_{2}{}^{in}\) = 5 vol %. Time duration of the oxidation phase = 100 s.  
buffering on the surface improves the oxygen storage capacity of the catalyst. Accordingly, the fraction of reduced copper sites (\(\theta_{4}\)) is low. In order to improve the catalyst regeneration, we changed the time length of the oxidation phase. In Figure 15, the trap simulation results are shown in terms of CO and CO\({}_{2}\) concentrations and site distribution as calculated by assuming a double oxidation phase duration (200 s), stopping the adsorption phase when CO concentration reaches 100 ppm as before. Under these conditions, CO adsorption can last more than the oxidation phase starting from the second cycle. This is due to a deeper re-oxidation of the copper sites (Figure 15d) and to the decrease of \(\theta_{4}\) going down to zero at the end of each oxidation phase. As a consequence, the CO adsorbed/reacted and the CO\({}_{2}\) formed can be the same at each cycle.