## 1 Introduction  
Proton exchange membrane (PEM) fuel cells represent a promising technology for electric power generation at low temperature from H\({}_{2}\). Especially in mobile applications, the hydrogen source is, generally, an organic compound (fossil or renewable), which is converted to hydrogen in a fuel processor (FP), consisting of a reformer and one or two water gas shift reactors [1; 2; 3; 4; 5; 6; 7; 8]. The so obtained H\({}_{2}\)-rich stream contains 0.5-1 vol.% CO, which is a poison for the fuel cell anode catalyst (Pt). To prevent catalyst deactivation, CO concentration has to be reduced to 10-100 ppm and then the stream has to be purified. To this aim, the catalytic PReferential OXidation of CO (CO-PROX) has been proposed [9; 10; 11; 12].  
CO-PROX catalysts have been studied to work under steady-state conditions and have been designed and selected to ensure high CO oxidation activity, selectivity, and stability. Pt- and Au-based catalysts have been first studied and some formulations have been proposed for commercial applications. Copper/ceria catalysts have been proposed as an alternative due to their good activity and selectivity at low temperatures [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32].  
The main drawback of CO-PROX is the competitive H\({}_{2}\) oxidation, showing a higher activation energy with respect to that of CO. At the high temperatures needed to obtain high CO conversions at reasonable contact times, H\({}_{2}\) oxidation becomes faster than CO oxidation [33; 34; 35]. In order to limit H\({}_{2}\) consumption, stringent control of the temperature profile is required and obtained by 2-4 adiabatic stages with intercooling [2; 3; 36; 37] or exchanging reactors [38]. For this reason, the volume of the CO-PROX section is very large and can correspond to more than 40% [36] or 50% [39] total volume of the fuel processor.  
Several efforts have been made in order to extend the temperature window corresponding to both high conversion and selectivity by doping CuO/CeO\({}_{2}\) with a proper cation [16; 21; 40; 41; 42; 43] or developing suitable preparation methods [14; 24; 44; 25] with successful results.  
In order to ensure a wide temperature window, the required O\({}_{2}\)/CO ratio should be higher than the stoichiometric value, whatever the catalyst. At these conditions, the CO-PROX reaction outlet stream would contain excess O\({}_{2}\), which would react with hydrogen at the PEM Pt anode. Accordingly, O\({}_{2}\) concentration should be kept lower than 10-100 ppm [45], thus increasing the sacrificial hydrogen.  
In this work, we propose a novel process for CO abatement over a CuO/CeO\({}_{2}\) catalyst based on 2 steps: CO adsorption and subsequent oxidation. In the CO adsorption step, the reactor acts as a trap for CO and a CO-free H\({}_{2}\) flow exits the reactor; in the second step the reactor works as a catalytic converter with oxidation of the CO adsorbed over the catalyst. Through this process, it is possible to achieve a CO concentration lower than 100 ppm without O\({}_{2}\) at the PEM fuel cell inlet, preventing H\({}_{2}\) consumption.  
This idea is based on previous papers showing that nano-ceria-based materials (CuO/CeO\({}_{2}\)) can adsorb CO at room temperature [46; 47]. More recently, we showed that nanometric copper/ceria is able not only to adsorb CO but also to partially oxidize it, releasing CO\({}_{2}\) at room temperature, due to easier oxygen mobility occurring in nanometric catalyst particles and also to the promotion of CO\({}_{2}\) desorption [48].  
We report both experimental tests and simulations of the proposed trap. Experiments were carried out on a structured monolith washcoated with nanometric CuO/CeO\({}_{2}\)[48].  
A mechanism was proposed and the kinetic model of both the adsorption and the oxidation phases was derived. Simulations of both adsorption and oxidation reactors were carried out at varying residence times, the CO inlet concentration and the duration of the oxidation step.