## 3 Results
### Temperature Programmed Reduction (TPR) by H\({}_{2}\)  
The H\({}_{2}\)-TPR measurements were performed to determine the reduction behaviors and oxidation state of various catalysts as depicted in Figure 3. Since the Al\({}_{2}\)O\({}_{3}\) support is irreducible from room temperature to 500 \({}^{\circ}\)C, the peaks of reduction in this temperature range should correspond to the reduction of the different kinds of metallic active phase species.  
Figure 3: H\({}_{2}\)-TPR (temperature programmed reduction) patterns of the different catalysts (\({}^{\circ}\)C).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Catalysts** & **Coding** & **Metallic Accessibility (\%)** & **\textless{}d\textgreater{}a (nm)** & **\textless{}d\textgreater{}b (nm)** \\ \hline
1\% Ir/Al\({}_{2}\)O\({}_{3}\) & Ir & 42 & 2.2 & - \\ \hline
1\% Pd/Al\({}_{2}\)O\({}_{3}\) & Pd & 40 & 2.4 & 2.9 \\ \hline
1\% Pt/Al\({}_{2}\)O\({}_{3}\) & Pt & 40 & 2.3 & 2.2 \\ \hline
1\% Rh/Al\({}_{2}\)O\({}_{3}\) & Rh & 41 & 2.2 & 2.8 \\ \hline
1\% Ru/Al\({}_{2}\)O\({}_{3}\) & Ru & 28 & 3.8 & 3.8 \\ \hline \hline \end{tabular}  
* Obtained by H\({}_{2}\) chemisorption analysis. \({}^{\text{b}}\) Estimated by TEM pictures.  
\end{table}
Table 3: Properties of catalysts: particles size and metallic accessibility.  
Figure 2: Transmission electron microscope (TEM) pictures and size distribution of the Al\({}_{2}\)O\({}_{3}\) supported Pd, Pt, Rh and Ru catalysts.  
The TPR profile of Pd supported is characterized by one broad pic centered at around 180 \({}^{\circ}\)C. According to [PERSON] and al. [18] this peak is associated with the reduction of PdOx species that strongly interacts with the support. However, no negative peak was observed at 80 \({}^{\circ}\)C corresponding to the desorption of hydrogen from PdHx which could be formed at low temperature [16; 19].  
The profile of Ru shows a peak with a maximum at about 194 \({}^{\circ}\)C, which is assigned to Ru oxide reduction already demonstrated by [PERSON] et al. [20].  
The Rh profile is composed of a broad reduction peak from ambient to 250 \({}^{\circ}\)C which is related to the reduction of surface RhOx species, according to the literature [21; 22].  
The Ir catalyst exhibits a broad reduction peak between 100-400 \({}^{\circ}\)C, which can be attributed to the reduction of IrO\({}_{2}\) phase [23; 24]. However, usually the reduction IrO\({}_{2}\) phase occurred at higher temperature.  
No reduction peak was observed for Pt meaning that Pt nanoparticles should mainly exist in the metallic form (Pt\({}^{\circ}\)) not accessible for further reduction [23].  
As all catalysts were reduced at the end of the preparation and stored under air, some metals can be re-oxidized at room temperature either only in the first layers of the particles surface or until to core oxidation, according to their oxidation resistance and their particles size. Checking metals oxidation state before NO/CO reaction using TPR technique is therefore of great interest. As expected, Pd was deeply re-oxidized as the hydrogen consumption evidenced whereas Rh and Ru were more lightly re-oxidized during the storage. For Ir, the peak temperature observed is lower than usual suggesting that an unstable oxide is formed probably on the first layers of metallic particles. As it concerns platinum, the lack of reduction peak means that the reduction step went to completion during synthesis and that no re-oxidation occurred under air at room temperature or so weakly that the Ar treatment before TPR can remove the oxygen [23].  
[PERSON] et al. [25] have shown that NO reduction by CO occurs on stable iridium metal sites. We can therefore assume that the more metallic the species is, the better the activity will be for the reaction studied. Thus, a good platinum activity is expected.