## 3 Results and Discussion
### Catalytic Tests  
N\(\mathrm{\SIUnitSymbolO}\) decomposition experiments were performed with a 1000 ppm N\(\mathrm{\SIUnitSymbolO}\)/He stream and the steady-state N\(\mathrm{\SIUnitSymbolO}\) decomposition rates and conversions obtained are plotted in Figure 4.  
According to these results, and in agreement with previous XRD, Raman spectroscopy and N\({}_{2}\) adsorption characterization, the decomposition conditions of cerium nitrate has no effect on the catalytic performance (profiles for Rh350 Ce25 and Rh350 Ce250 are almost equal) while rhodium nitrate \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Lattice** & **Crystal Size** & **Crystal Size** & **BET Surface Area** \\  & **Parameter (nm)** & _by [PERSON]_ (nm)** & **by [PERSON] (nm)** & **(m\({}^{2}\)/g)** \\ \hline Rh25/Ce25 & 0.5401 & 12 & 14 & 64 \\ Rh250/Ce25 & 0.5412 & 12 & 13 & 66 \\ Rh350/Ce25 & 0.5401 & 12 & 13 & 66 \\ Rh350/Ce250 & 0.5424 & 12 & 13 & 69 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: XRD and N\(\mathrm{\SIUnitSymbolO}\) adsorption characterization results. Figure 4: Steady-state N\(\mathrm{\SIUnitSymbolO}\) decomposition rates and conversions as a function of temperature. decomposition affects the activity. The catalyst where rhodium nitrate was decomposed in ramp (starting heating at 25 \({}^{\circ}\)C (Rh25 Ce25)), which is the most conventional calcination procedure, presents lower activity than catalysts calcined by a flash procedure (Rh250 Ce25 and Rh350 Ce25 catalysts). As postulated in a previous publication [18], this can be tentatively attributed to the improved interaction between rhodium and ceria particles obtained by flash calcinations and this interaction is studied in detail in the coming sections. Additional information about the catalyst performance is obtained from the N2O decomposition profiles as a function of time for the different temperatures, that is, from the behavior of the N2O decomposition profiles before the steady-state is achieved. The steady state was reached after 20-30 min of reaction for all temperatures and catalysts, except for experiments performed at 225 \({}^{\circ}\)C, where longer times were required to reach a constant N2O decomposition level. These curves are included in Figure 5 for selected catalysts (Rh25 Ce25 and Rh250 Ce25; rhodium decomposed by ramp and flash calcinations, respectively). From a qualitative point of view, both profiles are similar, with a first step of 35-40 min where the N2O decomposition level increased until a pseudo steady-state was reached. The conversions then increased again for 20-25 additional minutes before the real steady-state level was stabilized. Despite both profiles on Figure 5 being qualitatively similar, the sample Rh250 Ce25 needed more time to reach the final steady-state than Rh25 Ce25, the former also being more active than the latter. The transformations suffered by these two catalysts before the steady state was achieved at 225 \({}^{\circ}\)C were studied by XPS. Characterization by XPS of Fresh Catalysts and after in Situ Pre-Treatments with N2O at 225 \({}^{\circ}\)C  
XPS spectra were recorded with the fresh Rh250 Ce25 and Rh25 Ce25 catalysts and with these catalysts pre-treated _in situ_ with 1000 ppm N2O/He at 225 \({}^{\circ}\)C for different periods of time. The profiles obtained for the Rh 3d and Ce 3d transitions were qualitatively similar to those previously obtained with some other rhodium/ceria catalysts [21; 22; 29]. The percentages of Rh (0) (with regard to total rhodium) and Ce (III) (with regard to total cerium) were calculated similarly to that reported elsewhere [21; 22; 29]. Figure 6 compiles these Rh (0) and Ce (III) percentages. Figure 5: N2O decomposition at 225 \({}^{\circ}\)C as a function of time. Rhodium appeared fully oxidized on both fresh catalysts (\(t=0\) min), which was expected since the catalysts were calcined at 500 \({}^{\circ}\)C, and the Ce (III) percentages (38%-40%) were in accordance with values typically obtained with these materials [21,22,29].