## 3 Results and Discussion  
### TG-MS Study of Metal Precursors Decomposition  
Ce(NO\({}_{3}\))\({}_{3}\cdot\)6H\(\beta\)O and Rh(NO\({}_{3}\))\({}_{3}\)/Ce25 decomposition was studied by TG-MS. The results obtained are plotted in Figure 1. In both experiments, most water molecules (_m/z_ 18) evolved below 250 \({}^{\circ}\)C with the corresponding weight loss. During Ce(NO\({}_{3}\))\({}_{3}\cdot\)6H\({}_{2}\)O decomposition (Figure 1a), three well-defined _m/z_ 18 peaks were observed assigned to coordination water release. In the Rh(NO\({}_{3}\))\({}_{3}\)/Ce25 decomposition experiment (Figure 1b) a broad _m_/_z_ 18 band with two maxima appeared. It must be outlined the difference in the Y-axis scales due to the low Rh(NO\({}_{3}\))\({}_{3}\) percentage of the sample. Cerium nitrate (Figure 1a) decomposed in a relatively narrow range of temperature (240-320 \({}^{\circ}\)C) with NO (_m_/_z_ 30), NO\({}_{2}\) (_m_/_z_ 46) and O\({}_{2}\) (_m_/_z_ 32, which is not shown for clarity) release. This result lead us to select 25 and 250 \({}^{\circ}\)C for the preparation of the ceria supports in ramp (slow water evolution) and flash (rapid water evolution) calcination conditions, respectively. Rhodium nitrate impregnated on ceria (Figure 1b) decomposed in the range 200-550 \({}^{\circ}\)C, and decomposition in such wide range of temperature is an evidence of the interaction of the noble metal species (nitrate and/or oxide) with the ceria support (otherwise a narrower range of decomposition temperatures would be expected). Taking the results of Figure 1b into account, three temperatures were selected to start the calcination treatments for catalysts preparation: 25, 250 and 350 \({}^{\circ}\)C. In the calcination from 25 \({}^{\circ}\)C, water release will be slow and the impregnated rhodium salt will be allowed to move on the ceria particles surface forced by the concentration gradients created during drying. Once dry, rhodium nitrate is expected to be distributed on the ceria surface more heterogeneously than if water is released very rapidly (as in calcinations starting at 250 or 350 \({}^{\circ}\)C), and this heterogeneity is expected to  
Figure 1: Metal precursors decomposition followed by TG-MS. (**a**) Ce(NO\({}_{3}\))\({}_{3}\cdot 6\)H\({}_{2}\)O and (**b**) Rh(NO\({}_{3}\))\({}_{3}\)/Ce25. affect the final rhodium particle size, as it will be demonstrated afterwards. In the calcination starting at 250 \({}^{\circ}\)C, most of the water release will be very rapid but not the rhodium nitrate decomposition, while starting at 350 \({}^{\circ}\)C both water release and rhodium nitrate decomposition will be fast. X-ray Diffraction, Raman Spectroscopy and \(N_{2}\) Adsorption at \(-\)196 \({}^{\circ}\)C Characterization  
All catalysts were characterized by XRD (Figure 2) and Raman spectroscopy (Figure 3), and both techniques provide complementary information about the structure of the ceria supports. X-ray radiation penetrates more than 1 \(\upmu\)m into the solid giving information about the bulk structure, in particular about the cations' sublattice. On the contrary, Raman spectroscopy usually gives information of 10-100 \(\upmu\)m deep, and in the case of cerium oxides, is useful to analyze the oxygen anions' sublattice. All X-Ray diffractograms (Figure 2) show fluorite structure characteristic reflections, corresponding to the planes (111), (200), (220), and (311). No other peaks but those of fluorite were observed in the  
Figure 3: Raman spectra of fresh catalysts. Figure 2: XRD patterns of fresh catalysts. diffractograms [23] indicating that rhodium species should be highly dispersed. The average crystallite sizes of the ceria particles were determined with the Scherrer and Williamson-Hall equations. The values obtained are included in Table 1 together with the BET surface areas. Similar crystallite sizes of around 13 nm and BET surface areas of \(66\pm 2\) m\({}^{2}\)/g have been found for all catalysts. Raman spectra included in Figure 3 are also similar for all catalysts.