## 2 Materials and Methods
### Characterization of the Catalysts  
Before any impregnation, the support used (Al\({}_{2}\)O\({}_{3}\)) was characterized by nitrogen physisorption. The surface area was determined from N\({}_{2}\) adsorption/desorption isotherms measured at \(-\)196 \({}^{\circ}\)C with a Brunauer-Emmett-Teller (BET) surface analyzer (Micromerities Model TRISTAR 3000) using BET and pore size distribution based on Barrett-Joyner-Halenda (BJH) methods. Before the analysis, the samples were outgassed at 250 \({}^{\circ}\)C under vacuum overnight.  
The H\({}_{2}\) chemisorption capacity was determined by a static volumetric vacuum technique at room temperature, except for Pd catalyst for which the experiment was run at 70 \({}^{\circ}\)C in order to avoid palladium hydride formation. Firstly, the catalyst was cleaned under ultra-high vacuum overnight to eliminate the adsorbed molecules. Then, the catalyst was reduced under 750 mbar of H\({}_{2}\) for 1 h at 300 \({}^{\circ}\)C, and purged under vacuum until reaching a low pressure (\(p<10^{-5}\) mbar). Two series of hydrogen adsorption isotherms were performed with an increasing hydrogen pressure, from 10 to 70 mbar in steps of 10 mbar. Between the two series, the sample was submitted to ultrahigh vacuum in order to remove weakly bound hydrogen. The adsorbed hydrogen quantity was calculated by extrapolating the isotherm to \(p=0\). The first isotherm allows determining the total amount of adsorbed hydrogen and the second one the amount of physisorbed hydrogen. The chemisorbed hydrogen amount was calculated by the difference between the two values for Ir/Al\({}_{2}\)O\({}_{3}\) and Pt/Al\({}_{2}\)O\({}_{3}\). Concerning the Rh/Al\({}_{2}\)O\({}_{3}\) and Pd/Al\({}_{2}\)O\({}_{3}\), only the first isotherm was used to determine the amount of adsorbed hydrogen due to a very low H\({}_{2}\) physisorption on these materials. The metal particle size was calculated considering a stoichiometry of H/M = 1, and assuming that particles correspond to cubes deposited on the support and exposing 5 faces [17], with an equal distribution between (1 1 1), (1 1 0) and (1 0 0) planes.  
In order to consolidate the results obtained with hydrogen chemisorption, transmission electron microscopy (TEM) analysis was performed with a JEOL 2100 electron microscope operating at 200 kV with a theoretical resolution of 0.19 nm. Catalysts were finely ground, dispersed in ethanol using an ultrasonic bath and finally deposited on the carbon film of a copper grid. The particle size distribution was estimated on the basis of TEM pictures analysis using ImageJ software and the average particle diameter was determined from the measurement performed on at minimum 100 particles.  
H\({}_{2}\) temperature programmed reductions (TPR) were performed on a Hiden Catlab equipped with QGA Hiden quadrupole mass spectrometer. The catalysts were first pretreated at 200 \({}^{\circ}\)C for 1 h under argon (20 mL/min Air Liquide 5.0). The sample was then cooled down to 80 \({}^{\circ}\)C and exposed to a mixture of 0.25% H\({}_{2}\) in an inert atmosphere. The TPR was then carried out using a ramp of 10 \({}^{\circ}\)C.min\({}^{-1}\) up to 500 \({}^{\circ}\)C.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Metal** & **Iridium** & **Palladium** & **Platinum** & **Rhodium** & **Ruthenium** \\ \hline precursor used & Ir(acac)\({}_{3}\) & Pd(acac)\({}_{2}\) & Pt(NO)\({}_{2}\)(NO)\({}_{3}\)\({}_{2}\) & Rh(NO)\({}_{3}\)\({}_{3}\) & Ru(acac)\({}_{3}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Metal precursors.