## 3 Materials and Methods
### Characterization of the Solution Combustion Synthesized Catalysts  
Reversed-flow inverse gas chromatography (RF-IGC) was employed to study the hydrogen adsorption phenomena [37; 38; 39]. During the measurements, helium was the carrier gas with a 24 cm\({}^{3}\)/min constant volumetric flow rate, 0.5 mL H\({}_{2}\) was the injected reactant at atmospheric pressure, and the volume of the catalytic powder in the diffusion column was 5 cm\({}^{3}\). In order to double the chromatographic signals, the direction of the carrier gas flow was reversed twice every two minutes, as that was a shorter time than the gas holdup. All the experiments conducted by RF-IGC were performed at least 3 times.  
The temperature evolution during SCS and the temperature of the combustion wave were monitored in situ with a high-speed infrared camera (model A655 SC, FLIR Systems, Wilsonville, USA).  
An FEI CM20 TEM operating at 200 kV was utilized to study the microstructure and morphology of the synthesized catalysts. The TEM specimens were prepared by drop-casting 1 \(\upmu\)L of catalyst suspension in ethanol onto carbon-support Cu TEM grids.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Material** & **Manufacturer** & **Assay/Purity** \\ \hline Nickel (II) nitrate hexahydrate for analysis (Ni(NO3)2-6H2O) & Merck & 99.0\%–102.0\% \\ Glycine for synthesis (CH2 NH2 COOH) & PanReacAppliChem & 99.0\% \\ Maleic acid (HOOCCH=CHCOOH) & Riedel-de Haén & 99.0\% \\ Hydrogen (H2) & Air Liquide & 99.999\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Materials used for solution combustion synthesis (SCS) catalyst production and the hydrogenation reaction.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Number of Experiment** & **Water Added in the Precursor Solution** \\ \hline
25 & 50 mL \\
27 & 75 mL \\
28 & 25 mL \\
29 & 100 mL \\ \hline \hline \end{tabular}
\end{table}
Table 2: The conducted experiments.  
In order to depict the changes in surface electronic and chemical properties of the material, X-ray photoemission spectroscopy (XPS) measurements were carried out, where the photon energy is fixed and the electron intensity is measured as a function of electron kinetic energy.  
X-ray photoemission spectroscopy is a highly sensitive surface analysis technique with the signal originating from a thin layer at the surface (\(<\)3 nm). All the photoelectron spectra reported here were recorded on a load-locked Kratos XSAM800 system equipped with a dual anode X-ray source. The excitation source that was used is a Mg K\(\alpha\) (1253.6 eV), operated at 12 kV and 10 mA,. Throughout the measurements, the base pressure of the chamber was maintained below 10\({}^{-9}\) mbar. The electron energy analyzer was set to the high magnification mode to collect electrons from the smallest possible area on the specimen, approximately 4.0 mm\({}^{2}\). All spectra were calibrated with respect to the adventitious carbon at 284.8 eV present in the sample due to the ambient exposure and to the Au 4f\({}_{72}\) core lines (84 eV) as well. Metallic gold was kept in direct contact with the sample to align the Fermi level. The photoelectron spectra thus obtained were analyzed using Casa XPS software using a Shirley background subtraction algorithm. Core lines were fitted with a 50% mixture of Gaussian and Lorentzian line shape functions to keep the residue (x\({}^{2}\)) \(<\)1.