## 2 Materials and Methods  
### Obtaining and Characterization of the Pd-Based Photocatalyst  
The Pd-based photocatalyst was obtained through Pd(II) adsorption from residual solution onto the Florisil impregnated with Cyphos IL-101. The Florisil and the Cyphos IL-101 were purchased from Sigma-Aldrich, Darmstadt, Germany. The adsorption of Pd (II) ions was studied in a previous work, and the dependence of the maximum adsorption capacity of the impregnated Florisil function of solid:liquid (S:L) ratio, pH, contact time and initial concentration of the Pd(II), was established [25]. It was put in evidence that the ionic liquid (Cyphos IL 101) enhance the adsorption capacity of Florisil in the removal process of Pd(II) ions from aqueous solutions. The adsorbent was obtained through impregnation of Florisil with trihexyl tetradecyl phosphonium chloride using the ultrasonication method of impregnation. Then, 0.1 g of IL was impregnated onto 1 g of Florisil. The sample was entitled F-IL). The maximum adsorption capacity developed by F-IL, of 133.3 mg/g, was obtained in the following conditions: solid:liquid ratio (S:L) of 2 g/L, pH = 3 and stirring time of 60 min [25]. Therefore, in the present paper, the same conditions were used to obtain the Pd-based photocatalyst. Were prepared higher quantities of photocatalyst containing different amount of Pd ions (1 mg Pd/g of F-IL--the sample being entitled F-IL-Pd1 and 2 mg Pd/g of F-IL--the sample being entitled F-IL-Pd2, respective).  
The used photocatalyst was analyzed by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which was coupled with energy-dispersive X-ray analysis (EDX) and by XPS analysis.  
The FTIR spectra (KBr pellets) of the samples were recorded on a Shimadzu IR Prestige-21 FTIR spectrophotometer (Shimadzu, Kyoto, Japan) in the range 400-4000 cm\({}^{-1}\). SEM micrographs were recorded using a Quanta FEG 250 microscope (Thermo Fisher Scientific, Waltham, MA, USA).  
TEM micrographs were obtained using an FEI Tecnai F20 G\({}^{2}\) TWIN Cryo-TEM (Thermo Fisher Scientific, Waltham, MA, USA). Initially, the samples were ground to reduce the size of the particles so as to make them compatible with TEM analysis. The powdered samples were placed over lacey carbon copper grids. After removing the excess powder, the samples were analyzed. The scanning transmission electron microscopy (STEM) feature of the microscope, together with the EDX detector (X-MaxN 80T model from Oxford Instruments, Abingdon, UK), allowed identifying elements present in the sample.  
X-ray photoelectron spectroscopy (XPS) was performed on a KRATOS Axis Nova (Kratos Analytical, Manchester, UK), using AlK\(\alpha\) radiation, with 20 mA current and 15 kV voltage (300 W). The XPS survey spectra for the Pd-based photocatalyst sample were collected in the range of \(-\)10-1200 eV with a resolution of 1 eV and a pass energy of 160 eV. The high-resolution spectra for all the elements identified from the survey spectra were collected using a pass energy of 20 eV and a step size of 0.1 eV. Data were analyzed using the Vision Processing software (Vision2 software, Version 2.2.10). The binding energy of the C1s peak was normalized to 285 eV.