## 3 Materials and Methods
### Flame Spray Pyrolysis  
Flame spray pyrolysis, often called FSP, is a method that allows us to obtain single or mixed metal oxides which are thermally stable, with a high degree of purity and an interesting specific surface area. The metal or metals whose oxide is to be obtained are contained in organometallic salts, called precursors. The precursors are dissolved in a \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Catalyst** & **Reaction Medium** & **Light Source** & \begin{tabular}{c} **Productivity** \\ **(mol/kg\({}_{\text{cat}}\) h)** \\ \end{tabular} & **Reference** \\ \hline \multirow{4}{*}{Co\({}_{3}\)O\({}_{4}\)} & \multirow{4}{*}{\begin{tabular}{c} Aqueous suspension in \\ water \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} LED lamp \\ 21 W, \(\lambda\) = 510 \\ \end{tabular} } & \multirow{4}{*}{[38]} \\  & & & & \\ \hline \multirow{4}{*}{ZnO-Cu\({}_{2}\)O} & \multirow{4}{*}{\begin{tabular}{c} Aqueous suspension in \\ water and \\ ZnO-Cu\({}_{2}\)O \\ 300 W Xe lamp \\ 0.2 M Na\({}_{2}\)CO\({}_{3}\) \\ \end{tabular} } & \multirow{4}{*}{[39]} \\  & & & & \\ \hline \multirow{4}{*}{TiO\({}_{2}\) FSP} & \multirow{4}{*}{\begin{tabular}{c} Aqueous suspension in \\ water, HS 1.66 g/L of \\ Na\({}_{2}\)SO\({}_{3}\) \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} Medium \\ Hg-pressure UV \\ lamp 250 W \\ \end{tabular} } & \multirow{4}{*}{13.9} & \multirow{4}{*}{This work} \\  & & & & \\ \hline \multirow{2}{*}{WO\({}_{3}\)/TiO\({}_{2}\) (40/60) & \multirow{2}{*}{\
\begin{tabular}{c} Aqueous suspension in \\ water, HS 1.66 g/L of \\ Na\({}_{2}\)SO\({}_{3}\) \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} Medium \\ Hg-pressure UV \\ lamp 250 W \\ \end{tabular} } & \multirow{2}{*}{36.5} & \multirow{2}{*}{This work} \\  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._combustible organic solvent, and the solution thus obtained is fed by means of a syringe pump through a needle in a vertical nozzle placed in the center of the burner. A scheme of the FSP apparatus is reported in Figure 7. The end of the capillary tube is coaxially lapped by a flow of O2, which acts as a combument and at the same time has the function of dispersing the solution into very fine droplets. The heterogeneous O2/solution mixture is ignited by twelve CH4/O2 flames, arranged in a cone around the nozzle. The instantaneous dispersion and vaporization of the solution droplets and the decomposition (pyrolysis) of the precursor take place in the flame, giving rise to the first oxide nuclei, which increase by coalescence and condensation. The primary nanoparticles thus obtained aggregate in the final part of the flame, forming the desired powders. The powders are collected from the dust collection glass chamber on which they are deposited. The size of the formed particles depends on the temperature of the flame, on the residence time and on the concentration of particles in the flame. When the values of these parameters increase, the probability that sintering phenomena will occur increases as well, leading to the formation of larger particles with lower surface areas. The FSP process is affected by many parameters, including the nature of the solvent, the concentration and flowrate of the precursor solution, the flow of the oxygen and methane and the overpressure of the oxygen used for the dispersion of the solution. The latter can play a significant role in the final surface area of the catalyst, and for this reason, a manometer is used to constantly control this value. FSP can be defined as one of the so-called non-conventional synthesis techniques. Several advantages over traditional synthesis methods like thermal annealing, sol-gel and wet impregnation are present, but it also comes with some challenges. One of its main strengths is the ability to produce nanoparticles in a single step, without the need for multiple drying and calcination stages. This translates as well to better control over the particle size and dispersion.