**2.2. Supported metal oxide composites via ultrasound-assisted synthesis**

Metal oxides find important application as catalysts, but agglomeration during their synthesis reduces the active surface area beneficial for catalysis. The combination of solid support and metal has been proposed, since composite materials can isolate the nanoparticle on the supporting material and effectively reduce its size. As a result, such nanocomposites can increase the surface area of metal oxide catalyst [14–16]. Silica, one of the most widely used catalyst supports, is synthesized in this study in the nanoscale and used as a metal oxide support material. Another material considered as support for nanocomposites is graphene oxide (GO). It is compatible with various organic/inorganic nanomaterials, taking advantage of possible chemical modification utilizing the oxygen-containing functional group on its sheet. In addition, it has high chemical stability and specific surface area [17–20]. The synthesis of silica and graphene oxide-based catalysts is presented, where ultrasound was used to reduce reaction time, simplify reaction steps, or perform synthesis in milder conditions.
