## 4 Discussion
In contrast, an important feature of Au/Al is the presence of gold with quite large particles size. This catalyst does not show any activity and this finding is not surprising. [PERSON] et al. [25] have also observed very low conversion of propene over Au/Al\({}_{2}\)O\({}_{3}\). The crucial impact of gold particles size on oxidation activity has been widely discussed [10]. Numerous studies have pointed out the role of highly dispersed gold particles stabilized over high surface area supports, in particular reducible oxides as CeO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), etc., to enhance VOCs oxidation activity. However, the deposition of Pd on inert Al\({}_{2}\)O\({}_{3}\) resulted in relatively high activity of Pd/Al (85% conversion at 100 \({}^{\circ}\)C). On the one hand, the explanation could be related to the ability of PdO to provide active oxygen. The CBO activity of Au/CeAl and Au/YCeAl is governed solely by enhanced active oxygen provision from affected by gold ceria or Y-doped ceria phase which is 30 wt.% only. Much better catalytic behavior in CBO of Pd-based catalysts originated from contribution to the oxidation activity of both Pd located on ceria or Y-doped ceria phase and Pd dispersed on alumina. The positive effect of Y-doping on oxygen supply from ceria was disclosed by XPS analysis (higher ratio Ce\({}^{3+}\)/Ce\({}^{3+}\)+Ce\({}^{4+}\)) and it agreed well with the observed superior activity of Pd/YCeAl catalyst (Figure 2a). On the other hand, the hydrocarbon activation is also appreciable. The role of gold and palladium on one side, and support on the other in both benzene and oxygen activation seems responsible for the differences in catalytic performance. As commented above, the role of gold for oxygen and benzene activation is via enhanced oxygen vacancies creation at ceria surface. Big gold particles on alumina do not contribute to the CBO activity. Concerning the high performance of Pd catalysts, [PERSON] et al. have concluded that it can be related to the ability of PdO to provide oxygen and also to the participation of metallic Pd sites in the VOCs decomposition [43] and references therein. Hydrocarbon adsorption is generally found to occur at Pd surface through \(\pi\) bonds and backdonation from the metal to the \(\pi^{*}\)-hydrocarbon orbitals [7] and references therein. Studying metal-promoted C-H activation, [PERSON] et al. have commented that electron poor metals in high oxidation states may react with the electron rich arenes--the vacant Pd\({}^{2+}\) orbitals will be attracted to the \(\pi\)-electron rich aromatic sites and the electron transfer will result in organic molecule oxidation and Pd\({}^{0}\) formation, transferring by the oxidizing system back to its initial Pd\({}^{2+}\) state [44]. Concerning benzene oxidation over zeolites supported Pd catalysts, [PERSON] at al. have proposed initial Pd\({}^{2+}\)O\({}^{2-}\) reduction by benzene followed by Pd\({}^{0}\) oxidation with O\({}_{2}\) from the stream and Pd\({}^{2+}\)O\({}^{2-}\) recovering [45]. Such redox process with gas phase oxygen supply prevails for unreducible supports like alumina, whereas for reducible supports like ceria, the lattice oxygen from ceria surface layers could participate in the redox transfer Pd\({}^{0}\leftrightarrow\) PdO. Differently from our previous finding of superior CBO activity of bimetallic Pd-Au catalysts in comparison with monometallic ones, in this study, the best performance was exhibited by Pd-based samples on alumina-supported ceria and particularly on alumina-supported Y-doped ceria. In addition to above commented redox transfer Pd\({}^{0}\leftrightarrow\) PdO, the reason could be also higher surface concentration of Pd and Ce\({}^{3+}\) in monometallic samples, evidenced by XPS analysis, as well as the presence of adsorbed superoxo species O\({}_{2}\)\({}^{-}\) close to the Ce\({}^{4+}\) ions registered in EPR spectra.