## 4 Discussion  
Generally, the catalytic activity of Au/ceria catalysts for VOCs oxidation is associated with the weakening of Ce-O bond [14]. The effect is stronger at the interface with gold, where the formation of oxygen vacancies is favored and consequently the surface oxygen mobility of ceria is enhanced. Resulting loosely bonded surface lattice oxygen and easy formed anion vacancies have been proposed as sites of oxygen adsorption and activation, favoring the oxygen supply in VOCs oxidation [10]. The promoted reactivity by Ce-O bonds weakening and easy oxygen vacancies formation can be also related to the ability of the formed oxygen vacancies to affect the binding energy between the catalysts and the reactant molecules lowering the energy barrier for their activation [42] and references therein. Aiming the exploration of appropriate and economically viable supports as a relevant approach to preparation of high performing and cost-effective formulations we applied impregnation or mechanical mixing for deposition of nanosized ceria or Y-doped ceria on high surface area alumina. Analysis of catalytic tests over gold-based catalysts provided clear evidence of strong effect of CeO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) preparation method on CBO activity. The differences in textural properties (Table 1), in particular, pore volumes and pore sizes, allowed a suggestion of the location of ceria particles inside the pores of alumina using the impregnation method, thus causing some limitations of the access of reactant to the active sites. In contrast, ceria crystallites with a smaller size were located mainly on the surface of alumina by applying mechanochemical mixing. This synthesis approach contributed to improved textural properties that affected CBO activity of gold-based catalysts on alumina- supported ceria or Y-doped ceria. The favorable role of Y-doping in CeO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) mixed support on activity of gold catalysts could be related to improved mobility of oxygen in ceria surface layers. The shift of T\({}_{\text{max}}\) in the TPR profile of Au/YCeAl to lower temperatures as well as slightly higher hydrogen consumption supports this statement. Figure 6: Electron paramagnetic resonance (EPR) spectra of samples after catalytic test: (**a**) Au/YCeAl; (**b**) Pd/YCeAl; (**c**) Pd-Au/YCeAl; (**d**) Au/Ce and (**e**) Pd/Ce. At first glance, the higher CBO activity over Au/Ce in comparison with that over Au/YCeAl suggests that the amount of Y-doped ceria phase, dispersed on alumina, could not provide enough active sites, while much more Au-ceria interfaces on Au/Ce maximized active sites between Au and ceria. This suggestion corroborates above discussed role of preparation method of mixed oxide supports on activity of gold-based samples. However, after looking again at Figure 2a, where all Pd-containing samples were compared, this hypothesis does not seem relevant. The Pd/YCeAl catalyst exhibited superior activity, higher than that of Pd/Ce. Obviously, the nature of noble metal plays a very important role. The Pd/Ce catalyst demonstrated lower activity then that of Au/Ce (Figure 3a) and this result is in agreement with the observed low temperature HC (0.54 mmol-g\({}^{-1}\) as compared to 0.43 mmol-g\({}^{-1}\), Table 3), e.g., a lower possibility of active oxygen supply. In all other cases, the performance of monometallic Au-based catalysts was inferior compared to Pd-based and bi-metallic Pd-Au catalysts. The deposition of gold was carried out by deposition-precipitation. The affinity of gold particles to nucleate preferentially on ceria surface by applying this preparation method was taken into consideration in order to avoid as much as possible deposition over alumina. Indeed, XPS analysis showed higher dispersion of gold on the surface of Au/Ce. The comparison of the XPS derived atomic ratio (0.06) and the nominal one (0.0175) implied a gold enriched surface, while the respective ratios for Au/CeAl and Au/YCeAl were just as the nominal ratio.