## 3 Results
### Catalyst Characterization
#### 3.2.4 Tpr  
The reducibility of the catalysts, estimated by TPR, reflects the catalyst oxygen mobility, which is crucial for the Mars-van Krevelen mechanism. The obtained results for monometallic Au and Pd catalysts are compared in Figure 5. TPR profiles of Pd-Au catalysts (not shown) cannot be informative for the role of bimetallic composition because in the presence of hydrogen small paladium and gold particles form non-active inter-metallic species having no effect on the low temperature ceria reducibility. Such behavior was already observed in our previous investigations [22; 23] and the decreased surface ceria reducibility was explained by formation of Au\({}_{x}\)Pd\({}_{y}\) alloy particles [22]. In Figure 5a are illustrated the TPR profiles of gold-containing catalysts. Reduction was not registered for Au/Al sample, because alumina is unreducible oxide and metallic state is characteristic for big gold particles (average size of 32 nm, Table 1). It is already well known that nanosized gold affects the ceria surface layer reduction leading to a significant shift of the position of the TPR peak characteristic for pristine ceria (at around 500 \({}^{\circ}\)C) towards lower temperatures. The surface ceria reduction of Au/Ce was completed up to 250 \({}^{\circ}\)C, while TPR peaks finished before 200 \({}^{\circ}\)C in case of alumina-supported CeO\({}_{2}\) and Y-CeO\({}_{2}\). The T\({}_{\text{max}}\) of the peaks are located at 103-115 \({}^{\circ}\)C. The lowest temperature reveals the positive effect of Y-doping. Differently, ceria surface reduction of Pd-containing catalysts (Figure 5b), preceded by reduction of PdO (Pd\({}^{2+}\) evidenced by XPS analysis, Table 2) to metallic Pd, occurred at very low temperatures (44-60 \({}^{\circ}\)C). Broad reduction peaks were also detected in the interval 300-500 \({}^{\circ}\)C. A very small negative peak visible in the TPR profiles was attributed to the formation of Pd \(\beta\)-hydride that decomposes at about 70-80 \({}^{\circ}\)C [32; 33]. The catalyst reducibility was also evaluated on the basis of experimental hydrogen consumption (HC) up to 300 \({}^{\circ}\)C which is of interest for the present study of CBO. The calculated values are shown in Table 3. In case of Au/Ce, the theoretical HC for ceria surface layers reduction without changing the cubic structure (17-20% degree of reduction [34; 35]) are 0.49-0.58 mmol-g\({}^{-1}\). Having in mind that the initial state of gold in the fresh samples is mainly metallic (XPS analysis), the HC related to gold is negligible and the experimental HC of 0.54 mmol-g\({}^{-1}\) obtained for Au/Ce has to be assigned to surface ceria reduction, which is in good agreement with the theoretical values. The HC of 0.26 mmol-g\({}^{-1}\) and 0.32 mmol-g\({}^{-1}\) respectively, in the case of 30 wt.% ceria and Y-doped ceria on alumina correspond to 30% and 37% degree of reduction (effect of Y-doping). This higher low temperature oxygen accessibility could be achieved for supported ceria phase with structure different from that of the bulk oxide. Though, the total oxygen availability is significantly less as compared to Au/Ce catalyst. Figure 5: Temperature programmed reduction (TPR) profiles of: (**a**) Au-based samples; (**b**) Pd-based samples. Concerning HC for ceria surface layers reduction of Pd catalysts, after subtraction of theoretically calculated 0.09 mmol-g\({}^{-1}\) for PdO (1.15 wt.%) reduction, the obtained values are: 0.34 mmol-g\({}^{-1}\) (12% reduction degree) for Pd/Ce, 0.14 mmol-g\({}^{-1}\) (16% reduction degree) for Pd/CeAl and 0.20 mmol-g\({}^{-1}\) (23% reduction degree) for Pd/YCeAl. The differences in the ceria surface layer reduction caused by gold or palladium are well visible: (i) in the presence of gold, the TPR peaks are broader with T\({}_{\rm max}\) located about 100 \({}^{\circ}\)C. The effect of gold is stronger when the reduction starts at the interface with ceria support.