## 3 Results and Discussion
### Microwave-Based Detection of Aging State Dependent Water Adsorption at Low Temperatures  
A further aging-dependent difference in the resonance data can be seen at low temperatures below 180 \({}^{\circ}\)C. Figure 4 shows the time dependent resonance frequencies and temperatures during heating under a constant feed gas atmosphere at \(\lambda\) = 1.02. The resonance frequencies at the starting point of the temperature ramp at \(T\) = 80 \({}^{\circ}\)C differ from each other depending on the aging state. With increasing temperature, the resonance frequencies increase. The steepness of the curves, or, in other words, the time-derivative \(\mathrm{d}\!f_{\mathrm{res}}/\mathrm{d}\!t\), respectively, the temperature dependency \(\mathrm{d}\!f_{\mathrm{res}}/\mathrm{d}\!T\), differ depending on the aging state. For temperatures \(T\) > 180 \({}^{\circ}\)C, however, all resonance frequencies have the same value. The observed resonance frequency increases at low temperatures and their dependence on the aging state can be explained by assuming an effect of adsorbed water on the microwave signal. Therefore, the water influence is investigated further. Since the oxidation state of ceria under rich conditions is dependent on temperature and on the oxygen partial pressure, the following experiments are conducted in lean atmosphere to ensure that all ceria exists  
Figure 4: Resonance frequencies of a fresh (**red**), at 850 \({}^{\circ}\)C aged (**blue**, aging state 1) and at 1050 \({}^{\circ}\)C aged (**black**, aging state 2) TWC-sample at the beginning of light-off tests with a ramp of 20 \({}^{\circ}\)C/min and a constant lean feed gas of \(\lambda\) = 1.02 as well as the corresponding feed gas temperatures. in form of Ce\({}^{4+}\). The gas mixture was calculated according to Equation (1) to \(\lambda\) = 1.05 with a water content of 10 vol. %. All feed gas species were then kept constant, with the exception of water, which was varied between 0 and 10 vol. % in steps of 2% balanced with N\({}_{2}\) to a total flow rate of 2500 L/h. Additionally, the feed gas temperature was varied between 70 and 300 \({}^{\circ}\)C. The water content at all tested conditions was below the dew point to avoid condensation. Figure 5 shows for the fresh catalyst sample the resonance frequency differences (\(\Delta f_{\text{res}}\)) between water containing and water free feed gas. A huge influence on the water content can be seen. With increasing temperature, this influence gets smaller. The dependence is in accordance to results on the influence of relative humidity between 13 \({}^{\circ}\)C and 80 \({}^{\circ}\)C obtained on measurements with diesel particulate filter substrates [25]. It is well known from Al\({}_{2}\)O\({}_{3}\)-based humidity sensors that Al\({}_{2}\)O\({}_{3}\), which is part of the washcoat to provide a high surface area, is able to adsorb water [26]. Temperature programmed desorption experiments on Al\({}_{2}\)O\({}_{3}\) in [27, 28] have shown that water still desorbs at temperatures above the here-tested 300 \({}^{\circ}\)C. Nanosized ceria is able to sorb water as well [29]. It is even considered as a material for humidity sensors [30] since its electrical impedance changes by decades with water in the ambience. As already shown in Figure 4, the aging state of the catalyst has an influence on the resonance frequency at low temperatures. Figure 6 compares the resonance frequency differences when 2 or 10 vol. % water are added to the feed gas to the ones without water in the feed for the three aging states. When aging proceeds, the influence of water decreases for both water concentrations. The influence of water on the high temperature aged sample (aging state 2) is about 50% smaller compared to the fresh sample. The different amounts of adsorbed water and therefore the different resonance behavior during desorption may be a measure for the TWCs' aging state. Figure 5: Resonance frequency differences, \(\Delta f_{\text{res}}\), between water free and 2–10 vol.