Apparently, at low concentrations anthracene could not reach all the active sites even if the total amount of anthracene molecules in solution was more than sufficient. Most likely, substantial amounts of anthracene were adsorbed on other surface sites not capable of generating the radical cations. The maximum anthracene solvability in toluene at room temperature obtained by us was about 0.04 M. When we heated the solution to 80°C we managed to dissolve more anthracene, reaching 0.25 M concentration. However, the total concentration of the generated radical cations did not change, indicating that 0.04 M concentration was sufficient to detect all the acceptor sites. So, we used the latter concentration in the subsequent experiments as the experimental procedure was much simpler in this case.
It is notable that substantially higher concentrations of the radical species were obtained after activation at 500°C compared to the activation at 300°C. The activation at higher temperatures in known to free higher percentage of the surface sites from chemisorbed water. To determine optimum activation temperatures we studied the dependence of the concentration of the radical cation generated after anthracene adsorption on alumina on the activation temperature (Fig. 2). The concentrations of the strong acceptor sites tested using toluene following the previously reported procedure are also shown for comparison.
One can see that anthracene as the spin probe is capable of detecting substantially higher concentrations of the acceptor sites. The concentration of the weak acceptor sites tested with anthracene exceeded by more than an order of magnitude that of the stronger acceptor sites tested using toluene. The determined concentration of the weak acceptor sites is below 1% of the monolayer. Here one should remember that using a donor with lower ionization potential, such as anthracene, we test the total concentration of the sites capable of ionizing this donor and possible polycondensation products formed from it. So, this concentration includes the strong sites as well. However, in this study we shall refer to such sites as weak. This is reasonable because the concentration of the sites tested using anthracene was substantially higher than that of the strong sites. Meanwhile, it is important that although the sites tested with anthracene are weaker, they are still obviously very active as they are capable of generating the radical cations at room temperature and must have the electron affinity close to 7.5 eV.
The measured concentration of the weak acceptor sites increased with the activation temperature in the whole studied temperature range. The highest temperature used in this study was 700°C as separate experiments showed that the phase composition of the sample could change after prolonged treatment at higher temperatures. Apparently, the activation at higher temperatures leads to deeper purification of the surface from adsorbed water and other impurities. The activation at 700°C leads to almost complete surface dehydration and yields the highest concentration of the registered radical anions. However, it was more important for us to work using lower activation temperatures that are closer to the ones used in the catalytic experiments. The minimum temperature that could be used for testing the strong acceptor sites capable of generating radical cations after toluene adsorption was 400°C. Meanwhile, the weak sites tested using anthracene were observed on the alumina surface already after activation at 200°C immediately after desorption of physisorbed water. This feature may be useful for investigation of the sample used at low temperatures or unstable samples obtained without calcination at high temperatures.
The evolution of the measured concentration of the paramagnetic species with time was studied for the alumina sample activated at 400°C (Fig. 3). The Inset in Figure 3 shows that a substantial concentration of the radical cations was generated immediately after adsorption. Then, this concentration gradually increased. It was found that maximum concentration of the radical cations was achieved approximately after 200 hours at room temperature and gradually decreased thereafter.
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