# Pentane Depletion by a Surface DBD and Catalysis Processing
## 3 Experimental Results
### Pentane Decomposition  
In what follows, we analyze pentane depletion according to treatment time \(t\), for the different experimental conditions, i.e., plasma power, initial pentane concentration, and catalyst activity. We found that relative to \(t\), pentane concentration strongly depends on the plasma power used and on the catalyst activity. In particular, without the catalyst and at 16.9 W plasma power, pentane was almost fully, but not completely, decomposed after 5 min. However, at 44.2 W plasma power, a full decomposition took about 2 min. It should be emphasized, however, that the full decomposition of pentane does not mean that pentane is completely converted to CO\({}_{2}\). As we discuss in Section 3.3, the plasma also produces intermediate reaction products such as acetylene and propane. In the presence of the catalyst, pentane is fully decomposed at the lower plasma power (16.9 W) in \(t\) = 5 min. At the higher plasma power (44.2 W), the catalyst activity is able to shorten the depletion time such that the pentane is completely decomposed in about 1 min. To understand the chemical dynamics, we plotted the experimentally measured pentane concentrations (having an error of about 10%), \(\rho_{5}(t)\), in Figure 5 relative to treatment time \(t\). The basic idea here is to represent the data in semi-log scale in order to see whether the data are consistent with an exponential decay [26], either a single one or a more complex decay. We therefore used the following basic function:  
\[\rho_{5}(t)=\rho_{5}(0)\,e^{-t/\tau_{5}}, \tag{1}\]  
where \(\rho_{5}(0)\) is the initial pentane concentration before plasma treatment, \(\tau_{5}\) is a parameter representing the characteristic time scale for depletion, which is related to the decomposition mean lifetime, \(t_{1/2}\), according to \(t_{1/2}=\tau_{5}\log(2)\). As a general feature, we found that \(\tau_{5}\) depends on the initial pentane concentration. As one can see from Figure 5a, the experimental values for the lower power, \(P=16.9\) W, scatter significantly. In particular, a double-exponential decay scenario seems to emerge at the initial pentane concentrations 600 ppm and 300 ppm. At the higher power of \(P=44.2\) W, considered in Figure 5b, the reduced time scale for depletion seems to be consistent with a single exponential decay for \(\rho_{5}(t)\). At plasma power \(P=16.9\) W, the reaction rates increase with decreasing initial pentane concentration (Figure 5a). At the highest pentane concentration (1200 ppm), we see a single decaying exponential, yielding a decomposition time of \(\tau_{5}\) = 5.3 min. At lower pentane concentrations, the decay becomes faster, and a second regime seems to take place after \(t\) = 2 min. At these low concentrations and long times \(t\), we expect the plasma to produce radical species more effectively. By increasing the residence time of the contaminated air in the plasma region (i.e., by prolonging the treatment time \(t\)), the availability of plasma-generated radical species for the air contaminant (pentane, in our case) increases. The availability of radical species becomes even higher when dealing with reduced concentrations of the air contaminant. Additional decomposition effects due to the catalyst are not apparent for pentane, such that pentane depletion seems to be dominated by the plasma to a large extent. Given the uncertainties implied by the measurements, the present suggestion for a double-exponential decay remains to be understood. Additional measurements, which are beyond the scope of our work, are therefore required in order to draw a more quantitative conclusion. At plasma power \(P=44.2\) W, much higher decomposition rates are found, and again, pentane depletion is plasma-dominated (Figure 5b). Now, we find single exponential decays for \(\rho_{5}\). The fact that we only have three experimental points in Figure 5b for the low initial pentane concentrations is a result of the fast depletion rate found. One should consider intermediate treatment times to have more data points.