5.4 Results and Discussion
5.4.1 Constraining kinetics model
Table 5.2: Experimental conditions for constraining kinetics model: 1) 10:1:100 N2O:CH4:He, 2) 1:1:10 N2O:CH4:He, 3)1:10:50 N2O:CH4:He, 4) 1:10:50 N2O:CH4:H2. These conditions were prepared in four premix cylinders to measure methane and N2O depletions as a function of excimer pulses.
In order to constrain the chemical kinetics model, we performed a series of photochemical experiments (Table 5.2) to confirm our understanding of the chemistry.
The first experiment was to estimate the 193nm fluence that interacts with the gas mixture in the quartz cell. 10 torr of pure N2O was photolyzed, and the depletion was monitored as a function of laser pulses. Here, the N2O depletions arise from both photolysis and reactivity (e.g. O(1D)+N2O). A MATLAB code was used to implement
N2O (molec cm-3) CH4 (molec cm-3) He (molec cm-3) H2 (molec cm-3) Total Pressure (Torr)
1 1.71x1017 1.93x1016 1.43x1018 0 50
2 3.27x1016 3.27x1016 2.59x1017 0 10
3 4.73x1015 4.63x1016 2.08x1017 0 8
4 4.60x1015 4.64x1016 0 2.08x1017 8
repetitive photolysis kinetics modeling with the Kintecus18 software. The data and the kinetics model are shown in Figure 5.2A.
Figure 5.2: A) Measured N2O depletion as a function of excimer pulses. A kinetics model was used to simulate N2O depletion from photolysis and chemical reactions. The pulse energy was the only fitted variable (68 mJ/pulse). B) Measured N2O and CH4 depletion using condition #1 from Table 2. The same kinetics model from A) was used, and a fitted pulse energy of 69 mJ/pulse gave good agreement at low pulse number <100 pulses. The deviation at higher pulse number arises from secondary reactions that are difficult to model.
The error bars on the x-axis are from the per pulse fluctuations in the excimer energy.
The fitted pulse energy of ~68 mJ/pulse provided the best agreement with the data. Next, CH4 was added into a mixture with N2O and He (experiment 1 in Table 5.2). In this case, both N2O and CH4 depletions were measured (Figure 5.2B). All the reaction rates in the kinetics model are taken from literature and fixed. The only fitted variable was the pulse energy, which sets the initial concentration of the reactive species O(1D). A kinetics model with 69 mJ/pulse energy reproduced the data and agreed with the pulse energy from the pure N2O experiments. We also observed that the model and the data deviated at higher pulse number, which is a signature of secondary chemistry that is not properly modeled. With these two experiments, we confirmed the validity of our kinetics model and extracted the excimer pulse energy interacting with the gas in the quartz cell.
Experiments in the following sections utilize the same kinetics model, with all reactions and kinetic rates fixed.
We attempted to quantify contributions from secondary chemistry for O(1D)+CH4
and OH+CH4. In general, we chose conditions (Table 5.2) so that we can tune the contributions from O(1D) and OH to CH4 depletions. These experiments not only aid in choosing optimal conditions to isolate O(1D) from OH, but also help constrain our chemical kinetics model. The mixing ratios 1-4 are listed in increasing OH contributions to CH4 depletion. Figure 5.3 shows the measured CH4 depletion for the four conditions with the corresponding kinetics modeling results. Here, the excimer pulse energies were adjusted within 5% of the 69 mJ/pulse, which is well within the uncertainty from the pulse energy fluctuations and joule meter measurement, to achieve the best agreement.
The good agreement for all four conditions confirms the validity of our kinetics model.
Since condition 1 is dominated by O(1D) and 4 is dominated by OH, they were chosen for KIE measurements.
Figure 5.3: Measured methane percent depletion as a function of excimer pulses for conditions #1-4 from Table 5.2. The conditions were tuned by increasing OH contribution from 1 to 4 such that O(1D)+CH4 dominates and 1 and OH+CH4 dominates in 4. The dashed lines show the kinetics model simulation of the observed behavior. Only the excimer pulse energy variable is floated to within 5% of the 69 mJ/pulse determined from the experiment in Figure 5.2 to obtain good agreement at low pulse energy. The disagreements at higher pulses are expected due to secondary reactions.
Figure 5.4: Kinetics model to estimate the contributions from OH and O(1D) for conditions 1 and 4. A) Kinetics model of methane percent depletion for condition 1 (O(1D)+CH4 dominant) when the OH+CH4
channel is turned off in the model. The differences in the integrated areas from the two curves (4.5%) provides an estimate for OH+CH4 contribution interfering with O(1D)+CH4. B) Kinetics model for condition 4 (OH+CH4 dominant) when the OH+CH4 channel is turned off in the model. From the
differences in the integrated areas from the two curves, 1.5% of the methane depletion is from O(1D)+CH4.
Finally, by kinetics modeling, we can estimate contributions from secondary OH and O(1D) reactions for both O(1D) and OH+CH4 channels. This was achieved by artificially
“turning off” chemical reactions within the kinetics model. Figure 5.4A shows the percent depletion of CH4 for condition 1 (O(1D)+CH4) as OH+CH4 reaction is turned off.
The small drop in the percent depletion per pulse is the contribution from OH+CH4. The differences in depletions were estimated from integrating under the curve. Similarly, Figure 5.4B shows the percent depletion of CH4 for condition 4 (OH+CH4) with and without OH+CH4. From modeling we estimate a ≈4.5% OH contribution to the O(1D)+CH4 and ≈1.5% O(1D) contribution to the OH+CH4 channel. These values are used to correct the observed KIEs to the actual KIEs through eq. 7:
O1D OH
KIEobserved KIE (1 )KIE (eq. 7)