*3.1. MOC Initial Performance*

To oxidize methane, in addition to the effective catalyst, high temperature is required. This was found to be true in the present study also. After preconditioning the catalyst for 48 h in the engine exhaust, the initial performance was defined by measurements upstream and downstream of the catalyst. Figure 2 presents the initial methane conversion as a function of exhaust temperature and at three different exhaust flows. At exhaust temperature of 400 ◦C, the methane conversion was found to be negligible (only 2%). However, when increasing the temperature, the methane conversion sharply increases, being above 20% at exhaust temperature of 460 ◦C, above 60% at exhaust temperature of 500 ◦C and at the highest temperature of 550 ◦C the methane conversion is approx. 70%. Furthermore, the lower exhaust flow (40 kg/h) studied at 550 ◦C increased the methane conversion to near 80% which is reasonable since the lower exhaust flows means more time for the catalytic reactions to occur. Roughly, changing the exhaust flow from 80 kg/h to 40 kg/h means similar effect on the performance that could be expected with doubling the catalyst size. In this case, the methane conversion increased from 60% (with 80 kg/h) to 80% (with 40 kg/h).

**Figure 2.** Initial methane conversion (the line is to guide eye only).

In practice, the engine out methane level of 1500 ppm compared to 1000 ppm had no effect on the MOC performance and the difference in methane conversions within these two cases was found to be less than 4%.

The ethane conversion was measured to be approx. 30% at exhaust T of 400 ◦C while at exhaust T of 460 ◦C and higher no ethane was found in the exhaust downstream of the MOC meaning near 100% conversion. Ethylene found in the engine out exhaust (see Table 1), was not found in the exhaust downstream of MOC in any of the test conditions meaning near 100% conversion for ethylene already at 400 ◦C. In addition, CO was in the level of only a few ppm downstream MOC, confirming nearly total CO conversion. From the FTIR measurements also formaldehyde was analyzed and was found to be below 2 ppm downstream of MOC in all test conditions (while the engine out level was approx. 55 ppm) confirming nearly total HCHO conversion as well. The NOx level was not affected by the MOC, as expected.

### *3.2. MOC Performance in Ageing*

The methane conversion of the MOC was found to decrease rather quickly over time. The conversions calculated form the GC measurement results are presented as a function of time in Figure 3. The GC measurement was done just before the regeneration and then again 3 h after the regeneration.

**Figure 3.** Methane conversion as a function of time, over the ageing experiment, calculated from GC results. GC measurements were done just before regeneration and 3 h after the regeneration. (The lines are to guide eye only).

The conversion was found to decrease by approx. 20 percentage units during the first 20 h of driving (see Figure 3). However, the regeneration, done once a day, was found to recover the methane efficiency of the catalyst. For example, in the case of "SOx trap + MOC" the methane conversion in the beginning was approx. 70%, after 20 h it decreased below 50%, but after the regeneration the conversion was found to be near 65%. During the following day (20–44 h) the conversion decreased again, now approx. to 40%; however, after the regeneration the conversion increased back to above 60% (see Figure 3, 44 h). Next, the third day was very similar to the second day's performance. After this, the next 2 days were conducted without regenerations (68–140 h), and the regeneration was done on the third day. The conversion decreased, without the daily regenerations, to 30%, but again, after the 5 min regeneration period, the conversion was back to above 60% (see Figure 3, 140 h).

The exhaust SO2 level had a clear influence on the methane catalyst efficiency. When MOC was aged with 1 ppm extra SO2 added to the exhaust, the methane conversion after each 24 h of driving was lower compared to the case where MOC was aged without any extra SO2 in the exhaust (see Figure 3 cases "MOC-no SO2" and "MOC + SO2"). The SOx trap was found to protect the MOC since when the same ageing was conducted with trap

installed upstream of the MOC the methane efficiency was similar to the case with no SO2 added to the exhaust (Figure 3, cases "trap + MOC + SO2" and "MOC-no SO2").

Note. All three MOCs of the tests were similar and were preconditioned for 48 h prior to the actual ageing tests start. However, the methane conversion in the start of the ageing test varied between 70–80% with the MOCs. In addition, for some reason, the first regeneration in the cases of "MOC-no SO2" and "MOC + SO2" was not successful, as it seemed not to recover the catalyst efficiency. For this, we do not have any clear explanation. Turning engine to stoichiometric mode was done manually and due to human factors, this might not have been done exactly similarly in all cases. In the case of "trap + MOC" the engine, unintended, faced changes in operation between 68–140 h, which we observed as an increased methane slip, while other emission components were not significantly changed. The methane slip in this case was near 2000 ppm at 140 h and forward. This was also the reason, why this test was ended at 164 h.

To have a closer look at the MOC performance from hour to hour, the methane conversion calculated from the simultaneously measuring FTIR devices' methane results are presented in the Figure 4 roughly for two days period. We started the period to examine only after the first day (due to the unsuccessful first regenerations discussed above). After the second regeneration, i.e., at the 44 h point, all the catalysts resulted in similar methane conversions. The conversion after the regeneration decreases more quickly in the case of "MOC + SO2" than for the two other cases ("MOC-no SO2" and "trap + MOC") that seem to behave very similarly. Again, after the regeneration the conversion is recovered, in all cases, and similarly to earlier day, the conversion after the regeneration (68 h) decreases more quickly in the case of "MOC + SO2" than for the two other cases. The methane conversion is approx. 10–15% units higher when the trap is placed upstream the MOC, meaning there are more active sites for methane to convert in the MOC downstream the trap. This indicates the SO2 level inside the MOC is lower when trap is involved compared to the MOC only case.

**Figure 4.** Methane conversion based on two simultaneously measuring FTIRs.

Although the methane conversion was significantly decreased within time, we saw, in practice, no change in the performance regarding the other emission components. The ethane and ethylene levels downstream of MOC stayed close to zero (below the detection limit) throughout the ageing test. CO level was only a few ppm downstream MOC and formaldehyde level stayed also in the level of 2 ppm (or below) throughout the ageing test.
