Abstract
Effects of engine oil additives on the activity of several commercial oxidation catalysts have been tested in the engine laboratory. In this study, engine oil additives were added to a non-leaded gasoline. The amount of additives added corresponded to the consumption of engine oil additives in a 40, 000km run. The oil consumption rate was assumed to be 2, 000km/l.
It was found that phosphorus from ZDTP (Zinc dialkyl dithiophosphate) caused a significant decrease in the catalyst activity after a 40, 000-equivalent-km test for the fuel containing ZDTP (phosphorus content 0.1wt%) alone (Figs. 3, 4). On the other hand, the poisoning effect of phosphorus was reduced with increasing atomic ratio of calcium and barium to phosphorus for the combination of calcium and barium detergents and ZDTP in the test oil. In particular, when the atomic ratio was 2.3, the decrease in the conversion efficiency was 15% even for the most degraded catalyst, and the poisoning effect of phosphorus was considerably reduced (Fig. 6).
The total amount of phosphorus found on used catalysts was different from catalyst to catalyst in spite of the constant consumption of phosphorus in all the tests run. As shown in Fig. 7, the conversion efficiency decreased with increasing amount of phosphorus found on the catalyst, and this amount showed a good correlation with the atomic ratio of calcium and barium to phosphorus in the test oil (Fig. 8). When the atomic ratio was zero (ZDTP alone), the amount of phosphorus found on the catalyst was most abundant where compared with other cases, and the amount reduced with increasing atomic ratio.
The result of electron probe micro analysis indicated that, in the case of ZDTP alone, phosphorus had penetrated about 30μm into the catalyst, but in the combination of calcium and barium detergents and ZDTP, phosphorus was found adhered only on the catalyst surface.
Consequently, in the case of ZDTP alone, it seemed that the catalyst was degraded not only by the large amount of phosphorus deposited on its surface but also by the phosphorus penetrated deeply into it. In the combination of calcium and barium detergents and ZDTP, it seemed that phosphorus neither deposited on the catalyst surface nor penetrated into the catalyst, thus suppressing catalyst degradation.
Table 6 shows X-ray diffraction analyses of accumulated compounds on the catalyst. When ZDTP alone was uscd, AlPO4 and BaZn2(PO4)2, possiblc reaction products of phosphorus and catalyst carrier, were formed together with the combustion products of ZDTP, e.g., β-Zn2P2O7. However, in the combined case of calcium and barium detergents and ZDTP, the reaction products of phosphorus and catalyst carrier were not formed, instead β-Ca3(PO4)2 was mainly formed.
When calcium and barium detergents are present, it is thought that phosphorus will react with calcium and barium in the combustion chamber, and it neither deposits on the catalyst surface nor reacts with inner active sites of the catalyst. As the amount of calcium and barium detergents increase the phosphorus tends to react with them. Thus, suppression of poisoning effects of phosphorus will be intensified.
