Effect of Na, K, Ca and P-impurities on diesel oxidation catalysts (DOCs)

Abstract

Impurities contained in biodiesel and motor lubricants reduce NO/NO x ratio, decreasing SCR and DPF efficiency. • Noble metals reducibility decreased in presence of alkali compounds. • Alkali compounds improve C 3 H 6 conversion related to their electronegativity. • The higher interaction of alkalis with CO decreased its reaction rate. • NO 2 /NO x ratio decreases in presence of additives. In order to reach the limits of CO 2 emissions set by Kyoto protocol, the use of biofuels is gaining momentum. However, the biofuels contains alkali and alkaline earth metals (Na, K and Ca) and phosphorus (P) species, which could decrease the efficiency of the after-treatment system. The main aim of this study is to increase the knowledge of the effect of impurities contain on biofuel on the catalytic activity of the diesel oxidation catalysts (DOCs). Both reference (PtPd/CeZrO 2 /La-Al 2 O 3 ) and modified catalysts were synthesized, characterized by several techniques and tested, concerning their physico-chemical, redox and catalytic properties. Based on the characterization results, the catalyst crystalline structure did not change after the impurities incorporation, however, the specific surface area was decreased in all cases. NO adsorption strength was increased due to the low electronegativity of Na, K and Ca species, according to H 2 -TPR and NO-TPD analyses. XPS analysis confirmed the formation of cerium phosphate, which can stabilize the Ce 3+ oxidation state related to its higher Ce 3+ /Ce 4+ ratio, diminishing the catalytic redox activity. The physico-chemical and redox properties modifications after Na, K and Ca impurities addition diminished the CO and NO removal, however, the C 3 H 6 conversion was improved. Besides, both CO and C 3 H 6 oxidation were enhanced, while the NO to NO 2 conversion decreased in presence of P species.