Automobile combustion processes pose serious environmental challenges due to the formation of NOx (NO, NO2). This issue is more prevalent in advanced combustion strategies which are designed to meet stricter emission regulations and attain higher fuel efficiency by operating at lower exhaust temperatures. NOx mitigation is a major issue during early stages of engine operation termed as “cold start” when the temperature is &lt; ~200 °C and downstream selective catalytic reduction (SCR) catalysts are inactive. Passive NOx adsorbers (PNA) offer solution to this issue by storing NOx during the cold start and releasing them as the engine warms up, and the SCR catalyst is also active thereby facilitating the complete conversion of NOx to N2. Since 2016, numerous reports have shown Pd-based catalysts to be the most promising PNAs. Since Pd-prices have been highly volatile and cost-prohibitive during recent times, search for alternative compositions is warranted. Our approach to address this issue is by exploration of bimetallic Pd-catalysts for PNA. To this end, using solution-phase ion-exchange method we synthesized equimolar Pd0.5Co0.5 /SSZ-13, Pd0.5Cu0.5/SSZ-13 and Pd0.5Ag0.5/SSZ-13 and catalytically evaluated them for PNA using U.S.DRIVE reaction feed. Interestingly, Pd0.5Co0.5/SSZ-13, Pd0.5Cu0.5/SSZ-13 and Pd0.5Ag0.5/SSZ-13 exhibited 1.2x, 1.6x, and 1.3x higher (NO:Pd)molar uptake than Pd/SSZ-13 respectively. In order to assess long-term activity, measurements were repeated for 10 trials. Trial-dependent decline behavior was observed in all cases with Pd0.5Co0.5 /SSZ-13, Pd0.5Cu0.5/SSZ-13 and Pd0.5Ag0.5/SSZ-13 retaining 64, 82, and 60% of their initial activity respectively. While synergistic advantages of mixing Co, Cu and Ag with Pd are clear, our goal is to understand the decline behavior which seems distinct for the three bimetallic systems. Pd0.5Cu0.5/SSZ-13 was catalytically most superior as in addition to the highest activity it showed best resistance upon 10-repeats. Our ongoing investigations on understanding the molecular interaction between individual reaction substrates and catalytic entities present amongst the three bimetallic systems will also be presented. This is especially relevant since our reaction feed is diverse and contains NO, CO, O2, hydrocarbons and H2O. Our future goals are attainment of long-term stability of such PdxCu1−x/SSZ-13 PNAs and their applications in conjunction with SCR and oxidation catalysts.

Pd-based bimetallic zeolites as highly efficient passive NOx adsorbers

ACS Spring Meeting 2021

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