The selective catalytic reduction (SCR) of NOx by hydrocarbons on noble metals is critically
important to the implementation of leaner-burning, more fuel-efficient combustion engines in
order to handle the increased amount of NOx that is produced. Understanding the reaction
mechanisms and pathways is essential for designing an effective catalytic system for exhaust
treatment. As one small part of this effort, we focus on the interaction of nitrogen atoms and
simple unsaturated hydrocarbons such as ethylene and acetylene on the Pt(111) surface under
ultra high vacuum conditions to understand the potential intermediates in NOx reduction.
In this study, we employ a variety of surface techniques, including temperature programmed
desorption (TPD), and reflection absorption infrared spectroscopy (RAIRS) in an attempt to
identify reaction pathways in hydrocarbon SCR. Three interesting observations have been made.
First, we observed the presence of π-bonded ethylene below 220 K, indicating a switch in the
preferred binding site for ethylene on N-Pt(111) as compared to the clean surface. This result
suggests that nitrogen could potentially serve as a promoter in metal catalyzed hydrogenation
reactions. Second, the formation of ammonia is observed through ND3 desorption by using
isotopically labeled ethylene or acetylene at 500 K. Because direct reaction between nitrogen
atoms and hydrogen does not proceed to form ammonia, the appearance of ammonia is believed
to be the result of a reaction between N atoms with coadsorbed ethynyl (CCH). This route to
ammonia synthesis has not been previously observed under UHV conditions. Third, above 560
K, CN coupling occurs as indicated by the desorption of HCN and the identification of CNH2 with RAIRS. In addition, a new dual UHV/“high-pressure” chamber has been constructed
and tested through two proof of principle experiments. First, nitrogen adsorption on Ni(110)
has been examined at pressures ranging up to the torr level. Second, we have studied the
hydrogenation of a nitrogen layer on Pt(111) at room temperature to determine the effects of
pressure on the ability to achieve a higher coverage of NH than what can be achieved under
UHV conditions. A detailed discussion about the system limitations are provided and possible
improvements are suggested
