Ammonia removal in selective catalytic oxidation: Influence of catalyst structure on the nitrogen selectivity.

Selective catalytic oxidation is regarded as an effective and favored method for the removal of hazardous ammonia. A number of M-Pt/USY (M=Mn, Fe, Ce and Pr) catalysts were prepared and the resulting materials were characterized using N2 adsorption/desorption, XRD, TEM, NH3-TPD, XPS and H2-TPR. It was found that the addition of non-stoichiometric metal oxides to Pt/USY leads to the generation of additional acid sites for ammonia chemisorption and that N2 selectivity improved with increased strong acidity of the bi-functional catalysts. The oxidation state of active Pt could be adjusted by the introduction of non-stoichiometric metal oxides with increased concentrations of oxidized Ptδ+ species observed in the order of FeOx >CeO2-x >MnO2-x >Pr6O11. High valence platinum surrounded by atomic oxygen that can act as a proton scavenger to drive ammonia activation, inhibiting O2 dissociation and therefore improve N2 selectivity. Fe-containing USY zeolite is demonstrated to be a preferred catalyst for the removal of ammonia, due to its high N2 selectivity and good hydrothermal stability.

Ammonia Formation over Pd/Rh Three-Way Catalysts during Lean-to-Rich Fluctuations: The Effect of the Catalyst Aging, Exhaust Temperature, Lambda, and Duration in Rich Conditions.

The formation of ammonia (NH3) as a byproduct during the operation of a three-way catalyst (TWC) in a simulated exhaust stream was investigated using a commercially available Pd/Rh TWC under steady-state and lean/rich cycling conditions. Ion molecular reaction-mass spectrometry was applied to determine NO, NO2, and NH3 concentrations at a time resolution of 0.6 s. Catalyst aging was shown to result in a significant increase in the amount of NH3 formed, which has received limited attention in the literature to date. The selectivity toward NH3 formation has been shown to increase with the decrease in the oxygen storage capacity (OSC) of a TWC induced by thermal aging. NH3 has been shown to mainly form within the exhaust temperature range of 250-550 °C. Typical lambda and rich operational condition duration periods found in vehicle test procedures were also employed to investigate their effects on NH3 formation. The results suggest that a decrease in the lambda and/or an increase in the duration of rich operating conditions will lead to an increase in the selectivity toward NH3 formation. Improving the OSC of TWCs and effectively controlling the lambda near to 1.0 with limited duration in rich operating conditions are therefore significant factors in the reduction of NH3 emissions.

Rational addition of capping groups to the phosphomolybdate Keggin anion [PMo12O40](3-) by mild, non-aqueous reductive aggregation.

Controlled reductive assembly of capped Keggin anions [PMo(12)O(40)(ML(m))(n)](3-) has been achieved by reduction of [PMo(12)O(40)](3-) with sodium-mercury amalgam in the presence of metal halides, as exemplified by the rational syntheses of mono-capped [PMo(12)O(40){Co(MeCN)(2)}](3-) and bi-capped [PMo(12)O(40)(VO)(2)](3-) and [PMo(12)O(40)Sb(2)](3-).