NO oxidation rates are limited by O2 activation steps that require transition state stabilization without concomitant inhibition by chemisorbed oxygen (O*). We present here two approaches that resolve these practical manifestations of the Sabatier principle.  The first approach uses two NO molecules to form termolecular transition states analogous to those in homogeneous routes, but stabilized by physisorption or confinement in siliceous solids.  Such van der Waals stabilization causes NO oxidation to occur at ambient temperatures without the involvement of specific binding sites and at rates that increase with decreasing temperature. Activation enthalpies and entropies become more negative as the confining voids become smaller and do so in a manner consistent with the adsorption properties of molecules similar in size to the proposed transition states; activation free energies decrease (and rates increase) upon confinement because enthalpy effects prevail over entropy losses at low temperatures.  Another approach recognizes that NO oxidation on metal surfaces requires kinetically-relevant O2 adsorption on minority vacancies (*) at surfaces nearly saturated with oxygen (O*). Such reactions occur at higher temperatures (~500 K) and their rates benefit from the weaker binding of O* on larger metal and oxide clusters and on nobler metals.  O* coverages and vacancy concentrations are dictated by oxygen chemical potentials set by NO2-NO equilibration at surfaces, instead of reflecting prevalent O2 pressures. Consequently, intimate mixtures of catalysts with BaO adsorbents, which scavenge NO2 as it forms, lead to markedly higher NO oxidation turnover rates. These synergistic effects depend, in turn, on the ability of metals, when present within diffusion distances of BaO, to promote the oxidation of nitrites to nitrates, thus allowing NO2 molecules to displace CO2 from BaO adsorbents.   Such mutual synergies enable intimate metal-adsorbent mixtures to function effectively at the trace NOx levels prevalent in exhaust streams.

Synergistic effects of adsorption and confinement and the mechanisms of NO oxidation catalysis

248th National Meeting (2014)

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