Pressure Domain Physical versus Chemical Quenching

Low-Pressure Domain Physical versus Chemical Quenching 

The quenching effect of gaseous NO, 02, CO, and N20 on the PL of Mo6+/ Si02 photocatalysts has been investigated at room temperature either under vacuum or as a function of pressure (0.003-66 mbar) of the quenching gas (Shelimov et al., 2003). Nonlinear plots of the relative PL intensity Iq/I (J0, initial intensity under vacuum) versus gas pressure were obtained. 

Paramagnetic NO and 02 molecules and reactive CO molecules efficiently quench the PL in the order NO 02 CO, whereas N20 only weakly affects the PL intensity. This work allowed confirmation of some important steps of the mechanism proposed earlier for the photocatalytic reduction of NO by carbon monoxide on Mo/Si02 (Subbotina et al., 1999). In particular, the NO photoreduction kinetics was consistently described by Subbotina et al. (1999) assuming first that the deactivation rate constant is much smaller than the quenching rate constant and second that the NO molecules efficiently quench the (Mo5+=0-) excited triplet state without chemical interaction (i.e., physical quenching ), in contrast to the chemical quenching by CO molecules to yield C02 molecules. The PL data demonstrate the correctness of those assumptions. 

The ratio of the quenching rate constants for NO and CO calculated from the computer best fits of the experimental pressure dependence is in agreement with earlier data indicating the kinetics and mechanism of the photocatalytic reduction of NO by carbon monoxide on Mo6+/Si02 photocatalysts reported earlier by Shelimov coworkers (Lisachenko et al., 2002 Subbotina et al., 1999,2001). 

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