Transient species, determining fractions

B. Determining Fractions of Transient Species. As in any spectroscopic method, the determination of a transient structure by the XTA method requires deconvolution of the signal of the transient species of interest from those of other species, such as any remaining ground state or photoproducts. In general, total signals detected via X-ray transmission or fluorescence can be expressed as ... 

The net rate of production of gas-phase species by heterogeneous reaction participates in the boundary condition through the surface mass balance (Eq. 17.24). However, since the heterogeneous production rates of gas-phase species depends on the surface composition, the transient state of the surface composition must also be determined. As discussed in Section 11.10 the surface site fractions Z are found from the solution of... 

In a catalytic (ycle (Fig. 9.1b), the active catalyst M is often rather unstable and is only formed in situ from the catalyst precursor (or precatalyst), M. If we monitor the system, for example, by NMR, we normally see only the disappearance of S and the appearance of P, not the transient catalytic intermediates. We may still see only M because only a small fraction of the metal is likely to be on the loop at any given time. Even if we appear to see an intermediate, we cannot be sure it is not M S, an off-loop species. If a species builds up steadily during the reaction, it might be a catalyst deactivation product M", in which case, the catalytic rate will fall as [M"] rises. Excellent reviews are available on the determination of mechanism in catalytic reactions. ... 

---