THE MECHANISM AND KINETICS OF WATER FORMATION ON PT(111)

We use modulated molecular beam relaxation spectroscopy (MMBRS) to identify the sequence of elementary reaction steps and to quantify the rate parameters for catalytic water (D2O) formation on Pt(111). This investigation is restricted to surface temperatures in the range 373 less-than-or-equal-to T(s) less-than-or-equal-to 723 K and oxygen and deuterium pressures on the order of 10(-5) mbar, where coverages of oxygen-containing species are low (curly theta less-than-or-equal-to 0.04 monolayer), and rate parameters can be assumed coverage-independent. Under these conditions, we find that adsorbed hydroxyl (OD(a)) formation is rapid and (nearly) irreversible; the rate-limiting, majority pathway for water production is D(a) + OD(a) --> D2O (E(a) = 16 kcal/mol): and water production by OD(a) disproportionation, i.e. 2 OD(a) --> D2O + O(a), contributes as a minority pathway (E(a) = 18 kcal/mol). From the results we construct potential energy diagrams that account for the energetics of nearly all elementary steps in these reactions.