Calculating the coverage of saturated and sub-saturated layers of carbon monoxide adsorbed onto platinum

Adsorption and subsequent stripping of COads from catalyst surfaces is an important process used in the determination of active surface area in a range of electrocatalyst systems. Crucial to this process is a measure of how to relate the stripping charge to the actual amount of CO which was adsorbed. In this paper a new experimental approach to measure CO coverage is introduced. By utilising an impinging jet (wall-jet) irrigation system with microelectrodes it is possible to operate under conditions of uniform access. This approach is used to accurately determine the saturated COads adlayer coverages using coulometric methods alone even in the presence of background Faradaic processes. A method for determining sub-saturated COads adlayer coverages prepared either by partial admission, or partial oxidation of a saturated adlayer is also presented. This approach is used to assess COads adlayer coverage on platinum. It is shown that at a CO admission potential of 100 mV failing to take into account the charges associated with CO adsorption, and only using the charges measured during the COads oxidation would result in an 34% underestimate of the COads coverage. A saturated coverage of CO on Pt of 0.68 is found, and these saturated layers are shown to be stable over a period of at least 100 min in the presence of low concentrations of oxygen (45 nmol dm−3). The CO adsorption transient is experimentally determined at 298 K, and a mathematical expression for the CO coverage with time under conditions of defined mass transport is derived. Fitting the experimental data to the theoretical adsorption transient equation yields a value for the CO adsorption rate constant, kad of 0.0086 cm s−1.