Most work modelling the reaction of coadsorbed species on surfaces uses simple Langmuirian kinetics, i.e. assumes that internal equilibrium exists in the layer, and that the chemical potential of adsorbates corresponds to that of an ideal gas. Coverage dependences of reacting species and of site blocking are usually treated with simple linear dependencies, even though it is well known that lateral interactions are strong in adsorbate and coadsorbate layers, so that considerable influences on kinetics are to be expected. We have in the past been concerned with many coadsorbate systems. In this connection we have collected a number of examples where strong deviations from simple coverage dependences exist, both in blocking and in promoting reactions. Interactions can range from those between next neighbors to larger distances, and can be quite complex. In addition internal equilibrium in the layer as well as equilibrium distributions over product degrees of freedom can be violated. While this behavior can usually be described by kinetic models, the deeper reasons will require detailed theoretical work.

Time allowing, part or all of the following examples will be discussed briefly:

-  Site blocking and nonlinear Q dependence:

-         Water formation reaction on Ru(001)

The reaction of O adatoms with hydrogen from the gas phase is drastically coverage-dependent: It is rapid for 0.25<QO<0.5 but virtually stops at 0.25 ML, at variance with the existing theoretical description of the O adlayer. Also a strong effect of steps is observed.

-  Promotor coadsorbate:

-         NCO formation on Ru(001)

The reaction proceeds between coadsorbed CO and NO via NO dissociation, and between coadsorbed N, O, and CO; but not between N and CO. The necessity of adsorbed O atoms is explained by electric field influences.

-         Toluene dissociation on Ru(001)

The sequential dissociation of H from adsorbed toluene differs in clean and in CO/O coadsorbate layers. The H from the methyl group always breaks off first, but for pure toluene all come off together, while with CO and O coadsorbates sequences of 1:1:1 H, and 1:2 H, resp., for the three methyl H’s are found. Steric and field influences play a role.

-  Nonequilibrium effects in the dynamics:

-         CO oxidation on Pt(111)

Very different distributions of translational energy and angle for the CO2 formed from O+CO or O2+CO layers are found with varied coverages. High ET peaked in angle are found in most cases. Only at high temperatures and low coverages a Langmuirian, equilibrated component is found. This proves direct reaction paths with non-equilibration of the reaction enthalpy over the degrees of freedom.

-  Site blocking and nonequilibrium:

-         O/NO coadsorbates on Ru(001): desorption and restructuring

Two wellknown coadsorbate structures of O and NO, the row-like (2x1)-(O+NO), and the honeycomb structure (2x2)-(2O+NO), for which the detailed geometries are known, are connected by desorption of 1 NO per (2x2) surface mesh upon heating. The reverse process happens upon cooling in NO; however the backreaction is exceedingly slow. The detailed interplay between de(ad)sorption and geometric restructuring, as monitored by in situ spectroscopy, has been modelled successfully and the strong asymmetry between reaction and backreaction explained.by a concerted mechanism.