Single-crystal surface kinetic parameters

Kinetic Parameters for the Decomposition of HCOOD or HCOOH on Single Crystal Surfaces... 

This almost trivial conclusion may, however, become invalid if the kinetics of a more complex reaction are no longer governed by a set of linear ordinary differential equations. Such a case is, for example, given by the CO oxidation reaction at a Pt(llO) single crystal surface where for certain sets of control parameters (p02, pCO, T) and by operation in a flow system the kinetics may become oscillatory or even chaotic, lliis is illustrated by Fig. 4 which shows the variation of the work function (which is a measure for the O-coverage as well as for the reaction rate) as a function of time for three slightly differing sets of control parameters [15]. le this quantity varies periodically with time in a), it is chaotic in b) and even more in c). The latter data reflect in fact a case of hyperchaos, in which Lyapounov exponents are positive. 

For single-crystal surfaces, the translational periodicity leads to the conservation of momentum hk parallel to the surface [18,19]. The parallel momentum is related to the experimental parameters emission angle and kinetic energy Skin through the relation hk = /ImEnn sin. The low kinetic energies usually encountered in two-photon photoemission justify the neglect of reciprocal lattice vectors of the surface in the momentum balance. 

The oxidation of CO by either 02 or NO was studied by Peden et al. and Oh et al. over Rh, Pd, Pt,and Ir single crystals (90-92). The CO + 02 reaction was relatively insensitive to the atomic structure of the surface, and the specific activities and kinetic parameters agreed for both crystal surfaces and for alumina-supported catalysts. The Rh surfaces deactivated at high 02 pressures due to the formation of a near-surface oxide (91, 92). On the other hand, the CO + NO reaction was very sensitive to... 

For many years it has been well known that CO electrooxidation on platinum is a structure-sensitive reaction. Studies with singlecrystal electrodes have shown that the kinetic parameters depend not only on the surface composition of the catalyst but also on the symmetry of the surface and that the presence of steps and defects alters significandy the reaction rate. As a consequence, the surface structure of the nanoparticles should also affect the performance for the oxidation of CO. Understanding how the different variables affect CO oxidation on Pt nanoparticles dispersed on carbon requires the control of the platinum surface in a similar way as has been achieved for single-crystal electrodes. In this sense, the influence of the surface site distribution on CO oxidation using nanoparticles of well-defined... 

The results presented in this chapter clearly demonstrate that the state of the siuface is a dynamic process depending on kinetics of surface transformation temperatiue, reactant concentration, time on stream, even on the reactor used, all of which determine the structure and composition of the Pt surface. In short T-O-S experiments, in which the surface is not equilibrated, the conditions can be different than in long-term experiments, where the surface reaches a quasiequilibrium state. Comparison of results from different research groups requires detailed examination of all the conditions used to draw valid conclusions. Although this is a common-sense conclusion, researchers often compare results at quite different conditions, e.g., single crystal versus supported catalysts, to cite a common example. Often-heated arguments about which interpretation is the correct one are just a reflection of the surfaces being under different states due to the use of different experimental parameters. 

Figure 6 shows the variation in the relative surface concentration of adsorbed N atoms with N2 exposure at 693 K for the Fe(llO), Fe(lOO), and Fe(lll) surfaces from which data for the initial (i.e., extrapolated to zero coverage) sticking coefficient was found to vary from 7 x 10 to 2 x 10 to 4 x 10 for the respective surfaces (21). The value for Fe(lll) is of the same order of magnitude as that derived by Emmett and Brunauer (22) for the doubly promoted catalyst. Even more remarkably, these numbers are also of the same order as the reaction probabilities derived in Somoijai and co-workers work (26). This agreement shows that kinetic parameters derived from single-crystal studies are transferable across the pressure gap (rate measurements were performed at 20 bar, whereas that on adsorption kinetics at 10 mbar ) and are also consistent with the behavior of a real catalyst. 

---