Chapter 4 Multiple Choice Questions

. The flux of NO molecules impinging (per adsorption site) upon a Pt{111} surface at a partial pressure of 10-7 mbar, with both surface and gas at a temperature of 300 K, is 27.67 s-1. If 15N18O were used instead, with the surface temperature increased to 450 K and all else remaining unaltered, what would be the new flux?

. A certain molecular species adsorbs intact on a given surface at room temperature, forming a single chemisorbed layer. After subsequent cooling of the surface to liquid nitrogen temperature, additional adsorbing molecules of the same species form a physisorbed multilayer on top of the first layer. On heating the resulting sample at a constant rate in ultra-high vacuum, what kinetic orders should be expected for desorption of the molecular species?

. A very low coverage of benzene is deposited on Au{111} at a temperature of 100 K, and the adsorbed molecules are believed to remain intact. In a subsequent TPD experiment conducted at a heating rate of 3 K.s-1, the maximum desorption rate occurs at 239 K. Estimate the activation barrier for desorption, assuming a pre-exponential factor of 1013 s-1.

. Desorption of some molecular species from a given surface is found to be first-order, with the maximum rate occurring at 750 K. Applying Redhead's formula, with an assumed pre-exponential factor of 1013 s-1, yields an estimate of 205 kJ.mol-1 for the desorption activation barrier. If the true pre-exponential factor were more like 1011 s-1, what would be your best estimate for the barrier?

. After adsorption onto a certain surface at some very low coverage, oxygen desorbs molecularly in a TPD experiment with a peak rate that occurs at 800 K. Upon repeating the experiment with a higher initial coverage, the peak desorption rate is found to occur at the lower temperature of 765 K and to be roughly twice the maximum rate from the previous experiment. Assuming that pre-exponential factors are the same in both cases, and that the activation barrier for desorption is constant at around 220 kJ.mol-1, by roughly what factor must the initial coverage in the latter case have exceeded that in the first?

. Which of these statements best describes the character of a true Rayleigh mode whose wavevector corresponds to a high-symmetry point within the surface 1BZ?

. Which of these materials is incapable of supporting a Fuchs-Kliewer mode at any of its surfaces?

. An isolated carbon monoxide molecule adsorbs perfectly upright at an atop site on some surface. At very low temperature (and ignoring any motion of the surface atoms) how many vibrational modes ought one to expect the adsorbate-substrate complex to display?

. A certain dehydrogenation reaction is found to occur on specific surfaces of two different metals with activation barriers of 1.24 eV and 1.67 eV, while the corresponding reaction enthalpies are found to be -1.54 eV and -1.21 eV on a per-molecule basis. Assuming perfect adherence to the Brønsted-Evans-Polanyi relation, what would one estimate for the reaction barrier on a third metal surface (of similar structure to the first two) for which the reaction enthalpy is -1.34 eV per molecule?

. Across the sequence of elements W, Re, Os, Ir, and Pt, the activation barrier for a certain catalytic reaction rises linearly with increasing nominal d-band occupancy from 0.82 eV to 3.22 eV, while the barrier for desorption of the reaction product falls linearly from 1.67 eV to 0.67 eV. Assuming that exponential factors dominate in both processes, for which metal would one expect the overall reaction rate to be maximised?

Back to top