The desorption kinetics of dissociatively chemisorbed diatomic molecules are examined from a kinetic-modeling point of view. A comparison is made between a one-step process, resulting in the usual second-order kinetics, and a two-step process which takes into account explicity recombination of atoms and subsequent desorption of molecules. The kinetics from the two-step mechanism are found to be equivalent to second-order desorption with a coverage-dependent activation energy which, in many cases, is a linear function of coverage. The two-step process leads to second-order kinetics with a constant activation energy only for special values of the model rate parameters, or if chemisorption is activated. The steady onate approximation for the intermediate in the two-step process is often adequate, but the transient period during which a steady state is reached also contains important kinetic information. The implications of these results for desorption kinetics and molecular beam reaction experiments are discussed.
This is a preview of subscription content, log in to check access.
Buy single article
Instant access to the full article PDF.
Price includes VAT for USA
Subscribe to journal
Immediate online access to all issues from 2019. Subscription will auto renew annually.
This is the net price. Taxes to be calculated in checkout.
D.Menzel: InInteractions on Metal Surfaces, ed. by R. Gomer, Topics Appl. Phys.4 (Springer, Berlin, Heidelberg, New York, 1975)
D.A.King: Surface Sci.47, 394 (1975)
R.Gomer: Sol. State Phys.30, 93 (1975)
L.D.Schmidt: Catal. Rev.9, 115 (1974)
M.R.Shanabarger: Surface Sci.44, 297 (1974)
Ch.Steinbrüchel: Surface Sci.51, 539 (1975)
D.A.King: Surface Sci.64, 43 (1977)
R.Gorte, L.D.Schmidt: Surface Sci.76, 559 (1978)
J.H.Seinfeld, L.Lapidus, M.Hwang: Ind. Eng. Chem. Fundam.9, 266 (1970)
D.D.Warner: J. Phys. Chem.81, 2329 (1977)
J.Lapujoulade, K.S.Neil: J. Chem. Phys.57, 3535 (1972)
J.Lapujoulade, K.S.Neil: Surface Sci.35, 288 (1973)
K.Christmann, O.Schober, G.Ertl, M.Neumann. J. Chem. Phys.60, 4528 (1974)
M.Balooch, M.J.Cardillo, D.R.Miller, R.E.Stickney: Surface Sci.46, 358 (1974)
L.R.Clavenna, L.D.Schmidt: Surface Sci.22, 365 (1970)
We assume that the experimental peak widths are due to the kinetics rather than to such instrumental effects as temperature inhomogeneity of the sample or non-differential pumping conditions.
Further examples for kinetics that are second order at low coverage but deviate from second order at higher coverage: H2 on W(100) ; N2 on W(100) ; H2 and N2 on Mo(100) ; H2 on Pt(111) [19, 20]
P.W.Tamm, L.D.Schmidt: J. Chem. Phys.51, 5352 (1969)
H.R.Han, L.D.Schmidt: J. Phys. Chem.75, 227 (1971)
K.Christmann, G.Ertl, T.Pignet: Surface Sci.54, 365 (1976)
R.W.McCabe, L.D.Schmidt: Surface Sci.65, 189 (1977)
H.A.Engelhardt, P.Feulner, H.Pfnür, D.Menzel: J. Phys. E. Sci. Instrum.10, 1133 (1977)
D.R.Olander: InProc. Int. Conf. Colloids and Surfaces, San Juan, P.R., ed. by M.Kerker, A.C.Zettlemoyer, and R.L.Rowell (Academic Press, New York 1977)
S.L.Bernasek, G.A.Somorjai: J. Chem. Phys.62, 3149 (1975)
K.E.Lu, R.R.Rye: Surface Sci.45, 677 (1974)
K.Christmann, G.Ertl: Surface Sci.60, 365 (1976)
Work performed under the auspices of the Office of Basic Energy Sciences of the Department of Energy
About this article
Cite this article
Steinbrüchel, C. Desorption rates and mechanisms for dissociatively chemisorbed molecules. Appl. Phys. 19, 413–420 (1979). https://doi.org/10.1007/BF00930105