PC IV • G re n zflä c h e n • W S 2011/12
Transcrição
PC IV • G re n zflä c h e n • W S 2011/12
PC IV • Grenzflächen • WS 2011/12 Heterogene Katalyse - mikroskopisch - PC IV • Grenzflächen • WS 2011/12 “The reaction which takes place at the surface of a catalyst may occur by interaction between molecules or atoms adsorbed in adjacent spaces on the surface, … or it may take place directly as a result of a collision between a gas molecule and adsorbed molecule or atom on the surface.” Langmuir, 1941 PC IV • Grenzflächen • WS 2011/12 Desorption Irvin Langmuir 1881 - 1957 Diffusion Chemisorption Cyril Hinshelwood 1897 - 1967 Reaktion PC IV • Grenzflächen • WS 2011/12 Eric Rideal 1890 -1974 Proc. R. Soc. London, Ser. A 178, 429 (1941) D.D. Eley, ~1955 100 Langmuir-Hinshelwood 3 RLH [10 ] 80 60 40 20 0 0 2 4 6 8 10 pA (with pB held constant) 1.0 Eley-Rideal 0.8 RER [] PC IV • Grenzflächen • WS 2011/12 Generic Kinetics 0.6 0.4 0.2 0.0 0 2 4 6 pA (with pB held constant) 8 10 Pd(110) Pt(210) 4.0 1 transition point CO 3.0 PCO2 2.0 LITD 1.0 0.25 T = 500 K -6 PO2 =17 x 10 mbar 0 0 0.5 1.0 2.0 3.0 4.0 5.0 O CO -6 0.75 Reaction Rate rco2 [rel. u.] O PCO2 [10 mbar] PC IV • Grenzflächen • WS 2011/12 CO2 Formation 0 100 300 500 700 Ir(110) 0 100 300 500 0 6.0 700 Pt(110) -6 PCO [10 mbar] 0 100 300 500 Temperature [ºC] PC IV • Grenzflächen • WS 2011/12 CO solo, Arrhenius Verhalten O voradsorbiert, CO Strahl CO sucht Reaktionspartner pseudo 1. Ordnung Ertl & coworkers, J. Chem. Phys. 73, 5862 (1980) Surf. Sci. 107, 207-219 (1981) PC IV • Grenzflächen • WS 2011/12 PC IV • Grenzflächen • WS 2011/12 ohne Katalysator E=500 CO(g) + 1/2 O2(g) 284 O 247 96 O C O E=-284 134 O 38 mit Katalysator C CO2(g) CO2(a) 0 ~ -33 Einheiten kJ/mol PC IV • Grenzflächen • WS 2011/12 13030 J. Phys. Chem., Vol. 100, No. 31, 1996 suspected, however, that certain highly facile gas-surface reactions might occur directly at the point of impact between an incident reagent and an adsorbate, by way of a so-called Eley-Rideal (ER) mechanism.70 In one limit, an ER mechanism could involve a single gas-surface collision. Alternatively, the incident reagent could “bounce” many times before reaction, being accommodated to varying degrees in the process.95 For us, the working definition of an ER reaction is that it should occur before the reagents have become equilibrated at the surface. Notice also that ER reactions are far more exothermic than their LH counterparts, with an excess energy close to the heat of adsorption of the incident species. The idea that reactions can occur directly as an incident reagent strikes an adsorbate is strongly supported by detailed theoretical calculations.96-103 Over 20 years ago, Elkowitz, McCreedy, and Wolken96 investigated the reaction of an incident H atom with an H atom adsorbed on tungsten by running classical trajectories on a model PES. Tully97 has examined the reaction of O atoms incident on adsorbed C atoms adsorbed on platinum using similar methods, but treating the flow of energy from the reaction zone into the crystal more accurately. The most recent theoretical studies of ER reactions have involved quantum mechanical models, beginning with model 2-D calculations,98,99 which clearly established that product molecules should show a high degree of vibrational excitation. Very recently, Jackson and Persson101,103 have included rotational motion for the first time by performing 3-D quantum calculations, using a flat-surface approximation. Until relatively recently, the experimental evidence for the ER mechanism came largely from kinetic measurements, relating the rate of reaction to the incident flux and to the surface coverage and temperature.70 For example, it has been found104 that the abstraction of halogens from Si(100) by incident H proceeds with a very small activation barrier, consistent with an ER mechanism. In order to proVe that a reaction can occur on essentially a single gas-surface collision, however, dynamical measurements are required. The first such studies concerned the determination of internal state distributions of product molecules. Highly vibrationally excited hydrogen molecules have been observed105,106 issuing from cells in which H atoms are produced on a hot tungsten filament. Considering the energetics of the possible reactions, the excited molecules have been attributed to ER reactions between the incident atoms and Rettner et al. Figure 8. Polar plot of the angular distributions of the HCl product of the reaction of H atoms with Cl/Au(111) for Ts ) 100 K. (Signal intensities are proportional to the distance from the origin.) Results are displayed for incidence energies of 0.07 and 0.37 eV, as labeled. The angle of the incidence beam (60°) is indicated graphically by the arrow. These results are attributed to an ER mechanism. The broken line corresponds to cos f, which is expected in the absence of dynamical factors. The tics are placed at 10° intervals. H atoms with Cl/Au(111). Clear evidence was again found110 for an ER mechanism. In this case, in addition to a fast, highenergy component in the TOF distributions (corresponding to the ER product), they found a second, low-energy component corresponding to an LH product. These observations confirmed an earlier proposal107 that this reaction occurs by concurrent ER and LH processes. The solid lines in Figure 8 show the angular distribution of the ER component. These clearly peak away from the surface normal and change with Ei, demonstrating a “memory” of the initial parallel momentum and energy. The dashed line shows a cosine distribution, which is close to that of the LH component for this system. The most recent studies of ER reactions have progressed beyond the issue of proving whether or not they occur, moving on to the task of characterizing the reaction dynamics of carefully chosen systems. In addition to velocity and angular distributions, quantum-state distributions have now been ob107 PC IV • Grenzflächen • WS 2011/12 PC IV • Grenzflächen • WS 2011/12 Modellrechnung 10 Model output NH 3 -%) PC IV • Grenzflächen • WS 2011/12 NH3 / Ru(0001) 1 Supported catalyst 0.1 0.01 0.001 0.0001 0.0001 Single crystal 0.001 0.01 0.1 1 10 Experimental output (NH3-%) Stolze & Nørskov, J. Catal. 110, 1 (1988) PC IV • Grenzflächen • WS 2011/12 (335) steps ( 11 13 19 ) kinks High Miller indexed fcc surface with steps / kinks BALSAC plot Au/Ru(001) Θ= 0.15 ML Behm & Co, Phys. Rev. Lett. 67, 3279 (1991) N2 -10 Ru(0001) Ea = 0.4 ± 0.1eV 0 -11 log S PC IV • Grenzflächen • WS 2011/12 Θ= 0.03 ML -12 -13 1-2% Au on Ru(0001) Ea = 1.3 ± 0.2eV -14 0.0015 0.0020 0.0025 0.0030 0.0035 1/T (1/K) Chorkendorf & Nørskov, Phys. Rev. Lett. 83, 1814 (1999)