J . Phys. Chem. 1988, 92, 429-433 weak chemisorption observed for clusters. Oxygen reacts readily with aluminum surfaces.45 The surprising feature from this work is that certain small clusters are much less reactive (but still quite reactive) toward O2 (Figure 7) compared to the atom, dimer, and larger (n > 25) clusters.
Summary and Conclusions The reactions of gas-phase aluminum clusters with different molecules have been studied. Very dramatic cluster size selective reactivities are observed. This work shows that the metal cluster reactivity depends not only on cluster size for a particular metal-molecule combination but also on the molecule for a particular metal. For the reactions studied, the relative reactivity for the most reactive cluster for different reactants is summarized in Table 111. As can be seen, several orders of magnitude variation in reactivity is observed between different reactant molecules with a general trend in reactivity CH4 < H2 < D 2 0 < CO < C H 3 0 H < 0,. As seen from the reactivity plots, e.g., Figures 5-8, a given size cluster may be extremely reactive toward one molecule but unreactive toward another. In Table 111 the primary product peaks observed at low extent of reaction are also tabulated. In the majority of the cases the
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product peaks correspond to the mass of the associated cluster plus molecule. For example, with D2, D20, 02,and C H 3 0 H the first adducts detected are equivalent to masses A1,(D2), Al,(D20), A1,(02), and Al,(CH30H), respectively. For hydrogen chemisorption onto metal clusters it has been argued that dissociative chemisorption is most 1ikely.I” In particular, it has been argued6*& that chemisorption bonds must be in excess of about 13 kcal/mol in order for the product species to be collisionally stabilized prior to undergoing unimolecular decomposition. Thus the nonreactivity of aluminum clusters (toward CH4 for example) may simply reflect the weakness of the chemisorption bonds. Such effects are examined in more detail for reactions of CO with several transition metals as well as aluminum.6 In summary, this work indicates that relative reactivities exhibit large nonmonotonic variations as a function of cluster size and as a function of molecular type. In cases where the aluminum surfaces are unreactive the larger clusters are also found to be unreactive. For the reactive clusters unexpected and large variations in reactivity arise for clusters smaller than 20-30 atoms in size. The ratios of relative reactivities are expected to be a good measure of the ratios of rate constants. Registry No. Al, 7429-90-5; D2,7782-39-0; H20,7732-18-5; 02, 7782-44-7; CH,OH, 67-56-1; CH4, 74-82-8; CO, 630-08-0.
(45) For a recent review of oxygen chemisorption on aluminum surfaces see: Batra, I. P.; Kleinman, L. J . Electron Spectrosc. Relar. Phenom. 1984, 33, 175.
(46) Geusic, M. E.; Morse, M. D.; O’Brien, S . C.; Smalley, R. E. Reu. Sci. Insrrum. 1985, 56, 2123.
Initial Radiolysis Effects on the T2-02 Gas Reaction R. A. Failor,* P. C . Souers, Lawrence Livermore National Laboratory, Livermore, California 94550
and S . G. Prussint Department of Nuclear Engineering, University of California, Berkeley, California 94720 (Received: December 12, 1986; In Final Form: August 6, 1987)
Model calculations were performed to examine the effects of self-radiolysis of pure T2on the reaction of T2 and 02.The pressure dependence of the steady-state concentrations of the major T2 self-radiolysis products and the times required to reach steady state are examined in detail. The largest variations from the modeling results obtained by assuming initially pure T2and O2are found with low total tritium concentrations where self-radiolysis produces the largest T/T2 concentration ratios. In general, the effects of inclusion of T2self-radiolysis effects are important only during the first few seconds following initiation of the T2 O2 reaction.
+
Introduction The rate of oxidation of molecular tritium to tritiated water is of great concern for the safe operation of tritium handling and nuclear fusion facilities. Two of the main reasons for concern are the increased health hazard of tritiated water vapor over the molecular form and the difference in chemical behavior of the two forms in tritium containment and removal systems. Tritiated water can be produced by the tritium oxidation reaction at room temperature because of radiolysis effects from the tritium fi decay. This stands in contrast to the reaction of H2and 02,which usually requires elevated temperatures to provide the intermediate free radicals. In a previous paper,’ we modeled the homogeneous gas-phase reactions of small amounts of tritium in oxygen at 298 K and with a total pressure of 1 atm. In that report we assumed the initial mixture contained only oxygen and tritium molecules and that the 6 radiolysis of the gas mixture began at the time of mixing. Consultant to Lawrence Livermore National Laboratory.
0022-3654/88/2092-0429$01.50/0
This is equivalent to instantaneous mixing of T2 and O2with the tritium stripped of its radiolysis products. It is possible to conceive of removing the atomic radiolysis products by adsorption on metal with a high surface area and removing the ionic radiolysis products with a high electric field. However, complete removal of all the tritium radiolysis products is a limiting case that would be difficult to accomplish in practice. Here, we consider the limit in which T, gas, in equilibrium with its radiolysis products, is instantaneously mixed with oxygen. We use the term self-radiolysis to mean the production of ions and atoms in initally pure T2 due to the slowing down of the tritiumdecay @ particle and all secondary electrons. We assume there are no effects of vessel walls which would result in losses and reactions of the active species. Although, this is a limit that is also difficult to accomplish in practice, this study permits assessment of the extent to which the mechanism and rate of TzO ( 1 ) Failor, R. A,; Souers, P. C.; Prussin, S. G . J . Phys. Chem. 1986, 90, 5914.
0 1988 American Chemical Society
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Failor et al.
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
TABLE II: Calculated Steady-State Concentrations (in mol/cm3) for the Products of T2 Self-Radiolysis at 298 K4 species 1 atm 10” a t m concn press. dependence T, 4.1 ( - 5 ) 2.9 (-11) first order TJt 1.5 (-14) 1.2 (-17) half-order e1.5 (-14) 1.3 (-17) half-order T 3.5 (-1 l ) 2.5 (-1 l ) insignif press. dependence Tt 1.1 (-18) 6.4 (-19) insignif press. dependence T2’ 7.6 (-22) 7.2 (-22) insignif press. dependence
TABLE I: Gas-Phase Reactions Used To Calculate Tritium Self-Radiolysis Effects rate const, cm3/(mol.s) reactions unless noted” ref notes T+TYT, 1.45 (15) 2 b Tt + T, T2+ T
