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Rn ADSORPTION ON TRANSITION METALS

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                                            Rn ADSORPTION ON TRANSITION METALS
                                                      R. Mole (Univ. Cambridge), R. Eichler (PSI)

     The adsorption properties of Rn on transition metal surfaces have been studied by gas chromatography. The
     determined adsorption enthalpies are in agreement with literature values only in the case of gold. The adsorption on
     oxidized metal surfaces and metal surfaces treated with pure H2 was studied.


The adsorption of Rn on transition metal surfaces is                                 shown in Fig. 1. Two getters, made of Ta turning, were
particularly of interest in advance of chemical investigation                        inserted directly at both ends of the chromatography
of the elements 112 (E112) and 114 (E114). Radon                                     column and held during the entire experiment at 1000°C.
represents the noble gas which should have the most similar                          This technique allowed us to measure the Rn adsorption on
chemical properties to the hypothetical noble gas elements                           surfaces down to temperatures of -180°C free of ice. From
E112 and E114. A second reason for studying the                                      the resulting thermochromatograms (see e.g. Figure 2,
adsorption of Radon is that the daughter products of the                             Rn/Au) the adsorption interaction of Rn with the surface
decay of 220Rn, namely 212Bi and 212Po, have decay                                   has been quantified as the standard adsorption enthalpy
properties which disturb or even disable the unambiguous                             using the Monte Carlo based kinetic approach [1]. The
detection of the heaviest elements by the measurement of                             results of the experiments are compiled in Table 1.
their correlated α-decay chains.                                                                           30                                                             50

The experiments to determine the chemical nature of E112                                                                                           Au
                                                                                                                                      MCS: ∆Hads (Rn)=-27 kJ.mol
                                                                                                                                                                   -1

                                                                                                           25
and E114 will be carried out using gas phase adsorption                                                                                                                   0

chromatographic techniques with transition metal surfaces                                                  20




                                                                                      rel. Rn activity/%
                                                                                                                                                                          -50
as stationary phases. For this reason it is advantageous to




                                                                                                                                                                                 Temperature/ C
know the behavior of Rn in a gas flow at low temperatures                                                  15
                                                                                                                                                                          -100
in contact with metal surfaces. This poses problems: Firstly,
the carrier gas may contain trace amounts of water,                                                        10




                                                                                                                                                                                          o
                                                                                                                                                                          -150
resulting in a coverage of all surfaces with ice. Secondly, a
                                                                                                           5
co-adsorption of carrier gas atoms or molecules at these low
                                                                                                                                                                          -200
temperatures may disturb the whole adsorption process. We                                                  0
investigated the adsorption of Rn on the pure metal surfaces                                                     0    10       20       30        40     50     60

of gold, silver, nickel, and palladium. Also studied were                                                                           distance/cm

palladium, silver, and nickel oxides, nickel hydride, and the                        Fig. 2: Thermochromatogram of 219Rn along the gold
solid solution palladium forms with hydrogen.                                                column (grey bars, left-hand scale). Monte-Carlo
                                                     Cooling: Liquid                         simulation is shown (stepped line, left-hand scale).
                                                     Nitrogen
                                  Heating                                                    The temperature gradient is indicated (doted line,
                                                                                             left-hand scale).
                                                                                     Table 1: Experimental results compared to literature data
                                    Heating:
                                                                                                                                                                         −∆Hads / kJ·mol
                                                                                                                                                                                                      -
 Mass Flow                                                                                                                 −∆Hads /kJ·mol         −∆Hads / kJ·mol
                                                                                                                                             -1                     -1
                                    Water bath                                                                  Surface
 Controller                                                                                                                                                                                       1
                                    30oC                                                                          Au           27±2                  29±2 [2]
                                                                                                                  Pd           25±2                  41±2 [2]                  37±4 [3]
                                                                                                                  Ag           33±2                  23±2 [2]
       227                                                                                                        Ni           27±2                  43±2 [2]
             Ac/219Rn Generator
                                            Metal                  Ta turnings                                   NiH           28±2
                                            column                 heated to                                    Pd (H2)        23±2
                                                                   1000oC
                                                                                                                 AgO           22±2
Fig. 1: Experimental Setup.                                                                                      NiO           28±2
The surfaces were prepared as follows: The pure metal                                                            PdO           29±2
surfaces were obtained by heating the metal foils under a
                                                                                     The results with the NiH and PdH are interestingly in
flow of a gas mixture of hydrogen (50 ml/min) and helium
                                                                                     agreement with the results for pure Pd and Ni metal.
(50 ml/min). This was followed by heating under a flow of
                                                                                     Obviously, the cleaning procedure of the metal surfaces
pure He, to ensure that no adsorbed or dissolved hydrogen
                                                                                     from H2 after their reduction was not successful. Surprising
remained. Finally, the column was cooled by taking it out
                                                                                     is the strong interaction measured for Rn on Ag, on NiO,
of the oven and allowing the He gas to continue flowing.
                                                                                     and PdO. This result has to be reproduced in further
To form nickel hydride or the palladium/hydrogen solid
                                                                                     experiments. The adsorption investigation of Rn on Au
solution, the last two steps were omitted. To form the oxide,
                                                                                     reproduced well the literature data, revealing once more the
the column was placed in a pre-heated oven under a flow of
                                                                                     primacy of the noble gold surfaces for adsorption studies.
oxygen for five minutes (50 ml/min). The only metal that
differed from these procedures was gold. Due to its inert                            REFERENCES
nature, gold was cleaned by heating in oxygen at 500°C for                           [1] I. Zvára, Radiochim. Acta 38, 95 (1985).
half an hour followed by heating under a flow of He for                              [2] R. Eichler et al., J. Phys. Chem. B 106, 5413 (2002).
half an hour. The experiments were performed in the set-up                           [3] B. Eichler, et al., Isotopenpraxis 21, 180 (1985).

				
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posted:9/23/2011
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