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 . 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
and E114 will be carried out using gas phase adsorption 0
chromatographic techniques with transition metal surfaces 20
rel. Rn activity/%
as stationary phases. For this reason it is advantageous to
know the behavior of Rn in a gas flow at low temperatures 15
in contact with metal surfaces. This poses problems: Firstly,
the carrier gas may contain trace amounts of water, 10
resulting in a coverage of all surfaces with ice. Secondly, a
co-adsorption of carrier gas atoms or molecules at these low
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).
Heating The temperature gradient is indicated (doted line,
Table 1: Experimental results compared to literature data
−∆Hads / kJ·mol
Mass Flow −∆Hads /kJ·mol −∆Hads / kJ·mol
Water bath Surface
30oC Au 27±2 29±2 
Pd 25±2 41±2  37±4 
Ag 33±2 23±2 
227 Ni 27±2 43±2 
Metal Ta turnings NiH 28±2
column heated to Pd (H2) 23±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
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