Journal of Nuclear and Radiochemical Sciences, Vol. 21, pp. 7-14, 2021

Articles
Isothermal gas chromatography study of Zr and Hf tetrachlorides using radiotracers of 88Zr
and 175Hf− Towards investigation of gas-phase chemistry of Rf −Kaori Shiraia, Shin-ichi Gotoa,*, Kazuhiro Ooeb, and Hisaaki Kudoca

Graduate School of Science and Technology, Niigata University, Niigata, Niigata 950-2181, Japan
Graduate School of Medicine, Osaka University, Suita, Osaka 565-0871, Japanc
Faculty of Science, Niigata University, Niigata, Niigata 950-2181, Japanb

Received December 22, 2020; Accepted January 29, 2021; Published online March 24, 2021
Experiments on gas-phase chemistry for chlorides have been conducted to clarify chemical properties of the firstsuperheavy element, rutherfordium (Rf), Z = 104. However, adsorption enthalpies obtained in each experimentare not the same, even for those of the homologues Zr and Hf. In the present study, off-line isothermal gas chromatography was done using long-lived radioactive tracers to obtain reliable adsorption enthalpies for ZrCl4 andHfCl4 when adsorbed on a quartz glass surface. To simulate a column surface modified by a chlorinating reagent
in on-line experiments, chromatography was also performed using a chlorinated column. A newly developed
analytical method which is independent of experimental parameters and uncertain physical quantities was applied
to determine the adsorption enthalpies. The obtained enthalpies for ZrCl4 and HfCl4 were −101.3 ± 4.0 kJ mol−1
and −98.1 ± 3.1 kJ mol−1 for the non-treated column, respectively, and those for the chlorinated column were −85.1± 4.5 kJ mol−1 and −84.2 ± 3.3 kJ mol−1, respectively.

1. Introduction
Chemical properties of the superheavy elements (Z ≥ 104)are not well known. Researchers have had a long-term interest
in investigating whether the periodic law is applicable to thesuperheavy element region. According to theoretical calculations, relativistic effects of orbital electrons become more pronounced with increasing atomic number,1 and hence it ispossible that chemical properties of the superheavy elements
are not reliably predicted from their homologues. Therefore,
numerous efforts have been made to obtain their chemical
properties experimentally, and they have been reviewed in theliterature.2
Experimental gas-phase chemistry techniques are used to
study the chemistry of superheavy elements. In gas-phasechemistry, elements of interest synthesized by nuclear reactions are converted to volatile species and a technique such asgas chromatography is used to obtain thermodynamic properties, for example adsorption enthalpy (∆ads H) of the volatile
species for the surface of the chromatography column. The
gas-phase chemical study of rutherfordium (Rf), Z = 104, hasbeen carried out for its tetrachloride together with the tetrachlorides of Zr and Hf, which are homologues of Rf in the
periodic table.3 Zvára4 pointed out that the reported values of∆ads H were quite different between experiments and attributed
it to the differences in the modification of the surface of thequartz glass column by different chlorinating reagents. On theother hand, Türler and Pershina3 argued that this variation was
due to different physical constants such as the period of intrinsic oscillation in adsorption (τ0) used in analysis of the experimental results. Then, they reanalyzed reported experimental
results and concluded that the values of ∆ ads H of ZrCl4 and
RfCl4 were about the same, while that of HfCl4 was smaller
than those of ZrCl 4 and RfCl 4. 3 However, this conclusion
seems to be too crude since the order of magnitude of ∆ads H*Corresponding author. E-mail: goto@chem.sc.niigata-u.ac.jp

for Zr and Hf differs among the experimental results. And thesequence of ∆ads Hs proposed by Pershina et al.5 could not be
reproduced by the calculation with applying various adsorption mechanisms.
The present study was aimed at obtaining reliable ∆ads H values of ZrCl4 and HfCl4 by using isothermal gas chromatography to overcome the above problem. In isothermal gas
chromatography, the retention time can be determined either
by continuously introducing a short-lived nuclide into the column as in an on-line experiment or by injecting a short pulseof chemical species into a carrier gas and measuring the timeat which it emerges at the column exit. Off-line isothermal gas
chromatography experiments were performed using long-lived
radioactive tracers, 88Zr and 175Hf, to determine more reliable∆ ads H of ZrCl4 and HfCl4 for adsorption onto a quartz glass
surface. Migration behaviors of the chlorides were also investigated using a chlorinated column to clarify the effects of thechlorinated surface. The chlorination of the column surface
was analyzed qualitatively with X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy with attenuated totalreflection (ATR-IR).
2. Experimental
2.1. Experimental setup. A schematic diagram of anexperimental apparatus for isothermal gas chromatography isshown in Figure 1. The apparatus consisted of four component
parts as indicated in the figure: (i) a reaction part for formationof Zr and Hf oxides and chlorides; (ii) a chloride collection
part for collection of the chlorides volatilized from the reaction
part; (iii) an isothermal part for chromatography; and (iv) a
measurement part for detection of the chlorides that passed
through the isothermal column into a cold trap. The parts (i)–(iii) were heated individually using an electric tube furnace. A
straight quartz glass tube 85 cm in length passed through thefour parts. To make the temperature of the isothermal part asuniform as possible, copper jackets were installed around the© 2021 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 03/24/2021

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Figure 1. Schematic diagram of the off-line isothermal gas chromatography apparatus; (1) CCl4, (2) 4-way valve, (3) pressure sensor, (4) carbon
filters, (5) electric tube furnaces, (6) Cu jackets, (7) quartz glass tube, (8) Pb shield, (9) water cooling jacket, (10) HPGe detector, (11) quartz
wool, (12) needle valve.

quartz tube at both ends of the tubular furnace in the isothermal part. The inner diameter of the quartz tube at the reaction
and chloride collection parts was 7 mm, and that at the isothermal and measurement parts was 4 mm. The length of the isothermal part was 30 cm.
An aliquot of tracer solution was dropped onto a carbon filter (Uni-Mitsubishi Pencil PFC, 80 ppi) inserted in a thinquartz tube (6 mm o.d., 4 mm i.d., 120 mm in length), and thistube was inserted into the reaction part. A carbon filter of thesame type as that in the reaction part was placed in the chloride collection part to collect the volatilized chloride. The
measurement part was plugged with quartz wool (TOHSO,
grade FINE 10) and cooled with a water-cooled jacket at room
temperature. An HPGe detector (CANBERRA, GC4018) was
installed in front of the jacket to measure γ-rays. The detector
was suitably shielded with Pb plates.
2.2. Isothermal gas chromatography. The radioactive
tracers of 88Zr (t1/2 = 83.4 d) and 175Hf (t1/2 = 70 d) were produced via the 89Y(d, 3n) and natLu(d, xn) reactions, respectively,by a 24 MeV deuteron beam supplied by the RIKEN K70 AVF
cyclotron. These tracers were chemically separated by anion
exchange chromatography6 and prepared as 1 M HCl solution.
The no-carrier-added stock solution containing both 88Zr
and 175Hf was suitably diluted with 1 M HCl, and 80 μL of thediluted solution was infiltrated into the carbon filter in the thinquartz tube. Then the tube was inserted into the reaction part.
Chloride oxides of Zr and Hf may be formed when their HCl
solution evaporates. Therefore, chloride oxides which may
have been formed were completely converted to oxides byheating at 650 °C for 40 min with mixed air and helium gas
f lowing through the reaction part. After that, the formedoxides of Zr and Hf were converted to chlorides as follows.
Carbon tetrachloride vapor was introduced instead of air in the
He mixed gas flow by bubbling at room temperature. Highpurity He dried with liquid nitrogen was used in the mixedgas. The flow rate of He was set to 0.5 L min−1 (0 °C, 1 atm)
using a mass flow controller (STEC, SEC-E40), and the pressure in the reaction part was kept to 111.3 kPa by adjusting theexhaust flow with a needle valve. Tetrachlorides of Zr and Hf
were formed at 600 °C for 90 min. The chlorides immediately
volatilized and migrated with the carrier gas flow, and werecollected by the carbon filter in the chloride collection part.
This part was kept at 75 °C to collect the Zr and Hf chlorides
completely and also to prevent the deposition of hexachloroethane crystals which are the main pyrolysis product formedby heating carbon tetrachloride at 600 °C.
When the conversion was completed, the supply of CCl4

was stopped, and the f low rate of He gas was changed to
1.0 L min−1 (0 °C, 1 atm). During the conversion, the isothermal part was kept at the temperature for isothermal chromatography.
As the sublimation temperatures of ZrCl4 and HfCl4 at 111.3
kPa are 337 °C and 321 °C, respectively,7 the collected chlorides were volatilized at 400 °C for introduction into the isothermal column. The rising temperature of the tube furnace
was sufficiently fast (about 95 °C min−1) and the time requiredfor volatilization of ZrCl4 and HfCl4 was short enough, so thatthe difference in the start time of the ZrCl4 and HfCl4 chromatograms was negligible. From the time when the temperature of the chloride collection part reached 320 °C, γ-ray
spectra were measured every 30 s until the γ-ray counting
rates from 88Zr and 175Hf were almost constant.
2.3. Chlorination of the column surface with CCl 4 .
Surface-chlorinated quartz columns and non-treated quartz
columns were examined to study the effect of the surface stateof the isothermal column on the behaviors of Zr and Hf tetrachlorides in isothermal chromatography. McDaniel8 studied
the chlorination of porous silica surfaces with various chlorinating reagents and reported that heating with carbon tetrachloride at 400–800 °C for 2 h resulted in the replacement ofsilanol and siloxane groups by chlorine atoms on the surface.
Based on this result, the chlorination of the isothermal column
was examined with carbon tetrachloride and the optimal conditions were identified. High-purity helium gas saturated withcarbon tetrachloride was introduced into the apparatus withthe flow rate of 0.5 L min−1 at room temperature, and the isothermal column was chlorinated while varying heating temperatures and durations. During chlorination, the reaction
part and the chloride collection part were kept at room temperature. After the chlorination under each condition set, theZrCl 4 and HfCl 4 chromatography was carried out and thechanges in their chromatograms were investigated.
2.4. Re-hydroxylation of the chlorinated column surface.
Because the column is repeatedly heated and cleaned in achromatography experiment, it is difficult to keep a uniform
chlorinated surface. Therefore, chlorine atoms on the surfaceof the column were re-substituted once with hydroxyl groups(re-hydroxylation) and newly chlorinated. Re-hydroxylation of
the isothermal column was performed at 300 °C for 24 h byf lushing air saturated with water as described in previous
reports.8–11 After this treatment, chromatography was carriedout to confirm that the surface was reliably hydroxylated andthe reproducibility was guaranteed.

J. Nucl. Radiochem. Sci., Vol. 21, 2021

Isothermal gas chromatography study of Zr and Hf tetrachlorides using

2.5. Analysis of the quartz glass surface. Quartz glass
was analyzed using X-ray photoelectron spectroscopy (XPS)
to verify the presence of chlorine atoms on the surface as aresult of the chlorination treatment. The hydroxyl groups on
the surface, which were reduced by the chlorination, were
measured using attenuated total reflection infrared (ATR-IR)
spectroscopy. Small pieces of quartz glass plate of the samematerial as the chromatography column were used instead of
the column itself for the convenience of observation. The
chlorination was performed in the same way as in the case of
the column.
The XPS measurements were performed by Quantum 2000
(ULVAC-PHI) using a monochromatic Al Kα X-ray source(1486.6 eV) with a spot size of 100 μm and a take-off angle of
45°. Narrow scan spectra in the Cl 2p range were measured ata pass energy of 58.7 eV.
The instrument used for ATR-IR spectroscopy was the
VIR-9500 (Jasco) with a diamond crystal ATR accessory(Specac). Spectra were stored in the 800–6000 cm−1 range
with a resolution of 8 cm−1 by accumulating 1000 scans.
3. Results and Discussion
3.1. Derivation of average retention time. Example
results of the isothermal chromatography experiments areshown in Figure 2, where cumulative yields of ZrCl4 and HfCl4
at the isothermal temperature (Tiso) of 140 °C using the nontreated column are plotted against elapsed time, and chromatogram values deduced from the cumulative yields are also
shown. The characteristic parameter for the chromatogram isaverage retention time ( tˉr ), which is the weighted average timefor volatile chemical species to pass through an isothermal column. The average retention time is defined as
n

tr =
i =1

(2i – 1)

t
yi
2

(1)

Figure 2. Cumulative yields of ZrCl4 (a) and HfCl4 (b) at an isothermal column temperature of 140 °C using a non-treated column (circles). The × and + symbols are chromatogram values obtained fromthe difference in cumulative yield. The dashed line indicates theaverage retention time.

9

where ∆t the measurement time, n is the number of measurements, yi is an independent yield defined as ci − ci−1, and ci isthe cumulative yield obtained in the i-th measurement. By
rearranging eq (1), the following is obtained.
tr =

t
2

n

(2i – 1)(ci – ci–1)
i =1= nt –nt2cn t

ci–1

i =1

By setting c0 = 0, cn = 1, and n∆t = tn, the following is obtained.
t
tr = tn –
2

n–1

ci

t
i =1

In actuality, however, the cumulative yield ci here is smaller
than the true yield because the time at which the volatile species are measured depends on the time at which they reach thecollection part during the measurement time. Correcting for
this, the average retention time can be approximated as
Equation (2).
ntr tn t

ci

(2)

i =1

In the case of Figure 2, the average retention times of ZrCl4and HfCl4 were 15.2 ± 0.2 min and 15.4 ± 0.2 min, respectively. Errors were derived from statistical errors in cumulative yield measurements. Hereafter, the weighted averageretention time is simply denoted by the retention time.
3.2. Effect of the chlorination of the column surface. In
order to determine the optimal conditions for chlorination of
the column, the surface was chlorinated with CCl4 at 400, 600,
and 700 °C for 2 h, and at 600 °C for 1, 2, and 5 h. Here ‘optimal conditions’ mean the minimum temperature and time thatwould yield sufficient chlorination. As preliminary experiments showed that the retention time for chlorinated columns
was shorter than that for non-treated columns, the conditions
which gave the shortest retention time were regarded as the
optimal conditions. The dependences of the retention time onchlorination temperature and on chlorination time for ZrCl4and HfCl 4 at Tiso = 125 °C are shown in Figures 3 and 4,
respectively. From these results, the optimal conditions for thecolumn surface chlorination was determined to be 600 °C and2 h.
Figure 5 summarizes the retention times of ZrCl4 and HfCl4

Figure 3. Average retention time of ZrCl4 (black circles) and HfCl4(white circles) at the isothermal temperature of 125 °C using a column chlorinated at various temperatures for 2 h.

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Figure 4. Average retention time of ZrCl4 (black circles) and HfCl4(white circles) at 125 °C in the isothermal part by using a column
chlorinated at 650 °C for various time.

Figure 6. XPS spectra of the Cl 2p region taken using a narrow scanmode for the non-treated quartz glass plate (black) and the chlorinated plate (red).

Figure 5. Retention times of ZrCl4 (black bars) and HfCl4 (whitebars) at the isothermal temperature of 125 °C for non-treated (A),heat treated (B), chlorinated (C), re-hydroxylated (D), and re-chlorinated (E) columns.

Figure 7. Infrared spectra of non-treated (black), chlorinated (red),and re-hydroxylated (blue) quartz glass plates. Normalized with theintensity of the Si-O-Si band around 1000 cm−1.

obtained for non-treatment (A), simply heated (B), chlorinated(C), re-hydroxylated (D) and re-chlorinated (E) columns. It
has been reported that some OH groups were desorbed from aquartz glass surface heated to 600 °C.12 When the isothermalcolumn was simply heated at 600 °C for 2 h, the retention time
was almost the same as in the case without treatment. Thisfinding implied that the OH groups desorbed by simple heating had little effect on the adsorption of the chlorides. When
CCl4 was introduced into the column under the same heatingc o n d it io n s , t h e r e t e nt io n t i m e b e c a m e ve r y s h o r t .
Re-hydroxylation of the chlorinated column resulted in a longer retention time, but that time was shorter than the retention
time of the non-treated column, which meant that there might
be an irreversible substitution of OH and Cl. When the rehydroxylated column was newly chlorinated, the retention
time was almost the same as that in the first chlorination.
Therefore, chlorination after rehydroxylation could reproduce
a uniform surface chlorination.

observed in the implantation of chlorine ions into silicon crystals.13,14
Figure 7 shows the IR spectra of variously treated quartz
glass plates. For the non-treated plate, the sharp band at 3750
cm −1 and the close-lying band to it originating from OH
groups11,15–17 could be clearly recognized. In contrast, the signal in this range was much smaller for the chlorinated plates.
This indicated a decrease in the amount of OH groups on thesurface. Re-hydroxylation after chlorination regenerated thebands, but their intensities were weaker than those for the nontreated plate. This result was consistent with the change in theretention time of ZrCl4 and HfCl4, that is, chlorination led toan irreversible change in a part of the OH groups on the surface of the quartz glass.
The results obtained by XPS and IR qualitatively confirmed
the substitution of OH for Cl on the surface of the quartz glassby chlorination.

3.3. Surface analysis of the quartz glass surface. Narrowscan XPS spectra around Cl 2p for the chlorinated and nontreated quartz glass plates are shown in Figure 6. A peakaround 200 eV was observed for the chlorinated plate, while
no peak was observed for the non-treated plate. It is difficult
to obtain definite information about the Cl chemical statebecause the peak positions are not clear due to poor statisticalprecision of counting rates. However, it was likely to be present as Si-Cl, since the peak position was very close to that

3.4. Effect of column-surface modification on ∆ads H. In
order to investigate the effect of by modification of the column
surface on the ∆ads Hs of ZrCl4 and HfCl4, isothermal chromatography experiments were performed using the non-treatedand chlorinated columns at various isothermal temperatures.
Examples of cumulative yield curves obtained for the two
types of columns are shown in Figures 8 and 9. In studying
the gas-phase chemistry of superheavy elements, adsorption
enthalpy (∆ ads H) of volatile species for a column surface isusually deduced by using a Monte Carlo simulation (MCS)Isothermal gas chromatography study of Zr and Hf tetrachlorides using

J. Nucl. Radiochem. Sci., Vol. 21, 2021

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Figure 8. Examples of cumulative yields for the non-treated column. The black and white triangles represent ZrCl4 and HfCl4, respectively.
Solid lines show the results of MCS. Dashed lines indicate the position of the average retention time.

Figure 9. Examples of cumulative yields for the chlorinated column. The black and white circles represent ZrCl4 and HfCl4, respectively. Solidlines show the results of MCS. Dashed lines indicate the position of the average retention time.

based on the column-migration model.18 In this method, usually the value of the oscillation period τ 0 is assumed and allvalues of experimental parameters are used as they are, and∆ads H is treated as a free parameter and determined from thevalue which yields the best fit of an experimental result. The
time dependence for cumulative yield was very well reproduced by MCS. This indicated the validity of the columnmigration model that MCS is based on for the present study.
Türler and Pershina3 reanalyzed the results of reported gasphase chemistry experiments for the halides of group-4 elements by MCS with τ0 = 2 × 10−13 s. Their analysis allowed the∆ ads H values of different experiments to be compared witheach other. In the present work, the ∆ ads H values wereobtained by using τ0 = 2 × 10 −13 s and they are summarized inTables 1 and 2 along with all the obtained retention times. The

obtained ∆ads H values were almost the same at all column temperatures for each column type and there seemed to be no difference between ZrCl4 and HfCl4. For the non-treated column,
the average ∆ads H values of ZrCl4 and HfCl4 were −92.6 ± 0.1
kJ mol−1 and −93.2 ± 0.1 kJ mol−1, respectively. On the other
hand, those for the chlorinated column were −83.7 ± 0.2 kJ
mol−1 and −84.2 ± 0.2 kJ mol−1 for ZrCl4 and HfCl4, respectively. These values are plotted in Figure 10 together withthose reported by Türler and Pershina.3 The reported results
were somewhat scattered in each experiment, and this mightcome from fluctuation among different column-surface conditions.
3.5. Parameter-free evaluation of ∆ads H. The experimental cumulative yields were well reproduced by MCS as shown

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TABLE 1: Retention times at various isothermal temperatures and adsorption enthalpies derived by MCS for the
non-treated column.

Tiso
/ °C
135
140

145

150

155

160

tˉr / min

ZrCl4

HfCl4

20.8 ± 0.2
23.9 ± 0.3
16.6 ± 0.1
15.3 ± 0.2
15.5 ± 0.3
10.2 ± 0.2
9.0 ± 0.2
10.2 ± 0.1
7.5 ± 0.2
6.9 ± 0.2
7.8 ± 0.1
5.6 ± 0.3
5.6 ± 0.2
5.7 ± 0.2
4.0 ± 0.1
4.1 ± 0.2
3.7 ± 0.1

20.4 ± 0.2
22.6 ± 0.2
16.0 ± 0.2
15.5 ± 0.2
15.7 ± 0.3
10.4 ± 0.3
8.9 ± 0.2
10.1 ± 0.2
7.4 ± 0.2
7.1 ± 0.2
7.9 ± 0.1
6.5 ± 0.3
5.8 ± 0.3
5.4 ± 0.2
4.2 ± 0.1
4.6 ± 0.2
3.8 ± 0.2

∆adsH (MCS)/ kJ mol−1
ZrCl4
HfCl4

TABLE 2: Retention times at various isothermal temperatures and adsorption enthalpies derived by MCS for thechlorinated column.

Tiso
/ °C

−93.0

−93.4

−93.1

−93.5

−93.1
−92.7

−93.6
−93.3

−93.0

−93.6

−92.6
−92.1

−93.2
−92.6

−92.4

−93.0

115

−92.7
−92.2

−93.3
−92.8

120

−92.6

−93.3

−92.6
−92.5
−92.3
−92.3
−92.6
−92.3

−93.6
−92.8
−92.8
−93.1
−93.6
−93.0

100
105
110

125
140

tˉr / min

ZrCl4

HfCl4

16.0 ± 0.3
16.0 ± 0.3
12.7 ± 0.2
10.1 ± 0.2
9.4 ± 0.3
9.6 ± 0.4
6.4 ± 0.1
5.4 ± 0.1
4.0 ± 0.1
3.2 ± 0.2
2.9 ± 0.3
2.8 ± 0.2
0.9 ± 0.1
0.8 ± 0.1

14.9 ± 1.0
16.0 ± 0.1
12.4 ± 0.3
9.9 ± 0.2
8.9 ± 0.4
9.9 ± 0.5
6.4 ± 0.1
5.6 ± 0.1
4.0 ± 0.1
3.4 ± 0.1
2.6 ± 0.2
2.8 ± 0.2
0.8 ± 0.1
0.9 ± 0.1

∆adsH (MCS)/ kJ mol−1
ZrCl4
HfCl4
−84.3
−84.8
−84.7
−85.2
−84.4
−84.8
−83.4
−84.0
−84.3
−84.6
−84.5

−85.2

−83.8
−84.3
−83.8
−83.2
−83.2
−83.2
−83.0
−81.9

−84.3
−84.8
−84.4
−83.9
−83.6
−83.6
−83.0
−82.8

in the previous section. However, the evaluation of ∆ads H by
MCS depends on experimental parameters and the oscillation
period τ 0. A new analytical method was proposed here toevaluate the ∆ads H value free from uncertain parameters suchas τ0.
In the column-migration model applied to isothermal chromatography,18 the average retention time ( tˉr ) is expressed as
the product of the average number of collisions with the wall
per unit length of the column ( vˉ ), the column length (L), andthe average time spent at the surface for a single adsorption( τˉa ). This is given as Equation (3):tr = L v a =LrT0 P
Q0 P0

2
 H
0 exp – ads
MTiso
RTiso

(3)

where r is the inner radius of the column, Tiso and P are theisothermal temperature and pressure at which chromatography
is performed, respectively, Q0 is the flow rate at the reference
temperature T0 and pressure P0, M is the molar mass of themolecule of interest, and R is the gas constant.
Equation (3) can be rearranged as H 1
LrT0 Pln( tr Tiso) = – ads+ ln
R Tiso
Q0 P0

2
0 .
M

(4)

Therefore, a plot of ln( tˉr Tiso) against 1/Tiso is expected to yielda straight line, and from its slope ∆ads H can be obtained independent of parameters such as τ0.
Figure 11 plots the results of ln( tˉr Tiso) against 1/Tiso, wherethe values of tˉr and Tiso tabulated in Tables 1 and 2 were used.
As seen in the figure, the plotted values showed good linearity
as expected. The straight lines were fitted using a weighted
least squares method. From the slopes of the fitted lines, thevalues of ∆ ads H free from parameters were obtained for the
first time and those of ZrCl4 and HfCl4 for the non-treated column were −101.3 ± 4.0 kJ mol−1 and −98.1 ± 3.1 kJ mol−1,
respectively. For the chlorinated column, ∆ads H of ZrCl4 was−85.1 ± 4.5 kJ mol−1 and that of HfCl 4 was −84.2 ± 3.3 kJ
mol −1. No significant difference was found between theadsorption enthalpies of ZrCl4 and HfCl4 for either type of col-Figure 10. Comparison of the obtained adsorption enthalpies.
Squares plot the enthalpies given by Türler and Pershina 3 in their
Table 2. Blue marks indicate the results obtained in the present study
by a MCS method with τ0 = 2 × 10−13 s. Red marks indicate the results
obtained by the new method of the present study. Triangles are fornon-treated column and circles are for chlorinated column. Lines
connect values for the same series of experiments.

umn and the absolute value of ∆ads H for the non-treated column was larger than that for the chlorinated column. Theseobtained values of ∆ads H were compared with literature values3
in Figure 10. It was found that the values of ∆ads H for the chlorinated column were about the same with those obtained by
MCS, whereas the MCS method yielded a sizably smaller
absolute value of ∆ads H for the non-treated column comparedto that obtained in the new method. The values of τ0 could be
estimated from the ordinate intercept of the fitted line of eq(4), although the evaluated values contained large uncertainties arising from the error propagation in the fitting process.
For the chlorinated column, the evaluated values of τ0 were 1.4
× 10 −13 s and 2.1 × 10 −13 s for ZrCl4 and HfCl4, respectively.
These values were similar to the value used in the MCS
method, 2 × 10−13 s. On the other hand, for the non-treated column the evaluated τ 0 values were smaller than those for thechlorinated column, that is, 1.7 × 10 −14 s and 5.2 × 10 −14 s for
ZrCl4 and HfCl4, respectively. These trends in τ0 might be thereason for the difference of ∆ads H between the MCS method
and the new method. However, discussion of the reason

J. Nucl. Radiochem. Sci., Vol. 21, 2021

Isothermal gas chromatography study of Zr and Hf tetrachlorides using

Figure 11. Plot of ln( tˉr Tiso ) versus 1/Tiso for ZrCl4 (a) and HfCl4 (b).
Triangles and circles indicate the type of the column, non-treatedand chlorinated, respectively. Solid lines are the results of a weighted
least-square fitting.

should be postponed until future experiments are madebecause of the large uncertainty of τ0.
The obtained trend in ∆ads H was consistent with the trend in
the sublimation enthalpy of macro amounts of ZrCl 4 andHfCl4.19 The theoretical calculation in the framework of therelativistic density functional theory by Pershina et al.5 predicted that the adsorption enthalpies of ZrCl4 and HfCl4 were
almost the same with the adsorption mechanism being a physisorption provided by the van der Waals force. In the case of
the chlorinated column, a mechanism for adsorption of not
only the tetrachloride MCl4 but also the hexachloride complex
MCl62−, which is formed by complexing with chlorine atoms
on the surface, is also possible. However, the magnitude of theobtained adsorption enthalpy is smaller than that for the nontreated column and too small for chemisorption to form acomplex. In the case of the chlorinated column, it is more
likely to be a physisorption of MCl4 as well. If this adsorption
mechanism is adopted, RfCl4 is expected to have an enthalpy
close to those of homologues according to the theoretical prediction by Pershina et al.5 Therefore, it is very important toobtain the adsorption enthalpy of RfCl4 experimentally with areliable accuracy.
4. Conclusions
In previous studies on the gas-phase chemistry of Rf chlorides, the chemical properties of Rf could not be fully discussed because the values of the adsorption enthalpies of thehomologues Zr and Hf for quartz glass and the relationshipbetween them are different in each experiment. In this study,
off-line isothermal gas chromatography was done using longlived radioactive tracers to obtain reliable adsorption enthalpies of ZrCl4 and HfCl4. In addition, chromatography by using
a deliberately chlorinated column was also performed to simulate the case of a column surface modification by a chlorinating reagent in an online experiment. The chlorination of theisothermal column was achieved by heating with helium gas
saturated with carbon tetrachloride and XPS and IR analyses

13

confirmed this chlorination for the same type of quartz glass
plate as the column. The retention time for the chlorinatedcolumn was shorter than that for the non-treated column,
which indicated the adsorption interaction for the chlorinatedcolumn was weaker than that for the non-treated column.
Detailed isothermal chromatography experiments were performed by using the non-treated and the chlorinated columns
at various isothermal temperatures. The time dependence incumulative yield was very well reproduced by MCS indicating
the validity of the column-migration model. A conventional
analysis using the MCS method found that there was no significant difference between the adsorption enthalpies of ZrCl4and HfCl4 for either type of column and the absolute value of∆ads H for the non-treated column was larger than that for thechlorinated column.
In the present study, an analytical method was developed toobtain the adsorption enthalpy without depending on theparameters of the experimental apparatus or uncertain physical quantities. According to this method, the adsorption
enthalpies of ZrCl4 and HfCl4 were −101.3 ± 4.0 kJ mol−1 and−98.1 ± 3.1 kJ mol−1 for the untreated column, respectively.
For the chlorinated column, −85.1 ± 4.5 kJ mol−1 for ZrCl4 and−84.2 ± 3.3 kJ mol−1 for HfCl4 were obtained. There was no
significant difference between the adsorption enthalpies of Zr
and Hf for the two types of columns. These results were consistent with the trend in the sublimation enthalpy of macro
amounts of ZrCl4 and HfCl4 and the theoretical calculation by
Pershina et al. 5, in which the adsorption mechanism wasassumed to be a physisorption provided by the van der Waals
force.
Acknowledgments
The 88 Zr and 175 Hf were supplied through the Supply
Platform of Short-lived Radioisotopes, supported by JSPS
Grant-in-Aid for Scientific Research on Innovative Areas,
Grant Number 16H06278. This work was partly supported by
the Sasakawa Scientific Research Grant from the Japan
Science Societ y and JSPS K A K EN HI G rant Nu mber
18K11917.
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