Radiolytic preparation and catalytic properties of platinum nanoparticles

Article abstract of DOI:10.1016/S0025-5408(01)00499-8

Stable Pt sols were prepared by the radiolytic reduction of H₂PtCl₄ in presence of protective water soluble polymer gelatin. The particle size and morphology were determined by TEM and found to be in the nm range. The catalytic activity of the sols was tested by hydrogenation of C₂H₄ at room temperature and compared with the standard Pt/Al₂O₃ catalyst. The effects of CH₃OH addition and glass support on the rate of the catalytic reaction were also investigated.

Full text of DOI:10.1016/S0025-5408(01)00499-8

Materials Research Bulletin 36 (2001) 145–151
Pergamon
Radiolytic preparation and catalytic properties of platinum
nanoparticles
A.D. Belapurkar, S. Kapoor, S.K. Kulshreshtha, J.P. Mittal*
Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400 085, India
(Communicated by C.N.R. Rao)
Received 16 March 2000; accepted 22 May 2000
Abstract
Stable Pt sols were prepared by the radiolytic reduction of H2PtCl4 in presence of protective water
soluble polymer gelatin. The particle size and morphology were determined by TEM and found to be
in the nm range. The catalytic activity of the sols was tested by hydrogenation of C2H4 at room
temperature and compared with the standard Pt/Al2O3 catalyst. The effects of CH3OH addition and
glass support on the rate of the catalytic reaction were also investigated. © 2001 Elsevier Science Ltd.
All rights reserved.
Keywords: A. Nanostructures; C. Electron microscopy; D. Catalytic properties
1. Introduction
Study of nanosized metal particles is of great interest because of their potential applica-
tions in optical, electronic and magnetic devices [1–4]. Finely dispersed metal particles have
found wide applications as catalyst materials. For heterogeneous catalysis, involving gas
phase reactions, the noble metal particles are finely dispersed on the surface of inert oxide
materials like Al2O3, SiO2, TiO2 etc [5]. Such catalyst are produced by impregnation of
oxide powder with metal salt solution and its subsequent reduction in hydrogen stream
leading to the formation of finely dispersed metal particles. For homogeneous catalysis
where the reactants and products are in liquid phase, the metal particles need to be finely
dispersed in the liquid phase in the form of stable sol. A variety of reducing agents have been
* Corresponding author. Fax: ϩ91-22-5505151.
E-mail address: mittaljp@magnum.barc.ernet.in (J.P. Mittal).
0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0025-5408(01)00499-8

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used for the reduction of metals ions and the agglomeration of metal particles is prevented
by using suitable capping reagent like PVA, PVP, gelatin and other polymers [6–8]. One of
the major limitation of this method is the presence of excess reducing agent and the by
products, produced during reduction process, which can affect the catalytic reactions. As
most of the organic reactions are carried out in organic solvents, efforts have been made to
produce finely dispersed metal particles in sol form by using alcohol reduction [9–11]
Radiolytic reduction of metal ions in aqueous phase is a clean process [12–16]. The hydrated
electron produced during the radiolysis of water is used for the reduction of metal ions. Here
also the agglomeration of metal particles is prevented by suitable capping reagents. The
shape and size of metal particles can be controlled by the relative concentration of metal ions
and capping reagent [17,18].
There are very few studies dealing with the evaluation of the catalytic activity of
radiolytically produced metal sols [19–22]. In the present communication, we report the
preparation of Pt sol in water using radiolytic methods and evaluate their catalytic activity
for a model reaction namely ethylene hydrogenation and results are compared with those
obtained for the standard Pt/Al2O3 catalyst. The effect of adding methanol on the catalytic
activity of Pt sol was also studied.
2. Experimental
Tetrachloroplatinic acid (Sigma) and gelatin (BDH) were used as received. All other
chemicals were of HPLC, AR or GR grade. IOLAR grade N2 gas (purity Ͼ99.9%) was used
for purging the solutions. All solutions were prepared in Nanopure water (conductivity 0.06
␮S cmϪ1) just before the experiments and kept in dark to avoid photochemical reactions.
Gelatin was allowed to swell in water for 15 minutes at ambient temperature and subse-
quently warmed at ϳ40–50°C for 2–3 min on water bath with continuos stirring so as to get
a clear solution. Millimolar solution of chloroplatinic acid was prepared and added to the
gelatin solution along with 0.1 mol dmϪ3 methanol as the OH radical scavenger. The mixed
solution was purged with N2 prior to gamma irradiation using 60Co source with a dose rate
of 20 Gy/min.
The formation of colloidal Pt nanoparticle was characterized by means of UV-vis spec-
trophotometry and transmission electron microscopy (TEM) (JEOL, JEM - 2000FX model).
Samples for TEM studies were prepared by putting a drop of Pt sol on a copper grid coated
with a thin amorphous carbon film. Samples were vacuum dried before putting them in TEM
specimen holder.
␥-Al2O3 of 60–80 mesh size was impregnated with aqueous H2PtCl6 solution and reduced
in 50% H2-Ar gas mixture at 300°C. Reduced catalyst was cooled in H2-Ar mixture to room
temperature and flushed with Ar for ϳ1 hr. Catalyst so obtained contained 6.2 wt% Pt.
The catalytic activity of radiolytically prepared Pt sol for ethylene hydrogenation was
studied in a closed glass reactor of 250 ml capacity at room temperature. The reactor was
evacuated and filled with high purity argon at atmospheric pressure. 8 ml aqueous Pt sol
containing different amount of Pt was injected in the reactor followed by injection of 10 ml
C2H4 (3.33% v/v) and 40 ml H2 (13.3% v/v). The reaction rate was monitored by analyzing

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gas mixture present in the reactor by gas chromatographic method (Chemito 850 model)
using 8 feet porapak Q-S column and FID detector. Catalytic activity of Pt/Al2O3 with and
without moisture was also evaluated using the same glass reactor.
Different amounts of methanol were added to aqueous Pt sol containing 40 mM Pt to study
its effect on C2H4 hydrogenation under static conditions at room temperature. In these
experiments same volume of methanol containing Pt sol having varying amounts of Pt, was
used. For comparison activity evaluation was also carried out using Pt sol diluted with equal
amount of water to account for the decrease in Pt concentration by methanol addition. Effect
of various reaction parameters such as Pt metal concentration and the relative ratio of water
and methanol present in Pt sol, was investigated with a view to see their effect on the reaction
rate and product yield.
3. Results and discussion
Fig. 1a shows the absorption spectrum of Pt sol which is consistent with the results
available in the literature [21]. Fig. 1b shows the TEM picture of these particles. The average
size of platinum particles is found to be in the range of 10–20 nm.
3.1. Effect of Pt concentration on catalytic activity. Fig. 2 shows the plot of C2H6 formation
as a function of time for two sols containing different concentrations of Pt along with the
results of control experiments having no Pt particles. No ethane formation was observed in
the absence of Pt. The amount of C2H6 formed increases monotonically with increase in
reaction time and Pt concentration in aqueous sol. Based on the analysis of gas samples
collected from the reactor at different time intervals, it was observed that ethane is the only
reaction product as the total concentration of ethane and ethylene was found to be constant
for all these samples.
3.2. Effect of methanol addition on catalytic activity. To study the effect of adding methanol
on catalytic activity, the particles are produced in water because the radiolytic yield of
solvated electron is more in water compared to methanol [23]. Ethylene hydrogenation
reaction was studied in presence of various amounts of methanol (up to 25%) in aqueous Pt
sol and the results are listed in Table 1. From this table it is observed that the catalytic activity
of Pt particles is not affected by addition of large amount of methanol.
3.3. Effect of ageing. To test the stability of Pt nanoparticles the sol was kept under ambient
conditions for three months. The extent of Pt precipitated was found to be almost negligible
and the sol was found to be still active with almost same efficiency for C2H4 hydrogenation.
3.4. Glass supported Pt nanoparticles. Pt nanoparticles were found to strongly adhere on the
surface of glass reactor. The Pt coating was found to be quite strong and could not be
removed by washing with water and acetone. Pt coated reactor was evacuated after washing
with water and acetone and used for C2H4 hydrogenation. The rate of C2H6 formation was
found to be 2050 ml/hr.g Pt compared to 339 ml/hr.g of Pt observed over aqueous sol of Pt

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Fig. 1. (a) Absorption spectrum of Pt sol obtained on ␥-irradiation of an N2-bubbled solution containing 1.0 ϫ
10Ϫ3 mol dmϪ3 K2PtCl4, 0.1% (wt/v) gelatin and 1.0 ϫ 10Ϫ2 mol dmϪ3 methanol at pH 7. Dose ϭ 480 Gy. (b)
TEM picture for Pt particles.

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149
Fig. 2. Amount of ethane formed as a function of reaction time over Pt sol of different metal concentrations. (a)
Œ 0.0 M (b) ■ 25 mM (c) F 40 mM.
nanoparticles. Lower activity of aqueous Pt sol may be due to lower mass transfer of the
rectants C2H4 and H2 to the surface of Pt nanoparticles which are surrounded by water
molecules. No such constrain exists for glass supported catalyst. hence it shows higher
activity.
3.5. Comparison of catalytic activity for Pt sol and Pt/Al2O3. Fig. 3(a) and (b) show the
results of ethylene hydrogenation over the surface of dry Pt/Al2O3 and Pt sol, prepared by
impregnation and radiolytic method respectively, for the same amount of Pt metal present in
the sample. These results are almost comparable. However when 2 or 8 ml of water was
Table 1
Effect of methanol concentration in Pt sol on the rate of C2H6 formation
Serial No.
Methanol concentration in sol
(mM)
Rate of C2H6 formation,
ml/hr. g Pt
1
2
3
10
789
4930
266.8
273.1
250.0

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Fig. 3. Amount of ethane formed as a function of reaction time in samples containing 6.2 wt% Pt. (a)  over dry
Pt/Al2O3 (b) F over Pt sol (c) ϩ over Pt/Al2O3 immersed in 2 ml water (d) ᭛ over Pt/Al2O3 immersed in 8 ml
water.
added to Pt/Al2O3 catalyst the activity was found to be decreased significantly as can be seen
from Fig. 3(c) and (d).
4. Conclusion
The above study shows that the aqueous sol of Pt prepared by radiolytic method shows
good activity for C2H4 hydrogenation. The activity of Pt/Al2O3 in wet state was found to be
significantly less than that of Pt sol. The hydrogenation activity of Pt sol is approximately
proportional to its Pt concentration and the Pt nanoparticles supported on glass surface
showed higher activity than aqueous Pt sol.
Acknowledgments
The authors are thankful to Dr. N.M. Gupta, Head, Applied Chemistry Division for
providing the experimental facilities to carry out ethylene hydrogenation reaction.
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