Catalytic oxidation of carbon monoxide over radiolytically prepared Pt nanoparticles supported on glass

Article abstract of DOI:10.1016/j.materresbull.2005.05.015

Platinum nanoparticles have been prepared by radiolytic and chemical methods in the presence of stabilizer gelatin and SiO₂ nanoparticles. The formation of Pt nanoparticles was confirmed using UV-vis absorption spectroscopy and transmission electron microscopy (TEM). The prepared particles were coated on the inner walls of the tubular pyrex reactor and tested for their catalytic activity for oxidation of CO. It was observed that Pt nanoparticles prepared in the presence of a stabilizer (gelatin) showed a higher tendency to adhere to the inner walls of the pyrex reactor as compared to that prepared in the presence of silica nanoparticles. The catalyst was found to be active at ≥150 °C giving CO₂. Chemically reduced Pt nanoparticles stabilized on silica nanoparticles gave ～7% CO conversion per hour. However, radiolytically prepared Pt nanoparticles stabilized by gelatin gave ～10% conversion per hour. Catalytic activity of radiolytically prepared platinum catalyst, coated on the inner walls of the reactor, was evaluated as a function of CO concentration and reaction temperature. The rate of reaction increased with increase in reaction temperature and the activation energy for the reaction was found to be ～108.8 kJ mol^-1. The rate of CO₂ formation was almost constant (～1.5 × 10^-4 mol dm ^-3 h^-1) at constant O₂ concentration (6.5 × 10^-3 mol dm^-3) with increase in CO concentration from 2 × 10^-4 mol dm^-3 to 3.25 × 10 ^-3 mol dm^-3. The data indicate that catalytic oxidation of CO takes place by Eley-Rideal mechanism.

Full text of DOI:10.1016/j.materresbull.2005.05.015

Materials Research Bulletin 40 (2005) 1654–1661
www.elsevier.com/locate/matresbu
Catalytic oxidation of carbon monoxide over radiolytically
prepared Pt nanoparticles supported on glass
b
a
a,
S. Kapoor a, , A.D. Belapurkar , J.P. Mittal , T. Mukherjee
*
*
a Radiation Chemistry & Chemical Dynamics Division, Bhabha Atomic Research Centre,
Trombay, Mumbai 400085, India
b Applied Chemistry Division, Chemistry Group, Bhabha Atomic Research Centre,
Trombay, Mumbai 400085, India
Received 24 May 2005; accepted 26 May 2005
Abstract
Platinum nanoparticles have been prepared by radiolytic and chemical methods in the presence of stabilizer
gelatin and SiO2 nanoparticles. The formation of Pt nanoparticles was confirmed using UV–vis absorption
spectroscopy and transmission electron microscopy (TEM). The prepared particles were coated on the inner walls
of the tubular pyrex reactor and tested for their catalytic activity for oxidation of CO. It was observed that Pt
nanoparticles prepared in the presence of a stabilizer (gelatin) showed a higher tendency to adhere to the inner walls
of the pyrex reactor as compared to that prepared in the presence of silica nanoparticles. The catalyst was found to
be active at ꢀ150 8C giving CO2. Chemically reduced Pt nanoparticles stabilized on silica nanoparticles gave ꢁ7%
CO conversion per hour. However, radiolytically prepared Pt nanoparticles stabilized by gelatin gave ꢁ10%
conversion per hour. Catalytic activity of radiolytically prepared platinum catalyst, coated on the inner walls of the
reactor, was evaluated as a function of CO concentration and reaction temperature. The rate of reaction increased
with increase in reaction temperature and the activation energy for the reaction was found to be ꢁ108.8 kJ molÀ1
(6.5 Â 10À3 mol dmÀ3) with increase in CO concentration from 2 Â 10À4 mol dmÀ3 to 3.25 Â 10À3 mol dmÀ3
The data indicate that catalytic oxidation of CO takes place by Eley–Rideal mechanism.
# 2005 Elsevier Ltd. All rights reserved.
.
The rate of CO2 formation was almost constant (ꢁ1.5 Â 10À4 mol dmÀ3 hÀ1) at constant O2 concentration
.
Keywords: A. Nanostructures; C. Electron microscopy
* Corresponding authors. Tel.: +91 22 25595234; fax: +91 22 25505331.
E-mail addresses: sudhirk@apsara.barc.ernet.in (S. Kapoor), mukherji@magnum.barc.ernet.in (T. Mukherjee).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.05.015

S. Kapoor et al. / Materials Research Bulletin 40 (2005) 1654–1661
1655
1. Introduction
In recent decades, there has been a considerable interest in the preparation and characterization of
nanosized particles, both from fundamental and practical points of view [1–7]. The nanosize-associated
catalytic activities have many potential applications. Worldwide attention has been aroused by the
discovery of Haruta concerning the catalysis of Au nanoparticles [8]. In particular, the oxidation of CO
has attracted a lot of interest because of its broad technological and fundamental importance [9–12]. The
catalytic activity has been found to be sensitive to the size of the Au nanoparticles, the nature of the
support and the method of preparation [13].
Central to the problem of oxidation of CO is the activation of oxygen molecules [14]. It is known that
Au nanoparticles adsorb CO molecules well, they do not strongly adsorb and activate O2 molecules [15].
Thus, the oxide support often plays an important role in the activation of O2. In this regard, large number
of catalysts have been evaluated for oxidation reaction of CO [16–27]. Also, a lot of work has been
carried out for the catalytic recombination of CO and O2 for application in sealed off CO2 laser [28–31].
Various mechanisms have been proposed for explaining CO + O2 reaction [32–34]. In spite of these
efforts, there is still scope for a better catalyst for application in sealed off CO2 laser. For example, if an
active, adherent, discontinuous film of small noble metal particles is coated inside a laser tube, it may be
possible to recombine CO and O2 generated and maintain the optimum laser output.
In the present work, we report on the catalytic efficiency of radiolytically prepared Pt nanoparticles for
CO–O2 recombination reaction. To mimic the conditions for practical applications, Pt nanoparticles were
coated inside a tubular glass reactor and studied for CO–O2 recombination reaction under static reaction
condition which is almost similar to one existing in CO2 laser tube. Also, an attempt was made to evaluate
the catalytic efficiency of coated Pt nanoparticles with time to check the propensity for poisoning [35].
2. Experimental
Tetrachloroplatinic acid (BDH), sodium borohydride (BDH), gelatin (BDH) and SiO2 (Aldrich) were
used as received. All other chemicals were of HPLC, AR or GR grade. Silica (SM-30, 30 wt.%) was
obtained from Aldrich. IOLAR grade N2 gas (purity > 99.9%) was used for purging the solutions. All the
solutions were prepared in nanopure water (conductivity, 0.06 mS cmÀ1) just before the experiments and
kept in dark to avoid photochemical reactions. Gelatin was allowed to swell in water for 15 min at ambient
temperature and subsequently warmed at ꢁ40–50 8C for 2–3 min on a water bath with continuous stirring
soasto get a clear solution. Millimolarsolution ofmetalsaltswas prepared and added to thegelatin solution
along with 0.1 mol dmÀ3 methanol as ꢂOH radical scavenger. The mixed solution was purged with N2 prior
to gamma irradiation using a 60Co source with a dose rate of 20 Gy/min. The total dose used for irradiation
was 1 kGy. Chemically reduced Pt nanoparticles were prepared by adding drop wise ice-cold sodium
borohydride solution (10À3 mol dmÀ3) to K2PtCl4 solution (10À3 mol dmÀ3) containing gelatin or SiO2
nanoparticles (1.5 Â 10À3 mol dmÀ3) which are used to prevent aggregation of Pt particles. These
nanoparticles were then coated on the inner walls of the reactor and used for CO oxidation activity.
Colloidal metal particles were characterized by UV–vis absorption spectroscopy and transmission
electron microscopy (TEM) [JEOL, JEM-2000 FX model]. Samples for TEM were prepared by putting a
drop of the metal solution on a copper grid coated with a thin film of amorphous carbon. Samples were
vacuum-dried before putting them in TEM specimen holder. A closed cylindrical glass reactor of 75 ml

1656
S. Kapoor et al. / Materials Research Bulletin 40 (2005) 1654–1661
capacity (24 cm length  2 cm diameter) was etched with 10% HF solution, washed and dried. This dry
reactor was soakedwiththe aqueous metalsol. The reactor was dried at 100 8C after draining allthe sol. The
procedure was repeated five times. Amount of Pt deposited was analysed by atomic absorption spectro-
scopy after dissolving in aqua regia and found to be 0.078 mg and 0.057 mg for radiolytically and
chemically prepared Pt nanoparticles, respectively. Surface roughness of the metal coated reactor
was evaluated using Mitutoyo Surftst-211 instrument. Catalytic activity of the glass-supported Pt
nanoparticles was studied under static reaction condition using a metal-coated glass reactor. The reactor
was evacuated and filled with high purity helium at atmospheric pressure. Different concentrations of
CO [2 Â 10À4 mol dmÀ3 to 3.25 Â 10À3 mol dmÀ3] were used in the presence of fixed concentration of O2
(6.5 Â 10À3 mol dmÀ3) during CO oxidation reaction. The progress of the reaction was monitored by
periodic analysis of the reactor content by gas chromatography [Chemito-8510] using TCD and 8 ft
spherocarb column maintained at 100 8C. The rate of the reaction was calculated on the basis of the time
required for 10% CO conversion. The effect of reaction temperature was studied by maintaining the reactor
at a desired temperature [100–350 8C]. For comparison, CO oxidation by Pt nanoparticles, prepared by
chemical reduction method and stabilized by SiO2 nanoparticles, was evaluated under identical conditions.
Also, thermal oxidation of CO was studied using the same reactor without metal coating.
3. Results and discussion
UV–vis absorption spectra for Pt sol prepared by g-irradiation or chemical reduction were consistent
with the reported data [36,37]. Fig. 1 shows a representative TEM picture of gelatin-stabilized
radiolytically prepared Pt nanoparticles used for catalytic reactions. It can be seen that the size of Pt
particles was in the range of 10–20 nm.
3.1. Catalytic oxidation of CO by gelatin-stabilized radiolytically prepared Pt nanoparticles
No detectable thermal reaction between CO and O2 was observed up to 500 8C in the absence of
metal nanoparticles. To examine the catalytic reactions, Pt nanoparticles were deposited on the reactor
Fig. 1. TEM picture for radiolytically prepared Pt nanoparticles.

S. Kapoor et al. / Materials Research Bulletin 40 (2005) 1654–1661
1657
walls. Cente line average (CLA) surface roughness of the Pt nanoparticles coated reactor was found
to be 0.41 mm. Glass supported Pt catalyst was found to be active for oxidation of CO at ꢀ150 8C.
Fig. 2a shows the amount of CO2 formed (normalized to per milligram of Pt) as a function of the
reaction time at 175 8C. It can be seen that the amount of CO2 formed varies linearly with the reaction
time.
The rate of catalytic CO oxidation was also studied as a function of temperature. It was found that the
rate of catalytic oxidation increased with increase in the reaction temperature. The plot of ln k versus 1/T
was found to be a straight line (Fig. 3). Activation energy of the reaction was calculated to be
ꢁ108.8 kJ molÀ1.
Fig. 4 shows the effect of CO concentration on the rate of CO2 formation at constant O2 concentration.
It can be seen from the figure that when CO concentration was increased from 2 Â 10À4 mol dmÀ3 to
3.25 Â 10À3 mol dmÀ3, the rate of CO2 formation remained constant at 1.5 Â 10À4 mol dmÀ3 hÀ1. This
indicates that reaction occurs via Eley–Rideal mechanism [38] between chemisorbed CO and gaseous/
physisorbed O2. As the rate of CO2 formation is constant, percent conversion of CO increases with
decrease in CO concentration (Table 1). It is important to mention here that CO oxidation activity of Pt
catalyst exposed to ambient air at room temperature for 9 months was found to be the same indicating that
the catalyst is stable under ambient conditions.
3.2. Catalytic activity of chemically reduced Pt nanoparticles
For comparison, K2PtCl4 was reduced with NaBH4 with and without gelatin. However, Pt nano-
particles so generated were found to agglomerate with time giving large particles that settled down easily.
Fig. 2. Variation in the yield of CO2 (normalized to 1 mg of Pt) as a function of reaction time for catalytic oxidation of CO at
175 8C, CO = 1.63 Â 10À3 mol dmÀ3 and O2 = 6.5 Â 10À3 mol dmÀ3: (a) gelatin-stabilized radiolytically prepared Pt nano-
particles and (b) chemically reduced Pt nanoparticles stabilized by SiO2 nanoparticles.

1658
S. Kapoor et al. / Materials Research Bulletin 40 (2005) 1654–1661
Fig. 3. Effect of temperature on the catalytic activity for the reaction of CO oxidation over Pt nanoparticles prepared
radiolytically in the presence of gelatin.
Hence, these particles were not used for measurements of catalytic activity. Therefore, K2PtCl4 was
reduced by NaBH4 in the presence of SiO2 nanoparticles as a dispersing medium. Pt nanoparticles so
generated did not agglomorate and remained in the sol form for more than 1 month. CO oxidation
activity of Pt nanoparticles so prepared was evaluated under an identical condition. It can be seen from
Fig. 2b that the yield of CO2 formed (normalized to per milligram of Pt) is higher for radiolytically
prepared Pt nanoparticles stabilized by gelatin as compared to that for the chemically reduced Pt
nanoparticles, stabilized by SiO2 nanoparticles. Gelatin-stabilized Pt nanoparticles were supported on
borosilicate (pyrex reactor) while chemically reduced Pt nanoparticles were supported on silica which in
turn were coated on the reactor wall. Catalyst-supports are known to affect the catalytic activity [13].
This may be the probable reason for higher catalytic activity of the radiolytically generated Pt
nanoparticles.
Fig. 4. Variation in the rate of CO2 formation with the change in the concentration of CO.

Table 1
Effect of CO concentration on the rate of CO2 formation over gelatin-stabilized radiolytically prepared Pt nanoparticles
S. no.
Catalyst
Reactant concentration
(10À3 mol dmÀ3
Reaction
temperature
(8C)
CO2 formed
(10À3 mol
CO
conversion
(% hÀ1
)
)
dmÀ3 hÀ1
)
CO
O2
1
2
Nil
Pt nanoparticle coated on
glass surface
3.25
3.25
6.5
6.5
500
175
0
0.15
0
4.6
3
4
5
6
Pt nanoparticle coated on
glass surface
1.63
0.83
0.41
0.2
6.5
6.5
6.5
6.5
175
175
175
175
0.16
0.13
0.13
0.14
10.2
15.5
30.5
70.9
Pt nanoparticle coated on
glass surface
Pt nanoparticle coated on
glass surface
Pt nanoparticle coated on
glass surface

1660
S. Kapoor et al. / Materials Research Bulletin 40 (2005) 1654–1661
4. Conclusions
Radiolytically prepared Pt nanoparticles on glass support are active for oxidation of CO at ꢀ150 8C.
Catalytic activity was found to increase with increase in reaction temperature. It appears that the
radiolytically prepared catalyst can be used for longer duration of time as its activity was found to be the
same even after 9 months. Also, the coating of gelatin-stabilized Pt nanoparticles over glass is more
effective as compared to chemically reduced Pt nanoparticles. The present work shows that very small
quantity of Pt is effective for oxidation of CO.
Acknowledgements
Authors thank Dr. N.M. Gupta, Head, Applied Chemistry Division, for providing the experimental
facilities to carry out catalytic oxidation studies. Authors also thank the referees for many useful
suggestions.
References
[1] C. Hayashi, Phys. Today 40 (1987) 44.
[2] H.C. Youn, S. Baral, J.H. Fendler, J. Phys. Chem. 92 (1988) 6320.
[3] P. Kumar, V. Pillai, D.O. Shah, Appl. Phys. Lett. 62 (1993) 765.
[4] D.H. Chen, J.J. Yeh, T.C. Huang, J. Colloid Interface Sci. 215 (1999) 159.
[5] M.P. Pileni, J. Phys. Chem. 97 (1993) 6961.
[6] J.H. Fendler, Chem. Rev. 87 (1987) 877.
[7] A. Henglein, Chem. Rev. 89 (1989) 1861.
[8] H. Haruta, Catal. Today 36 (1997) 153.
[9] F.G. Dwyer, in: H. Heinmann (Ed.), Catalysis Reviews, vol. 6, Marcel Dekker Inc., New York, 1972, p. 261.
[10] J. Wei, in: D.D. Eley, H. Pines, P.B. Weisz (Eds.), Advances in Catalysis, vol. 24, Academic Press, 1975, p. 57.
[11] M. Katz, Advances in Catalysis, vol. 5, Academic Press, 1953,, p. 177.
[12] Low-Temperature CO Oxidation Catalysts for Long Life CO2 Lasers, D.R. Schryer, G.B. Hoflund (Eds.), NASA
Conference Publication No. 3076, 1990, p. 405.
[13] G.C. Bond, D.T. Thompson, Catal. Rev. Sci. Eng. 41 (1999) 319.
[14] M.M. Schubert, S. Hackenberg, A.C. Van Veen, M. Muhler, V. Plazak, R.J. Behm, J. Catal. 197 (2001) 113.
[15] Z.-P. Liu, P. Hu, A. Alavi, J. Am. Chem. Soc. 124 (2002) 14770.
[16] J.S. Close, J.M. White, J. Catal. 36 (1975) 185.
[17] T. Matsushima, J.M. White, J. Catal. 29 (1975) 265.
[18] T. Matsushima, C.J. Mussett, J.M. White, J. Catal. 41 (1976) 397.
[19] H. Heyne, F.C. Tompkins, Proc. R. Soc. Lond. Ser. A 292 (1966) 460.
[20] C.A. Becker, J.P. Cowin, L. Wharton, D.J. Auerbach, J. Chem. Phys. 67 (1977) 3394.
[21] T. Matsushima, D.B. Almy, J.M. White, Surf. Sci. 67 (1977) 89.
[22] T.E. Madey, H.A. Engelhardt, D. Menzel, Surf. Sci. 48 (1975) 304.
[23] C.T. Campbell, J.M. White, J. Catal. 54 (1978) 289.
[24] C.T. Campbell, S.K. Shi, J.M. White, Appl. Surf. Sci. 2 (1979) 382.
[25] M. Haruta, T. Kabayashi, H. Sano, N. Yamada, Chem. Lett. 2 (1987) 405.
[26] M. Haruta, S. Tsubota, T. Kobayashi, M. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 173.
[27] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301.
[28] D.S. Stark, A. Crocker, G.J. Steward, J. Phys. E: Sci. Instrum. 16 (1983) 158.

S. Kapoor et al. / Materials Research Bulletin 40 (2005) 1654–1661
1661
[29] A.K. Tripathi, N.M. Gupta, U.K. Chatterjee, R.M. Iyer, Ind. J. Technol. 30 (1992) 107.
[30] F.M. Taylor, A. Lombardo, W.C. Eppers, Appl. Phys. Lett. 11 (1967) 180.
[31] P.G. Browne, ALS. Smith, J. Phys. D Appl. Phys. 7 (1974) 2464.
[32] C.J. Zhang, P. Hu, A. Alavi, J. Chem. Phys. 114 (2001) 8113.
[33] J. Jia, J.N. Kondo, K. Domen, K. Tamura, J. Phys. Chem. B 105 (2001) 3017.
[34] I. Meusel, J. Hoffmann, J. Hartmann, J. Libuda, H.-J. Freund, J. Phys. Chem. B 105 (2001) 3567.
[35] P.C. Biswas, Y. Nodasaka, M. Enyo, M. Haruta, J. Electroanal. Chem. 381 (1995) 167.
[36] A.D. Belapurkar, S. Kapoor, S.K. Kulshreshtha, J.P. Mittal, Mater. Res. Bull. 36 (2001) 145.
[37] A. Henglein, B.G. Ershov, M. Malow, J. Phys. Chem. 99 (1995) 14129.
[38] R.J. Farrauto, C.H. Bartholomew, Fundamentals of Industrial Catalytic Processes, 1997, p. 37.