富山大学水素同位体科学研究センター研究報告34：45－50，2014.

Research Note
溶媒洗浄と焼成による Pt ナノ粒子の PVP 保護ポリマー除去
田口 明，長岐友介，米山優紀

富山大学水素同位体科学研究センター
930-8555

富山市五福 3190

Solvent washing and calcination for effective PVP-cap removal
on Pt nanoparticles

Akira Taguchi, Yusuke Nagaki, Yuki Yoneyama

Hydrogen Isotope Research Center, University of Toyama
Gofuku 3190, Toyama 930-8555(Received April 17, 2015; accepted July 10, 2015)
Abstract
Repeated washing with ethanol and hexane of polyvinylpyrrolidone (PVP)-capped Pt nanocolloid could
generate an uncovered Pt surface. Pt dispersion was significantly improved by this washing as compared tounwashed Pt. IR spectrometry revealed that PVP-free Pt supported on SiO2 could be obtained by calcination
at 200 ºC, while unwashed Pt required more than 250 ºC.

Research note
Polyvinylpyrrolidone (PVP) is one of the most commonly used polymeric capping agentsfor metal nanoparticle preparation [1,2]. The PVP used during synthesis is usually removed
before or after deposition on a catalyst support. While UV-ozone treatment [3] and oxygen145

田口明・長岐友介・米山優紀

plasma treatment [4] are interesting methods for PVP removal, thermal treatment is the mostcommon method for this purpose. Free PVP starts to decompose at about 280 ºC in the presenceof O2 [5]. Metal nanoparticles enhance the combustion of PVP and decrease the PVP
decomposition temperature to 180 ºC. However, about 380 ºC is needed for the full combustionof PVP [5]. Thus, the removal of PVP under mild conditions is still a challenging issue.
Recently, washing by ethanol-hexane cycles has been found to remove PVP from Pt surface[6,7]. Although PVP partially remained on metal nanoparticles, the metal even in this condition
was effective in catalyzing ethylene hydrogenation [5,6] and the ring opening reaction ofbenzocyclopentane [8].
We investigated the PVP removal from Pt nanocolloid using ethanol/hexane washing andcalcination. Pt nanocolloid with and without PVP washing were supported on SiO2 byimpregnation, followed by calcination at different temperatures. The solvent washing was
found to be effective for catalytic combustion of PVP. The changes in infrared spectra and Pt
dispersion of the obtained SiO2 supported Pt nanoparticles are reported.
Commercially available SiO2 beads (CARiACT Q-50, Fuji Silysia Chemical Ltd.; surface
area 76.2 m2/g [9]) was ground to powder and used in this study. PVP-capped Pt nanocolloid(Pt diameter of about 2 nm, 0.195 wt% in H2O/ethanol) was purchased from Wako Pure
Chemical Industries, Ltd. An incipient wetness impregnation method, where the loading
amount of Pt was controlled to 2.0 wt%, was carried out for preparation of SiO2 supported Pt
samples as follows. The as-received PVP-capped Pt nanocolloid solution was concentrated byevacuation. Then, the obtained solution was re-dispersed in an adequate amount of methanol(typically, 1.66 mL for 1.00 g of SiO2). After impregnation this methanol solution into SiO2,
the sample was dried under vacuum, and subsequently at 60 ºC in an oven overnight. Theobtained catalyst was denoted as Pt-PVP (Table 1). Portions of the Pt-PVP were then calcined246

溶媒洗浄と焼成によるPtナノ粒子のPVP保護ポリマー除去

at 250 ºC or 400 ºC for 8 h. These are denoted

Table 1 Sample designations and Pt dispersionCalcinationSamplePt dispersion
temperature
0.23−
Pt-PVP
1.92
250 ºC
Pt-2508.13
400 ºC
Pt-4000.98−Pt(wsh)4.83
200 ºCPt(wsh)-200
9.36
400 ºCPt(wsh)-400
n.d.
−
SiO2

as Pt-250 and Pt-400, respectively (Table 1).
Solvent washing was carried out asdescribed

inliterature

withaslight

modification [6]. An adequate amount of thePVP-capped Pt nanoparticle solution wasdiluted with acetone (1/9 in volume) and centrifuged at 5,000 rpm for 10 min. The obtained
black oily phase was dispersed into ethanol under ultrasonication (5 min). Then, hexane wasadded into ethanol solution (1/3 of ethanol/hexane in volume), followed by ultrasonication andcentrifugagion again. The dispersion and centrifugation were repeated 3 times. Finally, the oilyphase was dispersed in methanol and used for impregnation in the same manner as mentionedabove. The sample obtained after drying was denoted Pt(wsh) (Table 1). Portions of Pt(wsh)was then calcined at 200 and 400 ºC for 8 h and denoted as Pt(wsh)-200 and Pt(wsh)-400,
respectively, as summarized in Table 1.
The FT-IR spectra were recorded on a Spectrum 100 instrument (Perkin Elmer). Selfsupported wafers were prepared by pressing, and the spectra were obtained with a resolution of
4 cm−1. Pt dispersion was measured by CO chemisorption using a pulse-flow reactor system.
About 0.1 g of the sample was reduced in advance under an H2 (50%)/N2 (balance) gas flow
(50 cm3/min) at 200 ºC for 1 h. A CO (2.0 %)/He (balance) gas mixture in 0.5 cm3 was injected
into the sample at 0 ºC. The concentration of CO in the outflow was monitored by an on-lined
thermal conductivity detector (TCD) in the gas-chromatograph (GC-8A, Shimadzu). Pt
dispersion was calculated using the following equation:

Pt dispersion =Number of CO molecules adsorbed� ���
Number of Pt atoms loaded347田口明・長岐友介・米山優紀

The CO sorption into SiO2 was below the TCD detection limit, and hence was neglected.
The IR spectrum of SiO2 is shown in Fig. 1. A broad absorption band around 3750−3000
cm-1 is in the hydroxyl-group region, possibly assignable to the asymmetric OH stretching ()vibration (abbreviated to OH) of Si-OH, adsorbed H2O and structural hydrogen-bonded OH[10]. The signals around 1990 and 1874 cm-1 are Si-O-Si. A signal around 1635 cm-1 isattributed to the OH bending () band (OH). A steep increase around 1250 cm-1 is due to theSi-O-Si framework. Impregnation with PVP-capped Pt gave the IR spectrum labeled Pt-PVPin Fig. 1, which had new bands around 3000−2800, 1525−1360 and 1660 cm-1 attributable tothe stretching bands of the PVP framework (PVP), the bending bands of the PVP framework(PVP), and the strong carbonyl group (C=O) band, respectively. Also, an enlargement of theOH band was observed in the Pt-PVP spectrum, reflecting the hydrophilic nature of PVP.
Calcination at 250 ºC (Fig. 1, Pt-250) lowered the signal intensities of PVP, PVP andC=O in the IR spectrum. CO chemisorption study revealed that the active Pt surface on
nanoparticles was 1.92 (Table 1). Further
calcination at 400 ºC (Fig. 1, Pt-400) resulted
Pt-400bands, suggesting a full combustion of PVP.

Pt-250Absorbance /a.u.

in the disappearance of PVP frameworkThis increased Pt dispersion to 8.13.
Incontrast,

whenethanol/hexane-Pt-PVP
Pt(wsh)-400Pt(wsh)-200Pt(wsh)washed Pt was loaded onto SiO2 (Fig. 1,Pt(wsh)),theIRspectrum

SiO2

showed4000

significantly smaller absorption bands of3500

3000 2500 2000
Wavenumber / cm-1

1500

1000

PVP, PVP and C=O, suggesting virtually Fig. 1 FT-IR spectra of SiO2-supported Pt calcined atdifferent temperatures after preparation from washedcomplete removal of PVP. Actually, the Pt and unwashed PVP-capped Pt nanocolloid.

448溶媒洗浄と焼成によるPtナノ粒子のPVP保護ポリマー除去

dispersion was evaluated to be 0.98, a value larger than that obtained with Pt-PVP (Table 1).
This indicates that ethanol/hexane washing generated uncovered Pt surfaces. Furthermore,
subsequent calcination at 200 ºC (Fig. 1, Pt(wsh)-200) led to a completely PVP-free IR
spectrum. The Pt dispersion of Pt(wsh)-200 increased to 4.83, showing remarkable
improvement over Pt-250. The attainment of this large Pt dispersion at a lower calcination
temperature suggests enhanced catalytic decomposition of PVP, since the ethanol/hexanewashed Pt nanoparticles had PVP-uncapped Pt on their surfaces.
Also, as expected, Pt(wsh)-400 showed an IR spectrum similar to those of Pt-400 and SiO2.
The Pt dispersion was evaluated to be 9.36, which was comparable to that of Pt-400. The Pt
dispersion considerably smaller than the expected level of more than 50 based on the particle
diameter of 2 nm was probably attributable to the aggregation of nanoparticles during
impregnation or thermal treatment. Indeed, Pt dispersion of Pt(wsh)-400 after H2 pretreatment
at 400 ºC was of 8.59. Therefore, another impregnation method other than the incipient wetness
method would be required to prevent the aggregation. On the other hand, the low Pt dispersion
on Pt(wsh)-200 as compared to Pt(wsh)-400 is not ignorable. This low Pt dispersion of Pt(wsh)200 suggests the presence of some IR invisible species, such as partially decomposed PVP [5,6].
However, the Pt nanoparticles prepared in the same washing and thermal conditions withPt(wsh)-200 showed good activities in hydrogen oxidation and hydrogen isotope exchange
reactions [11], indicating that not fully cleaned Pt was required in these catalyses. The detailsof these catalyses will be reported elsewhere.
In conclusion, our experiments demonstrated the removal of PVP from Pt nanocolloid
supported on SiO2 by direct combustion and the effect of solvent washing. As evidenced by IR
spectroscopic and CO chemisorption studies, the ethanol/hexane washing prior to impregnation
effectively decreased the burn out temperature of PVP to as low as 200 ºC, while 250 ºC was549

田口明・長岐友介・米山優紀

insufficient for the conventionally impregnated sample. This lowering of calcination
temperature is expected to expand the application of PVP-capped Pt for catalysts with relatively
low thermal resistance [11].

Acknowledgement
The authors thank Fuji Silysia Chemical Ltd., Aichi, for their kind supply of porous SiO2
material. This work was supported under the NIFS Collaboration Research programs[NIFS14KESA016] and [NIFS13KOBA029].

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