富山大学研究推進機構水素同位体科学研究センター研究報告 36：45－52，2016.

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α-Al2O3 担持 Pt ナノ粒子触媒の CO 酸化活性と着火現象

田口 明*，尾崎 智弘

富山大学

研究推進機構〒930-8555

水素同位体科学研究センター
富山市五福 3190

Ignition-extinction phenomena in CO oxidation reaction over “dry” impregnatedplatinum nanoparticles on α-Al2O3

Akira Taguchi*, Tomohiro Ozaki

Hydrogen Isotope Research Center
Organization for Promotion of Research, University of Toyama
Gofuku 3190, Toyama 930-8555(Received January 31, 2017; accepted July 7, 2017)

Abstract
Catalytic CO oxidation activity of α-Al2O3 supported Pt, prepared by the sputter deposition
method was investigated. The oxidation activity showed the structure sensitivity withnegative particle effects. An ignition-extinction phenomenon was observed in CO oxidation
reactions arising from the “mass-transfer limitation” owing to the high turn-over frequency.

45

田口明・尾崎智弘

Research note
Recent advances in the synthesis and characterization of nanosized particles have promoted
an extensive search for methods to prepare highly efficient nanostructured catalysts [1-3].
Sputter deposition, a “dry” impregnation method that does not use any solvent or liquid forcatalyst preparation, has gained attention because it has several advantages as compared to theconventional “wet” impregnation method [4-6].

We have investigated the preparation andgrowth of Pt nanoparticles on Al2O3 by FE-SEM and CO chemisorption study [7].

Ptnanoparticles formed in the early stages of sputter deposition were well isolated, and the particle
size distribution of Pt nanoparticles was narrow. Longer sputter deposition resulted in growthand coalescence of Pt nanoparticles.
In this note, the catalytic activity of these obtained Pt/Al2O3 was studied from the oxidationof CO to CO2. An intrinsic ignition-extinction phenomena has been observed.
The α-Al2O3 supported Pt (Pt/Al2O3) used in this study was prepared by a barrel-sputteringsystem as described in our previous paper [7]. The RF power and Ar gas pressure were 25 W
and 0.8 Pa, respectively. Depending on the duration of the sputter deposition, the deposition
amount and the diameter of Pt particle obtained were changed. The number of Pt atom onsurface, corresponding to the amount of CO adsorbed, and the Pt dispersion calculated from
CO chemisorption study were listed in Table 1. Here, the catalysts are denoted as Pt(x), wherex represents the Pt dispersion determined by CO chemisorption study [7].
The CO oxidation reaction was studied in a fixed-bed flow reactor (6 mm inner diameter)
under atmospheric pressure. Two thermocouples were used, one located in the catalyst in the
fixed bed to measure the temperature, and the other on the outside of the quartz reactor tocontrol the reaction temperature via a heater. Reactor effluent gas was analyzed with an on-

46

α-Al2O3 担持 Pt ナノ粒子触媒の CO 酸化活性と着火現象

Table 1 Preparation, Pt particle diameter and Pt dispersion of Pt/Al2O3 [7].

SputterPt deposition Pt diameter
Pt
Sampledeposition
Pt surface
amount(FE-SEM)
dispersion*
ID
time
(/g)
(wt%)
(nm)
(-)
(min)
Pt(0.70)
4
0.021
1.8
2.181018
0.70
18Pt(0.50)
5
0.032
2.7
1.57100.50
Pt(0.31)
7
0.056
3.9
0.951018
0.31
18
Pt(0.25)
9
0.075
4.5
0.77100.25
Pt(0.18)
20
0.136
5.1
0.571018
0.18* Pt dispersion was investigated at 0 ºC after thermal treatment at 250 ºC under
H2/He flow.

lined gas chromatograph (Shimadzu; GC-8A, column; MS-5A) equipped with TCD.

For thereaction test, a mixture of Pt/Al2O3 (0.10 g) and unsupported α-Al2O3 (2.90 g) as the dilutant
was loaded into the reactor. The catalyst was heated to 250 ºC at a ramp rate of 5 ºC/min underHe flow. At this temperature, fluid gas was changed to a mixture of H2 (50 %) and He balance,
and this temperature was maintained for additional 1 h.

After cooling to room temperature

under He flow, the fluid gas was switched to the mixture of CO (0.5%), O2 (5.0%) and He
balance with a total flow rate of 120 ml/min to start the reaction. After 30 min, the effluent
gas was analyzed by gas chromatography. The temperature was raised from room temperature
to an appropriate one at a ramp rate of 5 ºC/min, and then the effluent gas was analyzed after
30 min.

Subsequently, the temperature was raised and catalytic activity was measured in thesame manner. This sequence was repeated up to the appropriate temperature where the COconversion reached 100 %.

After reaching 100 % conversion, the temperature was lowered atdecrements of 5 ºC until the conversion decreased to a certain value. The CO conversion was
calculated from the following equation:

Conv. =([CO]in - [CO]out )[CO]in× 100

Here, [CO]in and [CO]out are the concentration of CO in the inflow and the effluent gas,47

田口明・尾崎智弘

respectively.
Figure 1 shows the oxidation of CO to CO2 over Pt(0.70) and Pt(0.25) as a function oftemperature. The CO oxidation activity of Pt catalysts was typically low as compared to othertransition metals and precious metals [8-10]. Thus, for instance, about 5 % conversion wasobserved at 165, 170, 180, 190 and 200 ºC for Pt(0.18), Pt(0.25), Pt(0.31), Pt(0.50) and Pt(0.70),
respectively. The CO oxidation conversion increased exponentially up to about 60 % with anincrease in the reaction temperature, and then the conversion suddenly increased to 100 % witha further increase of 5 ºC in the reaction temperature. This sudden increase of conversion was
not due to the lack of the reactant, but rather due to the ignition phenomenon.
The conditions where the conversion reached 100% (Conv.100) can be predicted using thefollowing relationship:[CO]feed = TOF × Ptsurface

Here, [CO]feed represents the number of CO molecules fed in a unit of time (2.68751017
100

temperature where the conversion reached

80

Thus,

the

100 % can be calculated from the Arrhenius

Conversion / %expected

number/sec).

60
40

equation as follows:

20E
1
T = - ( a) ∙ ()
Rln(TOF) - ln(A)E

1R[CO]feedln() - ln(A)Ptsurface= - ( a) ∙ (00 50 150)

Fig. 1200
Temperature / ºC

250

The conversion of CO to CO2 as a functionof reaction temperature over Pt/Al2O3 prepared.
(●) Pt(0.70) at rising temperature, (○) Pt(0.70) at
lowering temperature, (■) Pt(0.25) at rising

The temperatures of Conv.100 for Pt(0.18),

48

temperature, (□) Pt(0.25) at lowering temperature.

α-Al2O3 担持 Pt ナノ粒子触媒の CO 酸化活性と着火現象

Pt(0.25), Pt(0.31), Pt(0.50) and Pt(0.70) were 233.3, 240.5, 242.5, 251.0 and 257.9 ºC,
respectively. These values were about 5–10 ºC higher than the empirical ones shown in Figure
1. This fact supports the idea that the sudden increase of conversion was not due to the lackof the reactant, but to the ignition phenomenon.
It should be noted that the catalysis experiments with different amounts of catalyst, i.e.,
Pt/Al2O3 (0.06 g) and CO (0.5 %), O2 (5%) and He balance with a total flow rate of 80 ml/min,gave almost the same reaction curves as in Figure 1.

Thus, it turn out that such an ignition1

phenomenon arises from the hot spot in the exothermic reaction ( CO + O2 →2

CO2 , ∆H = -283.0 kJ/mol). According to the detailed study by Bourane and Bianchi [11-13],
the dominant species on a Pt surface change before and after ignition temperature. In thatstudy, although adsorbed CO species were dominant on Pt surface (Pt-CO) before the ignition
temperature, strongly adsorbed oxygen species were covered on Pt surface (Pt-O) above theignition temperature. This was interpreted from the “mass-transfer limitation” owing to thehigh TOF [12,13].
Lowering the reaction temperature in

2.0

steps of 5 ºC from the one where 100 %1.0

conversion was achieved resulted in the

In Pt(0.70), for example, 100 % conversionln(TOF)hysteresis in the reaction curve in Figure 1.

0.0
-1.0
-2.0

was maintained from 250.3 to 230.0 ºC, then

-3.0

suddenly decreased to 17.4 % at 225.3 ºC.

-4.0

A similar hysteresis was observed in other
Pt/Al2O3

catalysts.

The

width

-5.0

of

hysteresis, defined as the temperature range
in which the CO conversion maintained at

Fig. 21.9

2.0

2.1 2.2 2.3 2.4
1000/T (1000/K)

2.5

Arrhenius plot obtained from the oxidationfrom CO to CO2 over Pt/Al2O3. (●) Pt(0.70), (▲)
Pt(0.50), (■) Pt(0.31), (○) Pt(0.25), (∆) Pt(0.18).

49

田口明・尾崎智弘

100 %, was about 20–25 ºC and
was independent of Pt/Al2O3.
The Arrhenius plot obtained
from TOF, defined as thenumberofCO2

molecules

Table 2 The Arrhenius parameter at CO oxidation over Pt/Al2O3

Sample ID
Pt(0.70)
Pt(0.50)Pt(0.31)Pt(0.25)Pt(0.18)

Activation energy(Ea kJ/mol)
114.93105.53
92.63
82.44
86.03

Frequency factor(-)
2.4857  1011
5.6359  1010
6.6465  109
8.4260  108
3.5210  109

formed over each surface Pt atom per second, is shown in Figure 2. The values of Ea and thefrequency factor A are listed in Table 2. The Ea was found to decrease with a decrease of Pt
dispersion, consistent with previous papers [14,15]. The frequency factor was in the range of1011–108 s-1, which was in good agreement with previous papers [15-18].

It is clear from

Figure 2 that the activity of CO to CO2 reaction was in the order of Pt(0.18) > Pt(0.25) > Pt(0.31)> Pt(0.50) > Pt(0.70) in the entire temperature region studied. This order indicates a particle
size dependence of activity, and this can be clearly seen in Figure 3.
In Figure 3, TOF at 180, 200, and 220 ºC is plotted against the Pt dispersion. Exponential
decrease of the activity depending on the Pt dispersion, i.e., the particle diameter of Pt, has beenclearly observed [15,19-21], indicating a structure sensitive reaction with negative particle
effects according to the classification by Che and Bennett [19]. In addition, Figure 3 reveals
that there were two different activities, which became clear when the reaction temperature wasraised.

They were in low Pt dispersion,corresponding to Pt(0.18), Pt(0.25) andPt(0.31), and in high Pt dispersion ofPt(0.50) and Pt(0.70).

Generating thisgrouping might give an indication of theeffect of agglomeration of Pt nanoparticles.
The dispersion dependency in Figure 3 has
two possible interpretations: The activity of

50

Fig. 3 Relation between TOF and the Pt dispersionof Pt/Al2O3. (●) 180 °C, (▲) 200 °C and (■) 220 °C.

α-Al2O3 担持 Pt ナノ粒子触媒の CO 酸化活性と着火現象

Pt with low dispersion was accelerated as compared to the one extrapolated from that of Pt withhigh dispersion, or, alternatively, the Pt with high dispersion was accelerated as compared to
the Pt with low dispersion.

An important point is that the value of Ea increased with anincrease of the Pt dispersion (Table 2).

That is, in Pt with high dispersion, there was aremarkable increase the activity as the reaction temperature increased, resulting in an increaseof Ea. Therefore, it can be found that Pt with high dispersion, i.e., small particle diameter, has
the potential ability to enhance the CO oxidation activity as compared to the one with low
dispersion. Also, it supposes that Pt with high dispersion is suitable for an exothermic reaction
such as hydrogen combustion under mild conditions.
In conclusion, CO oxidation activity of α-Al2O3 supported Pt catalyst, prepared by a dry
impregnation method was studied. An ignition-extinction phenomenon was observed under O2rich conditions of O2/CO=10.

The activation energy decreased with a decrease of Pt

dispersion, i.e. an increase of Pt diameter.

Further, the catalytic activity increased with adecrease of Pt dispersion, showing a structure sensitivity with negative particle effect. Two
different activities were observed in the relation between TOF and the Pt dispersion: thePt/Al2O3 with rather high dispersion exhibited higher TOF than the expected one extrapolated
from the low dispersion catalysts.

This indicates that Pt nanoparticles with small particle

diameter possess a potential ability to enhance CO oxidation activity.

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