Pressurized physical activation method for
production of high-performance activated carbons
based on microdomain pore structure model
Jin Miyawaki*
Institute for Materials Chemistry and Engineering, Kyushu University
6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
miyawaki@cm.kyushu-u.ac.jp
Growing demand for high-performance porous carbon materials stimulates developments of new
preparation and modification technologies. Activated carbon (AC), one of representative porous
carbon materials, is also desired to improve the physicochemical properties, especially theporosity, for upgrading the application characteristics and exploiting the new application fields.
To produce AC, activation is a key process, because it governs the porosity. In this article, a simple
production method of AC with a highly developed pore structure is introduced. An idea of the“pressurized physical activation” was come up with an understanding of physical and chemical
activation mechanisms based on a recognition of microdomain, which is a basic structure unit ofartificial carbon materials. This pressurized physical activation method gave AC with a specificsurface area larger than 2600 m2/g, which is difficult to achieve by a conventional atmosphericpressure physical activation. Although chemical activation method can produce AC with higher
degree of pore development in high activation yield, the pressurized physical activation isconsidered as an option to provide a high degree of pore development at a relatively low-cost way.
Together with the high porosity, the pressurized physical AC showed a characteristic pore size
distribution and a high bulk density. Taking these advantages, a superior potential of thepressurized physical AC was demonstrated as a relatively-inexpensive and high-performance
adsorbent in adsorption heatpump system using ethanol as arefrigerant. Traditional AC still has
huge rooms to boost its
performance and broaden its fieldof application. New idea for facile
production
andmodification
methods of AC with purposedesigned porosity and surface
property is eagerly anticipated.
Keyword: Activated carbon, Microdomain, Pore, Pressurized physical activation
Jin Miyawaki received his Dr. Sci. in 2001 from Chiba University under the supervisionof Prof. Katsumi Kaneko. He started his academic career in 2001 as a postdoctoral
fellow of The Energy Institute at The Pennsylvania State University, USA, in a research
project conducted by Prof. Ljubisa R. Radovic. Then, he studied as a postdoctoral
fellow from 2003 to 2008 in a Solution-Oriented Research for Science and Technology
program of The Japan Science and Technology (JST-SORST) organized by Prof.
Sumio Iijima. In 2008, he moved to Institute for Materials Chemistry and Engineering,
Kyushu University as an assistant professor, and promoted to an associate professor
in 2013. His research interests include arbitrary controls of porosity and surface functionality of porous
carbon materials together with developments of porosity assessment technology.
Acc. Mater. Surf. Res. 2024, No.9 Vol.2, 46-57.

46

Pressurized physical activation method for production ofhigh-performance activated carbons based on microdomainpore structure model
Jin Miyawaki*
Institute for Materials Chemistry and Engineering, Kyushu University
6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
miyawaki@cm.kyushu-u.ac.jp
1. Prolusion

confirmed that most of artificial carbon

Porous carbon materials as represented by
activated carbon (AC) are widely used inmaterials including various ACs consisted of
a

basic

structure
1)–4)

unit

named

various application fields, such as gas“microdomain”.

On the basis of thepurification/separation/storage,

water-

recognition that the artificial carbon materials

treatment, solvent recovery, catalyst support,

are an aggregate of microdomains, a newand electric double layer capacitor. The highpore structure model of AC has been

performance and multifunctionality of the

proposed as shown in Figure 1.2)

porous carbon materials stand on the highly

Based on this microdomain pore structuredeveloped pore structure with the hugemodel, in this article, differences in the pore

specific surface area (SSA). To further extend

development mechanisms between physicalthe performance and exploit new applicationand chemical activation processes for AC

areas of porous carbon materials, therefore,

preparation are firstly discussed from a

the increase of degree of pore development

structural point of view.4) The understanding

based on a precise understanding of poreofstructure is indispensable.

suggested an idea of a novel pressurized

Throughobservationsofphysical

activation

scanning

physical activation method: The effectivenesselectron microscope (SEM) and scanningof pressurization in physical activation is thentunnelingdemonstrated as a simple production method

microscopeusing

mechanism(STM),

wehave

Figure 1. (Left) Scanning tunneling microscope image of activated carbon fiber and (center and right)
new pore structure model of activated carbons based on recognition of microdomains.
Acc. Mater. Surf. Res. 2024, No.9 Vol.2, 46-57.

47

of AC

with astructure.5)

highlyFinally,developed poreanapplicabilityofpressurized physical AC as an adsorbent in
adsorption heat pump (AHP) is introduced.6)
2. Structural elucidation of physical andchemical activation mechanisms basedon microdomain structure model4)
Activation is a process to develop pore
structure in carbonized substances derived

Figure 2. Relationship between weight loss

from various raw materials, and is classified

and developed micropore volume of (blueinto physical and chemical methods. A lower

square) steam- and (red circle) KOH-activatedAC production cost is an advantage ofcarbons

physical activation, while chemical activation

temperatures, Ta.4) (Reprinted from Carbon,

can provide AC with highly developed pore

114,

structure in high yield. Although many

permission from Elsevier.)prepared

98–105.

atdifferent

Copyright

activation(2017),

withresearches have been carried out to elucidate

C6Sx and C6Kx, respectively, where x

reasonspore

indicates the number of the first digit of thedifferences

activation temperature, Ta: e.g. C6S8 wasmechanisms

obtained by steam activation at Ta = 800°C.

between physical and chemical activation

Figure 2 shows the relationship between

processes have not been well clarified,weight loss (= 100 – activation yield [wt.%])especially from a structural point of view. In

and developed micropore volume from C6 by

this study, therefore, an elucidation of theactivation.

differences in the structural mechanisms ofdevelopment of steam-AC was apparently

these two activation processes was carriedobserved as compared to KOH-AC. In detail,out, based on the microdomain pore structure

at Ta = 700°C, KOH activation induced a large

model.

weight loss than steam activation. On theofhigherdegreedevelopment for chemical AC,
in

the

pore

development

7)–9)

of

As a starting material, a spherical carbon

other

hand,

Lower

much

degreelargerofpore

increaseof(C6) prepared by the carbonization of a

micropore volume was found for C6K7 (1.21

spherical phenol resin (BEAPS series, ASAHI

cm3/g), whereas C6S7 showed a negligible

YUKIZAI CORPORATION, Japan) at 600°C

increment (0.03 cm3/g). This indicates thatfor 1 h in N2 was used. Relatively uniformthe weight loss of about 20 wt.% by steam

particle size and shape of C6 allowed toactivation at Ta = 700°C was mainly due to a

discuss influences of the activation process

gasification of solid carbon without micropore

on size and shape of carbon particles. For

development, whilst KOH activation at thephysical and chemical activations, steam andsame activation temperature gave rise to an

potassium hydroxide (KOH) were used as

effective pore development. At a higher

activating agents, respectively. The prepared

activation temperature, degree of poresteam- and KOH-ACs were abbreviated as

development increased by both steam andAcc. Mater. Surf. Res. 2024, No.9 Vol.2, 46-57.

48

KOH activations. However, the increasing
ratio of micropore volume against weight loss
was more apparent for KOH-AC than forsteam-AC. When KOH activation was carriedout at Ta = 900°C, the micropore volume
increased sharply without noticeable weightloss (4 wt.% reduction). This is likely due todifferent

poredevelopment

mechanism,

namely potassium intercalation mechanism,
Figure 3. Change of mean sizes of (circle)

as previously reported.7), 10)
SEM and STM observations revealed thatparticles and (square) microdomains by steam

spherical shapes of carbon particles andand KOH activations depending on activation

microdomains of C6 were nearly unchanged

temperature, Ta. The solid (blue) and open

by both steam and KOH activations. However,(red) symbols denote steam- and KOH-

a distinct difference was found in size

activated carbons, respectively.4) (Reprinted

depending on the activation method. Figure

from Carbon, 114, 98–105. Copyright (2017),

3 summarizes changes of mean sizes ofwith permission from Elsevier.)

carbon particles and microdomains by two
activation methods depending on Ta. Even athigh Ta, and thus, at high degree of poredevelopment, both particle and microdomain
sizes of KOH-AC remained almost the samewith those of the pristine C6. In contrast,
steam-AC showed significant decreases inboth particle and microdomain sizes with anincrease of Ta.
Based on the above-mentioned findings, aschematic model of the structural mechanismof pore development during physical andchemical activation is proposed, as shown in
Figure 4. The STM observation showed that

Figure 4. Structural mechanism model of poreeach particle of C6 was an aggregate of adevelopment for steam- and KOH-activatedlarge number of nanometer-size sphericalcarbons.4) (Reprinted from Carbon, 114, 98–

microdomains. By KOH activation at Ta =105. Copyright (2017), with permission from

700°C, almost no size changes of particles

Elsevier.)andmicrodomains

were

induced.

This

indicates that, for C6K7, pores developed in
the intra-microdomain region, giving rise to alargely increased micropore volume. On the
other

hand,

steam

activation

atAcc. Mater. Surf. Res. 2024, No.9 Vol.2, 46-57.

lowtemperature of 700°C did not induce the poredevelopment, regardless of the weight loss ofabout 20 wt.% and apparent decreases ofparticleandmicrodomain

sizes.

This

suggests that the oxidative reaction of steam
49

progressed preferentially at the externalof activating agent (activating gas molecule)surfaceandinto core parts of carbon particles and theirmicrodomains, resulting in a gasification of

microdomains causes the inhomogeneous

the outer parts. An increase of Ta gave thereaction,

micropore development also for steam-AC,

physical activation at atmospheric pressure is

although the pore developing efficiency was

diffusion-limited

less than that for KOH-As. At Ta = 900°C, a

pressurize the activation gas for improving

widening of pore size was occurred by thethe diffusivity, and thus, for leading moreintercalation mechanism for KOH-AC. In theuniform activation reaction. Previously, wecase of steam-AC, however, the preferential

have reported that pressurization improved

gasification of microdomains located at the

diffusivity of oxygen gas in pitch fibers, giving

periphery of carbon particles progressed

rise

to

the

homogeneous

together with the pore development in theeffect.

11)

In

this

microdomains, giving rise to the large weighteffectiveness of pressurization in physicalloss and remarkable size-reductions ofactivation was investigated to propose aparticle and microdomains of C6S9.

simple production method for AC with a highly

of

the

carbonparticles

In summary, KOH activation (chemical
activation)induces

uniformotherwords,

reaction,study,

conventional
we

tried

to

stabilization

therefore,

thedeveloped pore structure.

pore

The same starting material of sphericaldevelopment overall for all microdomains

carbon (C6) was used in the same way as thecomprising carbon particles; however, in theprevious section. C6 was physically activatedcase of steam activation (physical activation),

at 700–900°C using pure CO2 as an

pores

are

a

in

developed

inremaining

activation gas at an absolute pressure of 0.1

after

inhomogeneousor 1.0 MPa for the desired time. The obtained

gasification from the outer surface of carbon

ACs were designated as PXTymZ, where X,

particlesTheY, and Z are the activation pressure [MPa],different pore development mechanisms oftemperature [°C], and holding time at 900°C

physical and chemical activations cause[min], if any, respectively: e.g. P0.1T900m5 isdifferences

AC prepared at 0.1 MPa and 900°C with 5-

microdomains
and

in

their

the

the

microdomains.

degreeofporedevelopment and the activation yield.

min holding.
Figure 5 shows a relationship between

3. Pressurized physical activation: A

activation yield and SSA of ACs prepared bysimple production method for activatedconventional

carbon with a highly developed porepressurized physical activations using CO2 as

5)structure

As discussed in Section 2, lower activation

atmosphericpressure

andactivating gas. Here, the ordinate is SSA per
weightofAC.

A

difference

betweenyield and limited pore development of

atmospheric pressure physical activation (0.1physically activated carbons are due toMPa) and pressurized physical activation (1.0

inhomogeneous gasification from the outerMPa) was apparently observed: At a similar

surface of carbon particles and microdomains.

yield,On the hypothesis that insufficient diffusivity(pressurized-AC) showed higher SSA thanAcc. Mater. Surf. Res. 2024, No.9 Vol.2, 46-57.

thepressurized

physicalAC50

Figure 5. Relationship of activation yield toFigure 6. Relationship of activation yield tospecific surface area on activated carbon

specific surface area on char weight basis forweight basis for activated carbons prepared by

activated carbons prepared by (black circle)(black circle) atmospheric pressure and (redatmospheric and (red square) pressurized

square)activation

physical activation methods with CO2 as an

methods with CO2 as an activating agent. The

activating agent. The result of the starting

result of the starting carbonized material (C6;carbonized material (C6; open triangle) is

open triangle) is shown for comparison.12)

shown for comparison.12)

thepressurizedatmosphericphysicalpressure(atmospheric-AC),

physical AC

indicatingtheeffectiveness of pressurization for poredevelopment. Of note, pressurized physicalactivation afforded to obtain AC with SSA of
2630 m2/g; such high value is difficult toachievebyconventional

atmosphericpressure physical activation.
To discuss how effectively the gasificationof solid carbon surely contributes to the poredevelopment, changes of SSA on a starting
material (char: C6) weight basis depending

Figure 7. Relationships of activation yield toon activation yield are shown in Figure 6. The

porosity for activated carbons prepared bychar-weight-basis SSA values of pressurized-(black circle) atmospheric pressure and (redAC increased up to 40 wt.% of activation yield,

square)whilst those of atmospheric-AC started tomethods. The result of the starting carbonizeddecrease above 60 wt.% of activation yield.

material (C6; open triangle) is shown forThiscomparison.12)

indicatesthat

pressurizationcansuppress the unwanted gasification of solidcarbon, giving rise to more efficient poredevelopment as compared with conventional
Acc. Mater. Surf. Res. 2024, No.9 Vol.2, 46-57.

pressurized

physical

activation

atmospheric pressure physical activation.
Effectiveness of pressurization on poredevelopment

wasconfirmed

also

from
51

Figure9.

Relationships

betweenmean

particle size and activation yield for (black
circle) atmospheric pressure and (red square)pressurized physical activations.5) (Reprinted
from Carbon, 183, 735–742. Copyright (2021),
with permission from Elsevier.)

Pressurization

provided

not

only

a

remarkable increase of degree of poredevelopment, but also a characteristic pore
Figure 8. Pore size distributions of (a)size distribution (Figure 8). Conventional

atmospheric pressure and (b) pressurizedatmospheric pressure physical activation

physically activated carbons. (Black circle)

using CO2 as an activating gas predominantly

P0.1T700 or P1T700; (red square) P0.1T800

developed narrow micropores less than 1.0

or P1T800; (blue rhombus) P0.1T900 or

nm of pore width. In contrast, CO2 activation

P1T900;or

at 1.0 MPa gave the development of wider

triangle)

micropores; especially, P1T900m5 showed astar)characteristic pore size distribution with a(pink

P1T900m5;

triangle)(orange

P0.1T900m15;

P0.1T900m5
invertedand(brownP0.1T900m90. Inset in (a): The results of thepeak pore width of about 1.6 nm.

starting carbonized material (C6) are shown

Figure 9 shows relationships between

for comparison.5) (Reprinted from Carbon,

activation yield and mean particle size

183,

estimated from SEM images for atmospheric-735–742.

Copyright(2021),

withand pressurized-ACs. Although mean particle

permission from Elsevier.)

porosity as shown in Figure 7. From theporosity values of AC with about 12–13 wt.%of activation yield, approximately one third ofvessel volume was considered to correspondto the developed pores for atmospheric-AC,
whereas almost half of vessel volume wasattributed

to

the

developedpores

pressurized-AC.
Acc. Mater. Surf. Res. 2024, No.9 Vol.2, 46-57.

forsize of atmospheric-AC decreased withdecrease in activation yield, no change wasfound for pressurized-AC, indicating thatpressurization allowed the uniform poredevelopment, at least at the particle level. On
the other hand, STM observation revealed
that mean microdomain sizes of P1T900m5
and P0.1T900m90, which showed similar
activation yields, were 5.0 and 6.1 nm,
52

Figure 10. Schematic diagram of differences in atmospheric pressure physical, pressurized physical,
and chemical activation processes.5) (Reprinted from Carbon, 183, 735–742. Copyright (2021), withpermission from Elsevier.)

respectively. Therefore, from the STM results,

outer surfaces of carbon particles andpressurization was considered not effective to

microdomains, resulting in low degree of pore

improve a diffusivity of activating gas intodevelopment and activation yield. In contrast,

microdomains.

observations of particles and microdomains

A schematic model of the difference in porerevealed that pressurization improved thedevelopment of the atmospheric pressure

activating agent diffusivity in carbon particles,

physical, pressurized physical, and chemical

but not in their microdomains. That is, theactivation methods was proposed based onremarkable increases in SSA and total pore

the microdomain structure model (Figure 10).

volume for pressurized-AC is very possibly

In the case of chemical activation, a sufficientdue to the development of not only intra-

diffusivity of the activating agent in both

microdomain pores for most of microdomains

carbon particles and microdomains allowed

consisting of carbon particles, but also inter-

uniform reaction of the activating agents withmicrodomaineach microdomain in the carbon particle.

pressurized physical activation method canTherefore, intra-microdomain pores are wellprovide a high degree of pore development,

developed, giving rise to a high degree ofbetween chemical and atmospheric pressure

pore development and activation yield forphysical activation methods.

pores.

Therefore,thechemical-AC. On the other hand, lowdiffusivity of activating agent in conventional

4. Applicability of pressurized physicallyatmospheric pressure physical activation

activated carbon as an adsorbent incauses local gasification reactions at theadsorption heat pumps6)

Acc. Mater. Surf. Res. 2024, No.9 Vol.2, 46-57.

53

Among various heatpump systems,

adsorption heat pump (AHP) has advantages,
such as low power consumption, minimal
vibration and noise, operable by solar heat orindustrial low-temperature waste heat. For
thewidespread

usage,

however,adownsizing of AHP devices is required.
Existing commercial AHP systems use
synthetic zeolite and water as an adsorbent
andarefrigerant,anFigure 11. Pore size distributions for starting

largercarbonized material and derived activatedadsorption capacity, AC-ethanol receives

carbons. (circle) C6; (triangle) PAC; (square)

attention.13–16) For example, we have reported

PPAC; (diamond) CAC.6) (Reproduced fromthat slit-shaped carbon micropores of 1.6 nm

Ref. 6 with permission from the Royal Society

in width showed the remarkable effectiveof Chemistry.)

alternative

respectively. As

promising

pair

with

adsorption amount of ethanol under acondition of 10°C/30°C/80°C for coolingwater/surrounding environment/regeneration
temperatures, respectively.17)
As introduced in Section 3, pressurized-AC
showed not only a high degree of poredevelopment, but also a characteristic poresize distribution with a peak for a pore widthof approximately 1.6 nm. In this study,
therefore, the applicability of pressurized-AC
as an adsorbent in AC-ethanol AHP system
was examined.
In

additiontoatmospheric-AC(P0.1T900m90)

andpressurized-AC(P1T900m5) shown in Section 2, another AC
was prepared by chemical activation method
at 900°C using KOH as an activating agent
from a spherical carbon (C6). In this section,
ACs prepared by atmospheric pressure
physical activation, pressurized physicalactivation,

andchemical activationaredesignated as PAC, PPAC, and CAC,
respectively.
Pore size distributions of C6, PAC, PPAC,
and

CAC

are

shown

in

FigureAcc. Mater. Surf. Res. 2024, No.9 Vol.2, 46-57.

11.

Conventional atmospheric pressure physicalactivation using CO2 as an activating gas
predominantly introduced narrow micropores(< 1.0 nm), but CAC prepared by chemical
activation using KOH as an activating agent
possessed a large amount of mesopores.
PPAC

showedabimodal

pore

size

distribution in micropore region.
To assess the applicability in AHP systems
with ethanol refrigerant, ethanol adsorption
and desorption isotherms were measured at
30°C.

Assuming

the

AHP

operation

temperatures as 10°C/30°C/80°C for coolingwater/surrounding environment/regeneration,a difference of adsorption amounts at 0.1–0.3
ofrelativepressure(P/P0)

ofethanol

adsorption isotherm at 30°C, which is called
as effective adsorption amount, is a goodindex to estimate a performance as AHP
adsorbent. Together with the saturated
adsorption amounts calculated from a value
at P/P0 = 0.95, the effective adsorption
amounts of each sample are tabulated inTable 1. C6 showed low value of effective
adsorption amount (10 mg/g). For PAC, the54Table 1. Saturated and effective ethanol adsorption amounts of starting carbonized material andderived activated carbons.6) (Reproduced from Ref. 6 with permission from the Royal Society ofChemistry.)
SampleSaturated adsorption

Effective adsorption amount of ethanol

amount of ethanol

On a weight basis

On a volume basis[mg/g][mg/g][mg/cm3]

C6

170

10

8

PAC

730

70

40

PPAC

1730

570

220

CAC

1630

580

120

saturated adsorption amount of ethanolincreased remarkably to 730 mg/g ascompared with C6 (170 mg/g), whilst an
increase of the effective adsorption amount
was limited (70 mg/g). This indicates that PAC
is not suitable as an adsorbent for the ethanol
AHP system, because mainly introduced
pores by conventional atmospheric pressure
physical activation were narrow micropores.
On the other hand, PPAC showed a

Figure 12. Ethanol adsorption and desorption

comparably

adsorption

isotherms on an adsorbent volume basis at

amount of ethanol (570 mg/g) with CAC. The

30°C of starting carbonized material, and thecharacteristic pore size distribution of PPAC

derived activated carbons (circle: C6; triangle:

having a larger number of optimum pore size

PAC;

of 1.6 nm should provide the high ethanol(Reproduced from Ref. 6 with permission fromuptake at P/P0 = 0.1–0.3.

the Royal Society of Chemistry.)largeeffective

In addition to the effective adsorption
amount of a weight basis of adsorbent, theeffective adsorption amount per unit volume
is important, considering a downsizing of AHP
devices. Figure 12 shows adsorption anddesorption isotherms of ethanol at 30°C on anadsorbent

volumebasis.

The

effective

adsorption amounts of ethanol on a volume
basis for C6 and the derived ACs are also
shown in Table 1. Here, the volume basis
adsorption amounts were calculated from theethanol adsorption amounts on a weightbasis and the bulk density (C6: 0.84 g/cm3,
Acc. Mater. Surf. Res. 2024, No.9 Vol.2, 46-57.

square:PPAC;

diamond:

CAC).6)

PAC: 0.57 g/cm3, PPAC: 0.39 g/cm3, CAC:
0.21 g/cm3). Although the effective adsorption
amount on a volume basis for CAC remained
120 mg/cm3, PPAC showed a high value of220

mg/cm3.

possessed

This

is

“effective”

because
pores

PPAC

for

an

enlargement of effective adsorption amountofethanol,

but

wide

pores

includingmesopores of CAC were less effective.
In summary, it was demonstrated thatpressurized physically activated carbon has agreat potential as an adsorbent for AHP
system with ethanol refrigerant.
55

5. Conclusion

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