Strength and pore structure of models for carbon catalyst supports

CataZysisTo&y, 7 (1990 ) 299-308 Elsevier Science Publishers B.V., Amsterdam-Printed

299

in The Netherlands

STRENGTH AND PORE STRUCTURE OF MODELS FOR CARBON CATALYST SUPPORTS B. McENANEY, I. M. PICKUP AND L. BODSWORTH School of Materials

Science, University of Bath, Bath, BA2 7AY, UK.

SUMMARY The porous structure, measured by image analysis, and mechanical properties (elastic modulus, flexural strength and toughness) of carbons prepared from phenolic resins (novolaks) by a powder pressing method are compared to those of glassy carbons prepared by casting of liquid resins. Carbons with pore volume fractions up to -0.65 were prepared with mechanical properties which are comparable with those of many other engineering carbon materials. Porosity and mechanical properties can be varied by alterations to the fabrication parameters, e.g., pressing conditions, addition of pore forming agents and carbonisation rate. The combination of high porosity and good mechanical properties suggests that these carbons may be suitable as catalyst supports. INTRODUCTION Activated industrial

carbons are being used increasingly

processes,

e.g.,

as catalyst supports in

hydrodesuphurization

(ref.

1,2)

flue

gas

desulphurization (ref. 3) and ammonia synthesis (ref. 4). Advantages of activated carbons include their large surface areas and the wide range of surface chemical functionality.

Disadvantages

of activated carbons as catalyst supports include

variability in their properties, which reflects the heterogeneous activated

carbon

particular

precursors,

nature of most

and limited ability to tailor pore structure for

applications. Currently, there is interest in developing porous carbon

catalyst supports from polymeric resins, e.g., phenolic novolak powders, since it may be possible to tailor the pore structure of these carbons by careful control of the fabrication process. However, little is known about the relationships between porosity

and strength

performance

of these carbons

and hence

about their mechanical

in industrial situations. This paper reports a study of this subject

which is part of a wider study of the potentiality of resin-based carbons as catalyst supports. Resin carbons prepared by powder manufacturing

routes are related to

glassy carbons prepared from liquid resins by casting (ref. 5). Previous studies of the mechanical

properties of glassy carbons have been principally confined to

commercial materials and the wide variation in mechanical properties reported e.g., flexural strengths ranging from 42 to 130 MPa, probably reflect differences in precursors

and in manufacturing

0920-5861/90/$03.50

processes. Thus there is a need to improve

0 1990 Elsevier Science Publishers B.V.

300

understanding

of strength-structure

relationships in the general class of carbon

materials prepared from polymeric resins, of which the novolak-based

carbons

are examples. Measurements

are reported

of the pore structure,

made using image

analysis, dynamic elastic modulus and flexural strength of models for porous carbon catalyst supports prepared from phenolic resins. Carbon beams suitable for mechanical testing were prepared from novolak resin powders. Some limited studies were also made on glassy carbon beams prepared from resol resin liquids and a commercial glassy carbon. EXPERIMENTAL Material6 Two types of resin were supplied by British Petroleum plc for this work: (i) a novolak powder (MRM 138) pre-mixed with hexamine hardener and (ii) a liquid resol resin J2018L which was hardened with a commercial catalyst, Phencat 10. In some cases polyethylene

glycol, PEG, was used as a pore-forming

agent. A

commercial glassy carbon, Vitreous Carbon VlO, manufactured by Le CarboneLorraine, France, was used for comparative studies; this carbon, which had been subjected to a nominal maximum heat-treatment temperature of 1000 oC, was reheated to this temperature in flowing nitrogen.

Manufacture ofthe carbon beama Two methods were used to produce carbon beams: a powder pressing route for those prepared from the novolak resin and a casting route for those prepared from the liquid resol resin. In the powder pressing method novolak resin powder (particle size up to 250 m)

was placed in a rectangular steel die, 50x10 mm, and

cold pressed at 580 MPa to produce resin beams with adequate green strength. The resin beams were carbonised in a horizontal tube furnace in an atmosphere of flowing nitrogen at a constant heating rate of either 15 or 60 oC/h to 1000 oC to produce rectangular carbon beams of approximate dimensions 40x9x3 mm. The influences of the following manufacturing and mechanical

properties

variables on the porosity

of the carbon beams were studied:

(i) pressing

conditions, (ii) carbonisation rate, (iii) addition to the resin of 10 wt% PEG as a pore-forming agent; (iv) it was also found that the properties of the carbons varied with the batch of the resin. This gave rise to four main classes of carbon designated by the letters B, C, D, and E, Table 1. Carbons in each class were also identified by a number denoting the carbonisation rate in oC/h, e.g., C.15.

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TABLE 1 Classification of carbons ---------------_____~~~~~~~~~--~~~~~~~~~~~---------------Carbon R.&II Pressing/curing class

batch

conditions

Carbonisation ratea. oc7h

_____---__------------------------------------_--___-_____ Novolak resin based carbons B.15 1 b Cl5 C ii C.60 : C 60 D.15 2 C 15 2 D.60 C 60 E.15 2+10wt%PEG c 15 E.60 2+10wt%PEG c 60 Glassy carbons d d F” JiOl8L 15 : J2018L 15 : aged J2018L f 15 ~------------------------------------_-------------------a. to 1000 OC in flowing nitrogen. b. cold press at 340 MPa + hot press at 4 MPa and 200 oC for 5 min. c. cold press at 580 MPa d. unknown. Vitreous carbon VlO, Le Carbone Lorraine, France. e. 4 wt% Phencat hardener, cured for 24 h at 35 oC. f. e + stirring with a hollow glass rod. Glassy carbon beams were prepared from the liquid resin containing 10 wt% Phencat 10 hardener by casting between melinex-lined

perspex strips

which were separated by aluminium spacers, 3.5 mm thick. The resin was cured for 24 h at 35 oC to produce sheets 300x40x3.5 mm. Beams, 50x9x3.5 mm, cut from these sheets using a band saw, were packed in activated

charcoal particles in a

graphite crucible and carbonised at 15 oC/h to 1000 oC to produce class F carbons. Carbons of class G were prepared in a similar way except that the resin was stirred

prior to casting

using

a hollow

glass rod to increase

porosity

by

introducing air into the resin as it cured. Class H carbons were prepared in a similar way to class G except that an aged (more viscous) resin was used. The commercial glassy carbon, class A, was supplied as sheets 100x100x2 mm; each sheet was cut into beams, 50x10~2 mm, using a diamond wheel. Microstructure and norositv Microstructures

of the carbons were examined in reflected

using a Zeiss ICM inverted

light microscope.

Sections

white light

for microscopy

were

prepared by mounting samples of the carbons in a cold-setting resin and then polishing them to 1 pm surface finish using diamond paste. Quantitative

pore

structure parameters (see below) were measured on the polished sections using a Joyce-Loebl

Magiscan

2A image

analyser

linked

to a Zeiss Ultraphot

II

microscope. Using a 40x objective lens the resolution of the image analyser was 1 km. The effect of porosity dominated

by large

on the mechanical

macropores

properties

so that the image

of the carbons is

analysis

technique

is

appropriate. However, complete assessment of these carbons as catalyst supports requires requires characterisation of micropores and mesopores. Mechanical ox-one&es Before mechanical testing the carbon beams were ground and polished to 600 grade surface finish to remove surface and edge defects introduced during preparation standard

of the beams. Dynamic elastic modulus, E, was measured using a

sonic resonance

technique

for carbon materials

(ASTM C742-74).

Flexural strength, B, was measured on a model 1122 Instron in 3-point bending mode using a span of 30 mm and a cross-head speed of 0.5 mm/min. RESULTS AND DISCUSSION Novolak Representative

micrographs

of carbons are in Fig. 1. Total pore area

fractions and histograms of pore area distributions

were obtained from image

analysis; statistics were collected from about 10 fields each containing about 1000 pores. The majority of the pore area histograms, Fig. 2, are unimodal, although a few are multimodal. The ratio of Feret diameters of the pores, r, defines an aspect ratio or pore shape factor. For all of the carbons the modal value of r was -1.5 and the

maximum

approximately

value

was -3,

equi-axed

indicating

in shape.

This

that the majority is consistent

with

of pores

are

the globular

appearence of the pores, Fig. 1. Pore area fractions may be equated with pore volume fractions, P, if pores can be considered as an isotropically-dist~buted

array of spheres (ref. 6). This

assumption, which is implicit in much published work on image analysis, is fully justified for the spherical pores of the glassy carbons, Fig. 1, and is a reasonable assumption

for the globular pores in the novolak-based

carbons. The linear

dimension of a pore of area A can be represented by an equivalent circle diameter, D = (4A/x)1’2. Values of P and D obtained from the pore area histograms are in Table 2, together with the mechanical property measurements expressed as the mean and standard deviation of results from 6 to 35 beams. Referring

to mechanical

properties,

the greater degree of scatter for o

compared with E may be attributed to its sensitivity to variations in flaw sizes in the tensile faces of the carbon test beams. By contrast, the values of E are

Fig. 1. Representative micrographs of the resin-based carbons; magnifications: a = 200 pm; b = 100 pm; c = 125 p.

0

123456

Fig. 2. Representative pore area histograms for resin-based carbons.

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TABLE 2 Mechanical properties and pore structure parameters ___-______-__--__--_--_----_--~------~---------------__--Flexural Carbon Elastic Pore modulusa,

class

strengtha,

E/GPa

volume fractiona. P

o/MPa

Equiv. circle

Toughness,

diameter, D/pm,

IU(J/mz) c

min mode max

---------------------------------------------------------Novolak resin-based carbons B.15 23.97f 2.63 6l.30f 24.19 18.32 + 1.13 35.96 f 15.65 c.15 14.14f 0.89 15.01+ 3.28 C.60 D.15 2099f 1.55 39.86 f 4.68 D.60 23.565 2.28 4456f 4.91 E.15 17.47f 1.41 29.77f 3.76 E.60 14.63f 0.98 27.83f 1.98

0.172f 0.512 + 0.629+ 0.305 f 0.318+ 0.392f 0.411+

0.058 0.024 0.057 0.027 0.056 0.023 0.024

8 100 280 12200360 12 b1000 12 80 300 12 80 300 12 90 360 12 140 360

138 80 50 ; 57 60

Glassy carbons 4 10 28.8 + 0.7 103.2 f 28.2 0.074f 0.014 2 4 10 16 ; 22.3 f 1.8 79.5 f 21.5 0.261 f 0.011 G 23.0 f 0.9 72.7 -+ 12.8 0.613x 0.018 4 b 80 H 22.9 f 0.7 64.9x 8.4 0.654x 0.016 3 b 100 -------------_---_---a. + one standard deviation; b. multi-modal pore area distribution. c. calculated from R = ( 02 x D,,, / E )-

12 14 :

determined by the resonant response of the whole beam and are therefore more representative significant

of the carbon

differences

between

microstructure. the properties

In the following of the carbons

discussion

refer to 95%

confidence levels using the t-test. Comparing pore structure parameters P and D for carbons C.15 and D.15 shows that porosity in the carbons varies with the batch of resin; carbon D.15 has a significantly smaller value of P and a narrower range of pore sizes, Table 2. The lower porosity for carbon D.15 is reflected in its significantly higher elastic modulus, E; no firm conclusions can be drawn about the differences in 0 because of the large scatter of results The majority of carbons were produced by cold pressing at 580 MPa. The effect of hot pressing, significantly,

c.f., carbons

Table 2, with significant

B.15 and C.15, is to reduce

P and D

increases in E and cr. Comparisons

carbon D.15 with E.15 and D.60 with E.60 show that volatilization

of

of the pore-

forming agent PEG during carbonisation produces significant increases in P and D, accompanied

by significant reductions in E and o. In the case of carbons of

class D and E a fourfold increase in carbonisation rate has small but significant effects upon the properties of the carbons. Increases in P when comparing carbon D.15 with D.60 and E.15 with E.60 are reflected in significant reductions in E,

305

although values of 0 are not significantly different from each other. For carbons of class D, there is no change in the range of pore sizes on increasing

the

carbonisation rate, although for carbons class E there is an increase in the modal value of D, Table 2. For class C carbons, however, increasing the carbonisation rate produces a significant increase in P and the development of long crack-like pores, Fig 1, which are reflected in the large D,,,

value for carbon C.60, Table 2.

The increase in porosity and the presence of large crack-like

pores result in

reductions in E and 0. The different effects of carbonisation rate between carbons of class C on the one hand and carbons of classes D and E on the other is another indication of the sensitivity of the properties of the carbons to resin batch.

Pores in the glassy carbons, Fig. 1, are spherical and smaller than those in the carbons formed from the novolak resins, Table 2. For carbon F the pores are relics of bubbles of gaseous condensation

products released during the curing

process which are trapped in the increasingly viscous resin. The morphology of the pores in the commercial glassy carbon A are similar to those of carbon F, suggesting that it has also been formed from a liquid resin by a casting route. The introduction

of bubbles into the resin by stirring with a hollow glass rod has

increased porosity in the carbons significantly, c.f., carbon F with carbons G and H, Table 2. However, the increased porosity of carbons G and H, compared to carbon F has no significant effect upon E, Table 2. This suggests that there are significant changes in the properties of the solid carbon which offset the expected decrement in properties due to increases in porosity. GENERAL DISCUSSION plastic Modulus and Porosity The variation of E with P for various brittle and semi-brittle materials has often been fitted to various equations, including linear and exponential equations of the form E= E,(l -bP)

(1)

E = E,‘exp (-b’P)

(2)

where E, and E,’ are estimates of the value of E at zero porosity and b and b’ are parameters related to pore shape. For the novolak-based carbons the reduction of E with increasing P, Fig. 3 is fitted equally well to either E / GPa = 27.6 (1 - 0.80 P)

(3)

or E/GPa=

29.5exp(-1.17P)

(4)

Eshelby (ref. 7) derived an equation which reduces to Equn (1) with b = 3 for

306

an isotropic body containing an isolated spherical pore. A form of Equn (2) has been derived by Buch (ref. 8) also for an isolated pore in an isotropic medium. For a spherical pore, b’ = 2.78. Note that, since P CC 1 for an isolated pore, both equations give similar values of b and b’ for spherical pores. Values of b and b’ less than -3 found for the novolak-based carbons show that decrements in E are less severe than predicted by these theories, which are however only valid for P CC 1. For large values of P the effects of porosity on E can be described (ref. 9) by E = E,(l - P)3

(5)

0 0.2

0.4

0.6

Pore volume fraction, P Figure 3. Effect of porosity on elastic modulus; open symbols, novolak-based carbons; closed symbols, glassy carbons; error bars show one standard deviation. a: Equn (6); b: Equn (3). Equn (5) has been used to describe the effect of P on E for porous glasses and cements (ref. 9). Equn. 5 is a poorer fit to the data than Equn. (3) and (4) giving E/GPa = 58.9 (l-P)3 This value of E, -59 GPa is unrealistically

(6) high, Fig. 3, which also shows that

Equn (6) over-predicts E at low P and underpredicts it at high P. This shows that the effects of P upon E for novolak-based carbons are less severe than those found for porous glasses and cements.The effects of P on E are also less than those found in a similar study of electrode graphites (ref. 10) This suggests that there are variations in the mechanical properties of the solid carbon material between the different carbon classes which partially offset the effects of porosity. A similar conclusion was reached in studies of the influence of porosity on the strength of metallurgical cokes (ref. 11). The few measurements of E and cs for glassy carbons

307

do not warrant fitting to Equn (1) and (21 but the small effect of P on E, Table 2, indicates that variations in the mechanical properties of the solid carbon material partially offset the effects of porosity in the case of the glassy carbons too. Flexural stre.nath and Porosity Although analogous expressions to Equn (1) and (21 have been proposed to fit Q - P data, the large scatter in the cr data for the resin carbons do not warrant such an approach. The strength of the carbons is influenced by the size of flaws in the tensile face of the specimen. In general the relationship between strength and flaw size, c, is given by the Griffith-Orowan equation a = (E~xcll”2

(71

where R is the fracture surface energy or toughness (J/m2) and is a measure of the resistance of the material to crack propagation. Lawn and Wilshaw (ref. 12) have classified materials with R values less than 10 J/m2 as brittle, while R values in the range lo-100 J/m2 denote semi-brittle materials. Crack propagation in a porous material is influenced by the size of the largest pores or cracks. The microstructure of the carbons, Fig. 1, and measurements of the pore shape factor, r, showed that the majority of pores in the carbons are approximately equi-axed. Thus a reasonable estimate of c in Equn. (7) is provided by Dmax, enabling R to be estimated, Table 2. Novolak-based

carbons are semi-brittle solids with R in the

range 50-80 Jlm2; hot pressing the resin, significantly increases toughness, c.f., carbons C.15 and B.15. The glassy carbons A and F are brittle materials whose R values are similar to those of soda-glass, while the toughness of carbons G and H approaches the values for novolak-based carbons. Values of E, cr and R for the resin carbons fall within the ranges found for many other engineering carbons and graphites: E -10-100 GPa; cr -10-100 MPa, R -50-300

J/m2 (ref. 10,13). Values of 0 for the resin carbons with P -0.5 are

greater than those found for metallurgical cokes with similar porosities using the diametral compression test (ref. 11). The combination of high porosity and good mechanical properties suggests that these carbons may be suitable as catalyst supports. CONCLUSIONS. Porous carbons can be prepared from phenolic resins (novolaks) by a powder pressing method. Carbons with pore volume fractions

up to P -0.65

can be

prepared with mechanical properties which are comparable with those of many other engineering

carbon materials.

Porosity

and mechanical

properties

are

308

dependent upon the batch of the resin and can be varied by alterations to the fabrication parameters, e.g., pressing conditions, addition of pore forming agents and carbonisation rata. The reasons for this are as follows. The successful

production

of resin-based

carbons requires

a carefully-

controlled conversion of the cross-linked structure of the resin to a cross-linked microfibrillar

structure of carbon layer planes in the carbon, so that the carbon

structure retains a “memory” of the molecular structure of the resin. The process is analogous to the conversion of a polymeric fibre to a carbon fibre, where a “memory” of the axial orientation oriented

carbon

layer planes.

of the polymer molecules is retained in the

It follows

that the physical

and mechanical

properties of the solid carbon material will vary with the chemical constitution and cross-linked

density of the cured resin. This in turn is influenced

by the

thermal history of the resin system from its fabrication up to the point of full cure and then by conditions during carbonisation. To produce monolithic

carbon artefacts with adequate strength requires

that the volatiles trapped in the resins during curing and those produced during carbonisation are able to diffuse through the porous structure of the material as it is converted carbonisation

to carbon. For glassy carbons this is achieved using very slow rates. This work has shown that satisfactory

carbons can be

produced from novolak resins using fast carbonisation rates, provided that open porous structures are developed in the resin at the curing stage. ACKNOWLEDGEMENTS We thank Mr S R Tennison and Dr T J Mays for useful discussions and British Petroleum plc and the SERC for financial support. REFERENCES. 1. F.J. Derbyshire, V.H.J. de Beer, G.M.K. Abotsi, A.W. Scaroni J.M. Solar, D.J. Skrovanek, Appl. Catal., 27, (1986) 117. 2. H. Juntgen, Fuel, 65, (1986) 1436. 3. K. Knoblauch, E. Richter and H. Juntgen, Fuel, 60 (1981) 832. 4. A I. Foster, P.G. James, J.J. McCarrol and S.R. Tennison, US Patent 4163775, 1979. G.M. Jenkins and K. Kawamura, Polymeric carbons - carbons, glass and char., Cambridge University Press, 1976. E.E. Underwood, Quantitative Stereology., Addison-Wesley, Boston, USA, 1970. J.D. Eshelby, Proc. Roy. Sot. A241, !957) 376; idem.. ibirl, A252 (1959) 561. J. D. Buch, Extended abstracts 16th American carbon conference, San Diego, American Carbon Society (1983) 400. K. Kendall, A.J. Howard, J.D. Birchall, Phil. Trans. Roy. Sot., London, A310, (1983) 139. 10. Y. Yin, M.Phil thesis, University of Bath, 1987. 11. J.W. Patrick and A. Walker, Carbon, 27, (1989) 117. 12. B.R. Lawn and T.R. Wilshaw, Fracture of brittle solids, Cambridge University Press, 1975. 13. J. E. Brocklehurst, Chem. Phys. Carbon, 13 (1977) 145.