Effect of metallic impurities on the gasification of graphite in water vapor and hydrogen

Carbon.

1974, Vol.

12, pp

45.V4-464.

Pergamon

Press.

Printed

in Great

Britain

EFFECT OF METALLIC IMPURITIES ON THE GASIFICATION OF GRAPHITE IN WATER VAPOR AND HYDROGEN Corporate

Research

DOUGLAS W. MCKEE and Development, General Electric York 12301, U.S.A.

Company,

Schenectady,

New

(Received 4 January 1974) Abstract-The catalytic effects of metallic impurities on the reactivity of graphite towards water vapor and hydrogen in the temperature range 25” to 1100°C have been investigated as a function of the oxidation state of the impurity. Iron, cobalt and nickel are active catalysts for the former reaction between 600”and 1000°C when the metal is kept in the reduced state by means of added hydrogen. Motion of the metallic particles on the basal plane surface of the graphite during the reaction leads to the formation of channels which with the smallest catalyst particles are oriented mostly in the (1120) directions. Vanadium and molybdenum are weak catalysts under these conditions, whereas copper, zinc, cadmium, silver, chromium, manganese and lead are inactive. When hydrogen is absent so that the metal remains in the oxidized state, the catalytic activity of all these impurities is low or negligible. The reaction of graphite with dry hydrogen occurs less readily but is again strongly catalyzed by metallic iron, cobalt and nickel. Manganese, chromium, molybdenum and vanadium show a slight catalytic effect in the hydrogenation reaction at temperatures around lOOO”C,whereas copper, zinc, cadmium. silver and lead are inactive. INTRODUCTION Because

of

their

the reactions carbon of

between

importance,

carbon

and hydrogen

many

and Hill [4], who showed

technological

and steam

have been

investigatons,

which

and

the subject have

mostly

greatly

accelerated

1000°C

when

at ternperatures

the graphite

with iron,

cobalt,

However,

iron

nickel and

lost their

activity

involved

parently

to conversion

on

commercial

carbon

types [2:]. Although these

reactions

purities, studies and

the

kinetics

samples

it is often

are catalyzed

there of the

have

been

behavior

mechanisms

stated

various

few

steam-carbon

the

present

metals,

is therefore

On

graphite

the other hand, the effects of metals, oxides and salts on the oxidation or combustion of

In

carbon

as

and

studied and are available One

of

graphite

have

been

several excellent review on this subject[3]. the

few

detailed

studies

widely

lead,

to inac-

many

molybdenum

the

The

graphite-water from

that

of metal

of

reaction

involved to

in he

oxides [3, 61.

additives.

and

was

mechanism

is known

metal

in the

rnetal

vapor

which

by a variety

also showed

as a catalyst

when

different

addition,

due ap-

form.

oxidation

accelerated

the reaction Sykes[5]

reaction

salts.

progressively

of the metals

was inactive

in oxidized

the catalyzed

reaction

obscure.

and

around

and vanadium

during

Long

that the iron

im-

systematic

of different remain

tive oxides.

that both

by metallic

by which

are accelerated

of

was

was impregnated

vanadium

emphasized

the complex equilibria [ I] and the kinetics of the reactions

that the reaction

copper.

such which

articles

actively promote graphite oxidation have not been reported as catalysts for the

of

graphite-water vapor reaction. Even less information is available on the role of catalysts

the

effects of catalysts on the graphite-water vapor reaction was carried out by Tudenham

in 35.7

the

hydrogenation

of

carbon,

although

454

DOUGLAS

mixed evaporated films of nickel and carbon have been found to react readily with hydrogen at temperatures below 3OO”C[7]. This paper describes the results of an investigation into the relative importance of a number of metallic impurities as catalysts for the gasification of graphite by water vapor and hydrogen and an attempt to correlate selectivity and catalytic activity with the oxidation state of the metal. EXPERIMENTAL

Measurements of the effects of metallic impurities on the reactivity of graphite were carried out with both microcrystalline graphite powder and large flake crystals as starting materials. As in previous work[S], a highly purified graphite powder (Ultra Carbon Corp., Type UCP-2,325 mesh), containing as impurities less than 5 ppm of Si and Mg, was impregnated with solutions of salts of a number of metals. The salts used included the acetates of nickel, cobalt, copper, zinc, manganese, chromium, lead, silver and cadmium, and ferric formate. Salts of organic acids were chosen as these decompose to the respective metal oxides at temperatures below 500°C without contaminating the carbon substrate with anions such as halides or sulfate which may have an effect on the reaction kinetics[9]. Graphite samples were also contaminated with vanadium and molybdenum by dry grinding with powdered V,O, and Moos, respectively. The metal impurity concentration was adjusted to about 1 wt% in most cases, although the effect of impurity concentration in the range 0.1-5 wt % was studied in the case of iron, nickel and cobalt. Following impregnation with the soluble metal salt, the samples were evaporated to dryness under a heat lamp and then homogenized for 4 hr in a Fisher Minimill blender. Thermogravimetric measurements of weight changes as a function of temperature in various gaseous environments were carried out in the Chevenard balance described previously, using sample weights of 200 mg, a heating rate of SOO”C/hr and a gas

W. MCKEE

flow rate of 250 ml/min. The gases used were: dry hydrogen (Matheson, Ultra Pure Grade), and nitrogen (Linde, pre-purified grade) passed through an activated Zr-Ti getter (Oxytrap, Alltech Associates) to remove traces of oxygen and then through a molecular sieve tower (Linde 13X pellets). Water vapor was introduced in some of the experiments at a partial pressure of 23 mm, by removal of the zeolite trap and passage of the gases through a sealed wash bottle fitted with a fine fritted disk and filled with carefully deaerated distilled water. Microscopic observations of the topographical effects of dispersed impurity particles on purified Ticonderoga graphite flakes were made with the Leitz 1750 hot stage described previously [S]. Freshly cleaved and purified flakes, cut to approximately 3x 3 mm, were mounted horizontally on the platinum sample holder of the hot stage. A tiny drop of a 0.02 per cent solution of the metal salt was placed on the basal plane surface of the specimen and evaporated to dryness in air by means of radiant heat from the platinum foil heaters of the hot stage. The quartz cover glass was then replaced and the subsequent motion of the metal impurity particles was observed directly during reaction of the graphite with water vapor and hydrogen at temperatures of 900-llOO”C, as measured by the Pt-Rh/Pt thermocouple mounted in contact with the lower surface of the specimen holder. Gas flow rates through the envelop of the hot stage were maintained at 100 ml/min during the observations. RESULTS The reaction of graphite with water vapor-Efect of metal impurities

As described above, weight loss measurewere carried with metalments out contaminated graphite samples in flowing streams of hydrogen and nitrogen which had been saturated with water vapor at room temperature (i.e. containing 23 mm of water vapor at 1 atm total pressure). Pure graphite and

graphite

impregnated

with

1 wt %

COP-

GASIFICATION

per,

zinc,

lead

and silver

chromium,

weight

when

1000°C

in either

other

hand,

cobalt

promoted

graphite

OF GRAPHITE

manganese,

cadmium,

showed

no detectable

heated

to

gaseous

temperatures

of

a rapid reaction

and moist hydrogen

of

On the

nickel

and

between

the

lower

at temperatures

Figure as

a

1 shows

function

graphite nated The

and

measured of

latter

began than

temperature rogen

The

of

reaction

water

vapor

shows

the results

carried graphitc-l ent

as metal).

at 750°C

gasified

1000°C.

In

and

when moist

was much

on

between in

of isothermal

iron-

graphite

and

Fig.

2, which

experiments

at 930°C with the .O wt % Fe sample in three differatmospheres.

Initially,

using

water

trap.

nitrogen

and

eratures

in the range

Above

750°C

in both

tnoist

graphite-O.32

wt %

The

hydrogen

though

weight

are

catalyst graphite,

for

sample shown

losses were

the

in

of graphite with water vapor in hydrogen-catalytic 1 wt % iron. Per cent weight loss vs temperature.

Fig.

4.

more

a and Al-

rapid of

also acted as a

hydrogenation which

of dry

in the presence of

the

will be discussed

TEMPERATURE 'C

Fig. 1. Reaction

gas.

hydrogen

in

much

the iron impurity

an effect

about 300°C

controlled.

at the lower temperatures water vapor,

effect

gasification

Fe

a

temp-

the gasification

in moist

diffusion of

from

as the carrier

the reaction rates

.4rvapor

700-1100°C.

with nitrogen

was apparently

moist

3 shows

at constant

was to cause

reof a

of water

to occur at temperatures than

was

obtained

series of rate measurements

lower

vapor

sample,

hydrogen.

ensued, to a much

by means

Figure

with this graphite-Fe

reaction

the

the

loss

dropped

gas stream

sieve

by moist hyd-

weight

plots for the reaction

Relative

hydrogen

of 30 min, at which

suddenly

the

nit-

reduced

samples showed only at temperatures in the

is illustrated

rhenius

rapid

when

from

of the hydrogen

out

gaseous

rate

the

of 1000°C.

effect

catalyzed

pure

(calculated

effect

and iron-impregnated slow weight losses

for

contami-

to lose weight

reached

changes

of graphite

40 per cent

this catalytic

neighborhood

weight

temperature

a sample

with 1 .O wt % iroi

was more

however,

moved

loss of the graphite

was replaced

very

molecular

as low as 700°C.

no weight for a period

point the nitrogen A

455

AND HYDROGEN

was detected rogen. which,

iron,

VAPOR

moist nitrogen,

loss in

atmosphere.

additions

IN WATER

effect

of

DOUGLAS W. MCKEE

456

14

tz-

I

I

I

I

" 10

20

30

40

GRAPHITE-I.O%Fe 930°c

I

I

I

I

I

50

60

70

80

90

IO-

4-

2-

0. 0

=

TIME

MINS.

Fig. 2. Reaction of graphite-l.0 wt % iron with moist and dry hydrogen. cent weight loss vs time at 930°C. Gas flow rate = 250 ml/min. Figure

5 shows the influence

further

below.

of iron

impurity

erature

and rate of the graphite-water

concentration

T lcm I

vapor

eoo

900 I

700 I

I

GRAPHITE-LOX Fe 2001119 23mm H$l VAPOR

t

0.74

on the temp-

0.76

0.62

0.66 IIT'K

0.90

0.94

0 96

X lO-3

Fig. 3. Reaction of graphite-l.0 wt ‘7%iron with water vapor. Weight loss rates in wet nitrogen and wet hydrogen vs l/temperature.

Per

In this series of experiments, it was likely that the graphite was reacting with both water vapor and hydrogen simultaneously, the former reaction being dominant. In separate experiments, it was found that at 900°C with a graphite sample containing 0.09 per cent iron impurity, the relative rates of the graphite-water vapor and graphite-hydrogen reactions were in the ratio 8.5 to 1. Low concentrations of iron had the most dramatic effect on reaction rates and little additional increase in reactivity was observed for iron concentrations in excess of 3 wt %. Figure 6 shows the effect of nickel additions on the graphite-water vapor reaction in a hydrogen atmosphere. This metal was some what less effective than iron for this reaction at the same concentration levels. Direct microscopic observations of the surface of graphite flakes during the catalyzed reaction with water vapor between 900 and 1100°C showed that active catalyst particles exhibited the curious mobility which has been previously reported with graphite gasification in oxygen[6, lo]. The most vigorous mobility was observed in the case of iron particles, the reaction.

GASIFICATION 35

30-

OF GRAPHITE

IN WATER

1 I I I GRAPHITE-0.32%Fe GAS FLOW RATE =250ml/min TEMP. RISE RATE=300"C/hr

VAPOR

1

I

ANI)

15i

Hl’DRO(;EN I

25-

,

Q20-I k $

15-

ap IO-

5-

OL 500

--1

I

I

I

600

700

600

900

TEMPERATURE "C

Fig. 4. Catalytic effect of 0.32 WC ‘; iron on the graphite-water vapor and graphite-hydrogen reactions. Per cent weight loss vs temperature.

WET He (23mm H20)250ml /min

30

800

900

1000

TEMPERATURE 'C

Fig. 5. Effect of iron impurity on the reaction of graphite (325 mesh) with water vapor in hydrogen. Per cent weight changes vs temperature. motion

being

accompanied

by the formation

of shallow channels in the graphite plane, as shown in Fig. 7. The smallest cles tended

to be the most

active,

basal parti-

excavating

mainly

straight

channels,

oriented

predomin-

antly in the (1120) directions (Fig. 8), whereas metal particles of 10 nm diameter or larger showed more irregular motion and curved

DOUGLAS W. MCKEE

458

WET Ii,l23mm Ii,01 25Omllmin

-600

700

900

600

1000

TEMPERATURE 'C

Fig. 6. Effect of nickel impurity on the reaction of graphite (325 mesh) with water vapor in hydrogen. Per cent weight changes vs temperature.

L

4

Fig. 7. Catalytic channeling produced by iron particles on graphite basal plane during reaction with water vapor in hydrogen at 1000°C X 300.

Fig. 8. Catalytic channeling produced by nickel particles on graphite basal plane during reaction with water vapor in hydrogen at 1000-l 100°C.

GASIFICATION

OF GRAPHITE

IN WATER

VAPOR

AND

HYDROGEN

459

channels (Fig. 7). Particles of nickel and cobalt required somewhat higher temperatures than iron to initiate the motion, although the channeling produced on the graphite substrate was similar, as shown in Figs. 8 and 9. Catalytic channeling and particle mobility were only observed in moist hydrogen, dry hydrogen and moist nitrogen giving no visible effects in the temperature range BOO-1000°C. The reaction of graphite

with dry hydrogen-Eflect

La

of metallic impurities

Gasification rates of impregnated graphite samples in dry hydrogen as a function of temperature and metal concentration are shown in Fig. 10 for iron and in Fig. 11 for nickel. In contrast to iron, nickel appeared to be more active in promoting the hydrogenation of graphite at the lower temperatures than as a catalyst in the graphite-water vapor reaction, although the rate of the latter accelerated rapidly at temperatures above 950°C. A similar effect was found for cobalt, as shown in Fig. 12. In this case, the hydrogenation

reaction

dominated

below

850°C.

Fig. 9. Catalytic channeling

produced by cobalt particles on graphite basal plane during reaction with water in hydrogen at 1000-I 100°C x f(OO.

14

12

DRY Hz 250ml/min TEMP. RISE RATE = 300”Clhr

8

6

0 600

700

800

900

1000

TEMPERATURE "C

Fig. 10. Effect of iron impurity on the reaction of graphite (325 mesh) with drv hydrogen. Per cent weight changes vs temperature.

460

DOUGLAS

I

I

I

W. MCKEE

I

I

,

I

DRY Ii,26Oml/min

I

2.2% Ni

TEMPERATURE 'C

Fig. 11. Effect of nickel impurity on the reaction of graphite (325 mesh) with dry hydrogen. Per cent weight changes vs temperature. 14

I I GRAPHITE+ l%Co FLOW RATE =250 ml/min

12-

ae 6-

1600

700

600 900 TEMPERATURE 'C

1000

Fig. 12. Effect of 1 wt % cobalt impurity on the reactivity of graphite (325 mesh) with water vapor and hydrogen. Per cent weight change vs temperature. whereas became tures. Table

the graphite--water vapor reaction more significant at higher tempera1 summarizes

qualitatively

the

ob-

served catalytic effects for the twelve different metallic impurities studied. As noted above, iron, cobalt and nickel were very active catalysts

for

the

graphite-water

vapor

reaction

GASIFICATION

Table

1. Effect of impurities graphite

Strong catalysts

OF GRAPHITE

on the reactivity of

Similar

(b) Graphite-hydrogen reaction Fe Mn Ni Cr co MO V

results

hydrogenation

Ag

ments

manganese

hydrogenation

conditions, at somewhat

higher

In both cases, the addition

these

metals

which least

decreased

gasification 200°C.

the

became

Certain 14r

other

I

tempera-

observable

I

at by at

metals,

l2-

Typical

also

obtained

re-

moderate

catalysts

with

Chromium activity

but were inert reaction.

This

to the metallic

at temperatures

but remain

in oxidized

A number

of other for either

in water

metals

were

reaction;

selectivele-

state in dry

around

form

as

in the

vapor

are reduced

a

and

due to the fact that both

gasification

idizing

conditions,

1 OOO”C, vapor.

inactive

as

it is interesting

purities

studied

in oxygen none

showed

for the graphite-water a vivid illustration impurities tion

I

I

I

range.

graphite-water

notably

200 mg GRAPHITE -I % MO

in the

that these include some, such as copper, lead and silver, which are very potent catalysts for

of 1 wt % of

temperature

a mod-

ity is probably

catalysts

and for graphite

tures.

were

exhibited

hydrogen

cu Zn Cd Pb

showed reactions

wt % Mo sample in wet are shown in Fig. 13.

wt % V sample.

graphite reducing

on both

temperature

graphite-l

Ag

under

effect

sults for a graphite-l and dry hydrogen

cu Zn Mn Cr Cd Pb

361

AND HYDROGEK

and vanadium,

catalytic

900-1000°C

Inactive

vapor reaction conditions) MO V

VAPOR

molybdenum erate

Weak catalysts

(a) Graphite-water (under reducing Fe Ni Co

IN WATER

[6]. Under

of the

significant

vapor

reaction.

of the selective

in different

I

I

I

-

TEMP. RISE RATE= 300Whr.

IO-

TEMPERATURE

lC

Fig. 13. Effect of 1 wt 96 molybdenum impurity on the reactivity of graphite (325 mesh) with water vapor and hydrogen. Per cent weight changes vs temperature.

oxirrr-

activity This is

behavior

types of carbon

reactions.

I

metallic

of

oxida

462

DOUGLAS

DISCUSSION

In the temperature range 700-lOOO”C, the rate of the reaction of pure carbons and graphite with water vapor is invariably much slower than the corresponding reaction with oxygen, but considerably faster than that with hydrogen. At 800°C and a pressure of O-1 atm, the relative rates of the uncatalyzed reaction of carbon with oxygen, carbon dioxide, water vapor and hydrogen have been estimated as 105, 1, 3 and 3 X lo-‘, respectively [l]. As expected from the equilibrium

the reaction with water vapor is often found to be retarded by the presence of hydrogen and a rate equation of the form

kJ& rate = 1 + kzPH2+ k,PH,o has been found to agree with the observed kinetics in many cases [ 11. As with gasification by oxygen, the presence of certain metallic impurities has a marked effect in catalyzing the carbon-water reaction, but in contrast to the situation with oxidation, the catalytic effect is only appreciable when the metal remains in the reduced condition, metal oxides being generally inactive. Thus the effects of iron, nickel and cobalt impurities are more pronounced in a moist hydrogen atmosphere than with moist nitrogen, whereas with pure graphite the reaction rate is decreased in the presence of hydrogen. Metals which are active catalysts for the graphite-water vapor reaction form oxides whose free energies of formation are within 30 kcal of that of water at temperatures of 1000°C and below. However, in the case of the active catalysts iron, cobalt and nickel, it appears that the metal itself is the active catalytic entity for when the ambient hydrogen pressure is reduced so that a bulk oxide grows on the metal surface, a sudden loss of catalytic activity occurs. It is likely that dissociation of water at the metal surface

W. MCKEE

takes place initially, leading to chemisorbed oxygen and hydrogen atoms; e.g. Ni + HZ0 = Ni - Oads+ 2(Ni - Hads) This may be the rate determining step in the overall reaction and in general the higher the affinity of the metal for oxygen, the more rapid will be this oxygen transfer step. Thus, catalytic activity for the graphite-water vapor reaction should increase in the order, nickel < cobalt < iron, as observed experimentally. Metals which form less stable oxides (e.g. copper, lead and silver) will not promote the dissociation of water, whereas if the oxygen affinity of the metal becomes so great that the bulk oxide is formed (as with chromium and manganese) the subsequent reaction with the graphite substrate Ni-Oads+C=Ni+C-Oads c - O& = CO(g) will not take place. From free energy data (1 l), the minimum HZ/H,0 ratios required to convert the lowest metallic oxides to the metallic state at 1000°C are approximately: Ni 1 x lo-*, Co 5 X lo-‘, Fe 3.0, Cr 5 X 10’. Thus with the wet hydrogen environment (HZ/H20 = 32), iron, cobalt and nickel will exist in the metallic state at equilibrium, whereas chromium will remain in the oxidized form. In wet nitrogen, all four metals will be present as oxides. This prediction was confirmed by measuring weight losses of samples of anhydrous nickel acetate on heating in the thermobalance in both nitrogen and hydrogen saturated with water vapor. Results obtained for a temperature rise rate of SOO”C/hr are shown in Fig. 14, together with the expected weight losses for complete conversion of the salt to metallic nickel and NiO, respectively. The thermograms show that in wet nitrogen, the final product of the decomposition of the salt at 1000°C was NiO, whereas in wet hydrogen almost complete conversion to metallic nickel had taken place at this temperature. As significant catalytic activity was only observed in the wet hydrogen atmosphere, it

GASIFICATION

OF GRAPHITE

IN WATER

VAPOR

463

AND HYDROGEN

Hz/H20 (32/l)

Ni

701 60N2/ H20

TEMP. RISE RATE:300°C/hr GAS FLOW RATE=250ml/min

200

300

500

400

700 800 600 TEMPERATURE OC

900

1100

1000

Fig. 14. Decomposition of anhydrous nickel acetate in wet nitrogen hydrogen. Weight changes vs temperature. is reasonable

to conclude

that metallic

cles are the active catalytic for

accelerating

the

entities

parti-

responsible

graphite-water

vapor

reaction. A similar Grabke

mechanism

has been proposed

[12] for catalysis

H, = CO-t

H,O

this case,

also,

of the reaction

at a temperature iron

is only

of 800°C.

when

it is kept

in the

excess

hydrogen.

If the HZ/H,0

gas mixture converted catalytic

falls below

reduced

between with

time

and

as the

the

oxidation

determined

phase

sharply.

effect

metal

According

transfer

from

sixty times

less

of iron on the

by CO,--CO

1000°C

and the

decreases becomes

mixtures rapidly oxidized.

However, revived

investiga-

out by Walker

graphite state

urements.

Metallic

of

oxidized

the

whereas oxidaFe@, resulted activity,

to a reducing

such as hydrogen

or CO. Thus,

elementary

involved

phases The

in

those of the the specificity

the

atmosphere although this

the

case

are

graphite-water of the catalyst

is identical. observed

on the basal during

which

by exposing

sample

different from vapor reaction,

as

meas-

iron and, to a lesser extent

be restored

steps

of

metal

susceptibility

loss in catalytic

however

activity

was correlated

FeO, were the active species, tion of the metal to magnetite could

Fe0

carried

by magnetic

in a dramatic

the catalytic

700

with

to the

in the

CO, to Fe0 at 800°C is about than that for metallic iron. of graphite

added

iron is

the rate of oxygen

Similarly,

with

a detailed

has been

et al. [3]. In this work the observed iron

ratio

activity decreases

oxidation

In

catalyst

state

reaction,

1.8 the metallic

to the wiistite

to Grabke,

by

CO, +

an active

graphite-CO, tion of which

and wet

mobility

plane

surface

the graphite-water

of catalyst

particles

of graphite vapor

flakes

reaction

is

the catalytic activity can again be intermittent hydrogen by treatment[l3]. Indeed, the catalytic behavior of the various oxidized and reduced states of

probably the result of the evolution of the gaseous products of the reaction at the catalyst-graphite interface. As with catalyzed

iron in the graphite-water closely paralleled in

most vigorous motion, moving generally in a rectilinear manner in preferred crystallog-

vapor reaction is the case of the

oxidation[6],

the smallest

particles

show the

464

DOUGLAS

raphic directions. Catalyst particles larger than about 10 /.L m showed more sluggish motion and tended to move in curved channels. The dominant direction of the narrowest channels appeared to be perpendicular or oriented at angles of 30” to the twin bands which run in the (lOi0) directions through the graphite crystals. The preferred orientation of the channels is therefore in the (1120) directions which implies that the ‘armchair’ edge carbon atoms on the (1121) faces are inherently more reactive in the presence of the catalyst particles than are the ‘zigzag’ edge carbon atoms on the {lOil} faces. It has been repeatedly shown that the uncatalyzed oxidation of graphite below 1000°C produces hexagonal pits at dislocation sites on the basal plane surfaces of graphite crystals which are exclusively oriented with pit sides parallel to the twin band directions. According to Thomas [ lo], this result implies that removal of carbon atoms at the {lOil} faces is consistently more rapid than at the (1121) faces. By contrast, it has been observed [6] that, in the presence of a dispersed iron catalyst, oxidation leads only to ‘perpendicular’ type hexagonal pits, in which preferential oxidation of the (1121) carbon atoms has occurred. The reason for this difference is not clear, although it is possible that the smallest catalyst particles are more strongly adsorbed on the ‘armchair’ (1121) faces with adjacent unsaturated carbon atoms, than on the {lOiZ> faces. Recent calculations by Abrahamson [14] have shown that the surface energy of the ‘armchair’ configuration is about 15 per cent greater than that of the ‘zigzag’ arrangement. In any case, the difference in activation energies for reaction in the two directions is likely to be small as the curved channels found with the larger catalyst particles implies that reaction occurs in both directions simultaneously. For the uncatalyzed oxidation reaction, the difference in activation energies for reaction at (1121) and {ioil) faces is only of the order of 4 kcal/mole [lo].

W. MCKEE

In the case of the graphite-hydrogen reaction, although dissociation of molecular hydrogen certainly takes place at the metal surface, hydrogen atoms migrate freely along the surface of graphite [ 151 so that gasification of the substrate is not restricted to the regions of contact between the solid phases and no motion of the particles is observed below 1000°C. At higher temperatures where the rate of the graphite-hydrogen reaction becomes more significant it is possible that catalytic mobility and channeling may occur. ‘Perpendicular’ type pits have also been observed in the reaction of pre-dissociated hydrogen atoms with graphite crystals [16] so it is likely that preferential attack on the (1121) ‘armchair’ carbon atoms also occurs in this case. REFERENCES 1. Walker P. L., Jr., Rusinko F., Jr. and Austin L. G., Adv. Catalysis 11, 133 (1959). 2. Ergun S. and Mentser M., Chemistryand Physics of Carbon, Vol. 1, p. 204. Marcel Dekker, New York (1965). 3. e.g. Walker P. L., Jr., Shelef M. and Anderson R. A., Chemistry and Physics of Carbon, Vol. 4, p. 287. Marcel Dekker, New York (1968). 4. Tudenham W. M. and Hill G. R., Ind. Eng. Chem. 47, 2129 (1955). 5. Long F. J. and Sykes K. W., Proc. Roy. Sot. London A215, 100 (1952). 6. McKee D. W., Carbon 8, 623 (1970). 7. Deitz V. R. and McFarlane E. F., Carbon 1, 117 (1964). 8. McKee D. W., Carbon 8, 131 (1970). 9. Gallagher J. T. and Harker H., Carbon 2, 163 (1964). 10. Thomas J. M., Chemistry and Physics of Carbon, Vol. 1, p. 121. Marcel Dekker, New York (1965). 11. Swalin R. A., Thermodynamics of Solids, p. 84. Wiley, New York (1962). 12. Grabke H. J., Ber. Bunsenges, Phys. Chem. 69,48 (1965); Wagner C., Adu. Catalysis 21, 323 (1970). 13. Turkdogan E. T. and Vinters J. V., Carbon 10, 97 (1972). 14. Abrahamson J., Carbon 11, 337 (1973). 15. Robe11 A. J., Ballou E. V. and Boudart M., J. Phys. Chem. 68, 2748 (1964). 16. McCarroll B. and McKee D. W., Carbon 9, 301 (1971).