In situ electron microscopy study of the palladium-amorphous carbon interaction in carbon dioxide and oxygen atmospheres

Applied Elsevier

Catalysis, 11 (1984) 117-122 Science Publishers B.V., Amsterdam

ELECTRON

IN SITU

IN CARBON

MICROSCOPY

DIOXIDE

M. BOUDARTa,

Joint

aTo whom queries University, b

Present

duPont

de Nemours

R.D. MOORHEAD

and Microstructure of Chemical

Research.

Engineering,

Stanford

U.S.A.

Technology

& Co., Wilmington,

31 January

INTERACTION

and H. POPPA

for Surface

CA 94305,

Engineering

CARBON

ATMOSPHERES

be sent: Department

Stanford,

address:

(Received

Institute

should

in The Netherlands

STUDY OF THE PALLADIUM-AMORPHOUS

AND OXYGEN

W.L. HOLSTEINb,

Stanford/NASA

117 - Printed

1984, accepted

Laboratory,

Delaware

5 March

Experimental

Station,

E.I.

19898, U.S.A.

1984)

ABSTRACT The interaction between palladium particles 10 to 300 nm in diameter and amorphous carbon in carbon dioxide and oxygen atmospheres was studied by in situ electron microscopy. Palladium-catalyzed gasification of the carbon was observed at 850 K in oxygen at 30 Pa. In carbon dioxide at 120 Pa, the palladium-amorphous carbon interaction was identical to that observed in vacuum: palladium catalyzed the conversion of amorphous carbon to graphitic carbon at 1150 K. No gasification of the carbon by CO2 was observed.

INTRODUCTION Recently,

we reported

and amorphous graphitic carbon

carbon

carbon

the effect

in z)ucuo resulting

[I]. This paper

interaction

by both dioxygen

the effect

of carbon

that palladium

[Z] and carbon

between

in the conversion

reports

on the gasification

It has been shown previously carbon

of the interaction

of amorphous

particles

carbon

to

of the palladium-amorphous

by dioxygen

is a catalyst

dioxide

palladium

and carbon

dioxide.

for the gasification

of

[3].

EXPERIMENTAL The RCA EMU-4 in described

previously

300 nm in diameter iously

described

Oxygen

electron

situ

through

carbon

a jet located

Gas Products,

99.99%)

just above

30 Pa with dioxygen

the specimen.

particles

10 to

has also been prev-

pressure

in the specimen

a liquid

nitrogen

0 1984 Elsevier

dioxide

chamber

The estimated

Publishers

B.V.

99.999%)

chamber

pressures

at the

dioxide.

was maintained

cold trap, and liquid

Science

(Airco,

into the UHV specimen

and 120 Pa with carbon

Orb Ion

0166-9834/84/$03.00

of palladium 70 nm thick

and carbon

The gases were admitted

the background pumps,

support

has been

[I].

(Scientific

were

used in the investigation

[4,5]. The in situ preparation

on an amorphous

were used as received.

specimen

microscope

nitrogen

For both gases, -2 at 10 Pa by two cooling

of the

118

FIGURE

1

Palladium

particles

gen at 30 Pa for 10 minutes.

on amorphous The channels

carbon

after

are regions

heating

where

to 850 K in dioxy-

the carbon

has been

gasified.

upper aperture be maintained

of the specimen in carbon

lower for this gas because carbon

chamber.

dioxide

A higher

because

specimen

the background

of the ability

chamber

pressure

of the liquid

pressure

could

nitrogen

could

be kept

cold trap to pump

dioxide.

Temperatures be regarded

were measured

only as estimations

the specimen,

as previously

The interaction it occurred.

between

The temperatures

reported

must

gradients

were present

in

and the carbon

was observed

as

[I].

the palladium

cooling

following

pyrometry.

since large temperature

discussed

micrographs

The electron

the experiments

by optical

particles

shown here were taken

of the specimen

upon completion

to room temperature

of

and evacuat-

ion.

RESULTS

AND DISCUSSION

Following

deposition

of palladium

7 x 10-6 Pa to 1150 K resulted carbon

[I]. The two forms of carbon

The driving ordered

graphitic

Heating oxidation

force

carbon

the specimen

been gasified The temperature

resulting (Figure

is the free-energy

from the disordered in dioxygen carbon

heating

amorphous

at 30 Pa resulted

in the formation

the catalytic

of curved

of

to graphitic

diffraction.

of formation

of the more

carbon. in the palladium-catalyzed

channels

with many previous oxidation

in a vacuum

carbon

by electron

at 850 K [6]. The palladium

I), in agreement

at which

carbon,

of amorphous

were distinguished

for the conversion

of the amorphous

on the carbon,

on amorphous

in the conversion

particles where

similar

is observed

move around

the carbon

has

observations.

is much

lower than the

FIGURE

2

Palladium

on amorphous

the amorphous

temperature

carbon

Heating

catalytic

conversion occurs

induced

which

to proceed

eration

which

of the oxygen

rapidly

observed,

about

combination

(Figure

2). The

NO gasificat-

from micrographs

of the experiment.

with those

or in a carbon by the beam

However,

must

be given to beam-induc-

of the specimen

or carbon

outside

dioxide

the carbon

that gas dissociation

occurs

the atomic

oxygen

occurs,

was observed

of the beam, but no atmosphere.

heat transfer. the atomic

If electron

oxygen

surface.

to the carbon.

of

less signifi-

does not occur appreciably diffuses

The accel-

to beam heating

but becomes

over the entire

by

of the beam or within

C-O2 reaction

is attributed

radiative

dioxide

or beam-

of the beam was checked

at low temperatures,

due to enhanced

with

concern

beam heating

The catalyzed

indicating

before

form

100°C lower in the presence

in vacuum

react directly

in the palladium-

or subsequently

gas. The effect

200°C.

is significant

cant at hiuh temperatures

would

experiments,

from electron

to about

were observed

dissociation

was not observed

from that in dacuo.

the process

in the beam region

cooling

to a graphitic

at the conclusion

in rate of the C-O2 reaction

the specimen,

carbon

in UCICUO.

may have occurred.

of the reactant

at temperatures

beam effects

dioxide

of some of

observed

at 120 Pa resulted

carbon

during

microscopy

may result

observations

the beam after

dioxide

jr, vacua

of gasification

dissociation

comparing

the conversion

of graphitic

indistinguishable

was detected

electron

In in. sita

to 1150 K in carbon

was previously

formation

of the amorphous

taken at room temperature

ed effects

graphitization

in carbon

in a manner

ion of the carbon

a small amount

heating

has catalyzed

of dioxygen.

the specimen

conversion

after

carbon.

the palladium-catalyzed

in the presence

catalyzed

to graphitic

at which

As a result,

carbon

The palladium

at 120 Pa for 10 minutes.

beam

produced This was not or that re-

120

FIGURE

3

followed

Palladium

on amorphous

by oxidation

for carbon particle

graphitization.

which

remained

active

for oxidation.

zation

and oxidation.

The results

presented

carbon

catalyzed

gasification

above

by dioxygen

particle

conversion

carbon

by carbon

dioxide.

In recent hydrogen proposed. ferred

kinetic

[7], water The metal

of graphitic

of carbon

The catalyst

step appears

The adsorbed

the catalyst

to form the product

surface

by carbon

dioxide,

mechanism

of

for

carbon

the transfer

is the

of carbon

diffusion

at the opposite

of car-

side of the of

steps could play a role in dioxide.

gasification

[S], the following

of carbon

mechanism

bonds and carbon

also dissociatively

equilibrated

gas atoms

oxidation

to graphitic

by carbon

carbon-carbon

surface.

both graphiti-

rate than the gasification

dioxide

tures and pressures.

the gasification

carbon

[8], and carbon

This adsorption

The proposed

of the platinum-catalyzed

breaks

during

by bulk or surface

at a faster

b) Pd became

rate than the palladium-

carbon

catalyst

to the catalyst

molecules.

dioxide.

was active

for oxidation.

that the palladium-catalyzed

Thus one or more of these

studies

which

Many such particles

inactive

at a much faster

of amorphous

gasification

at 1150 K in ucct?o

inactive

bonds by the palladium,

steps all occur

the palladium-catalyzed

later

remained

to the other

and the formation

[I]. These

were

graphitization.

by carbon

of carbon-carbon

graphitization

at 30 Pa. a) Pd particle

which

indicate

occurs

from one side of the palladium bidic carbon,

during

of carbon

the palladium-catalyzed breakage

after

Such particles

inactive

c) Pd particle

amorphous

direct

carbon

at 850 K in dioxygen

carbidic

has been

atoms are trans-

adsorbs

at sufficiently

and adsorbed

by

gas

high tempera-

carbon

react on

gas (CH4 for HZ, CO for HZ0 and CO*).

at pressures

from 0.4 kPa to atmospheric

For

and

121

temperatures around 900 K, the rate of carbon gasification is controlled by both the rate of carbon-carbon bond breakage and the rate of reaction of adsorbed oxygen with adsorbed carbon on the catalyst surface. The above mechanism is supported by the results presented here for the palladium-catalyzed gasification of carbon by carbon dioxide. The palladium catalyst clearly breaks carbon-carbon bonds. A plausible reason why no gasification of the carbon was observed is that at the low carbon dioxide pressure used (120 Pal, the rate of re~oval of carbon on the palladium by reaction of adsorbed carbon with adsorbed oxygen is much slower than the rate at which the adsorbed carbon is precipitated in the graphitic form. The activity of palladium for the catalyzed gasification of active carbon, a disordered carbon, by carbon dioxide is reported to decrease rapidly during the initial stage of gasification [3J at 1173 K and atmospheric pressure. This may be due to formation of graphitic carbon on the catalyst surface, which prevents adsorption of carbon dioxide. The fact that the carbon-carbon bonds are being broken by the palladium and that carbon is being transferred to the palladium surface can be seen only when non-graphitic carbon is used. For palladium particles on graphite, the carbon is already in its lowest free energy form and graphitization does not occur. In vacuo, the rate of carbon-carbon bond breakage is equal to the rate of the reverse step and no changes in the carbon are apparent. The discussion above for the mechanism of the palladium-catalyzed gasification of carbon by carbon dioxide is not applicable to the catalyzed oxidation of carbon in dioxygen, since the latter reaction occurs at a much faster rate than the rate of catalyzed carbon-carbon bond breakage. It is likely that the catalyzed oxidation proceeds through a different mechanism, perhaps spillover of oxygen from the metal to the carbon following dissociative adsorption of dioxygen on the metal [9J. In one experiment, catalyzed graphitization of the carbon was carried out at 1150 K. The specimen was then cooled to room temperature, oxygen was admitted at a pressure of 30 Pa, and the specimen was heated to 850 K. A micrograph taken after cooling the specimen to room temperature and evacuation of the oxygen is shown in Figure 3. Some of the palladium particles which were not previously active for graphitization or which had spread out an& wetted the amorphous carbon became active for catalytic oxidation. However, only a fraction of the particles became active. Palladium particles which ~ad become encapsulated with carbon and skeletons of the initial particles were uniformly inactive for oxidation. This is probably due to their being covered with graphitic carbon, preventing the adsorption of oxygen. Active particles preferentially carved channels in the amorphous carbon when reaching an amorphous carbon-graphitic carbon interface, but some gasification of graphitic carbon was observed. This observation indicates a difference in the rates of palladium-catalyzed oxidation depending on the degree of crystallinity of the

122 carbon.

This may be due to preferred

These carbons

results

indicate

by dioxygen,

may be greatly recognized

ing reaction kinetics

carbon

influenced

problem

dioxide

and other

by the pre-treatment

of catalyst

may also result

of gasification

wetting

of the disordered

that the rate of catalyzed

sintering,

from these

of carbon

gases,

such as water

procedure.

changes

carbon

gasification

by the metal.

of disordered and dihydrogen,

In addition

to the long-

in the rate of gasification

processes.

These transient

will be discussed

elsewhere

aspects

dur-

of the

[lo].

ACKNOWLEDGEMENT This work support

is part of the Ph.D. Dissertation

by the U.S. Department

of Energy

of W.L.H.,

under Grant

who acknowledges

Number

DE AS03-SF-00326.

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