Thermodynamic properties of surface carbon on ruthenium

Surface Science 133 (1983) 31 l-320 North-Ho~and Publishing Company

311

THERMODYNAMIC PROPERTIES OF SURFACE CARRON ON RANT Henry WISE and Jon G. MCCARTY Solid State Catalysis L.aboratoty, SRI International,

Menlo Park, California 94025, USA

Received 19 April 1983; accepted for publication 1 June 1983

The thermodynamic properties of surface carbon on alumina-supported ruthenium have been determined from measurements of the Boudouard equilibrium in the temperature range from 525 to 675 K. The results indicate that the enthalpy of surface-carbon formation on Ru is positive relative to graphite and insensitive to the fractional coverage with carbon. Measurements of the chemical reactivity of surface carbon towards hydrogen indicate that the ~~-en~~py carbon forms methane at relatively low temperatures (275 K).

1. Introduction Only recently the multif~ous properties of carbon species on the surface of transition metals have been recognized. Thus, on the one hand surface carbon appears to be the precursor to catalyst coking [1,2], on the other it is the intermediate in methanation [3,4] and Fischer-Tropsch synthesis [5,6]. The chemical and physical properties of surface carbon coordinated to metal sites are of considerable importance to the elucidation of reaction mechanisms. The morphology of large surface-carbon deposits formed by the interaction of hydrocarbons (alkanes, olefins, aromatics) with such metals as Fe and Ni has demonstrated a range of structures, including filaments, platelets, amorphous carbon, and graphite [7]. Surprisingly the carbon surface structures are relatively unaffected by the chemical nature of the source material used in forming the carbon deposit {8,9]. But, crystal orientation [lO,l I], and defect structure [12] strongly influence the nature and rate of growth of the carbon deposit. An important observation was made by Dent et al. [ 131 on the basis of his studies of the Boudouard equilibrium in the presence of a nickel surface. He concluded that the thermodynamic activity of the carbon present on Ni at multilayer coverage is not unity, i.e., the eq~b~um constant measured does not correspond to that expected in the presence of graphite. The carbon surface structure was not identified, but is often identified as “Dent carbon”. More recently Rostrup-Nielson [14] examined the thermodynamics of the 0039-6028/83/0000-0000/$03.00

0 1983 North-Holland

312

H. Wise, J. G. McCariy

/ Thermodynamic properries of C on Ru

Boudouard equilibrium on small nickel crystallitbs dispersed on various support materials including magnesia, magnesia-alumina spinel, ~-alumina, and a-alumina. Again the equilibrium constants measured at high multilayer surface coverage indicated significant deviations from the Boudouard reaction involving graphite. In addition they were found to be a function of the Ni crystallite diameter. The carbon layers formed exhibited highly disordered whisker-like structures. In all these measurements the carbon surface density represented many monolayers of carbon. In the present study the thermodynamics of the Boudouard equilibrium are examined at fractional surface coverage of carbon on ruthenium crystallites. Also the reactivity of the surface carbon is examined in a temperature programmed surface reaction with hydrogen.

2. Experimental details For study of the equilibrium: 2 CO(g) + c(a) + W(g),

(1)

where (a) refers to adsorbed and (g) to gaseous species, a closed loop gas recirculation system was employed which operated at a total pressure of one To Vent

ing Loop

Reactor with Catalyst

nator, ID

t

CO/He CO$He

Fig, 1. Schematic diagram of apparatus for equilibrium studies.

H. Wise, J. G. McCarty / Thermodynamicproperties of C on Ru

313

atmosphere, but at very low partial pressures of CO and CO2 diluted with He. The recirculating system (fig. 1) contained a Pyrex glass reactor (< 10 ml in volume). The powdered catalyst sample was deposited on a fritted Pyrex disk contained in the reactor. Auxiliary lines isolated by suitable valves were attached to the recirculating system to (1) admit H, for reduction of the catalyst, (2) introduce CO/He and COJHe gas mixtures for the equilibrium studies, and (3) withdraw samples for quantitative chemical analysis by gas chromatography. The system was designed to operate either in a continuousflow mode or closed-loop recirculating mode. For gas recirculation, a pump with stainless-steel bellows was used (Metal Bellows Corp., Sharon, MA). In order to increase the sensitivity for detection of the low concentrations of CO and CO, used in our studies, these gases were converted to methane and analyzed with a flame ionization detector (FID) [ 151. The aliquot of gas withdrawn for analysis from the reactor was first passed over a chromatographic column operating at 350 K to separate CO from COz. Subsequently, H, was admixed to the gas stream which entered a small methanation reactor for conversion of CO and CO, to CH,. The electrically heated methanator, containing a small mass of 5 wtS Ni/Al,O, and operating at 550 K, was located inside the GC oven on the downstream side of the separation column. The efflux from the methanator passed through a Hz/air flame ionization detector that detected sequentially with high precision (~-0.2 ppm) the CH, formed by catalytic conversion of CO and CO2 initially contained in the sample. By this means CO and CO2 levels as low as 1 ppm could readily be determined. The gases used in the measurements were of high purity. Removal of trace quantities of oxygen from helium was accomplished by passing He over Ti-sponge maintained at 750 K followed by a liquid nitrogen cooled trap. The CO/He gas mixture (1.05 ~01% CO) contained only a low level of CO, ( < 10 ppm). The catalyst (5 wt% Ru/Al,O,) was prepared by impregnating to incipient wetness a sample of y-Al,4 with the required amount of aqueous acidified (0.6N HCl) solution of ruthenium trichloride. The catalyst support (Reynolds alumina RA-I) was pretreated by calcination in air at 1300 K for 1 h [ 161. Preceding each equilibrium study a sample of catalyst was reduced in H, for 2 h at 475 K, 2 h at 675 K, and 10 h at 725 K. Then the catalyst was flushed with flowing He at 725 K for 5 h to remove chemisorbed H,, followed by cooling to the temperature chosen for the equilibrium measurements. The recirculating pump was in operation while flushing with He. The adsorption capacity of the catalyst for carbon monoxide was determined by exposing it to a series of pulses of CO (1.05 ~01% CO in He) injected into the He carrier stream. Saturation coverage was determined by the breakthrough of the CO pulses. The maximum uptake of CO amounted to 2.0.x 10m4 mol CO/g catalyst.

314

H. Wise, J. G. McCarty

/ Themoa’ynamic properties of C on Ru

In a typical experiment for determination of the equilibrium composition the reduced catalyst was isolated in the recirculating loop filled with He only. A known concentration of CO and/or CO* (1 to 3 ~01% in He) were introduced and recirculated over the catalyst until the gas composition attained a constant value. The attainment of equilibrium at a given temperature was monitored by withdrawing samples for GC analysis over a period of time. The equilibrium gas phase composition was determined in this manner and the equilibrium constant calculated over a range of temperatures. From the measured concentration of CO and CO2 in the gas phase and a carbon and oxygen balance, the surface coverage of the catalyst sample with C and 0 was evaluated. Under our experimental conditions CO adsorption would be expected to occur, based on adsorption studies with single-crystal [17] and SiO,-supported [18] ruthenium. To examine this aspect in more detail we carried out a series of temperature programmed desorption (TPD) measurements for CO adsorbed on Ru/Al,O, shown (fig. 4). The results indicate that CO populates two binding states which desorption energies of - 84 and - 113 kJ mol-‘, similar to the observations made with single-crystal Ru [17] and Ru/SiO, [ 181. Also a small amount of CO, was detected during TPD, most likely formed by CO disproportionation. In the correlation of the thermodynamic properties of surface carbon with coverage coexistence of chemisorbed carbon and carbon monoxide was taken into consideration. 3. Experimental results For reaction (1) at a specified temperature given by: K = a, Pco,/P,‘o

the equilibrium constant K is (2)

9

where p represents the partial pressure of the gas-phase components (CO and CO& and (rc the thermodynamic activity of carbon. With graphite chosen as the reference state the carbon activity is equal to unity. However, for the equilibrium with Ru the carbon activity a, * 1. This parameter can be evaluated with C@c, as the reference state from the function: % = ( Pco,/p:o

)*J(

P,o,/p,‘o),,

(3)

9

where the numerator involves the equilibrium gas composition in the presence of graphite, and the denominator in the presence of the Ru surface. An analysis of the experimental data obtained in our study is presented in fig. 2. Each point represents the equilibrium surface carbon activity (cw,) at a specified fractional coverage and temperature. The data can be fitted to a single straight line despite the range of surface carbon coverages * (indicated by the *

The surface coverages are expressed in terms of a fractional monolayer, saturation coverage at 300 K.

equivalent

to the CO

H. Wise, J. G. McCarty / Thermodynamic properties

of C on Ru

315

lOOOK (K-l)

Fig. 2. Thermodynamic activity of carbon on ruthenium surface. Numbers on experimental points identify fractional carbon coverage.

50

I

I

Y 0 12.5 -

0 400

I

I

500

600

TEMPERATURE Fig. 3.

700

(K)

Variation of partial molar free energy of surface carbon on ruthenium with temperature.

316

H. Wise, J. G. McCarty / ~he~rn~narnic~r~e~~ies

of C on Ru

r

0.006 P B E

450 K

620 K

= 0.004 1?! s $ F g

0.002

a x t; z

/_

0 300

400

500

600

TEMPERATURE

700

800

900

(K)

Fig. 4. Temperature programmed desorption following CO adsorption to saturation coverage at 300 K on Ru/AI,O, (1.5 wtl). Rate of temperature rise is 1 K s-‘.

fractional number attached to each point). Cons~uently enthalpy of surface, given by: AHe = R d(ln o,)/d(

the partial molar

l/T),

can be determined by graphical analysis of the data in fig. 2. It is found to be Age = 102.4 f 7.1 kJ mol-‘. The partial molar free energy (or chemical potential) of surface carbon is given by: Apt = RT In cy,. This function has been evaluated (fig. 3). Again one finds little dependence of Age on carbon surface coverage. Finally, the partial molar entropy of surface carbon can be evaluated by

Table 1 ChemicaI potential and partial molar entropy of surface carbon on ruthenium ‘) Temperature (K) 500 550 600 a) Ru/A1,03 catalyst.

Ate (kJ mol-‘))

AS, (J mol-’ K-‘)

41.4 35.5 29.3

124.0 121.6 121.8

H. Wise, J. G. McCarty

/ Thermodynamic properties

of C on Ru

317

means of the relationship:

As, = (ARC - A&T. From the experimental data one obtains a value of 123 f 2 J mol-

’ K- ’ (table

1).

4. Discussion Some information can be derived from our experimental results on the distribution of surface adspecies. The data indicate a decrease in the total surface population with increasing temperature (table 2). At the same time the fraction of surface carbon atoms grows relative to that of CO ad molecules. Based on the data obtained we have evaluated an adsorption isotherm relating the amount of surface carbon C(a) to the equilibrium partial pressure of gaseous CO (fig. 5). Although the data are semi-quantitative, because of some variation in temperature (545 f 15 K), we note the gradual approach to saturation coverage with adsorbed carbon as the CO pressure is increased. The question may be raised whether the surface carbon formed on Ru/Al,O, at low coverages exhibits high chemical reactivity for interaction Mth hydrogen. In experiments involving temperature programmed surface reaction (TPSR) of surface carbon with H,, we observed two distinct states of reactivity as indicated by the two maxima in reaction rate (fig. 6) . The relative distribution in surface population between the two states depends on exposure time and temperature. Of special interest is the formation of methane from surface carbon and H, at temperatures as low as 250 K. The observed high reactivity of surface carbon on Ru is interpretable in terms of a carbon

Table 2 Distribution

of surface adspecies on Ru/Al,O, Temperature (K)

qa)+Co(a) (mol g-‘)X

1.35

492 502 530 560

185 163 129 99

11.9 23.3 39.5 61.6

1.68

527 543 552 604 624

223 217 205 129 108

17.9 18.4 22.0 62.0 78.7

Icoil,

a)

(mol crnm3 g-‘)X

lo6

a) Initial CO dose injected into reactor.

lo6

70

60

50 cp 0 ”

40

7 01 p

30

10

0 0

1

2 p,,

3

4

5

(atm x IO41

Fig. 5. Surface carbon isotherm on Ru/Al,O,

at 545 f 15 K.

(a)

I

I

1

I

I

I

300

400

500

600

700

600

CATALYST

TEMPERATURE

(K)

Fig. 6. TPSR of hydrogen with surface carbon adspecies formed by exposure of Ru/A1,03 (a) 3.4 pmol CO at 553 K; (b) 3.4 cmol CO at 623 K.

to CO:

H. Wise, J. G. McCarty

/ Thermodynamic properties

of C on Ru

319

overlayer consisting of isolated surface carbon atoms or clusters. The existence of such surface species has been confirmed by Auger electron spectroscopy for carbon on Ni [2q and Fe [21]. Similar to the case of Ru, the presence of surface carbon species with positive partial molar enthalpy has been observed on Ni [22]. However significant variations exist between the Ru and Ni systems. First, the enthalpy of surface carbon is considerably more positive on Ru than on Ni. Secondly, in the case of Ru the value of AH= remains nearly constant over a wide range of surface carbon coverages, while for Ni it declines and approaches zero, i.e., the graphite reference state. It amy be inferred that on Ru the progressive population of surface with carbon does not lead to attractive C-C interactions. On the contrary, the results suggest the existence of isolated carbon atoms coordinated to Ru without formation of carbon islands and of graphitic overlayers. In fig. 7 we have constructed an energy diagram for the surface adspecies on ruthenium. The positive heat of formation of surface carbon relative to

800

700

‘i 5 E

600

200

a g

loo

$ K

0 LL

0

C (gr, ref. state.1

“0 -100

5 9

-200

co 0 (a)

CO

5 w

j

Hz0 (9)

-300

-400

-600 Fig. 7. Energy diagram for surface and gas species; (a) refers to adsorbed and (g) to gaseous species.

320

H. Wise, J. G. McCarty

/ Thermodynamic properties of C on Ru

graphite yields a binding energy * of carbon to ruthenium of 607 kJ mol-‘, compared to a binding energy of carbon in graphite of 710 kJ mol- ‘.

as

Acknowledgement Support of this study by the National acknowledged.

Science

Foundation

is gratefully

References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ 1 l] [12] [13]

[ 141 [15] [16] [17] [18] [19] [20] [21] (221

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* The binding surface-bound

energy is defined as the energy required to remove state to an infinite distance from the surface.

a carbon

Inst.

Gas

atom

from

Eng.

the