Rare earth oxides as carbon oxidation catalysts

Rare earth oxides as carbon oxidation catalysts

Carbon Vol. 23, No. 6, PP. 707-713. Pnnted in the U.S.A. ooO8-6223185 $3.00 + .oO 0 1985 Pergamon Press Ltd. 1985 RARE EARTH OXIDES AS CARBON OXID...

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Carbon Vol. 23, No. 6, PP. 707-713. Pnnted in the U.S.A.

ooO8-6223185 $3.00 + .oO 0 1985 Pergamon Press Ltd.

1985

RARE

EARTH OXIDES AS CARBON OXIDATION CATALYSTS

D. W. MCKEE General Electric Corporate Research and Development, P.O. Box 8, Schenectady,

NY 12301, U.S.A

(Received 8 February 1985)

Abstract-The

behavior of a number of rare earth oxides as catalysts for the oxidation of graphite in air has been investigated by the methods of thermal analysis. Of the oxides studied, only CeO, showed significant activity in accelerating the gasification of graphite by oxygen between 500 and 1000°C. Cerium salts, which decompose to a finely dispersed oxide phase at low temperatures, e.g. Ce (III) nitrate and ammonium Ce (IV) nitrate, were found to be very active catalysts. The catalytic effect may be due to a redox process involving the cyclic conversion of the oxide from the Ce (IV) to the Ce (III) oxidation state, or the oxide particles may provide sites for the dissociative chemisorption of oxygen. Key Words---Carbon catalysis, oxidation, rare earth oxides, cerium

1. INTRODUCTION

Many different types of inorganic species are known to promote the reaction of carbonaceous materials with gaseous oxygen at elevated temperatures[ 1, 21. Active catalysts for this process include members of every group in the Periodic Table, with the exception of the halogen elements, which act as oxidation inhibitors[3]. Known catalysts include oxides and salts of the alkali metals (Group I); the Group IB metals copper, silver and gold; the alkaline earths (Group II); arsenic, antimony and bismuth in Group V; the transition metals of Group VIII; the platinum group metals; and the oxides of mid-group elements such as lead, vanadium, tellurium, molybdenum and tungsten. A common property of all these elements is that they are capable of existing in several oxidation states at the temperature of the catalyzed oxidation process. Indeed, it is generally agreed[2, 4, 51 that the catalytic effects result from the occurrence of cyclic redox reactions on the carbon surface. Thus, although the mechanistic details are not known in many cases, active catalysts are apparently reduced to a lower oxidation state by reaction with the carbon substrate at gasification temperatures, and are then reoxidized to the initial state by reaction with ambient oxygen. The net effect of this sequential process is the gasification of carbon at the catalyst-substrate interface. This process is often facilitated by movement of catalyst particles over the carbon surface during the oxidation reaction, resulting in the formation of channels in the substrate surface[6]. In other cases (e.g. with chromium oxide), the catalyst particles appear to act as sites for the dissociation of the oxygen molecule, and mobile oxygen atoms then migrate over the carbon surface to edge sites where gasification of the carbon takes place[7]. In spite of a large literature on this subject, the behavior of oxides and salts of metals of the lanthanide (rare earth) group has not apparently been studied. These metals form stable refractory oxides, which are reduced by carbon only at very high temperatures. However, the oxides of 707

a few of these metals, notably cerium, are known to occur in more than one oxidation state, suggesting that a catalytic redox process might occur with carbon in the presence of oxygen. This paper describes the results of a thermogravimetric and kinetic study of the behavior of a series of rare earth oxides on the oxidation of graphite in air in the temperature range 500-1ooo”C. The observed catalytic behavior of cerium oxide led to a more detailed investigation of the effects of cerium oxysalts in this gasification process.

2. EXPERIMENTAL Two different types of graphite material were used in this work. For kinetic and thermal analysis studies, spectroscopic-grade graphite powder (Type UCP-2, 100-mesh, 2.3 m2/g surface area initially; Ultra Carbon Corp.) was doped with known amounts of finely powdered rare earth oxides and cerium salts, and the mixtures were blended in a Fisher Minimill before the reactivity measurements. Additives investigated included reagent-grade oxides of cerium CeO, (Pfaltz and Bauer Inc.), europium Eu?O, (Alfa Products), gadolinium Gd,O, (Genera1 Electric Co.), lanthanum La,O, (Lindsay Chemical Co.), samarium Sm,O, (Semi-Elements Inc.), ytterbium Yb,O,, terbium Tb,O, and neodymium Nd,O, (Alfa Products). Cerium salts used included Ce (III) carbonate Ce?(CO,), (Alfa Products), Ce (III) sulfate Ce,(SO,), . 5H20 and Ce (III) oxalate Ce,(C,O,), . 9Hz0 (Pfaltz and Bauer Inc.), Ce (IV) sulfate Ce(SO,), . 4H20 (Research Org/Inorg. Chemical Co.), Ce (III) nitrate Ce(NO,), . 6Hz0 and ammonium Ce (IV) nitrate (NH&Ce(NO,), (Fisher Scientific Co.), ammonium Ce (IV) sulfate (NH,),Ce(SOJ, . 2HZ0 (G.F. Smith Chemical Co.) and Ce (IV) hydroxide 2Ce02 3H20 (K & K Chemical Co.). Optical observations of catalyst particle behavior during graphite oxidation in flowing air were made with a Leitz hot-stage microscope. For this purpose, flakes of Ticonderoga graphite were purified by repeated boiling with hydrofluoric acid and concentrated hydrochloric acid,

708

D. W.

followed by thorough washing with distilled water. Graphite single crystals, -3 x 3 mm in size, were mounted on the hot-stage specimen holder so that the basal plane surfaces were horizontal. Catalysts were introduced to the exposed surface of the flakes in dilute aqueous solution by means of a micrometer syringe and evaporated slowly to dryness in flowing air. The graphite crystals then were heated slowly to temperatures in the range 500900°C in dry flowing air, direct observations of catalyst particle behavior being made through the quartz cover glass of the hot stage during the catalyzed oxidation process. As in previous studies[8, 91, kinetic measurements of gasification rates for doped graphite powder samples were made in a Mettler Thermoanalyzer TA-2 controlled atmosphere thermobalance. Measurements were carried out with lOO-mg samples in flowing air (1 atm, 0.1 MPa) at 200 ml/min. The balance was operated either isothermally in the temperature range 50&800”C, with weight losses being measured as a function of time at each temperature, or with a linearly increasing furnace temperature of 10°C min, to a maximum temperature of 1000°C. Some thermal analysis experiments were carried out with pure cerium salts and with mixtures containing equal weights of graphite and cerium salts heated at a constant temperature rise rate in flowing dry air. A few high temperature (to 1400°C) thermal analysis experiments were carried out with graphite-CeO, mixtures heated in flowing nitrogen in a Netzsch Thermal Analyzer STA 409 provided with an alumina furnace tube and silicon carbide heating elements. The sensitivity of both balances was -CO.02 mg for a lOO-mg load.

3. RESULTS AND DISCUSSION

A summary of the effects of adding 5% by weight of a series of rare earth oxides on the oxidation behavior of graphite powder is shown in Table 1. For each additive, the temperatures required to achieve 1 and 5% gasification of the graphite in flowing air are listed. All these experiments were carried out with lOO-mg samples of the doped graphite and with a temperature rise rate of lO”C/min. With the exception of CeO,, Yb,O, and Tb,09, the oxides of the other rare earth metals exhibited very small catalytic effects, the gasification temperature of the

Table 1. Catalytic behavior of rare earth oxides: 100 mg doped graphite samples (5 wt.% salt), flowing air 200 ml/mitt; temp. rise rate = lO”C/min

Additive None J-a,& CeO, [email protected] [email protected], Sm,Q NdG Yb,Q Tb,Q

Temperature Required for C Bum-off (“C) 5% 1% 764 759 703 758 746 762 760 722 730

815 810 768 810 808 815 810 770 780

MCKEE

Table 2. Catalytic behavior of cerium salts for graphite oxidation: 100 mg doped graphite samples (10 wt.% salt), flowing air 200 mlimin; temp. rise rate = 10Wmin

Additive

None CeLCOJ, CeLSQ), Ce(SOJI Ce(I’Q), (NIL),Ce(SO,), CeO, Ce(OI-0, (NH&Ce(NO,), Ce,(C,O,), Oxalate

Temperature Required for C Bum-off (“C) 1% 5%

764 733 752 708 655 720 695 693 560 700

815 793 802 762 727 790 760 759 636 745

graphite being lowered by <2O”C in each case. Cerium (IV) oxide was somewhat more active, physical mixtures with the graphite reducing the gasification temperature for a given extent of bumoff by 50-60°C. Under these experimental conditions, the oxides of ytterbium and terbium showed intermediate behavior. Table 2 shows similar data for a series of cerium salts added to graphite powder at a concentration level of 10 wt%. It is clear from these results that these additives show a wide spectrum of behavior. The carbonate and sulfates of cerium exhibit modest catalytic activity, whereas the nitrates, hydroxide and oxalate are very active catalysts. As it seemed likely that these salts would decompose, at least partially, to oxide phases at temperatures within the catalytic oxidation range of graphite, experiments were performed to determine the thermal decomposition ranges of these salts in air in the presence and absence of added graphite. Figure 1 shows the results of thermal analysis experiments with the salt Ce,(SO,), . 5H,O on heating in air in the presence and absence of added graphite powder. As shown by curve S, the anhydrous sulfate began to decompose in air at temperatures above 750°C and was completely converted to CeO, at a temperature of 950°C. The curve marked G shows the weight losses of a lOOmg sample of graphite powder on heating in flowing air at a temperature rise rate of lO”C/min. Curve D shows weight losses observed with a mixture of 100 mg Ce,(SO& . 5H,O and 100 mg graphite powder on heating in air. This curve is almost identical to the sum of curve G and S, as shown by the dotted curve marked G + S. This result indicates that the Ce (III) sulfate exerts a very small catalytic effect on the gasification rate of graphite in air. A similar set of data is shown by the thermogravimetric curves in Fig. 2 for the salt (NH,),Ce(SO,), * 2H,O and a mixture of the salt with an equal weight of graphite. In this case, decomposition of the salt to cerium oxide began at -720°C and was complete at 870°C. Curve E indicates that gasification of graphite in the presence of this salt took place at a temperature at least 50°C lower than expected for a mixture of graphite and salt with no catalytic effect (curve G+Z). Thus, the decomposition product of the ammonium ceric sulfate has a definite catalytic effect on the oxidation of the graphite.

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Rare earth oxides as carbon oxidation catalysts

G S D

100mg GRAPHITE 100mg Ce2tS0413.5H20 100mg GRAPHITE+ lOOmgCe2~S04~3.5H20

0

TEMPERATURE,"C

Fig. 1. Thermograms (weight losses vs temperature) for 100 mg pure graphite (G), 100 mg cerium (III) sulfate (S) and a mixture of 100 mg graphite + 100 mg salt (D) in flowing air. Dotted curve (G t- S) shows calculated weight losses for a graphite-salt mixture in the absence of catalysis.

Figure 3 summarizes a series of thermogravimetric experiments with Ce (III) oxalate, which decomposes sharply to CeO, at temperatures between 400 and 450°C. Again, a moderate catalytic effect on the oxidation of graphite in air was observed, curve F obtained for a 1: I mixture of the salt and graphite showing weight losses at 50-60°C lower temperatures than those expected in the absence of a catalytic effect (curve 0 + G).

Ce02

A more dramatic catalytic effect is illustrated by the data in Fig. 4, obtained with Ce(NO& . 6Hz0 with and without added graphite. The Ce (III) nitrate salt decomposed on heating in air to give CeOz at the low temperature of 300°C. In the presence of graphite, gasification in air was detectable even at 450°C and became rapid at higher temperatures (curve C), whereas in the absence of an interaction between the two phases, no weight losses

AIR 5 200ml/min

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I

I

200

300

I I I 400 500 600 TEMPERATURE,'C

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700

800

'*-*..._; \ '$

"..* ': i \

900

000

Fig. 2. Thermograms (weight losses vs temperature) for 100 mg pure graphite (G), 100 mg ammonium cerium (IV) sulfate (2) and a mixture of 100 mg graphite + 100 mg salt (E) in flowing air. Dotted curve (G + Z) shows calculated weight losses for a graphite-salt mixture in the absence of catalysis.

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D. W. MCKEE

CeOp

AIR = 200ml/min LI = 10Wmin At G 0 F

xot,

t00mg GRAPHITE t00mg Ce2(C204)3.9H20 1OOmg GRAPHlTEt 100mg Ce2(C20413.9H20

I

I 100

200

I

300

I

I

I

500 600 400 TEMPERATURE,%

I

700

I

I

I

800

900

1000

Fig. 3. Thermograms (weight losses vs temperature) for 100 mg pure graphite (G), 100 mg cerium (III) oxalate (0) and a mixture of 100 mg graphite + 100 mg salt (F) in flowing air. Dotted curve (0 + G) shows calculated weight losses for a graphite-salt mixture in the absence of catalysis.

would be expected below 700°C (curve N + G). Ce (III) nitrate thus appears to be a powerful catalyst for graphite oxidation, as indicated also by the results shown in Table 2 for a salt concentration of 10 wt%. Another very potent catalyst was the salt (NH,),Ce(NO,),, which also decomposed in air to give CeO, at 13OO”C, as shown in Fig. 5. In the presence of

added graphite, gasification of the carbonaceous phase began at -450°C (curve B) or at a temperature -300°C lower than expected in the absence of a catalytic effect (curve A + G) . The catalytic effect of low concentrations (1, 5 and 10 wt%) of the (NH,),Ce(NO,), salt on the gasification rates of graphite powder in air is illustrated in Fig. 6, which

*.

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'=*....N +G _ *. : : :

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I

200

I

300

I

I

I

600 500 400 TEMPERATURE,'C

I

700

'\ '\ I

800

:

\ bd

900

1000

Fig. 4. Thermograms (weight losses vs. temperature) for 100 mg pure graphite (G), 100 mg cerium (III) nitrate (N) and a mixture of 100 mg graphite + 100 mg salt (C) in flowing air. Dotted curve (N + G) shows calculated weight losses for a graphite-salt mixture in the absence of catalysis.

711

Rare earth oxides as carbon oxidation catalysts 0.

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1400

I 100

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200

I

300

I

I

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400 500 600 TEMPERATURE,'C

I

700

‘b BOO

900

1000

Fig.5. Thermograms

(weight losses vs temperature) for 100 mg pure graphite (G), 100 mg ammonium cerium (IV) nitrate (A) and a mixture of 100 mg graphite + 100 mg salt (B) in flowing air. Dotted curve (A+G) shows calculated weight losses for a graphite-salt mixture in the absence of catalysis.

500°C I

600 I G A B C D E

AIR 200ml/min GRAPHITE G-lO%(NH&Ce(NO& G- 5%(NH4k$e(NO3)6 G- I%(NH4)2CdNO3)6 G-IO%Ce02 G-!O%Ce(NO&

I/T,K Fig. 6. Air oxidation kinetics (rates vs 1 lT) for pure graphite powder (G) and graphite doped with 10 wt.% ammonium cerium (IV) nitrate, 10 wt% CeO, and 10 wt% cerium (III) nitrate.

1, 5 and

D. W. MCKEE

712

also shows the effects of 10 wt% CeOz and Ce(NO&. In the figure, the data are plotted as log (oxidation rate) versus reciprocal temperature in K. It is evident that as little as 1% by weight of the (NH,)Ce(NO& salt added to the graphite increases the gasification rate in the temperature range 5OO-700°C by more than 20-fold. One percent of this salt is about as effective as 10% of added CeO, or Ce (III) nitrate, in spite of the fact that all three salts formed CeOz on heating. The wide differences in catalytic activity exhibited by the three additives are probably related to the temperature of decomposition of the salt and the degree of dispersion of the resulting oxide on the graphite surface. Increasing concentrations of salt resulted in further increases in gasification rates, but the catalytic activity was not proportional to the amount of salt present, 10 wt% of added salt being only four times as effective as 1% at 600°C. Little change in apparent activation energy resulted from the introduction of these catalysts, the values lying in the range of 35 2 2 kcali mol for pure graphite and all the cerium-doped samples studied. Visual observations of the behavior of cerium oxide particles [formed by the thermal decomposition in situ of (NH&Ce(NO& from aqueous solution] on the basal plane surfaces of graphite single crystals were made in the hot-stage microscope. On heating in flowing air at -9OO”C, the particles were observed to execute irregular motion on the graphite surface. However, the particles remained amorphous and irregular in shape and did not assume the molten droplet appearance exhibited by other catalyst species[6,10]. Some moving particles were observed to excavate curved channels, but the predominant topographical features of the catalyzed oxidation process were the obvious increase in the rate of edge recession of the graphite flakes and the formation, on prolonged oxidation, of hexagonal etch pits on the basal plane having the “perpendicular” orientation (i.e. bounded by {lOiZ} faces). In contrast, graphite single crystals oxidized under the same conditions but in the absence of the cerium catalyst developed only pits with the “parallel” configuration (i.e. bounded by (1121) faces). A similar change in orientation of hexagonal etch pits on graphite basal plane surfaces has been previously reported in the presence of iron[ lo] and tungsten[ 1 l] catalysts. It appears from these results that the catalytic activity of the cerium oxysalts was most marked when decomposition to a finely dispersed CeO, phase occurred at low temperatures. The mechanism of the catalytic process is, however, not obvious. By analogy with redox processes, which are believed to occur with other catalytic spccies[2], it seems possible that the observed catalytic effect of finely dispersed CeO, results from a sequential process of the type 2Ce0,

+ C = Ce,O, + CO

Ce,O, + wo, = 2CeO,, giving the overall reaction c + No, = co.

Although CeO, was the stable oxide phase in air at temperatures below - 15OO”C, the reduction of this oxide to the lower oxide Ce,O, by carbon is possible under conditions of low oxygen and CO partial pressures, such as might exist beneath catalyst particles in physical contact with the carbon substrate. Figure 7 shows the results of heating a mixture of equal weights of graphite powder and CeO, in flowing pure helium at increasing temperature. At temperatures of Z9OO”C, weight losses resulting from this carbothermic reduction reaction became measurable and reaction (1) approached completion at temperatures of > 1400°C. The conditions for the occurrence of reaction (1) were calculated from published values for the thermodynamic properties of CeO, and Ce,O,[ 121, and assuming reasonable values of C, for Ce,O,. Figure 8 shows the calculated stability regions of the two cerium oxides, according to the calculated free energy values of reaction (l), as functions of temperature and CO partial pressure in the ambient atmosphere. It is apparent that at, for example, a temperature of 9OO”C, reaction (1) could take place if the ambient value of P(C0) fell below -30 mm (40 KPa). It is conceivable that such a condition might exist at the interface between the catalyst particle and the gasifying carbon substrate. However, as the free energy decrease involved in reaction (2) is so large (- 56 kcal/mol at 1200 K in air), a reduced species such as Ce,O, could have only a transitory existence under gasification conditions. Another possibility is that the oxide particles provide sites for the dissociation of molecular oxygen. As with CrZO, catalyst particles[7], the lack of channeling activity and the predominance of edge recession and the formation of hexagonal etch pits suggest that migration of chemisorbed oxygen atoms to sites remote from the catalyst particles may be taking place. CeO, is known to act as a catalyst in a variety of heterogeneous oxidation reactions (e.g. oxidation of CO, SOZ, olefins and aromatic hydrocarbons) that involve the dissociative chemisorption of the 0, molecule[ 131, and it is conceivable that a similar process operates in the case of the ceria-catalyzed oxidation of carbon.

.2Ce02

1

+ C = Ce203 + CO

100 mg GRAPHITE + IOOmg CeOt HELIUM

(1)

12 -

(2)

0

$

= IO*C/min I

1 400

I

I I I 800 1200 TEMPERATURE, ‘C

I 1600

Fig. 7. Thermogram (weight losses vs temperature) for a mixtare of 100 mg graphite and 100 mg CeO, heated in helium.

713

Rare earth oxides as carbon oxidation catalysts 600

1

700

,

TEMPERATURE, 'C 800 900 1000 1100

I

1

I

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o-I o-2 -

trate and ammonium cerium (XV) nitrate. Under these conditions, 1% by weight of the cerium salt increased the gasi~cation rate of graphite powder in air by more than 2O-fold at 600°C. Observations of graphite single crystals during the catalyzed oxidation process showed that the predominant modes of attack were accelerated recession rates of the graphite crystal edges and the growth of oriented hexagonal etch pits on the basal plane surfaces. Possible mechanisms of the catalytic effects were considered. REFERENCES

2CeOZ+C - Cez03+CO (I) -6

800

I

I

I

,

L

I

900 1000 II00 1200 1300 1400 TEMPERATURE, *K

Fig. 8. Calculated stability regions of CeO, and CezO,, assuming the occurrence of reaction (l), as functions of temperature and P(C0).

4. SUMMARY

Cerium oxide has been found to be unique among a series of rare earth (lanthanide) oxides in exhibiting appreciable catalytic activity for the gasification of graphite in air at temperatures in the range 500-1000°C. The activity was most marked when the oxide phase was formed in situ on the graphite surface by the low temperature decomposition of salts such as cerium (Ill) ni-

1. I? L. Walker, Jr., M. Shelef and R. A. Anderson, In ChemiszryandPhysics @Carbon, Vol. 4 (Edited by P. L. Walker, Jr.). p. 287. Marcel Dekker, New York (1968). 2. D. W. McKee, In Che~z~sfryaad Physics of Carbon, Vof. 16 (Edited by P. L. Walker, Jr.), p. 1 (1981). 3. J. R. Arthur and J. R. Bowring, Indust. Eqn~. Chem 43, 528 (1951). 4. H. Amariglio and X. Duval, Carbon 4, 323 (1966). 5. B. J. Wood and K. M. Sancier, Caral. Rev. Sci. Engng. 26. 233 (1984). 6. R. T. K. Baker, Catal Rev. Ski. E~gn.g. 19, 161 f1979). '7.R. T. K. Bakerand J. J. Chlu~inski, Carbon 19.75 (1981). 8. D. W. McKee and C. L. Spiro, Carbon 22, 285 (1984). 9. D. W. McKee, Carbon 22, 513 (1984). 10. D. W. McKee, Carbon 8, 623 (1970). 11. R. T. Yang and C. Wong, 1. Amer. Inst. Chem. Engng. 29, 338 (1983). 12. 0. Kubaschewski. E. L. Evans and C. R. Alcock, Metaf~z~r~i~~~ ~her~~ochemisfry, 4th ed. Pergamon Press. New York (1967). 13. 0. V. Krylov, Cutu~~sir by Nonmetals, pp. 170-183. Academic Press, New York (1970).