Ind. Eng. Chem. Res. 1993,32, 1359-1366
1359
Intrinsic Rates of NOAarbon Reactions Xi Chu and Lanny D. Schmidt' Department of Chemical Engineering and Material Science, University of Minnesota, Minnesota 55455
The noncatalytic reactions of NOz,NO, NzO, and 02 with graphite between 400 and 700 "C have been studied by scanning tunneling microscopy to obtain quantitative kinetics by measuring the number and size of monolayer pits on the basal plane versus temperature and time. At low temperature, the reaction initiates exclusively from the point defects on the basal plane to form monolayer pits. The monolayer etching rates follow the order of rNO, > rNzO > r N O > roz. The activation energies for these reactions are determined to be 60,90,74, and 127 kJ/mol, respectively. At high temperatures, instead of the recession of the monolayer pits, direct abstraction of the atoms on the basal plane of graphite becomes significant. This produces more active sites for monolayer pit nucleation, thus enhancing the total rate of reaction. N20 has the highest rate of basal plane attack. The rates of basal plane abstraction follow the order of RN,O> RNO> Ro, > RNQ. Also, the formation of stable (CN), polymer during the graphite-NO, reaction below 700 OC has been confirmed by transmission electron microscopy, atomic force microscopy, Auger electron spectroscopy, and electron energy loss spectroscopy. 1. Introduction NO, removal from high temperature combustion sources has attracted increasing attention due to mounting environmental concerns (Offen et al., 1987, and references therein; Hjalmarsson, 1990). There are many examples such as heavy oil burners and diesel engines where NO, and soot are both in the exhaust. Thermodynamically, the NO, + carbon reaction is favorable, and use of carbon to convert NO to COz and NZis a possible way of reducing NO,. Many investigators have examined the reactions of amorphous carbon with NO (Bedjai et al., 1958; Ewards, 1971;Watts, 1958;Smithet al., 1958;Radovichand Walker, 1984; Chan et al., 1983; Furusawa et al., 1980; Teng et al., 1989, 1992; Suuberg et al., 19901, Nz0 (Madely and Strickland-Constable, 1953; Smith et al., 1957), and NO2 (Arthur et al., 1956)and found them effective for reducing NO, emissions. However, thus far only scattered kinetic data have been published based on the overall rate measurements on different carbon materials. To optimize this process, precise kinetics are needed, especially the relative rates of NO,- and 02-carbon reactions. Graphite gasificationhas been used as a model for carbon gasificationsbecause graphite has a well-definedstructure and an easily prepared atomically flat surface. Attack is most rapid at low coordination edge sites and slowest on the (O001) basal plane. Based on the decorated transmission electron microscopy (TEM) technique (Hennig, 1964;Evans et al., 1971; Feates and Robinson, 1971; Yang and Wong, 1981a,b;WongandYang, 19821,extensive work has been published on graphite basal plane reactions with mainly 0 2 (Hennig, 1964; Evans et al., 1971; Feates and Robinson, 1971;Y ang and Wong, 1981a,b;Wong and Y ang, 1982), H2O (Yang and Wong, 1983a,b; Duan and Yang, 19841, and COz (Yang and Yang, 1985a,b). This subject has also been summarized in several excellent reviews (Yang, 1984; Walker et al., 1991). Recently, scanning tunneling microscopy (STM) has been used to study graphite oxidation in air (Chang and Bard, 1990, 1991) and to characterize quantitatively the monolayer pitting of the basal plane of graphite in NO, HzO, 02, and COz because at low temperatures etching nucleates exclusively at point defects so that measurement
* To whom correspondence should be addressed.
of pit diameter versus time and temperature gives accurate kinetics (Chu and Schmidt, 1991, 1992). We observed that, for monolayer pitting at high pressure (>50 Torr) 02 or NO, the pits are circular. Also, pita formed in the second and higher layers were tangent to each other because the process begins when a defect is exposed in the second and higher layers, and growth rates in all layers are identical. We also observed attack at line defects, forming deep hexagonal holes and resulting in higher gasification rates than by monolayer etching. At lower gas pressures, monolayer pita become hexagonal rather than circular and multilayer etching becomes much faster than monolayer etching. Recently STM and AFM (atomic force microscopy) have been used to study the catalyzed gas-graphite reactions: C/HdPt, C/HdPd, C/OZ/VZO~, CICOdKzCO3, and C/NO/Rh (Chu et al., 1993a) and MoSz gasification (Chu and Schmidt, 1992b). More recently, we have systemically studied the noncatalytic and catalytic reactions of NO, with graphite. Preliminary results have been presented ina recent conference (Chu and Schmidt, 1992~). The metal-, oxide-, and salt-catalyzed NO, + graphite reactions will be published in a separately paper (Chu and Schmidt, 1993). In this paper we report a comparison of noncatalytic reactions of graphite with NO, NO2, and NzO with STM, AFM, TEM (transmission electron microscopy), AES (Auger electron spectroscopy), and EELS (electron energy loss spectroscopy). We have found that NO2, NO, and N2O have much higher intrinsic rates of reaction with graphite than does 02.We have also found several differences in these reactions at different temperatures and pressures: the monolayer pits change their shapes from triangular at low temperature to hexagonal at intermediate temperatures and finally to circular at high temperatures. More importantly, we have found that the formation of (CN), polymers during the carbon-NO, reaction is probably responsible for the relatively low reaction rates of NO, with amorphous carbon. 2. Experimental Section
Highly oriented pyrolytic graphite from Union Carbide (grade ZYA) was used in this study. Typically, the freshly cleaved surface has 3-4 basal plane defects/pm2 or (3-4) X 108/cm2as determined by counting the number of pits after heating in gases. For graphite reaction with NO and NO2, freshly cleaved samples were heated in a mixture of
Q888-5885/93/2632-1359$04.00/00 1993 American Chemical Society
1360 Ind. Eng. Chem. Res., Vol. 32, No. 7,1993
5% NO or NO2 in He at atmospheric pressure a t specified temperatures for different times. Reactions in NzO were carried out in 10% N20 in He standard gas. For comparison, reactions in 0 2 wereconducted in high-purity oxygen a t 760 Torr. Since the presence of 0 2 enhancesthe etching rate, highpurity Hegas (99.998%)wasfirstflowedoverthesamples for at least 30 min to pump out the residual air and heat the sampletothe reactiontemperature. Thereactinggases were admitted when the system achieved the reaction temperature. After reaction, the reacting gases were removed and the samples were quenched in He to room temperature. This procedure asswes that there is no significant reaction caused by impurities or by reactions during heating and cooling. After reaction, the samples were transferred to a Nanoscope I1scanningprobe microscope which is capable of being operated in both STM and AFM modes with the same software system. All surface examinations were in air at 300 K. For STM the constant-height mode was used with scanning rates adjusted for best images as describedpreviously(ChuandSchmidtl991,1992a,b;Chu et al., 1993a). Unlike the STM, AFM can readily image electric insulators. AFM scans a sample with a shard of a Sic cantilever. The electroncloud of the cantilever tip (which may end in a single atom) presses against the clouds of individual atoms in the sample, generating a repulsive force thatvarieswiththesurfacerelief. The forcedeflects the tip, whose movements are monitored by a laser beam reflected from the top of the arm, to a path by activating a piezoelectric control which adjusts the sample height so that the deflection of the arm remains constant. The sample movements are translated into a surface profile. The surfaces were randomly imaged in a large 15 X 15 pm2areafiitforabout20differentregionsforeachsample. Those showing interesting structures were closely scanned with lower 2 range and slower scanning rate. Repeated Calibrations were made for the step height of graphite. An exponential output of the image was chosen so that the monolayer step could he easily distinguished from doublelayer or multilayer structures. Surfaces were imaged and the etching rates were determined on the basis of data averaged over several samples with 150-200 pits recorded in a typical measurement. Repeated calibrations verified that step heights were characteristic of exclusively single monolayer pits except for dislocation attack and at multilayer steps on the original surface. TEM samples were prepared by successively cleaving a graphite flake using tape. The samples were washed several times using acetone before being placed on the gold TEM grids. The reactions of TEM samples were conducted in the same tube reactor as that for the STM samples. Transmission electron microscopy was done using a Phillips CM 30 microscope which is capable of detecting light elements with electron energy loss spectroscopy (EELS). Auger electron spectra were obtained using a Perkin-Elmer PHI 595 scanning Auger system which permitted analysis on a particular region of the sample. 3. Results 3.1. Monolayer Reactions. Figure 1 shows a lowmagnification STM picture of a sample heated in NO2 a t 600 'C for 10 min. Monolayer pits of uniform diameter are randomly distributed over the basal plane of graphite. For the reactions in NO, N20, and 0 2 , similar pictures
I
Figure 1. Low-magnification new of a sample heen heated in N G at 600 OC for 10 min. Circular monolayer pits of constant diameter (220 nm) are randomly distributed over the banal plane of graphite because all formed from point defeets in the original cleaved surface.
were routinely obtained. Since pits have different shapes a t different temperatures, the average diameters of the pita were recorded and calibrated to circular for rate calculations. Figure 2 shows the monolayer pits formed on the graphite basal plane after heating in NO2 a t different temperatures. It was found that at temperatures from 375 to 400 "C the monolayer pits were approximately triangular. A typical picture is shown in Figure 2a, which was taken from a sample after heating in NO2 a t 375 "C for 50 min. However, at temperatures between 400 and 500 "C, the monolayer pits have nearly hexagonal shapes as shown in Figure 2b, which was taken from a sample after heating in NO2 at 450 OC for 15 min. Increasing the temperature to 5OC-550 O C forms perfect monolayer hexagons as shown in Figure 2c, which was taken from a sample after heating in NO2 at 500 "C for 40 min. Figure 2d shows round hexagons formed in a sample after heating in NO2 a t 550 "C for 20 min. However, at temperature? above 600 "C, the pits are exclusively circular as shown in Figure 2e, which was taken from a sample after reaction with NO2 at 650 'C for 5 min. We observed similar changes in N20, NO, and 02 reactions, hut the transition in pit shape is not as distinct as in this reaction. Occasionally the coexistence of hexagonal and circular pita in the same sample was found. The pressures of the reacting gases also affected the shape of the monolayer pits greatly. Pits tended to he hexagonal at low pressures ( RNO> Ro, > RNO~: 3.4. The Formation of (CN), We noticed that for the NO-amorphous carbon reaction there was a brown material deposited downstream on the glass walls. X-ray photoelectron spectroscopy (XPS) analysis showed that
Ind. Eng. Chem. Rea.,Vol. 32, No. 7,1993 1363
Figure 8. TEM picture of a sample alter heating in 5 5 NO for45 min at 650 O C showing (CNJ. ciuslen, formed at graphite edges and steps.
Figumi'. AFM imsgeof(CN,, pdymcr formed during NOdraphite reaction at650 "C for45 min. ( 8 , Low-maKmficntronview; (b) a1 an atomic atsp.
it containedmainlycarbonand nitrogen. Thisstimulated us to find out a possible solid compound formed in NO,carbon reactions. Figure l a shows an atomic force microscopy image of a graphite sample after heating in NO at 650 O C for 45 min. Evidently some solid material accumulates at the edge of all the steps. This kind of picture was more frequently found during the first few scans in STM, but the replusive forces between the tip and the sample sweep these small clusters out of the field of view. Figure 7b shows a small region at a monolayer step where the left side is one atomic layer lower than the right. Small clusters of this material sitting at the monolayer step are evident. For reactions in N 2 0 and NO*,similar deposita were observed. However, the amount of this material after reaction at the same temperature and pressure was significantly lower than that in NO reaction. It has been found that the favorable temperature of forming this material is between 600 and 650 "C. although traces of it were found in samples after reaction at 450 ' C . Figure 8 shows a TEM micrograph of a graphite sample heated in NO at 650 O C for 25 min. The thin graphite sheet is a single crystal with thickness of about 0.05 pm. After heating in NO, many small clusters formed at the edges of graphite single crystal as shown in the figure. This confirms the AFM findings shown in Figure 7. It is noteworthy, however, that very large clusters always accumulateat thecornersofthesinglecrystal. Allofthese are irregular in shape and exhibit nearly amorphous characteristics from electron diffraction. Microregion electron energy loss spectroscopy (EELS) analyses were performed in the TEM. Using the high spatial resolution EELS, many different particles of this material were examined. In addition to a strong carbon peak, a small peak at energy loss from 350 to 400 eV was
Figure 9. SEM image of a step region in the same sample as s h o w in Figure 8. The inset Auger spectra B and b were taken from points a and b, respectively. A nitrogen peak is shown in spectrum b. indicating that the solid material accumulated at the step is (CN). polymer.
also found. Thispeakcould befromeitherCaorN.Energy dispersive spectroscopy was then carried out to confirm this peak because EDS can detect Ca but not N, and it was confirmed that this peak was from nitrogen. For double-checking, XPS was first used to examine the sample. Due to the low coverage (scattered distribution) ofthematerialasshownin Figure7a,XPSanalysis of these graphite samples failed to indicate any elements other than carbon from the sample. Auger analyses of small regions were then performed by using a PerkinElmer PHI 595 scanning Auger system which is capable of examining the microstructure and composition of the same region in the sample. As all the solid material formed along the edges on graphite surfaces was indicated by both AFM and TEM, analyses were concentrated on the edge regions. Figure 9 shows a region where analyses were carried out at points a and b, respectively. Curve a in the insert was taken from point a, which is the flat surface of the sample, and no Auger peak was found between 350 and 400 eV. However, a strong N peak was found for the analysis at point b, where some accumulated material is indicated. The corresponding Auger spectrum is shown as curve bin the insert. This indicates that there is indeed a nitrogencontaining compound sitting in the step region. Thus we confirm that the solid material formed in NO,-carbon reactions is a high melting point (CN), polymer.
1364 Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 4. Discussion 4.1. (CN),Formation and theRates of NO,-Carbon Reactions. The reaction between NO, and amorphous carbon has been studied extensively (Bedjai et al., 1958; Ewards, 1971; Watts, 1958; Smith et al., 1958; Radovich and Walker, 1984;Chan et al., 1983;Furusawa et al., 1980; Teng et al., 1989,1992; Suuberg et al., 1990; Smith et al., 1957; Madely and Strickland-Constable, 1953; Arthur et al., 1956). It has been found that thereaction ofamorphous carbon with NO is about 2 orders of magnitude lower than that with oxygen. No quantitative rate data have been reported for the N20-carbon reaction, but it is generally believed that this reaction is also very slow. However, it has been reported that for the NOacarbon reaction the rate is about 100 times higher than that of the 02-carbon reaction (Arthur et al., 1956). In a previous paper we reported the measurement of the rate of the NO-graphite reaction using STM. We found that this reaction has a higher intrinsic rate compared with the Ozgraphite reaction (Chu and Schmidt, 1992a). We found in this study that N2O has a high reaction rate when reacted with graphite, and we also obtained a very high rate for the NO2-graphite reaction. For reactions between NO, and carbon, the possible products are N2, C02, CO, and C2N2. Thermodynamically C2Nz formation is also favorable in the NO, + C reaction at T > 400 "C. Thus, a considerable amount of C2N2 as a by-product of side reaction in the C + NO, reaction is predicted. The polymerization of C2Nz to form high melting point (CN), polymers has been reported previously at a low temperature around 300-500 O C (Holliday et al., 1973). Therefore, in NO, + C reactions the formation of (CN), polymer is expected. This has not been realized probably due to the difficulties in identifying this material because it was well mixed with carbon after reaction. Our experiments confirmed the formation of this material in the NO, + C reactions, especially in the NO + C reaction. It is understood that once this high melting point material formed in amorphous carbon, it would stay where it formed. Thus, the diffusion of reactant NO, is hindered by the presence of (CN), in the carbon sample resulting in the low apparent reactionrates. This explainsthe widely reported results of NO + carbon reactions. It has been found (Teng et al., 1992) that nitrogen is a significant high-temperature (>lo00 K)product during desorption from NO-oxidized chars indicating that there is a stable material formed in the sample. This gives additional proof for (CN), production in the NO + carbon reaction. For reaction in NO2 or NO + 02, no significant amount of (CN), was found. This explains their highest rates among all NO, + C reactions. Therefore, we attribute the rate difference between NO,-graphite and NO,-amorphous carbon reactions to the formation of (CN), polymer during the reactions. 4.2. The Shape Change of Monolayer Pits. Monolayer pitting is one of the basic modes of graphite basal plane etching. We found that monolayer pits have different shapes in different cases. Generally speaking, monolayer pits tend to be triangular or hexagonal at low reaction rates (low pressure and/or low temperature) and circular at high rates. It is generally believed that there are two processes in graphite monolayer pitting (Feates and Robinson, 1971; Yang and Wong, 1981a,b): (1)direct collision of reacting gases with the edge atoms of monolayer pits; (2) reacting gases absorbed on the surface migrating to the active sites. These two processes compete in the pitting process. The first process is more straightforward. The second mech-
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anism, proven by the density-dependent etching rate of monolayer pita, is now widely recognized (Yang and Wong, 1981a,b). In the graphite structure, each carbon atom is equally bound to three neighbor carbons. Suppose that the monolayer pit is initiated from a basal plane vacancy which may intrinsically exist in the sample or created by abstraction by the reacting gas at high temperature. It can be seen that once a carbon atom is missing in the perfect graphite basal plane, three carbon atoms with each of them having only two bonds will be exposed. These three carbon atoms have higher energy than those in the perfect sites. Thus, they will be removed f i s t by reacting with the coming gas in the gasification process. This exposes more two-bond carbon atoms which will be removed at the next time sequence. It should be mentioned that the two-bond carbon atoms at the middle of the monolayer edge were exposed fist. If the reacting gases pick up the two-bond atoms according to their order of appearance, a triangular monolayer pit is formed. This should be the case when the reaction rate is low, due to a low reacting gas flux (at low temperatures and/or low pressures). When the reaction rate is increased (by increasing either temperature or the pressure), the total flux of reacting species is increased. The removal of the two-bond carbon atoms (edge atoms) will not be solely from the middle of the edge. Rather, those near the corners will be removed at the same time. Thus a hexagonal pit as described previously (Yang and Wong, 1983b) is formed. At high reaction rates (hightemperature and pressures), the reaction is so fast that there is no selectively for the removal of carbon atoms at the edge of the monolayer pit. Thus, the monolayer pit behaves as circular. It should be mentioned that, under these conditions, the edges of the formed monolayer pita are very rough. We observed the shape changes for NOz, N20, NO, Con, and 0 2 . Only NO2 gives the sharpest transition. Similar shape changes should also be observed in the H20 reaction. These will require much higher temperatures. By the above consideration and wing the NO-arbon reaction as a reference, we predict a circular pit in the H+~arbon reaction at 1200 "C with 30 Torr of HzO. It should be mentioned that the interfacial energies between the edge of the monolayer pit and different reacting gases are different. This will affect the monolayer pit shape transition. 4.3. Multiple Layer Reaction. Among the different modes of reactions on graphite surface,multilayer reactions are the most complicated. Several factors are involved in this process: (i) the total flux of the reactant; (ii) the interaction of the absorbed reacting species with the strain field of the dislocation if the deep pita are generated from screw dislocations; (iii) lower gas fluxes in deep multilayer pita. The latter becomes significant a t the early stage of the reaction when the diameter of the pit is small. In this study, we observed that, for almost all reactions, at low pressure and low temperature there is always a gap between the first-layer pit and the exposed second-layer pit for double-layer pitting. This is different from the high-pressure case in which the first- and the second-layer pits are all tangential. In order to create a gap between ' the first-layer pit and ita exposed second-layer pit, more reacting species should react with the atoms at edge of the first-layer pit. The gasification of the edge atoms can be regarded through two different pathways as described in section 4.2: direct collision of the reacting gas with the edge atoms
Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 1365 and the surface migration of reacting species to the edge of the pita. The only explanation of the gap is that the reaction contributed by surface migration is more important at low pressure (
