Synthesis and catalytic properties of vanadium interstitial compounds

Applied Catalysis A: General 168 (1998) 47±56

Synthesis and catalytic properties of vanadium interstitial compounds Jeong-Gil Choia,*, Joseph Hab, Jin-Who Hongc a

Department of Chemical Engineering, Hannam University, 133 Ojeong-dong, Taedung-gu, Taejon, South Korea 300-791 b Department of Mechanical Engineering and Applied Mechanics, University of Michigan, Ann Arbor, MI 48109, USA c Department of Polymer Science and Engineering, Chosun University, 375 Seosuk-dong, Dong-gu, Kwangju, South Korea 501-759 Received 22 January 1997; received in revised form 22 August 1997; accepted 22 August 1997

Abstract Vanadium nitrides and carbides were synthesized by the temperature-programmed reaction of vanadium oxide (V2O5) with pure NH3, and with pure CH4 or a mixture of 49.9% CH4 in H2, respectively. Based on the XRD results, the materials contained VN or VN with a negligible amount of V2O3 in the bulk after the nitridation of vanadium oxides and only V8C7 after the carburization. These results indicated that the structural properties of these materials were strong functions of the heating rate and space velocity employed. The vanadium nitrides and carbides proved to be active NH3 synthesis and decomposition catalysts. Since the activity varied with changes in the surface area and particle size, ammonia decomposition over the vanadium nitrides and carbides appeared to be structure-sensitive. This response was considered to be due to variations in the surface stoichiometry with particle size. In general, the activities of the vanadium carbides were about 1 to 2 orders of magnitude lesser than that of Mo nitride catalyst while vanadium carbides had higher activities by a factor of 1 or 2 than Pt/C catalyst. However, the activity of the lowest surface area vanadium carbide is similar to that of Mo nitride catalyst. # 1998 Elsevier Science B.V. Keywords: Ammonia synthesis and decomposition; Vanadium nitride; Vanadium carbide

1. Introduction An increasing interest has developed in exploring the catalytic properties of transition metal nitrides and carbides since these materials possess a variety of unique physical and chemical properties owing to the presence of small atoms like nitrogen and carbon in the interstitial sites of the parent metal lattice. Molybdenum and tungsten nitrides and carbides in particular have received a great deal of attention due to their

*Corresponding author. Tel.: 82 42 629 7856; fax: 82 42 623 9489; e-mail: [email protected] 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00332-3

competitive activities for reactions including hydrodenitrogenation [1,2] CO hydrogenation [3], and NH3 synthesis [4]. In addition, their function as catalysts for dehydrogenation, hydrogenolysis and isomerization reactions resembles that of platinum [5±7]. The catalytic and adsorptive properties of these materials are governed by their structure and stoichiometry near the surface. For instance, the properties of transition metal carbides also changes with the amount of carbon present in the interstitial sites of the host metal lattice [8]. Leclercq et al. [5] reported that WC and W2C were active for cyclohexane dehydrogenation and the most active catalyst was slightly carbon de®cient at the surface. One might expect that the stoichiometry near

48

J.-G. Choi et al. / Applied Catalysis A: General 168 (1998) 47±56

the surface could in¯uence the catalytic function. Choi et al. [9] reported that the pyridine HDN activities over the molybdenum carbides increased with decreasing surface area, suggesting a structure-sensitive reaction. They concluded that the surface stoichiometry was a function of particle size, resulting in the production of different activities. Despite a great interest in transition metal nitrides and carbides, not many materials have been investigated as catalysts or catalyst supports. Amongst those nitrides and carbides, very little is known concerning catalysis by vanadium nitrides and carbides. Here we describe the surface and bulk, and catalytic properties of vanadium carbides and nitrides. These carbides and nitrides were prepared by the temperature-programmed reaction of vanadium oxide precursor (V2O5) with pure NH3, and with pure CH4 or a mixture of 49.9% CH4 in H2, respectively. Characterization techniques including X-ray diffraction, BET total surface area measurements, and chemisorption were employed to evaluate the structural and sorptive properties of these materials. Finally, the catalytic properties for NH3 synthesis and decomposition were evaluated.

2. Experimental Vanadium nitrides and carbides were synthesized via the TPR of V2O5 (99.95%, Junsei) with pure NH3 and a mixture of 49.9% CH4 in H2 (Taedug Gas), respectively. Varying the heating rate and molar hourly space velocity were needed to obtain these materials with different structural and compositional properties. For the vanadium carbides, two heating rates of 0.033 and 0.067 K/s, and two CH4 molar hourly space velocities of 5.1 and 10.2 hÿ1 were used in syntheses. The space velocity is de®ned as the CH4 molar ¯ow rate divided by the molar amount of V2O5. A summary of the synthesis conditions and sorption properties of the vanadium nitrides and carbides is given in Table 1. The reaction temperature for the synthesis of carbides was quickly increased from room temperature to 763 K in 1 h. The temperature was then increased from 763 to 1323 K at 0.033 or 0.067 K/s and held at 1323 K for an additional hour. These synthesis conditions are similar to those employed previously to prepare vanadium carbides [10]. For the synthesis of nitrides, the temperature was quickly increased from room temperature to 573 K in

Table 1 Effect of synthesis factors on properties of vanadium nitrides and carbides Heating ratea (K/s)

H2/CH4

VC-1 VC-2

0.033 0.067

VC-3

0.067 ± 0.033 0.067 0.033 0.028 0.030 0.082 0.042 ±

Catalyst code

VC-4 VC-5 VC-6 VC-7 VCg VN-a VN-b V 2 O5 a

Space velocity (hrÿ1)b

Surface area(m2/g)c

Total

CH4

Fresh

Usedd

1.004 1.004

20.4 10.2

10.2 5.1

1.004

20.4

10.2

1.004 0 0 1.004 5 ± ± ±

10.2 10.2 10.2 61.2 67 000 25 25 ±

5.1 10.2 10.2 30.6 13 400 ± ± ±

10.7 7.2 7.5e 7.8 9.4f 8.5 4.2 8.4 31.4 60 6.5 11 3.9

9.3 5.0 ± 10.3 ± 8.4 ± ± ± ± 17.7 ± ±

Particle size(nm)

99 148 142 137 113 125 253 127 34 19 151 89 454

Heating rates for synthesis of V carbides and nitrides indicate that the linear temperature increases from 763 to 1323 K and from 873 to 1123 K, respectively. b Ratio of the molar flow rate of gas to moles of precursor. c Measured after pretreatment in H2 flow at 673 K for 3 h. d Samples were used for NH3 synthesis and/or decomposition. e,f Measured following H2 treatment at 673 K for 4 h and 6 h, respectively. g See [15].

J.-G. Choi et al. / Applied Catalysis A: General 168 (1998) 47±56

30 min. The temperature was then increased from 573 to 873 K at 0.042 K/s, and from 873 to 1123 K at 0.042 or 0.082 K/s, and held at 1123 K for an additional hour. After synthesis, the product was quenched to room temperature and passivated in a mixture of 0.5% O2 in He (Taedug Gas) ¯owing at 20 cm3/min to prevent bulk oxidation. This passivation was continued for 2 h. After passivation, the product was removed from the reactor for subsequent analysis. Nitrogen BET surface areas were measured using a Quantasorb model Chembet 3000 sorption analyzer. Prior to N2 BET surface area measurements, the material was pretreated isothermally in H2 (20 cm3/ min) at 673 K for 3 h, purged in ¯owing He (20 cm3/ min) for 10 min then cooled to room temperature. Standard single point BET measurements were made at 77 K using a 30.1% N2 in He mixture (Taedug Gas). Pulses of puri®ed N2 (99.998%, Taedug Gas) were used to calibrate the amount of adsorbed N2. The average particle size, Dp, was estimated using the equation Dpˆ6/Sg, where Sg is the BET surface area, and  is the density of the primary bulk phase (ˆ3.36, 5.64, and 6.13 g/cm3 for V2O5, V8C7, and VN, respectively). The bulk structure of the materials was evaluated using a computer controlled Rigaku Rota¯ex DMAX-B rotating anode with a CuKa radiation source. To determine the average crystallite size we used line broadening analysis based on the most intense peak in the diffraction pattern. The average crystallite sizes, Dc, was calculated using the Scherrer equation [11] Dcˆ0.9/( cos), where  is the waveÊ ),  is the length of the CuKa radiation (1.5405 A Bragg angle, and is the peak full width at halfmaximum intensity of the most intense peak in the pattern, corrected for instrumental broadening. Approximately 200 mg of catalyst was spread over a plug of glass wool packed into the reaction zone of a 9 mm o.d. pyrex glass ¯ow reactor. A chromel±alumel (K type) thermocouple was used to monitor the temperature. The catalyst was heated in H2 from room temperature to 673 K at a rate of 0.033 K/s, held at 673 K for at least 14 h and then cooled to the reaction temperature. After reduction, a feed mixture of 25% N2 (99.995%) in H2 (99.995%), and NH3 (99.995%) were passed over the catalyst at atmospheric pressure with the same inlet space velocity based on the bed volume of 7500 hÿ1 for NH3 synthesis and decomposition reactions, respectively. The activities were

49

measured at temperatures between 633 and 843 K. All of these measurements were in agreement with an estimated experimental error of 10%. External mass transport limitations were absent at the present reaction conditions since negligible changes in the reaction rate were observed with the variation of the reactant ¯ow rate. The reaction conditions employed in this study were such that NH3 conversions were less than 5%. The catalytic properties of vanadium carbides and nitrides were compared to those of MoN and 0.5 wt% Pt/C (Engelhard) that were pretreated under similar conditions. In particular, the molybdenum nitride (named as MoN) was synthesized using conditions similar to those in the literature [12]. For the synthesis of molybdenum nitride, the temperature was increased from 623 to 723 K at 0.011 K/h during the ®rst heating ramp. The temperature was then increased from 723 to 973 K at 0.028 K/h during the second heating ramp, and held at 973 K for an additional hour. The molar hourly space velocity (SV), de®ned as the ratio of the molar ¯ow rate of NH3 to the moles of MoO3, was 8.5 hÿ1. The reactor ef¯uent was analyzed using an on-line Donam gas chromatograph (DS 6200) equipped with both ¯ame ionization and thermal conductivity detectors. The products were separated using Porapak Q packed columns (80/100, 80 1/800 , CRS) connected to a gas chromatography detector. 3. Results The BET surface areas of the vanadium carbides, which ranged from 4.2 to 31.4 m2/g, depended on the synthesis conditions employed (Table 1). In general, the surface areas of these materials increased with increasing space velocity. This is consistent with previous studies which indicated that high space velocities were necessary for the synthesis of high surface area carbides and nitrides [9,12]. Choi et al. [9] reported similar behavior during the preparation of Mo2C. This increase in surface area may have been caused by the fact that the high space velocities removed H2O vapor from the reaction interface. The heating rates employed during nitriding and carbiding of vanadium oxide also had a signi®cant effect on the resulting surface areas. Increasing the heating rate resulted in the production of vanadium carbides and nitrides with lower surface areas, regard-

50

J.-G. Choi et al. / Applied Catalysis A: General 168 (1998) 47±56

less of the space velocity used. Variations in the surface area with heating rate are not uncommon, since one would expect these factors to in¯uence the product selectivity of the solid-state reactions [12]. The surface areas of vanadium carbides were also dependent of the H2 treatment condition. As the treatment time increased from 3 to 4 h or 6 h, the surface areas also increased by 21%. One reason for this behavior was considered to be the pore opening caused by the progressive conversion of the oxycarbide passivation layer into either the denser vanadium carbide or vanadium metal. The dissolution of oxygen into the vanadium carbide during passivation could cause the expansion of the lattice, closing off some of the pore structure. On the contrary, removal of oxygen from this oxycarbide would cause contraction of the lattice and pore opening. Another reason might be related to the removal of impurities (C and O) on the surface by H2 treatment. For a constant concentration of CH4/H2 or a ®xed space velocity, the higher synthesis temperature used during carburization may accelerate the formation of amorphous or graphitic carbon through the reaction, CH4!C‡2H2 [13]. The deposition of polymeric carbon on the product surface may prohibit the synthesis of high surface area vanadium carbides. Therefore, the removal of polymeric carbon via H2 treatment at high temperature could create the increase in the surface area of these materials. It is known that there are three different kinds of carbonaceous surface species produced by methane decomposition. They can be distinguished by their different hydrogenation temperatures. Carbidic surface carbon can be hydrogenated below 400 K and a less reactive amorphous carbonaceous layer is hydrogenated around 500 K. Unreactive graphitic carbon reacts at higher temperatures than 650 K to produce only methane. In this study we did not perform other experiments using NMR (nuclear magnetic resonance) spectroscopy or AES (atomic emission spectroscopy) to ®gure out which is which. Nonetheless, from separate experiments, the carbon removal was con®rmed by the observation of production of CH4 from vanadium carbides during H2 TPR. Fig. 1 shows the methane peak obtained from TPR (temperatureprogrammed reaction) between H2 and V8C7. The V8C7 sample showed one methane peak produced at 700 K by hydrogenating the surface carbon, suggesting the presence of graphitic carbon formed at carbide

Fig. 1. The methane peak obtained from TPR (temperatureprogrammed reaction) between H2 and V8C7.

surface. The area under this peak gives the total CH4 production per initial surface area of 1.17 10ÿ2 mmol/cm2. This corresponded to an atom ratio of C/Vs (Vs: surface V atom) of 7 assuming the surface V number density of 1.091015 V/cm2. The high value of C/Vs indicates that multilayer polymeric carbons were deposited on the sample, which resulted in the production of low surface area materials. Moreover, since a high temperature was used in the synthesis of vanadium carbides, a higher reduction temperature may be needed to remove the polymeric carbon from the sample. The presence of graphitic carbon was also checked by independent XPS (X-ray photoelectron spectroscopy) experiments. These results show that the graphitic carbon peak appears at a binding energy of 284.3 eV, and the carbidic and adsorbed carbons are observed at 282.2 eV and 285.8 eV, respectively. Table 1 also shows the surface areas of the fresh and used vanadium carbides for ammonia synthesis and/or decomposition reactions. Pretreatment conditions for the used samples were the same as those for fresh samples. With the exception of VC-3, the surface areas of the used vanadium carbides decreased. VC-3 appeared to be a stronger function of H2 treatment conditions than other carbides. As the treatment time increased from 3 h to 6 h, the surface area of VC-3 increased by 21%. Before being used for catalytic reactions, VC-3 was reduced with H2 for at least 14 h. Therefore, the increase in the surface area of used VC3 catalyst might have been related to the prolonged

J.-G. Choi et al. / Applied Catalysis A: General 168 (1998) 47±56

51

Table 2 Oxygen chemisorptive properties of carbides and nitrides Catalyst code

Surface area (m2/g)

Oxygen uptakea (mmol O2/g)

Oxygen capacity10ÿ13 (molecules O/cm2)

Surface Coverage (%)

References

VC-1 VC-1 VC-2 VC-3 VC-4 VC-5 VC-6 VC-7 VN-a VN-b MoN Mo2C W2C W2C

10.7 10.7 7.2 7.8 8.5 4.2 8.4 31.4 6.5 11 3 7 37.3 38.7

2b 1.77c 0.80 1.09 0.97 0.79 0.47 3.1 20.1 28.5 1.33 9 2.9 2.7

2.25 1.99 1.34 1.68 1.37 2.25 0.67 1.14 37.2 31.2 5.34 16 0.94 0.84

2.1 1.9 1.2 1.5 1.3 2.1 0.6 1.0 34.1 28.6 4.9 14.7 0.9 0.8

This This This This This This This This This This This [9] [22] [22]

a

work work work work work work work work work work work

Measured at room temperature after 3h H2 reduction. Measured after 3 h and 2 h H2 reduction, respectively.

b,c

pretreatment with H2. Similar result has also been found for the vanadium nitride catalyst. The oxygen chemisorption was made at room temperature on all vanadium carbides and nitrides (Table 2). For the vanadium carbides, there was a nearly linear relationship between the O2 uptake and the BET surface area. This corresponded to an oxygen capacity of 1.811013 O/cm2. Using an assumed surface vanadium number density (1.091015V/cm2), the average oxygen uptake was equivalent to 2% surface coverage by the atomic oxygen. The uptake values of vanadium carbides are comparable to those of tungsten carbides, but signi®cantly lower than those of vanadium nitrides and molybdenum carbides and nitrides. The low uptake values might be due to the surface blockage by polymeric carbon and/or residual oxygen at surface. This is supported by the fact that the oxygen uptake increased with the increase of reduction period as shown in Table 2. The vanadium carbides between xˆ0.5 and 1 in VCx are thermodynamically stable below 1900 K according to the V±C phase diagram [14]. Therefore, under our preparative conditions, the only crystalline phase observed from all of the materials was V8C7. A typical XRD pattern of V8C7 is given with that of V2O5 for comparison (Fig. 2). V8C7 is simple cubic, while V2O5 is orthorhombic. It was previously pro-

Fig. 2. Typical XRD patterns of (a) vanadium oxide (V2O5) and (b) vanadium carbide (V8C7).

posed that the synthesis of vanadium carbide involves two steps: the formation of a single suboxide intermediate, V2O3, from reduction of V2O5 at 800 K, and the reduction and subsequent carburization of V2O3 to vanadium carbide at 1180 K [15]. In this study, XRD results showed that V2O3 was not formed from reduction of V2O5 by methane up to 763 K, but was produced at 973 K. For the synthesis of vanadium nitrides, based on XRD results we proposed the following reaction pathway: V2O5!(NH4)2V3O8! V2O3!VN. The VN-b contained only VN phase, a

52

J.-G. Choi et al. / Applied Catalysis A: General 168 (1998) 47±56

Table 3 Crystal properties and particle size of selected vanadium carbides Catalyst code

Phase present

I(222)/I(400)

Lattice constant(pm)

Ê )a,b Crystalline size (A Dc1

VC-1 VC-2 VC-3 VC-4 VC-7 V8C7c VCd

V8C7 V8C7 V8C7 V8C7 V8C7 V8C7 VC

0.97 0.96 1.012 1.080 1.21 0.94 ±

834.6 834.7 834.7 834.7 834.7 833.4 ±

a

224 180 260 236 250

Particle size (nm)

Dc2a 187 171 241 221 230

99 148 137 125 34 19

a

Dc1: measured using XRD peak with (222) plane. Dc2: measured using XRD peak with (400) plane. c From the powder diffraction file (see [16]). d See [15]. b

mononitride -phase with the NaCl (cubic closepacked) structure while VN-a had VN phase with a negligible amount of V2O3. Table 3 lists some of the structural features of vanadium carbides. The speci®c volume (per gram of V) decreases by 61% during the transformation of V2O5 to V8C7. This signi®cant decrease in the speci®c volume would result in the evolution of cracks, and the exposure of signi®cant amounts of internal surface area. In this study, it was found that the transformation of V2O5 to V8C7 caused the surface area to increase by a factor of 2.7. The increase in the surface area by carburization can also be supported by comparison of the shortest metal±metal distance for vanadium metal Ê ) with that for vanadium carbide (4.16 A Ê ). The (2.62 A insertion of carbon in the interstitial spaces of vanadium metal expands the metal lattice, resulting in an increase in the metal±metal bond distance. The increase in the metal±metal distance causes the variation in structure, leading to the creation of higher surface area of the ®nal products. For the production of texturing in the process of transformation of vanadium oxides to vanadium carbides, the ratio of the intensities of the (222) and (400) re¯ections of V8C7, I(222)/I(400), was used as a measure of the texturing of the V8C7 crystallites. The I(222)/I(400) ratios for VC-1 and VC-2 were close to 0.94, the value expected for randomly distributed V8C7 crystallites of uniform dimensions [16]. However, VC-3, VC-4, and VC-7 showed texturing for which I(222)/I(400) was 1.1. This value indicates that the crystallites in VC-3, VC-4, and

VC-7 were not of uniform dimensions and/or had a preferential orientation. Note that the highest surface area vanadium carbide (VC-7) had the largest I(222)/ I(400) ratio of 1.21. Vanadium oxide (V2O5) used in this study is typically elongated in the 001 direction. Lattice parameters of vanadium carbides were estimated based on a cubic structure. The lattice parameter averaged 834.7 pm for all vanadium carbides, which are in agreement with the reported literature value of 833.4 pm. Since the intensities of (222) and (400) planes are similar, we estimated the crystallite size of vanadium carbides using each different plane. The Dc1 and Dc2 represent the crystallite sizes based on the (222) and (400) planes, respectively. Using the averaged crystallite size, a comparison of the crystallite and particle sizes suggested the presence of polycrystalline aggregates. There was no direct correlation between the crystallite and particle sizes. The vanadium carbides were active for NH3 synthesis and decomposition. Figs. 3 and 4 show the catalytic behavior of vanadium carbides for the synthesis and decomposition of ammonia, respectively. Fig. 3 shows the conversion of N2 as a function of time at 633 K. The freshly prepared vanadium carbide exhibits the highest initial conversion, but then loses activity gradually over time. In Table 4, the kinetic properties of the vanadium carbides are compared to those of the Mo2Nc and Fe±Al2O3±K2O catalysts. Activity of the vanadium carbides was similar to that of Mo2N catalyst but lower by a factor of two than that of the Fe±Al2O3±K2O catalyst. Similar

J.-G. Choi et al. / Applied Catalysis A: General 168 (1998) 47±56

Fig. 3. N2 conversion with time on stream for NH3 synthesis over vanadium carbide at 633 K.

catalytic behaviors of vanadium carbides and nitrides with time were observed for NH3 decomposition. The NH3 decomposition reaction rates decreased to the steady-state activities during the ®rst 40 min on stream and remained constant for several hours on stream. Arrhenius plots of the activities measured on vanadium carbides are given in Fig. 4. These plots show that the activities increased with increasing particle size (Fig. 5). In Table 4 and Fig. 6, we compared the kinetic properties of the vanadium carbides with those of the vanadium nitrides. Even though vanadium carbides generally exhibited the lower reaction rates on a

53

Fig. 5. Activities versus particle sizes over vanadium carbides.

per gram basis, the activities for vanadium carbides bracketed those determined for the vanadium nitrides. On an oxygen uptake basis, in general activities of the vanadium carbides were about 1 to 2 orders of magnitude lesser than that of MoN catalyst, while vanadium carbides had activities that were higher by a factor of 1 or 2 than Pt/C catalyst. However, the activity of the lowest surface area vanadium carbide is similar to that of Mo nitride catalyst. The ammonia decomposition activation energies for all the vanadium carbides were similar and averaged 35 kcal/ mole, suggesting that ammonia decomposition over vanadium carbides proceeded via a common mechan-

Fig. 4. Arrhenius plots of the NH3 decomposition over vanadium carbides.

54

J.-G. Choi et al. / Applied Catalysis A: General 168 (1998) 47±56

Table 4 Catalytic reaction rates for vanadium carbides and nitrides for NH3 synthesis and decomposition Catalyst code

Reaction typea

Reaction rate (mmol/m2/s)

Areal rate (mmol/m2/s)

Activity (1/s)


Eact (kcal/mol)

References

VC-2 b Mo2N c Fe±Al2O3±K2O VC-1 d VC-2 VC-3 VC-4 VC-5 VC-6 VC-7 VN-a VN-b MoN Pt/C Mo2N e VN e

SYN SYN SYN DES DES DES DES DES DES DES DES DES DES DES DES DES

2.2x10ÿ4 ± ± 11.9 4.6 4.2 2.9 10.2 1.7 1.55 16.2 11.1 16.8 0.74 ± 5.1

3.10x10ÿ3 ± ± 1.12 0.65 0.55 0.34 2.43 0.21 0.05 2.48 1.01 5.83 6x10ÿ4 4.04 0.20

2.8x10ÿ4 3.5x10ÿ4 14x10ÿ3 5.99 0.46 3.18 3.92 12.9 3.7 0.5 0.80 0.39 12.6 0.07 ± 0.18

20 ± ± 34 36 37 32 35 39 30 52 43 36 24 35 33

This [23] [24] This This This This This This This This This This This [17] [18]

c

work work work work work work work work work work work work

a

SYN and DES indicate the reactions of NH3 synthesis and decomposition, respectively. Reaction rates measured at 633 K and 673 K, respectively. d All NH3 decomposition reaction rates were measured at 101 kPa and 843 K. e Reaction rates measured at 101 kPa and 800 K. b, c

Fig. 6. A comparison of the activities for NH3 decomposition over vanadium carbides and nitrides.

ism. The average vanadium carbide activation energy was lower than the average activation energy (47 kcal/ mol) for vanadium nitrides, indicating that the reaction rates over vanadium nitrides were more temperaturesensitive. However, the activation energies of the vanadium carbide catalysts were similar to those reported in the literature for ammonia decomposition

reaction [17,18]. The Arrhenius expressions for NH3 decomposition, assuming zero order kinetics, were obtained in the units of mmol/m2/s for vanadium carbide catalyst: kcˆ5.59108exp(ÿ17489/T). This expression was in agreement with that published in the literature (kˆ1.65108exp(ÿ16427/T)) for NH3 decomposition [18].

J.-G. Choi et al. / Applied Catalysis A: General 168 (1998) 47±56

4. Discussion 4.1. Structure-sensitivity Vanadium carbides and nitrides exhibited the catalytic activity for NH3 synthesis and decomposition. The results indicated that the activities normalized by the surface area and oxygen uptake increased with decreasing surface area. The plots of vanadium carbide activities as a function of particle size show a nearly linear relationship between activity and particle size (Fig. 5). Hence, we concluded that NH3 decomposition over the vanadium carbides was structuresensitive. It is generally considered that structure-sensitivity is due to the differences between the activities of the various crystallographic planes exposed at the catalyst surface. However, for catalysts like transition metal nitrides and carbides it is also believed that there are some differences between the activities of different composition and structures that can coexist. Given the observed variations in activity with different surface area but with the same bulk structure, it appears that the surface stoichiometry was a function of the particle size in the metal carbides. A recent investigation reported that the signi®cant variations in the near surface compositions and structures were observed for a series of molybdenum nitrides with a wide range of particle size: from 5.5 to 159.6 nm [19]. While the same bulk structure (g-Mo2N, fcc) for all these materials was found with N/Moˆ0.5, the structure in the near surface was body-centered with different N/Mo ratios ranging from 0.48 to 1.3. Likewise, for vanadium carbides, we expect the variations in the surface composition and structure, resulting in signi®cant variations in the catalytic properties. Preliminary separate XPS and XRD results showed that the bulk composition of C/V for all the vanadium carbides was 0.875 with the same bulk structure (V8C7), while the different surface compositions were observed ranging from 0.26 to 0.78. These results indicated that the surface stoichiometry was a function of the particle size in the vanadium carbides. Therefore, for NH3 decomposition the variations in catalytic activity of vanadium carbides with particle size suggest structure-sensitivity. It was previously reported that NH3 decomposition was structure-sensitive over tungsten metal [20] and molybdenum

55

nitrides [21]. In particular, for molybdenum nitride thin ®lms Lee et al. [21] reported that in NH3 TPD and decomposition reactions, the NH3 saturation capacity increased in the following order: d-MoN (hex(101))< b-Mo16N7 (bct(400))
56

J.-G. Choi et al. / Applied Catalysis A: General 168 (1998) 47±56

for the vanadium carbides. We found that for the vanadium nitrides and carbides, the NH3 decomposition activities normalized by the surface area and oxygen uptake increased with the decreasing surface area, suggesting that NH3 decomposition over these materials appeared to be structure-sensitive. This response was considered to be due to the variations in the surface stoichiometry with particle size. The activities of the vanadium carbides were about 1 to 2 orders of magnitude less than that of MoN catalyst while vanadium carbides had higher activities by a factor of 1 or 2 than Pt/C catalyst. The ammonia decomposition activation energies for all the vanadium carbides were similar and averaged 35 kcal/ mole. This average activation energy was in good agreement with those reported in the literature. Acknowledgements This work was ®nancially supported by 1996 Korea Science and Engineering Foundation (Project No. 961-1108-049-2). The author thanks Mr. M-K Jung for his assistance in measuring activities of V carbides and nitrides. References [1] D.J. Sajkowski, S.T. Oyama, Prep. Petrol. Chem. Div., 199th ACS Nat. Meeting, 1990. [2] J.C. Schlatter, S.T. Oyama, J.E. Metcalfe III, J.M. Lambert Jr., Ind. Eng. Chem. Res. 27 (1988) 1648.

[3] L. Leclercq, K. Imura, S. Yoshida, T. Barbee, M. Boudart, in: B. Delmon (Ed.), Preparation of Catalysts II, Elsevier, New York, 627 (1978). [4] L. Volpe, M. Boudart, J. Phys. Chem. 90 (1986) 4874. [5] L. Leclercq, M. Provost, H. Pastor, J. Grimblot, A.M. Hardy, L. Gengembre, G. Leclercq, J. Catal. 117 (1989) 371. [6] R.B. Levy, M. Boudart, Science 181 (1973) 547. [7] J.H. Sinfelt, D.J.C. Yates, Nature Phys. Sci. 229 (1971) 27. [8] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. [9] J.-G. Choi, J.R. Brenner, L.T. Thompson, J. Catal. 154 (1995) 33. [10] S.T. Oyama, J.C. Schlatter, J.E. Metcalfe, J.M. Lambert Jr., Ind. Eng. Chem. Res. 27 (1988) 1639. [11] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Company, Reading, MA, 1978. [12] J.-G. Choi, R.L. Curl, L.T. Thompson, J. Catal. 146 (1994) 218. [13] J.S. Lee, S.T. Oyama, M. Boudart, J. Catal. 106 (1987) 125. [14] E. Rudy, S. Windisch, A.J. Stosick, J.R. Hoffman, Trans. TMS-AIME 239 (1967) 1247. [15] R. Kapoor, S.T. Oyama, J. of Solid State Chem. 120 (1995) 320. [16] W.F. McClune (Ed.), Powder Diffraction File; Alphabetical Index Inorganic Materials, International Centre for Diffraction Data, Swarthmore, PA, 1991. [17] R.S. Wise, E.J. Markel, J. Catal. 145 (1994) 335. [18] S.T. Oyama, J. Catal. 133 (1992) 358. [19] B.G. Demczyk, J.-G. Choi, L.T. Thompson, Appl. Surf. Sci. 78 (1994) 63. [20] J. McAllister, R.S. Hansen, J. Chem. Phys. 59 (1973) 414. [21] H.J. Lee, J.-G. Choi, C.W. Colling, M.S. Mudholkar, L.T. Thompson, Applied Surf. Sci. 89 (1995) 121. [22] K.E. Curry, L.T. Thompson, Catal. Today 21 (1994) 171. [23] L. Volpe, S.T. Oyama, M. Boudart, in G. Poncelet, P. Grange, P.A. Jacobs (Eds.), ``Preparation of Catalyst III,'' p. 147. Elsevier, New York, 1983. [24] S.T. Oyama, Catal. Today 15 (1992) 179.