바나듐 질화물의 촉매반응

정문기, 오현균, 최정길

한남대학교 화학공학과, 대전광역시 대덕구 오정동 300-791

Catalysis by Vanadium Nitrides

Moon-Ki Jung, Hyun-Gyun Oh, and Jeong-Gil Choi

Department of Chemical Engineering, Hannam University, 133 Ojeong-dong,

Taedug-gu, Taejon, Korea 300-791

Introduction

It was reported that transition metal nitrides were very active for reactions including hydrodenitrogenation [1,2], CO hydrogenation [3], and NH3 sythesis [4]. Moreover, their function as catalysts for dehydrogenation, hydrogenolysis and isomerization reactions resembles that of platinum based catalysts (Group 8-10 metals)[5-7]. Recent investigation reported that the vanadium nitride was active for decomposition of ammonia[8]. The kinitic study over vanadium nitrides showed that the rate parameters of vanadium nitride were similar to those of iron and platinum. Even though the vanadium nitride is one of the promising candidates as the substitute for Pt group metal in catalysis, very little is currently known about the catalytic properties of vanadium nitride. Several vanadium nitrides were prepared using different synthesis conditions and characterized using a series of characterization techniques.

The vanadium nitrides were prepared by the temperature programed reduction of vanadium oxide percursor(V2O5) with pure NH3. The X-ray diffraction and BET surface area measurement were employed to characterize these meterials. The catalytic properties were evaluated using the NH3 decomposition and compared to the MoN and Pt/C catalysts.

Experimental

The vanadium nitrides were synthesized using the temperature-programmed reaction of V2O5 (99.95%, Junsei Chemical Co. Ltd.) with pure NH3. Two heating ramps and two NH3 molar hourly space velocities were utilized to synthesize various kinds of vanadium nitride with different structures and compositions. The space velocity is defined as the NH3 molar flow rate divided by the molar amount of V2O5.

The BET surface areas of the materials were measured using the Quantasorb model Chembet 3000 sorption analyzer. Prior to N2 BET surface area measurements, the material was reduced isothermally by H2 at 673 K for 3 hour. The average particle size, Dp, was estimated using the equation Dp=6/Sg□, where Sg is the BET surface area, and □ is the density of the primary bulk phase. A compuer controlled Rigaku Rotaflex DMAX-B (Cu K□ source) x-ray diffractometer was used to evaluate the bulk structure of the materials. The average cristallite size, Dc, was estimated using the Scherrer equation [9] Dc=0.9□/(□cos□), where □ is the wavelength of the Cu K□ radiation (1.5405 Å), □ is the Bragg angle , and □ is the peak full width at half maximum intensity of the most intense peak in the XRD pattern.

For ammonia decomposition activity measurement, ∼0.2g of catalyst was used and loaded over a plug of glass wool packed into the pyrex glass flow reacter. After H2 pretreatment, NH3(99.995%) was passed over the catalyst bed at atmospheric pressure. Activity measurments were performed at appropriate temperatures.

Result and discussion

The BET surface areas of the vanadium nitrides synthesized in this study ranged from 10.5 to 33 m2/g. Since the BET surface area of the vanadium oxide was measured to be 3 m2/g, an increase in surface area was ascribed to the change in density of materials during transformation of vanadium oxide to vanadium nitride. The effect of synthsis condition employed on BET surface area was shown in Table1. It is obvious that the synthesis conditions influenced the surface area. The heating rate employed during the first and second heating ramps had a significant effect on the resulting surface areas. A similar result was observed for the synthesis of molybdenum nitrides via TPR of molybdenum oxides [10].

During the first 50 minites on steam, the ammonia decomposition reaction rates decreased significantly then leveled off to a nearly constant value. The initial decomposition rates were up to 10-fold higher than the steady state rates measured after∼350 min on stream. Among vanadium nitride catalysts, the VN-1 had the highest steady state rates for NH3 decomposition. The activity of VN-3 catalyst was about 4 times higher than that of VN-b. This suggested that either the distribution of active species or the active species in VN-1 catalyst might be different from that in VN-b catalyst. In fact, the most active VN-3 catalyst contained the largest particle size while VN-b had the smallest particle size.

Catalytic activities of vanadium nitride are compared to those of other vanadium nitride (denoted as VN-c, Pt/C, and MoN (Table 2). Table 2 shows the NH3 decomposition activities expressed on a per gram or surface area basis. Generally the catalytic activities of vanadium nitrides in this study were in good agreement with that of vanadium nitride(VN-c) in the literature [8]. The reaction rates and activities for vanadium nitrides were comparable or inferior to those determined for MoN catalyst, but were even higher than those of Pt/C. On a per gram basis, the most active catalyst of VN-1 was more active than Pt/C and MoN catalysts. Futhermore, on a surface area basis the activity of VN-1 was two times lower than that of MoN catalyst. These results suggested that the characteristics of active sites in the vanadium nitrides were similar to those in the Group 8-10 metal-based catalysts.

The activity results of vanadium nitride catalysts indicated that the activities increased with decreasing surface area or increasing the particle size. There is a linear relationship between activity and the particle size. The linear relationship suggested that NH3 decomposition over the vanadium nitrides was structure-sensitive. Similar conclusions over Mo nitride catalysts have been drawn for ammonia synthesis [4, 11]. Choi et al.[12] also reported that the pyridine HDN activities increased with increasing the particle size over the Mo carbides. They concluded that the pyridine HDN over the Mo carbides was a structure-sensitive reaction. We believe that these responses were associated with the grain boundary length which is proportional to particle size/(crystallite size)2. However, it can not be ruled out that variations in the surface stoichiometry/composition with particle size influenced the activity. The difference in the surface composition with particle size was confirmed by performing the x-ray photoelectron spectroscopic experiments using vanadium nitrides.

Conclusions

It was found that vanadium nitrides were active for NH3 decomposition. The activity normalized by the surface area increased with decreasing surface area, suggesting that NH3 decomposition over vanadium nitrides appeared to be structure-sensitive. On a per gram basis, the most active catalyst of VN-1 was more active than Pt/C and MoN catalysts. Furthermore, the activity of VN-1 was two times lower than that of MoN catalyst. These results suggested that the characteristics of active sites in the vanadium nitride were similar to those in the Group 8-10 metal-based catalysts.

Acknowledgements

This work was supported by Non-Directed Research Fund of Korea Research Foundation, 1995.

References

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Table 1. The Effect of Synthesis Conditions on BET Surface Areas of

Vanadium Nitrides

Sample	Heating Ramp (K/h)		Molar Hourly Space Velocity (1/h)	Surface Area * (m2/g)
	B1	B2		fresh sample	used sample
VN-1 VN-2 VN-3 VN-4 VN-5 VN-6 VN-7 VN-8 V2O5 VN-a+ VN-b VN-c	150 75 150 75 150 75 150 75 - 30 60 119	300 300 150 150 300 300 150 150 - 120 120 -	25 25 25 25 12.5 12.5 12.5 12.5 - 30 30 487	10.5 11.7 18 14 13.7 12.8 13.6 13.2 3 27 33 25.2	11.9 13.3 19 17.1 12.2 9.8 13.2 15.1 - - - -

+ Pretreatment in flowing hydrogen at 673 K for 3 h. Used sample indicates catalyst

used for ammonia decomposition.

* VN-a and VN-b were prepared using the final temperature of 973 K;

VN-c: Oyama, 1992

Table 2. Catalytic Reaction Rates of Selected Vanadium Nitrides for NH3	decomposition

Sample	Reaction Rate* (nmol/g/s)	Areal Rate (nmol/m2/s)	Activity (1/s)
VN-1 VN-2 VN-3 VN-5 VN-a VN-b VN-c MoN Pt/C	30.8 28.2 22.9 47.7 19.2 24.0 5 17 0.74	2.93 2.41 1.27 3.48 0.71 0.73 0.2 5.8 6x10-4	2.99 2.14 1.7 2.74 0.8 0.7 0.18 12.6 0.07

* Reaction rates measured at ~101 kPa and 843 K.