Catalytic Decomposition of Hydrazine over r-Mo2C/γ-Al2O3 Catalysts

Xiaowei Chen, Tao Zhang,* Mingyuan Zheng, Liangen Xia, Tao Li, Weicheng Wu,Xiaodong Wang, and Can Li*

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,P.O. Box 110, Dalian 116023, China

A series of R-Mo2C/γ-Al2O3 catalysts with different Mo loadings were prepared by temperature-programmed reaction and characterized by X-ray diffraction, BET specific surface areadetermination, and NH3 temperature-programmed desorption. Their ability to decomposehydrazine was tested using a thruster and a microreactor. The catalytic activity for hydrazinedecomposition of the fresh R-Mo2C/γ-Al2O3 catalyst is higher than that of the passivated R-Mo2C/γ-Al2O3 catalyst. The catalytic activity also decreased with the reaction time at 303 K, and thedeactivation is mainly due to strongly adsorbed NHx species. Flushing with N2 at 773 K recoversthe activity of the catalyst. It was shown that hydrazine decomposes to NH3 and N2 at lowtemperatures, and the produced NH3 dissociates into N2 and H2 at elevated temperatures.

Introduction

Transition-metal nitrides and carbides exhibit cata-lytic behavior resembling that of columns 8-10 metalsin a number of reactions,1,2 such as NH3 synthesis anddecomposition,3-6 hydrotreatment,7-13 hydrogenolysis,14

and hydrogenation.14-16 These reactions are usuallycatalyzed by the noble metals. Therefore, the possibilityof using nitrides and carbides of cheaper metals ascatalysts has economic as well as academic interest.

The catalytic decomposition of hydrazine to gases N2,H2, and NH3 has been used to modify the orbit ofspacecraft since the 1970s. A commercial catalyst forhydrazine decomposition is Ir/γ-Al2O3 (20-40 wt %).17,18

However, iridium is expensive and of limited avail-ability. Therefore, a novel, highly active and low-costcatalyst for hydrazine decomposition is economicallydesirable.

Recently, bulk Mo2N, W2N, NbN, and W2C have beeninvestigated for hydrazine decomposition.19-22 The tung-sten carbide was found to be more efficient than Ir/γ-Al2O3 catalyst for the same consumption of hydrazine.19

Furthermore, the catalytic activity of a MoNx/γ-Al2O3catalyst was found to be similar to that of a 31.6 wt %Ir/γ-Al2O3 catalyst.23 Also, γ-Al2O3-supported R-Mo2Cand â-Mo2C catalysts exhibited activity comparable tothat of Ir/γ-Al2O3 catalyst.24

Hydrazine can be catalytically decomposed by twodifferent routes as shown in eqs 1 and 2.25,26

Previous studies have reported that the selectivitybetween reactions 1 and 2 depends on the catalyst andthe temperature range.26 The decomposition pathwayand the product distributions for hydrazine decomposi-tion using the nitride and carbide catalysts in a thrusterare not well established. The objective of this work is

to determine the catalytic performances of R-Mo2C/γ-Al2O3 catalysts for hydrazine decomposition in both athruster and a microreactor. The catalytic activities ofboth passivated and fresh R-Mo2C/γ-Al2O3 catalyst werecompared in the microreactor. The pathway of catalyticdecomposition of hydrazine over the R-Mo2C/γ-Al2O3catalyst has been defined for the first time.

2. Experimental Section

Catalyst Preparation. The MoO3/γ-Al2O3 precursorswere prepared with different Mo loadings by repeatedimpregnation of γ-Al2O3 (SBET ) 198 m2/g, 20-30 mesh)with an aqueous solution of (NH4)6Mo7O24‚4H2O. Thesamples were dried at 393 K for 12 h and calcined at773 K for 4 h.

The R-Mo2C/γ-Al2O3 catalyst with face-centered cubic(fcc) structure was prepared by carburizing the Mo2N/γ-Al2O3 precursor with a mixture of 20% CH4/H2(v/v).16,27,28 A four-stage heating ramp was used: thetemperature was first raised from room temperature to573 K at a rate of 10 K/min, then to 823 K at a rate of0.5 K/min, and further from 823 to 973 K at a rate of 1K/min and finally maintained at 973 K for 1 h. Thesupported R-Mo2C/γ-Al2O3 catalysts with different Moloadings were passivated under a flow of 1% O2/N2 for10 h to form a protective oxide layer on the surface.

BET Specific Surface Area and X-ray Diffrac-tion. The BET specific surface areas of the samplesbefore and after reaction were measured by nitrogenadsorption at 77 K using a Micromeritics ASPA-2000adsorption analyzer. The structure of the catalysts wasdetermined by X-ray diffraction (XRD) using a RigakuRotaflex (Ru-200b) powder X-ray diffractometer (Cu KRradiation).

Evaluation of Catalytic Activity in a Thruster.The catalytic activity of hydrazine decomposition wasevaluated following the procedure described previ-ously.23 The initial temperature in the catalyst bed was373 K. The continuous test lasted 30 s, and then thecatalyst bed was allowed to cool to 373 K before the next30 s startup-shutdown cycle began. For the pulsedtests, the frequency was 1 Hz and the periods duringwhich the electromagnetic valve was open were 100 and200 ms. The chamber pressure (Pc), catalyst bed tem-

* To whom correspondence should be addressed. Tel.: 0086-411-84379015 (T.Z.), 0086-411-84379070 (C.L.). Fax: 0086-411-84691570 (T.Z., C.L.). E-mail: taozhang@dicp.ac.cn (T.Z.),canli@dicp.ac.cn (C.L.).

N2H4 f N2 + 2H2 (1)

3N2H4 f N2 + 4NH3 (2)

6040 Ind. Eng. Chem. Res. 2004, 43, 6040-6047

10.1021/ie034262w CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 08/21/2004

perature (Tc), and ignition delay (t0) were recorded witha frequency of 1 kHz and calculated automatically bycomputer.

Pc and Tc are the parameters describing the extentof the hydrazine conversion. t0 is the time taken for thepressure of the chamber to reach 10% of its stable-statepressure. It directly reflects the initial catalytic activity.The smaller the value of t0, the higher the initial activityof the catalyst. In this work, the specific activity isdefined as the chamber pressure produced per mass ofhydrazine feeding [(MPa/g)/s].

Evaluation of Catalytic Activity in a Micro-reactor. Hydrazine decomposition was also carried outat atmospheric pressure in a U-shaped fixed-bed con-tinuous-flow microreactor made of quartz. The ap-paratus is shown in Figure 1. The temperature of thereaction was controlled by a water bath below 373 K.The oven was used to adjust the reaction temperatureabove 373 K. The temperature of the liquid hydrazinewas kept at 303 K. A flow of Ar gas was passed throughthe liquid hydrazine. Thus, the composition of thefeedstock, which was about 3 vol % hydrazine in argon,can be estimated from the saturation pressure of hy-drazine at 303 K. The flow rate of N2H4/Ar was 85 cm3/min. About 0.1 cm3 of catalyst and 0.2 cm3 of silica weremixed and put in the microreactor. Before each reactiontest, the catalyst was pretreated with a flow of 20% CH4/H2 to recarburize the oxycarbide samples. The temper-ature was raised to 973 K at a rate of 10 K/min andmaintained for 1 h. The sample was cooled to reactiontemperature, and then the feedstock was introduced intothe reactor. The reaction temperature was 303 K at first.After reaction at 303 K for 7 h, the temperature wasgradually raised.

The feedstock and the products of hydrazine decom-position were analyzed by an on-line gas chromatograph

(Agilent-6890A) with a thermal conductivity detectorand an automatic injection valve. Argon was the carriergas. A Chromosorb 102 column separates ammonia andhydrazine and a molecular sieve 13 X column separateshydrogen and nitrogen.

NH3 TPD Characterization. A 200 mg passivated26.9 wt % R-Mo2C/γ-Al2O3 sample was heated to 973 Kat a rate of 10 K/min and held at this temperature for1 h in a mixture of 20% CH4/H2 to recarburize thesurface oxycarbide layer. When the temperature wasdecreased to room temperature, NH3 was injected usinga syringe and adsorbed saturatedly. The sample washeated at a rate of 10 K/min to 1173 K in a flow of Heat a rate of 40 cm3/min. The 31.6 wt % Ir/γ-Al2O3catalyst was reduced at 773 K for 1 h and then flushedfor 0.5 h at 773 K with He. At room temperature, NH3was adsorbed saturatedly on the Ir/γ-Al2O3 catalyst, andthen the temperature was increased from room tem-perature to 1173 K at a rate of 10 K/min. During theTPD process the desorbed products were monitored bymass spectrometry. The signals of m/e ) 2, 14, 15, 16,17, 18, and 28 were detected simultaneously.

3. Results and Discussion

BET and XRD Results. The BET specific surfaceareas of both the Ir/γ-Al2O3 catalyst and of the R-Mo2C/γ-Al2O3 catalysts with a series of Mo loadings arepresented in Table 1. The BET specific surface area ofthe γ-Al2O3 support is 198 m2/g, and the BET specificsurface areas of the R-Mo2C/γ-Al2O3 catalysts are in therange of 150-170 m2/g. It is found that the XRDpatterns of the R-Mo2C/γ-Al2O3 catalysts with a Moloading lower than 26.9 wt % exhibit some broad peaksand the R-Mo2C phase could only be identified for theR-Mo2C/γ-Al2O3 catalyst with a Mo loading of 26.9 wt

Figure 1. Experimental apparatus for hydrazine decomposition in a microreactor: (1) 20% CH4/H2, (2) Ar, (3) stopcock, (4) mass flowmeter, (5) water bath, (6) N2H4 bubbler, (7) three-path valve, (8) oven, (9) four-path valve, (10) U-shaped reactor, (11) gas chromatograph.

Table 1. Performance of the Catalysts for Hydrazine Decomposition during the First Startup-Shutdown Cycle and theInitial Temperature of the Catalyst Bed at 373 K

BET specific surface area (m2/g)

catalyst

metalcontent(wt %)

beforereaction

afterreaction

flow rateof hydrazine

(g/s)t0

(ms)Tc(K)

Pc(MPa)

specificactivity

[(MPa/g)/s]

R-Mo2C/γ-Al2O3 8.7 170 150 6.22 165 393 1.03 0.16512.9 164 143 4.70 142 1171 1.08 0.23015.4 160 143 4.59 122 1147 1.08 0.23523.0 163 138 4.95 126 1075 1.19 0.24026.9 152 135 4.70 92 1130 1.10 0.234

Ir/γ-Al2O3 31.6 165 123 4.63 28 1071 1.07 0.232

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6041

% (Figure 2). The theoretical monolayer coverage ofMoO3 on γ-Al2O3 corresponds to a loading of about 12.9wt % Mo in the catalyst, which has been estimated byusing a simple close-packed monolayer model.29 Thisresult suggests that molybdenum carbide on the surfaceof the alumina is well dispersed.

Hydrazine Decomposition over r-Mo2C/γ-Al2O3Catalysts in the Thruster. The dynamic tendenciesof Pc and Tc versus reaction time over the passivatedR-Mo2C/γ-Al2O3 catalysts and the Ir/γ-Al2O3 catalystduring the first startup-shutdown cycle are shown inFigure 3, and the experimental results of steady Pc andTc, ignition delay (t0), and specific activity are sum-marized in Table 1. For the R-Mo2C/γ-Al2O3 catalystwith a Mo loading of 8.7 wt %, Pc increases quickly, andthen declines slightly during the first 30 s continuousfeeding of the hydrazine. The temperature in thecatalyst bed remained constant at 393 K. The specificactivity for hydrazine decomposition is only 0.165(MPa/g)/s. When the Mo loading is increased to 12.9 wt%, corresponding to the monolayer coverage of MoO3 onthe surface of γ-Al2O3, the specific activity increases to0.230 (MPa/g)/s and is close to that of Ir/γ-Al2O3 (0.232(MPa/g)/s). The specific activity does not alter greatlywith a further increase of the Mo loading. As seen inFigure 3, there is no obvious difference between thesteady-state chamber pressures for the R-Mo2C/γ-Al2O3catalysts when the Mo loading is higher than 12.9 wt% and the Ir/γ-Al2O3 catalyst under the same reactionconditions. These results suggest that the R-Mo2C/γ-Al2O3 catalysts are as active as Ir/γ-Al2O3 for catalyticdecomposition of hydrazine. The high activity of theR-Mo2C/γ-Al2O3 catalyst with a Mo loading higher than12.9 wt % can be attributed to the R-Mo2C phase.

Table 1 represents the t0 values during the firststartup-shutdown cycle. It can be seen that the t0 valuevaries significantly with the Mo loading, and as the Moloading increases, the t0 value becomes shorter. Thismeans that the initial activity increases with theincreasing Mo loading. Figure 4 also shows the t0 valuesduring a series of startup-shutdown cycles. After threecycles the t0 value reaches a stable level in the range of

40-60 ms. As mentioned earlier, the R-Mo2C/γ-Al2O3catalysts were passivated before exposure to air, causingan oxycarbide layer to form on the catalyst surface. Thisoxycarbide layer would be reduced by the hydrazine andfurther ammonia formed during the hydrazine decom-position reaction. This accounts for the activationprocess for the R-Mo2C/γ-Al2O3 catalysts at the begin-ning of hydrazine decomposition. The t0 values of theIr/γ-Al2O3 catalyst remained constant between 20 and30 ms. The lower t0 value for the Ir/γ-Al2O3 catalystimplies that the initial activities of supported R-Mo2C/γ-Al2O3 catalysts are lower than that of Ir/γ-Al2O3catalyst. These results are similar to those for the MoNx/γ-Al2O3 catalysts.23

The ammonia produced by hydrazine decompositioncan further dissociate into H2 and N2 at elevatedtemperatures. The ammonia decomposition is endother-mic, and the hydrazine decomposition is exothermic. Sothe temperature of the catalyst bed (Tc) is affected byboth the ammonia and hydrazine decomposition. Asshown in Figure 3, Tc increases when hydrazine decom-poses, and finally rises to a stable level. The Tc of theIr/γ-Al2O3 catalyst is lower than that of the R-Mo2C/γ-Al2O3 catalyst. The hydrazine decomposes completelyover both Ir/γ-Al2O3 and R-Mo2C/γ-Al2O3 catalysts andgenerates equal energy over both Ir/γ-Al2O3 and R-Mo2C/γ-Al2O3 catalysts. The lower Tc for the iridium catalystis possibly due to the difference in the conversion ofproduced NH3. The endothermic NH3 decompositionrequires energy and could lower the temperature in thecatalyst bed. It appears that the ammonia conversionis higher over the Ir/γ-Al2O3 catalyst than over theR-Mo2C/γ-Al2O3 catalysts.

Pulsed tests were also performed to evaluate theactivity of the catalyst when the hydrazine was injectedinto the catalyst bed in pulses. The catalytic propertyof the catalyst during pulse treatment is very importantfor the practical application of the catalyst. Figure 5shows the catalytic behaviors of the R-Mo2C/γ-Al2O3catalyst with a Mo loading of 12.9 wt % and the Ir/γ-Al2O3 catalyst for the same consumption of hydrazineduring a series of pulsed tests. The Pc lines of theR-Mo2C/γ-Al2O3 catalysts with other Mo loadings are notshown in Figure 5 during the pulsed tests because theyare so similar to those for a Mo loading of 12.9 wt %. Itcan be seen that the Pc line produced by the Ir/γ-Al2O3catalyst is close to that produced by the R-Mo2C/γ-Al2O3catalyst under the same reaction conditions. This resultalso confirms that the R-Mo2C/γ-Al2O3 catalyst is asactive as the Ir/γ-Al2O3 catalyst.

Table 1 also lists the BET specific surface areas ofthe Ir/γ-Al2O3 catalyst and R-Mo2C/γ-Al2O3 catalystsafter six startup-shutdown cycles of reaction test in thethruster. Compared with those before reaction, the BETspecific surface areas of R-Mo2C/γ-Al2O3 catalysts afterreaction diminish to the range of 130-150 m2/g. Underthe same reaction conditions, the BET specific surfacearea of the Ir/γ-Al2O3 catalyst also decreases afterreaction. During the process of hydrazine decompositionin the thruster, the reaction temperature can rise toabout 1000 K. The decrease of the BET specific surfacearea of the catalysts is probably attributed to a decreaseof the surface area of the alumina at high temperatures.

Hydrazine Decomposition in the Microreactor.The catalytic properties of R-Mo2C/γ-Al2O3 catalysts forhydrazine decomposition were also investigated usinga microreactor. The decomposition by reactions 1 and 2

Figure 2. XRD patterns of (a) γ-Al2O3, (b-f) R-Mo2C/γ-Al2O3catalysts with different Mo loadings [(b) 8.7 wt %, (c) 12.9 wt %,(d) 15.4 wt %, (e) 23.0 wt %, (f) 26.9 wt %], and (g) bulk R-Mo2C.

6042 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004

coupled with the ammonia decomposition (reaction 3)can be combined and expressed as eq 4.

The selectivity of the catalyst, X, is taken as thehydrazine decomposition percentage according to reac-tion 1, and X can be calculated from the amount ofhydrogen and ammonia:

The selectivity of the catalyst is also equal to theconversion of produced ammonia decomposition in nu-merical value. Hence, the selectivity of the catalyst, X,can be used to evaluate how much ammonia decomposesaccording to reaction 3.

Figure 6 compares the hydrazine decomposition overpassivated and fresh R-Mo2C/γ-Al2O3 (8.7 wt %) catalystat 303 K. The conversion of hydrazine decompositionon passivated R-Mo2C/γ-Al2O3 catalyst is about 36% at

the beginning of reaction and then deceases to around10% and becomes stable as the reaction time increases.However, the conversion of hydrazine decomposition isabout 100% for the first 20 min of reaction over freshR-Mo2C/γ-Al2O3 catalyst and then decreases with the

Figure 3. Comparison of the catalytic performance of hydrazine decomposition over 31.6 wt % Ir/γ-Al2O3 catalyst and passivated R-Mo2C/γ-Al2O3 catalysts with different Mo loadings during the first 30 s continuous feeding of hydrazine in a thruster. The initial temperatureof the catalyst bed was 373 K.

2NH3 f N2 + 3H2 (3)

3N2H4 f [4(1 - X)]NH3 + (1 + 2X)N2 + (6X)H2 (4)

X )1/2[H2]

1/2[H2] + 3/4[NH3]× 100% )

2[H2]

2[H2] + 3[NH3](5)

Figure 4. Comparison of the t0 values of the 31.6 wt % Ir/γ-Al2O3catalyst and passivated R-Mo2C/γ-Al2O3 catalyst with different Moloadings during a series of startup-shutdown cycles. The initialtemperature of the catalyst bed was 373 K.

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6043

reaction time to around 20%. The lower activity of thepassivated R-Mo2C/γ-Al2O3 catalyst is due to the oxy-carbide layer formation on R-Mo2C during the passiva-tion process. Fresh R-Mo2C/γ-Al2O3 catalysts with dif-ferent Mo loadings were selected for study in themicroreactor.

Figure 6 also shows the conversion of hydrazinedecomposition over a series of fresh R-Mo2C/γ-Al2O3catalysts at 303 K. For R-Mo2C/γ-Al2O3 catalyst with aMo loading of 8.7 wt %, the reaction time of hydrazineconversion kept around 100% is 20 min. When the Moloading was up to the monolayer of MoO3 on alumina,the conversion of hydrazine decomposition remained at

100% for 120 min. With an increase of the Mo loading,the 100% conversion can be maintained longer; forexample, the conversion of hydrazine decomposition was100% for 250 min for R-Mo2C/γ-Al2O3 (26.9 wt %)catalyst, which was the most active among the seriesof R-Mo2C/γ-Al2O3 catalysts.

The effect of reaction temperature on the activity ofhydrazine decomposition is shown in Figure 7. The datain Figure 7 were taken after a reaction of 420 min at303 K; the hydrazine conversion increases with anincrease in reaction temperature. When the tempera-ture is up to 333 K, the hydrazine conversion is 100%for all the catalysts.

Figure 8 represents the effect of reaction temperatureon the selectivity of the catalyst. The selectivity of theIr/γ-Al2O3 catalyst is around 0.5% when the reactiontemperature is below 573 K. The conversion of ammoniabegins to rise at 573 K and then increases sharply witha further increase of the reaction temperature. At 773K, the conversion of ammonia is close to 95%. ForR-Mo2C/γ-Al2O3 catalyst, only a small amount of hydro-gen is detected and the main products of hydrazinedecomposition are N2 and NH3 below 673 K. A dramaticincrease in the selectivity to hydrogen is found with areaction temperature above 673 K. When the temper-ature is 823 K, the selectivity is 100%. The reaction

Figure 5. Comparison of the catalytic performance of hydrazinedecomposition over (a) 31.6 wt % Ir/γ-Al2O3 catalyst and (b)passivated 12.9 wt % R-Mo2C/γ-Al2O3 catalyst during a series ofpulsed tests in a thruster.

Figure 6. Activity of hydrazine decomposition over passivated8.7 wt % R-Mo2C/γ-Al2O3 catalyst, fresh R-Mo2C/γ-Al2O3 catalystswith different Mo loadings, and 31.6 wt % Ir/γ-Al2O3 catalyst as afunction of reaction time at 303 K, SV ) 17000 h-1.

Figure 7. Conversion of hydrazine decomposition over freshR-Mo2C/γ-Al2O3 catalysts with different Mo loadings and 31.6 wt% Ir/γ-Al2O3 catalyst at different temperatures, SV ) 17000 h-1.

Figure 8. H2 selectivity of hydrazine decomposition over R-Mo2C/γ-Al2O3 catalysts and 31.6 wt % Ir/γ-Al2O3 catalyst at differenttemperatures, SV ) 17000 h-1.

6044 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004

routes of hydrazine decomposition over Ir/γ-Al2O3 andR-Mo2C/γ-Al2O3 catalysts are similar. This result con-firms the previous supposition that the selectivity of Ircatalyst is higher than that of R-Mo2C/γ-Al2O3 catalystat the same temperature.

To explore the cause of the decrease of catalystactivity with reaction time at 303 K, the R-Mo2C/γ-Al2O3(8.7 wt %) catalyst, after the hydrazine decompositionfor 420 min, was flushed with N2 flowing at 773 K for180 min. During the process of N2 flushing, the gasesfrom the outlet were analyzed by gas chromatography.H2 was found besides N2 at 773 K. The hydrazinefeedstock was reintroduced into the reactor at 303 K,and as seen in Figure 9, the conversion of hydrazinewas restored to 100%, although after the earlier reactionfor 420 min the activity of the catalyst was reduced to20%. The activity of the catalyst was recovered after theN2 flush treatment to that of fresh 8.7 wt % R-Mo2C/γ-Al2O3 catalyst. These results suggest that traces ofoxygen or H2O are not the cause of the catalystdeactivation since the inert gas nitrogen could notremove the oxygen species. It seems that other speciesare formed on the catalyst surface instead of oxygen-containing species. As seen in the results for themicroreactor, the main products of hydrazine decompo-sition are N2 and NH3 at 303 K. It is likely that thespecies remaining on the surface are NHx, which arebasically the intermediate species. These surface NHxspecies might block the active sites with an increase ofreaction time, and as a result, the activity woulddecrease. After the N2 flushing at 773 K, the adsorbedNHx species were removed, and consequently, the activ-ity was recovered. Therefore, the NHx species formedon the catalyst causes the deactivation of the catalystat lower temperatures. With higher Mo loading, thecatalyst has more active sites, a longer time beingrequired to block them all. This is an indication thatthe surface NHx species can be quickly removed atrelatively high temperatures so that the catalyst is nolonger deactivated.

NH3 TPD. Figure 10 shows the NH3 TPD profiles ofγ-Al2O3, Ir/γ-Al2O3, and fresh 26.9 wt % R-Mo2C/γ-Al2O3catalysts recorded by mass spectroscopy. The curves ofmass 2 (H2), 16 (NH3), and 28 (N2) versus temperatureare shown in Figure 10. The NH3 desorption appears

at a temperature around 370 K for the γ-Al2O3 support.No N2 and H2 desorption peaks are observed. NH3desorbs from Ir/γ-Al2O3 catalyst from 330 to 430 K. Thedesorption of N2 commenced in the temperature regionof 500-700 K; meanwhile, a H2 desorption peak ap-peared. Therefore, the desorption peaks of N2 and H2are attributed to NH3 decomposition on Ir/γ-Al2O3catalyst. For fresh 26.9 wt % R-Mo2C/γ-Al2O3 catalyst,an NH3 desorption peak appears at 370 K. A major H2desorption peak appears in the temperature region of550-750 K. N2 desorption is observed together with H2desorption. The H2 and N2 peaks are attributed to aresult of NH3 decomposition at elevated temperatures.In other words, there are active sites for NH3 decom-position on both the Ir/γ-Al2O3 and R-Mo2C/γ-Al2O3catalysts.

Figure 9. Comparison of the conversion of hydrazine decomposi-tion over fresh R-Mo2C/γ-Al2O3 catalyst and postreaction R-Mo2C/γ-Al2O3 catalyst after a N2 flush at 773 K for 180 min.

Figure 10. NH3 TPD profiles (mass spectrometry) of γ-Al2O3, Ir/γ-Al2O3, and 26.9 wt % R-Mo2C/γ-Al2O3 catalysts.

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6045

From the results of NH3 TPD, two types of adsorbedNH3 species are formed on the Ir/γ-Al2O3 and freshR-Mo2C/γ-Al2O3 catalysts. One is weak adsorption at 370K both on Ir/γ-Al2O3 and on fresh R-Mo2C/γ-Al2O3catalysts. Another type is strongly bonded NH3 on Ir/γ-Al2O3 and fresh R-Mo2C/γ-Al2O3 catalysts. This kindof strongly adsorbed species decomposes to N2 and H2in the temperature region of 550-750 K over R-Mo2C/γ-Al2O3 catalyst, while the strongly adsorbed NH3 onIr/γ-Al2O3 catalyst begins to dissociate into N2 and H2in the temperature region of 500-700 K. NH3 candecompose into N2 and H2 on Ir/γ-Al2O3 catalyst moreeasily than on fresh R-Mo2C/γ-Al2O3 catalyst. This resultagrees well with the reaction results obtained with themicroreactor.

As mentioned before, the main products of hydrazinedecomposition are NH3 and N2 below 673 K in themicroreactor. In the temperature region of 673-823 K,the H2 selectivity increases dramatically. When thetemperature is up to 823 K, the selectivity of the catalystis 100%. The main products are H2 and N2. NH3 TPDresults for the fresh R-Mo2C/γ-Al2O3 catalysts areconsistent with the results of hydrazine decompositionin the microreactor. These results suggest that hydra-zine decomposes to NH3 and N2 in the low-temperatureregion (below 673 K), and the produced NH3 decomposesat high temperatures. This result agrees with that ofBrayner et al.; at 333 K, the reaction proceeds by route2 on niobium nitride catalyst.20 The mechanisms ofhydrazine decomposition over Ir/γ-Al2O3 and freshR-Mo2C/γ-Al2O3 catalyst seem to be similar.

4. Conclusions

The catalytic activities of R-Mo2C/γ-Al2O3 catalysts forhydrazine decomposition are close to that of Ir/γ-Al2O3catalyst in both continuous and pulsed injection ofhydrazine in the thruster. The initial activities of theR-Mo2C/γ-Al2O3 catalysts increase during the early stageof hydrazine decomposition due to the reduction ofpassivated oxycarbide.

A series of fresh R-Mo2C/γ-Al2O3 catalysts with dif-ferent Mo loadings were thoroughly studied using amicroreactor for the first time. The activity of passivatedR-Mo2C/γ-Al2O3 catalyst is much lower than that offresh R-Mo2C/γ-Al2O3 catalyst. It is found that thehydrazine decomposes via the same pathway on theR-Mo2C/γ-Al2O3 catalyst and Ir/γ-Al2O3 catalyst. Below673 K the main products of hydrazine decomposition areN2 and NH3; when the temperature is elevated to 673K, the hydrazine decomposes to N2 and NH3 and thenthe produced NH3 further decomposes into N2 and H2.This result shows that the NH3 decomposition is a slowstep of the hydrazine decomposition.

Acknowledgment

This work was supported by a grant from the NaturalScience Foundation of China (NSFC) for OutstandingYouth (No. 20325620). We thank Professor Malcolm L.H. Green (University of Oxford) for helpful discussionand careful revision of this paper.

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Received for review November 21, 2003Revised manuscript received April 25, 2004

Accepted July 6, 2004

IE034262W

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6047