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Surface Science 600 (2006) 461–467

Cluster size effects on hydrazine decomposition on Irn/Al2O3/NiAl(110)

Chaoyang Fan, Tianpin Wu, William E. Kaden, Scott L. Anderson *

Department of Chemistry, University of Utah, 315 S. 1400 E. Rm 2020, Salt Lake City, UT 84112-0850, United States

Received 18 July 2005; accepted for publication 24 October 2005Available online 7 December 2005

Abstract

A series of planar model catalysts were prepared by deposition of size-selected Irþn on Al2O3/NiAl(110), and hydrazine decompositionchemistry was used to probe their size-dependent chemical properties. Small Irn (n 6 15) on Al2O3/NiAl(110) are able to induce hydra-zine decomposition at temperatures well below room temperature, with significant activity first appearing at Ir7. Both activity and prod-uct branching are strongly dependent on deposited cluster size, with these small clusters supporting only the simplest decompositionmechanism: dehydrogenation and N2 desorption at low temperatures, followed by H2 recombinative desorption at temperatures above300 K. For Ir15, we begin to see ammonia production, signaling the onset of a transition to clusters able to support more complexchemistry.� 2005 Elsevier B.V. All rights reserved.

Keywords: Iridium; Catalysis; Hydrazine; Surface chemical reaction; Nanocluster

1. Introduction

Decomposition of hydrazine on metal surfaces and sup-ported metal particles is important in monopropellantthrusters and gas generators, and potentially interestingas a source of pure hydrogen for fuel cells. Catalysts forthruster and gas generator applications must be stable athigh temperatures, and are typically refractory transitionmetals dispersed on a refractory oxide support. Iridium dis-persed on a high surface area aluminum oxide support isthe benchmark commercial catalyst (Shell 405). Issues ofimportance include particle sintering, support degradation,and activity over a wide temperature range. Even thoughthe thrusters and gas generators operate at high tempera-tures, it is important to have good low temperature activityto insure smooth cold starts in intermittent operation inspacecraft environments. As part of a program to explorefundamental properties of such catalysts, we are studyingsize-selected Ir particles supported on planar epitaxialAl2O3 supports. Here we report hydrazine decomposition

0039-6028/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.susc.2005.10.054

* Corresponding author. Tel.: +1 801 585 7289; fax: +1 801 581 8433.E-mail address: anderson@chem.utah.edu (S.L. Anderson).

results under temperature-programmed desorption (TPD)conditions, for Irn, n < 15.

There are a number of studies in the literature that pro-vide interesting points of comparison with this work.Hydrazine decomposition has been studied under UHVconditions for metal surfaces such as Ir(111) [1],Ru(0001) [2], Pd(100) [3], Pt(111) [4], and Ni(111) [5].There are also a number of temperature-programmeddesorption studies of NH3 and H2 on various bulk metalsurfaces that provide important insights for interpretationof the hydrazine results [6–9]. It would be nice to compareour results for Irn/Al2O3/NiAl with results for real highsurface area catalysts. The only detailed work on hydra-zine/Ir/Al2O3 high surface area catalysts is that of Falconerand Wise [10], who studied TPD and reactions under stea-dy flow conditions, but only above room temperature andfor hydrazine exposures much higher than in our experi-ments. We previously reported a study of TPD of hydra-zine on a model catalyst prepared by depositing enoughIr on a planar Al2O3/NiAl(110) support to grow some dis-tribution of non-size-selected Ir clusters [11], and foundchemistry qualitatively similar to that on bulk metal sur-faces. We also reported preliminary results for reaction of

462 C. Fan et al. / Surface Science 600 (2006) 461–467

a few size-selected Irn under steady state conditions. Thechemistry observed here for TPD from small Irn is quitedifferent from that seen for bulk metals and the high cover-age model catalyst, indicating that both activity and prod-uct branching are strongly affected by cluster size in thissystem.

2. Experimental methodology

The experiments were carried out in a UHV chamber(base pressure �2 · 10�10 Torr) that has been describedpreviously [11,12]. The chamber is attached to the end ofa mass-selected, ion deposition beam line that is used to de-posit Irþn on the sample surface. Irþn is generated by laservaporization of a rastering Ir target, with the resulting Irvapor confined and cooled by a pulsed helium flow. Cat-ions exiting the source are transported and mass-selectedby a series of quadrupole ion guides, and deposited at anenergy of 1 eV/atom through a 2 mm exposure mask posi-tioned just in front of the sample surface. The system con-tains facilities for in situ sample preparation, andcharacterization by X-ray photoelectron spectroscopy(XPS), Auger electron spectroscopy (AES), and ion scatter-ing spectroscopy (ISS). The XPS measurements used anAlKa source and XPS binding energies were calibratedusing the O and Al peaks of the Al2O3/NiAl substrate.

The sample substrate is a 7 · 7 mm NiAl(110) singlecrystal (Surface Preparation Laboratory), spot welded toa pair of tantalum heating wires, and suspended by tung-sten rods from a liquid nitrogen cryostat. The sample tem-perature can be controlled in the range from �90 K to>1300 K. Sample temperatures were measured by a K-typethermocouple spot welded to the back of the NiAl(110)crystal. The substrate for these experiments was an aluminafilm grown epitaxially on the NiAl(110) crystal using aprocedure described by Kulawik et al [13]. The methodhas been shown to give alumina films �5 A thick, withgood local order and a structure similar to that of c-Al2O3 [14–17]. In brief, the NiAl(110) single crystal wasinitially cleaned by repeated cycles of Ar+ bombardmentand 1270 K annealing. Before each experiment, the NiAlcrystal was Ar+ sputtered to remove deposited Ir, then an-nealed to 1270 K for 5 min in UHV. The Al2O3 film wasgrown by exposure to 1200 L of O2 at a sample tempera-ture of 550 K, followed by annealing at 1070 K for 5 min.To insure that there were no open patches in the Al2O3

layer, the oxidation treatment was repeated. The as-pre-pared Al2O3 thin film was characterized with XPS andISS, the latter showing that the film is continuous, withno exposed Ni atoms. (To avoid sputter damage, the ISSanalysis was not done on samples used in the depositionexperiments.) After the Al2O3 thin film was prepared, Irþnclusters were deposited on the thin film at room tempera-ture, to a density equivalent to 10% of a close packed Irmonolayer (1.6 · 1014 atoms/cm2). The sample was thencooled to 100 K for dosing hydrazine prior to study bytemperature-programmed desorption.

The main UHV chamber has a mass spectrometer tomonitor gas composition, and a second, differentiallypumped mass spectrometer is used to monitor speciesdesorbing from the sample. This spectrometer views thesample through a 3 mm aperture at the end of a skimmercone. We have found [18] that species desorbing from thecluster-containing spot are detected substantially more effi-ciently than those from the surrounding substrate area,probably because of additional collimation from the ion-izer geometry. One serious problem is that hydrazinedecomposes on internal surfaces of the mass spectrometer,creating background at the product masses. To minimizethis background, the differential pumping skimmer cone,including all wall surfaces in the vicinity of the ionizer, iscooled to �87 K by flowing liquid nitrogen. In addition,hydrazine cracks in the mass spectrometer ionizer yieldingions at the masses of interest for the decomposition reac-tion on the surface, thus it is critical to have an accuratemethod for subtracting this background.

In this work, hydrazine was introduced through an in-ert, pulsed inlet system [19], to minimize decompositionof hydrazine in the inlet system. At the beginning of eachday, the liquid hydrazine sample (N2H4, 98.5% Alpha, orN2D4, 95%, C/D/N, Isotopes Inc.) was purified with sev-eral freeze–pump–thaw cycles using both liquid nitrogenvapor and dry ice/acetone baths to freeze N2H4 (m.p. =275 K) while maintaining substantial vapor pressure forthe NH3 decomposition product (b.p. = 240 K). Duringuse, the hydrazine container is kept in an ice–water bathto maintain constant vapor pressure of �4.5 Torr [20].Even though the inlet system is constructed entirely ofglass, teflon, and perfluoroalkoxy materials, there is stillsome hydrazine decomposition. Before each sample dosingoperation, the hydrazine is additionally purified by repeat-edly evacuating the head space of the hydrazine containerto preferentially pump away the higher vapor pressureNH3, N2, and H2 decomposition products. With theseprecautions, the purity of the delivered hydrazine is excel-lent [19].

3. Results and discussion

Before analyzing the decomposition of hydrazine on Irn/alumina, it is useful to review the TPD behavior of hydra-zine from an inert surface. The inset to Fig. 1 shows a seriesof hydrazine TPD spectra from a clean (Ir-free) alumina/NiAl(110) surface. For unit sticking probability, �1.8 Lexposure results in deposition of a monolayer [11]. The firstmonolayer desorbs in a broad peak around 181 K. Forcoverages between 1 and �3 ML, the monolayer desorp-tion peak is preceded by a sharp peak (153 K) assignedto desorption of multilayer (i.e., mostly 2nd layer) hydra-zine. For thicker films, a third peak at 166 K appears, be-tween the multilayer and monolayer peaks. The smooth0th order desorption behavior expected for evaporationof a structureless multilayer film is observed only for thickfilms (>10 ML). The origin of the 166 K desorption feature

Fig. 1. Inset: Coverage-dependent TPD spectra of hydrazine from cleanAl2O3/NiAl(110). Main figure: TPD of hydrazine from Ir7/Al2O3/NiAl(110). Data points show the intensities for masses corresponding toN2H4, N2, NH3, and H2. Solid traces through the N2, NH3, and H2 spectrashow the ionizer cracking contribution to each mass.

C. Fan et al. / Surface Science 600 (2006) 461–467 463

is not entirely clear, but it may indicate either an phasetransition within the film during the heating process [21],or possibly some layer structure in the multilayer hydrazinefilm [22]. For our purposes, the important point is thatmultilayer hydrazine desorbs intact, even from Ir/alumina[11], while monolayer hydrazine may decompose. The mul-tilayer hydrazine, thus, provides an internal standardneeded to correct for background at product masses origi-nating from cracking of hydrazine in the mass spectrometerionizer.

3.1. Decomposition during TPD

The main figure shows desorption of hydrazine and itsdecomposition products from a sample prepared by depos-iting 0.1 ML equivalent of iridium in the form of Irþ7 .Hydrazine was dosed at 100 K sample temperature, andthe exposure was set to produce approximately 2 MLhydrazine coverage. The sample temperature was thenramped at 3 K/s, while monitoring masses correspondingto hydrazine, NH3, N2, and H2. The top trace showsN2H4 signal vs. temperature, and is quite similar to thatfor desorption from clean alumina for similar hydrazine

coverage. This is to be expected, because our Ir coverageis only 0.1 ML, so that even in the monolayer, most hydra-zine is adsorbed on Ir-free regions of the alumina substrate.

For the decomposition products, there are contributionsfrom both desorption of decomposition products, andfrom ionizer cracking of intact hydrazine desorbing fromthe surface. If no decomposition occurs on the surface,then the TPD spectral shape for the products should simplytrack that of the hydrazine parent. Consider the NH3 prod-uct. There is a sharp peak at 153 K, attributed entirely toionizer cracking of intact multilayer hydrazine desorbingfrom the surface. If we scale the hydrazine spectrum tomatch the intensity of this multilayer peak, then it can beseen that the entire NH3 spectrum (data points) simplytracks the scaled hydrazine signal (solid curve), indicatingthat hydrazine ionizer cracking is the only significantsource of mass 17 signal.

In contrast, for N2, the shape and intensity of the mono-layer desorption component of the spectrum (data points)is substantially different than what would be expected ifionizer cracking were the only source of mass 28 signal(solid curve). Clearly, there is significant N2 desorptionfrom the sample, for temperatures ranging from just afterthe multilayer desorption peak to �400 K. Finally, forH2, the two signal components observed below room tem-perature can be mostly attributed to ionizer cracking ofdesorbing N2H4, but there is a broad H2 signal componentfrom �300–550 K that must result from H2 desorptionfrom the sample. In this figure the intensities have not beencorrected for the variation in electron impact ionizationefficiency with product mass. For the ionizer conditionsused, the H2 intensity should be multiplied by �2.3 to bringit into proper scale with respect to the N2 product, and thisgives approximately the 1:2 N2:H2 stoichiometry expectedfor complete decomposition of N2H4. Note that the highernoise levels for N2 and H2, relative to N2H4 and NH3, are aconsequence of the relatively high background at thesemasses (i.e. CO and H2) typical of UHV systems. For someexperiments, we also used N2D4 to better observe thehydrogen product (i.e., D2).

It is interesting to compare these results with what is ob-served in analogous experiments with a ‘‘high coverage’’model catalyst prepared by deposition of 0.5 ML of Ir+

on Al2O3/NiAl(110), where some distribution of largerclusters was assumed to form by agglomeration [11]. Inthose experiments, all three products (N2, H2, and NH3)are observed, and each product has both a relatively sharplow temperature feature (<250 K), and a broad high tem-perature component (>350 K). This behavior is similar towhat is seen in hydrazine TPD from bulk Ir [1], Ru [2],Pd [3], and Rh [23] surfaces. From comparison with theseTPD results for hydrazine, and TPD of NH3 and H2 onvarious bulk metal surfaces [6–9], the sharp low tempera-ture desorption features were attributed to primary hydra-zine decomposition reactions, while the high temperaturefeatures are clearly the result of recombinative desorptionof Hads, Nads, and NHx,ads species left on the surface by

Fig. 2. N2 desorption signal corrected for ionizer cracking background.Vertical lines indicate temperatures of multilayer (153 K) and monolayer(181 K) hydrazine desorption features.

464 C. Fan et al. / Surface Science 600 (2006) 461–467

the primary decomposition chemistry. In contrast, thehydrazine chemistry for the Ir7/Al2O3(110) sample is muchsimpler. After accounting for ionizer cracking, no NH3

desorption is observed, N2 is observed only at low temper-atures, and H2 is observed mostly at high temperatures.These results suggest a quite simple decomposition mecha-nism for N2H4 on Ir7/Al2O3:

LowT : N2H4;ads ! N2 " þ4Hads

HighT : 2Hads ! H2 "

The low temperature N2 feature for Ir7 is quite broad,however, with a tail extending to �300 K, suggesting thatthe low temperature N2 production mechanism may in-volve several processes. For example, the dehydrogenationreaction ultimately generating N2 + 4Hads may passthrough various N2Hx intermediates, and there may besome fission to NHx with recombination as well (thoughnone resulting in NH3 desorption). The fact that N2 contin-ues to evolve well above the temperature where N2H4

desorption stops (300 K), indicates that at least some ofthe N2 is produced by secondary reactions of species lefton the surface after all the intact N2H4 has desorbed.The absence of a high temperature N2 recombinativedesorption feature for the Ir7/alumina sample indicatesthat there are no Nads or NHx,ads species left on the surfaceat temperatures above 300 K, unlike the situation for thehigh coverage catalyst or for bulk surfaces.

For H2, in contrast, it is the low temperature featurethat is missing in the Ir7/alumina results (or at leaststrongly attenuated compared to the 0.5 ML Ir/Al2O3/NiAl(110) sample). This observation indicates that the pri-mary N2H4 decomposition chemistry generates Hads that ismostly stable with respect to recombination until tempera-tures >300 K are reached.

Low temperature ammonia production on the modelhigh coverage catalyst [11] occurred in a sharp feature at215 K, similar to what is observed in hydrazine TPD onbulk metal surfaces [1,23]. Several reactions generatingNH3 can be proposed:

N2H4;ads + 2Hads ! 2NH3 ", orNH2;ads +Hads !NH3 "

where Hads and NH2,ads are generated by decomposition ofN2H4. The absence of ammonia desorption for the Ir7/alu-mina sample indicates that for small clusters, such reac-tions are not possible. The absence of a high temperatureN2 recombinative desorption feature is also consistent withno NHx,ads being generated in the decomposition mech-anism.

Fig. 2 shows the N2 signal observed for a series ofanalogous experiments performed on samples preparedby deposition of Irþn ; n ¼ 1; 3; 5; 7; 10; 15, on Al2O3/NiAl(110). In this figure, the contribution from ionizercracking has been subtracted by scaling the N2H

þ4 signal re-

corded during each TPD run such that the multilayerdesorption peak intensity matches the Nþ

2 signal from themultilayer desorption, then subtracting the scaled N2H

þ4

spectrum from the Nþ2 spectrum. For comparison, we also

include an analogously corrected N2 TPD spectrum forhydrazine on the clean (Ir-free) Al2O3/NiAl(110) sub-strate. The data shown for Ir10 and Ir15 were taken usingN2D4. The vertical lines in the figure indicate the positionsof the multilayer and monolayer hydrazine desorptionfeatures.

Note that there is a small amount of N2 production onthe Al2O3 substrate, i.e., the Nþ

2 :N2Hþ4 intensity ratio is

slightly higher during desorption of monolayer N2H4, rela-tive to the multilayer. This observation indicates that theAl2O3/NiAl(110) film is not completely inert, howeverthe relatively low N2 intensity compared to those for the0.1 ML Irn/Al2O3/NiAl(110) samples, suggests that thedecomposition is probably limited to reaction at a low den-sity of defects inevitably present in such epitaxial films (e.g.domain boundaries [13,24]). The N2 intensities for samplesprepared with Ir1, Ir3, and Ir5 are only slightly greater thanthat for the Ir-free Al2O3 film, indicating that the Ir in thesesamples is rather unreactive. In contrast, for samples pre-pared with Ir7, Ir10, and Ir15, the N2 production activityis substantially higher than on the alumina film, indicatingthat these cluster sizes have significant activity for hydra-zine decomposition. In addition to indicating an activityonset between Ir5 and Ir7, this pattern also shows thatwhatever diffusion/agglomeration processes occur in thesesamples is not sufficient to wipe out the dependence ondeposited cluster size, at least for temperatures up to�300 K, where most of the hydrazine decomposition andN2 desorption has occurred.

It should be noted, however, that the heating used inTPD does alter the samples significantly. This effect is mostobviously seen by simply running sequential TPD experi-ments on the same sample. Take the Ir7 results in Fig. 1as an example. In the second TPD run on this sample,the N2 production is reduced by a factor of �2.5. Similar

Fig. 3. O1s XPS after TPD for all samples.

C. Fan et al. / Surface Science 600 (2006) 461–467 465

decreases are seen for all cluster sizes. The most likelyexplanation is that the initially highly dispersed Ir clustersare sintering at the high temperatures reached in the firstTPD run, resulting in large Ir particles. Samples containingonly large particles would have a substantially lower den-sity of exposed Ir sites, and thus activity is reduced sub-stantially. It is also possibly that some Ir is diffusingsubsurface, however, Libuda et al. have reported [25] thatthis process is significant only for temperatures above ourmaximum TPD temperature (600 K), at least for Pt. It isinteresting to compare the sintering behavior of Ir/Al2O3/NiAl(110) with sintering experiments for Aun/Al2O3/NiAl(110) [26]. For Au, significant sintering was observedto occur during deposition at room temperature, whereasfor Ir, the size-dependent TPD results suggest that sinteringis significant only when the sample is heated. Given themuch higher melting point of Ir (2683 K vs. 1338 K forAu), it is perhaps not surprising that Ir requires higher tem-peratures to activate diffusion and agglomeration.

The shapes of the corrected desorption features in Fig. 2provide some additional insight into the decompositionmechanism. For all samples, and particularly for thosewith active size Irn (n = 7, 10, 15), there is a sharp onsetof N2 desorption at a temperature just above that wheredesorption of the multilayer hydrazine film is complete.(The derivative line shape seen for some spectra at the po-sition of the multilayer desorption peak (153 K) resultsfrom error in subtracting this sharp feature in the ionizercracking background.) The sharp onset indicates eitherthat hydrazine decomposition in the monolayer film is trig-gered by desorption of the multilayer, or that some hydra-zine decomposition has occurred at lower temperatures,with N2 desorbing as soon as the overlayer is gone. Ineither case, it is clear that these small clusters are activefor hydrazine decomposition/N2 desorption at tempera-tures below 200 K. In contrast, for the model catalyst pre-pared with 0.5 ML Ir deposition [11], the low temperatureN2 desorption peaks sharply at 215 K, although there issome N2 desorption at lower temperatures. N2 evolutioncontinues up to �300 K—well after intact desorption ofboth multilayer and monolayer hydrazine is complete(�220 K), indicating that some NxHy species remains onthe surface, continuing to generate N2.

The TPD behavior for the H2 product is more problem-atic to interpret. For all Irn/Al2O3 samples, H2 desorptionis observed in a broad feature running from �300 to 550 K(Fig. 1). Closer examination suggests that this feature hastwo components—one peaking around 350 K, and onepeaking around 475–500 K. In hydrazine TPD from theIr-free alumina film, we do not observe H2 in the tempera-ture range below 400 K, but there is H2 desorption at high-er temperatures with intensity comparable to that observedfor T > 400 K for the Irn/Al2O3 samples. We conclude thatthe H2 desorption from 300–400 K results from hydrazinedecomposition on Irn, but the desorption at higher temper-atures is from hydrazine decomposition on the aluminafilm. The actual hydrazine decomposition occurs at low

temperatures (Fig. 2), and evidently the recombinativedesorption of H2 occurs at lower temperatures on Irn/Al2O3, compared to Ir-free Al2O3. Interpretation of thishigh temperature desorption behavior is complicated bythe fact that the Ir clusters are probably sintering duringheating through this temperature range, as discussedabove.

3.2. Reactions with the alumina support

To further probe the effect of deposited Irn size on sam-ple properties, X-ray photoelectron spectra (XPS) were re-corded for each sample. Unfortunately, the Ir XPS isdifficult to analyze quantitatively because the Ir coverageis low, and the most intense Ir transitions (4d and 4f) havesubstantial interference from Al and Ni transitions of thesubstrate. About all we can say is that the apparent posi-tion of the Ir4d5/2 feature is at �298 eV—�1 eV higherthan the 297 eV binding energy reported for bulk Ir [27].A shift to higher binding energy is expected for small Irclusters, because of reduced final state screening in smallerclusters. Unfortunately the combination of low Ir signaland interference from an Al2p MgKa ghost line makes itdifficult to tell if there is any shift with deposited cluster sizefor these samples.

The O1s region of the XPS taken after TPD is plottedfor all samples in Fig. 3. Prior to TPD, the O1s binding en-ergy is independent of deposited cluster size. Note that thepeaks observed for the samples prepared by deposition ofIr1, Ir3, and Ir5 are consistently shifted �0.25 eV to higherbinding energy compared to the samples prepared with Ir7,Ir10, and Ir15. In other words, the final average O chemicalenvironment is slightly different for samples prepared withclusters that are active or inactive for hydrazine decompo-sition, presumably because the more active clusters induceproduction of some hydrazine decomposition product thatreacts with the alumina film. The two obvious possibilities

Fig. 4. TPD of ammonia (ND3) following N2D4 exposure to samplesprepared with Ir10 and Ir15, showing raw data and signal remaining aftersubtraction of ionizer cracking contribution.

466 C. Fan et al. / Surface Science 600 (2006) 461–467

are reaction with hydrogen to partially hydroxylate the sur-face, or reaction with nitrogen to generate some AlxOyNz

species. McCafferty and Wightman report [28] that theO1s binding energy for OH groups on alumina films areshifted �1.2 eV to higher binding energy relative to the alu-mina O2� centers. In our earlier work on TPD of hydrazinefrom the 0.5 ML Ir/Al2O3/NiAl(110) model catalyst, N1sXPS showed a small amount of Ir-induced nitrogen uptakeby the substrate, forming a nitride-like compound. In thepresent work, no distinct N1s signal is observed in thepost-TPD analysis, but this is not unexpected, becausethe Ir coverage is five time smaller. It is not clear what effectnitrogen incorporation into the alumina film would haveon the O1s binding energy, however, the very low(398 eV) N1s binding energy indicates that the nitrogen iselectron rich, and this tendency to withdraw electron den-sity should raise the binding energies of other elements inthe film. Therefore, the slight O1s binding energy shiftcould result from incorporation of either H or N into thefilm in small concentrations.

3.3. Cluster size effects

Two different types of cluster size effects are observed inthis study. There is a significant increase in hydrazinedecomposition activity as the deposited cluster size is in-creased from Ir5 to Ir7, as shown by the N2 TPD resultsin Fig. 2. A more dramatic size effect can be inferred bycomparing the results for the present Irn/Al2O3 samples(e.g. Fig. 1) with analogous results for the high coverage0.5 ML Ir/Al2O3 sample [11], or for bulk Ir surfaces [1].As outlined above, the decomposition chemistry is muchsimpler for the small, size-selected clusters, with no NH3

production, no N2 recombinative desorption at high tem-perature, and little if any H2 production below roomtemperature.

There are several ways of thinking about the size depen-dence of both activity and the decomposition mechanism.The onset of activity may relate to changes in the Irn elec-tronic properties with increasing cluster size, and it wouldbe interesting to examine such effects by valence photo-emission and/or theory. It is also likely that both activityand mechanism are affected by the number and type of sitesavailable for binding both hydrazine and various reactionintermediates and products. Consider N2 production. Forthe recombinative desorption mechanism to occur, the ini-tial hydrazine decomposition must break the N–N bond togenerate separated NHx species that subsequently dehydro-genate and recombine to generate both N2 and H2. The ab-sence of this chemistry for samples with small Irn maysimply result from the clusters being too small to bind sep-arated NHx fragments. Instead, the nitrogen is lost as N2 atlow temperatures, and only Hads is left on the surface torecombine at high temperatures.

A similar argument may account for absence of signifi-cant ammonia production at any temperature. As outlinedabove, any mechanism for NH3 production will require

either reaction of one hydrazine molecule with Hads gener-ated in decomposition of a neighboring molecule, orrecombination of NH2 with Hads. The first mechanismcan only occur on clusters with enough binding sites to sta-bilize at least two hydrazine molecules and their decompo-sition products. The second pathway might occur with onlya single hydrazine parent, but still requires enough bindingsites to accommodate NH2, Hads, and also the NHads orNads + Hads produced in concert with the Hads reactant.Evidently, the small clusters simply cannot accommodatethis chemistry.

As cluster size increases, at some point we should beginto see more complex chemistry, and the most sensitiveprobe is probably ammonia production, because of lowbackground in the mass spectrometer. Fig. 4 shows that,indeed, a small amount of ammonia is generated for thelargest cluster size studied here: Ir15. The figure comparesND3 production from N2D4 TPD from Ir10/Al2O3 andIr15/Al2O3 samples. The raw NDþ

3 signal is shown forIr10, and the two upper traces show the NDþ

3 signals forboth Ir10 and Ir15 after subtracting the background fromionizer cracking of desorbing N2D4. Clearly for Ir10, thereis no significant ammonia production, however, for Ir15there is a small ND3 desorption feature visible between�250 and 350 K. We conclude that Ir15 is just at the limitwhere the complex chemistry observed on large Ir particlesand bulk surfaces begins to be possible.

We previously reported a study where Irn/Al2O3/NiAl(110), n = 1, 5, 7, 10, model catalysts were used todecompose hydrazine under steady state conditions, i.e.,constant temperature and 5 · 10�8 Torr of hydrazine inthe chamber background. These conditions resulted in highand rising background of hydrazine decomposition prod-ucts from wall reactions, but by moving the sample so that

C. Fan et al. / Surface Science 600 (2006) 461–467 467

the mass spectrometer alternately viewed the Ir-containingspot, and an adjacent region of Ir-free alumina, it was pos-sible to observe signal attributable to Ir-catalyzed chemis-try. It is interesting that under these conditions, ammoniawas observed at Tsurface = 300 K and 400 K. There are sev-eral reasons why the product branching might be differentfor these experiments, compared to the present TPD re-sults. The NH3 production probably resulted from reactionof impinging N2H4 with Hads (or NHx,ads) already presenton the surface from previous decomposition reactions.Note that the temperatures studied were low enough thatreaction intermediates like Hads, NHx,ads, etc. would be sta-ble on the surface to react with impinging hydrazine. It isalso possible that the clusters sintered to large clustersduring the �60-min reaction period, although the presentresults suggest that sintering is not facile at room tempera-ture, at least in the absence of adsorbates. We plan to studythese effects with our new inlet system, using pulsed hydra-zine dosing to improve the signal/background ratio.

4. Conclusions

We have shown that small Irn (n 6 15) on Al2O3/NiAl(110) are able to induce hydrazine decomposition attemperatures well below room temperature. Both activityand product branching are strongly dependent on depos-ited cluster size, with the small clusters supporting onlythe simplest decomposition mechanism: dehydrogenationand N2 desorption at low temperatures, followed by H2

recombinative desorption at temperatures above 300 K.For Ir15, we begin to see ammonia production, signalingthe onset of a transition to clusters able to support morecomplex chemistry.

In future work we plan to examine the hydrogen chem-istry in greater detail, and to try to passivate defects in theAl2O3/NiAl(110) film apparently responsible for spuriousH2 desorption features at high temperatures.

Acknowledgment

This work was supported by the US Air Force Office ofScientific Research, under Grant F49620-03-1-0062.

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