Ultrafine nickel particles generated by laser-induced gas phase photonucleation

NanoSeuctured Materials, Vol. 8, No. 7, pp. 879-888, 1997 Elsevicr Science Ltd Q 1998 Acta Metallurgica Inc. Printed in the USA, All rights remed 09654773197 Sll.00 + .co

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PII SO965-9773(98)00016-6

ULtTRAFINE NICKEL PARTICLES GENERATED BY LASER-INDUCED GAS PHASE PHOTONUCLEATION H. He*, R.H. Heist*, B.L. McIntyre+ andT.N. Blantou* *Department of Chemical Engineering, University of Rochester Rochester, New York 14627-0166 t!Schoolof Engineering and Applied Sciences, University of Rochester Rochester, New York 14627 *AnalyticalTechnology Division, Eastman Kodak Company Rochester, New York 14652-3712 (AcceptedDecember 2,1997) Abstract -A W laser-assistedgas phase photonucleationprocess is used to generate nickelultr@neparticles (UFPs)fromambienttemperatureNi(CO)4precursor. The nanostructure and morphologyof the UFPs are characterizedby transmissionelectronmicroscopy(TEM) and X-ray dtffractometry(XRD). Using ethane hydrogenolysisto probe the reactivityof the nickel UFPs as heterogeneouscatalysts,wefind thattheseUFPs showhigh reactivityand extraordinary resistancetodeactivation.XRDanalysisrevealsa hexagonalNiJCphase in thoseUFPs thatwere used to catalyzethe ethane hydrogenolysisreactionin excess ethane. 01998Acta Metallwgica Inc.

INTRODUCTION Nickel ultrafimeparticles (UFFs)have attractedconsiderableinterest over the last decade or so due to interesting optical (1). magnetic (2) and chemical (3) properties that have been reported. The two general classes of techniques used to produce these particles am based on gas phase nucleation and growth of particles and liquid solution precipitation and growth of particles using appropriate precursors in each case. Regarding the former method, the more commonly reported methods for producing nickel particles include gas evaporation(4), vapor thermal decomposition, and laser dielectric breakdown (5) methods. Wereport here results of gas phase UV-light induced nucleation studies in which we induce particle formation under comparatively mild conditions (e.g., lower temperature) which differs from conditions typically used in those methods just mentioned. As such, we might expect to produce particles that exhibit properties that differ from those associated with particles produced using the more conventional, (usually higher temperature) methods. We note that similar observations of particle production under these ambient conditions have been reported in other studies of gas phase photochemical reactions (6,7). In this study we use UV laser-assisted gas phase photonucleation to generate nickel ultrafIne particles from commercially available Ni(CO)s precursor at room tempemture; and we use ethane 879

6

Mechanica

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He

Pump

Cold Trap

CI

Ethane Cylinder

I Quartz

Cell

Thermocouple Gauge

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Sampling by a syringe for GUMS 8&C

FL”,” (optional)

Laser Excimer

XeCl Excimer

Figure 1. Schematic of experimental setup with the cell photochemical reactor (CPR).

o-4

laser

UURAFINE NICKEL PARTICLES GENERATED BYLASER-INDUCED GASPHASE PtiomrwcmnoN

881

hydrogenolysis as a model reaction to probe the catalytic activity of these nickel UFPs. The objective of this investigation is to examine the morphology and catalytic activity of (unsupported) ultrafine nickel particles produced by nickel vapor nucleation and growth following the photodecomposition ojf ambient nickel carbonyl precursor (8).

EXPERIMENT

DE!KRII’TION

We describe here the use of a cell photochemical reactor (CPR), shown in Figure 1, for the laser induced decomposition of gas phase Ni(C0)4 to generate the nickel UFPs. The CPR is. a cylindrical optical cell 10 cm in length and4 cm in O.D. and is constructed using a quartz tube with commercial grade quartz windows fused to each end. ‘llvo standard 3/V’ (O.D.) quartz tubes attached to one: side of the CPR serve as ports for gas and vapor handling. One port is capped with a septum, and the other is connected to a high-vacuum stopcock using a Cajon Ultratorr fitting. In a typical photoinduced nucleation experiment, the reactoris first connected to the vacuum line and evacuated and then backfilled with background gas (e.g., helium or hydrogen) at least three times. The entire system consisting of the reactor, the vacuum line, and the Ni(CO)4 supply canister connector is then evacuated. The Ni(CO)4 canister valve is then opened slightly and vapor is allowed to flow into the cell, The canister valve is closed when the desired pressure of the vapor is reached. The reactor is isolated from the vacuum line and any vapor remaining in the vacuum line is collected in a dry ice - acetone cold trap for later processing. The sealed reactor is then detached from the vacuum line and mounted in the light path of an XeCl excimer laser (Lambda Physik LEXtra 50). The wavelength used in our experiments was 308 nm. The maximum laser repetition rate was 30 Hz and the maximum intensity was 200 mJ/pulse with a pulsewidth of 17 ns. The collimated laser beam was approximately 3 cm wide by 1 cm high and was passed co-axially through the cell where it was used to decompose the Ni(C0)4 precursor. UFPs were formed by theresulting nickel vapor nucleating and growing to nanoparticle size. The UFPs tended to aggregate as they continued growing, thus forming aggregates of these UFPs which, in tum, agglomerated due to the small amount of convective motion and mixing observed within the CPR to form the micron-size entities eventually visible as the solid deposit along the bottom and side of the CPR. Following the laser irradiation, the CPR was evacuated to remove residual gaseous material and backfilled with helium or hydrogen to prevent oxidation of the nickel particles. No pre-conditioning of the particles was carried out. A JBOL 2000FX transmission electron microscope (TBM) was used to analyze the nickel UFPs morphology and for (partial) structure determination. It was important, in order to study the particles as formed, to avoid exposure to air. When a TBM analysis was to be carried out, the CPR containing the particles was first evacuated and then backtilled with nitrogen. It was then moved to a glove box filled with nitrogen. Particle samples were transferred to a TBM carbon-coated, copper grid, which was then positioned on a TBM sample holder in the glove box. A plastic glove bag purged with nitrogen was used to protect the sample during transfer to the TBM. A second plastic glove bag was wrapped around the TBM at the entrance to the sample chamber and purged with flowing nitrogen. The well-isolated particle sample was then loaded into the TBM without exposing the sample to air. X-ray diffractometry (XRD) was used for crystal phase determination. Argon BET at liquid nitrogen temperatures was used to estimate the surface area of the UFPs. These two latter measurements were carried out ex situ.

882

H HE, RH HEIST,BL MCINTYRE, TN

(a)

BLANTON

(b)

Figure 2. TEM micrographs of the UFPs formed in the CPR under XeCl laser irradiation: (a) bright field image shows many small UFPs (black dots cl0 nm) making up aggregates; (b) dark field image shows that the crystal diffraction domains (white spots with sizes in the range 1 to 5 nm) are even smaller. When etbane hydrogenolysis was carried out to probe the reactivity of the nickel UFPs, the reactant mixture was backfilled into the evacuated CPR containing the UFPs immediately after they wereproduced. Reactant mixtures with various ethane/hydrogen ratios were used (i.e. C2He: H2 = 0.45.0.77 and 2.44) and a constant total pressure of 1000 Torr was maintained. The CPR was immersed in a heated sand bath and maintained at the desired reaction temperature (i.e. 255°C for the investigation described here). A type Tthermocouple was used to measure the wall temperature of the reactor. As the reaction proceeded, the gas composition was monitored as a function of residence time using a HPGC/MS (5890 Series II GC with 5970 MSD). A 100 micron-l Dynatech Pressure-I-ok syringe was used for gas sampling. Known amounts of argon in the mixture were used as an internal reference to quantify the product ion signals detected by the qua&pole mass spectrometer. Surface specific rates of methane formation and conversion were determined using the GCjIvlS data and the BET measurement of particle surface area. RESULTS TBM bright field imaging showed a mixed morphology for the ultrafme particles. Some particles appeared rather spherical while others appeared more fractal-like. Many of the UFP aggregates were smaller than 50 nm as is seen in Figure 2-a, and appeared to be made up of smaller ultrafine particles. Using TBM dark field imaging, we determined the size of these smaller

UCrwmE NICKELPARTICLES GENERATED BY ~ASER~NDUCED GASPHASEPtiomwxmnm

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Figure 3. Electron diffraction analysis of the UFPs: (a) SAD ring pattern is indexed to nickel fee stmcturc; (b) interplanarspacings for nickel UFPs vs. bulk nickel interplanar spacings; the measured values correspond to the ring radii R according to the equation &neasured = h L/R, where h is the camera constant. particles to be in the range of 1 to 5 nm as illustrated in Figure 2-b. The selected area diffraction (SAD) ring patterns, illustrated in Figure 3-a, were obtained using a 0” - 20” sample rotation and were indexed to a consistent lattice parameter in the range, a = 3.51- 3.54 A, which was assigned to the fee nickel (aNi@&= 3.5238A) as shown in Figure 3-b. Wo quite faintrings in the diffraction pattern are best matched by fee NiO (220) and (420). The presence of weak NiO diffraction is consistent with the observation (3,9,10)that fresh nickel particles are easily oxidized to form a

084

H HE, RH HEIST,BL MCINTYRE, TN BLANTON

Figure 4. X-ray diffraction spectra of the UFPs confirms nickel crystal phase and dual size distriiution analysis finds microcrystalline grains of 26 nm along with smaller sizes reflected by the peak tail broadening. TABLE 1 Dependence of CJ& Formation Rate and Conversion on C&&J2 Ratio C&i/Hz ratio

CH4 formation specific rate (molecules/cm2 * set)

conversion

0.45

4.85 x lOI

0.83

0.77

6.89 x 1013

0.80

2.44

9.98 x lOI

0.76

passivated surface layer. Some oxidation of particles was expected since, even though considerable care was exercised, the post-formation sample isolation was not perfect. We note that our freshly prepared nickel particles were pyrophoric. Sparks were observed when air was bled into the reactor containing fresh nickel particles initially under vacuum.

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Residence Time (Hr.) Figure 5. The CT&Imolecule production is seen to increase as the reaction proceeds and there is no significant drop in the rate for over 10 hours.

The X-ray diffraction spectrum shown in Figure 4 is consistent with our TEM electron diffraction results confirming the presence of the nickel crystal phase in the UFPs. The average size of the microcrystalline grams (domains) was estimated from the XRD spectra using the Scherrer formula. Dual size distribution analysis based on the Scherrer method showed that the nickel UFP ag:gregates had an average size of 20 _ 40 run with smaller crystal grains, e.g. c 6 nm, which is consistent with our TEM imaging results. Our BET measurements indicated that 3 mg of the nickel UFPs had approximately 0.11 m2 surface area which, after etbane hydrogenolysis at 255°C dropped to approximately 0.066 m2. The ethane hydrogenolysis catalyzed by these nickelUFPs resulted in more than 75% conversion when hydrogen was in slight excess in the reactant mixture. Our observed CIQ formation rates ranged from 4 x 1013 to 1 x 10” CH4 molecule&cc-cm2, which compare favorably with reported data for both supported and unsupported nickel catalysts (8). The data in Table 1 show that the conversion increases as more hydrogen is present in the reactant mix and that the reaction rate tracks the change in the Cm2 ratio. The data shown in Figure 5 illustrate that when there is excess hydrogen in the reactant mixhue, these reaction rates last for more than 10 hours before conversion ceases (also due, in part, to the decrease in reactant concentration in the CPR).

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H HE, RH HEIST,BL MC~NWIE,TN BLANTON

DISCUSSION The gas phase Ni(CO)4 optical absorption spectrum peaks at about 205 nm with a long tail extending to about 380 nm (7). Although higher energy W irradiation would probably result in more efficient loss of CO ligands (ll), it is probable, in our experiments, that a coherent multiphoton photodissociation process occurred (12,13), resulting in the cleaving of all four CO ligands from the Ni core under intensive 308 nm irradiation and giving rise to nickel and carbon monoxide as products of the gas phase decomposition: Ni(C0)4 + h ~308 + Ni + 4CO

VI

Once formed, the Ni atoms nucleate and grow to form nanoscale UFPs. These growing particles may also interact as they continue growing, forming aggregates of the UFPs. These aggregates may, in turn, agglomerate due to the slight convective motion and mixing observed in the reactor during irradiation, forming the larger entities eventually visible along the bottom and sides of the CPR. Although carbon monoxide does not absorb light at 308 nm (14), the gas phase in the CPR experiences a moderate temperature change due to the absorption of light and decomposition of the precursor and absorption of light by the metal. It is not completely clear what effect the W irradiation has on the nickel atoms and the nickel particles, particularly during the initial stages of nucleation and growth. Both electron diffraction and X-ray difBaction confirm the presence of the nickel crystalline phase in the UFPs. Also, the EDXA spectra exhibit dominant nickel peaks with only small carbon signals. This suggests that this method of generating nickel nanoparticles has an advantage over the Coz laser-initiated dielectric breakdown method since the latter typically results in an impurity phase of Ni3C in the deposited metal film (5). Data listed in Table 1 summarize results of the ethane hydrogenolysis studies. These data illustrate how the conversion (to methane) and the reaction rate vary with initial reactant concentration (ethane/hydrogen ratio). We observe that the reaction rate drops as the hydrogen concentration increases, which may be explained by competition of ethane with hydrogen for active surface sites. This observation is consistent with results from other well-documented heterogeneous catalysis studies (15). The data in Table 1 also reveal that the conversion increases with hydrogen concentration. This is also consistent with the well-known ability of hydrogen to remove surface carbonaceous deposits from the reaction, thus keeping the surface relatively clean. The results of this investigation illustrate that the nickel UFPs, when catalyzing ethane hydrogenolysis, behave in a manner consistent with the traditional supported and unsupported nickel catalysts. A somewhat more unusual property of these ultrafine particles is theirrather long activity as illustrated in Figure 5. Although the reactant concentration in this investigation was significantly larger than that used in earlier studies (15,16) and the surface area of the sample was significantly smaller, our relatively high reaction rates were maintained for over 10 hours (as shown in Figure 5). This behavior may imply a greater resistance to catalyst deactivation for these UFPs. We note that Verhaak, et al. (17) recently reported that an H/C (hydrogen to hydrocarbon) ratio larger than 20 was needed to keep the supported nickel catalysts active for 10 hours in the gas phase acetonitrile hydrogenation reaction. The mass spectral analysis of the final product gases for the ethane hydrogenolysis does not show the presence of ethane, indicating a near total consumption of the ethane in the reaction with excess hydrogen. However, the data in Table 1 show less than 85% conversion to methane. It is

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Figure 6. XFU) spectrum of the UFP after hydrogenolysis reaction with excessive CzI& in the reactant (a) two phases observed: hexagonal Ni$ and fee Ni; (b) a magnification of part of the spectrum highlighted in (a) resolves a split of Ni{ 111) and Ni3C( 101). possible to close the mass balance on carbon if the rest of the carbon ends up as another product(s) of the UFP catalysis. Analysis of the XRD spectrum in Figure 6 of the UFPs taken following the hydrogenolysis: reaction clearly identifies the presence of a hexagonal NisC phase in addition to the nickel and indicates that roughly 25% of the C& formed nickel carbide that remained with the UFR (for this analysis we used the conversion data from the C2IWH2 = 2.44 experiments). That the NisC phase is detectable using XRD suggests that the Ni$ most likely is not just on the particle surfaces. This is reasonable since a carbonaceous deposit created on the nickel surface during reaction may well diffuse into the interior and form Ni3C. The fact that we observe Ni3C formation during our nickel UFP catalyzed ethane hydrogenolysis may also imply that these

888

H HE, RH HEIST,BL MCINTYRE, TN BLANTON

materials am particularly active catalysts. progress.

Further investigation

of this issue is currently

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CONCLUSIONS A W laser-assisted gas phase photonucleation process has been developed and used as a method for generating ultralimenickel particles using ambient Ni(CO)das a precursor. The nickel composition of the UFPs was confirmed by TEM-SAD and XRD. These particles have been shown to be a particularly active catalyst for etbane hydrogenolysis. The ability of these particles to maintain a high level of reactivityover long residence times may suggest a significantresistance to deactivation. We observed the formation of Ni3C during the ethane hydrogenolysis reaction (with excess ethane), and this presence of nickel carbide appears to be related to the observed resistance to deactivation.

ACKNOWLEDGMENTS We thank Dr. Beth Gaillardand Dr. Bill Herkstroeterfrom the NSF Center for Photoinduced Charge Transfer at the University of Rochester for assistance with the XeCl excimer laser. We gratefully acknowledge many stimulating discussions with Professor Howard M. Saltsburg. Financial support from the Department of Chemical Engineering at the University of Rochester is greatly appreciated.

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