Structured and Nanoparticle Assembled Co−B Thin Films Prepared by

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J. Phys. Chem. C 2008, 112, 6968-6976

Structured and Nanoparticle Assembled Co-B Thin Films Prepared by Pulsed Laser Deposition: A Very Efficient Catalyst for Hydrogen Production Nainesh Patel,* Rohan Fernandes, Graziano Guella, Ashwin Kale, Antonio Miotello, Barbara Patton, and Cristian Zanchetta Dipartimento di Fisica, UniVersita` degli Studi di Trento, I-38050 PoVo, Trento, Italy ReceiVed: October 29, 2007; In Final Form: February 19, 2008

Amorphous Co-B-based catalyst powder, produced by chemical reduction of cobalt salts, was used as the target material for Co-B thin film catalyst preparation through pulsed laser deposition (PLD). A comparative kinetic analysis of the sodium borohydride (NaBH4) hydrolysis by using Co-B catalyst added to the hydride solution as powder or as thin film was carried out. Both forms of catalyst (powder and film) were heattreated at 623 K for 2 h under various atmospheric conditions (in vacuum or by using Ar, H2, and O2 gases) in order to study their effects on H2 generation rate. Surface morphology of the catalyst was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM), while compositional and bond formation analysis were carried out using X-photoelectron (XPS) and Fourier transform infrared spectroscopy (FT-IR), respectively. Structural characterization of catalysts was performed using the X-ray diffraction (XRD) technique. It was observed that nanoparticles produced during laser ablation process act as active centers in the catalyst films, producing significantly higher rate (about 6 times) of H2 generation than the corresponding Co-B powder. No significant changes were observed for Co-B powder treated in an inert atmosphere (vacuum and Ar) while it caused structural changes in Co-B films. Co2B phase formation in films makes them more efficient catalysts with 28% increase in rate of H2 generation as compared to untreated film. Heat treatment in an oxygen atmosphere causes complete inactivation of powder catalyst, while film still showed excellent catalytic activity with just a longer induction time. The AFM and SEM analysis of the heat-treated films did not show drastic change in surface morphology, indicating that changes in catalytic activity of the films were possibly connected to structural modification and formation of boron oxide on the catalyst surface. We report that by using suitable thin film Co-B catalyst the maximum H2 generation rate of about 5000 mL/(min g of catalyst) can be achieved. This can generate about 0.9 kW (0.7 V) for proton exchange membrane fuel cells (PEMFC), a critical requirement for portable devices.

Introduction In the next few decades, global energy resources will be facing a major breakdown due to increasing requirement for energy. Hydrogen is very important as future energy vector whose application areas range from the vehicular transport to energy batteries. Pure hydrogen is adopted as the fuel in proton exchange membrane fuel cells (PEMFC).1 However, on an industrial level, H2 is mostly produced by steam reforming of natural gas which, however, contains carbon contamination (CO2 and CO). The presence of carbon monoxide (even at ppm level) in the hydrogen gas reduces the performance of PEMFC due to catalyst poisoning.2 Therefore, especially for the mobile application, a fast and clean hydrogen supply method is required. Sodium borohydride (NaBH4) is a very attractive material for hydrogen supply to the fuel cells at room temperature because of its high hydrogen storage capacity (10.9 wt %).3 A large amount of pure hydrogen gas is released during the hydrolysis of chemical hydrides in the presence of certain acid or some catalyst like transition or precious metals and their salts. The hydrolysis reaction rate can be effectively increased by using many organic and inorganic acids; however, the reaction usually became uncontrollable. On other hand, solid-state catalysts such as precious or transition metals (generally functionalized with * Corresponding author. E-mail: [email protected].

carbon), and their salts are found to be very efficient in accelerating the hydrolysis reaction in a controllable manner. Catalysts like Ru supported on anion-exchange resin,3 fluorinated Mg-based alloy,4 Pt5 and Pd supported on carbon,6,7 Pt-Ru supported on metal oxide,8 Raney Ni and Co, and even nickel and cobalt borides9 are generally used as catalysts in hydrolysis reaction of the NaBH4. Interest toward Co-B-based catalyst comes from their high catalytic activity under proper reaction conditions, even due to their low cost and easiness of preparation.10 Co-B amorphous alloy catalyst can be effortlessly prepared by using chemical reduction of its salts in aqueous solution.11 Co-B amorphous compounds had been widely used as very efficient catalyst for selective hydrogenation of many compounds such as cinnamaldehyde (CMA),12 sulfolene,13 benzene,14 and citral.15 It has been reported16-18 that the Co-B compound acts as very active catalyst for H2 generation during hydrolysis of NaBH4 as compared to most of precious metals catalyst. Structural changes induced in the Co-B catalyst by thermal annealing are likely to increase the H2 production rate even if there is not a clear-cut explanation for this outcome since the mechanism for the catalytic hydrolysis reaction on Co-B catalyst is far to be understood. Schlesinger et al.19 in the 1950s first gave the mechanistic details of the BH4- acid-catalyzed hydrolysis, but the mechanism for the metal-catalyzed BH4hydrolysis has been largely overlooked: indeed, the true

10.1021/jp7104192 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008

Co-B Thin Films Prepared by Pulsed Laser Deposition chemical nature of the catalyzing species (metal, metal borides, metal hydride transient species) is not established. A new insight into the mechanism of Pd/C catalyzed hydrolysis of NaBH4 using 11B NMR measurements has been recently reported by Guella et al.,6 but the Co-B catalytic system is likely to work in a different way. Generally, most of the previous catalysts were used in form of homogeneous powder which has some issues to be still solved like, for example, (1) the separation of the catalyst from the suspension after the reaction could be difficult, (2) the suspended particles tend to aggregate, especially when they are present at high concentrations, and (3) the particulate suspensions are not easily applicable to continuous flow systems. Co-B amorphous samples obtained via direct reduction of Co(II) salts by BH4in aqueous solution usually display low surface area, broadly distributed particle size, and poor thermal stability against crystallization due to aggregation because of vigorous and exothermic nature of reduction reaction.20 Catalyst in the form of thin film, which has extra degrees of freedom to change the surface morphology and structure, can be easily recovered and reused, thus being suitable as on/off switch for generation of H2. For all these reasons, catalysts in form of thin films are used to solve the above-cited powder problems.21 Catalytic activity is mainly a surface effect which can be enhanced by using nanocatalysts. Pulsed laser deposition (PLD) has emerged as viable method for the production of nanoparticles on surface of the thin films.22 By changing the deposition parameters in PLD, the morphology and the structure of the film can be optimized for a given application. In our previous study,16 we have clearly showed that thin film catalysts of cobalt boride, synthesized by using PLD, work efficiently for hydrogen generation by hydrolysis reaction of NaBH4, producing higher H2 generation rate than that obtained with chemically synthesized powder using the reduction method. In the present paper, we will try to go in deeper details on qualification of our previous results by performing new and systematic experiments on catalytic effects. Specifically, we will report on (1) the effect of thermal treatment on Co-B catalyst films under different atmosphere conditions, (2) their structural changes under annealing, and (3) the corresponding changes in H2 generation rate. Experimental Methods Co-B catalyst powder for the H2 generation was prepared by using NaBH4 as reducing agent of aqueous solution of Co(II) salts. The black powder separating from solution during reaction course was filtered and then extensively washed with distilled water and ethanol before drying it in vacuum condition at room temperature. In order to be used as a target for the film deposition, the Co-B powder was cold pressed in the form of cylindrical disks. PLD was performed using a KrF excimer laser (Lambda Physik) at the operating wavelength of 248 nm, pulse duration of 25 ns, and repetition rate of 30 Hz. The ablation was carried out under vacuum with a base pressure of 2 × 10-4 Pa by using a laser fluence of 10 J/cm2. The target-to-substrate distance was maintained at 4 cm. Details of the deposition apparatus are reported in ref 23. Films were deposited on silicon substrates for the characterization and on glass substrates for checking their catalytic activity. Weight of the catalyst films was evaluated by measuring the weight of the glass slab (76 × 46 mm), before and after deposition, and it was kept approximately constant (∼2 mg) for all the produced samples. The obtained Co-B powder and film samples were heat-treated at 623 K for 2 h in vacuum, Ar, H2, and O2 gas atmosphere. Gas pressure during annealing was 0.1 MPa.

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6969 The surface morphology and the particle sizes of the Co-B catalyst were observed by using scanning electron microscope (SEM, JEOL), and particle size distribution was analyzed through the SEM images using an analysis 3.2 (SIS) software. The changes of surface roughness and area in the Co-B films before and after thermal treatment were measured by atomic force microscopy (AFM) using silicon as a cantilever operating in the contact mode. The relative surface area of the films was directly computed from their AFM images obtained from scanned area of 15 × 15 µm2. The structural characterization of catalyst powder and films was carried out in a BraggBrentano (θ-2θ) configuration using the X-ray diffraction (XRD) technique (Cu KR radiation, λ ) 1.5414 Å). The composition of the film was studied by using X-ray photoelectron spectroscopy (XPS). Details on XPS analysis are reported in our previous paper.16 Here we only summarize the main obtained results which will contribute to give new insights into the surface properties of the deposited Co-B films which could help to explain the observed catalytic properties. Bruker (Equinox 55) Fourier transform infrared spectroscopy (FT-IR) was used to identify the eventual presence of chemical species like B-O bonds in the films. For catalytic activity measurements, an alkaline-stabilized solution of sodium borohydride (pH 13, 0.026 ( 0.001 M) (Rohm and Haas) was prepared by addition of NaOH. The titer of reagent was independently measured through the iodometric method.24 The hydrogen generation rate was measured through a gas volumetric method in an appropriate reaction chamber with thermostatic bath, wherein the temperature was kept constant within accuracy (0.1 K. The chamber was equipped with a pressure sensor, a stirrer system, and a catalyst insertion device and also coupled with an electronic precision balance to accurately measure the weight of water volumes displaced by the hydrogen volumes produced during the reaction course. A detailed description of the measurement apparatus is given in ref 25. In all the runs, the catalyst (in the form of film or powder) was placed on the appropriate device inside the reaction chamber, and the system was sealed. In order to make a comparison, we plotted the time dependences vs stoichiometric hydrogen production yield (%) instead vs volume of hydrogen (mL). Co-B catalyst, both in forms of powder or as PLDgenerated thin films, was added to 200 mL of the above solution, at 298 K, under continuous stirring. The efficiency of the catalyst film was compared with the powder by using an analogous amount of catalyst (∼2 mg). Thus, in our tests the molar ratio between hydride and catalyst is about 170. The H2 generation rate was measured at different temperatures of 298, 308, 318, and 323 K in order to determine the activation energy involved in the catalytic hydrolysis reaction sustained by the Co-B film. In general, the rate of chemical reaction depends upon many factors like concentration of reactants, pressure (for gases), surface area, nature of the reactants, temperature, and presence or absence of a catalyst. In the case of catalytic hydrolysis reaction of NaBH4, determining the order of the reaction is fundamental to establish its rate constant. This point is very important because in the literature one can find that the hydrolysis reaction is either a first-order reaction6,26 or a zeroorder reaction3,19 with respect to NaBH4 concentration. In the zero-order reaction, plotting H2 production yield as a function of time results in a linear dependence, and the rate of reagent [BH4-] consumption is given by

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-

d[BH4-] ) k0[BH4-]0 ) k0 dt

Patel et al.

(1)

Since every mole of BH4 produces 4 mol of H2, the rate of hydrogen production is thus given by

d[H2] ) 4k0 dt

(2)

In the first-order reaction, plotting H2 production yield as a function of time results in an exponential dependence as described by

[H2](t) ) [H2]max(1 - e-k1t) ) 4[[BH4-]0(1 - e-k1t)]

(3)

where [BH4-]0 is the initial molar concentration of sodium borohydride in the solution and k1 is the overall rate constant of the reaction given by

-d[BH4-] ) k1[BH4-] ) khyd[H2O][catalyst][BH4-] dt

(4)

or in terms of hydrogen concentration

d[H2] ) 4k1[BH4-] ) khyd[H2O][catalyst][BH4-] dt

(5)

By fitting the curve of H2 production yield as a function of time by using eq 2 or eq 5, both reaction order and rate constant k1 can be evaluated. The last quantity may be used to calculate the H2 generation rate in terms of mL/(min g of catalyst) by taking into account the concentration of the NaBH4 (molarity of the solution, M) and of catalyst. In fact, the initial molar concentration of sodium borohydride in the solution is known, and when the reaction is exhausted (100% conversion), it is simple to calculate the total volume of produced H2 (VH2). By considering a first-order hydrolysis reaction and rewriting eq 3 in terms of produced volume of H2 gas, instead of molar concentration, we have

V(t) ) VH2(1 - e-k1t)

(6)

with production rate given by

dV ) VH2k1e-k1t dt

(7)

The maximum H2 generation rate (mL/(min g of catalyst)) was estimated, when [BH4-] is maximum (first-order reaction), i.e., for t ) 0, as follows:

maximum rate )

dV VH2k1 ) dt gcatalyst

(8)

Results and Discussion The surface morphologies of the Co-B powder and of deposited film at laser fluence of 10 J/cm2 in vacuum, as obtained by SEM, are reported in parts a and b of Figure 1, respectively. Film surface shows the presence of spherical particulates. The average particle size obtained from SEM images is about 300 nm. Such morphology can be explained by the laser-induced phase explosion process, in which at higher laser fluence the target material under the surface reaches temperature of ∼0.9Tc (thermodynamic critical temperature), causing a very high homogeneous nucleation of vapor bubble.

The target surface then makes a rapid transition from superheated liquid to a matrix of vapor and liquid droplets, which leave the irradiated target surface and get deposited on the substrate.27 The size and the number density of these particles increase with the laser energy as discussed in our previous paper.16 Identical surface morphology was observed in the AFM image (Figure 2), which shows a very rough surface, with average rms roughness of 264 nm (Table. 1), due to the particulates formation. In Figures 1b-f and 2 we present the SEM and AFM images of the Co-B catalyst films thermally treated at 623 K for 2 h, under various gas atmospheres. The images show no significant changes in surface morphology following the thermal treatment. The average rms roughness only varied by 20-30 nm between films treated under different atmospheres, which is a normal behavior shown by the films deposited in similar condition by PLD.28 Because the catalytic activity depends on the surface available for the reaction, then the value of the surface area is required. The AFM results presented here give an estimation of the films surface roughness and relative surface area Rsa ) A3D/A2D,29 where A3D is the effective area described in threedimensional (3D) space and A2D is the surface area of the projection of 3D surface onto a film, i.e., the solid geometric area. Calculation of Rsa was based on two main assumptions: (1) the particles on the surface are spherical (here, Figure 2 gives an indication on this point); (2) the height of the nodules observed in AFM images is equal to the diameter of the spherical particulates. The average values of the rms roughness and of Rsa are summarized in Table 1 for untreated as well as for thermal-treated Co-B films. We have to remark that calculations used here are qualitative, and the obtained results are only meaningful to make comparison between different films. Note that neither thermal nor atmosphere treatment was able to cause any change on the surface morphology or on the area value of the films. Previous XPS analysis,16 carried out on the surface of the catalyst, indicates that Co, B, and even oxygen (in form of oxide) are present on the surface of both powder (Figure 3) and film (Figure 4). No significant changes in XPS spectra were observed in both forms of catalyst, except for the presence of chlorine atoms (Cl) contamination on the surface of film catalyst. (This is explained on the basis of PLD process which involves subsurface layers of the target material where indeed Cl is present.16) The XPS spectra in Co 2p3/2 levels show the presence of cobalt species in form of metallic and oxide state corresponding to binding energies (BE) of 778.8 and 783.3 eV, respectively.30,31 In the B 1s level, however, only oxidized boron species were observed with the BE of 192.5 eV.30 In addition, XPS spectra in O 1s level clearly show the existence of cobalt and boron oxide on the surface of catalyst film and powder. Notice that XPS only samples the very few surface layers of the films, and so from such an analysis we cannot infer on boron oxidation in the volume of our samples. To detect the presence of oxygen into the bulk of the catalysts, we performed secondary ion mass spectroscopy (SIMS) measurement (the relevant figure is reported in the Supporting Information). Oxygen is only signaled up to a depth of 80-100 nm, and so the relevant portion of the bulk catalyst is free from oxygen. By using the XPS spectra, the atomic ratio of B and Co on a given (small) area of the surface of catalyst powder is determined as 2.75, while in the case of the film the ratio changes to 0.75. According to authors of ref 30, the Co-B amorphous alloy powder produced by the chemical method exhibits surface enrichment of the boron species: however, in our case, the film was deposited by ablating

Co-B Thin Films Prepared by Pulsed Laser Deposition

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6971

Figure 1. SEM micrographs of untreated Co-B (a) powder, (b) film, and Co-B films treated in (c) vacuum, (d) Ar, (e) H2, and (f) O2 gas atmosphere at 623 K for 2 h.

the target material (compressed powder) also beneath the surface. In addition, higher laser fluence (10 J/cm2) causes the preferential ablation of the Co from the target which could be related to the different thermophysical properties of Co and B:32 Co has lower melting and boiling temperatures as compared to those of B. Further, latent heats of vaporization of Co and B are 6276 and 35000 J/g, respectively, which could help to justify the surface enrichment on Co with respect to B in the deposited films. Preferential ablation of Zn with respect to Cu is reported by Mao et al.33 in the brass ablating process. Figure 5 shows the XRD patterns of freshly prepared and thermal-treated Co-B powders at 623 K for 2 h, under different gas atmospheres. (Notice that the use of techniques which samples volume properties gives average information which is not conclusive on surface layers but helps in following sample properties evolution.) Untreated powder shows only a broad peak around 2θ ) 45° in the XRD spectra, indicating an amorphous nature which is useful in enhancing the catalytic activity due to its unique short-range ordering but long-range disordering structure.34 No significant changes in the XRD pattern were observed when the catalyst powder was thermaltreated at 623 K in vacuum, H2, or Ar atmosphere. Note indeed that Co-B powder crystallizes at temperature above 773 K.35 In Co-B powder treated in O2 atmosphere, the broad peak disappears while sharp diffraction peaks corresponding to metallic Co appear. When Co-B powder is heated at elevated temperatures in anaerobic condition, it undergoes crystallization to form Co2B and Co3B phase, but simultaneously this phase decomposes to form metallic Co.35 On the contrary, when heattreated in aerobic (O2 atmosphere) condition boron, which has high affinity to oxygen, get oxidized to form B2O3 which causes only formation of crystalline metallic Co without formation of Co2B or Co3B phase at even low temperature of 623 K. These results have been considered to produce Co metal from the Co-B compound by treating it in aerobic condition, at moderate temperature, by following the reaction36

4Co2B + 3O2 f 8Co(s) + 2B2O3

(9)

To check the above mechanism in our present samples, FTIR spectra have been reported in Figure 6 for fresh and thermal-

treated Co-B powders, in different atmospheres at 623 K for 2 h. The bands are observed at frequencies attributable to the normal vibration mode of B-O species28 in all the powders. But the signal related to these bands are very weak and broad for both untreated powder and treated in vacuum, H2, and Ar, indicating the formation of marginal amount of boron oxide during the reduction reaction of cobalt salt. However, this band is much more intense and sharp in the case of oxygenatmosphere-treated powder, indicating a large amount of boron oxide formation as compared to the other powders. These results should not be compared with previous XPS analysis because now, with FTIR, we are sampling volume properties and not surface layers. XRD spectra reported in Figure 7 show a single peak of metallic Co in untreated as well as thermal-treated thin film samples. The peak related to the Co2B phase is observed in XRD pattern for films thermal-treated in vacuum, H2, and Ar atmosphere. A higher amount of Co on the films surface, as compared to powder, helps to understand the formation of Co2B phase even at lower temperature which was not observed in the Co-B powder. In Figure 8, we present the H2 yield and its production rate from hydrolysis of NaBH4 by using the Co-B powder and corresponding PLD-prepared thin films. The expected total amount of H2 has been measured, no matter of the used catalyst. The generation rate initially increases to a maximum value and then decreases with time because the NaBH4 concentration decreases in the solution: this suggests the nonzero order of the reaction kinetics. The hydrogen evolution data have been well reproduced by using the exponential function of eq 3, thus proving that the reaction is first order with respect to NaBH4 concentration. The maximum H2 generation rate was obtained by using eq 8. Several films synthesized by using the same PLD parameters showed almost identical ((5%) H2 generation rate, thus establishing the reproducibility of the process. During the reaction, the catalyst films were quite stable in terms of hydrogen production and their bonding with substrate. Maximum H2 generation rate obtained by using the untreated catalyst films (∼3590 mL/(min g of catalyst)) is about 6 times higher than that obtained using the powder (∼606 mL/(min g

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Figure 2. 3D-AFM images of Co-B films (a) untreated and treated in (b) vacuum, (c) Ar, (d) H2, and (e) O2 gas atmosphere at 623 K for 2 h.

of catalyst)) of same weight (2 mg) as in the film: this could be explained in terms of the prevailing surface morphology effects in nanoparticle assembled films (Figures 1 and 2). The average size of the nanoparticles is around 300 nm for the all films, and they are thought to be highly active sites for the hydrolysis reaction even at low concentration of NaBH4 and room temperature. These active Co-B nanoparticles promote the hydrogen production immediately after coming in contact with the solution, as seen by the high initial H2 generation rate (Figure 8). In our previous work,16 different laser parameters were used to deposit films with variable particle size and density. Increase in H2 generation rate was observed by increasing particle size and density, which in turn increase the average roughness (Ra) of the film. The relative surface area calculated on the basis of the AFM image of the untreated film shows 65% increment in the case of nanoparticle-assembled films. Thus, the Co-B particulate formation on the film surface by PLD increases the effective surface area while acting as efficient site to enhance the catalytic activity. Another possible minor

contribution to the increase in the H2 production rate may be attributed to the higher concentration of Co sites on the surface of the film. H2 generation volume as a function of time is measured at different solution temperatures using alkaline NaBH4 and 2 mg of Co-B catalyst film (Figure 9). As expected, H2 generation rate increases with the temperature. The Arrhenius plot of the hydrogen production rate using Co-B catalyst films (inset of Figure 9) permits to obtain activation energy of about 30 kJ mol-1. This value is lower than the activation energy found by Amendola (47 kJ mol-1)3 using higher NaBH4 and NaOH concentrations. Kaufman and Sen,37 by using different bulk metal catalysts, obtained 75 kJ mol-1 for cobalt, 71 kJ mol-1 for nickel, and 63 kJ mol-1 for Raney nickel. Lee et al.18 obtained 45 kJ mol-1 with structured Co-B catalyst powder. The favorable activation energy value obtained in the present work proves that nanoparticle-assembled Co-B film acts as efficient centers to enhance the catalytic reaction.

Co-B Thin Films Prepared by Pulsed Laser Deposition

Figure 3. XPS spectra of Co 2p, B 1s, and O 1s level for untreated Co-B powder catalyst (figure is taken from our previous work16).

In Figure 10, we compare the H2 generation rate for untreated and treated Co-B catalyst films, while the maximum H2 generation rates for both films and powders are summarized in Table 2. There is no change in the H2 generation rate for the Co-B powder treated in different atmospheres as compared to the untreated powder, except for powder treated in O2 atmosphere. The oxidized Co-B powder is almost shattered since it produces the H2 with a rate of about 33 mL/(min g), and it does not reach 100% on the H2 generation yield. Co-B films treated in inert atmospheres, i.e., vacuum and Ar atmospheres, show the highest H2 generation rate of 5016 and 4268 mL/(min g), about 40% and 19% greater, respectively, than the generation rate of the untreated films. On the contrary, the film treated in reducing atmosphere (H2 atmosphere) exhibits marginal increase in H2 generation rate of about 5% while the film treated in oxidizing atmosphere (O2 gas) shows decrease by 10% as compared to the untreated film. Another important observation for film treated in O2 atmosphere is that it required about 40 min to reach the maximum generation rate; this interval time is almost double with respect to that of other films. As discussed earlier, SEM and AFM images (Figures 2 and 3) did not show any change in the surface morphology, and the calculated relative surface area (Table 1) remained the same for heat-treated films. Thus, it can be argued that changes in H2 generation rate

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6973

Figure 4. XPS spectra of Co 2p, B 1s, and O 1s level for untreated Co-B film catalyst (figure is taken from our previous work16).

Figure 5. XRD pattern of Co-B powder (a) untreated and treated in (b) vacuum, (c) Ar, (d) H2, and (e) O2 gas atmosphere at 623 K for 2 h.

due to thermal treatment could be connected to structural changes in the films. XRD results (Figure 5) clearly indicate the amorphous nature of Co-B powders even after thermal treatment in vacuum, Ar, and H2 atmosphere. They also exhibit (Table 2) identical H2 generation rates as compared to the untreated powder. In the case of Co-B powder thermal-treated in O2 atmosphere, XRD

6974 J. Phys. Chem. C, Vol. 112, No. 17, 2008

Figure 6. FT-IR spectrum of Co-B powder (a) untreated and treated in (b) vacuum, (c) Ar, (d) H2, and (e) O2 gas atmosphere at 623 K for 2 h.

Patel et al.

Figure 8. Hydrogen generation yield as a function of reaction time with Co-B catalyst powder and film. Inset shows the comparison of H2 generation rate (mL/(min g)) between film and powder. AFM image showed 65% increment in relative surface area of the as-deposited Co-B film as compared to a flat surface.

Figure 9. Hydrogen generation yield as a function of reaction time with Co-B catalyst film measured at four different temperatures. Inset shows Arrhenius plot of NaBH4 hydrolysis rates with Co-B film.

Figure 7. XRD pattern of Co-B films (a) untreated and treated in (b) vacuum, (c) Ar, (d) H2, and (e) O2 gas atmosphere at 623 K for 2 h.

results show crystallization of the volume metallic Co phase. We have to investigate the reasons why, in the present case, there is a relevant reduction of the hydrogen production (Table 2). Here, certainly, surface properties are not anticipated by volume properties. During catalytic hydrolysis of NaBH4, metallic Co gets easily oxidized due to lack of protecting boron which results in lowest H2 generation rate. To prove this mechanism, catalytic activity was tested for pure Co films16 deposited using laser parameters as in Co-B films. Hydrogen yield with catalyst-Co films is only 20% of the Co-B films, and the H2 generation rate is very low. This shows that Co metal alone cannot be used as efficient catalyst probably because of surface oxidation which inhibits catalytic activity even though having nanoparticle morphology similar to Co-B films. FTIR spectra (Figure 6) show indeed the presence of higher amount of boron oxide in the O2-treated powder as compared to other powders. In a recent review article, Wee et al.38 reported that one of the major difficulty for the development of the borohydridePEMFC is the catalyst tolerance to deactivation. During the time

Figure 10. H2 generation rate as a function of reaction time with untreated Co-B catalyst films and treated in different atmospheres at 623 K for 2 h.

the catalyst remains exposed to the ambient atmosphere, deactivation may intervene because of the oxide formation. To investigate the effect of ambient atmosphere on our new developed catalysts, severe oxidizing conditions were realized

Co-B Thin Films Prepared by Pulsed Laser Deposition

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6975 carbon-supported Pt (Pt/C) shows higher H2 generation rate than our catalyst films.5 However, Pt is one of the costliest materials and not preferable for commercial use. Conclusion

Figure 11. H2 generation rate as a function of reaction time with Co-B catalyst (a) powder and (b) films treated in O2 gas atmosphere at 623 K for 2 h.

TABLE 1: Surface Roughness and Relative Surface Area Calculated on the Basis of the AFM Images for the Co-B Catalyst Films Treated in Different Atmospheres at 623 K for 2 h treatment of Co-B catalyst

rms roughness (nm)

rel surface area

untreated treated in vacuum treated in Ar treated in O2 treated in H2

∼264 ∼285 ∼294 ∼288 ∼274

∼1.65 ∼1.57 ∼1.56 ∼1.54 ∼1.57

TABLE 2: Maximum Hydrogen Generation Rate Obtained with Co-B Powder and Films Treated in Different Atmospheres at 623 K for 2 h maximum H2 generation rate (mL/(min g)) treatment of Co-B catalyst

Co-B powder catalyst

Co-B film catalyst

untreated treated in vacuum treated in Ar treated in O2 treated in H2

∼606 ∼604 ∼791 ∼33 ∼604

∼3589 ∼5016 ∼4268 ∼3225 ∼3762

by thermal-treating them in the O2 atmosphere at 623 K for 2 h. Figure 11 shows the plot of H2 generation rate as a function of time for Co-B powder and film catalyst samples treated in such conditions. With powder, the maximum observed H2 generation rate is negligible (33 mL/(min g)) while in the case of film (3225 mL/(min g)) it decreases by 10%, and the time required to reach the maximum H2 production rate is twice as compared to the untreated film. Because the Co-B powder has large amount of boron on the surface, this leads to formation of boron oxide under treatment in O2 atmosphere. It is also more porous as compared to the film (SEM images of Figure 1a,b) which allows the O2 to penetrate deep in the powder for a complete oxidization. On the contrary, the film is not porous and only few layers on the surface are oxidized. These oxidized layers cause a much higher incubation time for the film, but as soon as the oxide layer is modified (we have planned to investigate this aspect), film behaves similar to a freshly prepared film and reaches the maximum H2 generation rate. This result proves that nanoparticle-assembled Co-B film may solve the deactivation problem of the catalyst. In conclusion, maximum H2 generation rate for our Co-B catalyst films, treated in inert atmosphere, is about 5000 mL/ (min g of catalyst) which may generate 0.9 kW for PEMFC (0.7 V): this is the requirement for the portable devices. Only

We have synthesized Co-B nanoparticle-assembled thin films using PLD, which act as efficient catalyst for hydrolysis of NaBH4. This Co-B catalyst film showed higher catalytic performance than bulk powder, which could be explained in terms of the prevailing surface morphological effects in nanoparticle-assembled films. Structural changes in film caused by thermal treatment in inert atmosphere at 623 K increase the catalytic performance even more. This effect is absent in powder. Morphology of the films remains unchanged after thermal treatment in different atmosphere. Films subjected to strong oxidizing conditions (treatment with O2 at 623 K) show longer induction time but little change in catalytic performance. On the contrary, powder treated under similar conditions shows no catalytic activity. The maximum generation rate for Co-B catalyst films, treated in inert atmosphere, was about 5000 mL/ (min g of catalyst), which will be able to generate 0.9 kW for PEMFC (0.7 V). The activation energy exhibited by film (about 30 kJ/mol) is comparable to that of precious metal catalyst. Dedicated XPS experiments are planned to infer on possible electronic states which may contribute to the enhancement of catalytic activity of PLD films. Acknowledgment. We acknowledge Romina Belli for SEM analysis, Cristina Armellini for the support in the XRD analysis, Paolo Bettotti for the AFM analysis, and Paolo Mazzoldi and Cinzia Sada for SIMS analysis. The research activity is financially supported in the framework of the Hydrogen-FISR Italian project. Supporting Information Available: SIMS profile for untreated Co-B film. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ogston, J. M.; Kreutz, T. G.; Steinbugler, M. M. Fuel Cell Bull. 2000, 3, 5. (2) Dicks, A. L. J. Power Sources 1996, 61, 113. (3) Amendola, S. C.; Sharp-Goldman, S. L.; Janjua, M. S.; Spencer, N. C.; Kelly, M. T.; Petillo, P. J.; Binder, M. Int. J. Hydrogen Energy 2000, 25, 969. (4) Suda, S.; Sun, Y. M.; Liu, B. H.; Zhou, Y.; Morimitsu, S.; Arai, K.; Tsukamoto, N.; Uchida, M.; Candra, Y.; Li, Z. P. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 209. (5) Bai, Y.; Wu, C.; Wu, F.; Yi, B. Mater. Lett. 2006, 60, 2236. (6) Guella, G.; Zanchetta, C.; Patton, B.; Miotello, A. J. Phys. Chem. B 2006, 110, 17024. (7) Patel, N.; Patton, B.; Zanchetta, C.; Fernandes, R.; Guella, G.; Kale, A.; Miotello, A. Int. J. Hydrogen Energy 2008, 33, 287. (8) Krishnan, P.; Yang, T. H.; Lee, W. Y.; Kim, C. S. J. Power Sources 2005, 143, 17. (9) Liu, B. H.; Li, Z. P.; Suda, S. J. Alloys Compd. 2006, 415, 288. (10) Jeong, S. U.; Kim, R. K.; Cho, E. A.; Kim, H. J.; Nam, S. W.; Oh, I. H.; Hong, S. A.; Kim, S. H. J. Power Sources 2005, 144, 129. (11) Lu, J.; Dreisinger, D. B.; Cooper, W. C. Hydrometallurgy 1997, 45, 305. (12) Li, H.; Li, H.; Zhang, J.; Dai, W.; Qiao, M. J. Catal. 2007, 246, 301. (13) Ma, Y.; Li, W.; Zhang, M.; Zhou, Y.; Tao, K. Appl. Catal., A 2003, 243, 215. (14) Yu, Z.-B.; Qiao, M.-H.; Li, H.-X.; Deng, J.-F. Appl. Catal., A 1997, 163, 1. (15) Chen, Y.-Z.; Liaw, B.-J.; Chiang, S.-J. Appl. Catal., A 2005, 284, 97. (16) Patel, N.; Guella, G.; Kale, A.; Miotello, A.; Patton, B.; Zanchetta, C.; Mirenghi, L.; Rotolo, P. Appl. Catal., A 2007, 323, 18.

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