BN Additive on Hydrogen Sorption by Ti under Mechanical Treatment

J. Phys. Chem. C 2008, 112, 5869-5879

5869

Effect of h-BN Additive on Hydrogen Sorption by Ti under Mechanical Treatment in H2/He Flow C. Borchers,*,† O. S. Morozova,‡ T. I. Khomenko,‡ A. V. Leonov,§ A. V. Postnikov,|,⊥ E. Z. Kurmaev,| A. Moewes,# and A. Pundt† Institute for Materials Physics, UniVersity of Go¨ttingen, Friedrich-Hund-Platz 1, D-37077 Go¨ttingen, Germany, SemenoV Institute of Chemical Physics RAS, Kosygin st. 4, 119991 Moscow, Russia, LomonosoV Moscow State UniVersity, Chemical Department, Leninskie Gory, 119899 Moscow, Russia, Institute of Metal Physics, RAS-Ural DiVision, KoValeVskaya st. 18, 620041 Yekaterinburg, Russia, Paul Verlaine UniVersity - Institute de Physique Electronique et Chimie, Laboratoire de Physique des Milieux Denses, 1 Bd Arago, F-57078 Metz, France, and UniVersity of Saskatchewan, Department of Physics and Engineering Physics, 116 Science Place, Saskatoon, SK S7N 5E2, Canada ReceiVed: September 20, 2007; In Final Form: February 12, 2008

The effect of h-BN additive on hydrogen sorption by Ti under mechanical treatment in H2/He flow and on the thermal stability of Ti-hydride produced during milling were studied using kinetic, structural, microscopic, and spectroscopic techniques and theoretical first-principles calculations. The results obtained were compared with the corresponding properties of pure Ti and Ti with graphite additive. Like graphite, hexagonal BN was found to be an effective additive to improve the hydrogen sorption capacity of Ti. New types of occupation sites available for hydrogen, very similar to those detected for Ti/C, were observed. These sites are characterized by a low H2 desorption temperature: 630-670 K instead of ∼1000 K.

1. Introduction Titanium is a potential ingredient of hydrogen storage materials. Because the storage capacity is only 4 wt % in pure TiH2, there is little hope to use it for automotive applications. However, the volume density of hydrogen in TiH2 is about 0.18 kg/L, which makes it in principle attractive for stationary applications. Unfortunately, the combination of delivery temperature, delivery speed, and cycle life is not yet satisfactory, which is a characteristic of most metal hydrides.1-6 Over the last few years, much work has been done by many research groups attempting to improve these drawbacks, refs 7-21 to cite just a few. These studies were accompanied by quite a few theoretical studies, for example, refs 22-28, but a breakthrough is, to our knowledge, not yet attained. Ball milling, possibly with additives, reduces particle as well as crystallite size and enhances intermixing of metal/hydride and additives.3,4,6,29 It is well known that nanocrystalline TiH2 can be prepared during reactive milling of Ti in H2 atmosphere.30,31 An addition of graphite accelerates the hydrogen uptake considerably.32-34 However, the formation of a thick carbon surface layer, or a saturation of the surface with TiC, may suppress the Ti-H2 interaction as the Ti surface gets blocked.34 Application of other materials with layered structures (in particular hexagonal BN, h-BN) to stimulate the mechanochemical synthesis of metal hydrides is poorly studied.16,35 The aim of the present work is to study the effect of h-BN additive on the kinetics of hydrogen absorption by Ti in the process of milling, as well as on the thermal stability of titanium hydride produced in this reaction. * Corresponding author. E-mail: [email protected]. † University of Go ¨ ttingen. ‡ Semenov Institute of Chemical Physics RAS. § Lomonosov Moscow State University. | Institute of Metal Physics. ⊥ Paul Verlaine University. # University of Saskatchewan.

This is done in a combined study using methods for (micro-)structure determination, thermal desorption spectroscopy, and ab initio electronic structure and chemical bonding calculations. 2. Experimental Section The initial ingredient powders were elemental titanium with a purity of 99.5% consisting of spherical particles of about 200 µm in diameter, and hexagonal boron nitride (h-BN) with a purity of about 99%. The h-BN fraction was varied as 0.6 and 16.7 wt %. Hydriding experiments were carried out under flow conditions in a flow mechanochemical reactor fixed to the vibrator. The following parameters were used for the milling process: A stainless-steel container was loaded with 1.8 g of Ti/h-BN reaction mixture together with 19.8 g hardened steel balls (diameter 3-5 mm); the vibration frequency was 50 Hz with an amplitude of milling of 7.25 mm, and the average energy intensity 1.0 W/g. The effluent gas contained H2 with 50 vol % He, and the flow rate was 8-10 mL/min at atmospheric pressure. Hydrogen absorption and effluent gas composition were monitored continuously by a gas chromatograph combined online with the milling device. The interval of mechanochemical treatment was 30-206 min. The specific surface area, S, was measured by low-temperature Ar adsorption. Surface chemistry was examined by diffuse reflectance Fourier-transform infrared spectroscopy (DRIFT). The bulk composition, morphology, and microstructure of as-milled powders were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HREM). The hydrogen desorption or additional hydrogen absorption were tested by temperatureprogrammed desorption (TPD) and temperature-programmed reaction (TPR) techniques under flow conditions. Details of these experimental methods are published elsewhere.34,36 The main focus of the investigations was laid on four samples

10.1021/jp077582s CCC: $40.75 © 2008 American Chemical Society Published on Web 03/26/2008

5870 J. Phys. Chem. C, Vol. 112, No. 15, 2008

Borchers et al.

TABLE 1: Parameters of Mechanochemical Treatment of Ti/h-BN Milled in H2/He Flow sample no. composition 1 2 3 4

1.5 g Ti & 0.3 g h-BN 1.5 g Ti & 0.3 g h-BN 1.5 g Ti & 0.3 g h-BN 1.79 g Ti & 0.01 g h-BN

H2 uptake H2 uptake milling total H/Ti ratio time during milling (mol/g Ti) (mol/g Ti) final (min) 30

1.5 × 10-3

2.1 × 10-3

0.2

66

5.1 × 10-3

9.2 × 10-3

0.88

206

2.0 × 10-2

2.0 × 10-2

1.95

150

1.2 × 10-2

1.6 × 10-2

1.5

(named samples 1-4) with different Ti to h-BN ratio and different milling times as listed in Table 1. For the sake of comparison, pure h-BN, pure Ti, and TiH2 were also examined. The chemical state and local electronic structure of Ti, B, and N atoms were studied by high-energy resolution soft X-ray spectroscopic (XES) measurements. These were performed at beamline 8.0.1 of the Advanced Light Source at the Lawrence Berkeley National Laboratory. We measured Ti L2,3 (3d4s f 2p transition), N and B KR (2p f 1s transition) X-ray emission spectra of Ti/h-BN, mechanically activated in H2/He flow for 206 min. The X-ray spectra of the reference samples Ti, h-BN, TiH2, TiB2, and TiN were measured at the same conditions. For the fluorescence spectra, the emitted radiation was collected by a Rowland circle-type spectrometer with spherical gratings and recorded with an area-sensitive multichannel detector. The total experimental resolution in the K X-ray emission region was 0.3 eV for boron and 0.75 eV for nitrogen. All emission spectra were normalized to the number of photons falling on the sample, monitored by a highly transparent gold mesh in front of the sample. Details of this method can be found elsewhere.37 3. Results 3.1. Kinetics of Ti-H2 Interaction under Mechanical Activation. The mechanical activation of pure h-BN in H2/He flow gave a highly dispersed powder. The specific surface area, S, changed from 12 to 159 m2/g. The H2 sorption after 66 min of treatment was 3.4 × 10-4 mol/g h-BN, which is about 1 order of magnitude lower than H2 sorption by Ti metal (∼3.3 × 10-3mol/g Ti). Figure 1 shows the mechanically induced H2 sorption of Ti/h-BN powders containing 16.7 wt % (sample 3) and 0.6 wt % (sample 4) h-BN. For the sample containing 16.7 wt % h-BN, hydrogen sorption started at the beginning of the milling without an incubation period. In the case of the sample with 0.6 wt % h-BN, the reaction starts after an incubation period of about 15 min (see Figure 1, inset). The H2 sorption rate increased for the first 35-40 min and finally remained constant because the total H2 from the flow gas mixture was absorbed. The process was completely finished after 206 min of milling, when a ratio H/Ti ) 1.95 was reached. In order to study details of the Ti/h-BN-H2 interaction, we interrupted the mechanical treatmen after 30, 66, and 150 min. As follows from mass balance on the assumption that H2 reacts predominantly with Ti, the total hydrogen sorption after 30, 66, and 205 min of treatment (samples 1-3) was 2.1 × 10-3 (H/Ti ) 0.2), 9.2 × 10-3 (H/Ti ) 0.88), and 2.0 × 10-2 mol H2/g Ti (H/Ti ) 1.95), respectively. Hydrogen uptake by sample 4 after 150 min of treatment was 1.6 × 10-2 (H/Ti ) 1.5). The most remarkable feature of the mechanically induced reaction was a post-milling effect: Hydrogen absorption continued after interrupting the milling. The post-milling uptake amounted up to 45% of total

H2 uptake. The parameters of Ti/h-BN-H2 interaction are listed in Table 1. Surface NHx vibrations at 1120 and 3438 cm-1 and 1660 and 3211 cm-1 were detected by DRIFT technique for h-BN and Ti/h-BN milled in H2/He flow for 1 h. Boron was not detected by DRIFT. 3.2. Structure, Morphology, and Microstructure of Milled Powders. Figure 2 shows XRD spectra of Ti/h-BN powders milled in H2/He flow for different times. XRD patterns of unmilled Ti and TiH2/h-BN milled in He flow are given for comparison. Only cubic δ-TiH1.5-1.95 (JCPDS 25-982) with some tetragonal distortion was newly formed under milling. The lattice constants of the respective phases are given in Table 2. No Ti-nitride or -boride was detected. According to mass balance, fitting of XRD patterns, and lattice parameters of TiH1.5-1.95 phase,14 Ti/ h-BN powder milled for 30 min consists of h-BN, R-Ti (∼75 wt %), and cubic δ-TiH1.5-1.95 with an average coherent domain size of ∼18 nm. Peaks of R-Ti are rather broad, as compared to these of unmilled R-Ti, but no change in peak positions was observed (Figure 2, curve 1). Ti/ h-BN powder milled for 66 min contains ∼90 wt % cubic δ-TiH1.5-1.95 with an average coherent domain size of ∼6 nm. By applying bulk lattice expansion, the increase of the lattice parameter from a ) 0.442 nm to a ) 0.444 nm points to an increase in H content in the Ti-hydride phase from 62 atom % (H/Ti ) 1.63) to about 64 atom % (H/Ti ) 1.78). About 10 wt % R-Ti is still present. Ti/h-BN powders milled for 206 and 150 min (curve 3, Ti-16.7 wt % h-BN and curve 4, Ti-0.6 wt % h-BN) consist of cubic δ-TiH1.95 with average coherent domain sizes of about 6 and 12 nm, respectively. The lattice parameters of this phase, a ) 0.446 nm, corresponds to Tihydride phase with 66 atom % H (H/Ti ) 1.94), again applying bulk lattice expansion. The spectrum of TiH2/h-BN milled in He flow exhibits the same peaks as samples 1-4. Figure 3 shows SEM images of Ti and Ti/h-BN powders asreceived and milled in H2/He flow for various times. The original Ti powder consists of spherical particles with an average diameter of about 200 µm (Figure 3a) and a surface area S ) 0.007 m2/g. When milled in H2/He flow without additions, Ti adopts a flake-like structure, see Figure 3b. The Ti/16.7 wt % h-BN powder milled for 30 min consists of small particles with an average size of about 0.3 µm (Figure 3c). Some of them are lumped together to larger particles, 10-20 µm in size. The specific surface area is S ) 24 m2/g. The enlargement of powder size that can be observed for Ti/ 16.7 wt % h-BN after 60 and 206 min of milling (Figure 3c and d) is in good agreement with a corresponding decrease of specific surface area: S ) 19 and 11 m2/g, respectively. Figure 4 shows TEM images of Ti/h-BN powders milled for 206 min in H2/He flow. In Figure 4a, an overview is shown. Particles about 50-100 nm in diameter form a larger agglomeration, the thicker part of which is dark in the micrograph. The agglomeration is larger than the area of the micrograph, wich is 1 µm2. The individual particles forming the agglomeration seem to consist of a BN matrix, in which nanometer-sized hydride fragments are embedded, which are recognizable by the dark contrast. In the HREM image of Figure 4b, lattice fringes from several hydride particles can be seen. In the middle of the micrograph, the cubic structure of the hydride is evident by two sets of fringes within one particle, with identical spacing and a 90° relationship to each other. At the bottom edge of the micrograph, there are a few parallel yet wavy fringes with a larger spacing. These are induced by boron and/or BN enveloping the hydride particles. Figure 5 shows TEM images of BN. In Figure 5a, an overview of the original h-BN powder in the

Effect of h-BN Additive on Hydrogen Sorption by Ti

J. Phys. Chem. C, Vol. 112, No. 15, 2008 5871

Figure 1. Kinetics of H2 uptake under the milling conditions by Ti/h-BN powders containing the following: sample 3, 16.7 wt % of h-BN; and sample 4, 0.6 wt % of h-BN. Inset: details of early H2 uptake period.

TABLE 2: Structural Parameters of Ti/h-BN Milled in H2/He Flow lattice constants (nm)

R-Ti (∼75 wt %) δ-TiH1.5-1.95 R-Ti (∼10 wt %) δ-TiH1.5-1.95 δ-TiH1.95

a ) 0.295 c ) 0.470 a ) 0.442 a ) 0.295 c ) 0.468 a ) 0.444 a ) 0.446

20

δ-TiH1.95 Ti traces

a ) 0.446

12

1

1.5 g Ti & 0.3 g h-BN

30

2

1.5 g Ti & 0.3 g h-BN

66

3

1.5 g Ti & 0.3 g h-BN 1.79 g Ti & 0.01 g h-BN

206

4

Figure 2. XRD patterns of Ti/h-BN powders milled in H2/He flow for: 1, 30 min (sample 1); 2, 66 min (sample 2); 3, 206 min (sample 3) and 4, 150 min (sample 4). For comparison, the spectra of original Ti and TiH2 milled in He for 66 min are added.

unmilled state is shown. BN adopts a disc-like morphology, giving a strong evidence for the perfect graphite-like crystalline structure. Figure 5b shows a HREM micrograph of a Ti(TiH2)/ h-BN powder particle milled for 206 min in H2/He flow. A hydride particle can be discerned in the upper, darker part of the micrograph. At the lower edge of the particle, BN can be seen. It adopts a morphology reminding of entangled ribbons, each consisting of 3-10 single layers, most probably BN(0001). In some places, the ribbon-like structure is so blurred that it might hint on partial amorphization of BN. 3.3. TPD and TPR of Mechanically Activated Powders. Figure 6 shows TPD spectra for samples 1-4 in comparison to the TPD spectrum of original Ti powder milled in H2/He flow for 66 min and TiH2/h-BN powder milled in He flow for 66 min. The TPD spectrum of original Ti powder milled in

domain size (nm) (Scherrer)

phase milling time composition sample no. composition (min) (from XRD)

150

18 20 6 10

H2/He flow for 66 min consists of one single peak with Tmax ≈ 1000 K (Figure 6a). The TPD spectra recorded for Ti/ h-BN and TiH2/h-BN powders are completely different because they exhibit several peaks. Peak positions were obtained by fitting Gaussians to the spectra. Sample 1 (30 min of milling, Figure 6b) and sample 2 (66 min of milling, Figure 6c) show a pronounced high-temperature peak at 934 and 920 K, and three low-temperature peaks, of which the one at 768 and 758 K is the most pronounced. For both samples, the high-temperature peak is the most pronounced single peak. For samples 3 (206 min of milling, Figure 6d) and 4 (0.6 wt % of BN, 150 min of milling, Figure 6e) and TiH2/h-BN (Figure 6f), the picture is quite different. In all three cases, the highest peak is at about 850 K, and in all three cases it is not the most pronounced peak. Two to four additional peaks were fitted to the spectra, but all three spectra exhibit a pronounced maximum at about 780 K and another, less-pronounced one at about 650 K. The parameters of the TPD process are listed in Table 3. As stated above, samples 1 and 2 were only partially filled by hydrogen after milling in H2/He for 30 and 66 min, respectively. Additional filling of these samples in the TPR regime was carried out on the milled powder to characterize


Borchers et al.

Figure 3. SEM micrographs of a, Ti original; b, Ti milled in H2/He flow for 150 min and Ti/h-BN powders milled in H2/He flow for c, 30 min (sample 1); d, 66 min (sample 2); e, 206 min (sample 3); f, 150 min (sample 4). Samples 1-3 contain 16.7 wt % of h-BN, and sample 4 contains 0.6 wt % of h-BN.

the residual sites available for hydrogen in milled Ti/h-BN powders (Figure 7, curves 1 and 2). For sample 1, additional H2 absorption was 6.9 × 10-3 mol H2/g Ti at a temperature Tmax ≈ 780 K, resulting to an increased H/Ti ratio of 0.9. For sample 2, additional H2 absorption was 3.5 × 10-3 mol H2/g Ti at Tmax ≈ 700 K and the H/Ti ratio increased to 1.2. Curve 3 shows the TPR spectrum of pure Ti. Here, Tmax ≈ 800K.

3.4. Structural Changes of Milled Powders after TPD and TPR. Figure 8a shows XRD patterns of samples 1-4 in comparison with those of Ti initial and milled in H2 /He flow after TPD. The inset shows the 2θ ) 50-75° region, which allows us to analyze the phase composition. The XRD spectra exhibit the following remarkable features: (1) A broadening and small angle shift of all titanium peaks; (2) a partial splitting of Ti (0002), {101h2}, and {101h3} peaks, while the Ti {101h0}


Figure 4. TEM micrographs of Ti/h-BN powder milled for 206 min in H2/He flow. (a) Overview, metal-hydride particles appear as small dark spots in a BN matrix appearing brighter. (b) HREM micrograph. The cubic structure of TiHx is clearly visible.

peak remains a single peak. This is due to a new hexagonal phase R′, besides R-Ti and h-BN, that appears after TPD. The concentration of R′ depends on the time of the preliminary mechanical treatment. After 150 and 206 min of milling, R′ is the only one detected by XRD. The phase is identified as hexagonal TiNx (nitrogen interstitials in R-Ti, JCPDS 44-1095 and JCPDS 41-1352). An expansion of the hexagonal lattice cell goes along [0001]: the a-lattice constant 0.296 nm remains equal to that of R-Ti while c (and consequently the c/a ratio) increases slightly with the duration of the mechanical treatment (see Table 3), probably because of an increase of the nitrogen content in this phase. Additionally, an amorphous halo appears at 2θ ) 32-48°, the intensity of which also increases with increasing milling time. The analysis of XRD patterns allows to attribute this halo to amorphous TiN. The lower the h-BN


Figure 5. TEM micrographs of BN. (a) BN original, unmilled powder. (b) HREM micrograph of Ti/BN powder milled for 206 min in H2/He flow. Details of the BN structure can be seen.

content, the lower the halo intensity (compare Figure 8a, curves 3 and 4). Figure 8b shows XRD patterns of samples 1 (30 min of milling) and 2 (66 min of milling) after TPR of the as-milled powders. The phase composition and structural parameters are listed in Table 4. Sample 1 consists of R-Ti and δ-TiH1.5-1.95, as before. As follows from a fitting procedure, the Ti-hydride portion increases from ∼25 wt% to ∼35 wt%, while the formal H/Ti ratio changed form H/Ti ) 0.2 to H/Ti ) 0.9. Again applying bulk values, according to the Ti-hydride lattice constant, which falls down from 0.443 to 0.441 nm, there is a decrease in hydrogen content from 62% to about 60 atom % in the hydride. The concentration of micro-deformations goes down from 〈〉1/2 ) 0.7% to 〈〉1/2 ) 0.3%, while the coherent domain size is as before, about 18 nm. Contrary to this, a decrease in coherent domain size of R-Ti from 20 to 8 nm was observed after TPR. As for sample 2, the phase composition and structural


Borchers et al.

Figure 6. TPD spectra of powders milled in H2/He flow: a, Ti, 60 min; b, Ti/h-BN, 30 min (sample 1); c, Ti/h-BN, 66 min (sample 2); d, Ti/h-BN, 206 min (sample 3); e, Ti/h-BN, 150 min (sample 4), f, TPD of TiH2/h-BN milled in He flow.

TABLE 4: Parameters of TPR Process and Phase Composition after TPR sample

Tmax (K)

H2 sorption (mol/g Ti)

H2 content (mol/g Ti)

no. 1

780

6.9 × 10-3

8.4 × 10-3

no. 2

703

3.5 × 10-3

1.58 × 10-2

phase composition R-Ti (∼ 65 wt%) δ-TiH1.5-1.95 R-Ti (traces) δ-TiH1.5-1.95

lattice constants (nm) a ) 0.295 c ) 0.470 a ) 0.441 a ) 0.443-0.444

changes were negligible. Traces of R-Ti were still detected by XRD after additional H2 sorption. 3.5. X-ray Emission Spectra. X-ray emission spectroscopy was used to clarify the mechanism of TiNx formation. Ti L2,3 X-ray emission spectra (XES, 3d4 f 2p1/2,3/2 transition) of Ti/ h-BN sample milled for 206 min in H2/He atmosphere and reference samples (Ti, TiH2, TiN, and TiB2) are presented in Figure 9. Besides the main maximum at 452.1 eV, which is close to that of metallic Ti, an additional subband at about 448

eV appeared in the spectrum of the milled Ti/h-BN sample, see Figure 9a. This additional feature of the spectrum is absent in the spectrum of TiB2, see Figure 9b, but is present in the spectra of TiH2 and TiN. It can be attributed to Ti-H and Ti-N bonding. Indeed, a simulation of Ti L2,3 XES of the Ti/h-BN milled sample by a superposition of the spectra of the reference samples shows that the experimental spectrum is reproduced quite well by an additive spectrum consisting of 40% Ti, 40% TiH2, and 20% TiN, see Figure 10a. Alternatively, we could not receive good results by a simulation of the experimental spectrum by a superposition of spectra of Ti, TiH2, and TiB2, see Figure 10b. Hydrogen atoms enter the Ti lattice by occupying tetrahedral interstitials, as occurs in the formation of hydride TiH2. In order to understand the role of h-BN additive under hydrogenization of the Ti/h-BN sample induced by ball milling, we measured boron and nitrogen KR (2p f 1s transition) XES, shown in Figures 11 and 12. The B KR spectrum of the Ti/h-BN ball milled sample is found to be identical to that of h-BN, and a



Figure 7. TPR spectra of Ti/h-BN powders milled in H2/He flow for 1, 30 min (sample 1) and 2, 66 min (sample 2); and 3, Ti powder milled in H2/He flow for 66 min.

TABLE 3: Parameters of TPD Process and Phase Composition after TPD

sample pure Ti no. 1

no. 2

no. 3

no. 4 TiH2/hBN

Tmax (K)

H2 desorption (mol/g Ti)

H2 fraction in peak (%)

1000

4.5 × 10-3

100

714 768 823 934 685 758 814 920 608 634 747 782 857 674 768 852 657 727 781 853

2.1 × 10-4 2.7 × 10-4 1.7 × 10-4 1.5 × 10-3 1.0 × 10-3 2.3 × 10-3 1.7 × 10-3 4.1 × 10-3 3.9 × 10-3 1.9 × 10-3 7.5 × 10-3 1.3 × 10-3 6.4 × 10-4 8.2 × 10-3 2.4 × 10-3 2.5 × 10-3 8.3 × 10-3 1.3 × 10-3 5.9 × 10-3 3.0 × 10-3

10 12 8 70 11 25 19 45 26 12 49 9 4 63 18 19 45 7 32 16

phase composition R-Ti R-Ti (∼35 wt %) hex TiNx R-Ti (∼30 wt %) hex TiNx hex TiNx

lattice constants (nm) a ) 0.296 c ) 0.470 a ) 0.296 c ) 0.470 a ) 0.296 c ) 0.477 a ) 0.296 c ) 0.470 a ) 0.296 c ) 0.479 a ) 0.296 c ) 0.478

hex TiNx

a ) 0.296 c ) 0.479

hex TiNx

a ) 0.297 c ) 0.476

contribution of TiB2 seems to be negligible, see Figure 11. The results of measurements of N KR XES, which probe occupied 2p valence states, are presented in Figure 12. The spectrum of Ti/h-BN sample milled for 206 min in H2/He atmosphere shows some differences with that of h-BN: There is an enhancement of the relative intensity of the maximum of N KR XES at 393 eV, where the main peak of N KR XES of TiN is located, see Figure 12a. The superposition of spectra of the reference samples h-BN and TiN taken in ratio of 89.7 and 10.3%, respectively, reproduces this experimental spectrum quite well, see Figure 12b. 4. Ab Initio Calculations of Chemical Bonding An apparent controversy (a noticeable amount of TiN according to XES, but no traces of the nitride phase in the diffraction study) reflects the fact that the X-ray spectroscopy is sensitive to the nearest chemical neighborhood. A contribution of TiN-like spectrum reveals merely that nitrogen atoms exist either in the boron nitride layers (on the surface of titanium

Figure 8. XRD patterns after (a) TPD and (b) TPR treatments. (a) XRD patterns of Ti original and Ti milled in H2/He flow for 60 min, and of Ti/h-BN powders milled in H2/He flow for 1, 30 min; 2, Ti/hBN, 66 min (sample 2); 3, Ti/h-BN, 206 min (sample 3); 4, Ti/h-BN, 150 min (sample 4). Inset: details of 2Θ ) 50-75° region. b: XRD patterns of Ti/h-BN powders milled in H2/He flow for: 1, 30 min (sample 1) and 2, 66 min (sample 2).

particles) or in the TiN-like neighborhood, that is, octahedrally coordinated by titanium atoms. The latter situation corresponds to a nitrogen atom in an octahedral interstitial within hexagonal (R) or cubic (TiH2-like) titanium lattice. Such a conclusion offers a foundation for a microscopic ab initio study of the electronic structure and chemical bonding. Namely, we adopt a structural model that does not contain BN in crystalline or molecular form inside the titanium bulk but allows single nitrogen and, for the sake of comparison, also boron and carbon atoms in octahedral interstitials. A single hydrogen atom is placed in tetrahedral interstitials at different distances from the 2p atom (N, C, or B). Our primary objective was the study of the interaction potential between H and a 2p atom in the crystal, or, put differently, how the hydrogen affinity of titanium is affected by the presence of interstitial N, B, or C. The calculations have been done within the density functional theory paradigm (using specifically local density approximation for the exchange correlation), applying the method and computer code Siesta.38,39 Norm-conserving pseudopotentials have been generated for nominal valence charge configurations (B 2s22p1, C 2s22p2, N 2s22p3, Ti4s13p63d2) by the Troullier-Martins scheme.41 Strictly confined basis functions were of the “double-ζ with polarization orbitals” quality.42,43 Calculations have been done on a supercell of 48 Ti atoms, with one hydrogen and one 2p atom added. The effects of spin polarization were tested to be unimportant here because all electronic states do efficiently hybridize into a common metallic-type band with low state


Borchers et al.

Figure 11. B KR XES of Ti/h-BN milled sample and reference samples.

Figure 9. Ti L2,3 XES of Ti/h-BN milled for 206 min and reference samples.

Figure 12. N KR XES of Ti/h-BN milled sample and reference samples (a). Comparison of experimental N KR XES of Ti/h-BN milled sample with superposition of spectra h-BN and TiN (b).

Figure 10. Simulation of Ti L2,3 XES of Ti/h-BN milled sample by superposition of spectra of (a) Ti, TiH2, TiN and (b) Ti, TiH2, TiB2.

density at the Fermi level, and no localized magnetic states are formed in any of the systems studied. A detail of the 48-Ti supercell is shown in Figure 13a. The octahedral interstitial, occupied by a 2p atom, is shown in the left with a black sphere. Tetrahedral interstitials in different settings with respect to it are shown as small gray spheres in positions “a” to “e”, where “a” is the nearest tetrahedral interstitial, and the “e” position is the most distant one. We analyzed the total energies in these five different arrangements of hydrogen around each of the 2p dopants, with full lattice relaxation taken into account in each case, as a measure of interaction potential between B/C/N, on one side, and H, on the other side; the results are shown in Figure 13b. The “e” position represents the “infinite” separation of hydrogen from

the 2p atom; it is seen that closer placement of hydrogen to C or N results in a quite appreciable energy gain of some tens of millielectronvolts (10 meV corresponds to 11.6 K, or 0.96 kJ/ mol). A N or C interstitial impurity in titanium creates a large region in which the hydrogen affinity of titanium metal is substantially increased, which may explain the observed enhancement of its sorption capability. It is noteworthy that the C-H or N-H “interaction potentials” are not smooth but reveal a clear tendency to trap hydrogen in “b” and “d” positions, rather than in “a” or “c”. In fact, we failed to stabilize hydrogen in the “a” position close to carbon or nitrogen; it invariably diffuses into the neighboring “b” interstitial. It is seen from Figure 13a that in the preferential settings “b” and “d” the hydrogen is bonded to a 2p atom via Ti bridges, a single intermediate Ti atom (in “d”), or a Ti pair, almost in-plane with H and the 2p atom (in “b”). These configurations favor an efficient hybridization of H-1s and N(C)-2p states with Ti-3d and gain in the binding energy. It is seen from Figure 13b that the placement of H in the vicinity of boron costs energy; however, all hydrogen placements “a” through “e” around boron are metastable,


Figure 13. (a) Positions of 2p dopant (B, N, or C) in octahedral interstitial sites, big sphere at left, and of hydrogen in tetrahedral sites, small gray spheres, in different distances from the 2p dopant: “a”, nearest position, “e”, most distant position. (b) Calculated total energies, relative to that in the “e” configuration, for different placements of the (tetra)-interstitial H relative to (octa)-interstitial B, N, or C. The “a” configuration of H near the N or C site is unstable; the H atom invariably migrates, in the course of sructure relaxation, through the Ti3 face into the adjacent “b” position.

without the abovementioned effect of hydrogen squeezed out of the “a” site. A detailed analysis of involved molecular orbitals will be given elsewhere; it can just be stated here that the results given in this work are confirmed. Summarizing, we found that the addition of nitrogen does indeed improve the bonding of hydrogen in Ti lattice; not less important, this effect is quite long-ranged and extends over many tetrahedral sites up to ∼0.5 nm around the N (or C) dopant. The effect of interstitial boron on the hydrogen absorption seems to be negative but relatively small. 5. Discussion From our results, we can summarize what happens phenomenologically upon h-BN addition to mechanically activated Ti in H2/He gas stream: (1) A significant increase in H2 uptake; (2) a shift of TPR/TPD peaks to low-temperatures, and (3) the formation of additional occupation sites available for hydrogen, which desorb at low temperatures. The most evident effect of h-BN addition is a dramatic Ti powder fragmentation, see Figures 3 and 4: instead of rather large and flat Ti particles of up to 200 µm in size obtained when Ti is milled without additives,44 Ti and/or TiH2 particles a few nanometers in size in roundish h-BN matrix particles some 200 nm in size are observed. This is due to lubricant and antisticking properties

J. Phys. Chem. C, Vol. 112, No. 15, 2008 5877 of h-BN, quite similar to what is observed when graphite is added to Ti upon the same treatment.34 It is well known that hydrogenation behavior of nanocrystalline metals with typical cluster sizes of less than 20 nm differs strongly from that of bulk material3,25,45-48 particle refinement. However, the mentioned phenomena cannot be attributed to the particle size reduction alone. The process of H2 absorption includes two important stages: The first is H2 dissociative chemisorption on the Ti surface, which is usually the rate-determining stage, because it requires a significant activation energy of Ea ) 163 kJ/mol.49 The next step is the diffusion of H atoms into the Ti bulk. The activation barrier of this stage is lower: Ea ) 104kJ/mol.50 Hydrogen diffusion is fast until Ti-hydride is formed at the surface because the diffusion coefficient of H in the hydride is about 1 order of magnitude lower than that in pure titanium.50 Thus, two important prerequisites should be realized to improve titaniumhydride formation from Ti and H2: (1) to create many stable active surface sites to stimulate chemisorption and (2) to produce small metal particles with a high concentration of bulk defects to ease the second stage by keeping diffusion distances low and perhaps by speeding up diffusion along defects. Kondo and coworkers used X-ray photoelectron spectroscopy (XPS) to examine surface states of milled Ti, Ti/ h-BN, and Ti/C powders.16 They found that metallic Ti appears on a mechanically activated surface of Ti powder usually covered by a TiO2 layer, whereas the surfaces of Ti/h-BN and Ti/C powders interact with N or C atoms dissolving interstitially on the Ti surface, suggesting that milling with h-BN or C decreases the electron density around metallic surface Ti.16 When Ti was milled with h-BN, Kondo and co-workers found that TiO2 can still be detected by XPS, even after 20 h of milling (without hydrogen gas).16 Such a type of surface may be visualized as an intermixture of TiO2 and Ti patches with a large interface area; the longer the milling, the larger the Ti-TiO2-interface. It is well known that the most active H2 chemisorption sites are located in metal-oxide or metal-carbide interfaces, where surface vacancies play a special role.8,4,21,51,52 Low-coordinated Ti atoms in the interfaces and Ti atoms with low electron densities on the surfaces can be considered as active sites for H2 adsorption and dissociation, where oxygen might play a vital role. The interstitial dissolution of N on the Ti surface prevents surface Ti-hydride formation, which obstructs hydrogen transport into the bulk.6 Therefore, dissociative chemisorption of H2 is stimulated at active centers located at surfaces and Ti-TiO2 interfaces, and hydrogen diffusion is eased by small particles with high defect concentrations,53 and by the dissolution of N on the Ti surface preventing the formation of surface hydride. Contrary to the single-peak TPD curve of pure Ti, which is indicative of one single type of hydrogen occupation site in the Ti lattice, the multipeak TPD curves observed for milled Ti/hBN and TiH2/h-BN (Figure 6) give direct evidence of the creation of new types of occupation sites available for H atoms. Recently, we demonstrated for mechanically activated Ti/C and TiH2/C that TPD curves of such a shape correspond to new occupation sites available for hydrogen, which were formed in response to C atoms interstitially dissolved in surface and subsurface Ti-lattice sites.34,36 On the basis of this study, the study of Kondo and co-workers,16 and on Ding at al. as well as Tao et al., where FeNx and TiNx formation under the milling of Fe and Ti with h-BN powder were reported,54,55 we propose the modification of the Ti surface and subsurface by interstitial N atoms. This, of course, requires the dissociation of h-BN. The mechanically induced energy is too low for direct dissocia-

5878 J. Phys. Chem. C, Vol. 112, No. 15, 2008 tion of the h-BN molecules whose dissociation energy is Ediss ) 386 kJ/mol.56 In our experiments, this energy is put into the system after more than 500 min of milling, which is far longer than the milling times in this work. However, h-BN dissociation can be eased under the milling in H2/He, in the course of which its surface area S increased from 12 to 159 m2/g. Along with this fragmentation, nanocrystallization accompanied by buckling of basal planes, as well as amorphization of h-BN are observed, see Figure 5b. Huang and co-workers studied ball-milling of h-BN alone.57 They also observed cleavage, buckling, and elongation along basal planes, turbostratic packing of basal planes, and the coexistence of nanocrystalline and amorphous phases. These structures certainly ease the dissociation of BN molecules. Apart from this, the presence of Ti or TiH2 is a stimulating factor of h-BN surface hydrogenation leading to dissociation because of dissociative chemisorption of H2 on the Ti surface and/or emission of H atoms during mechanically induced TiH2 decomposition.36 In turn, the formation of B-H or N-H surface groups may be a key stage for a modification of the Ti lattice by B or N atoms: Indeed, surface NHx vibrations at 1120 and 3438 cm-1 and 1660 and 3211 cm-1 were detected by DRIFT technique for h-BN and Ti/h-BN milled in H2/He flow for 1 h. In contrast, boron was not detected by DRIFT. As stated above, Huang and co-workers also observed a modification of Ti by N atoms after milling Ti/h-BN, but they, too, could not detect any boron or boron-related phases.57 The introduction of N atoms into the Ti lattice seems preferable to the introduction of B atoms. The solubility of B atoms in Ti is less than 0.1 atom %,58 and the atomic radius of B of 0.09 nm is larger than 0.071 nm reported for N.59 This is supported by the formation of a hexagonal-phase TiNx observed by XRD, consisting of nitrogen interstitials in the Ti-lattice detected after TPD. Nitrogen atoms introduced into the Ti lattice can trap hydrogen atoms, which makes the hydrogenization of Ti more efficient in the presence of h-BN additive. On the basis of these findings, we can conclude that under ball milling the BN molecules are partially decomposed into nitrogen and boron atoms at the Ti surface, and consequently the nitrogen atoms enter the Ti lattice, occupying octahedral interstitials or perhaps vacancies in the Ti lattice. Surprisingly, this seems to be the case for Ti + 16.7 wt % BN as well as for Ti + 0.6 wt % BN. The main difference between these samples is that for the former composition amorphous TiN can be detected additionally, which is the case to a much lesser extent for the latter composition. These results are also corroborated by XES measurements: As shown in Figure 11, the N KR-XES can nicely be fitted by the addition of 10% of TiN and 90% of h-BN. As stated above, we are aware of the fact that TiN was not detected in XRD, but the TiN-curves shown in Figure 11 only reflect the existence of Ti-N bonds, and not the structure of TiN, so this is not a contradiction. Furthermore, it can be concluded that N atoms have already entered the Ti lattice during milling. The nitrogen bonds to the hydrogen orbital by hybridization of N-2p H-1s states with Ti-3d. From these results, it can be concluded that hydrogen gives up its metallic character partially, in favor of a more covalent bonding to nitrogen. Hybridization of N-2p with H-1s orbitals is also reported in the case of LiNH2.60 Even though in the case of LiNH2 a reduction of hydrogen desorption temperatures is achieved by weakening covalent N-H bonds,23 in the case of hydrogen in Ti N-H bonds seem to weaken the Ti-H bonds, leading to a reduction of desorption temperature in this system. The results of first-principle simulations of XES spectra support the fact of Ti-N bond formation in course of

Borchers et al. Ti/h-BN mechanochemical activation. Incidentally, the B KRXES revealed that B is only bonded as BN, and the simulations showed that bonding B-H is rather unfavorable in the Ti lattice. It should be mentioned once more that no B-H or B-Ti bonding was found by XRD in this work nor by Kondo and co-workers,16 either. The TPD spectra of Ti/h-BN milled for various times in H2/ He flow and of TiH2/h-BN all exhibit three to four more or less distinct peaks: (1) a high-temperature peak at 900-1000 K, (2) a peak at 800-850 K, (3) a peak at ∼750 K, and (4) a peak at ∼650 K, see Figure 6. The comparison with pure Ti shows that peak 1 is a Ti bulk peak. This is reflected by the fact that samples 3, 4, and TiH2, which contain no pure Ti, do not have this peak. In a previous study,36 where TPD spectra of TiH2 with and without milling treatment, the latter with carbon addition, were compared, it was found that the unmilled TiH2 desorbed 90% of the hydrogen at 817 K, whereas a milled TiH2/C mixture desorbed exhibiting a three-peak spectrum with its maximum at 674 K.36 This indicates that peak 2 stems from desorption of pure TiH2. The two low-temperature peaks 3 and 4 owe their existence to structural defects and to interstitials in Ti and TiH2, respectively, where peak 3 is most probably due to structural defects. This statement is confirmed by the fact that although peak 3 is present already after short milling times, peak 4 shows the most dramatic growth with increasing milling times and the assumption that structural defects evolve much faster upon milling than the emplacement of interstitials, because the latter requires diffusion and the former does not. Additionally, it can be noted that TiH2 is modified by interstitial N or C atoms as well as Ti.36 Some open questions remain: Where is the boron after decomposition of BN? Kondo and co-workers suggest that residual B atoms may exist as amorphous B in the composites,16 but they give no experimental evidence for this assumption. Can the sorption-desorption of hydrogen by Ti/h-BN be repeated in further cycles? This was investigated for Ti/graphite mixtures.44 In that study, it was found that C atoms enter the Ti lattice interstitially as well. However, after the first heating cycle, TiC was formed, which supressed further Ti-hydride formation in a second cycle. The fact that no TiN was found after TPD in the XRD spectra indicates that with h-BN addition, further hydrogen loading cycles might be possible for Ti, but this has yet to be confirmed. Further work needs to be done to address these issues. Studies with boron alone as an additive to Ti are underway. 6. Conclusions We have found that hexagonal BN is an effective additive to improve hydrogen sorption-desorption properties of Ti. While h-BN is added, H2 sorption significantly increases due to formation of long-lived H2 activation centers and reducing diffusion hindrances. TiH2 mechanically prepared can be decomposed at lower temperatures because of a modification of the Ti lattice by interstitial N atoms. Acknowledgment. We gratefully acknowledge the Russian Academy of Sciences Program, Project 01.2.006 13395. This work was partly supported by INTAS, project no. 05-10000057672 and RFBR, project nos. 07-03-00610 and 05-02-16438, which is gratefully acknowledged. We also gratefully acknowledge the Research Council of the President of the Russian Federation (grant NSH-4192.2006.2), Canada Research Chair Program and Natural Sciences and Engineering Research Council of Canada (NSERC) for support.

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BN Additive on Hydrogen Sorption by Ti under Mechanical Treatment

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