Adsorption and Desorption of Hydrogen by Gas-Phase Palladium

Article pubs.acs.org/JPCA

Adsorption and Desorption of Hydrogen by Gas-Phase Palladium Clusters Revealed by In Situ Thermal Desorption Spectroscopy Masato Takenouchi, Satoshi Kudoh, Ken Miyajima, and Fumitaka Mafuné* Department of Basic Science, School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan S Supporting Information *

ABSTRACT: Adsorption and desorption of hydrogen by gas-phase Pd clusters, Pdn+, were investigated by thermal desorption spectroscopy (TDS) experiments and density functional theory (DFT) calculations. The desorption processes were examined by heating the clusters that had adsorbed hydrogen at room temperature. The clusters remaining after heating were monitored by mass spectrometry as a function of temperature up to 1000 K, and the temperature-programmed desorption (TPD) curve was obtained for each Pdn+. It was found that hydrogen molecules were released from the clusters into the gas phase with increasing temperature until bare Pdn+ was formed. The threshold energy for desorption, estimated from the TPD curve, was compared to the desorption energy calculated by using DFT, indicating that smaller Pdn+ clusters (n ≤ 6) tended to have weakly adsorbed hydrogen molecules, whereas larger Pdn+ clusters (n ≥ 7) had dissociatively adsorbed hydrogen atoms on the surface. Highly likely, the nonmetallic nature of the small Pd clusters prevents hydrogen molecule from adsorbing dissociatively on the surface.

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INTRODUCTION

speed when Pd {111} of the Pd nanoparticles is exposed to hydrogen.3 Taking the discussion one step further, the question arises whether subnanometer-sized Pd clusters also exhibit hydrogen storage capacity and how many Pd atoms are required for the clusters to acquire said capacity.7−10 Atoms in subnanometersized clusters are predominantly exposed to the surface, forming a flexible framework, and hence the clusters can provide adsorption sites to hydrogen. In contrast, electrons that are to be donated to the antibonding orbital of the hydrogen molecule are limited due to their nonmetallic nature. In the present study, we investigated adsorption and desorption of hydrogen by Pd clusters and their hydrides.

Hydrogen storage is one of the key technologies required for the practical application of fuel cells. In particular, hydrogen storage by metals has been studied both intensively and extensively in terms of storing hydrogen easily and safely.1 The storage capacity of metals is known to depend on the d-band structure of the host metal above the Fermi level.2 Hence Pd, which has the highest electron density of state at the Fermi level of any pure metal, has been an important subject of research.2 The hydrogen adsorption of bulk Pd is considered to comprise several steps: (i) a hydrogen molecule dissociates into individual atoms on the surface of Pd by accepting, into its antibonding molecular orbital, conduction electrons from bulk Pd. (ii) The atoms penetrate the subsurface, and (iii) they diffuse into the octahedral interstitial site of the Pd lattice.3 In terms of the adsorption processes, nanosized Pd particles should show distinct differences in their hydrogen storage capacities because hydrogen likely occupies surface and subsurface adsorption sites, apart from the regular interstitial sites, in the nanoparticles.4 It is also known that small nanoparticles exhibit dilated lattices that would provide larger interstitial volumes for hydrogen storage.4 In addition, the smaller dimensions enhance fast kinetics for hydrogen adsorption.5 In fact, nanoparticles of different morphologies (spherical and platelet) with dimensions in the 4−10 nm range have been observed to store 10−20% more hydrogen than the bulk at 10 bar due to additional surface and subsurface adsorption sites.6 Further, rapid diffusion from the surface to the subsurface has been found to accelerate the adsorption © XXXX American Chemical Society

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COMPUTATIONAL SECTION To estimate the desorption energy of a hydrogen molecule from PdnDm+, DFT calculations were performed using the Gaussian 09 program.11 The LANL2DZ effective core potential and basis set12 was used to describe Pd atoms, while the 631G(d,p) basis set was used to describe H atoms. Becke’s threeparameter hybrid density functional13 with the Lee−Yang−Parr correlation functional14 (B3LYP) was used for all calculations. Several hundred initial configurations for each cluster were generated by placing Pd and H atoms at random and fully optimized with DFT calculations. We adopted the optimized structures having the lowest energies as the most stable isomers Received: April 24, 2015 Revised: June 5, 2015

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DOI: 10.1021/acs.jpca.5b03926 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. Experimental apparatus used in the present study.

digitized using an oscilloscope (LeCroy LT374). The averaged TOF spectra (typically 500 sweeps) were sent to a computer for analysis. The mass resolution (m/Δm) was sufficiently high (>1000 at m = 1000) to distinguish an O atom from a H2O molecule that may appear in the mass spectrum as an impurity. Figure 2 shows mass spectra of Pd5Dm+ after passing through the extension tube heated at (a) 300 K, (b) 368 K, (c) 577 K,

for each cluster. The vibrational frequencies were calculated for the lowest energy structure of each cluster to obtain zero-point vibrational energy (ZPVE) and check the optimized structure. All of the lowest energy structures had zero imaginary frequencies, suggesting that these structures corresponded to the equilibrium structures. All of the calculated energies described below included the ZPVE correction.

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EXPERIMENTAL SECTION To study adsorption and desorption of hydrogen by Pd clusters on the basis of thermal responses, newly developed in situ thermal desorption spectroscopy (TDS) was used to investigate gas-phase Pd cluster ions in combination with time-of-flight (TOF) mass spectrometry (see Figure 1).15−17 A Pd metal rod with purity of 99.9% was vaporized using the focused second harmonic of a Nd:YAG pulse laser at a typical pulse energy of 10 mJ and a repetition rate of 10 Hz. Pd cluster ions, denoted as Pdn+, where n indicates the cluster size (n = 2−14), were formed in the gas flow of helium from a pulsed valve with a stagnation pressure of 0.8 MPa. In addition, hydrogen was adsorbed on Pdn+ by including 1% D2 gas in the He carrier gas, thus forming PdnDm+ (n = 2−14) at room temperature. As control measurements with H2 revealed very similar behavior with D2, the results obtained by using D2 are discussed in this Article as hydrogen. After the adsorption, all of the cluster ions were introduced into an extension tube (4 mm inner diameter, 120 mm long) before expansion into a vacuum chamber. The typical gas pressure inside the tube mostly contributed by He was monitored by using a pressure gauge, rising to almost 1 × 103 Pa during pulsing. The number density of He gas was estimated to be ∼1018 molecules cm−3, giving a collision frequency of 3 × 108 s−1. As the residence time of the cluster ions in the reaction gas cell was estimated to be ∼100 μs irrespective of temperature and mass, the clusters are subjected to 30 000 collisions there. Hence, thermal equilibrium of the clusters was considered to be achieved through collisions with the He carrier gas well before expansion into the vacuum.18−20 The temperature of the extension tube was varied at 7 K min−1 in the range of 298−1000 K using a resistive heater and monitored using thermocouples. The changes of the cluster ions caused by heat were monitored by mass spectrometry. During the mass analysis, the cluster ions gained kinetic energy of 3.5 keV in the acceleration region. After traveling in a 1-m field-free region, the ions were reversed by the reflectron and detected using a Hamamatsu double-microchannel plate detector. Signals from the detector were amplified with a preamplifier (Stanford Research Systems SR445A) and

Figure 2. Mass spectra of Pd5Dm+ after passing through the heated extension tube at (a) 300, (b) 368, (c) 577, and (d) 978 K.

and (d) 978 K. Mass peaks shift to the smaller m/z value with an increase in temperature, indicating that a fragment was released into the gas phase by heating. However, the mass peak pattern is so complicated that the intensity change of each Pd5Dm+ appears unidentified, because a Pd atom has isotopes whose natural abundances are 102Pd (1.02%), 104Pd (11.14%), 105 Pd (22.33%), 106Pd (27.33%), 108Pd (26.46%), and 110Pd (11.72%). Conversely, the known isotope ratios enable us to estimate the contribution of each Pd5Dm+. The mass peak was deconvoluted into each peak of Pd5Dm+ by least-squares fittings, considering the isotope ratios of the Pd atom. The deconvolution was reasonably well, and the intensity of the cluster was thus evaluated (see Supporting Information Figure S1). The intensity of the cluster of interest then was obtained as a function of temperature, the changes of which show quantitatively cluster that disappeared and newly formed by the thermal desorption as the temperature rose, corresponding to a temperature-programmed-desorption (TPD) curve for an adsorbate on a solid surface. Note that the cluster ion remaining after thermal desorption was monitored, and neutral species desorbing from the cluster ion was not probed. In addition, nascent cluster ions were always supplied to the temperature controlled extension tube, and hence our TPD curves were given as an integrated form. For comparison with B


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Pd4Dm+ that remained after heating as a function of the temperature. Here, pristine Pd4Dm+ prepared at room temperature involved either even or odd numbers of hydrogen atoms, probably because hydrogen molecules had been adsorbed on bare Pd4+ or prehydrated Pd4D+ formed by the laser ablation in the cluster source. In both cases, nascent Pd4+ had up to 10 hydrogen atoms, losing 5−6 hydrogen atoms at temperatures below 400 K. Most hydrogen atoms were released in a narrow temperature window, so that it is difficult to infer whether hydrogen was released atom by atom or molecule by molecule from the TPD curves alone, although a molecular release is known to be energetically favorable. At 500 K, Pd4Dm+ (m = 0−5) remained after heating, albeit Pd4D4+ and Pd4D5+ were less abundant. The intensity of Pd4D2+ decreased at 700 K, whereas the intensity of bare Pd4+ increased to the same extent. This concomitant change evidently indicates that a hydrogen molecule is desorbed from a cluster by heating as

conventional TPD curves of the bulk materials, the obtained raw data were processed numerically into a differential form to show the amount of hydrogen molecules leaving the clusters. Experimentally, the detection efficiency of the cluster ions by mass spectrometry lowered gradually with an increase in temperature, because ion bunch in the gas flow expanded by the thermal diffusion. Hence, the intensity was normalized by setting the total intensity of PdnDm+ (m = 0−16) equal to unity, ∑m[PdnD+m] = 1, where the square brackets represent the intensity of the cluster ion in them. In other words, the intensity was expressed as the intensity ratio. It deserves to be noted that the intensity was normalized for each n. The change of the detection efficiency, more or less, can be mass-dependent in the wide mass range (m/z = 100−2000), but the change in a limited mass range, for instance, between Pdn+ and PdnDm+, was regarded as negligible. In addition, there was no evidence for thermal dissociation of Pdn+ itself in our experiments. It is consistent with the result of the DFT calculation that the dissociation energy of Pd7+ → Pd4+ + Pd3 exceeds 3.1 eV, which was almost unreachable by thermal desorption spectroscopy at 1000 K.

Pd4D2+ → Pd4 + + D2

(1)

This was also the case for Pd4D3+, which exhibited a concomitant change above 600 K. In this case, Pd4D+ was generated upon heating as

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RESULTS AND DISCUSSION Thermal Desorption Spectroscopy. Figure 3a and b shows the TPD curve as an integrated form, the intensity of the

Pd4D3+ → Pd4D+ + D2

(2)

The release of hydrogen at low and high temperatures was supported by the DFT calculations. For Pd4+, a hydrogen molecule dissociatively adsorbed onto a pyramidal Pd 4+ structure, one atom on the hollow site and the other on a bridge site. The adsorption energy for Pd4+ + D2 → Pd4D2+, the difference in the formation energy between Pd4+ + D2 and Pd4D2+, was calculated to be −0.85 eV. By contrast, the second hydrogen molecule adsorbed outside in molecular form, suggesting that D2 was weakly bound to the cluster. In fact, the adsorption energy for Pd4D2+ + D2 → Pd4D4+ was calculated to be −0.32 eV. The third and subsequent hydrogen molecules also loosely adsorbed outside the cluster in molecular form. For the case of an odd number of hydrogen atoms, one hydrogen atom adsorbed onto a hollow site of Pd4+ to form Pd4D+. A hydrogen molecule then dissociatively adsorbed onto the Pd4D+ structure, forming Pd4D3+, with three hydrogen atoms attached onto each hollow site. The adsorption energy for Pd4D+ + D2 → Pd4D3+ was calculated to be −0.7 eV. The second hydrogen molecule adsorbed in molecular form with an adsorption energy of −0.3 eV, showing that D2 was loosely bound. Thus, although up to 10 hydrogen atoms appeared to be involved in nascent Pd 4 + based on simple mass spectrometry, the DFT calculations revealed that Pd4+ can possess at most three hydrogen atoms in atomic form; it takes the other hydrogen in molecular form. This is consistent with the results of the TDS experiments, which showed that Pd4Dm+ (m = 0−3) predominantly existed at temperatures greater than 500 K and became Pd4+ and Pd4D+ at temperatures greater than 800 K. The release of atomic hydrogen was not observed in the temperature desorption experiments. Energy as high as 3.0 eV was required to remove one hydrogen atom from Pd4D2+ according to the DFT calculations. Hence, it is possible to consider that hydrogen was released from the clusters as hydrogen molecules, indicating that there were two series in the adsorption and desorption of hydrogen, one series containing

Figure 3. Intensity of Pd4Dm+ as a function of the temperature. (a) A series of Pd4Dm+ with an even number of hydrogen atoms and (b) a series of Pd4Dm+ with an odd number of hydrogen atoms. (c) Geometrical structures of Pd4Dm+ (m = 1−6) determined by density functional theory (DFT) calculations. C


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The Journal of Physical Chemistry A an even number of hydrogen atoms and the other containing an odd number. Hereafter, let us consider just the series comprising an even number of hydrogen atoms. For n = 7, nascent Pd7+ possessed as many as 12 hydrogen atoms at room temperature as shown in Figure 4a. It released three hydrogen

... → Pd nDm + 2+ → Pd nDm+ + D2 → Pd nDm − 2+ + 2D2 (3)

→ ...

occurred in a stepwise manner in a residence time of 100 μs inside the tube, where PdnDm+ increased by the desorption of a hydrogen molecule from PdnDm+2+ and decreased by the desorption from PdnDm+ into PdnDm−2+ as well. The reaction rate of the unimolecular reaction, PdnDm+ → PdnDm−2+, depends on the rate constant, km, which is the function of temperature, T, as km = A m exp( −Em /kBT )

(4)

where Am, Em, and kB are the pre-exponential factor of the Arrhenius equation, the threshold energy for the reaction, and the Boltzmann constant, respectively. Hence, the reaction rate increases with an increase in temperature, accelerating desorption of hydrogen at higher temperature. The shape of the TPD curve is determined by the threshold energies and the pre-exponential factors of a series of the unimolecular reactions. Indeed, the threshold energies for the D2 release were obtained from the TPD curves. The threshold energies were 0.2 and 1.2 eV for Pd7D2+ → Pd7+ + D2 and Pd7D4+ → Pd7D2+ + D2, respectively. For quantitative analysis of the TPD curve, not only the forward but also the following backward reactions should be taken into consideration, because D2 in the carrier gas was present in the extension tube.16 Figure 4. (a) Intensity of Pd7Dm+ with an even number of hydrogen atoms as a function of the temperature. (b) Geometrical structures of Pd7Dm+ (m = 0, 2, 4, 6, and 8) determined by the DFT calculations.

Pd nDm+ → Pd nDm − 2+ + D2

forward reaction

(5)

Pd nDm − 2+ + D2 → Pd nDm+

backward reaction

(6)

As the rate of the backward reaction depends on the number density of D2 in the extension tube, the backward reaction contributes significantly when the partial pressure of D2 in the carrier gas is high enough. In fact, the TPD curves are known to shift to the higher temperature, when the backward reaction proceeds in greater extent at higher partial pressure.16 In contrast, in the present study, the partial pressure of D2 was set sufficiently low that the TPD curve was confirmed to remain almost unchanged with the slight variation of the partial pressure. Hence, the backward reaction 6 is deduced to be negligible in this treatment. Figure 5 summarizes the threshold energy for the desorption of D2 from PdnDm+ forming PdnDm−2+ (n = 4, 7, and 10, m = 2, 4, 6, ...) together with the desorption energy obtained by the DFT calculations for n = 4, 7. The desorption energy equals the difference in the formation energy between PdnDm+ and PdnDm−2+ + D2, whereas the threshold energy equals the energy difference between PdnDm+ and the transition state, if any, giving the energy barrier in the reaction pathway leading to desorption. Hence, the threshold energy must be higher than the desorption energy. However, in some cases, threshold energy exhibits lower values than the desorption energy, probably due to uncertainty in the experiments and calculations. Most likely, there are basically no energetic transition states that form higher activation barriers in the reaction pathway, so the threshold energy mostly equals the desorption energy.

molecules in a temperature range of 300−400 K, and then sequentially released one hydrogen molecule at temperatures above 400 K. At 550 K, Pd7 possessed four hydrogen atoms, obviously indicating that more hydrogen atoms were able to adsorb strongly onto Pd7+ than onto Pd4+. It should be noted that the most abundant cluster transferred from Pd7D8+ to Pd7D6+ and from Pd7D6+ to Pd7D4+ with increasing temperature. However, the transfer from Pd7D4+ to Pd7D2+ was not so clear; instead, Pd7+ tended to increase at 600 K. These changes indicate that the threshold energy of the hydrogen release from Pd7D2+ was lower than that from Pd7D4+, and a hydrogen molecule was readily released at 600 K, which will be discussed later. The DFT calculations suggested that the most stable structure of Pd7+ was a pentagonal bipyramid (see Figure 4b). A hydrogen molecule dissociatively adsorbed onto Pd7+, which significantly distorted the framework of the Pd cluster. The adsorption energy was calculated to be −0.68 eV. Another hydrogen molecule also dissociatively adsorbed, bridging Pd atoms with an adsorption energy of −0.86 eV. The next hydrogen molecule adsorbed outside the cluster in molecular form. Thus, Pd7+ was able to possess at most four hydrogen atoms in atomic form, taking the other hydrogen in molecular form. Desorption Energy. In the present study, all of the cluster ions that had passed through the extension tube were observed by mass spectrometry. Unimolecular reactions: D


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Figure 5. Energies required to desorb a hydrogen molecule from PdnDm+, PdnDm+ → PdnDm−2+ + D2, for different values of n and m. The blue bars represent the threshold energy for desorption estimated from the TPD curves (see Supporting Information Figure S2), and the errors, caused upon fitting the model equations, were displayed by vertical bars. The red bars represent the desorption energy calculated by DFT. The arrows show the number of hydrogen atoms of the clusters having the highest desorption energy in each n.

Figure 6. Amount of hydrogen molecules leaving the cluster ions into the gas phase as a function of temperature, which was obtained by differentiating the average number of hydrogen atoms adsorbed on Pdn+ (see Supporting Information Figure S4). These curves correspond to the representation of a conventional TPD curve.

When compared to the results by DFT calculations, the first peak is assignable to hydrogen that is molecularly adsorbed onto the surface. With a desorption energy of ∼0.2 eV, the weakly bound hydrogen molecules are released into the gas phase by moderate heating. The second peak is assignable to hydrogen that has been dissociatively adsorbed onto the clusters. With a desorption energy of >0.6 eV, the hydrogen molecules are released into the gas phase when they are heated at temperatures above 500 K. Our TPD results show that bare clusters having no hydrogen atoms become abundant at temperatures above 700 K for all of the clusters except for Pd3+. There is a clear size dependence in the plot shown in Figure 6. The second peak increases with increasing cluster size, indicating that many more hydrogen molecules can adsorb dissociatively onto the clusters as the cluster size increases. To estimate the number of hydrogen atoms dissociatively adsorbed onto Pdn+, the average number of hydrogen atoms remaining in the clusters at 500 K was counted as an index because the second peak started at 500 K. These hydrogen atoms were released into the gas phase by intensive heating, resulting in the second peak. As shown in Figure 7, the average number of hydrogen atoms does not change significantly for n ≤ 6, whereas it increases almost linearly with cluster size for n ≥ 7 at 500 K (plot at 309 K is also indicated as a reference). The definitive change of the slope indicates that large clusters with n ≥ 7 are able to adsorb hydrogen stably. In other words, n = 7 is

Moreover, it deserves to be emphasized that both the threshold energy and the dissociation energy exhibit similar and strong m dependence. For n = 4, the desorption energy of the first hydrogen molecule, PdnD2+ → Pdn+ + D2, is more than that of the second hydrogen molecule, PdnD4+ → PdnD2+ + D2. In contrast, the energy magnitude is reversed at n = 7; the desorption energy of the second hydrogen molecule from Pd7D4+ is higher than that of the first one from Pd7D2+. Hence, Pd7D4+ was predominantly formed by heating the clusters to 550 K, transitioning to Pd7+ by slipping quickly through Pd7D2+ when Pd7D4+ was heated to 600 K. Pd7D2+ could have released the hydrogen molecule at temperatures lower than 600 K. Because desorption energy corresponds to the hydrogen affinity, Pd7D2+ is considered to have a higher hydrogen affinity than Pd7+. This is also the case for n = 10; the energy of the third hydrogen molecule from Pd10D6+ is higher than that of the first and second hydrogen molecules. Pd10D4+ is considered to have a higher hydrogen affinity than Pd10+ and Pd10D2+. Thus, there is a general propensity that the large Pd clusters will acquire higher hydrogen affinity by being hydrogenated. This propensity is considered to relate to the hydrogen adsorption capacity of Pd. Size Dependence. Figure 6 illustrates the amount of hydrogen molecules leaving the cluster ions into the gas phase as a function of temperature, which was obtained by differentiating the average number of hydrogen atoms adsorbed on Pdn+ (see Supporting Information Figure S4). The decrease of the number of hydrogen atoms adsorbed on Pd n + corresponds to the hydrogen molecule release into the gas phase. The curves thus obtained correspond to the conventional TPD curves in a differential form for solid materials. Pd4+ exhibits a first peak at 300−400 K, followed by a long tail in the higher-temperature region and a small peak at 750 K. This finding suggests that many hydrogen atoms are adsorbed so weakly on Pd4+ that they are readily released upon moderate heating, and that a small number of hydrogen atoms are adsorbed strongly. By contrast, Pd11+ exhibits a prominent second peak at 600 K. Many hydrogen atoms adsorbed onto Pd11+ remain after the clusters have been heated to 500 K. The contribution of the second peak increases with increasing cluster size.

Figure 7. Average number of hydrogen atoms remaining in the clusters, PdnDm+, at 309 K (○), 500 K (●) as a function of the cluster size. The number does not change significantly for n ≤ 6, whereas it increases almost linearly with increasing cluster size for n ≥ 7. E


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energy of −0.68 eV. Another hydrogen molecule also dissociatively adsorbed, bridging Pd atoms with an adsorption energy of −0.86 eV. The next hydrogen molecule adsorbed outside the cluster in molecular form, indicating that Pd7+ was able to possess at most four hydrogen atoms in atomic form, taking the other hydrogen in molecular form. Extensive analysis of the TPD curve for each Pdn+ indicates that smaller Pdn+ clusters (n ≤ 6) tended to have weakly adsorbed hydrogen molecules, whereas larger Pdn+ clusters (n ≥ 7) had dissociatively adsorbed hydrogen atoms on the surface. Highly likely, the nonmetallic nature of the small Pd clusters prevents the hydrogen molecule from adsorbing dissociatively on the surface. In terms of the hydrogen storage capacity, the smaller dimensions should enhance fast kinetics for hydrogen adsorption, but the small clusters are not able to transfer an electron to the hydrogen molecule. On balance, relatively large palladium clusters (n ≥ 7) are considered to have hydrogen storage ability similar to that of bulk Pd.

the threshold size for taking on hydrogen adsorption capacity. The slope of this increase, 0.75 ± 0.05, means that at n ≥ 7, Pdn+ has absorbed hydrogen atoms, such that the composition is expressed as PdD0.75. Comparison with the Bulk Phase. For a bulk Pd, it is known that a hydrogen molecule chemisorbs on the surface adsorption sites following dissociation into individual atoms even at low temperature (100 K). After filling most of the surface sites, hydrogen atoms start to occupy subsurface region, where the hydrogen atoms are bound weaker than the adsorbed atoms on the surface.21−29 In fact, the adsorption energies of a hydrogen molecule on the saturated surface and saturated subsurface are −0.89 and −0.71 eV, respectively, according to the theoretical study.26 The adsorption energy on the surface is consistent with the one for the gas-phase clusters having one or two hydrogen molecule(s) on the surface with the adsorption energy from −0.8 to −1.0 eV. In relation to the hydrogen storage capacity of the bulk Pd, the small gas-phase Pd clusters exhibit a bottleneck in the dissociative adsorption process of a hydrogen molecule. The Pd clusters are able to dissociate a few hydrogen molecules and have the other hydrogen as a molecular form. The propensity is ascribable to the nonmetallic nature of the small clusters. For bulk Pd, a hydrogen molecule dissociates into individual atoms on the surface of Pd by accepting, into its antibonding molecular orbital, conduction electrons from Pd. Hydrogen atoms thus formed on the surface then are negatively charged by −0.13e.27 On the basis of its analogy, the gas-phase Pd clusters with one or two hydrogen molecules dissociatively adsorbed are considered to have been electron-deficient, so that they are not able to transfer an electron to the hydrogen molecule. The average number of hydrogen atoms adsorbed on the surface increases with cluster size for n ≥ 7, because the clusters can afford to transfer electrons to the hydrogen molecules more readily, as the cluster size increases. In terms of the hydrogen storage capacity, the smaller dimensions should enhance fast kinetics for hydrogen adsorption,5 whereas the smallest clusters are not able to transfer an electron to the hydrogen molecule. On balance, it has been indicated that relatively large palladium clusters (n ≥ 7) have hydrogen storage ability similar to that of bulk Pd.

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ASSOCIATED CONTENT

S Supporting Information *

Deconvolution of mass spectrum to estimate abundance of each cluster ion; relative ion intensity of Pd5Dm+ (m = 0−10) as a function of temperature obtained from the mass spectra; TPD curves in Figure S2 shown in a differential form; average number of hydrogen atoms calculated from Figure S2 as a function of temperature; and the amount of hydrogen leaving the cluster into the gas phase and its differential form corresponding to representation of a conventional TPD curve. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpca.5b03926.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-3-5454-6597. E-mail: [email protected]. jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (A) (no. 25248004) and (C) (no. 24550010) and for Exploratory Research (no. 26620002) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), and by the Genesis Research Institute, Inc., for the cluster research.

CONCLUSION Adsorption and desorption of hydrogen by Pdn+ were investigated by TDS experiments and DFT calculations. The desorption processes were examined by heating the clusters that had adsorbed hydrogen at room temperature as a function of temperature, forming TPD curve. For Pd4+, up to 10 hydrogen atoms attached to Pd4+ at room temperature, losing 5−6 hydrogen atoms at temperatures below 400 K. At 500 K, Pd4Dm+ (m = 0−5) remained after heating. The intensity of Pd4D2+ decreased at 700 K, whereas the intensity of bare Pd4+ increased to the same extent, indicating that a hydrogen molecule is desorbed from a cluster by heating as

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REFERENCES

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Pd4D2+ → Pd4 + + D2

The threshold energy of the desorption was found to be 0.7 eV by the analysis of the TPD curve. The DFT calculations were consistent with the experiments, indicating that Pd4+ can possess at most three hydrogen atoms in atomic form; it takes the other hydrogen in molecular form. For Pd7+, a hydrogen molecule dissociatively adsorbed onto Pd7+, with the adsorption F


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Adsorption and Desorption of Hydrogen by Gas-Phase Palladium

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