J. Phys. Chem. B 2006, 110, 21783-21787
21783
Chemical Activity of Palladium Clusters: Sorption of Hydrogen Sheng-Yang Huang,† Chin-Da Huang,† Boh-Tze Chang,† and Chuin-Tih Yeh*,‡ Department of Chemistry, National Tsing Hua UniVersity, Hsinchu, Taiwan 30013, Republic of China, and Fuel Cell Center, Yuan Ze UniVersity, Taoyuan, Taiwan 32003, Republic of China ReceiVed: May 30, 2006; In Final Form: August 23, 2006
Samples of Pd/SiO2 with average cluster size of palladium (dPd) ranging from 1 to 10 nm were prepared by different methods, such as incipient wetness impregnation (IW), ion exchange (IE), and sol-gel (SG). The dispersion (DPd) for prepared Pd/SiO2 samples varied with the method of preparation and showed a trend of IW < IE < SG. Chemical interaction between the dispersed nanopalladium and hydrogen gas was calorimetrically studied as a function of hydrogen uptake. Measured profiles of interaction energy vary with the dPd of studied samples. The initial heat of hydrogen chemisorption (QHi) also steeply increases for particles with dPd smaller than 2 nm.
1. Introduction In the last two decades, nanoscience is expected as one of the great leaps in the civilization. A practical application of nanomaterials relies profoundly on the advance of their preparation and properties. Many physical properties such as mechanic strength,1 melting point,2 absorption spectra,3-5 magnetic susceptivity,6,7 and quantum confinement effect,8,9 as well as electronic structure,10,11 of nanomaterials have been reported to vary with size. Since the chemical property of a material depends heavily on its energy levels in the valence shell, it is expected that the chemical activity of nanoparticles should also vary with their size. Hydrogen-rich gas (HRG) used in the proton exchange membrane fuel cell (PEMFC) generally contaminated with a small fraction of CO.12 Unfortunately, CO is a poison to the Pt electrocatalyst and must be reduced. An extensive research has been the focus on palladium metals because the absorption of hydrogen by palladium metals can proceed at low pressures. Moreover, hydrogen gas of ultrahigh purity is achieved by permeating low graded gas through palladium thimbles in laboratories. Palladium powders with primary particles in a size of dPd ≈ 10 nm can be prepared by the reductive precipitation (RP) of palladium ions in an aqueous solution.13,14 However, primary particles prepared in the precipitation are thermally unstable and tend to aggregate into secondary particles and subsequently coalesce into large particles (∼µm) in the solution. The incipient wetness impregnation (IW)15 is a physical method that deposits the primary particles on porous solids, such as SiO2. Vast surface area of porous solid (support) separates the deposited primary particles and prevents their coalescence. The IW is a convenient and generally accepted method for the preparation of supported metallic catalysts. Supported Pd/SiO2 catalysts with Pd particles of ∼6 nm may be prepared. The size of Pd particles on supported Pd/SiO2 may be further reduced by a chemical method of ion exchange (IE).16 Palladium * To whom correspondence should be addressed. E-mail: ctyeh@ mx.nthu.edu.tw. Fax: 886-3-5711082. † National Tsing Hua University. ‡ Yuzn Ze University.
cations in aqueous solution are chemically bonded to a silica surface:
Pd2+ + 2H-OSis f Pd-(OSis)2 + 2H+
(1)
Sis indicates silicon atoms exposed on the surface of silica. The extent of bond formation varies with pH of the impregnating solution. Pd/SiO2 samples with an average palladium size of dPd ≈ 3 nm have been prepared by the IE method at pH ) 9. The extent of reaction 1 may also be enhanced by an increase in density of siloxy groups (HOSis) pendant on the surface of silica. The density varies significantly on the history of silica preparation. A freshly prepared silica gel generally has a maximum density. However, it diminishes upon a formation of inert siloxane bonds through the following dehydration:
2HOSis f SiOSi + H2O
(2)
during treatments of drying and calcinations. Accordingly, Pd/ SiO2 samples with palladium dispersed in very small size may be prepared by a sol-gel (SG) method.17,18 The SG method allows reaction 1 at the stage of gel formation when the extent of reaction 2 is low. In this report, the chemical activity as a function of particle size has been measured by sorption of hydrogen into palladium. We discover that the initial heat of hydrogen adsorption steeply increases for particles with dPd smaller than 2 nm. The size of dispersed metallic clusters affects not only their chemical activity but also their application in technologies of fuel cell, gas sensor, and heterogeneous catalysis. 2. Experimental Section 2.1. Sample Preparation. Table 1 lists palladium samples prepared in this study. A sample of pure palladium powders was prepared by the reductive precipitation of PdCl2 (Merck).13 Supported Pd/SiO2 samples were prepared by three methods, that is, IW, IE, and SG, described in the Introduction section. Commercial SiO2 (Cab-o-sil M-5, SA ) 200 m2 g-1) was used as support in preparation of IW and IE samples. The IW samples were prepared by impregnating the SiO2 with an aqueous
10.1021/jp063321r CCC: $33.50 © 2006 American Chemical Society Published on Web 10/10/2006
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TABLE 1: Physical Properties Characterized for Samples Prepared in This Study pretreatment
characterization
sample
Pd wt %
Tca
Trb
DPdc
DPd,TEMd
QHie
QHabf
QOig
Pd powder (RP) 20Pd/SiO2 (IW) 10Pd/SiO2 (IW) 05Pd/SiO2 (IW) 20Pd/SiO2 (IE) 10Pd/SiO2 (IE) 20Pd/SiO2 (SG) 10Pd/SiO2 (SG) 10Pd/SiO2 (SG) 10Pd/SiO2 (SG) 05Pd/SiO2 (SG)
100 1.86 0.87 0.45 1.84 0.90 2.05 0.95 0.94 0.98 0.47
373 723 723 723 723 723 723 573 723 873 723
373 573 573 573 573 573 573 573 573 573 573
7 18 20 21 29 34 57 60 75 81 85
10
94 92 93 95 107 113 120 115 131 149 183
39 37 37 38 42 44 43 48 45 70 65
272 272 280 324 320 321 335 377 369 376
25 33 60 82 90
a
Tc: calcination temperature (in K). b Tr: reduction temperature (in K). c DPd: dispersion of palladium obtained using H2 chemisorption (in %). d DPd,TEM: dispersion of palladium obtained using TEM (in %) according to DPd(%) ) 90/dPd(nm). e QHi: initial heat of hydrogen adsorption (in kJ mol-1). f QHab: heat of hydrogen absorption (in kJ mol-1). g QOi: initial heat of oxygen adsorption (in kJ mol-1).
solution of PdCl2. The IE samples were prepared by suspension; the SiO2 was added to an aqueous solution of Pd(NO3)2 (Strem) followed by an ion-exchange step at pH ) 9.0 (controlled by NH4OH). Silica support used in the SG samples was prepared, at the laboratory, from tetraethoxysilane (TEOS) (Acros). After refluxing TEOS with ethanol at 353 K, a gelation was formed upon addition of PdCl2 and NH4OH. All of the prepared samples were subsequently dried at 373 K, calcined at 723 K, and stored as fresh samples of IW, IE, and SG, respectively. Column 2 of Table 1 lists palladium loadings analyzed by ICP for the fresh samples. 2.2. Sample Characterization and Hydrogen Chemisorption. Physical properties of the freshly prepared samples were characterized by inductively coupled plasma-atomic emission spectrometer (ICP-AES), X-ray diffraction (XRD), and analytic transmission electron microscopy (AEM). The XRD analysis was performed on a MAC Science MXP18 X-ray diffractometer over a 2θ range 20-80° with Cu KR radiation. A tube voltage of 40 kV and a current of 100 mA were used for the scanning. TEM characterization was performed on a JEOL-2010 microscopy equipped with a LaB6 electron gun source operated at 200 kV. The dispersion of palladium (DPd), that is, fraction of atoms exposed to surface of palladium crystallites, on prepared samples was determined from isotherms of hydrogen chemisorption at 313 K. Each sample of 0.5 g was inserted into an adsorption cell connected to a volumetric system. The sample was pretreated with a reduction in flowing hydrogen (30 mL min-1) for 1 h and an evacuation at 10-3 Pa for 2 h prior to hydrogen uptake. Supported Pd samples were generally pretreated at 573 K. A mild pretreatment at 373 K was specially selected for the Pd powders to prevent severe coalescence. Controlled doses of hydrogen gases were sequentially introduced to an evacuated sample to pursue its isotherm of hydrogen uptake. Hydrogen uptake (∆NH) by the sample upon each dose was calculated from the pressure changes monitored by a Texas pressure gauge (with a resolution of 1 Pa). Heats evolved on these uptakes were measured by a Setaram C-80 differential microcalorimeter embracing the sample cell. Table 1 lists the fresh catalysts prepared and results on their characterization in this study. 3. Results and Discussion 3.1. Sorption of Hydrogen into Palladium. Figure 1 compares isotherms of hydrogen uptake volumetrically monitored for five samples. Each isotherm showed three consecutive stages of hydrogen uptake (NH) on increasing PH2: (stage 1) initial uptake at low overpressure (PH2 < 100 Pa); (stage 2)
Figure 1. Adsorption isotherms of hydrogen uptake at 313 K for different palladium samples: (a) Pd powder (RP); (b) 20Pd/SiO2 (IW); (c) 20Pd/SiO2 (IE); (d) 10Pd/SiO2 (SG); and (e) 05Pd/SiO2 (SG).
median uptake of NH between 0.1 and 3 kPa; (stage 3) final increase of NH between 3 and 5 kPa. The contribution of these three stages in isotherms of Figure 1 varies with the method of sample preparation. Stage 1 is obscure for the Pd powers [isotherm a] while stages 2 and 3 become negligible for the SG samples. Hydrogen uptake at the first stage has been assigned to chemisorption on palladium surface.19 The dispersion of palladium in each sample can be estimated from the uptake in this stage and the number of palladium atoms in sample (NPd) through
DPd ) NH (at PH ) 100 Pa)/NPd
(I)
Column 5 of Table 1 lists the DPd estimated for all of the samples freshly prepared in this study. The unsupported palladium powders exhibited a low dispersion of DPd < 10%. High dispersions were found from supported Pd/SiO2 samples. Estimated DPd for the supported samples varied significantly with the methods of preparation and showed a trend of
IW < IE < SG
(II)
Among supported samples prepared by the same method, DPd slightly decreased on increasing the loading of palladium.
Chemical Activity of Palladium Clusters
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Figure 2. Experimental heat flow monitored from Setaram C-80 DSC on introduction of 12 consecutive doses of hydrogen into palladium powders (see isotherm a in Figure 1).
Figure 3. Differential heat of hydrogen uptake for hydrogen chemisorption on samples of (a) Pd powder (RP), (b) 20Pd/SiO2 (IW), (c) 20Pd/SiO2 (IE), (d) 10Pd/SiO2 (SG), and (e) 05Pd/SiO2 (SG).
The uptake of hydrogen on palladium is an exothermic process. Figure 2 presents a sequence of heat flow monitored by the Setaram C-80 calorimeter during the isotherm measurement for the sample of Pd powders. The system temperature was held at 313.00 ( 0.01 K for the whole 5 h of monitoring period (a steady temperature has been found essential for maintaining decent DSC baseline). Twelve doses of hydrogen were sequentially introduced to the sample, and each dose caused an exothermic peak that returned to the baseline in 1 × 103 s. Heat (q) evolved in each dose may be integrated from its peak area. Differential heats of hydrogen uptake (QH) were calculated from the q measured from DSC and the ∆NH monitored from the volumetric measurement, according to
QH ) q/∆NH
(III)
Calculated QH of an isotherm generally decreased on increasing NH. Figure 3 compares uptake profiles of calculated QH for the five samples illustrated in Figure 1. Each profile exhibited three stages on increasing NH: (stage A) QH decreases from an initial value (QHi, which is shown in Figure 4 and increases with the DPd of sample) to a level off value of QH ≈ 90 kJ (mole H2)-1. This decline feature is not prominent in profiles a and b for samples of low DPd. These two profiles showed a short stage A with a constant QH ≈ 90 kJ (mol H2)-1. (stage B) QH decreases from 90 kJ (mole H2)-1 to a valley approximately 25 kJ (mole H2)-1. (stage C) A small peak occurs at the end of each profile. The width of this end peak is broad for the palladium powders (curve a) but narrows on increasing DPd (curves b-e) of the sample. Behm et al. proposed, based on a temperature programmed desorption study,20 that hydrogen in absorption systems may stay in four different environments, molecules in the gaseous phase (Hg), atoms dissociatively chemisorbed on surface (Hc), atoms incorporated at the subsurface of palladium crystallites (Hs), and hydride dissolved in the bulk of palladium (Hh). The features observed in Figures 1 and 3 can be correlated to absorption of Hg to different environments. Upon collision on palladium crystallites, hydrogen gases initially introduced to the system (at PH2 < 100 Pa) were chemisorbed dissociatively on
Figure 4. Effect of the average particle size of nanopalladium (dPd) on the initial heat of hydrogen chemisorption (QHi).
palladium surface through a fast equilibrium of
Hg S Hc
(3)
The chemisorption is an exothermic step [Qc > 90 kJ (mol H2)-1] that does not require significant activation energy. After the surface was saturated with a layer of chemisorbed hydrogen, NH increases mildly in stage B (PH2 between 10 and 3 × 103 Pa). In this stage, accompanied heat gradually decreases to QH ) 25 kJ (mol H2)-1. The uptake in this stage results from a penetration of gaseous hydrogen into the subsurface layer of palladium crystallites:
H g S Hs
(4)
Low-energy electron diffraction (LEED) studies suggested that a reconstruction of surface palladium occurred during the penetration.21 An R-phase hydride22 generally mentioned in absorption of hydrogen into bulk palladium should include combination of reactions 3 and 4.
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Figure 5. Transmission electron microgram for 05Pd/SiO2 (SG) by hydrogen at 573 K.
In isotherms a, b, and c (from samples with low DPd) of Figure 1, NH increases abruptly again on raising PH2 > 3 kPa and levels off around 5 kPa. The additional uptake in this stage reflects an absorption of hydrogen into bulk palladium to form palladium hydride (β-hydride), that is,
Hg S Hh
(5)
For samples of crystalline palladium, PdHh is a bulk hydride with an enthalpy of formation approximately -36 kJ (mol H2)-1.23,24 Equations 3 to 5 describe the three elementary stages involved in the absorption of hydrogen into palladium. They explain the three stages observed in the isotherms of Figure 1 and correlate well to the convolution of the three stages distinguished in calorimetric profiles of Figure 3. Noteworthy, the average size of palladium [dPd, estimated from 90/DPd(%)]22,25 significantly affects the phenomena of the absorption. All of the palladium atoms in cluster are exposed to the cluster surface as the size decreases to dPd ≈ 1 nm. Equations 4 and 5 therefore become negligible. Figure 4 correlates effects of the dPd on the QHi for the samples prepared in this study. On decreasing dPd, the QHi remains at 90 kJ mol-1 for samples with dPd ≈ 10 nm, rises mildly at medium dPd, and rises abruptly as dPd < 2 nm. Chou et al.24 also measured the adsorption heats of hydrogen (integral heat) on Pd samples. The highest value, 100 kJ mol-1, was obtained on one of the most highly dispersed samples, and the tendency of data is in good agreement with our results. In this study, a 2-fold increase in the QHi (from 90 to 183 kJ mol-1) is noticed on decreasing the dPd from 10 nm to subnanometer. 3.2. Potential Energy Diagram. Figure 5 presents a transmission electron micrograph for a sample (DPd ) 85%) prepared by the SG method. A small fraction of palladium particles in the micrograph showed a size larger than 1 nm. However, the size of the majority (over 90% in number) of the palladium particles was less than 1 nm. The size of the subnanometer palladiums is so tiny that the diffraction cannot be found from XRD. The XRD patterns have been provided in the Supporting Information (Figure S2). Figure 6 presents potential energy profiles obtained from the calorimetric study for the process of hydrogen absorption into palladium. A vertical line at the center of the figure not only provides the interaction energy of absorbed hydrogen but also represents the surface of the palladium cluster. Hydrogen molecules (Hg) at the right end of the figure do not interact
Figure 6. Potential energy diagram for uptake of hydrogen on palladium. The abscissa indicates the position of hydrogen relative to the surface layer of palladium, while the ordinate indicates the energy of hydrogen relative to hydrogen molecules. The upper curve describes the energy surface of palladium crystallites with d ≈ 10 nm, while the bottom curve describes the energy change on a palladium cluster with a size of d ≈ 1 nm. The ripples in the palladium bulk indicate the variation of energy on diffusion hydrogen atoms into bulk. The potential energy remains negative for all transition states because negligible activation energy was found in our laboratory for the absorption of hydrogen into palladium.
with palladium. The potential energy of hydrogen decreases on moving toward left to interact with palladium. The energy of interacted hydrogen, that is, Hc, Hs, and Hh, varies with their physical environment and the particle size of palladium. Two energy profiles are presented in Figure 6 to show the effect of palladium size. The upper profile describes absorption of hydrogen into palladium crystallites with dPd ≈ 10 nm while the lower profile illustrates an absorption into a palladium cluster with a size ∼1 nm. According to Figure 6, the penetration of hydrogen gas into the sublayer (eq 4) and the absorption of hydrogen into the bulk (eq 5) of palladium crystallites are exothermic steps. However, a substantial overpressure of hydrogen (PH2) is essential in Figure 1 for these two incorporation steps. The overpressure is required for the expansion of the palladium lattice (from 0.389 to 0.403 nm)26 that accompanies the incorporation. The energy (Eexp) required for expansion can be supplied from the heat of hydride formation (Qf).27 Conceivably, Qf of a single hydrogen molecule is generally insufficient to overcome the required Eexp. The absorption becomes spontaneous at high PH2 when an ensemble of hydrogen molecules becomes available. They incorporate simultaneously into a palladium crystallite as the sum of Qf from the hydrogen ensemble exceeds the required Eexp. As a consequence, the heat of absorption becomes less than the heat of adsorption. 3.3. Size Effect on Adsorption on Metal Clusters. The size of palladium is seen to affect the energies of all kinds of interacted hydrogen in the sorption system (Figure 6). The most prominent effect is found in QHi. A similar size variation has also been found in this study for the heat of oxygen adsorption on the prepared palladium samples. The last column of Table 1 describes variations of the initial heat of oxygen adsorption (QOi) measured in this study. Measured QOi is 280 kJ (mol O2)-1 for palladium particles of large size (d ≈ 10 nm) and increases on
Chemical Activity of Palladium Clusters decreasing dPd. A steep increase in the QOi is again found on samples with dPd < 2 nm. The drastic increase in heat of chemisorption (Qc) on decreasing the particle diameter of metal to subnanometer is conceivable from the following points of view: 1. Vacancy in d-Orbital. Heat for chemisorption of adsorbate on transition metals generally increases with the vacancy in their valence d-orbitals. Experiments of X-ray absorption indeed found an increase in the vacancy of the 5d orbit of palladium atoms on decreasing the size of Pd crystallites.20 2. Support. A large fraction of atoms in metal clusters finely dispersed on support would interact with support. The activity of metal atoms on the interphase may affected by the interaction. The extent of interaction increases on decreasing their size. 3. AVerage Coordination Number (Nc). The Nc of surface atoms decreases dramatically on decreasing cluster size from 10 nm (Nc ≈ 9) to subnanometer (Nc < 4). Atoms with low Nc are packed loosely and, consequently, the energy required to reconstruct atoms in a small cluster during adsorption should decrease. Dispersed nanometals have been widely used as catalysts in the chemical industry, pollution abatement, fuel cell technology, and sensor technology. The chemical activity of metallic catalysts may tailor with many parameters, such as, the kind of transition metal, its alloy formation, and nature of dispersing support. It is the theme of this study that the activity was also affected by another parameter such as the size of metal cluster. The effect is most prominent when the size is decreased to ∼1 nm. 4. Conclusions In conclusion, supported and nonsupported Pd samples with different dPd have been prepared in this study. The dPd was limited by the method of preparation and showed a trend of RP > IW > IE > SG. The potential diagram of hydrogen absorption and the enthalpies of chemisorption obtained from the prepared samples vary significantly with their dPd. A 90 kJ mol-1 increment in the enthalpy of chemisorption was generally noticed on a decreasing dPd from 10 nm to subnanometer. Acknowledgment. The authors appreciate the National Science Council and the Education Ministry of the Republic of China for their financial support on this study.
J. Phys. Chem. B, Vol. 110, No. 43, 2006 21787 Supporting Information Available: Full description of the material. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Irie, M.; Ohara, H.; Tsujioka, M.; Nomura, T. Mater. Chem. Phys. 1998, 54, 317. (2) Buffat, P.; Borel, J. P. Phys. ReV. A: At., Mol., Opt Phys. 1976, 13, 2287. (3) Bigioni, T. P.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 6983. (4) Russier, V.; Pileni, M. P. Appl. Surf. Sci. 2000, 162, 644. (5) Bednarkiewicz, A.; Maczka, M.; Strek, W.; Hanuza, J.; Karbowiak, M. Chem. Phys. Lett. 2006, 418, 75. (6) Shinde, S. R.; Kulkarni, S. D.; Banpurkar, A. G.; Nawathey-Dixit, R.; Date, S. K.; Ogale, S. B. J. Appl. Phys. 2000, 88, 1566. (7) Felner, I.; Nomura, K.; Nowik, I. Phys. ReV. B: Condens. Matter 2006, 73, 064401. (8) Kim, T.-Y.; Park, N.-M.; Kim, K.-H.; Sung, G. Y.; Ok, Y. W.; Seong, T.-Y.; Choi, C.-J. Appl. Phys. Lett. 2004, 85, 5355. (9) Luppi, M.; Ossicini, S. Phys. ReV. B: Condens. Matter 2005, 71, 035340. (10) Wu, L.; Wiesmann, H. J.; Moodenbaugh, A. R.; Klie, R. F.; Zhu, Y.; Welch, D. O.; Suenaga, M. Phys. ReV. B: Condens. Matter 2004, 69, 125415. (11) Mason, M. G. Phys. ReV. B: Condens. Matter 1983, 27, 748. (12) Mo, L.; Zheng, X.; Yeh, C. T. Chem. Commun. 2004, 1426. (13) Chou, C. W.; Chu, S. J.; Chiang, H. J.; Huang, C. Y.; Lee, C. J.; Sheen, S. R.; Perng, T. P.; Yeh, C. T. J. Phys. Chem. B 2001, 105, 9113. (14) Gonzalez, M. M.; Gallego, M.; Valcarcel, M. Talanta 2001, 55, 135. (15) Huang, S.-Y.; Chang, S.-M.; Yeh, C.-T. J. Phys. Chem. B 2006, 110, 234. (16) Zou, W. Q.; Gonzalez, R. D. Catal. Lett. 1992, 12, 73. (17) Gommes, C. J.; Jong, K.; Pirard, J.-P.; Blacher, S. Langmuir 2005, 21, 12378. (18) Heinrichs, B.; Noville, F.; Schoebrechts, J. P.; Pirard, J. P. J. Catal. 2003, 220, 215. (19) Chou, S.-C.; Lin, S.-H.; Yeh, C.-T. J. Chem. Soc., Faraday Trans. 1995, 91, 949. (20) Behm, J.; Penka, V.; Cattania, M. G.; Christmann, K.; Ertl, G. J. Chem. Phys. 1983, 78, 7486. (21) Okuyama, H.; Siga, W.; Takagi, N.; Nishijima, M.; Aruga, T. Surf. Sci. 1998, 401, 344. (22) Aben, P. C. J. Catal. 1968, 10, 224. (23) Kiraly, Z.; Mastalir, A.; Gerger, F.; Dekany, M. Langmuir 1998, 14, 1281. (24) Chou, P.; Vannice, M. A. J. Catal. 1987, 104, 1. (25) Huang, S.-Y.; Chang, C.-M.; Yeh, C.-T. J. Catal. 2006, 241, 400. (26) Alefeld, G.; Volkl, J. In Hydrogen in Metals I. Basic Properties; Springer-Verlag: Berlin, 1978. (27) Kang, B.-S.; Sohn, K.-S. Physica B 1996, 217, 160.
