ARTICLE pubs.acs.org/JPCC
Water Adsorption and Dissociation on Cu Nanoparticles Ching S. Chen,*,† Chen C. Chen,† Tzu W. Lai,† Jia H. Wu,† Ching H. Chen,† and Jyh F. Lee‡ † ‡
Center for General Education, Chang Gung University, 259 Wen-Hwa first Road, Kwei-Shan Tao-Yuan, Taiwan, 333, Republic of China National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, Republic of China ABSTRACT: The reaction of H2O dissociation on Cu nanoparticles prepared by an atomic layer epitaxy (ALE) technique is discussed in this article. The activation energy of H2O dissociation, desorption energy of H2O, active sites for H2O adsorption, and structural changes of the Cu surface were studied using temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), in situ IR spectroscopy, and X-ray absorption spectroscopy (XAS). The reduced Cu nanoparticles of the ALE-Cu/SiO2 catalyst possess a slightly positive charge (Cuδ+) due to the effect of the nanosized particles. The very low activation energy of H2O dissociation (23 kJ/mol) and the exothermic dissociation heat were obtained from a series of H2-TPR experiments on Cu nanoparticles. It is suggested that the Cu nanoparticles might be partially oxidized to Cu2O, while some oxygen atoms are proposed to be located on the surface of the Cu particles over the course of water adsorption.
1. INTRODUCTION The interaction of water with metal surfaces has received considerable attention because of its fundamental importance in various fields of science. Water chemistry on metal surfaces can involve several catalytic reactions of industrial importance, such as the watergas shift (WGS, CO + H2O f H2 + CO2) reaction, steam reforming of methanol (CH3OH + H2O f 3H2 + CO2), and steam reforming of methane (CH4 + H2O f 3H2 + CO). On the other hand, the watermetal interactions applied to the fields with respect to atmospheric corrosion, electrochemistry, and hydrogen production for fuel cells have promoted an enormous number of studies. Recently, density functional theory (DFT) calculations have become a powerful tool for understanding water adsorption on metal surfaces.112 In general, a weak water adsorption on the Cu surface is usually associated with low chemical activity for water dissociation.7 The reactivity of water dissociation on transition metals has also been reported in the literature in the order Au < Ag < Cu < Pd < Rh < Ru < Ni.7 Water adsorption on transition metals under ultrahigh vacuum (UHV) conditions has been reported to show that the H2OOH complex generated from partially dissociated H2O dominates as the major dissociation reaction.1315 On the basis of this phenomenon, it was suggested that the hydrogen bonding of OH + H2O is stronger than that between two water molecules. In our previous investigations, an alternative route for preparing uniform Cu nanoparticles on SiO2, namely, atomic layer epitaxy (ALE) or atomic layer deposition (ALD), was used to obtain nanoparticles with an average diameter of 2.43.4 nm and a narrow size distribution ( 0).8,34,35 Ren and Meng reported an exothermic reaction of water dissociation on Cu(110), in agreement with our results, but they also revealed a very high activation energy.35 Recently, a model of autocatalytic water dissociation on Cu(110) was suggested in which the formation of strong hydrogen bonds in the H2OOH complex can dominate water dissociation, but the rate decreases with temperature above 380 K.1315
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The H2OOH complex has been proposed to be the dominant species for enhancing water dissociation. Wang et al. indicated that an oxygen-preadsorbed Cu surface could lead to a lower energy barrier and an exothermic reaction over the course of water dissociation.7 They further suggested that the hydrogen abstraction mechanism by the preadsorbed oxygen atoms included hydrogen abstracted from water by oxygen to generate hydroxyl species (H2O + O f 2OH) to further induce water dissociation. In addition, Zhang et al. studied the adsorption and dissociation behaviors of H2O on Cu(111) with different surface charges, finding that the H2O molecule preferentially adsorbs on the top site when the surface charge is positive.11 The slightly positive charge on the Cu surface could lead to a lower activation energy for H2O dissociation in the first step (H2O f H + OH), but it does not favor the second step (OH f H + O). The XANES spectra of the reduced Cu/SiO2 sample in Figure 12 show that the reduced Cu nanoparticles on SiO2 usually provided higher absorption energy than Cu foil, even if the catalyst was reduced by H2 at 773 K for 5 h. It is suggested that the Cu nanoparticles might have a slightly positive charge (Cuδ+) attributable to the effect of the nanosized particles. This was reflected in the TPR profile of the oxidized ALE-Cu/SiO2 catalyst, which featured two reduction peaks at 512 and 773 K.19,20 The reduction of Cu2+ species at 512 and 773 K could lead to the generation of reduced L1 and L2 sites for CO adsorption. The low-temperature peak at 512 K might correspond to the reduction of Cu2+ species on small CuO particles containing defect sites, while the second peak at the higher temperature might be attributed to the reduction of Cu2+ species that provide strong interactions with the SiO2 support and form sites with highly dispersed Cu particles and/or isolated Cu atoms.19,20 Thus, the small Cu particles or isolated Cu atoms on the oxide supports could be rendered partially electropositive as a result of interactions with oxygen atoms at the surface of the support, even if the copper is reduced. The near-edge fine structures of Cu pretreated with water, shown in Figure 12A, provided a linearly 2-fold coordinated Cu+ complex in the case of Cu2O, where each Cu atom was linearly coordinated by two oxygen atoms, whereas O was tetrahedral in structure toward four Cu coordination atoms.31 In Table 1, the coordination number of CuCu bonds on the reduced Cu/SiO2 sample (N = 7.5) was low compared to the bulk coordination number (N = 12),36 which implies that the Cu particle size might be around 1.5 nm based on the literature.37 Thus, Cu nanoparticles containing low coordination numbers may lead to the presence of large numbers of defect sites on the surface, enhancing water dissociation. On the other hand, it was observed that the bond distance of these Cu nanoparticles (2.52 Å) was close to the bulk interatomic distance in Cu. Note that the CuCu and CuO bond distances in Cu2O are 3.02 and 1.86 Å, respectively, and the observed CuO distances for the Cu nanoparticles oxidized by water were around 1.84 Å after 30 min of exposure (Table 1). This observation indicates that the local environment around the Cu+ seems to resemble that in Cu2O. Nevertheless, the CuCu distances for the Cu nanoparticles, which increased slightly from 2.53 to 2.61 Å with water exposure time, were obviously shorter than the typical CuCu distance in a Cu2O structure (3.02 Å). The coordination numbers of CuO for Cu nanoparticles exposed to water for more than 30 min were slightly larger than that of a Cu2O structure (N = 2). On the basis of the results described above, it is suggested that the Cu nanoparticles might partially oxidize to a Cu2O structure, while 12899
dx.doi.org/10.1021/jp200478r |J. Phys. Chem. C 2011, 115, 12891–12900
The Journal of Physical Chemistry C some oxygen atoms were concluded to be located at the surface of the Cu particles, creating an oxygen-rich Cu surface. Thus, it could be proposed that the H2-TPR process of Cu nanoparticles (Figures 5 and 6) undergoing water dissociation at the surface might contain both a Cu2O structure and an atomic oxygencovered Cu surface, supporting the very low activation energy of H2O dissociation.
5. CONCLUSIONS In the present work, we have discussed the activation energy of H2O dissociation, the desorption energy of H2O, active sites for H2O adsorption, and structural changes in the Cu surface during H2O dissociation for the reaction of H2O adsorbed on the Cu nanoparticles. The reduced Cu nanoparticles of the ALE-Cu/ SiO2 catalyst possessed a slightly positive charge (Cuδ+) due to the effect of the nanosized particles, as demonstrated by XANES spectroscopy. This Cuδ+ might be the most important factor for inducing H2O dissociation. The Cu nanoparticles of the ALECu/SiO2 catalyst bound the H2O molecules strongly and caused rapid H2O decomposition at room temperature. There were two main peaks with maxima near 405438 K (R peak) and 539609 K (β peak) for H2O-TPD on the ALE-Cu/SiO2 catalyst. The R peak, with a desorption energy of 23 kJ mol1, was ascribed to molecularly chemisorbed H2O. The β-type was closely associated with dissociated H2O, which was confirmed as the dominant species on the Cu surface. The defect sites on the Cu nanoparticles were assumed to be the main active sites for H2O dissociation. The very low activation energy of H2O dissociation (23 kJ mol1) and the exothermic dissociation heat (4 kJ mol1 for 373 K and 8.7 kJ mol1 for 473 K) were obtained from a series of H2-TPR experiments on Cu nanoparticles. The EXAFS results demonstrated that the Cu nanoparticles might have been partially oxidized to a Cu2O structure, while some oxygen atoms were proposed to be located at the surface of the Cu particles during water adsorption. ’ AUTHOR INFORMATION Corresponding Author
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’ ACKNOWLEDGMENT Financial support from the National Science Council of the Republic of China (NSC 98-2113-M-182-001-MY2) is gratefully acknowledged. Dr. Pin C. Yao is acknowledged for operating the F-120C ALE equipment in the material and chemical research laboratories at the Industrial Technology Research Institute. We also thank the National Synchrotron Radiation Research Center (NSRRC) for X-ray absorption spectroscopy support. ’ REFERENCES (1) Wang, J. G.; Hammer, B. J. Catal. 2006, 243, 192–198. (2) Wang, Y.; Truong, T. N. J. Phys. Chem. B 2004, 108, 3289–3294. (3) Feibelman, P. J. Science 2002, 295, 99–102. (4) Michaelides, A.; Alavi, A.; King, D. A. J. Am. Chem. Soc. 2003, 125, 2746–2755. (5) Tatarkhanov, M.; Ogletree, D. F.; Rose, F.; Mitsui, T.; Fomin, E.; Maier, S.; Rose, M.; Cerda, J. I.; Salmeron, M. J. Am. Chem. Soc. 2009, 131, 18425–18434.
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