Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Fast Oxidation of Porous Cu Induced by Nano-Twinning Kazue Nishimoto,† Marian Krajčí,‡ Takayuki Sakurai,† Hirone Iwamoto,† Mitsuko Onoda,§ Chikashi Nishimura,§ Jeng-Ting Tsai,∥ Sea-Fue Wang,∥ Satoshi Kameoka,† and An-Pang Tsai*,† †
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University Sendai 980-8577, Japan Institute of Physics, Slovak Academy of Science, Dúbravská cesta 9, Bratislava 84511 Slovak Republic § National Institute for Materials Science, Tsukuba 305-0047, Japan ∥ Department of Materials and Mineral Resource Engineering, National Taipei University of Technology, Taipei 10608, Taiwan ‡
S Supporting Information *
ABSTRACT: The fcc lattice of porous Cu prepared by dealloying Al2Cu with HCl aqueous solution exhibits a high density of twinning defects with an average domain size of about 3 nm along the ⟨111⟩ directions. The high density of twinning was verified by X-ray diffraction and qualitatively interpreted by a structural model showing the 5% probability of twinning defect formation. Most of the twinning defects disappeared after annealing at 873 K for 24 h. Twinned Cu reveals much faster oxidation rate in comparison to that without (or with much fewer) twinning defects, as shown by X-ray diffraction and hydrogen differential scanning calorimetry. Using ab initio DFT calculations, we demonstrate that twinning defects in porous Cu are able to form nucleation centers for the growth of Cu2O. The geometry of the V-shaped edges on the twinned {211} surfaces is favorable for formation of the basic structural elements of Cu2O. The fast oxidation of porous Cu prepared by dealloying can thus be explained by the fast formation of the Cu2O nucleation centers and their high density.
1. INTRODUCTION
Raney Ni and Cu have been known for a long time as unsupported catalysts.1,2 However, due to a decrease in their activity resulting from sintering at high temperatures, their usefulness in practical applications is relatively less in comparison with the supported catalysts. The attention to “leaching” was revived after a report that porous Au obtained by “dealloying” of Ag from Ag−Au solid solutions exhibits high catalytic activity for CO oxidation even under ambient conditions.3 (In this study, we use the term “dealloying” in order to be consistent with other studies.) The exhibition of high catalytic activity of porous Au was very surprising, and the origin of active sites and the atomistic scenario behind the catalytic activity have not yet been completely explained. In most studies of porous Au the samples were prepared by dealloying from Ag−Au solid solutions.4 In addition to the Au− Ag solutions, intermetallic Cu3Au is also a promising precursor for porous Au, which also shows high activity in spite of its low surface area.5 However, in both cases, the dealloying was not fully completed, since a considerable amount of Ag or Cu remained in porous Au. Hence, one cannot rule out a contribution of residual Ag or Cu to the catalytic activity. It has been difficult to determine what and where are the most active atomic sites responsible for the observed extraordinary catalytic properties. Recently, we have reported studies on
It has been almost a century since Murray Raney for the first time prepared metals with an unusual spongy microstructure Raney metal by leaching samples. Raney metal has been become the representative name of nonsupported metallic catalysts with a highly porous structure. Raney metals have been extensively studied from various points of view. Some of the Raney metals showed great catalytic performance, which were consequently used in industry, and some of them revealed interesting morphologies such as fine porosity or a morphology of packed nanoparticles.2 The basic process used to prepare Raney metals is “leaching”, which is a simple process widely used in chemistry and the mine industries (mineral dressing) aiming to purify raw materials, in which the less noble metal atoms would preferentially/selectively dissolve into the solution and simultaneously the relatively noble metal atoms would rearrange their positions and form a porous microstructure. The process of leaching itself and its principle are simple, but the resultant structures and morphologies can be very different depending on the solution used and the structure of precursor alloys. Moreover, the leaching process at the atomic level is very complicated, since the residual elements can be sensitive to the solution and during leaching the remaining elements can form also oxides in addition to the metallic phases. The difference in sensitivity to the solution used can also have an influence on the structure and morphology. 1
© XXXX American Chemical Society
Received: January 7, 2018
A
DOI: 10.1021/acs.inorgchem.7b03225 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
oxidation behavior. Raney Cu is studied also with synchrotron powder X-ray diffraction, and the results are compared with a simulation of the stacking disorder (ABC) model. The oxidation behavior of Raney Cu is interpreted in terms of the nano-twinned structure. Our results are supported also by an ab initio DFT study of the formation of nucleation centers for oxide growth.
porous Au with a negligible amount of Al obtained from an intermetallic Al2Au. Our samples of porous Au also exhibited high catalytic activity for CO oxidation.6−8 We have found that this high activity is associated with the defects in the fcc lattice of Au such as twinning and stacking faults, as observed by X-ray diffraction (XRD) and transmission electron microscopy (TEM).6−8 This has helped in the understanding of the origin of the active sites and provided a new insight into the mechanism of catalytic activity of porous Au. The lattice defects in porous Au are generated by the dealloying process. The presence of lattice defects is characterized by peak broadening in the XRD patterns.7 However, it is not easy to distinguish the structural types and to estimate quantitatively the amount of the defects from the XRD patterns. On the other hand, the twin boundary (TB) defects in the ligaments of the porous Au have been frequently observed by TEM.7,8 We have found that, out of all lattice defects possibly present, only the TB defects play a crucial role in creating the active sites for catalysis. As they are planar defects, they must intersect with surfaces around the ligaments and they modify the arrangement of surface atoms. We have shown that on the surface of ligaments TBs are able to create new kinds of highly reactive low-coordinated atomic sites, denoted W-chains. Our previous studies have shown that the lattice defects as TBs are the major resource of the catalytically active sites in porous Au prepared by dealloying Al2Au.6−8 If this is valid also for other porous metals, this will give a new insight into the mechanism of catalytic activity for the bulk catalyst, and hopefully it will also provide principles for the design of new unsupported bulk catalysts. In this sense, the confirmation of the validity of the correlation between the lattice defects and the catalytic activity in porous metals would be very significant. In other studies Raney metals such as Raney Cu and Raney Ni prepared by dealloying with NaOH aqueous solutions from Al2Cu and Al3Ni, respectively, have been investigated with the aim of understanding their morphologies,9 catalytic properties,10 and kinetics of dealloying.11,12 However, in these experiments the homogeneity and the particle size of precursors often were not sufficiently carefully treated. Consequently, the dealloying was not completed and a significant part of the precursor alloy still remained in the samples. Although Raney metals reveal porous morphologies and high catalytic activities, e.g., Raney Cu for steam reforming of methanol, the origin of high activity has been an open question for a long time due to the insufficiently detailed investigation of their microstructure. Recently, in Raney Cu we have observed a high density of parallel TBs with a domain size of about 2−3 nm. The Raney Cu sample was prepared by dealloying intermetallic Al2Cu with HCl aqueous solution. Surprisingly, our Raney Cu samples with the nano-twinned structure showed very fast spontaneous oxidation in air at room temperature. As in previous studies,6,7 we have found that porous Au exhibits a correlation between high catalytic reactivity toward CO oxidation and the density of twins, and one can expect that the twin boundaries could have significant influence also on the observed fast oxidation of Raney Cu. We note that the nanotwinned structure was also observed in nanowires prepared via a solution-phase chemical synthesis method1,2 and in severely deformed bulk Cu.13,14 Au nanowires were prepared to study the atomistic arrangement of TBs, and the study of deformed Cu was focused on the mechanical properties induced by TBs. In this report, we have investigated in detail the microstructure of Raney Cu and a possible contribution of TBs to the
2. EXPERIMENTAL PROCEDURES The intermetallic compound with a nominal composition of Al2Cu was prepared from 99.9% Al and Cu in an arc furnace under an Ar atmosphere. The alloy was annealed once at 823 K for 24 h and then crushed and classified into powder form with particle sizes of
