Abstract
New important applications of copper metal, e.g., in the areas of hydrogen production, fuel cell operation, and spent nuclear fuel disposal, require accurate knowledge of the physical and chemical properties of stable and metastable copper compounds. Among the copper(I) compounds with oxygen and hydrogen, cuprous oxide Cu2O is the only one stable and the best studied. Other such compounds are less known (CuH) or totally unknown (CuOH) due to their instability relative to the oxide. Here we combine quantum-mechanical calculations with experimental studies to search for possible compounds of monovalent copper. Cuprous hydride (CuH) and cuprous hydroxide (CuOH) are proved to exist in solid form. We establish the chemical and physical properties of these compounds, thereby filling the existing gaps in our understanding of hydrogen- and oxygen-related phenomena in Cu metal.
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Copper in compounds may exist in oxidation states ranging from +1 to +4, although the +2 (cupric) and +1 (cuprous) are the most common ionic states of copper (1).
At ambient conditions, the most stable compounds of copper with oxygen and hydrogen are copper(II) oxide CuO (stable) and copper(II) hydroxide Cu(OH)2 (metastable) (1, 2). Among the compounds of monovalent copper with oxygen and hydrogen, cuprous oxide Cu2O is known to be the only stable and therefore best-studied compound (1–5). Cu2O has the cuprite crystal structure (Fig. 1A), in which the copper(I) cations, Cu+, are arranged into a face-centered cubic (fcc) sublattice and the oxygen anions, O2-, reside on the sites of a body-centered cubic (bcc) sublattice (6). Each O2- is tetrahedrally coordinated by Cu+, whereas the coordination of Cu+ ions is linear (7).
- pnas.1115834109fig1.jpeg (60.59 KiB) Exibido 643 vezes
Fig. 1.
Phonon spectra and heat capacity of copper(I) compounds. (A–C) Phonon density of states of (A) cuprite Cu2O calculated at the lattice parameter a = 4.27 Å, (B) wurtzite CuH calculated at the lattice parameters a = 2.904 Å, and (C) CuOH, calculated at molecular volume of 30.7 Å3. (D and E) Calculated and experimental (1, 15, 23, 24) heat capacities CP, of Cu2O (D), and CuH (E). Note that the break in the experimental specific heat curve (E) at about 60 K is due to differences in the specimens’ composition and cooling techniques used above and below that temperature (15).
The equilibrium solubility of hydrogen in fcc Cu is very low, and there is no stable hydride in the CuH binary system (8, 9). However, a metastable copper hydride CuH with the hexagonal ZnS (wurtzite) structure (10) can be formed chemically (11) or electrochemically (12, 13). Although CuH is the oldest metal hydride known (synthesized by Wurtz in 1844) (11), there have been very few experimental (10–16) and practically no theoretical (see, however, ref. 17) studies of CuH. Copper(I) hydride decomposes readily into Cu and H2 (gas) in vacuum or ambient air, as well as under electron or laser beam. Similar to other metal hydrides, copper(I) hydride is pyrophoric, but unlike them, it is not water reactive. Therefore, CuH may safely be kept in water at a T of approximately 0 °C or in an inert gas atmosphere at a T of approximately -50 °C (13–16).
Cuprous hydroxide CuOH has not been proven to exist in a solid form, and its properties are completely unknown. Experimental information on thermodynamic properties is available only for charge-neutral molecular CuOH species in aqueous solution (2). The formation of submonolayer of adsorbed OH species on Cu surfaces has been reported recently (18, 19).
In the present work, we apply first-principles calculations to search for (meta-) stable Cu–O–H compounds and to compute their electronic and phonon spectra. Such compounds are then synthesized using wet-chemical methods and characterized by nondestructive experimental techniques. The details of theoretical and experimental procedures used in this study are given in the SI Text.
Results and Discussion
Electronic and Phonon Spectra Calculations.
Our search for locally stable configurations of Cu+, O2-, H-, and/or H+ ionic species was performed using electronic structure calculations based on density-functional perturbation theory (20, 21), taking into account the contribution from lattice vibrations to the thermodynamic properties (22). Initial configurations were based on the close-packed (fcc and hexagonal) sublattice of Cu+ ions and, alternatively, on the bcc sublattice of O2- ions. The study yielded solid-state configurations and phonon spectra of cuprous oxide Cu2O, cuprous hydride CuH, and cuprous hydroxide CuOH (Fig. 1 A–C). Their local stability is consistent with the calculated semiconducting electronic structure (Fig. S1) that is indicative of saturated chemical bonding in each of these compounds.
The phonon spectra of the three compounds contain (i) low-frequency peaks (below 200 cm-1), which are due to the acoustic and optical phonon modes that involve motion of the relatively heavy Cu+ ions, and (ii) high-frequency peaks (above 400 cm-1), which are due to the optical modes associated with vibrations of lightweight species (O2- and/or H+/-). Thus, a group of peaks (band) situated at 500–650 cm-1 in the phonon spectrum of Cu2O (Fig. 1A and Fig. S2) corresponds to the vibrations of O2- inside a tetrahedron formed by the four neighboring Cu+ ions in the cuprite structure. The spectrum of copper hydride CuH (Fig. 1B and Fig. S3) has a high-frequency band lying between 900 and 1,140 cm-1 that corresponds to the optical vibrations of a hydride ion, H-, surrounded by a tetrahedron of Cu+ cations. The large difference in mass between the 63Cu and 1H isotopes creates a wide gap between the corresponding optical modes in the phonon spectrum of CuH. The high-frequency part of the CuOH spectrum (Fig. 1C and Fig. S4) is represented by two peaks situated around 500 cm-1 (and associated with the optical vibrations involving motion of a hydroxyl OH- group as a whole), a band at about 900 cm-1 (Cu–O–H bond bending mode, associated with changing the angle between the Cu–O and O–H bonds), and a high-frequency band at about 3,400 cm-1 (O–H bond stretching mode).