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Cu-based catalysts are widely employed for CO or CO2 hydrogenation into methanol. However, their catalytic performance highly depends on supports, and the real evolution of Cu species is still cove...

A crystal from the type locality Ajo, AZ, yielded just enough intensity from streaked diffractions using synchrotron x-rays at the Advanced Photon Source to solve the crystal structure with composition (K + Na) 3 Cu 20 Al 3 Si 29 O 76 (OH) 16 ⋅∼8H 2 O; triclinic, P1̄, a = 13.634(5) Å, b = 13.687(7), c = 14.522(7), α = 110.83(1)°, β = 107.21(1), γ = 105.68(1); refined to a final R = 12.5%. Electron microprobe analysis yielded a similar chemical composition that is slightly different from the combined chemical and electron microprobe analyses in the literature. The ajoite structure can be described as a zeolitic octahedral-tetrahedral framework that combines the alternate stacking of edge-sharing octahedral CuO 6 layers and curved aluminosilicate layers and strings. Channels bounded by elliptical 12-rings and circular 8-rings of tetrahedra contain (K and Na) ions and water. The Al atoms occupy some of the Si tetrahedral sites. Each Cu atom has near-planar bonds to four oxygen atoms plus two longer distances that generate a distorted octahedron. Valence bond estimates indicate that 8 oxygen atoms of 46 are hydroxyl. Only one alkali atom was located in distorted octahedral coordination, and electron microprobe analyses indicate K and Na as major substituents. The water from chemical analysis presumably occurs as disordered molecules of zeolitic type not giving electron density from diffraction. The high R factor results from structural disorder and many weak intensities close to detection level. The crystal chemistry is compared with shattuckite, Cu 5 (SiO 3 ) 4 (OH) 2 , and planchéite, Cu 8 Si 8 O 22 (OH) 4 ⋅H 2 O, both found in oxidized copper deposits of Arizona but only the former directly with ajoite.

A unique Cu-bearing chain silicate, Li2(Mg,Cu)Cuz[Si206b was synthesized, and the structure was determined by single-cry~tal X-ray diffraction techniques. The structure was found to be triclinic, spacoe group PI, with unit-cell parameters a = 5.7068(7), b = 7.4784(9), c = 5.2193(3) A, ex = 99.911(8), J3 = 97.436(8), 'Y = 84.52(1)°, and Z = 1. The arrangement of zweier single chains, [Siz06], differs significantly from chain arrangements in the pyroxene and pyroxenoid structures, and the "I-beam" description of the pyroxene structure is not applicable. The structure may be classified as a new derivative type of the pyroxene structure, with an "oblique I-beam". Cu atoms are coordinated by four ° atoms in a square-planar arrangement with 1.94-2.00 A for Cu-O and two ° atoms with longer Cu-O distances of 2.41-2.92 A, consistent with the crystal-field stabilization ~f the d9.electroni~ structure of Cu2+. The square-planar Cu04 units form a [CunOZn+2] nbbon with n = 3 ill the structure, which is also found in Cu-bearing chain silicates such as shattuckite and plancheite with n > 3. Mg is octahedrally coordinated by ° atoms, but the configuration is affected by the partial replacement by Cu.

Using reflection extended X-ray absorption fine structure spectroscopy (REFLEXAFS) and X-ray photoelectron spectroscopy (XPS) it has been established that Cu(II) in low concentration in aqueous media can be chemisorbed onto the (0001) surface of muscovite mica. From the XPS studies it is suggested that the Cu species is in a similar bonding environment to the copper in copper hydroxide. A depth profile of the reacted mica surface was also carried out using XPS and this suggests that there had been no diffusion of the Cu into the mica surface. REFLEXAFS studies of the reacted mica surface provided information about shells of O, Al/Si, and Cu surrounding a central Cu absorber. The best-fit bond distances are as follows: O–Cu, 1.98 A; Cu–Cu, 2.64 A; Al/Si–Cu, 3.09 A. This provides direct evidence for Cu being bound to the surface at aluminate or silicate groups and suggests that the Cu species adsorbed onto the surface are similar to the copper species found in copper hydroxide, plancheite, and shattuckite. It is proposed that Cu is chemisorbed at atomic imperfections, such as steps and kinks, on the mica surface.

Shattuckite Cu(5)(SiO(3))(4)(OH)(2) is a copper hydroxy silicate and is commonly known as a 'healing' mineral. Three shattuckite mineral samples from three different origins were analysed by Raman spectroscopy. Some Raman bands are common in the spectra of the minerals. Raman bands at around 890, 1058 and 1102 are described as the ν(3) -SiO(3) antisymmetric stretching vibrations. The Raman band at 670cm(-1) is assigned to the ν(4) bending modes of the -SiO(3) units and the band at around 785cm(-1)is due to Si-O-Si chain stretching mode. Raman (and infrared) spectroscopy proves that water is in the molecular structure of shattuckite; thus the formula is better written as Cu(5)(SiO(3))(4)(OH)(2)·xH(2)O.

Optical and EPR measurements are carried out on shattuckite, a copper bearing silicate. The EPR spectrum observed at 120 K reveales hyperfine structure for parallel and perpendicular components. The measured values are g ‖ = 2.402, g ⊥ = 2.038, A ‖ = 112 × 10 -4 cm -1 and A ⊥ = 49.5 × 10 -4 cm -1 and they indicate a tetragonal distortion for the Cu 2+ ion . Further, the constants P and K are evaluated and found to be 0.014 cm -1 and 0.0112 respectively. The bands observed at 8160, 13330 and 16125 cm -1 are assigned to the electronic states of Cu 2+ , in a crystal field subjected to tetragonal (C 4V ) distortion. The crystal field parameters evaluated are Dq = -1333, Ds = -1565 and Dt = -380 cm -1 . Correlating the EPR and optical data, the orbital reduction factors are calculated as k ‖ = 0.89 and k ⊥ = 0.59. It is confirmed from EPR and optical studies that the Cu 2+ ion in shattuckite is sited in an elongated octahedron. Similar studies are carried out for plancheite. The observed optical absorption spectrum of plancheite is attributed to Cu 2+ sited in an elongated octahedron and tetragonal field parameters are evaluated.

The crystal structure of shattuckite (Cu5(OH)2(SiO3)4), space group Pcab, a=9.88, b=19.82, c=5.40A, Z=4, has been investigated by three dimensional intensity data. The newly determined structure is the same as the structure studied by Evans and Mrose (1966) and Le Bihan (1967) in the projection parallel to the c-axis, but the z-parameters of silicon, copper and oxygen atoms are different. The configuration of the nearest neighbours of copper atoms are also different. The copper atoms stay at the centers of the rectangles of oxygen atoms, showing the usual coordination of Cu2+ in copper bearing minerals in general.

The orthorhombic crystal structures of shattuckite, Cu(5)( SiO(3))(4)(OH)(2) and planchétite, Cu(8)(Si(4)0(11))(2)(OH)(4) H(2)O, have been solved. Shattuckite contains silicate chains similar to pyroxene in a complex association with copper atoms, while the closely related planchéite contains silicate chains similar to amphibole.