David G. Billing

Inorganic–organic hybrid materials incorporating primary cyclic ammonium cations: The lead iodide series

Six inorganic–organic hybrids have been synthesized and characterised by single-crystal X-ray diffraction experiments. The inorganic component is based on lead(II) iodide units and the organic component various cyclic hydrocarbons with only a primary ammonium group as a ring substituent. If the organic component is cyclopropylammonium, cyclobutylammonium, cyclopentylammonium and cyclohexylammonium, the inorganic motif observed is based on the cubic perovskite structure type and consists of 2-D layers of corner-sharing octahedra, in the ratio of 1∶2 inorganic–organic. lead(II) iodide and cycloheptylammonium combined to give 1-D chains of corner-sharing lead iodide octahedra and similarly, lead(II) iodide and cyclooctylammonium gave 1-D chains of face-sharing octahedra. A quantitative measure of the steric effects of the size of the cyclic rings on the tilting of the inorganic layers is proposed.

Inorganic–organic hybrid materials incorporating primary cyclic ammonium cations: The lead bromide and chloride series

Twelve inorganic–organic hybrids have been synthesized and characterised by single-crystal X-ray diffraction experiments. The inorganic component is based on lead(II) bromide and lead(II) chloride units and the organic component on various cyclic hydrocarbons, each with only a primary ammonium group as a ring substituent. When the organic component is cyclopropylammonium, cyclobutylammonium, cyclopentylammonium and cyclohexylammonium, the inorganic motif observed is based on the cubic perovskite structure type and consists of 2-D layers of corner-sharing octahedra, in the ratio of 1 : 2 inorganic–organic. Lead(II) bromide and cycloheptylammonium combined to give 1-D chains of corner-sharing PbBr6 octahedra and similarly, lead(II) bromide and cyclooctylammonium gave 1-D ribbons of corner-sharing PbBr6 octahedra. Lead(II) chloride and cycloheptylammonium have a ribbon motif, and lead(II) chloride and cyclooctylammonium have 2-D layers of corner-, edge- and face-sharing octahedra. These results are compared with a similar study involving lead(II) iodide units and the same set of six cations. General trends and conclusions are discussed.

Synthesis, characterization and phase transitions in the inorganic–organic layered perovskite-type hybrids [(CnH2n + 1NH3)2PbI4], n = 4, 5 and 6

Three inorganic-organic layered perovskite-type hybrids of the general formula [(C(n)H(2n+1)NH(3))(2)PbI(4)], n = 4, 5 and 6, display a number of reversible first-order phase transitions in the temperature range from 256 to 393 K. [(C(4)H(9)NH(3))(2)PbI(4)] has a single phase transition, [(C(5)H(11)NH(3))(2)PbI(4)] has two phase transitions and [(C(6)H(13)NH(3))(2)PbI(4)] has three phase transitions. In all three cases, the lowest-temperature phase transition is thermochromic and the crystals change colour from yellow in their lowest-temperature phase to orange in their higher-temperature phase for [(C(4)H(9)NH(3))(2)PbI(4)] and [(C(6)H(13)NH(3))(2)PbI(4)], and from orange to red for [(C(5)H(11)NH(3))(2)PbI(4)]. The structural details associated with this phase transition have been investigated via single-crystal X-ray diffraction, SC-XRD, for all three compounds.

Synthesis and crystal structures of inorganic–organic hybrids incorporating an aromatic amine with a chiral functional group

In this paper we report the synthesis and the crystal structure of inorganic–organic hybrids containing various lead halides as the inorganic motif and a primary amine as the organic constituent. The organic molecule investigated is (C6H5C*H(CH3)NH2) and both the R and S as well as the racemic (±) forms were used. Within the structures obtained, three different inorganic motifs are displayed by the lead halide octahedra: 1-D polymeric face-sharing chains of formula PbCl3((R)–C6H5CH(CH3)NH3), PbBr3((R)–C6H5CH(CH3)NH3), PbI3((R)–C6H5CH(CH3)NH3) and PbI3((S)–C6H5CH(CH3)NH3); 1-D polymeric corner-sharing chains of formula PbCl5((±)–C6H5CH(CH3)NH3)3 and PbBr5((±)–C6H5CH(CH3)NH3)3; and 2-D corner-sharing layers of formula PbI4((S)–C6H5CH(CH3)NH3)2 and PbI4((R)–C6H5CH(CH3)NH3)2. The changes in geometry and intermolecular interactions such as hydrogen bonding and pi stacking are discussed and compared between the eight structures.

Synthesis, characterization and phase transitions of the inorganic–organic layered perovskite-type hybrids [(CnH2n+1NH3)2PbI4] (n = 12, 14, 16 and 18)

The room temperature single-crystal structures of the inorganic–organic layered perovskite-type hybrids of general formula [(CnH2n+1NH3)2PbI4] (n = 12, 14, 16 and 18) have been determined. The four compounds each display two reversible phase transitions above room temperature, with phases labelled III, II and I. The single-crystal structures of phase II have also been determined. The phase transition from phase III to phase II is a first-order transition and corresponds to a change in the conformation of the alkylammonium chains and a shift of the inorganic layers relative to each other. The phase change from III to II is thermochromic for all compounds, going from a yellow to an orange colour.

Autoreduction and Catalytic Performance of a Cobalt Fischer–Tropsch Synthesis Catalyst Supported on Nitrogen-Doped Carbon Spheres

Carbon materials have been investigated in a wide variety of applications due to their good mechanical stability and electrical conductivity. They have also been used as a catalyst support but in order to establish a uniform coverage of metal particles on the surface of pure carbon materials, it is necessary to activate the chemically inert surface. Generally, different acid or oxidizing treatments have been used to functionalize the carbon surface and to create carboxylic, carbonyl and hydroxy groups that are able to bind the carbon surface to metal clusters. Unfortunately, these treatments can considerably reduce the mechanical and electronic performance of the carbon due to the introduction of a large number of defects. Recently, the doping of heteroatoms into carbon materials has been used as an alternative procedure to successfully bind metals to carbon materials. Nitrogen-doped carbon materials contain sites that are chemically active and allow for the attachment of metal precursors onto the surface of carbon materials without functionalization by strong acid treatments. Following on from the discovery of fullerenes and later the seminal studies on carbon nanotubes by Iijima, the role of curved sp-hybridized carbon atoms was seen to play a key role in the new graphitic carbon structures. 6] This should also apply to carbon materials with other shapes, such as carbon spheres (CSs). Indeed, although carbon spheres have been known for decades, recent studies have paved the way for a reinvestigation of the synthesis, chemistry, and properties of spherical carbons. In the past few years, new synthesis methods have been reported to make a variety of carbon spheres (hollow, solid, core/shell) and these new carbon spheres are expected to exhibit excellent physical and chemical properties. Their use as a catalyst support has, however, hardly been studied. Co and Fe catalysts have been used in Fischer–Tropsch synthesis (FTS) 13] and NH3 decomposition studies. [14] The reduction of the metal oxide to the metal is an indispensable step in activating the catalyst, and is closely related to catalytic performance. 15] However, reduction is affected by the strong metal–support interaction (SMSI). This process typically inhibits the metal reduction process, leading to a lower catalytic activity. Herein, we report for the first time that cobalt oxide supported on nitrogen-doped carbon spheres (N-CSs) can be autoreduced completely by the carbon support. The autoreduced cobalt catalyst pretreated in Ar showed superior FTS catalytic performance to a Co catalyst reduced in H2. Nitrogen-doped carbon spheres (N-CSs) were prepared by chemical vapor deposition (CVD) through the pyrolysis of acetylene and NH3 at 900 8C. [9] This synthesis gave smooth, round carbon spheres with a uniform diameter (ca. 700 nm, BET surface area = 3.4 m g ; see the Supporting Information, Figure S1). Elemental analysis revealed that the nitrogen content of the as-prepared carbon spheres was approximately 2 wt % (see the Supporting Information, Table S1). The N-CSs-supported cobalt catalyst (Co/N-CSs) was prepared by a homogeneous deposition precipitation method using urea as the deposition agent at 90 8C. After filtration, the material was dried in an oven at 100 8C for 12 h and was found to contain 2.3 wt % Co (measured by ICP-AES). The Co/N-CSs had an average cobalt oxide particle size of approximately 13 nm and transmission electron microscope (TEM) images show that the cobalt species were retained on the surface of the carbon spheres even after sonication for 4–5 min (Figure 1 a). Addition of cobalt to CSs that did not contain nitrogen led to large Co particles, even at loadings below 1.5 wt % Co (Figure 1 b). The catalyst was characterized by thermogravimetric analysis (TGA, Perkin–Elmer STA 6000) under N2 using a heating rate of 10 8C min . Figure 2 displays the TGA curves of the nitrogendoped carbon spheres and the 2.3 wt % Co/N-CSs catalyst under N2. A weight loss of approximately 6 % was detected when the N-CSs were heated to 900 8C (Figure 2), owing to the loss of the nongraphitic carbon. The weight loss of greater than 10 % detected for 2.3 wt % Co/N-CSs in the temperature range 400–900 8C can be attributed to CO2 formation as the cobalt oxide is reduced. A possible cobalt-catalyzed loss of carbon from the matrix may also have contributed to the weight loss. The effect of the pretreatment temperature (prior to catalyst testing) on the reduction behavior of the resulting 2.3 wt % Co/N-CSs was monitored by hydrogen temperatureprogrammed reduction (TPR, Micromeritics Auto Chem II) under 5 % H2/Ar. Figure 3 presents the TPR profiles of 2.3 wt % Co/N-CSs pretreated in a flow of high purity Ar at different temperatures. As can be seen, the TPR profile of 2.3 wt % Co/N-CSs after pretreatment at 250 8C (Figure 3 a) has two reduction peaks, corresponding to the reduction of Co3O4 and a mixture of Co3O4 and CoO, respectively. [17] The first peak, [a] Dr. H. Xiong, Prof. D. G. Billing, Prof. N. J. Coville DST/NRF Centre of Excellence in Strong Materials University of Witwatersrand, Johannesburg 2050 (South Africa) Fax:(+27) 11-7176749 E-mail : neil.coville@wits.ac.za [b] Dr. H. Xiong, M. Moyo, M. K. Rayner, Prof. D. G. Billing, Prof. N. J. Coville School of Chemistry, University of the Witwatersrand Johannesburg 2050 (South Africa) [c] M. Moyo, Dr. L. L. Jewell School of Chemical and Metallurgical Engineering University of the Witwatersrand, Johannesburg 2050 (South Africa) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.200900309.

Lead halide inorganic–organic hybrids incorporating diammonium cations

Diammonium cations of general formula (H3N–R–NH3) have been used as templating moieties on lead(II) halide motifs, forming a variety of inorganic–organic nanocomposites. The R group can have simple, straight alkyl chains, which if even-membered and greater than ethane, form 2-D layers based on the RbAlF4 structure type, as found in [(H3N(CH2)4NH3)PbBr4] (1), [(H3N(CH2)4NH3)PbI4] (2), [(H3N(CH2)8NH3)PbI4] (4), [(H3N(CH2)10NH3)PbBr4] (5), and [(H3N(CH2)12NH3)PbI4] (6). If the R group has fused aromatic rings like naphthalene, the 2-D layered perovskite-type motif is still adopted, as in [(H3NC10H6NH3)PbI4] (7). If the chain is odd-membered, a 0-D inorganic motif, consisting of isolated blocks of face-sharing PbI6 octahedra and isolated iodide anions is seen, as in [(H3N(CH2)7NH3)4Pb3I12·2I−] (3). 1-D motifs, which have edge-sharing twin-anionic chains of corner-sharing ribbons are also observed in the compounds [(H3NC2H4NH3)4PbBr4] (8) and [(H3NC6H4CH2C6H4NH3)PbI6] (9) respectively.

Effect of heteroatoms in the inorganic–organic layered perovskite-type hybrids [(ZCnH2nNH3)2PbI4], n = 2, 3, 4, 5, 6; Z = OH, Br and I; and [(H3NC2H4S2C2H4NH3)PbI4]

Ten inorganic–organic hybrids have been synthesized and characterized by single crystal X-ray diffraction experiments. The inorganic component is based on lead(II) iodide units and four different types of alkylammonium cations. The structural motif adopted by the inorganic component has 2-D layers of corner-sharing PbI6 octahedra, which are closely related to the K2NiF4 and RbAlF4 structure types. This motif is observed in hybrids containing alkylammonium cations (ZCnH2nNH3) that contain three different heteroatoms: [(HOC2H4NH3)2PbI4] and [(HOC3H6NH3)2PbI4], [(BrC2H4NH3)2PbI4] and [(ICnH2nNH3)2PbI4] (n = 2–6). Additionally the hybrids [(H3NC2H4S2C2H4NH3)PbI4] and [(NH3C2H4S2C2H4NH3)2PbI5·I] crystallized from a single solution and have two distinct inorganic motifs, the former has 2-D layers of corner-sharing octahedra and the latter has 1-D chains of corner-sharing octahedra. The identity of the heteroatom and the chain length have an effect on the overall packing exhibited by the hybrid structures.

Carbon Spheres Prepared by Hydrothermal Synthesis—A Support for Bimetallic Iron Cobalt Fischer–Tropsch Catalysts

Carbon spheres (CSs) synthesised by the hydrothermal approach were explored as a model support material for a bimetallic Fe–Co Fischer–Tropsch (FT) catalyst. The CSs were characterised by N2 adsorption–desorption, thermogravimetric analysis, FTIR spectroscopy and powder XRD. If annealed at 900 °C for 4 h, the CSs exhibited an improved surface area, thermal stability and crystallinity. A series of Fe–Co bimetallic FT catalysts supported on the annealed CSs were prepared by co‐precipitation. A variety of Fe‐to‐Co ratios were used with the total metal loadings maintained at 10 %. Catalyst reducibility studies were performed by H2 temperature‐programmed reduction and in situ powder XRD. Catalysts with a Fe/Co ratio of 5:5 (w/w) showed Co–Fe alloy formation upon reduction at >450 °C. Interestingly, the presence of this alloy did not correlate with high C5+ selectivities during FT synthesis; rather the Co‐rich/Fe‐poor catalyst gave the best selectivity. The CSs allowed the metal–metal interactions in the bimetallic systems to be monitored because of the weak interaction of the metals with the support.

Poly[bis­[2-(1-cyclo­hexen­yl)ethyl­ammonium] di-μ-iodo-diodo­plumbate(II)]

The title compound, (C(8)H(16)N)(2)[PbI(4)], crystallizes as an inorganic-organic hybrid perovskite, adopting the unusual 2a(p) x 2a(p) superstructure. As such, the structure consists of two-dimensional sheets of corner-sharing PbI(6) octahedra in the ab plane, separated by bilayers of 2-(1-cyclohexenyl)ethylammonium cations. The ethylammonium groups are not in the plane of the cyclohexenyl rings.