M.J. McKelvy

Synthesis and characterization of small platinum particles formed by the chemical reduction of chloroplatinic acid

Abstract The preparation of platinum particles by the chemical reduction of chloroplatinic acid has been investigated in detail. The morphology and size of the product depends on the nature of the reducing agent, the presence and type of protective agent, and the reduction conditions. By carefully controlling these parameters, platinum spheres (⋍20–30 A diam), strands (⋍60 A diam), and composites (⋍100–200 A) can be synthesized. Polyvinylpyrrolidone was found to be the optimum protective agent due to its low protective ratio and high purity. The distribution of particle diameters for the spherical particles can be described by a log-normal distribution function. This work lays the foundation for studies of the chemical and physical properties of these particles. The applicability of this chemical-reduction approach to other systems is also outlined.

Synthesis and characterization of nearly stoichiometric titanium disulfide

Abstract Nearly stoichiometric titanium disulfide has been synthesized by direct reaction between the elements using an optimal procedure and characterized by thermogravimetric and chemical analyses, ammonia intercalation rate, magnetic susceptibility measurements, and X-ray diffraction. The most nearly stoichiometric TiS 2 that can be prepared is Ti 1.0021±0.0010 S 2 , so that a slight excess of titanium is always present. This excess titanium inhibits the initial opening of the van der Waals gap during ammonia intercalation. The small titanium excess is the primary source of conduction electrons in TiS 2 , which supports the view that TiS 2 is an extrinsic semiconductor rather than a semimetal.

Deintercalation and reintercalation energetics of ammoniated titanium disulfide

Abstract The deintercalation and reintercalation processes in ammoniated TiS2 have been studied by thermogravimetric analysis, differential scanning calorimetry, vapor-pressure measurements, and powder X-ray diffraction. The enthalpies determined calorimetrically for complete NH3 and NH+4 deintercalation of (NH+4)0.24(NH3)0.23TiS0.24−2 are approximately 10.5 and 22 kcal/mole, respectively. These enthalpies are in good agreement with those reevaluated from a previous calorimetric study of ammoniated TaS2. Ammonia vapor-pressure curves for deintercalation and reintercalation of (NH+4)0.24(NH3)y″TiS0.24−2 exhibit hysteresis, and the enthalpies for these reactions are estimated to be 15.5 and −13 kcal/mole, respectively. The absolute values of these enthalpies decrease progressively as NH3 is deintercalated and then reintercalated. The structural changes that accompany these processes are relatively complex and involve at least two phases. Further structural studies are necessary to help elucidate the energetics of these intercalation compounds.

Ammonia oxidation and charge compensation in the metal ammonia intercalates Li+x(NH+4)y′(NH3)y″TiS(x+y′)−☆

Abstract The lithium-ammonia intercalates of TiS2, Li+x(NH+4)y′(NH3)y″TiS(x+y′)−, where 0.00 ≤ x ≤ 0.20, have been investigated by thermogravimetric analysis (TGA), vapor pressure measurements, X-ray powder diffraction, and SQUID magnetometry. TGA and vapor-pressure measurements indicate that ammonia deintercalation occurs by a distinct two-step process consistent with the ionic nature of these compounds. These materials are monophasic and crystallize in a 3R-type structure. The c lattice parameter increases linearly with increasing ammonia content, which may be associated with the diminution of the ion-dipole interaction of the cation(s) with the lone pair of ammonia. Compositional analysis by TGA shows that charge compensation occurs such that the total cationic concentration (x + y′) is constant at 0.22 ± 0.02. The Pauli paramagnetism of the conduction electrons corresponds to complete ionization of both lithium and ammonium, so that the driving force for the charge-transfer phenomenon is the transfer of 0.22 ± 0.02 electrons to the conduction band of TiS2. The degree of NH3 oxidation depends upon the relative intercalation rate of metal and NH3.

On the intercalation and deintercalation mechanisms for ammoniated titanium disulfide

Abstract The redox mechanism associated with ammonia intercalation of TiS 2 has been clearly demonstrated, with essentially the only reaction products being (NH 4 + ) y ′ (NH 3 ) y ″ TiS 2 y ′ - and N 2 , (NH 4 + ) y ′ (NH 3 ) y ′(NiS 2 y ′ - undergoes thermal deintercalation by the sequential removal of NH 3 and “NH 4 + ” at progressively higher temperatures. Only small amounts of polysulfides and H 2 S were observed during intercalation and deintercalation, respectively, indicating the near quantitative nature of the above mechanisms.

A structural investigation of (ND+4)0.22(ND3)0.34 TiS0.22-2 by time-of-flight neutron powder diffraction

Abstract The structure of the intercalation complex (ND 4 + ) 0.22 (ND 3 ) 0.34 TiS 2 0.22- has been determined by Rietveld refinement of time-of-flight neutron powder diffraction data taken at 298 and 12 K. This compound belongs to space group R3m, and the lattice parameters are a = 3.4170(3) A and c = 26.723(4) A at 298 K. No phase change was observed on cooling to 12 K. However, the c lattice parameter was observed to contract about 1.2%. Both ND 4 + and ND 3 occupy trigonal prismatic interlayer sites, with ND 4 + exhibiting random spherical disorder and ND 3 adopting a planar geometry. Two of the ND 3 deuterium atoms are hydrogen bonded to host TiS 2 sulfur atoms, whereas the third is found in the interlayer midplane and is not hydrogen bonded. The implications of this structure for the properties of such intercalation compounds is discussed.

Incoherent inelastic neutron scattering investigation of ammoniated titanium disulfide

Abstract Incoherent inelastic neutron spectra of the intercalation compounds (NH 4 + ) 0.20 (NH 3 ) 0.36 TiS 2 0.20- and (NH 4 + ) 0.20 TiS 2 0.20- have been measured below 560 cm −1 . Bands due to NH 3 occur at 295, 355, 470, 520 and 575 cm −1 in (NH 4 + ) 0.20 (NH 3 ) 0.36 TiS 2 0.20- . A band at 215 cm −1 in (NH 4 + ) 0.20 TiS 2 0.20- is assigned to the NH 4 + torsional fundamental. The results of this study are compared to those found in a previous study of ammoniated TaS 2 .

A structural investigation of Ag0.167TiS2 by time-of-flight neutron powder diffraction

Abstract The structure of Ag 0.167 TiS 2 has been determined at several temperatures from time-of-flight neutron powder diffraction data. At 305 K, Ag 0.167 TiS 2 is a stage-II complex which belongs to the space group P3¯m1 , with a = 3.4161(4) Aand c = 12.100(2) A. When the temperature is lowered to 13 K, the lattice parameters decrease, but no phase change or superlattice formation is observed. Upon heating to 1123 K, this compound transforms to a stage-I structure belonging to the space group P3¯m1 , with a = 3.4676(1) Aand c = 6.2247(4) A. No further phase changes are observed when the temperature is raised to 1323 K. When this compound is slowly cooled to ambient temperature from 1323 K, it transforms quantitatively to the original stage-II structure. The Rietveld refinements of Ag 0.167 TiS 2 at six temperatures are presented and discussed.

Structure and dynamics of ammonia in Li-ammonia intercalated TiS2: a proton NMR study

Abstract Measurements of NMR line widths and spin-lattice relaxation times for protons, as functions of temperature and NMR frequency, have in recent years yielded much information about the ammonia molecule intercalated into TiS 2 . In conjunction with neutron powder diffraction studies, NMR has been used to probe: (1) intercalated ammonias molecular motions, (2) distortions in its structure and (3) bonding interactions with its host. The effects of intercalation with Li-ammonia solutions, instead of pure NH 3 , have been found to be especially large on the first and third of these, indicating a restricted range for rotational motion of the molecule during diffusion and at the same time weakened bonding of the ammonia protons to TiS 2 sulfur layers. Frequency and temperature dependences of spectral density functions (from T 1 measurements) have been examined for evidence of two-dimensional diffusion. Current findings from NMR bearing on these and related subjects will be reviewed, and comparisons with NH 3 -intercalated TiS 2 will be made.

Synthesis, characterization and properties of the new ionic intercalation compound (NH+4)0.22TiS0.22−2☆

Abstract The new ionic intercalation compound (NH+4)0.22TiS0.22−2 has been synthesized by vacuum deintercalation of ammoniated TiS2, characterized by thermogravimetric analysis and powder X-ray diffraction, and examined by SQUID magnetometry (indirectly), differential scanning calorimetry, incoherent inelastic neutron scattering, and nuclear magnetic resonance. Although the diffraction pattern of this compound resembles a stage-II structure in which every other van der Waals gap is occupied by NH+4 it cannot be indexed completely on this basis. Magnetic and calorimetric measurements show that one electron is transferred to the host TiS2 conduction band per NH+4 cation and that the deintercalation enthalpy of NH+4 is 22 kcal/mol NH+4, respectively. Neutron scattering has provided evidence for an NH+4 torsional fundamental mode at 215 cm−1. The proton NMR linewidth and spin-lattice relaxation time are independent of temperature between 100 and 540 K, and the linewidth of 2.6 ± 0.3 G is in good agreement with that calculated for isotropic reorientation of the NH+4 cations.