R. T. Obermyer

Magnetic, Mössbauer, and catalytic properties of the zeolite catalyst ZSM‐5(Fe)

Iron‐impregnated medium pore zeolite ZSM‐5 (pore opening of 5.5 A) is an efficient catalyst for the conversion of synthesis gas (CO+H2) to high octane gasoline. Activity and selectivity of the ZSM‐5 (Fe) depends upon the manner in which Fe is impregnated on ZSM‐5. Magnetic and Mossbauer studies were conducted on catalysts prepared by two methods: (a) ferric nitrate impregnation and (b) carbonyl Fe3(CO)12 impregnation on the zeolite. These studies were conducted for various stages of preparation and after use of the catalysts. The carbonyl impregnated sample in the as‐prepared form contained ultrafine γ‐Fe2O3 of d = 60–66 A, as evidenced by superparamagnetic behavior seen in magnetic and Mossbauer studies. In contrast, the as‐prepared, nitrate impregnated sample showed α‐Fe2O3 of approximate particle size of 100 A. Both samples were reduced to about 80% metallic Fe in flowing H2. On exposure to synthesis gas, the carbonyl impregnated sample yielded a substantial amount of Fe3O4 in addition to χ carbide, wheres the nitrate impregnated sample exhibited e, χ, and ϑ carbides. The steady catalytic activity and selectivity of the carbonyl impregnated sample can be related to the ultrafine iron dispersions leading to the observed phases in the used catalyst.

Magnetic properties of pseudo‐ternaries of the composition Nd(Mn1−xCrx)2Si2

Structural and magnetic properties of a number of pseudo‐ternaries of the composition Nd(Mn1−xCrx)2Si2 have been examined. In the composition range 0<x<0.6 they crystallize in tetragonal BaAl4‐type structure. Magnetic properties were examined between 4.2 K and 400 K in applied fields up to 20 kOe. NdMn2Si2 exhibits anomalies in the magnetization versus temperature plots at about 40 K and 380 K—the former due to the ordering of the rare earth moments and the latter due to the ordering of the transition metal sublattice. Addition of chromium to this compound increases the crystal cell volume significantly and modifies the magnetic behavior. The transition metal sublattice orders antiferromagnetically at 380 K, 260 K and 200 K for compositions of X=0, 0.2 and 0.3, respectively. Compounds of the composition X=0.2, 0.3 and 0.4 exhibit unusual thermomagnetic hystereses. The thermomagnetic curves for these compositions in low external fields (<10 kOe) exhibit an increase in the magnetization upon warming the sam...

Effect of partial substitution of Tb on the spin reorientation of the Er2−xTbxFe14B system

Numerous articles have been published concerning spin reorientation in R2Fe14B (R=rare earth) compounds. Previous measurements have suggested the presence of axial, conical, and planar regions in the magnetic phase diagrams. In the present systems the magnetization versus temperature (10–300 K) has been measured for aligned Er2−xTbxFe14B samples for x=0, 0.2, 0.5, and 1.5 in applied fields of 0–15 kOe. Transitions were determined from the inflection point as obtained from the derivative of the magnetization versus temperature. Earlier theoretical predictions of the magnetic phase diagrams for Er1.8Tb0.2Fe14B indicate that both plane‐to‐cone and cone‐to‐axis transitions should be present. Spin reorientations were observed at 194 K for plane to cone and at 212 K for cone to axis, in agreement with the predicted values. There was a weak‐field dependence in the transition temperatures, and the strength of the transitions steadily decreased with increasing field. In the case of Er1.5Tb0.5Fe14B, a cone‐to‐axis ...

NMR evidence of metal‐support interaction in syngas conversion catalyst Co‐TiO2

To examine the relation between catalytic and magnetic properties, the zero‐field NMR spectra and hysteresis loops of cobalt supported on silica, alumina, magnesia, titania, and ZSM‐5 with and without the promoter thoria were investigated. Cobalt was incorporated on the support by simple physical admixture of precipitated cobalt and support, and by aqueous impregnation technique. Our studies indicate that the particle sizes are consistently lower in the presence of thoria. Of all the catalysts examined, the Co/Th/TiO2 catalyst exhibits a high saturation magnetization value—about 20% higher than pure cobalt. In addition, the NMR spectrum of the aqueous impregnation Co/TiO2 catalyst is distinctly different from the rest. All the NMR lines are shifted to a higher frequency by about 4 MHz. These two features—enhancement of the magnetic moment of cobalt atoms and increases in the hyperfine field at the Co nucleus—clearly indicate that there occurs strong metal‐support interaction between cobalt and titania sup...

Zero‐field NMR studies of physically admixed Co/Co‐ThO2 ZSM‐5 catalysts

The Co‐ZSM‐5 and Co‐ThO2‐ZSM‐5 catalysts are a particularly promising group of bifunctional zeolite catalysts used for the conversion of synthesis gas to gasoline‐range hydrocarbons. Catalytic properties of these materials, such as activity and selectivity, depend upon the amount of the cobalt on the medium‐pore zeolite ZSM‐5, as well as upon the presence of promoters such as ThO2. These studies were undertaken to ascertain the effect of thoria upon the magnetic and structural nature of these catalysts. Zero‐field nuclear magnetic resonance measurements have been made on a series of zeolite cobalt and cobalt‐thoria catalysts with three different concentrations of Co (3, 6, and 9 wt. %). The catalysts were prepared by making physical admixtures of precipitated cobalt, or cobalt‐thoria, and ZSM‐5. The amount of thoria introduced ranged from 0.5 to 1.5 wt. %. After reduction of the catalyst samples, the normal metallic cobalt resonance line at 213.0 MHz (fcc) and the fault lines at 215.5 and 218.6 MHz were o...

Magnetic behavior of some rare earth germanides of the type RFe2Ge2

Magnetic properties of tetragonal rare earth germanides of the type RFe2Ge2, where R=Pr, Gd, Tb, Er and Th, have been investigated in the temperature interaval 4.2 to 273 K and in magnetic fields up to 20 kOe. The magnetization of ThFe2Ge2 is small and weakly temperature dependent. The compounds with Pr, Gd and Tb show antiferromagnetic ordering with Neel temperature of 14 K, 11 K and 7.5 K, respectively. The Pr sublattice in PrFe2Ge2 and the Tb sublattice in TbFe2Ge2 both undergo a transition, which at 4.2 K occurs at a field strength of about 15 kOe. The Gd compound does not show a transition up to a field of 20 kOe. The compound ErFe2Ge2 is paramagnetic in the temperature range investigated. However, some magnetization from Fe sublattice seems to be present in all the compounds.

Temperature‐compensated Pr1−x−ySmxRyCo5−δ permanent magnets (R=Er, Dy, Ho, Gd, and Tb; x=0.24; y=0.2, 0.3, and 0.4)

Application of permanent magnets in devices such as traveling wave tubes requires a high degree of flux stability over a wide temperature range (−50 to 150 °C). PrCo5‐based permanent magnets with near zero temperature coefficient of remanence (α) can be fabricated by the substitution of a suitable heavy rare‐earth element for Pr in Pr1−x−ySmxRyCo5 (R=Er, Dy, Ho, Gd, or Tb). Pr0.76−ySm0.24RyCo5 magnets with y=0.2, 0.3, and 0.4 and R=Er, Dy, Ho, Gd, and Tb were fabricated and the temperature coefficient of remanence (α) and the temperature coefficient of coercivity (β) were determined. Among all the heavy rare‐earth elements examined, the substitution of Er resulted in magnets with maximum Br and (BH)max at all temperatures (22–150 °C). Magnets with the substitution of Gd exhibited the highest Hci at all temperatures. The substitution of Dy and Tb gave magnets with low values for Br and (BH)max. Optimally processed magnets with a typical composition Pr0.46Sm0.24Er0.3Co4.6 exhibited Br =8000 G, Hci =15 800 O...

Nmr Studies of Cobalt-Thoria-Zsm-5 Catalysts

Abstract Co-ZSM-5 and Co-ThO 2 -ZSM-5 are promising bifunctional zeolite catalysts used for the conversion of synthesis gas to gasoline range hydrocarbons. Previous catalytic tests have shown that small amounts of the promoter thoria (0.4 wt%) significantly increase the liquid hydrocarbon yields. Zero-field nuclear magnetic resonance studies were undertaken to ascertain the effect of thoria upon the magnetic and structural nature of these catalysts. In Co-ZSM-5, NMR lines corresponding to fcc and hcp phases were found. When the promoter thoria was added, there was a systematic absence of the hcp line before use as catalyst. The change in catalytic behavior can be related to the changes in crystallite species and crystallite size.

Crossover from divergent to non‐divergent nature of the ordered to paramagnetic transitions of α‐MnCl2⋅4H2O as a function of applied field

The thermodynamic functions CH, (∂M/∂T)H and [(∂M/∂H)T– (∂M/∂H)S] have been determined as a function of temperature over the range 0.5 to 2.0 K for a spherical single crystal of α–MnCl2⋅4H2O with magnetic fields up to 20 kG applied parallel to the c‐crystallographic axis. The data clearly show that the lamba anomaly in [(∂M/∂H)T−(∂M/∂H)S] associated with the transition from the ordered to paramagnetic state changes character at the bicritical point and completely disappears for fields greater than 8.4 kG. The region for fileds greater than 8.4 kG also corresponds to apparent non‐power law behavior of the heat capacity and (∂M/∂T)H. The bicritical behavior is shown to be consistant with that of a antiferromagnet with orthorhombic anisotropy.

Magnetic Properties of Some Rare Earth Silicides of the Type RFe2Si2

RFe2Si2 compounds (where R = Rare Earth, Y or Th) crystallize in tetragonal BaAl4‐type structure. Magnetic properties of these compounds with R = Y, Tb, Ho and Er have been examined in the temperature range 4.2–950 K. TbFe2Si2 exhibits a Neel temperature of 5.5 K indicating that the rare earth sublattice orders antiferromagnetically. In HoFe2Si2 and ErFe2Si2, the rare earth sublattice does not order at temperatures down to 4.2 K. The iron sublattice orders magnetically at nearly 800 K in all the compounds investigated. Saturation magnetization studies on YFe2Si2 indicate partial ordering of iron moments confirming earlier work that a majority of iron atoms are diamagnetic. Crystal field interactions seem to be significant in TbFe2Si2.