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Chemisorption on Metal Films.

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Kinetic Investigations with Solid Electrolytes
By H . Rickerti*]
Solid state galvanic cells have two properties that are important for kinetic studies: (1) reaction velocities can be
measured in suitably constructed cells by means of the
electric current and (2) the E M F of the cells provides thermodynamic information, particularly about chemical potentials
or activities. The combination of velocity measurements with
determination of thermodynamic parameters often permits
analysis of kinetic processes. Four typical examples were
discussed :
a) Diffusion of oxygen in solid and liquid metals.
b) Evaporation and condensation of sulfur, selenium, and
iodine from and o n t o their solid metal compounds (including
description of electrochemical Knudsen cells for study of the
thermodynamics of vapors, e.g., sulfur and selenium vapor).
c) Passage of Ag ions and electrons through the phase
boundary Agsolid/Ag2Ssolidd) The kinetics of formation of NiSsolid from Ni at 400 "C.
Lecture at Hannover (Germany), on November 23, 1967 [VB 125 IEI
German version: Angew. Chem. 80, 292 (1968)
[*I
Doz. Dr. H. Rickert
Institut fur Physikalische Chemie der Universitiit
75 Karlsruhe, Kaiserstr. 12 (Germany)
Heterocyclic Azo Dyes in Complex Chemistry and
in Analytical Chemistry
By L. Sommer [*I
For many years azo dyes have been used as sensitive reagents
in analytical chemistry (e.g., for trace analysis and spectrophotometric determination of metals, also as indicators for
metals in chelatometric titrations). N-Heterocyclic azo dyes
containing the characteristic donor grouping have won a
special place. Dyes of the pyridine, thiazole, triazole, and
tetrazole series normally form red or reddish-orange metal
chelates, and derivatives of p-cresol, p-methoxyphenol, and
chromotropic acid blue or green metal chelates which are
more or less soluble in water. Metal ions with empty or with
full or partly filled d-orbitals are particularly reactive.
Analytically the most valuable reactions of heterocyclic dyes
are those with ions of Cull, Zn, Cd, Sc, lanthanides, TirV,
Vv, NbV, Fe", Fell1, Co", Ni", Pd", Ga, In, TllI1, Pb", and
UO22+. N o analytical importance attaches to the reactions
of Be, Ca, Sr, Ba, Al, SbIT1, 0111, MovI, or WvI unless
additional donor groups are present in the dye molecule.
Complex formation in solution has been studied for 4-(2pyridy1azo)resorcinol (PAR) and 4-(2-thiazolylazo)resorcinol (TAR) and compared with that of 1-(2-pyridyIazo)-2naphthol (PAN) and its thiazole analog (TAN).
PAR and T A R give protonated and "normal" 1:l metal
chelates over a broad p H range; at p H > 5 in solutions
containing a n excess of reagent, 1:2 chelates are also formed,
but only with difficulty and not always by Cu'I and not at
all by uranyl.
Loss of the proton from the hydroxyl group on self-dissociation of the dye, and formation of a protonated metal
chelate, must be taken into account when formulating the
complex constants; so must the formation of hydroxychelates of In, Ga, and T1"' in the hydrolysis range of these
Angew. Chem. internat. Edit. / Vol. 7 (I968) 1 No. 4
ions. The acidity of the p-hydroxyl proton is appreciably
increased by formation of a stable protonated metal chelate
as well as by presence in solution of an excess of metal ions
of higher valence. PAN and T A N preferentially afford
coordinately saturated metal chelates, which dissolve also in
slightly polar solvents. Formation of 1:3 and 1:4 chelates
of the lanthanides is an exception t o the otherwise strictly
maintained tridenticity.
The stability of metal chelates of N-heterocyclic azo dyes,
as well as the tendency to complex formation of metal ions
with this group of ligands, are with few exceptions directly
proportional to the electronegativity of the metals (calculated
with regard to their valence): PdrI > Cu" > U022+ >
Fe" > Nill
Co" > Pb" > Zn > Cd
Mn"; TI"'>
Bi > G a > Fell'> In > Sc > A1 > lanthanides > Y > La
The thiazole azo dyes are found to give chelates that are less
stable than those obtained from pyridine azo dyes.
-
-
Coordination of the metals proceeds in two steps: the first
involves bonding of the heterocyclic nitrogen, and the second
is closure of the chelate ring by way of the o-hydroxyl oxygen
and the azo nitrogen atom situated farthest from the heterocyclic nitrogen.
Lecture at Maim (Germany), o n November 23, 1967
IVB 126 IE]
and at Sadrbrucken (Germany), on November 24, 1967
German version: Angew. Chem. 80, 291 (1968)
[*] Prof. Dr. L. Sommer
Institute for Analytical Chemistry of the University
Kotlarska 2
Brno (CSSR)
Chemisorption on Metal Films
By G. Wedler *I
Changes in the physical properties of thin metal films on
adsorption of gases can often be used for a study of the
nature of the bonding between adsorbent and adsorbate.
Since properties, such as the electrical conductivity and
photoeffect, of evaporated films often differ markedly from
those of the compact material, interpretation of adsorption
effects requires a knowledge of the properties of films. We
have studied N i films of systematically varied thickness that
were condensed at 77'K in ultra-high vacuum at a rate of
ca. 10 A,min and then annealed at increasing temperatures.
Electron-microscopical and X-ray-diffraction measurements
show that the crystals are small plates and increase in size
with increasing layer thickness and annealing temperature;
the crystallites are arranged mostly with the 1111). and to a
smaller extent with the {loo}, faces parallel to the glass
support 111. The texture of the crystallites is dependent o n the
conditions of preparation. Because of the different coefficients of thermal expansion of the glass support and the metal
film the annealed films are under a stress which leads to a
decrease in the separation between the network planes that
lie parallel t o the support.
By applying Sondheimer theory t o the dependence of electrical resistivity of the film on layer thickness the mean free
path lo of the conduction electrons and the specific resistance
po can be determined for compact material that has the same
volume density of impurity centers as the film. lo, PO, and
analysis of the line profile of the X-ray reflections lead t o the
same conclusions about increase in the degree of order of the
originally disordered films with increasing temperature.
Study of the dependence of the electron work function o n
layer thickness and temperature shows that for unannealed
Ni films condensed at 77 "K the work function is the same,
'p = 4.59 V, down t o layer thicknesses of 40 A. With rise in
annealing temperature cq increases - more for thin than for
thick films - so that films annealed at 373 "K give a constant
work function of 4.95 V only at thicknesses of > 200 A.
When considering adsorption effects it is advisable either t o
study their dependence o n the thickness of the metal films o r
307
simultaneously to carry out other measurements that afford
thermodynamic values.
Thus it is found, for instance, with the system NijCO that the
increase in resistance observed as a consequence of adsorption cannot be explained by a change in the electron concentration or a decrease in the effective film thickness
due to formation of a compound on the surface. However, if the fact is taken into consideration that a change in the
number of free electrons must alter their mean free path, then
the dependence of the increase in resistance on the film
thickness, as measured, can be described correctly and
quantitatively.
Information about the various species of an adsorbate
occurring side by side in one system can be obtained by
combining measurements of the electrical resistance, electron
work function, and differential heats of adsorption and, if
possible, studying their temperature dependence. Thus it can
be shown that in the system Ni/Hp at equilibrium pressures
of < 10-4 torr and 273 OK there is no molecularly adsorbed
hydrogen (as there is at 77’K) but that account must be
taken of a second atomically sorbed species.
Lecture at Hamburg (Germany) on November 3, 1967
[VB 129 IEI
German version: Angew. Chem. 80, 289 (1968)
[*] Prof. Dr. G. Wedler
Jnstitut fur Physikalische Chemie
der Universitat Erlangen-Nurnberg
8520 Erlangen, Fahrstr. 17 (Germany)
[l] Cf. G . Wedler, Angew. Chem. 78, 827 (1966); Angew. Chem.
internat. Edit. 5, 848 (1966).
Rhenium(v1r) Oxide and “Perrhenic Acid”
By B. Krebs [*I
The structure and coordination state of solid Rez07 and of
solid “perrhenic acid” have hitherto been unknown. Attempts to predict the constitution on the basis of spectral
data and thermodynamic data did not lead to unambiguous
results. The structure of both compounds has now been
clarified in detail by X-ray structure analysis; this gave at the
same time the composition of “perrhenic acid” that was not
determined analytically in previous work. Both structures
are characterized by unusual coordination relations. Re06
octahedrons and R e 0 4 tetrahedrons, largely covalently bound,
occur together.
Dirhenium heptoxide crystallizes in space group P212121
(D;) with a = 12.508, b = 15.196, c = 5.448 A, Z = 8, and
presents a new type of structure that has no direct relation
to other oxide structures [l]. Chains of Re06 octahedrons,
joined together at corners, are linked by corners to Re04
tetrahedrons in such a way that a polymeric double-layer
lattice arises. The double layer can be described as an
arrangement of rings comprising four polyhedrons linked
at their corners (0-T-0-T)
and in turn linked to one
another by octahedrons. The separations in the tetrahedrons
are between 1.68 and 1.81 A, the terminal bonds being
shorter than the bridge bonds in accord with their higher
bonding order. The octahedrons are strongly distorted: Re
is shifted from the center parallel to one of the three-fold
octahedral axes (three Re-0 distances between 2.06 and 2.16,
the remaining three between 1.65 and 1.75 A). Re207 thus
constitutes a transition between the structure of the more
strongly ionic oxides Moo3 and W 0 3 (distorted octahedral
six-coordination) and the molecular structures of RU04 and
O s 0 4 (tetrahedral four-coordination). The mechanism of
evaporation of the oxide with formation of Re207 molecules
with tetrahedral coordination can be readily deduced from
the structure.
Solid “perrhenic acid” (which can be recrystallized from
organic solvents) has a molecular structure in which also
distorted Re06 octahedrons occur alongside Re04 tetrahedrons (in the binuclear compound one octahedron is
308
joined by a corner to one tetrahedron) [2J. The fact that two
of the terminal Re-0 bonds of the octahedron are very much
longer (2.21 A) than any of the others (octahedron, terminal
1.73-1.76 A; bridge 2.10 and 1.80 A; tetrahedron, terminal
1.73-1.77 A) is in agreement with the broad-line 1H-NMR
spectrum and the IR spectrum in indicating that the compound is correctly formulated as dihydratoheptaoxidodirhenium Re207(OH2)2 with trebly linked oxygen in the H20.
Lecture at Gottingen (Germany) on December 14, 1967
[VB 132 IE]
German version: Angew. Chem. 80, 291 (1968)
-~
[ * ] Dr. B. Krebs
Anorganisch-Chemisches Institut der Universitat
34 Gottingen, Hospitalstr. 8-9 (Germany)
[I] B. Krebs, A. Miiller, and ff. Beyer, Chem. Comrnun., in press.
121 H. Beyer, 0. Glemser, and B. Krebs, Angew. Chem. 80, 286
(1968); Angew. Chem. internat. Edit. 7, 295 (1968).
Preparative Chemistry of Chalcogenides
By A . Rabenau (Lecturer) and H. Rau [*I
If hydrogen sulfide is passed into an aqueous solution of a
lead salt in the presence of free hydrohalogen acid, the first,
precipitate is not black PbS but materials whose color
changes from yellow via orange to red and finally via brown
to black. The nature of these precipitates is an old problem
of preparative chemistry. As early as 1836 HiinefeZd[lJ
ascribed the composition 3 PbS.2 PbCl2 to a red precipitate
obtained in this way from a solution containing hydrochloric
acid. Parmentier [21 isolated from solutions containing hydrochloric or hydrobromic acid red precipitates which, according
to analyses, should have formulas PbS.PbC12 or PbS.PbBr2,
respectively. Lenher [31 found PbS.4PbI2. Since the low
stability of the precipitates makes it impossible to separate
preparatively the several phases constituting the mixture
and X-ray control was not available at that time, the question
of the existence of lead sulfide halides remained open.
On hydrothermal synthesis of PbS in concentrated hydrohalogen acids [41 we have isolated deep red single crystals of
the composition Pb.&I6 ( I ) and Pb7S2Br10 (2) [51. Thermoanalytical and X-ray studies show that these are stable
ternary phases in the pseudobinary systems PbS-PbI2 and
PbS-PbBr2; they decompose peritectically at 418 ( 1 ) and
381 “C (2),respectively. Analysis of the d-values of a compound “PbS.PbBr2” [61 showed that the preparation consisted of ( 2 ) and PbBr2.
Experiments carried out on precipitates obtained from
solutions containing hydrobromic acid showed that the
precipitates consisted of PbS, (21,and a still unidentified
further phase in proportions that varied with the concentration of HBr.
No crystals other than PbCI2 or PbS were observed during
hydrothermal synthesis in concentrated hydrochloric acid.
However, a ternary phase, Pb4SC16, was found o n rapid
cooling of melts of the system PbS-PbC12; it was obtained
in pure crystalline form on quenching a melt of this composition from 600°C. This compound is metastable and
decomposes into the binary components when kept at
temperatures between 200 and 450 OC (eutectic).
[VB 131 IE]
Lecture at Hannover (Germany), on December 14, 1967
German version: Angew. Chem. 80, 292 (1968)
[*I Priv.-Doz. Dr. A. Rabenau and Dr. H. Rau
Philips Zentrallaboratorium GmbH.
Laboratoriurn Aachen
51 Aachen, Postfach 450 (Germany)
[ l ] J. Hiinefeld, J. prakt. Chem. 7, 27 (1836).
[2] F. Parmentier, C . R. hebd. Seances Acad. Sci. 114,298 (1892).
[3] V. Lenher, J. Amer. chem. SOC.17, 511 (1895).
[4] H. Rau and A . Rabenau, Solid State Cornmun. 5, 331 (1967).
[5] A. Rabenau, ff.Rau, and G . Rosenstein, Naturwissenschaften,
55, 82 (1968).
[6] ASTM-Index 1958, 4-0383.
Angew. Chem. internat. Edit. / Yol. 7 (1968) / No. 4
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