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International Congress on Fuel Cells.

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substances have been measured as functions of the molecular
structure by the pulse radiolysis technique. The same technique has been used to determine the position of the acid-base
equilibria of a number of radicals. Thus, according to J. Rabani (Jerusalem, Israel), the p K value of the equilibrium
-OH + OH- S 0- -t H 2 0 is 11.9, and that of the equilibrium
.HOz $ H’ + 0 - 2 , according to M. S. Mutheson (Argonne,
Ill., U.S.A.) is 4.45.
Ch. S. Bagdassuryan (Moscow, USSR), detected the formation of stable radical-cations by a spectroscopic method,
when amines were exposed to y-rays in organic glasses. The
quantum yield for the formation of the radical cation can be
as high as 3. Thus in certain cases each primary ionization
results in the formation of a radical cation. G. 0. Phillips
(Monmouthshire, U.K.), discussed the energy transfer during
irradiation of carbohydrates in the presence of aromatic
compounds. It can be shown by ESR spectroscopy and by
yield measurements that the decomposition of the carbohydrates decreases in the presence of aromatics. The nature
of this protection is not yet clear.
Yu. A . Kolbnnovsky (Moscow, USSR), demonstrated the
formation of polycarboxylic acids in the polymerization of
carbonic acid and ethylene with y-irradiated catalysts. The
products (molecular weight = 3000) contain one carboxyl
group per 50 ethylene units, and are soluble in dimethylformamide. A . J. Swallow (Manchester, U.K.), in collaboration with D. Hummel (Cologne, Germany), reported on the
radiation-induced addition of hydrogen chloride to olefins,
which may proceed as a chain reaction. D. Schulte-Frohlinde
and F. Merger (Karlsruhe, Germany) studied the mechanism
of the radiation-induced hydroxylation of nitrophenol. The
first step in this reaction is the extremely selective addition of
the hydroxy radical to the benzene nucleus to form a cyclohexadienyl radical.
V. N . Kondrutyev (Moscow, USSR), in his lecture on “Problems in the Study of Elementary Processes in Low-Tempera .
ture Plasmas”, reviewed the present positjon of the theory
and the experimental determination of data for the following
processes: the thermal dissociation of diatomic and polyatomic molecules, exchange reactions, the excitation and
ionization of atoms by electrons, collision ionization, recombinative ionization, electronic excitation, electronic
energy transfer and charge transfer in collisions of atoms
and molecules, reactions of ionized molecules, dissociative
recombination, and associative ionization. Equilibrium
constants cannot yet be determined with sufficient accuracy.
W . Lochte-Holtgreven (Kiel, Germany), in his lecture on
“Simple Chemical Reactions in Plasmas”, dealt with the
formation of negative ions. The ions H-, C-, N-, 0-, C1-,
Ca-, and Al- have been detected by their continuous radiation. The preferred temperature range for the formation of
these ions is beetwen 3000 and 8000°K, i.e. higher than
flame temperatures and lower than luminous-arc temperatures. This temperature range is reached by supplementary
electric heating of flames; a quantity of electrical energy that
roughly corresponds to the energy of combustion is supplied
to the flame. Reaction products can be isolated from a
plasma in various ways: .by cooling a streaming plasma, by
cooling the anode flame of a luminous arc, or by adiabatic
cooling in vacuum. The last method in particular was used by
Lochte-Hollgreven for the reduction of oxides a t a high
throughput: castings 15 cm long made of the oxide powder,
charcoal, and tar when placed in a carbon tube anode, were
reduced within a few seconds. The resulting metal was not
pure, however, since the carbon anode partly disintegrated.
M . W. Thring (London, U.K.), in his lecture o n “Plasma
Engineering”, surveyed the possible methods of using a
plasma as a “commercial reaction system”. He discussed
inter alia the following endothermic processes which, although unable at present to compete with conventional
methods of synthesis, may acquire some importance in the
future, r . g . the preparation of acetylene in a plasma beam by
reactions (a) and (b). The best yields of acetylene are ob-
+ ClH2; AH = -543 kcal
2 C + Hr
2 CH4 + CZH2 3 Hz; AH = -86 kcal
tained if argon or helium is used as the carrier gas. At a
plasma temperature of 12000 OK, 80 % of methane could be
converted into acetylene.
Dicyanogen is formed in accordance with Equation (c) in a
nitrogen plasma which reacts with the graphite cathode; the
+ (CN)2; AH = -71 kcal
2 C+ Nr
yield is 15
based o n the carbon used. Hydrocyanic acid
has been obtained by the reaction of a graphite cathode with
a hydrogen-nitrogen plasma. In this reaction (d), 50 %, of the
2 C + H2
+ NZ
-60.2 kcal
carbon was converted into hydrocyanic acid and acetylene,
which is the most important by-product formed.
Uranium carbide can be prepared from uranium dioxide and
carbon at a plasma temperature of from 4000 to 5000°K
UOz+ 3 C
+ UC$- 2 CO; AH
-179 kcal
[Equation (e)]. In this case the uranium dioxide and carbon
form the rod anode.
M. J. Joncich and J . W. Vaughn (Dekalb, Ill., U.S.A.) have
prepared iodides, sulfides, halides, and carbides of magnesium
and aluminum, as well as nitrides of magnesium, titanium,
zirconium, tantalum, and aluminum, by exploding metal
wires in the presence of a non-metal (e.g. sulfur, iodine). The
yields were from 20 to 60 %. The nitrides of iron, rhodium,
platinum, copper, zinc, and cadmium could not be obtained
by this method. This synthesis of binary compounds has the
following advantages: Lt permits the preparation of very
hygroscopic substances; it can also be used t o prepare
compounds that are not formed under normal laboratory
conditions, since the required activation energy is available
only in the case of a wire explosion. The experimental
arrangement is simple. The reactions are complete in 100
ysec or less, and the products are very rapidly cooled to the
ambient temperature, particularly when the reaction is carried
out in a liquefied gas.
[VB 955/262 IE]
German version: Angew. Chem. 77, 968 (1965)
Translated by Express Translation Service, London
International Congress on Fuel Cells
I. Low and Medium Temperature Cells
The Belgian Societk d’Etudes, de Recherches et d’Applications pour I’lndustrie organized a meeting in Brussels
(Belgium) of scientists working o n the development of fuel
cells. This meeting was the largest o n this subject so far to be
held o n European soil: 60 lectures were delivered t o the 380
participants from June 21st to 24th, 1965.
Angew. Chem. internat. Edit.
VoI. 4 (1965) 1 No. I 1
C. Berger, Newport Beach, Calif. (U.S.A.) reported on organic and inorganic ion-exchange membranes. Single-mernbrane cells are easy to construct and are compact (representative specifications: 0.72 volt, 75 mA/cm2, 110 kW/m3, and
80 W/kg). Their disadvantages are the difficulties encountered
in controlling the temperature and moisture. Attempts have
96 1
been made to avoid these difficulties with double-membrane
cells containing mobile electrolyte between two ion-exchange
membranes connected to the electrodes, but the power density
falls to 46 kW/m3. All organic exchange membranes are
prone to decomposition by high temperatures and oxidative
attack. The search for a n ion-exchange material stable
between 25 and 150 'C led to inorganic exchangers, particularly zirconyl phosphates. Membranes were made either
from the pure inorganic material or by embedding it in organic supports. Zeolytes can also be incorporated in order to
improve the moisture control. The results of prolonged experiments (300 h) at 150°C are of interest for the direct conversion of hydrocarbons, which is possible only at
high temperatures. Current densities of about 10 mA/cm2
at a cell potential of 0.4 V were obtained with propane and
butane at platinum electrodes using inorganic ion-exchange
membranes. These figures are promising. Attempts were
made to economise in the platinum consumption by direct
impregnation of the membrane surfaces with the platinum
K . Schwabe, Dresden (Germany) discussed the use of oxygen
electrodes in fuel cells. Reduction of oxygen with high current
densities at catalyst-free carbon electrodes can be achieved
only in the strongly alkaline region. A platinum coating is
required in the acidic range. Reduction of the Pt content
from 10-20 mg/cmz to one-tenth these amounts with maintenance of the activity of the electrodes was achieved by a
coating of an Al-V spinel. Satisfactory operation in longterm experiments depends largely on the type and amount of
agent used for making the electrodes hydrophobic and on the
method of impregnation. Application of a 3 % solution of
polyethylene in toluene proved to be suitable.
propane and butane and then falls with increasing chain
length. The same electrode can also be used as cathode. Because of the short diffusion path, operation with air instead
of oxygen does not involve any significant reduction in
output. Current densities of about 20 mA/cm2 at 0.5 V were
obtained with methane or propane in complete cells containing 85 % phosphoric acid as electrolyte at 150 "C. Attempts
to replace the acidic electrolytes with another CO2-repellant
but less corrosive electrolyte have so far remained without
success. The major task is now to reduce the platinum content of the electrodes, to augment the activity with additives
to the platinum, or to replace the expensive noble metal
entirely. At present, about 500 g of Pt would be required to
achieve an output of 1 kW.
H. Binder, A . Kohling, and G. Sandstede, Frankfurt/Main
(Germany) studied the anodic oxidation of hydrocarbons in
acids. In 3 N sulfuric acid, complete direct oxidation of
methane, ethane, and propane can be achieved at 90 "C and
+0.5 V (vs. a reversible hydrogen electrode). Ethylene is only
90% converted to carbon dioxide and water judging from
measurements on porous gold electrodes containing 100 mg/
cm2 of Raney platinum as catalyst. Ethane and propane are
the most reactive alkanes. The polarization potentials in 3 N
sulfuric acid are lower at 100°C than at 120 and 155OC in
26 N phosphoric acid. When the unit is switched off, a small
amount of hydrogen and COz is reformed but no methane
can be detected. With ethylene, the back reaction leads to
C02 and ethane. Propane can displace some of the adsorbed
hydrogen from the platinum surface judging from the current
YS. potential curves determined by the potentiostatic potential
method. The oxidation of the hydrocarbon occurs on the
free surface, i. e. in the double-layer portion.
0. Vohler and R. Martina, Meitingen near Augsburg (Ger-
many) have adopted a new method for making carbon electrodes for fuel cells with a sharp maximum in the pore distribution curve. Extraneous binding agents can be dispensed
with by starting out from cellulose of uniform grain size;
after coking, samples of satisfactory rigidity are obtained. The
integral pore volume and the pore diameter can be influenced
by the pressure used and the nature of the hydraulic fluid,
e.g. Hz0,CzHsOH. The hydraulic press can be used simultaneously to introduce a catalyst into the electrode. This method
was used to prepare double-layer carbon electrodes with two
layers of different pore sizes. U p to 75 X of the pore volume
of the sintered carbon is accessible to the electrolyte; the
specific surface area lies between 5 and 10 m2/g depending on
the conditions used for its preparation.
J . M. Auclair, Marcoussis (France) found that sintered Ni/Ag
electrodes can be used without activation on both the oxygen
and hydrogen sides of hydrogen fuel cells. Industrial gases
give current densities of 50 mA/cmz and cell potentials of
0.7 V at 20 "C. The current density rises to 300 mA/cmz on
raising the temperature to 80 'C. CO contents of 1-5 % in the
gas d o not interfere with the operation of the element; the
current decrease is still reversible at 20-30% CO. In intermittent operation (current drawn for 10% of the time), the
life of the cell is 22 months; in continuous operation at 50
mA/cm2 it is 6 months.
H. A . Lieblzafsky, Schenectady (U.S.A.) dealt with the problem of direct hydrocarbon combustion cells. Anodic oxidation of hydrocarbons occurs in a series of steps: adsorption,
cleavage of C-H and C-C bonds, charge transfer, reaction
of intermediates with the solvent, and removal of the reaction
products. Since C-H bonds are split much easier on the
catalyst than C-C bonds, there is a danger that carbon-rich
intermediates accumulate and block the reaction. It was
therefore surprising that complete oxidation of unbranched
saturated hydrocarbons to carbon dioxide and water was observed. A gauze electrode containing platinum black as catalyst was developed as anode in the General Electric Research
Laboratory. A thin porous layer of Teflon on the gas side
maintains a stationary position of the three-phase boundary
within the electrode. The reactivity increases from methane to
11. High Temperature Cells
G. H . J . Broers and M . Schenke, Amsterdam (Holland) dealt
with high temperature cells run with paste electrolyte. The
life and load capability of such cells have been considerably
improved over the past few years but Broers maintains that
there is still a long way to go before they will be ready for
commercial exploitation. Greater demands are placed on the
stability of the electrolyte (alkali metal carbonates + magnesium oxide) and electrodes at the high operating temperatures
(about 72OoC) than in low temperature cells. Pulse
and impedance measurements were made on cells with
porous nickel anodes and silver or copper oxide cathodes.
The material transport in the film of electrolyte on the electrodes is the rate-determining step. The ohmic potential drop
Ri in the cell represents the major proportion of the total
polarization. In prolonged experiments iR increases owing to
evaporation losses from the carbonate electrolyte but the remaining polarization becomes constant after two months.
Small units could be run for 6 months at 100 mA/cm2,
but the cell potential decreased from 750 to 480 mV
during this period. A battery of ten elements in series utilizes
90 % of a 1:1 hydrogen/carbon dioxide fuel mixture when the
electrodes are worked a t 62 mA/cmz. The utilization under
the same conditions for the gas at the cathode (2.5 parts air +
1 part C02) is 94X.
P. Lovy, La Plaine-St. Denis (France) reported that Gaz de
France has worked since 1961 on the development of high
temperature cells using fused alkali metal carbonates as electrolyte in attempts to use natural gas as a n electrochemical
fuel. Corrosion of the porous silver cathode used was suppressed by applying a layer 0.1 mm thick of sintered AI2O3.
However, the electrodes cannot be piled up as in a filter press
because they are so fragile. At present, concentric arrangements are being tested. A carbon tube activated with palladium is used as anode; this is covered with the sintered alumina layer containing the electrolyte and then surrounded by
the silver electrode. The fuel gas flows through the carbon
tube while the air/carbon dioxide mixture flows around the
external cathode. Satisfactory mechanical stability is achieved
Angew. Chem. internat. Edit. / Vol. 4 (1965) 1No. 1I
with this arrangement but its electrochemical properties are
poor. At 600°C and 20 mA/cmz, the life of the cell is only a
few days.
111. Cells with Solid Electrolyte
D . W . White, Schenectady (U.S.A.) explained that Zr02
doped with 7-10 mole-% Y203 has a higher conductivity a t
800-900°C than ZrOz containing 11-14 mole-% CaO. The
gradual drop in conductivity can be reversed by brief heating
at 1200-1400°C. A cell with this electrolyte in the form of a
crucible containing molten silver as oxygen cathode was run
for 5 months using natural gas as fuel. Methane is pyrolysed
at the outer wall, and the pyrolytic carbon is oxidized electrochemically to CO. The carbon monoxide and hydrogen pass
out of the cell and must be burnt in another cell.
H . H . Mobius and B. Rohland, Greifswald (Germany) proposed the use of a bundle of 2500 tubes, each 1 m long, per m*
as a suitable construction for a high temperature fuel element
which is supplied on the inside with fuel gas and on the
outside with air. Large units are expected to work with high
efficiency only when the heat dissipated and remaining heat of
combustion can be utilized in a coupled power generator.
The production of stable electrodes presents some problems.
Addition of cerium, uranium, and praseodymium oxides to
the surface regions of the ZrO;? was proposed in order to
augment its electronic conductivity. Connection to the ceramic oxide electrodes could then be made through metallic
point contacts.
IV. Nonporous Electrodes
M . A. Vertes and A . J . Hartner, New York, found that when
fuel reformation is allowed to proceed inside the cell in the
gas space of a nonporous palladium/silver foil electrode, the
"lost heat" developed in the cell can be used for the endothermic reaction in the gas space. Since the foil electrode
works satisfactorily at even low hydrogen partial pressures,
temperatures of 200-250 "C are high enough in the reforming
space. A nickel carrier catalyst is used for hydrocarbon fuels
and a zinc/copper oxide catalyst for methanol as fuel. A
limiting current density of 85 mA/cmz was found with hexane
a t 250OC. Current densities of 100-200 mA/cmz with very
little polarization were observed at temperatures as low as
200 "C with methanol/water vapor mixtures. Bacon electrodes
were used as oxygen cathodes in complete cells with 85%
K O H as electrolyte. With gasoline and oxygen, the cell
can be run at 100 rnA/cmz and 0.7 V at 250 'C.
S. M. Chodosh and H. G. Oswin, New York, investigated the
activation of nonporous Pd/Ag electrodes as hydrogen anodes.
The entry of the hydrogen into the Pd phase is known t o be
kinetically inhibited. Various possibilities for activating the
foil surface o n the gas side, e.g. by etching, oxidative or reductive treatment at high temperature, roughening with alumina powder, or the application of pouros layers of Pd or Pt,
were discussed. Activating layers of noble metals were applied
by galvanic methods or by immersing the palladium foil
charged with hydrogen in solutions of the noble metal salts.
Foil anodes 40-400 p thick activated in this way exhibited
diffusion-controlled limiting currents which were inversely
proportional to the foil thickness. The stability ofthe activating
layer was studied for several hundred hours at a load of 100
mA/crnz at 250 'C.
A. Kiissner, Erlangen (Germany) examined irreversible
changes in nonporous palladium/silver foil hydrogen electrodes. The diminution in activity and load capacity of hydrogen foil electrodes in long-term experiments led to the discovery of irreversible processes in the foil. The uptake of hydrogen in the palladium/silver alloy is accompanied by a con-
Angew. Chem. internat. Edit./ VoI. 4 (1965) / No. I1
tinuous expansion of the lattice. If the foil electrode is now
subjected to sporadic changes in load, pressure gradients
arise in the proximity of the foil surface and lead to plastic
deformations of the foil material. The diffusion of hydrogen
is adversely affected by the resultant lattice defects. The
alternating expansions and contractions also result is localized separations of the activated material from the base
material. It is hoped that hardening of the material will bring
V. Electrocatalysis
C . E. Heath, Linden, N.J. (U.S.A.) assumed that the oxidation of methanol on platinum catalysts proceeds via surface
oxides. Since ordinary platinum black does not sufficiently
accelerate the reaction in acidic solution, attempts were made
to modify the Pt surface by incorporation of other metals, i. e.
to increase its specific surface x e a , and to alter its band
structure or introduce defects. The experiments did not
have the desired results. On the other hand, the introduction
of immobile redox couples into the catalyst surface have an
activating effect and determine the potential of the electrode.
Methanol/air cells with a platinum/ruthenium catalyst were
built which could be loaded with 50 mA/cm2 at 50-60 "C and
a cell potential of 0.5 V. Slightly lower cell potentials were
observed in a battery of 16 cells.
F. P. Dousek, Prague (Czechoslovakia) found that the
handling of Raney nickel hydrogen electrodes is complicated
by the pyrophoric nature of this catalyst. This difficulty can be avoided by expelling most of the adsorbed
hydrogen either by anodic polarization or by treatment with
organic compounds that can be hydrogenated. However, the
catalytic activity is always depleted by this treatment. Dousek
observed that when Raney nickel is dissolved out of the
Al/Ni alloy with 5 N sodium hydroxide and the solution is
subsequently brought to p H 6.5 with tartaric acid, hydrogen is
released. The desorption proceeds very rapidly when the
nickel is heated to 90 "C with distilled water. After this mild
treatment, the catalyst is still active but no longer pyrophoric.
Electrodes similar to those of the Bacon type were made from
this catalyst by sintering at 450 OC in a n atmosphere of hydrogen and gave good current vs. voltage curves for the oxidation
of hydrogen in long-term experiments, where they remained
stable for over 10000 h of operation.
R. Jusinski, Waltham, Mass. (U.S.A.) reported that nickel
boride of composition NizB is a n active catalyst for the hydrogen and hydrazine electrode. Its activity is greater than
that of Pd or Pt for the hydrazine electrode.
The nickel borides NiB, Ni4B3, NiZB, and Ni3B were studied
by H. Jahnke, Stuttgart (Germany) by X-ray and metallurgical
methods. When embedded in a nickel or gold support, Ni3B
is the best catalyst for the oxidation of hydrogen and methanol.
Ternary Ni/B/AI alloys were made by sintering and activated
like Raney alloys. The polarization of the methanol electrode
with such a catalyst at 80 OC and 50 mA/cmz is 215 mV compared with 330 mV with Ni3B. Since Raney nickel alone cannot catalyse the oxidation of methanol, the activity was ascribed to the NiB12 and B incorporated in the nickel catalyst.
I . Lindholm, Vaster& (Sweden) also tested the suitability of
catalytically active nickel borides as hydrogen electrodes.
Sodium borohydride precipitates a black nickel boride of
sum formula Ni2.35B with a high specific surface area (31.7
m2/g) from solutions of NiC12. Some of the boron can be
dissolved out with potassium hydroxide solution with concomitant increase in the specific surface area. This catalyst was
dried in vucuo at lOO"C, mixed with nickel powder as an
embedding medium, and the mixture was sintered. Hydrogen
anodes were also obtained by precipitation of NiBz in preformed sintered nickel supports. In operation at 8OoC and
150 mA/cm2, these had a polarization of 100 mV. In long-
term experiments with single cells, the cell potential fell from
0.9 to 0.8 volt at a load of SO m/cm2 within 4000 h. Replacement of the noble metals in low temperature cells with the
activated nickel boride catalyst produced a significant reduction in costs. However, the total cost of a battery cannot yet
be given.
[VB 9521256 IE]
German version: Angew. Chem. 77, 971 (1965)
Radical Phenylations with Aromatic
Diazo Compounds
C. Ruchordt, Munich (Germany)
Aromatic radical phenylations occur by the addition-disproportionation mechanism (a). The nature of X . and Y - is
essentially clarified for phenylations by dibenzoyl peroxide or
triphenyl(phenylazo)methane, but not for those by aromatic
diazo compounds. Assuming that the phenyl radicals are
generated from diazohydroxides (Gomberg reaction) or from
diazoacetates (thermolysis of N-nitrosoacetanilide), or that
hydroxyl or acetoxyl radicals constitute X., contradicts a
variety of experimental results [I].
Kinetic investigation of the Gomberg reaction showed that
the rate of liberation of nitrogen is proportional to the square
of the diazonium salt concentration and is greatest at the p H
equal to the pKa of the diazonium ion-diazotate equilibrium.
The results were interpreted as slow formation of diazo
anhydrides ( I ) in the aqueous phase and their rapid decomposition in the organic phase. In agreement therewith the
formation of biphenyl from benzenediazonium chloride at
p H 8.5 is greatly accelerated by addition of p-nitrobenzene-
diazonium chloride, and the yield of 4-nitrobiphenyl under
similar conditions increases o n addition of benzenediazonium chloride,
The almost complete absence of evolution of COz o n thermolysis of N-nitrosoacylanilides, whose acyloxy-radicals must
undergo decarboxylation without appreciable activation
energy (as, for instance, phenylacetoxyl, phenoxyacetoxyl, or
ethoxyoxalyloxy-radicals),is considereda convincing argument
against homolytic decomposition of diazo esters formed
as intermediates.
Acid anhydrides increase the yield of carbon dioxide from
the decomposition of nitrosoacetanilide. The evolution of
nitrogen from benzene solutions is first order only in very
dilute solutions (0.01 M), and the rate is that of the formation
of the diazo ester. The rate of evolution of nitrogen is decreased by the presence of vinylic monomers (acrylonitrile or
styrene) and depends greatly on the nature of the solvent. The
reaction hardly occurs at all in CC14. By reason of these
results it was proposed that in the decomposition of diazo
esters the formation and decomposition of diazo anhydrides
occur in chain reactions. It is known that all steps of this chain
reaction proceed rapidlyin inert solvents.The diazotate radicals
(2) were detected and identified by their ESR spectra, which
showed them to have a relatively long life, as expected o n the
basis of chemical results [l].
From 4-chloro-N-nitrosobenzanilide
in CC14 there is formed,
besides p-chlorobenzoic anhydride and benzenediazonium
chloride, a thermolabile compound which, on the basis of its
physical and chemical properties, is perhaps the first isolated
diazo ester.
[Chemical Colloquium, Universitat Heidelberg, June 28th,
[VB 949/258 IE]
Gcrman version: Angew. Chem. 77, 974 (1965)
The Acid-Base Catalysis of the Mutarotation of
Herrnann Schmid and Ginther Barrer, Vienna (Austria)
The negative entropy of activation of the water-catalysed
mutarotation of glucose is ascribed to the facts that the
activating process is the transfer of a proton from the
hydroxyl group on the carbon atom adjacent to the ring
oxygen to a water molecule and that the electrical field
between the ions formed tends to align the solvent dipole in
the activated complex.
Initial reaction
[ l ] C . Riichardt, B. Freudenberg, and E. Merz: Organic Reaction
Mechanisms. The Chemical Society, Special Publication No. 19,
London, 1965, p. 168.
+ HzO ?!!
activated complex ( G O . . . HzO.. . H,O@)
HG, denotes a-glucose and GO is the glucose anion in the
activated complex.
The activation enthalpy of the catalysis with ammonia is
almost 4 kcal/mole lower than that of the catalysis with
water because the ammonium ion formed in the reaction of
glucose with ammonia has a lower tendency to align the
solvent dipole in the activated complex than the hydronium
ion formed on activation of the a-glucose with water.
The enthalpy of activation for the catalysis with hydronium
ions is identical with that for the catalysis with water, but
its entropy of activation is less negative. The catalysis with
hydronium ions is in fact water catalysis in which the solvent
dipole entering the activated complex is "pre-aligned" by the
hydronium ions added as catalyst. The activation enthalpies
of the mutarotation of glucose catalysed by anions (formate,
acetate, a-gIucosate, hydroxyl) also correspond to the activation enthalpy of the catalysis with water. These reactions
are also water catalyses in which the first step is a reaction of
the hydration shell of the ions with the a-glucose.
The rate of mutarotation of glucose in water was measured
with and without addition of finely divided copper powder
under otherwise identical conditions. The energies of acAngew. Chem. internut. Edit.
1 Vol. 4 (1965) I N o . I I
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