Kinetics and mechanisms of the catalytic reactions of formaldehyde with copper oxides and a copper ion complex in aqueous alkali

Article abstract of DOI:10.1134/S0023158411010046

The kinetics of the autocatalytic reactions of formaldehyde with copper(II) and copper(I) oxides and with the Cu²⁺ ion of the copper EDTA complex, as well as formaldehyde disproportionation in the presence of copper metal, have been investigated in aqueous solutions of sodium hydroxide. Two likely reaction mechanisms are presented. The difference between these mechanisms does not alter the observed kinetics of the processes, whose rate is determined by their first, slow step, namely, the oxidation of the methylene glycol anion adsorbed on the copper surface into formic acid. In the slow step of the first mechanism, a hydride ion is abstracted from the methylene glycol anion and is transferred to copper. In the slow step of the second mechanism, the methylene glycol anion undergoes anodic oxidation, releasing a hydrogen atom and an electron. In the rapid steps of the first mechanism, the hydride ion undergoes anodic oxidation to hydrogen, the copper compound undergoes cathodic reduction to copper metal, and, simultaneously, the electron and hydrogen are transferred to a nonionized formaldehyde molecule to yield methanol. Mathematical models are suggested for the reactions. The effective rate constants and activation energies of the slow steps of the reactions have been determined. The effective rate constants of the noncatalytic reduction reactions of the copper compounds and the ratios of the rates of the rapid hydrogen and methanol formation reactions have been estimated.

Full text of DOI:10.1134/S0023158411010046

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2011, Vol. 52, No. 1, pp. 26–33. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © O.A. Demchenko, D.I. Belkin, 2011, published in Kinetika i Kataliz, 2011, Vol. 52, No. 1, pp. 28–35.
Kinetics and Mechanisms of the Catalytic Reactions
of Formaldehyde with Copper Oxides
and a Copper Ion Complex in Aqueous Alkali
O. A. Demchenko* and D. I. Belkin
Institute of Chemical Technologies, East Ukrainian National University, Rubezhnoe, Lugansk oblast, Ukraine
*eꢀmail: seredaoa@bk.ru
Received March 26, 2010
Abstract—The kinetics of the autocatalytic reactions of formaldehyde with copper(II) and copper(I) oxides
and with the Cu2+ ion of the copper EDTA complex, as well as formaldehyde disproportionation in the presꢀ
ence of copper metal, have been investigated in aqueous solutions of sodium hydroxide. Two likely reaction
mechanisms are presented. The difference between these mechanisms does not alter the observed kinetics of
the processes, whose rate is determined by their first, slow step, namely, the oxidation of the methylene glycol
anion adsorbed on the copper surface into formic acid. In the slow step of the first mechanism, a hydride ion
is abstracted from the methylene glycol anion and is transferred to copper. In the slow step of the second
mechanism, the methylene glycol anion undergoes anodic oxidation, releasing a hydrogen atom and an elecꢀ
tron. In the rapid steps of the first mechanism, the hydride ion undergoes anodic oxidation to hydrogen, the
copper compound undergoes cathodic reduction to copper metal, and, simultaneously, the electron and
hydrogen are transferred to a nonionized formaldehyde molecule to yield methanol. Mathematical models
are suggested for the reactions. The effective rate constants and activation energies of the slow steps of the
reactions have been determined. The effective rate constants of the noncatalytic reduction reactions of the
copper compounds and the ratios of the rates of the rapid hydrogen and methanol formation reactions have
been estimated.
DOI: 10.1134/S0023158411010046
The reduction of the Cu2+ ion with formaldehyde
has long been known and is now widely used in the
deposition of copper coatings onto various materials.
Copper hydroxide precipitation in this process is ruled
out by introducing ligands capable of binding the Cu2+
ion, such as the disodium salt of ethylenediaminetetꢀ
raacetic acid. The data available on the kinetics and
mechanism of this reaction [1–3] are controversial
and do not cover the range of conditions in which copꢀ
per precipitates from the reaction system. We failed to
find any information on the kinetics and mechanism
of the reaction of formaldehyde with copper oxides.
We have found only patents concerning the use of copꢀ
per [4, 5], copper oxalate [6, 7], copper(II) hydroxide
and oxide [8, 9], and copper(I) oxide [8, 10] in penꢀ
taerythritol synthesis for preventing the formation of
byꢀproducts from formaldehyde. There is no informaꢀ
tion about the kinetics and mechanism of formaldeꢀ
hyde disproportionation in the presence of copper.
This imposes a limit on the understanding of the above
reactions and on their applications, including the
purification of polyols. Here, we attempt to make up
for part of this lack of information.
EXPERIMENTAL
The chemicals used in this work were formalin conꢀ
taining 37 wt % formaldehyde and 10 wt % methanol,
sodium hydroxide, and a number of copper comꢀ
pounds. Copper(II) oxide was obtained by reacting
sodium hydroxide with copper(II) acetate; copper(I)
oxide, by reacting glucose with copper sulfate in the
presence of sodium hydroxide; the copper EDTA
complex, by the reaction between the disodium salt of
ethylenediaminetetraacetic acid and copper sulfate.
Reaction mixtures containing formaldehyde condenꢀ
sation products were also used.
Appropriate amounts of an aqueous solution of
sodium hydroxide and formalin were placed in a temꢀ
peratureꢀcontrolled glass flask with a magnetic stirrer,
and a copper compound was then added. The reaction
temperature was maintained with an accuracy of
0.5°С
.
The reactions were conducted under stirring such
that a decrease or an increase in stirring intensity
exerted no effect on their rate. The initial reaction
conditions were as follows: formaldehyde concentraꢀ
tion, 0.5–3.6 mol/l; sodium hydroxide concentration,
0.35–2.5 mol/l; reactant molar ratio, 1 : 1 to 1 : 0.4;
copper ion complex concentration, 0.05–0.17 mol/l;
copper oxide weight, 0.5–3.1 g; volume of the reacting
26

KINETICS AND MECHANISMS OF THE CATALYTIC REACTIONS
27
solution, 0.05–0.9 l; the temperature ranges for the vessel connected with the flask. In calculations, we
reactions involving copper(II) oxide, copper(I) oxide, took into account that Cu2+ reduction to copper metal
and the copper ion complex were 3–18, 5–30, and is not accompanied by Cu+ accumulation [11], used
13–40°С, respectively.
the material balance data presented below, and
assumed that the amount of the resulting copper is
equimolar to the amount of released hydrogen. The
amount of copper that was formed upon the compleꢀ
tion of the reaction was also determined gravimetriꢀ
cally. The amounts of copper oxides, copper ion comꢀ
plex, and methanol during the reactions were deterꢀ
mined using material balance equations.
The reaction mixture was sampled during the reacꢀ
tions. Each sample was poured into a flask containing
a known, excess amount of 0.1 N hydrochloric acid,
and the mixture was titrated with 0.1 N sodium
hydroxide in the presence of phenolphthalein to deterꢀ
mine the sodium hydroxide concentration in the iniꢀ
tial sample. Next, the formaldehyde concentration
was determined. To do this, a prescribed amount of an
aqueous solution of hydroxylamine hydrochloride was
added to the solution in the flask, and the resulting
solution was titrated with a 0.1 N alkali in the presence
of bromophenol blue.
The current concentrations of hydroxide ions
(
) and methylene glycol anions (
) were
CH2(OH)O−
COH−
C
calculated using Eqs. (1) and (2), which were derived
from the ion equilibrium and material balance equaꢀ
tions for the methylene glycol anionization reaction
The amount of released hydrogen was determined with the electroneutrality of the solution taken into
as the volume of water displaced by the gas from the account [12]:
2
− 1 + 6(C
[
− CNaOH) + 1 + 6(C
− CNaOH) + 24C
]
[
]
CH2O
CH2O
NaOH
(1)
(2)
COH−
=
,
12
6COH−
,
СCH
= C
2((OH)O−
CH O1 + 6COH−
2
containing formaldehyde condensation products [13]
does not change the induction period or the kinetics of
the reactions examined.
where
and
are the sodium hydroxide and
CCH O
2
CNaOH
formaldehyde concentrations in the reacting solution.
As is clear from Figs. 1–3, the reduction reactions
of copper(II) oxide, copper(I) oxide, and the Cu2+ ion
are autocatalytic. These reactions, as well as formaldeꢀ
hyde oxidation and reduction (disproportionation),
are catalyzed by copper metal, which results from slow
reaction steps during the induction period:
RESULTS AND DISCUSSION
An analysis of the material balances for the reacꢀ
tions of formaldehyde with copper(II) oxide, copꢀ
per(I) oxide, and the copper ion complex in aqueous
sodium hydroxide (table) shows that the reduction of
the copper compounds to copper metal is accompaꢀ
nied by formaldehyde oxidation to formic acid, hydroꢀ
gen evolution, formaldehyde reduction to methanol,
and formaldehyde condensation.
The formation of an additional amount of hydroꢀ
gen, which exerts no significant effect on the kinetics
of the reactions examined, is unlikely to be due to the
reduction of the initial copper compounds, because,
as follows from the data presented in the table, approxꢀ
imately the same amount of hydrogen forms in the
presence of Cu. The causes of the formation of this
extra amount of hydrogen will not be considered here.
CuO + 2CH2(OH)O–
Cu + 2HCOO– + H2 + H2O,
(I)
Cu2O + 2CH2(OH)O–
2Cu + 2HCOO– + H2 + H2O,
(II)
Cu2+ + 2CH2(OH)O–
Cu + 2HCOO– + H2 + 2H+.
(III)
Copper metal crystallizes on the surface of oxide
particles or, when the copper ion complex is used, in
the bulk of the reacting solution.
It is difficult to understand a priory whether the
formaldehyde condensation products are weaker or
stronger reductants than formaldehyde, even though
The length of the induction period under fixed iniꢀ
glucose is known to be capable of reducing Cu2+ only tial reaction conditions is not always the same. This is
to Cu+. Our experimental data demonstrate that these most likely due to the uncontrolled effects of various
products, like formaldehyde, reduce the copper comꢀ factors on copper crystallization. For example, the
pounds and undergo disproportionation in alkaline reduction of the copper ion in the complex in the presꢀ
solution in the presence of copper. However, these ence of a copper “seed” occurs without any induction
reactions proceed very slowly and show themselves period. The reaction kinetics after the induction
only after the complete disappearance of formaldeꢀ period (in the absence of a “seed”) is the same as in the
hyde (Figs. 1, 2). Replacement of water with a solution reaction without an induction period (in the presence
KINETICS AND CATALYSIS Vol. 52
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28
DEMCHENKO, BELKIN
Material balances for the reactions
Amounts of components in the reaction mixture, mmol
before reaction after reaction
CuR CH2O NaOH CH2O NaOH H2 HCOONa CH3OH
Reaction with CuO
Volume
of the reaction
mixture, l
n
H2
ΔnCH O
ΔnH
ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ
2
2
n
CH3OH
0.2
0.2
0.2
0.4
0.4
0.2
0.2
0.2
6.3
12.7
12.6
384.0 184.0
364.0 232.0
752.0 372.0
7.0
0.0
0.0
29.0
58.0
41.0
15.0
52.0
45.0
6.7
13.3
13.1
13.3
13.2
25.6
28.0
34.4
155.0
174.0
331.0
339.0
304.0
149.0
332.0
188.0
141.6
147.4
304.8
312.4
277.6
97.8
80.4
42.6
0.4
0.6
0.5
0.8
0.7
0.6
2.9
3.1
0.047
0.090
0.043
0.043
0.048
0.262
0.101
0.289
116.2
230.6
231.4
71.2
12.5 1733.0 354.0 851.0
12.5 1703.0 356.0 890.0
25.0
25.1
37.5
388.0 194.0
720.0 459.0
376.0 194.0
70.0
20.0 127.0
276.0
119.2
92.0
0.0 6.0
68.8
Reaction with Cu2+
0.2
0.2
0.2
0.2
8.5
16.0
16.0
33.0
418.0 266.0 112.0 119.0
376.0 248.0 0.0 69.0
746.0 510.0 187.0 246.0
8.9
16.6
17.1
35.3
147.0
179.0
264.0
179.0
192.2
145.8
229.8
108.6
29.8
51.2
65.2
22.4
0.4
0.6
1.1
2.0
0.046
0.113
0.074
0.320
380.0 235.0
70.0
56.0
Reaction with Cu2O
0.4
0.8
12.2
850.0 630.0
0.0 240.0
0.0 584.0
12.7
26.1
390.0
656.0
364.6
603.8
95.4
0.5
1.1
0.035
0.043
25.0 1600.0 1240.0
340.2
Reaction with Cu from CuO
0.2
0.4
0.4
0.2
0.2
1*
1*
1*
2*
3*
356.0 180.0
707.0 359.0
2.0
24.0
73.0
15.0
52.0
71.0
0.5
0.7
0.7
1.6
2.9
156.0
286.0
337.0
130.0
94.0
155.0
284.6
335.6
126.8
88.2
43.0
101.4
194.4
54.2
–
–
–
–
–
–
–
–
–
–
35.0
1550.0 352.0 683.0
362.0 182.0
309.0 165.0
51.0
75.0
51.8
Reaction with Cu from Cu2+
0.2
0.2
1*
3*
372.0 278.0
0.0 109.0
0.6
2.8
169.0
130.0
167.8
124.4
35.2
48.6
–
–
–
–
418.0 270.0 115.0 140.0
Reaction with Cu from Cu2O
13.0 116.0 0.5 316.0
0.2
1*
756.0 432.0
315.0
125.0
–
–
* Amount of Cu, g.
of a “seed”). The induction period shortens with an metallic surface; that is, an electrochemical mechaꢀ
nism takes place.
Several Cu2+ reduction mechanisms have been
considered in the literature [1–3].
increasing weight of the copper compound, methylene
glycol anion concentration, or temperature.
The explanation of the catalytic action of copper
presented in earlier publications [2, 3] is based on the
quantum mechanical theory of charge transfer in polar
media: electron transfer will require less energy if the
reductant and the ion of the oxidizer are localized near
the metallic surface, not in the bulk of the solution.
Calculations demonstrated that the adsorption of a
methylene glycol anion or hydride ion on the copper
surface is accompanied by almost complete transfer of
one electron. This is evidence that electron transfer to
According to the electrochemical mechanism, two
independent electrode reactions occur on the copper
surface. One is the anodic oxidation of formaldehyde,
2HCHO + 4OH–
2HCOO– + 2H2O + H2 + 2e–,
(IV)
and the other is cathodic Cu2+ reduction. The released
electrons are transferred via the copper surface to the
the oxidizer is more likely mediated by the conducting Cu2+ ions, reducing them to copper metal.
KINETICS AND CATALYSIS Vol. 52
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KINETICS AND MECHANISMS OF THE CATALYTIC REACTIONS
29
H2, mol
0.025
Ci,
mol/l
1
H2, mol
0.010
1.5
1.0
0.5
Ci
,
mol/l
2
0.020
0.015
0.010
3
3
1
1.5
1.0
0.5
0.005
2
0
0
50
0.005
0
Time, min
Fig. 2. Copper(I) oxide reduction kinetics: (
hyde, ( ) sodium hydroxide, and ( ) hydrogen. The points
represent experimental data, and the curves represent calꢀ
culated data. Reaction conditions: 25°С, 1.24 g of Cu O,
1) formaldeꢀ
2
3
0
20
40
60
80
2
Time, min
–6 –1
–1
k = 3 × 10 l g min , V = 0.8 l.
Fig. 1. Copper(II) oxide reduction kinetics: (
hyde, ( ) sodium hydroxide, and ( ) hydrogen. The points
represent experimental data, and the curves represent calꢀ
culated data. Reaction conditions: 5°С, 2 g of CuO,
1) formaldeꢀ
2
3
99045
8.314T
−
1014e
where
mol l–1 min–1 is the
k5 = (2.9 0.2) ×
k
=
–14 –1
–1
rate constant of this reaction. The consumption of
NaOH in the reaction accompanied by hydrogen evoꢀ
lution can be estimated as the amount of gas released.
6
×
10
l g min , V = 0.2 l.
According to the chemical mechanism, formaldeꢀ
hyde dehydrogenation takes place on the copper surꢀ
face to yield hydrogen molecules or atoms or hydride
ions, which act as electron donors for the Cu2+ ions. It
is not impossible that the copper ion complex and
formaldehyde interact directly through electron transꢀ
fer from the latter to the former.
According to experimental data, sodium hydroxide
consumption at a constant amount of copper depends
on the methylene glycol anion concentration and temꢀ
perature and is independent of the formaldehyde conꢀ
centration. At a fixed temperature, the NaOH disapꢀ
pearance rate is similar to a typical adsorption isoꢀ
therm and is described by the equation
dnNaOH
Under the given experimental conditions, the
reduction rates of the copper oxides and the copper
ion after the induction period depend on the initial
amounts of the compounds, on the extent of their
reduction to copper metal, on the reaction temperaꢀ
ture, and on pH.
−
= krS1, 0 kadsCn
CH2(OH)O−
dt
= krSsp1, 0m0, CukadsCn
(4)
= k1, 0 m0, CuCn
,
CH2(OH)O−
CH2(OH)O−
where
kr is the formaldehyde oxidation rate constant,
n
S1, 0 is the total surface area of copper,
is
kadsC
CH2(OH)O−
Formaldehyde disproportionation uncomplicated
by the reduction of copper compounds was observed in
the presence of copper metal that had resulted from
the reduction of copper(II) or copper(I) oxide or the
copper ion in the complex.
the methylene glycol anion concentration on the copꢀ
per surface (here, the adsorption equilibrium is
described in terms of Freundlich’s adsorption isoꢀ
therm equation and
is the specific surface area of copper, m0,Cu is the weight
of copper, and k1, 0 krSsp1, 0kads is the effective rate
n is the empirical exponent), Ssp1, 0
=
The effect of the other reactions on the kinetics is
insignificant, and it can be taken into account.
Sodium hydroxide consumption in the Cannizzaro
reaction is given by the equation
constant of formaldehyde oxidation; the subscript 0
means that the given quantity refers to then point in
time at which the formation of a copper layer on the
particle surface is complete.
dCNaOH
The experimental data suggest that formaldehyde
disproportionation in the presence of copper metal
,
(3)
= k5C
C
CH2O
CH2(OH)O−
dt
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30
DEMCHENKO, BELKIN
H2, mol
0.025
logCCH (OH)O
1.5
−
[
mol/l
]
Ci
2.5
,
mol/l
2
−
−
1.0
−
0.5
0
−
−
1
2
2.0
0.020
0.015
0.010
3
2
1
1
1.5
1.0
0.5
−
−
3
4
2
3
−
5
dnNaOH
[mol/min]
logꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ
dt
0.005
0
Fig. 4. Determination of
k
and n from Eq. (4) for the
) copper from copper(II) oxide at 5°С
) copper from copper(I) oxide at 25°С, and (3
from the copper ion complex at 25°С. The initial formalꢀ
dehyde and sodium hydroxide concentrations are 2.0 and
1.6 mol/l, respectively; the copper content is 1 g.
1,0
reactions on (
1
,
(
2
) copper
0
50
100
150
Time, min
2+
Fig. 3. Reduction kinetics of Cu in the complex: (
1)
nonionized formaldehyde and the rate equation is secꢀ
ondꢀorder with respect to the formaldehyde concenꢀ
tration [14].
For an ion strength of 1.2–1.5 and an ion activity of
0.72–0.75 in the solution [15], we determined the
formaldehyde, ( ) sodium hydroxide, and ( ) hydrogen.
The points represent experimental data, and the curves
represent calculated data. Reaction conditions: 25°С, iniꢀ
tial copper ion complex concentration of 0.11 mol/l,
2
3
k =
–5 –1
–1
7 × 10 l g min , V = 0.2 l.
numerical values of the coefficient n and constant k1, 0
(Fig. 4). If copper forms from copper(II) oxide, then,
takes place in steps. We think that the most likely
mechanism is that whose first, slow step is the loss of a
hydride ion by the methylene glycol anion adsorbed on
the copper surface:
76490
−
1012e
8.314T ; if copper
(1.94 0.3)
at
n
= 0.7 k1, 0
=
×
forms from copper(I) oxide, then, at n = 0.5
75200
−
k1,0 = (5.1 0.7) ×1011e
8.314T ; if copper forms from
k
1
CH2(OH)O–
HCOOH + H–.
(V)
the copper ion complex, then, at
n = 0.67 k1, 0 =
This hydride ion stays on the copper surface and
reacts with nonionized formaldehyde to yield methaꢀ
nol (second, rapid step):
55946
−
106e
8.314T
(2.95 0.5)
×
.
The difference between the k1,0 values is evidence
that the three copper specimens differ in catalytic
activity. It is possible that this difference arises from
the difference between the crystal structures of the
specimens. Note that the copper specimens obtained
by Cu2+ ion reduction with formaldehyde and zinc
show nearly the same catalytic activity. According to
gravimetric data, the copper specimens obtained from
copper(I) oxide and from the copper ion complex are
almost free of impurities, while the impurity content
of copper from copper(II) oxide is as high as 5 wt %.
The impurities were not identified.
k
2
H– + CH2O
H3CO–.
(VI)
Also possible is the mechanism whose first, slow
step is the anodic oxidation of the methylene glycol
anion adsorbed on the copper surface. In this process,
the anion loses a hydrogen atom and an electron:
CH2(OH)O–
HCOOH + H + e.
(VII)
In the second, rapid step, the hydrogen and elecꢀ
tron interact with nonionized formaldehyde to yield
methanol:
CH2O + H + e
H2COO–.
(VIII)
The difference between these mechanisms does not
alter the observed reaction kinetics.
In the interaction of formaldehyde with the copper
compounds, formaldehyde disproportionation is
Clearly, the formaldehyde disproportionation accompanied by the reduction of these compounds to
mechanism in the presence of copper differs from the copper metal with hydrogen evolution. Again, two
Cannizzaro mechanism, in which the hydride ion is mechanisms are possible here. In the first mechanism,
transferred directly from the methylene glycol anion to the first, slow step is methylene glycol anion dehydroꢀ
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KINETICS AND MECHANISMS OF THE CATALYTIC REACTIONS
31
genation into formic acid with hydride ion transfer to
copper (reaction (V)), which is followed by the rapid
anodic oxidation of the hydride ion into hydrogen with
electron transfer through metallic copper to the copꢀ
per compound:
dC
dn
dC
CH3OH
+
⎛
⎜
⎝
⎞
⎟
⎠
1
CH2O
H2
(11)
−
dt
The parameter
= 2.4
.
dt V
dt
l
in Eqs. (6) and (9) is the empirical
exponent in the Freundlich equation for adsorption
on the surface of the oxides.
In the case of the reduction of the copper ion comꢀ
plex, the model should include, apart from Eqs. (1)
and (2), the following equations:
k
3, 1
H–
H + e.
(IX)
In the second mechanism, the first, slow step is the
anodic oxidation of the methylene glycol anion into
formic acid with hydrogen abstraction and electron
transfer to copper:
dCNaOH
dt
+ k Cn
S
CH2(OH)O−
)
1
1
V
, (12)
−
= k5C
C
CH2(OH)O−
(
CH2O
1
CH2(OH)O–
HCOO– + H + e.
(X)
⎛
⎞
⎟
⎜
In both cases, the reaction ends with the rapid
cathodic reduction of the copper compound to copper
metal:
dC
n
CH3OH
1
1
V
⎜
⎟
= k1C
S1
C
CH2(OH)O−
2k3CCu
dt
(13)
⎜
⎜
⎝
⎟
⎟
⎠
1 +
k2CCH O
2
k
3, 2
CuO + 2e + H2O
Cu2O + 2e + H2O
Cu2+ + 2e
Cu + 2OH–,
2Cu + 2OH–,
Cu.
(XI)
(XII)
+ k5C
,
CH2(OH)O−
CH2O
k
3, 2
k3C
dnH dCCH OH
2
3
Cu2+
,
V
(14)
=
k
3, 2
(XIII)
dt
dt
k2C
CH2O
The difference between these mechanisms does not
alter the observed reaction kinetics.
dnH
dt
dnCu
dt
2
,
(15)
(16)
(17)
= kCCH (OH)O−
C
V +
Cu2+
2
The way the ratio of the resulting amounts of
hydrogen and methanol varies with the ratio of the iniꢀ
tial amount of the copper compound to the initial
formaldehyde concentration (table) suggests that
hydrogen and methanol form via parallel reactions
and that the common initial reactant for these prodꢀ
ucts is either the hydride ion or the methylene glycol
anion. From experimental data alone, it is rather diffiꢀ
cult to conclude whether this is actually the case
because the surface area of the resulting copper varies
during the reaction. For this reason, it is pertinent to
use a mathematical model in further investigation.
dnCu2+
dnCu
dt
1
,
−
=
dt
dC
dn
dC
CH3OH
+
⎛
⎜
⎝
⎞
⎟
⎠
CH2O
H2
.
−
= 2.4
dt
In Eqs. (6)–(17),
dt V
dt
,
, and
are, respecꢀ
CCH O CCH OH
CCu2+
tively, the formaldehyde, methanol, and copper ion
complex concentrations in the reacting solution; k1
k2 and k3 are the effective rate constants of the reacꢀ
2
3
k, ,
,
tion between copper oxide or the copper ion and the
methylene glycol anion, methylene glycol anion oxiꢀ
dation, formaldehyde reduction, and copper comꢀ
Under the assumption that the reactions in quesꢀ
tion have a common slow step described by the rate
equation
pound reduction, respectively; S, S1, and S2 are the
areas of contact between copper oxide and the reacting
solution, between copper and the reacting solution,
and between copper and copper oxide, respectively;
dn
dn
CH3OH
dnNaOH
dt
H2
,
(5)
−
= 2
+
dt
dt
and
V is the volume of the reacting solution.
the oxide reduction model should include, along with
Eqs. (1) and (2), the following equations:
Note that these models disregard the reaction
yielding the additional amount of hydrogen, which,
according to the data presented in the table, does not
exert any insignificant effect on the reaction kinetics.
In addition, the models do not include the formaldeꢀ
hyde condensation reaction, and formaldehyde disapꢀ
pearance due to this reaction is taken into account by
introducing an empirical coefficient in Eqs (11)
and (17).
dCNaOH
dt
−
= kCl
S + k1Cn
S
CH2(OH)O−
1
,
1
V
(6)
(7)
(8)
CH2(OH)O−
(
)
⎛
⎞
⎜
⎟
dCCH OH
= ⎜k1Cn
S1
⎟ ,
1
1
3
CH2(OH)O−
2k3S2
dt
V
⎜
⎜
⎝
⎟
⎟
⎠
1 +
n
k2CCH OS1
2
Because the surface areas of the solid reactants
were unknown, they were replaced by the weights of
these reactants:
dn
dC
CH3OH
k3S2
H2
=
V,
k2CCnH OS1
dt
dt
2
k3S2
m2
,
,
,(18)
kS = Sspkm k1S1 = Ssp1k1mCu
= KspK3,2
dnH
dt
dnCu
dt
= kCCl H (OH)O−S +
,
2
(9)
(10)
2011
k2S1
mCu
2
where
m
,
mCu, and m2 are the weights of copper oxide,
dn
dnCuO dnCu
dnCu
copper, and solid phase, respectively (m2 determines
the copper/copper oxide contact area); sp and sp1 are
Cu2O
or
−
=
−
= 0.5
,
S
S
dt
dt
dt
dt
KINETICS AND CATALYSIS Vol. 52
No. 1

32
DEMCHENKO, BELKIN
the specific copper oxide/reacting solution and copꢀ the copper ion in the complex is described by a more
per/reacting solution contact areas, respectively; sp is complicated equation:
K
the ratio of the copper/copper oxide and copꢀ
per/reacting solution specific contact areas; and
dCNaOH
= k5C
C
CH2O
−
CH2(OH)O
К3, 2
= k3/k2. Obviously,
dt
(22)
−p
q
Cn
m
.
1
V
dmCu
dt
dnCu dmCuO
dnCuO
dt
⎡
⎣
⎤
⎦
+ k1,0 x 1 + KС
Сu2+
CH2(OH)O
−
(
)
Cu
= 64
,
= 80
,
dt
dt
dn
(19)
This is likely due to the effect of the complex on the
adsorption equilibrium. According to experimental
data, the empirical coefficients in Eq. (22) are as folꢀ
dm
Cu2O
Cu2O
= 144
,
dt
dt
are the weights of copper(II)
where mCuO and
oxide and copper(I) oxide.
m
Cu2O
lows: q = 0.5 and К = 4.4 0.5 (25°С).
The specific copper/oxide contact area also varies
during the reaction. According to experimental data,
As was noted above, the copper contact areas vary
continually during the reaction. We did not find any
models or methods for taking into account these variꢀ
ations, so we had to carry out additional investigations.
According to experimental data, the variation of
the specific copper/reacting solution contact area is
described by the equation
k3S2
k3S2
m
mCu
at
х
≤
0.6
; at
х
≥
0.6
.
= KspK3, 2
= KspK3, 2
k2S1
k2S1
For copper(II) oxide, К3,2 = 0.014 0.001; for copꢀ
per(I) oxide,
is less reactive than CuO. At
К3, 2 = 0.0075 0.0005; that is, the latter
х
≤
0.6 Кsp = 2; at
х
≥
0.6
,
k1Ssp1 = k10x−p
Ksp = A3x(1 − x) −0.33
,
(20)
is the conversion of the copper compound and
is 0.33 0.02 for the copper oxides and 0.37 0.04 for
.
(23)
where
р
x
For copper(II) oxide, А3 = 2.95; for copper(I) oxide,
А3 = 3.28.
the copper ion complex.
The character of this dependence can be explained
This character of specific contact area variation for
the copper oxide reduction reactions can be explained
by assuming that the oxide particles are identical
spheres and that the copper particles forming in these
spheres are identical hemispheres. For the reactions
involving the copper ion complex, it should be
assumed that the reduction of the copper ions yields
identical spherical particles. Under these assumpꢀ
tions,
by assuming that, at
х
≥
0.6, the oxide particles turn
into identical bilayer spheres. In this case, taking into
account the volume and the surface area of the oxide,
which is the core of the particle, and the surface area
of copper, which is in contact with the reacting soluꢀ
tion, we can write the following equation:
r2
x
S2mCu
Ksp
=
= A4
S1,0m
2 1 − x
R0
S1m0, Cu
S1, 0mCu
R0
R
(24)
k1Ssp1 = k1,0
= Ak1, 0
0.66
⎛
⎞
ρCu
(21)
0.33
m
x
= A5 ⎜
= A3x(1 − x) −0.33
,
⎟
0.33
0.33
0.33
ρox m0,Cu
1 − x
m
⎛
⎞
⎛
⎞
⎛
⎞
x0
x
⎝
⎠
0, Cu
N
N
N
= A1k1, 0
= A2k1, 0
,
⎜
⎝
⎟
⎜
⎟
⎠
⎜
⎝
⎟
(
)
mCu
N
0
⎠
⎝
⎠
0
where A3, A4, and A5 are correction factors; ρCu and ρox
are the copper and oxide densities; and
of the particle core.
r is the radius
where А1
,
А2, and А3 are correction factors;
R
and R0
are the copper particle radii during the reaction and
after the completion of copper layer formation,
respectively;
cles in the initial oxide particle during the reaction and
after the completion of copper layer formation,
respectively; and is the oxideꢀtoꢀcopper conversion
According to experimental data, for the reaction
involving the copper ion complex, 3,2 = 71.4 5
N and N0 are the numbers copper partiꢀ
К
.
Systems of differential equations were solved by
numerical integration, setting the initial amount of
copper (“seed”) so that it corresponded to a converꢀ
sion value of 0.01 (end of the induction period). Next,
the effective rate constants of the noncatalytic reacꢀ
x0
upon the completion of copper layer formation.
The observed approximate constancy of the
0.33
tions were estimated under the assumption that l = n.
⎛
⎞
N
N
value is explained by the marked differꢀ
≈ 1
A2
⎜
⎝
⎟
⎠
A comparison between the calculated and experimenꢀ
tal data and the corresponding estimates of the rate
constants of the noncatalytic reductions of the copper
compounds are presented in Figs. 1–3.
0
ence between the rates of the noncatalytic and cataꢀ
lytic reactions and by the low probability of the
appearance of new crystallization centers after the forꢀ
mation of the copper particles.
The above assumptions and reaction mechanisms
As compared to the reduction rate of the copper provide an explanation for the experimental data
oxides, the rate of the limiting step of the reduction of reported here.
KINETICS AND CATALYSIS Vol. 52
No. 1
2011

KINETICS AND MECHANISMS OF THE CATALYTIC REACTIONS
33
9. US Patent 2329514.
10. US Patent 2206379.
REFERENCES
1. Vankovskaya, K.M., Metallicheskie pokrytiya, nanesenꢀ
nye khimicheskim putem (Chemically Deposited Metalꢀ 11. Jack, J., Cavalier, B., and Bloch, O., Electrochim. Acta
lic Coatings), Leningrad: Mashinostroenie, 1985.
1968, vol. 13, p. 1119.
,
2. Kuznetsov, A.M., Elektrokhimiya, 1991, vol. 27, p. 151. 12. Belkin, D.I., Rozkin, M.Ya, and Shepel’, L.I., Zh.
Prikl. Khim., 1979, vol. 52, p. 472.
3. Petrova, T.P., Soros. Obraz. Zh., 2000, vol. 6, p. 57.
13. Balezin, S.A. and Surykina, E.K., Zh. Prikl. Khim.
1966, vol. 39, p. 1826.
,
4. Czech Patent 181494.
5. US Patent 2818442.
6. US Patent 2186372.
7. US Patent 2240734.
8. US Patent 2401749.
14. Ingold, C.K., Structure and Mechanism in Organic
Chemistry, Ithaca: Cornell Univ. Press, 1969.
15. Butler, J.N., Ionic Equilibrium, Reading, Mass.: Addiꢀ
sonꢀWesley, 1964.
KINETICS AND CATALYSIS Vol. 52
No. 1
2011