1984
TITOVA et al.
reduced. It is the low concentration of metallic particles
in solution that is the reason why the stability of the
Pd(acac) –0.25NaBH system is comparatively high and,
Thus, the unconventional double-peak dependence of
the catalytic activity of the Pd(acac) –NaBH system on
the molar ratio between the reagents is due to different
relative stabilities of palladium nanoclusters being formed
and to the inhibiting effect of elementary boron and(or)
tetrahydroborate anions.
2
4
2
4
as a consequence, the initial rate of hydrogenation exceeds
that for the Pd(acac) –NaBH system at equimolar
2
4
amounts of the reagents. This suggestion is confirmed by
data on reduction of Pd(acac) in DMF with hydrogen,
2
CONCLUSIONS
when substances capable of stabilizing palladium
nanoparticles are absent in the system and are not formed
in the course of hydrogenolysis. Indeed, at a Pd(acac)2
concentration of 1 mM, the reduction with hydrogen
yielded a black-brown solution containing, according to
TEM data, palladium particles 20–30 nm in size and a
precipitate was formed in the course of hydrogenation.
As a result of the interaction of Pd(acac) with NaBH at
(
1) A high-efficiency Pd(acac) –NaBH catalyst for
2 4
hydrogenation of the double and triple bonds, nitro group,
and unsaturated aldehydes was suggested.
(
2) The main stages of formation of a palladium
catalyst for hydrogenation were determined. These
stages include reduction of palladium(II) to palladium
nanoclusters, catalytic decomposition of borane being
formed into elements, and its hydrolysis (or ethanolysis).
2
4
B/Pd ≤ 2.0, first, smaller particles are formed and, second,
a side process, hydrolysis of borane to give polyboric
acids acting as stabilizers of palladium particles, occurs
to a greater extent.
It is shown that not only NaBH , but also borane being
4
formed, act as a reducing agent for Pd(acac) at small
molar ratios between the components (B/Pd < 2.0).
2
The inhibiting effect of an excess amount of sodium
tetrahydroborate on the catalytic properties of the
Pd(acac) –nNaBH system indicates that a sufficient
(
3) It is shown that the double-peak dependence of
the catalytic activity of the Pd(acac) –NaBH system
on the molar ratio between the reagents is due to the
different relative stabilities of palladium nanoclusters
and to the inhibiting effect of elementary boron and(or)
tetrahydroborate anion.
2
4
2
4
amount of a catalytic poison, which possibly is elementary
boron formed in catalytic decomposition of diborane
and covering palladium, appears in the reaction system.
This assumption is based on recent data on the nature of
nickel boride catalysts, according to which the catalysts
are composed of nickel nanocrystals “cemented” together
by amorphous boron or its compounds [24], rather than
REFERENCES
1
2
3
. Pomogailo, A.D., Rozenberg, A.S., and Uflyand, I.E.,
Nanochastitsy metallov v polimerakh (Metal Nanoparticles
in Polymers), Moscow: Khimiya, 2000.
being constituted by nickel borides of composition Ni B,
2
as it has been believed during a number of decades. In
addition, it is known that hydrolysis of NaBH , catalyzed
4
. Glavee, G.N., Klabunde, K.J., Sorensen, C.M., and
Hadjapanayis, G.C., Langmuir, 1992, vol. 8, pp. 771–
by metal nanoparticles, occurs via stages in which surface
compounds are formed:
7
73.
. Glavee, G.N., Klabunde, K.J., Sorensen, C.M., and
Hadjapanayis, G.C., Langmuir, 1994, vol. 10, pp. 4726–
BH – + 2Pd ' BH Pd + PdH.
–
4
3
4
730.
Therefore, it cannot be ruled out that the catalyst
4. Belykh, L.B., Titova, Yu.Yu., Rokhin,A.V., et al., Zh. Prikl.
Khim., 2008, vol. 81, no. 6, pp. 917–925.
can be poisoned by an excess of NaBH as a result of
4
formation of intermediate surface compounds of the
type [25]:
5
. Belykh, L.B., Titova, Yu.Yu., Rokhin, A.V., and
Shmidt, F.K., Zh. Prikl. Khim., 2008, vol. 81, no. 7,
pp. 1075–1081.
6
. Bonnemann, H., Brijoux, W., Brinkmann, R., et al., Rev.
Roum. de Chim., 1999, vol. 44, no. 11–12, pp. 1003–
1
010.
7
. Belykh, L.B., Goremyka, T.I., Skripov, N.I., et al., Kinet.
Kataliz, 2006, vol. 47, no. 3, pp. 773–780.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 11 2009