Catalytic activities of 1% Pt/Cr 2 O 3 and 1% Pt/γ-Al 2 O 3 have been studied in total methane oxidation under lean conditions. The catalysts were characterized by DRS, XPS, and HRTEM. Low-conversion catalytic tests performed at 310 °C revealed a TOF (h −1 ) value 28 times larger for 1% Pt/Cr 2 O 3 in comparison with 1% Pt/γ-Al 2 O 3 . The difference in TOF (h −1 ) and the differences in T 50 observed through the light-off curves (352 °C on 1% Pt/Cr 2 O 3 and 460 °C on 1% Pt/γ-Al 2 O 3 ) are explained on the basis of strong differences in the methane oxidation rate dependences of the pre-exponential factor in the Arrhenius type equations. XPS analysis of 1% Pt/Cr 2 O 3 revealed the presence of stable Pt 0 −Pt 4+ catalytic sites of dipolar nature at the Pt/Cr 2 O 3 interface. These sites are capable of increasing the probability of CH 4 polarization, resulting in an increase of the strength and oriented collisions between the molecules and the catalyst surface, lowering the C−H bond energy, facilitating the abstraction of the first hydrogen in the adsorbed methane: the rate-determining step of methane oxidation. The high stability of the Pt 0 −Pt 4+ sites has been associated with the electronic interactions between platinum and n-type semiconductor Cr 2 O 3 at their interface.
Noble metals such
as Au, Ag, and Cu supported over semiconducting
ZnO are well-known heterogeneous oxidation catalysts. All of them
have been utilized for the oxidation of diesel soot with varied success.
However, Au-supported ZnO is seen to be superior among them. Here,
we present a comparative study of all these three catalysts for diesel
soot oxidation to explain why Au/ZnO is the best among them, demonstrating
the contribution of electronic states of metals in composite catalysts.
The electronic states of Cu, Ag, and Au determined by X-ray photoelectron
spectroscopy on 1 wt % Cu/ZnO, 1 wt % Ag/ZnO, and 1 wt % Au/ZnO catalysts
were correlated with their diesel soot oxidation activities. Although
all three catalysts present reasonable diesel soot oxidation activities
at relatively low temperature, 1% Cu/ZnO and 1% Ag/ZnO oxidize only
about 60% of the deposited diesel soot around 250 °C and 1% Au/ZnO
oxidizes 100% of the deposited diesel soot, at a temperature as low
as 230 °C. The activity of the catalysts is attributed to the
formation of stable M
0
–M
δ+
bifunctional
catalytic sites at the metal–ZnO interface, which enhances
the contact efficiency of solid diesel soot on M
δ+
and generates the superoxide species on M
0
moieties.
The stability of the bifunctional M
0
–M
δ+
sites is controlled by the electronic interactions between the metal
(M) and n-type semiconductor ZnO at their interface. Very high activity
of 1% Au/ZnO is attributed to the presence of Au
3+
at the
catalyst surface, which generates a stronger Coulombic force with
diesel soot electrons. We demonstrate a direct relation between the
diesel soot oxidation activity of these three metals and their electronic
states at the catalyst surface.
A simple and efficient one-pot, three-component synthetic method for the preparation of coumarin-3-carboxamides was carried out by the reaction of salicylaldehyde, aliphatic primary/secondary amines, and diethylmalonate. The protocol employs piperidine-iodine as a dual system catalyst and ethanol, a green solvent. The main advantages of this approach are that it is a metal-free and clean reaction, has low catalyst loading, and requires no tedious workup.
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