4 research outputs found
Understanding the Optimal Adsorption Energies for Catalyst Screening in Heterogeneous Catalysis
The fundamental understanding of
the activity in heterogeneous
catalysis has long been the major subject in chemistry. This paper
shows the development of a two-step model to understand this activity.
Using the theory of chemical potential kinetics with Brønsted–Evans–Polanyi
relations, the general adsorption energy window is determined from
volcano curves, using which the best catalysts can be searched. Significant
insights into the reasons for catalytic activity are obtained
Mechanistic Study of 1,3-Butadiene Formation in Acetylene Hydrogenation over the Pd-Based Catalysts Using Density Functional Calculations
Green
oil, which leads to the deactivation of the catalysts used
for the selective hydrogenation of acetylene, has long been observed
but its formation mechanism is not fully understood. In this work,
the formation of 1,3-butadiene, known to be the precursor of green
oil, on both Pd(111) and Pd(211) surfaces is examined using density
functional theory calculations. The pathways containing C<sub>2</sub> + C<sub>2</sub> coupling reactions as well as the corresponding
hydrogenation reactions are studied in detail. Three pathways for
1,3-butadiene production, namely coupling plus hydrogenation and further
hydrogenation, hydrogenation plus coupling plus hydrogenation, and
a two step hydrogenation followed by coupling, are determined. By
comparing the effective barriers, we identify the favored pathway
on both surfaces. A general understanding toward the deactivation
process of the industrial catalysts is also provided. In addition,
the effects of the formation of subsurface carbon atoms as well as
the Ag alloying on the 1,3-butadiene formation on Pd-based catalysts
are also investigated and compared with experimental results
Origin of the Increase of Activity and Selectivity of Nickel Doped by Au, Ag, and Cu for Acetylene Hydrogenation
Activity and selectivity are both important issues in
heterogeneous
catalysis and recent experimental results have shown that Ni catalysts
doped by gold exhibit high activity for the hydrogenation of acetylene
with good selectivity of ethylene formation. To unravel the underlying
mechanism for this observation, the general trend of activity and
selectivity of Ni surfaces doped by Au, Ag, and Cu has been investigated
using density functional theory calculations. Complete energy profiles
from C<sub>2</sub>H<sub>2</sub> to C<sub>2</sub>H<sub>4</sub> on Ni(111),
Au/Ni(111), Ag/Ni(111) and Cu/Ni(111) are obtained and their turnover
frequencies (TOFs) are computed. The results show that acetylene adsorption
on Ni catalyst is strong which leads to the low activity while the
doping of Au, Ag, and Cu on the Ni catalyst weakens the acetylene
adsorption, giving rise to the increase of activity. The selectivity
of ethylene formation is also quantified by using the energy difference
between the hydrogenation barriers and the absolute value of the adsorption
energies of ethylene. It is found that the selectivity of ethylene
formation increases by doping Au and Ag, while those of Cu/Ni and
Ni are similar
Ambient Temperature Hydrocarbon Selective Catalytic Reduction of NO<sub><i>x</i></sub> Using Atmospheric Pressure Nonthermal Plasma Activation of a Ag/Al<sub>2</sub>O<sub>3</sub> Catalyst
Atmospheric pressure nonthermal-plasma-activated
catalysis for
the removal of NO<sub><i>x</i></sub> using hydrocarbon selective
catalytic reduction has been studied utilizing toluene and <i>n</i>-octane as the hydrocarbon reductant. When the plasma was
combined with a Ag/Al<sub>2</sub>O<sub>3</sub> catalyst, a strong
enhancement in activity was observed when compared with conventional
thermal activation with high conversions of both NO<sub><i>x</i></sub> and hydrocarbons obtained at temperature ≤250 °C,
where the silver catalyst is normally inactive. Importantly, even
in the absence of an external heat source, significant activity was
obtained. This low temperature activity provides the basis for applying
nonthermal plasmas to activate emission control catalysts during cold
start conditions, which remains an important issue for mobile and
stationary applications