12 research outputs found
Acid Properties of Nanocarbons and Their Application in Oxidative Dehydrogenation
Carbon is emerging as an important
metal-free catalyst for multiple
types of heterogeneous catalysis, including thermocatalysis, photocatalysis,
and electrocatalysis. However, the study of mechanisms for carbon
catalysis has been impeded at an early stage due to the lack of quantitative
research, especially the intrinsic kinetics (e.g., intrinsic TOF).
In many carbon-catalyzed reactions, the surface oxygenated groups
were found to be the active sites. Recently, we have shown that these
oxygenated groups could be identified and quantified via poisoning
by small organic molecules; however, these small molecules were toxic.
As most of the oxygenated groups are acidic groups, they could also
be identified and quantified with respect to the acid properties.
More importantly, the method based on acid properties is very green
and environmentally benign, because only inorganic bases are added.
In this work, the acid properties of carbon nanotubes (CNTs) treated
by concentrated HNO<sub>3</sub> were thoroughly studied by mass titration
and Boehm titration. The two titration methods were also compared
to the conventional methods for acidity analysis including NH<sub>3</sub> pulse adsorption, NH<sub>3</sub>-TPD, and FT-IR. Boehm titration
was very effective to quantify the carboxylic acid, lactone, phenol,
and carbonyl groups, and the findings were consistent with the results
from XPS and NH<sub>3</sub> pulse adsorption. These CNTs were applied
in the oxidative dehydrogenation (ODH) of ethylbenzene, and the activity
of these catalysts exhibited a good linear dependence on the number
of carbonyl groups. The value of TOF for the carbonyl group obtained
from Boehm titration was 3.2 Ć 10<sup>ā4</sup> s<sup>ā1</sup> (245 Ā°C, atmosphere pressure, 2.8 kPa ethylbenzene, 5.3 kPa
O<sub>2</sub>). For better understanding the acidity of nanocarbon,
these CNTs were also applied in two acid-catalyzed reactions (Beckmann
rearrangement and ring opening), and a good linear relationship between
the conversion and the number of acidic sites was found
Enhanced Chemoselective Hydrogenation through Tuning the Interaction between Pt Nanoparticles and Carbon Supports: Insights from Identical Location Transmission Electron Microscopy and Xāray Photoelectron Spectroscopy
Ultrasmall-sized platinum nanoparticles
(Pt NPs) (ā¼1 nm)
supported on carbon nanotubes (CNTs) with nitrogen doping and oxygen
functional groups were synthesized and applied in the catalytic hydrogenation
of nitroarenes. The advanced identical location transmission electron
microscopy (IL-TEM) method was applied to probe the structure evolution
of the Pt/CNT catalysts in the reaction. The results indicate that
Pt NPs supported on CNTs with a high amount of nitrogen doping (Pt/H-NCNTs)
afford 2-fold activity to that of Pt NPs supported on CNTs with oxygen
functional groups (Pt/oCNTs) and 4-fold to that of the commercial
Pt NPs supported on active carbon (Pt/C) catalyst toward nitrobenzene.
The catalytic performance of Pt/H-NCNTs remained constant during four
cycles, whereas the activity of the Pt/oCNTs was halved at the second
cycle. Compared with Pt/oCNTs, Pt/H-NCNTs exhibited a higher selectivity
(>99%) in chemoselective hydrogenation of halonitrobenzenes to
haloanilines
due to the electron-rich chemical state of Pt NPs. The strong metalāsupport
interaction along with the electron-donor capacity of nitrogen sites
on H-NCNTs are capable of stabilizing the Pt NPs and achieving related
catalytic recyclability as well as approximately 100% selectivity.
The catalyst also delivers exclusively selective hydrogenation toward
nitro groups for a wide scope of substituent nitroarenes into their
corresponding anilines
Insight into the Enhanced Selectivity of Phosphate-Modified Annealed Nanodiamond for Oxidative Dehydrogenation Reactions
Due to a lack of fundamental understanding
of the surface properties
of nanocarbons, tuning their surface active sites for higher selectivity
of oxidative dehydrogenation reactions has always been a great challenge
for carbon catalysis. In this contribution, annealed nanodiamond was
controllably grafted by phosphate, which was demonstrated to be an
efficient way to adjust the nanodiamond surface and significantly
improve the propene selectivity in the oxidative dehydrogenation reaction.
We conducted an in-depth study to explore the role of phosphate modification
in the reaction in terms of the interactions between phosphate and
carbon surface, the evolution and preferential location of phosphorus
species, the promotion mechanism, and the impact on the reaction pathway.
The results revealed that phosphate preferentially reacts with the
phenol groups initially present on the nanodiamond surface, and then
it selectively blocks the defect sites that lead to COx formation
with an increased propene selectivity. During this process, the catalyst
active sites (ketonic carbonyl groups) were not affected. Such effects
originated from the formation of covalent CāOāP bonds
on the carbon surface, which was estimated as 15 wt % loading
Revealing the Janus Character of the Coke Precursor in the Propane Direct Dehydrogenation on Pt Catalysts from a kMC Simulation
As the commercial
catalyst in the propane direct dehydrogenation
(PDH) reaction, one of the biggest challenges of Pt catalysts is coke
formation, which severely reduces activity and stability. In this
work, a first-principles DFT-based kinetic Monte Carlo simulation
(kMC) is performed to understand the origin of coke formation, and
an effective method is proposed to curb coke. The conventional DFT
calculations give a complete description of the reaction pathway of
dehydrogenation to propylene, deep dehydrogenation, and CāC
bond cracking. The rate-limiting step is identified as the dissociative
adsorption of propane. Moreover, a comparison between different exchange-correlation
functionals indicates the importance of van der Waals corrections
for the adsorption of propane and propylene. The lateral interactions
between the surface adsorbates are significant, which implies that
mean field microkinetic modeling might not adequately describe the
reaction process. There are two distinct stages in PDH, which are
quick deactivation and steady state, respectively, as revealed from
the kMC simulation. The precursor of coke mainly formed during the
quick deactivation. The calculations indicate that the geometries
of the active sites for the dehydrogenation and deep reactions are
different. Therefore, the availability of surface sites is a crucial
factor in the formation of propylene and side products. The active
sites from quick deactivation are mainly occupied by C<sub>2</sub>/C<sub>1</sub> species, which are hard to remove. On the other hand,
the surface sites that are left are mainly active toward dehydrogenation
to propylene due to the geometry constraint. Therefore, a stable activity
and selectivity is reached. Furthermore, the effect of hydrogen molecules
in the input stream is also explored. The calculations indicate that
the inclusion of hydrogen in PDH reactants not only enhances the forward
reactions to the propylene formation but also reduces the consumption
of the resulted propylene during the reaction. Therefore, hydrogen
is very helpful to the selectivity increase in PDH in addition to
other effects. Overall, the current study lays out a solid base for
the future optimization of the Pt catalysts in PDH and we propose
that the fine control of the surface sites on Pt has paramount importance
in reducing coke formation
Efficient Metal-Free Catalytic Reaction Pathway for Selective Oxidation of Substituted Phenols
Selective oxidation of substituted
phenols to <i>p</i>-benzoquinones is known to be inefficient
because of the competing
CāO coupling reaction caused by phenoxy radicals. The poor
stability of conventional metal-based catalysts represents another
bottleneck for industrial application. Here, we describe a metal-free
reaction pathway in which onion-like carbon (OLC) as a low-cost catalyst
exhibits excellent catalytic activity and stability in the selective
oxidation of mono-, di- and trisubstituted phenols to their corresponding <i>p</i>-benzoquinones, even better than the reported metal-based
catalysts (e.g., yield, stability) and industrial catalysts for particular
substrates. Together with XPS, Raman, DFT calculations, and a series
of comparative experiments, we demonstrate that the zigzag configuration
as a type of carbon defects may play a crucial role in these reactions
by stabilizing the intermediate phenoxy radicals
Enhanced Stability of Immobilized Platinum Nanoparticles through Nitrogen Heteroatoms on Doped Carbon Supports
Catalysts in the
form of dispersed platinum nanoparticles (Pt NPs)
immobilized on carbon usually suffer from deactivation through sintering
under reaction conditions. In this contribution, we report the enhanced
stability of highly dispersed Pt NPs on surface-modified carbon nanotubes
(CNTs) against thermal and electrochemical sintering by N heteroatoms
in the N-doped carbon support. The improved antisintering property
of Pt NPs under thermal condition is characterized by <i>in situ</i> transmission electron microscopy (TEM), while the stability in electrochemical
methanol oxidation reaction (MOR) is further examined at <i>identical
location</i> (IL) using an advanced IL-TEM technique. A correlation
of the Pt NP growth with the electrochemical surface area (ECSA) and
the mass activity in MOR has been inferred. Our results indicate that
both the surface oxygen groups and nitrogen-doped species are responsible
for the fine dispersion of Pt NPs on the surface-modified CNTs, while
the Pt NPs can be effectively stabilized under thermal and electrochemical
conditions through the strong metalāsupport interaction <i>via</i> N heteroatoms. We further reveal that the mass activity
of Pt NP is closely associated with the ECSA rather than directly
affected by N-doping to CNTs
Assembly of Three-Dimensional Hetero-Epitaxial ZnO/ZnS Core/Shell Nanorod and Single Crystalline Hollow ZnS Nanotube Arrays
Hetero-epitaxial growth along three-dimensional (3D) interfaces from materials with an intrinsic large lattice mismatch is a key challenge today. In this work we report, for the first time, the controlled synthesis of vertically aligned ZnO/ZnS core/shell nanorod arrays composed of single crystalline wurtzite (WZ) ZnS conformally grown on ZnO rods along 3D interfaces through a simple two-step thermal evaporation method. Structural characterization reveals a ā(01ā10)<sub>ZnO</sub>//(01ā10)<sub>ZnS</sub> and [0001]<sub>ZnO</sub>//[0001]<sub>ZnS</sub>ā epitaxial relationship between the ZnO core and the ZnS shell. It is exciting that arrays of single crystalline hollow ZnS nanotubes are also innovatively obtained by simply etching away the inner ZnO cores. On the basis of systematic structural analysis, a rational growth mechanism for the formation of hetero-epitaxial core/shell nanorods is proposed. Optical properties are also investigated <i>via</i> cathodoluminescence and photoluminescence measurements. Remarkably, the synthesized ZnO/ZnS core/shell heterostructures exhibit a greatly reduced ultraviolet emission and dramatically enhanced green emission compared to the pure ZnO nanorods. The present single-crystalline heterostructure and hollow nanotube arrays are envisaged to be highly promising for applications in novel nanoscale optoelectronic devices, such as UV-A photodetectors, lasers, solar cells, and nanogenerators
Controllable Synthesis of Cobalt Monoxide Nanoparticles and the Size-Dependent Activity for Oxygen Reduction Reaction
In this work, carbon-supported cobalt
monoxides with an average
size of 3.5, 4.9, and 6.5 nm are synthesized via a facile colloidal
method avoiding any surfactants of long chains. Along with controlling
the CoO particle size, we investigate the dependence of ORR activity
on particle size of the CoO/C composite. It is discovered that the
turnover frequency of the ORR per CoO site is largely independent
of the particle size in the range of 3ā7 nm, and the enhanced
ORR activity for the smaller CoO particles is attributed to the enlarged
interface between CoO and carbon
Mesoporous and Graphitic Carbide-Derived Carbons as Selective and Stable Catalysts for the Dehydrogenation Reaction
Dehydrogenation
of ethylbenzene to styrene is one of the most important
catalytic processes in chemical industry. While it was demonstrated
that nanocarbons like nanotubes, nanodiamond, or nanographite show
high performance, especially selectivity, these powders give rise
to handling problems, high pressure drop, hampered heat and mass transfer,
and unclear health risks. More common macroscopic carbon materials
like activated carbons show unsatisfying selectivity below 80%. In
this study, mesoporous, graphitic, and easy to handle carbon powders
were synthesized on the basis of the reactive extraction of titanium
carbide in a novel temperature regime. This resulted in extraordinary
properties like a mean pore diameter of up to 8 nm, pore volumes of
up to 0.90 mL g<sup>ā1</sup>, and graphite crystallite sizes
exceeding 25 nm. Exceptional styrene selectivities of up to 95% were
observed for materials synthesized above 1300 Ā°C and pretreated
with nitric acid. Furthermore, the long-term stability of these non-nanocarbon
catalysts could be demonstrated for the first time during 120 h of
time-on-stream