19 research outputs found
Extent of Pseudocapacitance in HighâSurface Area Vanadium Nitrides
Early transitionâmetal nitrides, especially vanadium nitride (VN), have shown promise for use in high energy density supercapacitors due to their high electronic conductivity, areal specific capacitance, and ability to be synthesized in high surface area form. Their further development would benefit from an understanding of their pseudocapacitive charge storage mechanism. In this paper, the extent of pseudocapacitance exhibited by vanadium nitride in aqueous electrolytes was investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The pseudocapacitance contribution to the total capacitance in the nitride material was much higher than the doubleâlayer capacitance and ranged from 85â% in basic electrolyte to 87â% in acidic electrolyte. The mole of electrons transferred per VN material during pseudocapacitive charge storage was also evaluated. This pseudocapacitive chargeâstorage is the key component in the full utilization of the properties of earlyâtransition metal nitrides for highâenergy density supercapacitors.Doubleâlayer capacitance vs. pseudocapacitance: the electrostatic doubleâlayer and pseudocapacitive charge storage mechanisms in highâsurfaceâarea vanadium nitride are investigated. The magnitude of the pseudocapacitive charge storage capacity and mole of electrons transferred are reported. The pseudocapacitive chargeâstorage mechanism is the key component in maximizing the energy density of supercapacitors based on transitionâmetal nitrides.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146597/1/batt201800050.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146597/2/batt201800050_am.pd
Role of the Support and Reaction Conditions on the Vapor-Phase Deoxygenation of <i>m</i>âCresol over Pt/C and Pt/TiO<sub>2</sub> Catalysts
The
catalytic deoxygenation of biomass fast pyrolysis vapors offers
a promising route for the sustainable production of liquid transportation
fuels. However, a clear understanding of the mechanistic details involved
in this process has yet to be achieved, and questions remain regarding
the role of the catalyst support and the influence of reaction conditions.
In order to gain insight into these questions, the deoxygenation of <i>m</i>-cresol was investigated over Pt/C and Pt/TiO<sub>2</sub> catalysts using experimental and computational techniques. The performance
of each catalyst was evaluated in a packed-bed reactor under two conditions
(523 K, 2.0 MPa and 623 K, 0.5 MPa), and the energetics of the ring
hydrogenation, direct deoxygenation, and tautomerization mechanisms
were calculated over hydrogen-covered Pt(111) and oxygen vacancies
on the surface of TiO<sub>2</sub>(101). Over Pt(111), ring hydrogenation
to 3-methylcyclohexanone and 3-methylcyclohexanol was found to be
the most energetically favorable pathway. Over TiO<sub>2</sub>(101),
tautomerization and direct deoxygenation to toluene were identified
as additional energetically favorable routes. These calculations are
consistent with the experimental data, in which Pt/TiO<sub>2</sub> was more active on a metal site basis and exhibited higher selectivity
to toluene at 623 K relative to Pt/C. On the basis of these results,
it is likely that the reactivity of Pt/TiO<sub>2</sub> and Pt/C is
driven by the metallic phase at 523 K, while contributions from the
TiO<sub>2</sub> support enhance deoxygenation at 623 K. These results
highlight the synergistic effects between hydrogenation catalysts
and reducible metal oxide supports and provide insight into the reaction
pathways responsible for their enhanced deoxygenation performance
Experimental and Computational Investigation of Acetic Acid Deoxygenation over Oxophilic Molybdenum Carbide: Surface Chemistry and Active Site Identity
Ex situ catalytic fast pyrolysis
(CFP) is a promising route for
producing fungible biofuels; however, this process requires bifunctional
catalysts that favor CâO bond cleavage, activate hydrogen at
near atmospheric pressure and high temperature (350â500 °C),
and are stable under high-steam, low hydrogen-to-carbon environments.
Recently, early transition-metal carbides have been reported to selectively
cleave CâO bonds of alcohols, aldehydes, and oxygenated aromatics,
yet there is limited understanding of the metal carbide surface chemistry
under reaction conditions and the identity of the active sites for
deoxygenation. In this paper, we evaluated molybdenum carbide (Mo<sub>2</sub>C) for the deoxygenation of acetic acid, an abundant component
of biomass pyrolysis vapors, under ex situ CFP conditions, and we
probed the Mo<sub>2</sub>C surface chemistry, identity of the active
sites, and deoxygenation pathways using in situ diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS), X-ray photoelectron
spectroscopy (XPS), and density functional theory (DFT) calculations.
The Mo<sub>2</sub>C catalyst favored the production of acetaldehyde
and ethylene from acetic acid over the temperature range of 250â400
°C, with decarbonylation pathways favored at temperatures greater
than 400 °C. Little to no ethanol was observed due to the high
activity of the carbide surface for alcohol dehydration. The Mo<sub>2</sub>C surface, which was at least partially oxidized following
pretreatment and exposure to reaction conditions (possibly existing
as an oxycarbide), possessed both metallic-like H-adsorption sites
(i.e., exposed Mo and C) and BrĂžnsted acidic surface hydroxyl
sites, in a ratio of 1:8 metallic:acidic sites following pretreatment.
The strength of the acidic sites was similar to that for H-Beta, H-Y,
and H-X zeolites. Oxygen vacancy sites (exposed Mo sites) were also
present under reaction conditions, inferred from DRIFTS results and
calculated surface phase diagrams. It is proposed that CâO
bond cleavage steps proceeded over the acidic sites or over the oxygen
vacancy sites and that the deoxygenation rate may be limited by the
availability of adsorbed hydrogen, due to the high surface coverage
of oxygen under reaction conditions. Importantly, the reaction conditions
(temperature and partial pressures of H<sub>2</sub> and H<sub>2</sub>O) had a strong effect on oxygen surface coverage, and accordingly,
the relative concentrations of the different types of active sites,
and could ultimately result in completely different reaction pathways
under different reaction conditions
Thermodynamic Stability of Molybdenum Oxycarbides Formed from Orthorhombic Mo<sub>2</sub>C in Oxygen-Rich Environments
Molybdenum
carbide (Mo<sub>2</sub>C) nanoparticles and thin films
are particularly suitable catalysts for catalytic fast pyrolysis (CFP)
as they are effective for deoxygenation and can catalyze certain reactions
that typically occur on noble metals. Oxygen deposited during deoxygenation
reactions may alter the carbide structure, leading to the formation
of oxycarbides, which can determine changes in catalytic activity
or selectivity. Despite emerging spectroscopic evidence of bulk oxycarbides,
so far there have been no reports of their precise atomic structure
or their relative stability with respect to orthorhombic Mo<sub>2</sub>C. This knowledge is essential for assessing the catalytic properties
of molybdenum (oxy)Âcarbides for CFP. In this article, we use density
functional theory (DFT) calculations to (a) describe the thermodynamic
stability of surface and subsurface configurations of oxygen and carbon
atoms for a commonly studied Mo-terminated surface of orthorhombic
Mo<sub>2</sub>C and (b) determine atomic structures for oxycarbides
with a Mo:C ratio of 2:1. The surface calculations suggest that oxygen
atoms are not stable under the top Mo layer of the Mo<sub>2</sub>CÂ(100)
surface. Coupling DFT calculations with a polymorph sampling method,
we determine (Mo<sub>2</sub>C)<sub><i>x</i></sub>O<sub><i>y</i></sub> oxycarbide structures for a wide range of oxygen
compositions. Oxycarbides with lower oxygen content (<i>y</i>/<i>x</i> †2) adopt layered structures reminiscent
of the parent carbide phase, with flat Mo layers separated by layers
of oxygen and carbon; for higher oxygen content, our results suggest
the formation of amorphous phases, as the atomic layers lose their
planarity with increasing oxygen content. We characterize the oxidation
states of Mo in the oxycarbide structures determined computationally,
and simulate their X-ray diffraction (XRD) patterns in order to facilitate
comparisons with experiments. Our study may provide a platform for
large-scale investigations of the catalytic properties of oxycarbides
and their surfaces and for tailoring the catalytic properties for
different desired reactions
The need for and path to harmonized life cycle assessment and technoâeconomic assessment for carbon dioxide capture and utilization
The use of carbon dioxide as a feedstock for a broad range of products can help mitigate the effects of climate change through longâterm removal of carbon or as part of a circular carbon economy. Research on capture and conversion technologies has intensified in recent years, and the interest in deploying these technologies is growing fast. However, sound understanding of the environmental and economic impacts of these technologies is required to drive fast deployment and avoid unintended consequences. Life cycle assessments (LCAs) and technoâeconomic assessments (TEAs) are useful tools to quantify environmental and economic metrics; however, these tools can be very flexible in how they are applied, with the potential to produce significantly different results depending on how the boundaries and assumptions are defined. Built on ISO standards for generic LCAs, several guidance documents have emerged recently from the Global CO2 Initiative, the National Energy Technology Laboratory, and the National Renewable Energy Laboratory that further define assessment specifications for carbon capture and utilization. Overall agreement in the approaches is noted with differences largely based on the intended use cases. However, further guidance is needed for assessments of earlyâstage technologies, reporting details, and reporting for policymakers and nontechnical decisionâmakers
High-Throughput Continuous Flow Synthesis of Nickel Nanoparticles for the Catalytic Hydrodeoxygenation of Guaiacol
The
translation of batch chemistries to high-throughput flow methods
addresses scaling concerns associated with the implementation of colloidal
nanoparticle (NP) catalysts for industrial processes. A literature
procedure for the synthesis of Ni-NPs was adapted to a continuous
millifluidic (mF) flow method, achieving yields >60%. Conversely,
NPs prepared in a batch (B) reaction under conditions analogous to
the continuous flow conditions gave only a 45% yield. Both mF- and
B-Ni-NP catalysts were supported on SiO<sub>2</sub> and compared to
a Ni/SiO<sub>2</sub> catalyst prepared by traditional incipient wetness
(IW) impregnation for the hydrodeoxygenation (HDO) of guaiacol under <i>ex situ</i> catalytic fast pyrolysis conditions (350 °C,
0.5 MPa). Compared to the IW method, both colloidal NPs displayed
increased morphological control and narrowed size distributions, and
the NPs prepared by both methods showed similar size, shape, and crystallinity.
The Ni-NP catalyst synthesized by the continuous flow method exhibited
similar H-adsorption site densities, site-time yields, and selectivities
toward deoxygenated products compared to the analogous batch-prepared
catalyst, and it outperformed the IW catalyst with respect to higher
selectivity to lower oxygen content products and a 31-fold decrease
in deactivation rate. These results demonstrate the utility of synthesizing
colloidal Ni-NP catalysts using flow methods that can produce >27
g/day of Ni-NPs (equivalent to >0.5 kg of 5 wt % Ni/SiO<sub>2</sub>), while maintaining the catalytic properties displayed by the batch
equivalent