(6dw) Theoretical Approaches to the Design of Clean-Energy Processes and Materials

Theoretical
Approaches to the Design of Clean-Energy Processes and Materials

Peter C.
Psarras, Stanford University

Introduction

This past half-century has brought much change to the
landscape surrounding the chemical processes that sustain our global
socio-economic framework. In the midst of this evolving climate, it has become
increasingly apparent that new motivations, such as those for greater
energy-efficiency and less water-intensive processes, are not necessarily
aligned to those of previous concern. Fortunately, this call to attention has
re-energized the field of materials research, whereby promising new
technologies can serve to bridge the gap between stale, out-of-date processes
and those who would fit better into this new energy and
environmentally-conscience landscape. Concurrent to this scientific
revitalization has been a related emergence in its own right – the dawn
of high performance computing environments. As such, theoretical methods
represent a powerful approach to the design of materials aimed at resolving the
shortcomings of essential industrial processes through revision and innovation.
These methods are often favorable in situations where there exists a need to
explore and screen a number of materials at low cost and risk, where conditions
prove unsuitable or even unfeasible via conventional methods, or where an
understanding is sought at molecular levels or timescales that are incompatible
with experimental measurement. This presentation will introduce how different
theoretical methods can be used to solve four issues related to climate change:
nitrogen-selective membranes for indirect CO₂ capture,
nitrogen-functionalized micro/mesoporous carbon sorbents for direct CO₂
capture, the assessment of carbon neutrality for irreplaceable industrial
processes, and the effect of late-transition metal doping on methane
selectivity in Fischer-Tr?psch (FT) catalysis.

Vanadium-Ruthenium
Alloys for Indirect CO₂ Capture

Membranes represent one promising technology for
carbon capture, whereby the flue stream is subjected to some material that can
selectively remove CO₂ and prevent its release. Many
intrinsic properties of the capture material can be investigated theoretically.
Here, we propose to investigate alloyed VRu membranes for selective nitrogen
capture. In this technology, rather than separating out CO­₂
directly, nitrogen is separated from the flue stream. Nitrogen is far more
efficient to separate, owing to its greater flue concentration and
corresponding pressure differential across the membrane. Consequently, the
remaining flue becomes more concentrated after nitrogen removal, CO₂
included, making all downstream capture points more efficient by the same
virtue. Naturally, a highly selective membrane with good mechanical stability
and high nitrogen permeability is sought. This study will prepare 14 V_aRu_b
alloys in concentrations spanning those considered in real application.
Vanadium is known to bind nitrogen strongly, making it a good candidate for
activation of the N-N triple bond. However, this tight binding inherently slows
membrane diffusion. Nitrogen permeability can be represented as a product of its
solubility and diffusivity. The former is sought by considering the effect of
Ru concentration on relative binding energies and vibrational frequencies of
interstitially-bound nitrogen, while diffusivity can be examined by calculating
all potential TS-pathways, tabulating their rates, and employing kinetic Monte
Carlo methods. Together, the results of these two methods will combine to
predict nitrogen permeability as a function of Ru content. These findings can
inform on membrane design, whereby theoretically optimal Ru composition can be
validated experimentally.

Nitrogen-Functionalized
Porous Carbons for Enhanced CO<sub>2</Sub> Capture

One of the most mature methods for
CO₂ capture involves CO₂
absorption via amine-based solvents; however, this technology is notoriously energy
intensive, as amine regeneration requires high regeneration temperatures.
Additionally, this process is water-intensive, a factor that is becoming
increasingly important in drought-ridden states like California. Solid sorbents
represent an alternative method for CO₂ capture, with
liquid solvents replaced by (typically) large surface area, carbon-based porous
frameworks. These sorbents are generally low-cost, easily fabricated, and are
far less energy intensive in terms of regeneration. Further, their design can
be customized through the inclusion of surface-functionalized groups.
Unfortunately, current sorbents display CO₂ uptakes that
are deemed too low to be cost-competitive with traditional solvent-based
scrubbing. Nitrogen-functionalization of these porous carbons could result in
more efficient CO₂ capture, owing to the same chemistry
exploited by basic solvents in capturing acidic CO₂.
Similar studies have examined the effects of oxygenated functional groups on
CO₂ uptake and selectivity over
N₂.

This study will employ grand canonical monte carlo
methods to explore the effect of quaternary, pyridinic, pyrollic, and
oxidized-N groups on CO₂ uptake in porous carbon
sorbents. Doping amounts will be varied to ascertain the optimum coverage for
CO₂ capture. Additionally, gaseous mixtures of
CO₂/N₂ and CO₂/N₂/H₂O
will be examined to assess CO₂ selectivity. Theoretical
performance will be validated through comparison with experimentally obtained
CO₂/N₂ isotherms over fabricated
hierarchial micro/mesoporous N-doped carbon sorbents. The combination of these
methods will help to inform on sorbent design by illustrating which groups are
most important for enhanced CO₂ uptake. Design to
include more of a particular functional group can be achieved through, for
example, a change in carbonization temperature.

Assessment
of the CO₂ Capture Potential from Irreplaceable
Industrial Sources

In 2013, CO₂ emissions from
all industrial processes totaled 163.0 MMT, equivalent to the annual
CO₂ emissions of roughly 35 million automobiles. This
figure excludes indirect emissions associated with electricity usage. The
heaviest emitters (coal power plants, natural gas combined cycle plants, etc.)
continue to receive the majority of attention and funding in terms of carbon
capture projects; however, these major sources may also benefit from less
CO₂-intensive alternatives. Unfortunately, many
industrial processes fall into a category by which there are no alternative
routes to product available. For example, steel and cement production both
involve processes that directly emit CO₂ as a by-product
(via the oxidation of metallurgical coke and conversion of calcium carbonate to
lime, respectively). As there materials constitute the irreplaceable fabric of
industrialization, CO₂ emissions from these, as from
other irreplaceable industrial processes, are projected to increase unabated.
With capture technology in place, these emissions can be diverted instead to
viable CO₂ reuse and sequestration opportunities, such
as oil refining, enhanced oil recovery, food processing, metal treatment, and
fertilizer production.

To assess the capture potential of irreplaceable
industry, it will be necessary to geo-reference these sources alongside all
current and potential future CO₂ users (sinks), with the
goal of making economically sound linkages between source and similar-sized
usage markets. This will entail a cost analysis of on-site capture plus
additional transport costs (freight versus pipeline, hazmat fees, etc.). Geographic
information systems (GIS) mapping will assist in defining the most
cost-effective mechanisms for CO₂ delivery. As these
costs are inventoried, the financial incentive gap necessary to compel the
targeted source-sink pairings to move forward is calculated. This effort will
develop a current economic assessment of moving irreplaceable industry toward
carbon-neutrality. Though these industries represent a small portion (ca. 3%) of total
CO₂ emissions, their permanence requires immediate
attention. As these low-hanging fruits are tackled, this study may serve as a
model for assessing carbon-neutrality in other sectors.

The Effect
of Late Transition Metal Doping on Methane Selectivity in Fischer-Tropsch Catalysis (Dissertation Work)

Fischer-Tropsch synthesis has endured a long and
industrious career as the premier chemical method for the synthesis of
hydrocarbons from non-petroleum-based precursors. Historically, a collection of
factors has dictated interest in the FT process, including foreign policy,
availability of precursor(s), availability of alternate fuel sources and
reserves, and environmental considerations. The effect of late transition metal
substitution into Fe(100), Ni(111), and Co(0001)
surface analogs is investigated using density functional theory (DFT) methods.
The surface is modeled using a 7-atom cluster, with
perimeter atoms saturated with hydrogen atoms to approximate surface
coordination and mitigate dangling bond artifacts. Ab initio calculations are performed to predict adsorption energies
and transition state geometries and energies for eight surface adsorbate
species: C, CH, CH₂, and CH₃ to
represent the hydrogenating steps on surface carbide, while C+ CH, CH + CH, C +
CH₃, and CH₂ + CH₂
represent species involved in four competitive coupling pathways. A review of
the effect of Cu, Ag, Au, and Pd on the reaction energies and barriers
associated with these critical steps is discussed. Methane selectivity can be
described by a single descriptor, the carbon binding strength; thus, a
comparison of these energies over different surface/promoter analogs is an
effective method to screen for promising candidates. In this representation, a
lowering of carbon binding strength is equivalent to the lowering of methane
selectivity.