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the field of materials research, whereby promising new technologies can serve to bridge the gap between stale, out-of-date processes and those who would fit better into this new energy and environmentally-conscience landscape.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-Tršpsch (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.