Browsing by Subject "Oxidation"

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Amine oxidation in carbon dioxide capture by aqueous scrubbing
(2013-05) Voice, Alexander Karl; Rochelle, Gary T.; Sexton, Andrew J; Reible, Danny D; Willson, Carlton G; Anslyn, Eric V
Amine degradation in aqueous amine scrubbing systems for capturing CO₂ from coal fired power plants is a major problem. Oxygen in the flue gas is the major cause of solvent deterioration, which increases the cost of CO₂ capture due to reduced capacity, reduced rates, increased corrosion, solvent makeup, foaming, and reclaiming. Degradation also produces environmentally hazardous materials: ammonia, amides, aldehydes, nitramines, and nitrosamines. Thus it is important to understand and mitigate amine oxidation in industrial CO₂ capture systems. A series of lab-scale experiments was conducted to better understand the causes of and solutions to amine oxidation. This work included determination of rates, products, catalysts, and inhibitors for various amines at various conditions. Special attention was paid to understanding monoethanolamine (MEA) oxidation, whereas oxidation of piperazine (PZ) and other amines was less thorough. The most important scientific contribution of this work has been to show that amine oxidation in real CO₂ capture systems is much more complex than previously believed, and cannot be explained by mass transfer or reaction kinetics in the absorber by itself, or by dissolved oxygen kinetics in the cross exchanger. An accurate representation of MEA oxidation in real systems must take into account catalysts present (especially Mn and Fe), enhanced oxygen mass transfer in the absorber as a function of various process conditions, and possibly oxygen carriers other than dissolved oxygen in the cross exchanger and stripper. Strategies for mitigating oxidative degradation at low temperature, proposed in this and previous work are less effective or ineffective with high temperature cycling, which is more representative of real systems. In order of effectiveness, these strategies are: selecting an amine resistant to oxidation, reduction of dissolved metals in the system, reduction of the stripper temperature, reduction of the absorber temperature, and addition of a chemical inhibitor to the system. Intercooling in the absorber can reduce amine oxidation and improve energy efficiency, whereas amine oxidation should be considered in choosing the optimal stripper temperature. In real systems, 2-amino-2-methyl-1-propanol (AMP) is expected to be the most resistant to oxidation, followed by PZ and PZ derivatives, then methyldiethanolamine (MDEA), and then MEA. MEA oxidation with high temperature cycling is increased 70% by raising the cycling temperature from 100 to 120 °C, the proposed operational temperature range of the stripper. PZ oxidation is increased 100% by cycling to 150 °C as opposed to 120 °C. Metals are expected to increase oxidation in MEA and PZ with high temperature cycling by 40 - 80%. Inhibitor A is not expected to be effective in real systems with MEA or with PZ. MDEA is also not effective as an inhibitor in MEA, and chelating agents diethylenetriamine penta (acetic acid) (DTPA) and 2,5-dimercapto-1,3,4-thiadiazole (DMcT) are only mildly effective in MEA. Although MEA oxidation in real systems cannot be significantly reduced by any known additives, it can be accurately monitored on a continuous basis by measuring ammonia production from the absorber. Ammonia production was shown to account for two-thirds of nitrogen in degraded MEA at low temperature and with high temperature cycling, suggesting that it is a reliable indicator of MEA oxidation under a variety of process conditions. A proposed system, which minimizes amine oxidation while maintaining excellent rate and thermodynamic properties for CO₂ capture would involve use of 4 m AMP + 2 m PZ as a capture solvent with the stripper at 135 °C, intercooling in the absorber, and use of a corrosion inhibitor or continuous metals removal system. Reducing (anaerobic) conditions should be avoided to prevent excessive corrosion from occurring and minimize the amount of dissolved metals. This system is expected to reduce amine oxidation by 90-95% compared with the base case 7 m MEA with the stripper at 120 °C.
Ensemble and single molecules fluorescence studies of polymers
(2007-12) Kim, Yeon Ho, 1973-; Vanden Bout, David A.
The effects of chain conformation on the photo-oxidation and green emission of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) are investigated at both single molecule and ensemble levels. Single molecule studies reveal the conformation of PFO chains to be more globular when cast from THF than from toluene. Intensity transients of single molecules show that the elongated molecules cast from toluene have more fluctuations due to a fewer number of emitting centers on the polymer. Photochemical degradation leads to intensity fluctuations for the elongated molecules, while the globular chains show monotonic decays. Emission spectra of the single molecules show that photochemical oxidation leads to reduction in the emission of the molecule with no change in the emission spectra. No green emission is detected for single molecules indicating that formation of emissive ketone defects occurs rarely. Ensemble studies show that molecule cast from THF develop some green emission upon photodegredation while those cast from toluene don't. The increase in green in the globular molecules suggests that interchain contacts are necessary for the photochemical formation of emissive ketone defects in the PFO. All emission spectra of the aggregated and nonaggregated PFO during photooxidation are also analyzed by using a modified FranckCondon progression model with an additional independent Gaussian component and fitting results from single PFO spectrum. While emission spectrum of single PFO molecule shows a good fitting result to the model, the other two bulk PFO films display needs to introduce an additional term for better fit. This additional independent Gaussian component implies that green emission comes from non-Franck-Condon process. Rotational dynamics of poly(methyl acrylate) is investigated by single molecule spectroscopy. Polarized fluorescence transients from single rhodamine 6G dye embedded in polymer matrix above glass transition are analyzed and the correlation function of reduced linear dichroism is fit by a stretched exponential function. The fitting results suggest that non-exponential decay of correlation function. However, more rigorous study is needed because of the intrinsic statistical error of limited experimental data and the effect of high numerical objective.
Mitigation methods for piperazine oxidation in post-combustion carbon capture
(2022-08-12) Wu, Yuying, Ph. D.; Rochelle, Gary T.; Allen, Dave; Hwang, Gyeong; Goff, George S
Piperazine is a promising second-generation solvent for amine scrubbing in post-combustion CO₂ capture. However, the oxidative degradation of PZ can cause environmental problems and economic loss. This work presents the effects of two mitigation methods: carbon treating and N₂ sparging, on the PZ oxidation in long-term operations. The species and their respective quantities adsorbed by the activated carbon were tested in a bench-scale device. The carbon was then tested in the High Temperature Oxidation Reactor (HTOR), where the solvent oxidizes at a reasonably fast rate. Pilot plant campaigns were also performed at the UT Austin SRP and the National Carbon Capture Center (NCCC) and the effects were verified. When dissolved Fe is removed by carbon treating or other methods, available soluble Fe in the system dissolves and replaces the dissolved Fe. Therefore, all available Fe and ligands need to be removed for the mitigation to be effective. The sources of soluble Fe include fly ash and the corrosion of stainless steel, and the ligands are degradation products. All PZ degraded solvents have two absorbance peaks at 320 nm and 538 nm. The 320 nm peak is caused by dissolved metals, especially Fe, complexed by degradation products. The 320 nm peak is related to the amine degradation level and can be used as a simple and efficient method to estimate the amine degradation rate. The pilot plant data suggest that NO₂ can oxidize PZ significantly, possibly through radical reactions. 0.01 mmol/kg-hr absorption of NO₂ increased the PZ oxidation rate from 1.2 mmol/kg-hr to 2.5 mmol/kg-hr.
Model catalyst studies of the CO oxidation reaction on Titania supported gold nanoclusters
(2004) Stiehl, James Daniel; Mullins, C. B.
The chemical nature of gold has been determined to be much richer than previously thought. Recent discoveries that properly prepared gold catalysts (i.e. gold particle diameters in 2 – 5 nm size range) can catalyze the CO oxidation reaction at low temperature have spurned a renewed interest in the chemistry of gold. Despite the extensive research that has been performed regarding CO oxidation on Au based catalysts, many issues still remain unresolved. The origin of the particle size dependence of the reaction is not well understood. Also, details concerning the reaction mechanism, specifically identification of the active oxygen species, remain unresolved. In the following studies, ultra high vacuum, molecular beam, surface science techniques are used to study the CO oxidation reaction on titania supported gold nanoclusters (Au/TiO2). Using a radio frequency generated plasma-jet, it is possible to simultaneously populate the Au/TiO2 samples with atomically adsorbed and molecularly chemisorbed oxygen species, allowing for the opportunity to investigate the reactivity of each respective species. The reaction of CO with atomically adsorbed oxygen has been studied over a range of temperatures from 65 – 250 K as a function of gold coverage and oxygen coverage. The reaction is observed to be a strong function of both of the sample temperature and the oxygen coverage. The reaction is also relatively independent of the gold coverage on the sample, in contrast to findings for the reaction employing gas-phase reactants under moderate pressures. The formation and reactivity of molecularly chemisorbed oxygen on the samples following exposure to the plasma-jet was also investigated. Evidence is presented showing that some molecularly chemisorbed oxygen is formed as a result of recombination of impinging atoms on the model catalyst surface. Evidence is also presented showing that adsorption of an oxygen atom on the sample influences the chemisorption of molecular species from the gas phase. Finally, evidence is presented showing that the molecularly chemisorbed oxygen species can participate in the CO oxidation reaction at 77 K. This finding reveals a reaction channel for CO oxidation on Au/TiO2 model catalysts that does not require the dissociation of oxygen.
Molecular beam studies of low temperature CO oxidation on gold
(2005) Kim, Tae Sang; Mullins, C. B.
Gold is considered as noble among other metals because of its resistance to oxidation and corrosion. It is the most electronegative metal and its electron affinity is actually greater than that of oxygen. For this reason gold will not react directly with other electronegative elements such as molecular oxygen. As a result, gold has not been given much attention as a potential active ingredient for heterogeneous catalysis until it was discovered that gold particles that are 2-5 nm in diameter have exceptional catalytic activity towards many reactions. Among these reactions, low temperature CO oxidation is one of the most unique regarding gold catalysts in that it cannot be matched by other metals. Although it is widely accepted that gold particles which are 2-5 nm in diameter exhibit the greatest activity in CO oxidation, there is still much debate on the nature of the active sites for these catalysts and also the details of the reaction mechanism. Using molecular beams in conjunction with a radio frequency generated plasma jet, I have studied CO oxidation with atomically adsorbed oxygen on Au/TiO2 and Au(111). It is shown that CO reacts readily with pre-adsorbed oxygen atoms on a Au/TiO2 planar model catalyst and on Au(111) to produce CO2 even at temperatures as low as 77 K. The results presented show that gold particle size seems to have little effect on CO oxidation when oxygen adatoms are pre-adsorbed. This suggests that if reactive oxygen is primarily supplied through dissociation of oxygen molecules on the surface, the rate-limiting step in CO oxidation over gold is likely to be the dissociation of molecular oxygen. Another notable aspect of low temperature CO oxidation is that the addition of water in the feed stream is believed to enhance the reactivity by as much as two orders of magnitude. Here, evidence is shown that water can participate in CO oxidation on Au(111) surface populated with atomic oxygen by directly supplying its oxygen to CO to form CO2 at low temperatures. The results strongly suggest the direct involvement and promoting role of water in CO oxidation on oxygen covered Au(111).
Nitrogen dioxide absorption into sulfite inhibited by thiosulfate
(2019-08) Suresh Babu, Athreya; Lawler, Desmond F.; Rochelle, Gary
Emulsified sulfur was found to be the most suitable form of sulfur for in-situ thiosulfate production for sulfite inhibition with a maximum sulfur-to-thiosulfate-conversion of 50 % and t₅₀ of 6 hours. Increasing the ionic strength of the solution reduces the reaction rate between sulfur and sulfite. Reaction rate reduced by 3 when ionic strength of the solution was increased from 0.225 M to 2.95 M. The rate of reaction between sulfur and sulfite was found to be first-order in sulfur, half-order in sulfite, and zero-order in thiosulfate with a reaction rate constant of 5.48 x 10⁻³ mM [superscript -0.5] min⁻¹. Increasing the reaction temperature from 40 to 75 °C increased the interpreted reaction rate by a factor of 17. The activation energy of the reaction was found to be 74.2 kJ/mol, and this high value indicates that the reaction might be kinetically limited and not mass transfer limited. The reaction rate model predicts experimental bench-scale reaction rates with an absolute average deviation of 6.5%. In the pilot-scale prescrubber, at coal conditions, pH was observed to decrease as a function of time with 3 linear regions. These regions corresponded to CO₂ absorption to form carbonate, conversion of carbonate to bicarbonate, and CO₂ liberation from solution by reaction of bicarbonate with SO₂ respectively. The characteristic times of these linear regions corresponded to the rate of the reaction in each of these regions. Rate of oxidation of thiosulfate under 0-1 ppm NO₂ conditions was 0.13 gmol/hr which was half the rate at 0-5 ppm NO₂ conditions. Thiosulfate loss by tank bleed was found to be directly related to the amount of gas processed. Thiosulfate loss by bleed reduced from 60 gmol to 17.2 gmol when the flue gas flow rate reduced by 1/2.25 due to the lesser amount of water condensing in the prescrubber. A minimum of 25 mM sulfite was required to maintain NO₂ removal of 90% even under low NO₂ conditions. At NGCC conditions, thiosulfate degradation rates were 0.112 gmol/hr and 0.127 gmol/hr before and after thiosulfate addition respectively. Sulfite and thiosulfate in the prescrubber increased with SO₂ coming into the prescrubber. Thiosulfate and sulfite were correlated by a power-law relation just as in the coal condition
Oxidation and thermal degradation of methyldithanolamine/piperazine in CO₂ capture
(2011-12) Closmann, Frederick Bynum; Rochelle, Gary T.; Bedell, Stephen; Lawler, Desmond F.; Ekerdt, John G.; Willson, Carlton G.
The solvent 7 molal (m) methyldiethanolamine (MDEA)/2 m piperazine (PZ) presents an attractive option to industry standard solvents including monoethanolamine (MEA) for carbon dioxide (CO₂) capture in coal-fired power plant flue gas scrubbing applications. The solvent was tested under thermal and oxidizing conditions, including temperature cycling in the Integrated Solvent Degradation Apparatus (ISDA), to measure rates of degradation for comparison to other solvents. Unloaded 7 m MDEA/2 m PZ was generally thermally stable up to 150 °C, exhibiting very low loss rates. However, at a loading of 0.25 mol CO2/mol alkalinity, loss rates of 0.17 ± 0.21 and 0.24 ± 0.06 mM/hr, respectively, for MDEA and PZ were measured. No amine loss was observed in the unloaded blend. Thermal degradation was modeled as first-order in [MDEAH⁺], and a universal Ea for amine loss was estimated at 104 kJ/mol. An oxidative degradation model for 7 m MDEA was developed based on the ISDA data. From the model, the rate of amine loss in 7 m MDEA/2 m PZ was estimated at 1.3 X 10⁵ kg/yr, based on a 500 MW power plant and 90% CO₂ capture. In terms of amine loss, the solvent can be ranked with other cycled solvents from greatest to least as follows: 7 m MDEA>7 m MDEA/2 m PZ>8 m PZ. Thermal degradation pathways and mechanisms for 7 m MDEA/2 m PZ include SN2 substitution reactions to form diethanolamine (DEA), methylaminoethanol (MAE), 1-methylpiperazine (1-MPZ), and 1,4-dimethylpiperazine (1,4-DMPZ). The formation of the amino acids bicine and hydroxyethyl sarcosine (HES) has been directly tied to the formation of DEA and MAE, respectively, through oxidation. As a result of the construction and operation of the ISDA for cycling of solvents from an oxidative reactor to a thermal reactor, several practical findings related to solvent degradation were made. The ISDA results demonstrated that increasing dissolved oxygen in solvents leaving the absorber will increase the rate of oxidation. A simple N2 gas stripping method was tested and resulted in a reduction to 1/5th the high temperature oxidation rate associated with dissolved oxygen present in the higher temperature regions of an absorber/stripper system. The ISDA experiments also demonstrated the need to minimize entrained gas bubbles in absorber/stripper systems to control oxidation. When the ISDA was modified to intercept entrained gas bubbles, the oxidation rate was reduced 2 to 3X.
Oxidative degradation of amine solvents for C02 capture
(2015) Liu, Hanbi; Rochelle, Gary T.
This thesis focuses on the oxidative degradation of amine solvents used in coal-fired power plant flue gas CO2 capture. The relationship between amine structures and their oxidative degradation was studied at low temperature absorber conditions in order to identify stable amines that should be more rigorously screened. Amongst primary amines, those with three carbons separating the amino group from the neighboring electron-withdrawing group (such as PDA and MPA) showed resistance to oxidation, while those with even number of carbons (such as MEA, EDA, and DGA®) were susceptible to oxidation. Secondary amines did not follow the same observations. Tertiary amines resisted oxidation regardless of the structural characteristics mentioned above. The production of major oxidation products such as formate, oxalate and nitrite closely track parent amine degradation. Formate and oxalate are produced in certain ratios, and the ratio is different depending on the amine. Amine loss and formate also approach a certain ratio over time. The mechanism of oxidation is currently not well understood, and these observations are important to understanding the mechanisms of oxidation, production of nitrosamines from oxidation, and explaining why certain structures are resistant or susceptible to oxidation.
Oxygen scavenging styrene-butadiene-styrene block copolymer films for barrier applications
(2013-08) Tung, Kevin; Paul, Donald R.; Freeman, B. D. (Benny D.)
This dissertation discusses the oxidation behavior of reactive membranes that were produced by solution casting and by melt extrusion. These films, containing styrene-butadiene-styrene (SBS) block copolymer that undergoes catalytic oxidation, are of potential use as an oxygen scavenging polymer (OSP) for barrier applications. A thin film kinetic model was developed to ascertain reaction parameters that were used to describe thick film oxidation behavior. Ultimately complex structures containing these scavengers need to be produced via melt-extrusion. Therefore, processing conditions were established to ensure that melt-processed films have the same oxidation kinetics and capacity as those prepared by solution casting. Blends containing a non-reactive styrene phase and an oxygen-scavenging SBS phase were extruded and, by uptake and permeation experiments, their oxidation behaviors were monitored. The flux behavior and time lag extension as a function of oxygen pressure, film thickness, SBS scavenger and photoinitator contents were measured and compared to the theoretical model. The permeation behavior of the reactive blend films containing SBS showed that time lags can be extended via an oxidative mechanism and barrier properties be improved compared to traditional packaging membrane of native polystyrene.
Solid electrolyte substrates for two-dimensional transition metal dichalcogenide growth, transistors and circuits
(2021-08-13) Alam, Md Hasibul; Akinwande, Deji; Banerjee, Sanjay K; Dodabalapur, Ananth; Incorvia, Jean Anne; Lai, Keji
The high surface charge carrier densities, accumulated by the electrostatic gating of two-dimensional (2D) materials with ionic liquids (ILs), have often been exploited in 2D transistors and devices. However, the intrinsic liquid nature, sensitivity to humidity, and the stress induced in frozen liquids prevent them from forming an ideal platform for electrostatic gating and surface probe techniques. This dissertation reports a lithium-ion (Li-ion) solid electrolyte substrate (or simply Li-ion glass) alternative to ILs, by demonstrating its application in high-performance transistors and circuits using 2D transition metal dichalcogenide (TMD). The back-gated n-type MoS² and p-type WSe² transistors resulted in sub-threshold values approaching the ideal limit of 60 mV/dec while maintaining a high ON/OFF ratio (> 10⁶) and a complementary inverter amplifier gain of 34 under a 1 V supply, the highest among comparable solid-state amplifiers. Microscopic studies using microwave impedance microscopy clearly show a uniform and homogeneous channel formation, indicating a smooth interface between the TMD and the underlying electrolytic substrate. This dissertation also reports the direct growth of few-layer (3-4L) single-crystal MoS² on lithium-ion solid electrolyte substrate by chemical vapor deposition (CVD) and demonstrates efficient gate control in the as-grown crystal via electrolytic gating. The gating efficiency of the transistors fabricated on the as-grown crystals, and back-gated by the solid electrolyte, are comparable to the devices with exfoliated and transferred material with an additional gain in mobility value. Field-effect mobility in the range of 42-49 cm²V⁻¹s⁻¹ with current densities as high as 120 μA/μm with 0.5 μm channel length has been achieved, as expected from devices free from material transfer-related damage and impurity. This CVD growth method can potentially be extended for other 2D TMDs to realize high-mobility transistors and study intrinsic device properties. To sum up, the dissertation demonstrates solid electrolytes as an ideal platform for 2D TMD synthesis, advanced thin-film transistors, and circuits, otherwise difficult to achieve with liquid electrolytes. The results, therefore, further establish solid electrolytes as a promising alternative to ILs for surface science experiments and advanced thin-film devices. Finally, based on the work presented in this dissertation, some future research directions have been proposed
Structural study of a kero base of formula C₁₆H₂₅N
(1934) Lackey, Robert Woodfin, 1899-; Bailey, James Robinson
Superadiabatic combustion in counter-flow heat exchangers
(2009-05) Schoegl, Ingmar Michael; Ellzey, Janet L.
Syngas, a combustible gaseous mixture of hydrogen, carbon monoxide, and other species, is a promising fuel for efficient energy conversion technologies. Syngas is produced by breaking down a primary fuel into a hydrogen-rich mixture in a process called fuel reforming. The motivation for the utilization of syngas rather than the primary fuel is that syngas can be used in energy conversion technologies that offer higher conversion efficiencies, e.g. gas turbines and fuel cells. One approach for syngas production is partial oxidation, which is an oxygen starved combustion process that does not require a catalyst. Efficient conversion to syngas occurs at high levels of oxygen depletion, resulting in mixtures that are not flammable in conventional combustion applications. In non-catalytic partial oxidation, internal heat recirculation is used to increase the local reaction temperatures by transferring heat from the product stream to pre-heat the fuel/air mixture before reactions occur, thus increasing reaction rates and allowing for combustion outside the conventional flammability limits. As peak temperatures lie above the adiabatic equilibrium temperature predicted by thermodynamic calculations, the combustion regime used for non-catalytic fuel reforming is referred to as 'superadiabatic'. Counter-flow heat exchange is an effective way to transfer heat between adjacent channels and is used for a novel, heat-recirculating fuel reformer design. An analytical study predicts that combustion zone locations inside adjacent flow channels adjust to operating conditions, thus stabilizing the process for independent variations of flow velocities and mixture compositions. In experiments, a reactor prototype with four channels with alternating flow directions is developed and investigated. Tests with methane/air and propane/air mixtures validate the operating principle, and measurements of the resulting syngas compositions verify the feasibility of the concept for practical fuel-reformer applications. Results from a two-dimensional numerical study with detailed reaction chemistry are consistent with experimental observations. Details of the reaction zone reveal that reactions are initiated in the vicinity of the channel walls, resulting in "tulip"-shaped reaction layers. Overall, results confirm the viability of the non-catalytic reactor design for fuel reforming applications.
The RailPAc arc
(2018-10-09) Gray, Miles Dalbom; Raja, Laxminarayan L.; Sirohi, Jayant; Goldstein, David; Varghese, Philip; Ezekoye, Ofodike
This dissertation will detail work examining the plasma arc of the RailPAc magnetohydrodynamic flow actuator. Initial studies of the RailPAc arc have shown that the arc formation and propagation processes are highly stochastic and in many cases unpredictable. This insight motivated most of the work in this dissertation to better understand the nature of arc propagation, towards the design of a predictable and well behaved high intensity gliding arc. The work consists of several experiment based studies examining the RailPAc plasma arc, focusing on electrical characterization, spectroscopic temperature analysis, narrow-band-imaging species evolution within the arc, and the effects of electrode surface oxidation states on the propagation of the arc. Additional experimental studies examined the effects of external magnetic fields and rod configurations, the effect of the wall near the electrodes, as well as long term damage on copper and elkonite electrodes. Computational studies examined phenomena which were difficult or not possible to characterize experimentally. This includes mechanisms of wall stabilization, root mobility over oxidized surfaces, and simulations of the arc column in two and three dimensions to examine coupling of the arc to surrounding gas. The key contributions of this work can be split into two parts, both of which have experimental and computational components. The first is the characterization of the RailPAc arc dynamics (electrically, chemically, and physically) and its coupling to the surrounding flow. This is examined experimentally with spectroscopy, high-speed narrow-band imaging, and electrical measurements, as well as computationally with commercial arc modeling software solving fluid flow coupled to Maxwell’s equations in potential form. The second is the examination of the RailPAc arc root interaction with the electrode surface, particularly the anode root which has seen very little examination compared to the cathode in previous research efforts directed at high intensity gliding arcs. Both of these are combined in a computational effort to model the RailPAc arc column in two and three dimensions.
A theoretical study of oxidation on copper (100) surface
(2015-12) Yang, Sheng-Che; Henkelman, Graeme; Makarov, Dmitrii E.
The further oxidations to subsurface of the 0.5 ML oxygen coverage copper (100) surface and MRR surface were investigated by DFT calculations. The two possible pathways for continuous oxidation on MRR and the c(2x2) phase could be separated into three parts: the adsorption of oxygen molecule, the dissociation of oxygen molecule and the oxygen atoms diffusion into subsurface. The missing row of MRR can be regarded as an active site for the adsorption and dissociation of oxygen molecule. The overall energy barriers of the two pathways of MRR and the c(2x2) phase are 0.75 and 0.9 eV respectively, which indicates that MRR is more likely to be oxidized to the subsurface. The convex hull analysis agrees with the three relatively stable surfaces of early oxidation in the previous experiments and the convex hull analysis further indicated that a special unit in both MRR and MRR-like surface may be the key element to the stability.
Thermal degradation and oxidation of aqueous piperazine for carbon dioxide capture
(2011-05) Freeman, Stephanie Anne; Rochelle, Gary T.; Maynard, Jennifer A.; Reible, Danny D.; Katz, Lynn E.; Critchfield, James
Absorption-stripping with aqueous, concentrated piperazine (PZ) is a viable retrofit technology for post-combustion CO2 capture from coal-fired power plants. The rate of thermal degradation and oxidation of PZ was investigated over a range of temperature, CO2 loading, and PZ concentration. At 135 to 175 °C, degradation is first order in PZ with an activation energy of 183.5 kJ/mole. At 150 °C, the first order rate constant, k1, for thermal degradation of 8 m PZ with 0.3 mol CO2/mol alkalinity is 6.12 × 10-9 s-1. After 20 weeks of degradation at 165 °C, 74% and 63%, respectively, of the nitrogen and carbon lost in the form of PZ and CO2 was recovered in quantifiable degradation products. N-formylpiperazine, ammonium, and N-(2-aminoethyl) piperazine account for 57% and 45% of nitrogen and carbon lost, respectively. Thermal degradation of PZ likely proceeds through SN2 substitution reactions. In the suspected first step of the mechanism, 1-[2-[(2-aminoethyl) amino]ethyl] PZ is formed from a ring opening SN2 reaction of PZ with H+PZ. Formate was found to be generated during thermal degradation from CO2 or CO2-containing molecules. An analysis of k1 values was applied to a variety of amines screened for thermal stability in order to predict a maximum recommended stripper temperature. Morpholine, piperidine, PZ, and PZ derivatives were found to be the most stable with an allowable stripper temperature above 160 °C. Long-chain alkyl amines or alkanolamines such as N-(2-hydroxyethyl)ethylenediamine and diethanolamine were found to be the most unstable with an allowable stripper temperature below 120 °C. Iron (Fe2+) and stainless steel metals (Fe2+, Ni2+, and Cr3+) were found to be only weak catalysts for oxidation of PZ, while oxidation was rapidly catalyzed by copper (Cu2+). In a system with Fe2+ or SSM, 5 kPa O2 in the inlet flue gas, a 55 °C absorber, and one-third residence time with O2, the maximum loss rate of PZ is expected to 0.23 mol PZ/kg solvent in one year of operation. Under the same conditions but with Cu2+ present, the loss rate of PZ is predicted to be 1.23 mole PZ/kg solvent in one year of operation. Inhibitor A was found to be effective at decreasing PZ loss catalyzed by Cu2+. Ethylenediamine, carboxylate ions, and amides were the only identified oxidation products. Total organic carbon analysis and overall mass balances indicate a large concentration of unidentified oxidation products.
Vacuum ultraviolet enhanced atomic layer etching, and area selective atomic layer deposition for next generation nanofabrication
(2021-03-14) Coffey, Brennan Muir; Ekerdt, John G.; Engstrom, James R; Sreenivasan, S.V.; Hwang, Gyeong S; Milliron, Delia J
Novel nanofabrication methods for area-selective (AS) deposition and atomic layer etching (ALE) are presented. The aims of these methods are to facilitate the integration of metal assisted chemical etching (MACE), and electroless Cu deposition into devices such as FinFETs, transparent conductive electrodes (TCEs), and patterned photonic structures, which are ubiquitous in next generation nanotechnology, and flexible electronics. MACE and electroless Cu deposition require high quality metal catalyst (e.g., Pd, and Ru) patterns for their implementation, however, ALE methods to remove metals are limited, and difficult to control on an atomic scale. Additionally, no ALE methods currently exist that offer selectivity to remove undesired metal deposition regions, while leaving desired regions largely untouched. Two solutions are presented to ameliorate these difficulties: the first demonstration of vacuum ultraviolet (VUV) enhanced ALE, which has the potential to remove one atomic layer of Pd and Ru at a time, while also being selective to undesired Pd and Ru deposition; and the second, is to control deposition using AS atomic layer deposition (ALD) for photonic devices to desired regions alone, circumventing the need for patterning the challenging to etch BaTiO₃, while still allowing surface phase epitaxy. The first demonstration of VUV ALE is shown, where etching of Pd and Ru is achieved at 100 – 150 °C, with approximate material removal rates of 2.8, and 1.2 Å/cycle, respectively. Etching is accomplished using an oxidation half-cycle, consisting of co-exposure of the metal substrate to VUV and O₂ at 1 Torr for 2 – 5 min, followed by an etching half-cycle, consisting of exposure to 0.50 Torr of HCOOH for 30 sec. Density functional theory (DFT) is used to explore oxidation mechanisms on Pd and Ru. DFT results indicate all oxidants readily incorporate into the surface of both metals, however, a large of concentration of atomic O in the near surface region is required to form an oxide that can be removed by HCOOH exposure. Nudged elastic band (NEB) calculations indicate this is due to difficulty in forming subsurface oxides when a surface oxide is present on Pd, as well as Ru. True self-limiting behavior is predicted, and observed for Ru ALE, while the amount of Pd etched is controlled with the temperature and time of VUV/O₂ co-exposure. Additionally, selectivity in oxidation, and thus material etched, is observed on Pd, and Ru, where at co-exposure times less than 2 min, Pd will not oxidize if it is a continuous (low surface area) thin film, while discontinuous Pd will oxidize. Ru etch rate is decreases as etching is performed, indicating that as roughness decreases (with ALE cycles), longer oxidation times are required to achieve oxidation. This is amenable to the fabrication of the TCE, which requires selectivity between undesired and desired deposition, and patterning of Pd or Ru catalyst layers. Finally, AS-ALD of difficult to pattern crystalline BaTiO₃ (BTO) is presented, where patterns are defined with one lithographic patterning step. Epitaxial crystallization to form a single crystal film with an out-of-plane c-axis orientation, should be observed after AS patterning. This process could eliminate the need for post-growth etching for devices in, for example, Si photonic device fabrication