H2 Reaction Research Articles

In-situ hydrogenation of dual function material for integrated CO2 capture and methanation with the presence of steam

The impacts of steam on hydrogenation of dual function materials (DFM) for Integrated CO2 Capture and in-situ methanation (ICCM) is a new area requiring detailed investigations prior to industrialization. This work investigated impacts from steams on hydrogenation of Ru-Na2CO3/γ-Al2O3 DFM for ICCM that containing Na2O adsorbent, Ru sites, and γ-Al2O3 support. DFM performance was examined in cyclic reactions as introducing external steam during hydrogenation, and the behaviors of adsorbed CO2 species during hydrogenation were characterized by in-situ DRIFTS and H2-TPSR. CH₄ selectivity decreased sharply from 84.3 % to 1.2 % as increasing external steam concentrations to 20 vol.%, and the conversion of adsorbent component decreased from 298.5 μmol g-1 to 167.1 μmol g-1. b-CO32- and m-CO32- formed at Na2CO3/γ-Al2O3 interface were the carbonate species that could be hydrogenated into CH4, some of which were desorbed into CO2 due to moisture-driven desorption effects. With the presence of external steam in H2 reactants, the conversion of carbonate species is a competing process between hydrogenation and moisture-driven desorption. In ICCM reaction with external steam present, b-CO32- was preferred to be desorbed into CO2; while for m-CO32-, desorption into CO2 by steam and hydrogenation into CH4 proceeded in parallel. Strong moisture-driven desorption effects from steam product were demonstrated in a fixed-bed reactor, which also led to rapid decrease of localized selectivity of CH4 along bed height.

A ternary-structured NiCo-LDH/Ni/BiVO4 photoanode with enhanced charge dynamics for photoelectrochemical water splitting

Solar-driven photoelectrochemical (PEC) water splitting is one of the promising technologies to produce green hydrogen as the carbon-free energy carrier. However, the ideal photoanode material BiVO4 still suffers from sluggish carrier dynamic problem. Here, NiCo-LDH/Ni/BiVO4 (NC/N/BVO) composite photoanode with a three-layer structure was successfully fabricated by introducing Ni layer followed by NiCo-LDH on BiVO4 by magnetron sputtering and electrochemical deposition, respectively. The optimized structure of NC/N/BVO exhibited a photocurrent of 4.15 mA cm−2 (1.23 VREH) in neutral solution, which was 2.8 times of pristine BiVO4. Besides, the photocurrent can be well maintained at 98 % for NC/N/BVO in 1h reaction, which dramatically decreases to 60 % for BiVO4. In this composite photoanode, NiCo-LDH as excellent water oxidation catalyst can effectively enhance the kinetic process of water oxidation on the surface, while the introduction of Ni layer can not only promote the following deposition of NiCo-LDH being a finer nanostructure with high surface area, but also increase the charge conduction efficiency and promote the charge transfer process between NiCo-LDH and BiVO4 substrate. The appropriate deposition sequence of Ni and NiCo-LDH layers ultimately highly improved the charge injection efficiency of NC/N/BVO resulting in a high performance of PEC water splitting.

Hydrogen Oxidation and Evolution Reaction on Platinum in Alkaline Electrolyte: Fixed Reference vs Overpotential and the Effect of H2 Concentration

Understanding the kinetics of the electrochemical hydrogen oxidation and evolution reaction (HOR/HER) in alkaline media is of great interest, but the underlying reaction mechanism and the dependency on the hydrogen partial pressure are still debated. In this work, we investigated the HOR/HER kinetics on a polycrystalline platinum electrode in alkaline electrolyte using the rotating disk electrode (RDE) technique. Unlike typically done, we rigorously performed data treatment and analysis on a fixed potential scale with a fixed reference potential, which has implications for the use and assessment of RDE mass-transport corrections and the definition of reaction orders. Analyzing the kinetic current curves on a fixed potential scale, we find reaction orders with respect to the hydrogen partial pressure of essentially one and zero for the HOR and HER, respectively, at larger overpotentials. In contrast, fitting the kinetic current curves in a potential range around equilibrium with a generalized Butler-Volmer model yields differing kinetic parameters and fractional reaction orders. Our findings can be explained with a potential-dependent transition in the HOR mechanism from Tafel–Volmer with a rate-determining Volmer step at small overpotentials to Heyrovsky–Volmer with a rate-determining Heyrovsky step at more positive overpotentials, with a concomitant change in the H2 reaction order from fractional to unity, thus reconciling previous propositions in the debate on the alkaline HOR/HER mechanism. Our results demonstrate the strength and advantages of using a fixed potential scale rather than the commonly employed overpotential, and we expect this approach to be beneficial also for kinetic studies of other electrocatalytic reactions.

Different enhancement of Ni doping on La0.3Sr0.7Fe0.9Ti0.1O3-δ electrodes in symmetrical solid oxide cell

Poor catalytic activity is a significant challenge that needs to be addressed to enable the effective use of perovskite electrodes in symmetrical solid oxide cell (SOC). In this study, high-activity Ni ions were doped into the La0.3Sr0.7Fe0.9Ti0.1O3-δ (LSFTi91) perovskite lattice, which endowed the materials with different facilitation effects in different atmospheres. In an oxidizing atmosphere, the introduction of Ni ions enables La0.3Sr0.7Fe0.8Ti0.1Ni0.1O3-δ (LSFTNi811) to exhibit higher conductivity and oxygen vacancy content, which enhances the adsorption and dissociation of gases. When used as symmetrical electrodes in a solid oxide electrolysis cell, the LSFTNi811 symmetric cell presents higher electrolysis performance than LSFTi91 (1.78 A cm−2 vs. 1.57 A cm−2) at 1.5 V and 800 °C. This is the highest level reported for a single-phase perovskite electrode, underlining its excellent catalytic activity. In a reducing atmosphere, although Ni doping weakens the structural stability of the material, a large number of NiFe nanoparticles precipitate the H2 reaction after material decomposition. In this situation, LSFTNi811 achieves better performance than LSFTi91 as a symmetrical electrode for the solid oxide fuel cell both before and after decomposition. Therefore, LSFTNi811 is a promising symmetrical electrode material for SOCs.

XPS and DFT studies of γ-Al2O3 modified by NO

The modification of γ-Al2O3 after NO treatment and during the NO + H2 reaction was studied using X-ray photoelectron spectroscopy (XPS) and density functional theory. The XPS N1s spectra contain features with binding energies BE = 399.0 and 403.0 eV; the first is preserved, and the second disappears when NO is removed from the gas phase. The features were identified from experimental and modeled O-KLL regions of possible modification products − NAln oxynitrides (n = 3; 4) of the γ-Al2O3(110) surface and adsorbed states. Calculations show that NAln adsorbs NO, forming normal states with a distance to the surface d = 1.32–1.70 Å and distant (d-NO) states (d = 1.91–2.06 Å) in which the spin-polarized NO molecule is retained due to magnetic NAln and Coulomb interaction. An increase in the similarity of the O-KLL spectra in the sequence of NO/γ-Al2O3(110), NO/NAln and d-NO/NAln at an adsorption heat of ∼ 0.53, 1.33 and 1.26 eV, respectively, suggests that the modification includes the formation of NO/NAl4 corresponding to BE = 399.0 eV and d-NO/NAl4, which corresponds to BE = 403.0 eV and, being chemically unbound, is able to leave the surface in the absence of NO in the reaction medium.

Ortho-Para Conversion for H+ + H2 Collision in Low Temperature: A Fully Close-Coupled Time-Dependent Wave Packet Study.

The collision-induced rate coefficients of ortho-para conversion for the H+ + H2 reaction provide accurate information to probe the lifetime of cold environments in interstellar media. Rotationally resolved reaction probabilities are calculated at the low collision energy regime (0 < Ecol ≤ 0.3 eV) by employing the coupled three-dimensional (3D) time-dependent wave packet (TDWP) formalism in hyperspherical coordinates on a recently constructed ab initio ground adiabatic potential energy surface of H3+ [J. Chem. Phys. 2014, 141, 204306] for the process H+ + H2 (v = 0, j = 0-5) → H+ + H2 (v' = 0, j'). Cross-sections are then computed from the converged reaction probabilities as a function of total angular momentum (J) over the same energy regime and subsequently employed to obtain the rate constants for the ortho-to-para (O-P) and para-to-ortho (P-O) conversions and their ratio. The ratio of ortho-para conversion shows (a) appropriate convergence with the inclusion of a higher number of initial rotational states as well as a reasonable agreement with the results from a quantum statistical method and (b) a peak at lower temperature that could be due to the available collision energy for transitions involving lower rotational states (j = 0 → j' = 1).

Molecular H2 in silicate melts

A series of hydrothermal diamond anvil cell experiments was conducted to constrain the equilibrium distribution of molecular H2 between H2O-saturated sodium aluminosilicate melts and H2O at elevated temperatures (600–800 °C) and pressures (317–1265 MPa). The distribution of H2 between the silicate liquid and the aqueous fluid was achieved through real-time monitoring of the H-H stretching vibration under in situ conditions using Raman vibrational spectroscopy. Results show that the solubility of H2 in silicate melts saturated with H2O decreases as the temperature increases, with control exerted by the mole fraction of H2O in the melt. The dissolution of H2 in the hydrous silicate melts appears to follow Henrian behavior, resembling that of an inert, neutral non-polar species. To express species solubility as a function of temperature (T in K) an empirical equation was developed:ln Km/f = 11.4 (±1.3) *1000 / T (K) − 18.7 (±1.1)where Km/f is the equilibrium constant for the reaction H2(g) = H2(melt). This equation was derived by integrating data from the current and prior experimental studies that include silicate melts with varying H2O saturation levels. It should be deemed applicable within the temperature range of 600–1450 °C and pressures ranging from 0.3 to 3 GPa. The implications are extended into developing an understanding of the H partitioning between H2-rich atmospheres blanketing magma oceans in the early history of planetary bodies. For example, transferring H from primordial atmospheric envelopes to the interior of rocky exoplanets may be less efficient than previously believed, which should be considered in models of volatile retention. Experimental data also suggest that minimal amounts of solar nebula H2 are likely to dissolve in the molten surface of primitive objects in the protoplanetary disk (∼10−5 to 10−9 mol faction H2 in melt), contradicting the highly reducing conditions observed in chondrule mineral compositions.

Effect of H[formula omitted] addition on NH[formula omitted] flame structure and dynamics in a mixing layer

Hydrogen addition has been widely used to improve the combustion performance of low-reactivity fuels such as NH3. In this study, a series of two-dimensional NH3/H2 flames in mixing layers were investigated numerically. The aim is to assess and interpret the effects of H2 addition on the structure and dynamics of ammonia flames. Detailed chemistry and transport models were considered in the simulations. It was shown that for pure hydrogen or ammonia flames, a classical tribrachial structure was observed. The non-premixed branch in the tribrachial flame of NH3 combustion features high heat release rate (HRR), while the two premixed branches are weak. The mixture fraction corresponding to the maximum HRR of NH3 combustion is significantly lower than the stoichiometric mixture fraction. When blending ammonia with hydrogen, a tetrabrachial flame structure consisting of two premixed branches and two non-premixed branches was observed, where one non-premixed branch corresponds to the reaction of NH3, while the other is formed due to the reaction of H2. It was suggested that the formation of the tetrabrachial flame is related to the preferential diffusion of hydrogen. After decreasing artificially the diffusivity of hydrogen to suppress the preferential diffusion effect, the flame exhibits only three branches with the merging of the two non-premixed branches. By analyzing the flame dynamics at the leading edge, it was found that the flame speed (Sd∗) is negatively correlated with the flame stretch (κ) and scalar dissipation rate (χ), with Sd∗ decreasing almost linearly with κ or χ, consistent with previous studies of methane and hydrogen flames. As the H2 concentration is increased, the values of Sd∗ increase due to the enhanced reactivity.

Treatment of bio-treated coking wastewater in a 3DEF system with Fe-loaded needle coke particle electrodes

As one of the most representative hard-to-biodegrade industrial wastewater, coking wastewater is poor in biodegradability and has a high concentration of organic matter. Consequently, even after treatment using biochemical technology, the discharge standards cannot be reached. We used the self-developed Fe-loaded needle coke electrodes (Fe-NCPEs) as particle electrodes and established a three-dimensional electro-Fenton (3DEF) system for the treatment of bio-treated coking wastewater (BTCW) in this study. The 3DEF system's conditions for treating BTCW were optimized using Response Surface Method (RSM). When the initial pH was 4.69, the applied voltage was 11.1 V, the Fe-NCPE dosage was 11.63 g L−1, and the COD was reduced from 387 to 57.3 mg L−1 and the colour removal rate reached 99 % after 3 h of reaction, meeting local coking wastewater discharge standards. The power consumption is only 15 kWh per ton of BTCW, indicating that this system can treat the BTCW with high efficiency and low energy consumption. UV-Vis, FTIR, and GC-MS analysis illustrated that cyclic aromatic compounds, esters, and ethers can be efficiently degraded to alkanes and olefins by the 3DEF system. The ·OH quenching experiments showed that in the 3DEF system, the ·OH is the main reactive group, which can oxidize the large hard-to-degrade organic compounds in BTCW to small organic compounds, and even to CO2 and H2O. Dynamic experiments showed that the 3DEF system can successfully remove pollutants from BTCW under continuous flow conditions with an HRT of 3 h, allowing it to meet emission standards.

A Pyridazine‐Based Cadmium(II) Organic Framework With pcu Topology as a Luminance Sensor to Nitrobenzene, Al(III), and Cu(II) Ion

ABSTRACTIn the field of water decontamination, the design of luminance sensor for sensitive detection of heavy metal ion toxic and nitroaromatic compounds are important and challenging. Herein, a new luminescent Cd(II) metal–organic framework (MOF), {[Cd2(H2O)3(4‐CPCA)2]·2H2O}n (SNUT‐27) (H2(4‐CPCA) = 1‐(4‐carboxyphenyl)‐4‐oxo‐1,4‐dihydropyridazine‐3‐carboxylic acid) was obtained by the reaction of flexible ligand H2(4‐CPCA) and Cd(II) ions, characterized by single crystal X‐ray diffraction, PXRD, FT‐IR, and TGA, the samples were further analyzed. The analysis of single crystal X‐ray diffraction showed that SNUT‐27 possesses a distinct three‐dimensional spatial arrangement and demonstrates impressive fluorescence properties. Particularly, SNUT‐27 showed a highly sensitive response to the nitro‐aromatic compounds nitrobenzene by fluorescence quenching, and in contrast, showed a fluorescence enhancement response to Al3+ and Cu2+. SNUT‐27 can be effortlessly recycled by simply rinsing with water post‐sensing experiments, showcasing its reusability. Additionally, potential detection mechanisms have been explored.

DFT modeling of reaction of H2 with O2 pre-adsorbed on In2O3(011) surface

The DFT approach is used for the first time to model the reaction of hydrogen with oxygen molecule adsorbed on the defective surface of In2O3(011). This reaction is crucial for hydrogen detection by In2O3 sensor. The activation energies are calculated using two mechanisms involving the formation of both an adsorbed water molecule and hydroxyl groups on the In2O3(011) surface. One hydroxyl group is formed due to the bonding of OH with the surface metal atom, and the other due to the bonding of hydrogen with oxygen of the lattice. Both reactions are characterized by the presence of a potential barrier and are exothermic. The activation energies of the two reactions are calculated by the climbing-image nudged elastic band method to be 0.99 eV and 0.98 eV. The results of the calculations are compared with available experimental data. It is also shown that the presence of a surface neutral oxygen vacancy leads to the formation of a vacancy state below the Fermi level, and the electron density is concentrated on the fourfold coordinated indium atom.

Influences of reformate on the performance of high temperature proton exchange membrane fuel cell and its optimization strategy

Due to the challenges of hydrogen production, storage and transport, hydrocarbon reformate-based high temperature proton exchange membrane fuel cell (HT-PEMFC) has wide application opportunities. However, the inevitable water vapor and CO in reformate significantly determines the fuel cell performance and durability. In this work, a three-dimensional, non-isothermal and multiphase model based on agglomerate sub-model is developed to investigate the effects of reformate components and operating conditions, in which multiphase transport, dissolution, diffusion, adsorption, desorption and reaction of H2 and CO are considered. The results show that the fuel cell performance increases with the rising of water vapor content as CO content higher than 2 % (dry basis, the same below) and water vapor content lower 20 % (wet basis). The alleviation effect of water vapor on CO poisoning is discovered due to reduced CO coverage rather than enhanced CO electrochemical oxidation. An elevated temperature leads to improved performance and the effect of CO poisoning (CO<3%) is weak at 180°C.

A density functional theory study of the formation of COS from CO and H2S on cesium sulfide

Alkali metals can promote the performance of MoS2 in methanethiol (CH3SH) synthesis from CO/H2/H2S mixtures. Recently, it has been found that alkali sulfides and most prominently Cs2S can also catalyze the reaction between CO and H2S to COS and H2, COS acting as an intermediate in CH3SH formation (M. Yu et al. J. Catal. 2022, 405, 116–128). Here, we study the nature of the active sites and the mechanism of the CO + H2S → COS + H2 reaction for the 6 low-index Miller planes of Cs2S. While CO adsorbs weakly, strong dissociative adsorption of H2S results in HS* and H* intermediates, which further stabilize the (001) facet as the dominant surface termination. The main reaction pathway towards COS involves the association of CO* and SH* to COSH* followed by its dehydrogenation in a Langmuir-Hinshelwood mechanism. Reactions of CO* with lattice S atoms have prohibitively high barriers due to the strong Cs-S bonds in Cs2S. Overall, the reaction rate is dominated by the (001) facet with small contributions of the (010), (011) and (101) surfaces. COSH* formation, its dehydrogenation to COS, and COS desorption compete as rate-controlling steps on these surfaces.

Non-adiabatic dynamics studies of the C+(2P1/2, 3/2) + H2 reaction: Based on global diabatic potential energy surfaces of CH2.

Global diabatic potential energy surfaces (PESs) of CH2+ are constructed using the neural network method with a specific function based on 18 213 abinitio points. The multi-reference configuration interaction method with the aug-cc-pVQZ basis set is adopted to perform the abinitio calculations. The topographical properties of the diabatic PESs are examined in detail. In general, the diabatic PESs provide an accurate quasi-diabatic representation. To validate the diabatic PESs, the dynamics studies of the C+(2P1/2, 3/2) + H2 (v0 = 0, j0 = 0) → H + CH+(X1Σ+) reaction are performed using the time-dependent wave packet method. The reaction probabilities, integral cross sections, differential cross sections, and rate constants are calculated and compared with the experimental and theoretical results. Non-adiabatic dynamics results are in good agreement with experimental data. In addition, the non-adiabatic effect in the C+(2P1/2, 3/2) + H2 reaction is significant due to the non-adiabatic results being obviously larger than adiabatic values. The reasonable non-adiabatic dynamics results indicate that present diabatic PESs can be recommended for any type of dynamics study.

Application and Cytotoxicity Evaluation of Fe-MIL-101 Nanozyme in Milk.

In this study, we used Fe-MIL-101 nanozyme to convert lactose into lactitol, and it was proved that Fe-MIL-101 nanozyme has lactase-like activity. Due to the potential health effects of nanomaterials, we evaluated the cytotoxicity of Fe-MIL-101 nanozyme. To reduce the potential toxicity of the nanozyme, we applied centrifugation and membrane filtration. When the membrane aperture size was 100nm, the residual content of Fe-MIL-101 nanozyme was 14.09μg/mL. The residual content of Fe-MIL-101 nanozyme was reduced by optimizing time, temperature, and Fe-MIL-101 nanozyme-to-substrate ratio. It was showed that the concentration of Fe was 38.47mg/kg and the concentration of H2BDC was 0mg/kg under optimized conditions (110℃, 2h of reaction and the ratio of Fe-MIL-101 nanozyme to substrate is 1:20). The result met the national standard of China. Experiments measuring cytotoxicity, oxidative stress, and cell membrane damage revealed that less than 20μg/mL Fe-MIL-101 nanozyme had no significant cytotoxicity. Our study findings showed that Fe-MIL-101 nanozyme reduced lactose content in milk.

Diabatic Potential Energy Surfaces of SrH2+ and Dynamics Studies of the Sr+(5s2S) + H2 Reaction.

Based on the ab initio energy points of both ground and excited states, a neural network fitting method combined with a specific function was successfully used to construct the diabatic potential energy surfaces (PESs) of the SrH2+ system. The topographical features of the diabatic PESs were examined in detail. The results indicate that the nonadiabatic transition characteristics between ground and excited states are accurately described by the newly constructed diabatic PESs. To verify the validity and applicability of the diabatic PESs, as well as the nonadiabatic effects during the reaction process, the quantum dynamics studies of the Sr+(5s2S) + H2 reaction were performed based on both adiabatic and diabatic PESs. The dynamics results indicate that adiabatic dynamics results are dozens of times larger than those of nonadiabatic. This illustrates the significant effect of nonadiabaticity, indicating that adiabatic dynamics results often overestimate the actual values. The integral cross sections (ICSs) were calculated and compared with the experimental data. The diabatic ICSs are in good agreement with the experimental results. The reasonable dynamics results indicate that the newly constructed diabatic PESs are suitable for the relevant dynamics studies.

(Invited) Mechanical Properties of Proton Exchange Membranes for Water Electrolysis Applications

Water electrolysers are critical to the advancement of sustainable hydrogen technologies. A key component of proton-exchange membrane water electrolysers (PEMWE) is the ionomer membrane which transports protons and water from the anode to the cathode and physically separates the H₂ and O₂ reaction products. Importantly, the barrier properties of the membrane must be maintained at high differential pressure over the lifetime of the PEMWE. To help predict the mechanical durability and operational safety of the PEMWE, it is important to understand the mechanical properties of the PEM as a function of temperature, differential pressure, and time.The main component of a typical modern PEM is a perfluorosulfonic acid (PFSA) ionomer. PFSAs have a hydrophobic perfluorinated backbone with pendant perfluorinated side chains terminating in sulfonic acid groups. The general chemical structure of PFSAs is understood to result in phase separation into hydrophilic, sulfonate-rich domains that act as transport pathways and hydrophobic, tetrafluoroethylene-rich domains that impart thermomechanical stability. In PEMWE applications, short-side chain PFSAs are a promising alternative to the benchmark Nafion, a long-side chain PFSA. At a given ion-exchange capacity (IEC), the ratio of perfluorinated backbone to sulfonic acid side chain increases as the side chain length decreases. Thus, short-side chain PFSAs may be more thermomechanically stable while maintaining good proton and water transport necessary for PEMWE efficiency.In this work, the mechanical properties of short-side chain PFSAs are investigated as a function of IEC, temperature, differential pressure, and time. Particular focus is given to compressive mechanical measurements to simulate the conditions experienced by the membrane in PEMWE applications. The findings are expected to help inform the design of next-generation membranes for hydrogen technology applications.

Reaction barriers at metal surfaces computed using the random phase approximation: Can we beat DFT in the generalized gradient approximation?

Reaction barriers for molecules dissociating on metal surfaces (as relevant to heterogeneous catalysis) are often difficult to predict accurately with density functional theory (DFT). Although the results obtained for several dissociative chemisorption reactions via DFT in the generalized gradient approximation (GGA), in meta-GGA, and for GGA exchange + van der Waals correlation scatter around the true reaction barrier, there is an entire class of dissociative chemisorption reactions for which GGA-type functionals collectively underestimate the reaction barrier. Little is known why GGA-DFT collectively fails in some cases and not in others, and we do not know whether other methods suffer from the same inconsistency. Here, we present barrier heights for dissociative chemisorption reactions obtained from the random phase approximation in the adiabatic-connection fluctuation-dissipation theorem (ACFDT-RPA) and from hybrid functionals with different amounts of exact exchange. By comparing the results obtained for the dissociative chemisorption reaction of H2 on Al(110) (where GGA-DFT collectively underestimates the barrier) and H2 on Cu(111) (where GGA-DFT scatters around the true barrier), we can gauge whether the inconsistent description of the systems persists for hybrid functionals and ACFDT-RPA. We find hybrid functionals to improve the relative description of the two systems, but to fall short of chemical accuracy. ACFDT-RPA improves the results further and leads to chemically accurate barriers for both systems. Together with an analysis of the density of states and the results from selected GGA, meta-GGA, and GGA exchange + van der Waals correlation functionals, these results allow us to discuss possible origins for the inconsistent behavior of GGA-based functionals for molecule-metal reaction barriers.

The Potential for Organic Synthesis in the Ocean of Enceladus

The Cassini spacecraft detected a soup of organics in the plume of Saturn’s moon Enceladus. Those compounds could provide building blocks for the potential emergence or sustenance of microbial life in Enceladus’ subsurface ocean. However, the sources and stabilities of organics in Enceladus’ ocean are still poorly understood. Here, we perform nonequilibrium thermodynamic calculations to assess the energetics of abiotic synthesis for a broad spectrum of small organic molecules under both cold oceanic and hydrothermal conditions on Enceladus. Most of the organics that we studied are thermodynamically favorable to synthesize at micromolal dissolved concentrations over wide ranges of pH (8.5–11) and redox conditions. This suggests that many of the organic compounds detected by Cassini may be derived from reactions of CO2 and H2. However, some widely assumed precursors of biomolecules, such as formaldehyde, hydrogen cyanide, and acetylene, are found to be unstable and therefore unfavorable to be synthesized. We found that higher temperatures also seem to favor the synthesis of organic species under Enceladus hydrothermal conditions. Detection of thermodynamically unstable species in the plume might reflect artifacts introduced during high-speed spacecraft flybys and/or active synthesis via degradation of primordial chondritic organics, or biological activities.

Construction of diabatic potential energy surfaces for the SiH2+ system and dynamics studies of the Si+(2P1/2, 3/2) + H2 reaction.

The construction of diabatic potential energy surfaces (PESs) for the SiH2+ system, related to the ground (12A') and excited states (22A'), has been successfully achieved. This was accomplished by utilizing high-level abinitio energy points, employing a neural network fitting method in conjunction with a specifically designed function. The newly constructed diabatic PESs are carefully examined for dynamics calculations of the Si+(2P1/2, 3/2) + H2 reaction. Through time-dependent quantum wave packet calculations, the reaction probabilities, integral cross sections (ICSs), and differential cross sections (DCSs) of the Si+(2P1/2, 3/2) + H2 reaction were reported. The dynamics results indicate that the total ICS is in excellent agreement with experimental data within the collision energy range studied. The results also indicate that the SiH+ ion is hardly formed via the Si+(2P3/2) + H2 reaction. The results from the DCSs suggest that the "complex-forming" reaction mechanism predominates in the low collision energy region. Conversely, the forward abstraction reaction mechanism is dominant in the high collision energy region.