Cl Coverage Research Articles

Surface chlorination of IrO2(110) by HCl.

The ability to controllably chlorinate metal-oxide surfaces can provide opportunities for designing selective oxidation catalysts. In the present study, we investigated the surface chlorination of IrO2(110) by HCl using temperature programmed reaction spectroscopy (TPRS), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. We find that exposing IrO2(110) to HCl, followed by heating to 650K in ultrahigh vacuum, produces nearly equal quantities of on-top and bridging Cl atoms on the surface, Clt and Clbr, where the Clbr atoms replace O-atoms that are removed from the surface by H2O formation. After HCl adsorption at 85K, only H2O desorbs at low Cl coverages during TPRS, but HCl begins to desorb in increasing yields as the Cl coverage is increased above about 0.5 monolayer (ML). The desorption of Cl2 was not observed under any conditions, in good agreement with the high barrier for this reaction predicted by DFT. A maximum Cl coverage of 1 ML, with nearly equal coverages of Clt and Clbr atoms, could be generated by reacting HCl with IrO2(110) in UHV. Our results suggest that a kinetic competition between recombinative HCl and H2O desorption under the conditions studied limits the saturation Cl coverage to a value less than the 2 ML maximum predicted by thermodynamics. XPS further shows that the partitioning of Cl between the Clt and Clbr states can be altered by subjecting partially chlorinated IrO2(110) to reductive or oxidative treatments, demonstrating that the Cl site population can change dynamically in response to the gas environment. Our results provide insights for understanding the chlorination of IrO2(110) by HCl and can enable future experimental studies to determine how Cl-modification alters the surface chemical reactivity of IrO2(110) and potentially enhances selectivity toward partial oxidation chemistry.

A transient site balance model for atomic layer etching

We present a transient site balance model of plasma-assisted atomic layer etching of silicon (Si) with alternating exposure to chlorine gas (Cl2) and argon ions (Ar+). Molecular dynamics (MD) simulation results are used to provide parameters for the model. The model couples the dynamics of a top monolayer surface region (‘top layer’) and a perfectly mixed subsurface region (‘mixed layer’). The differential equations describing the rates of change of the Cl coverage in the two layers are transient mass balances. Model predictions include Cl coverages and rates of etching of various species from the surface as a function of Cl2 or Ar+ fluence. The simplified phenomenological model reproduces the MD simulation results well over a range of conditions. Comparing model predictions directly to experimental optical emission spectroscopy data, as reported in a previous paper (Vella et al 2023 J. Vac. Sci. Technol. A 41, 062602), provides further evidence of the accuracy of the model.

The role of chlorine promoters in mediating particle size effects in silver-catalyzed ethylene epoxidation

Measured chlorine coverages over Ag/α-Al2O3 catalysts with varying Ag weight loadings (1.3–35 wt%) and different Ag particle size distributions when parsed in terms of Cl coverages over particles of varying size constituted in the distribution reveal that Ag particles of size below ∼30 nm are covered in more than 1 monolayer (ML) of chlorine in presence of 3.5 ppm C2H5Cl. These assessments of Cl coverages were made possible by correlating the cumulative Cl coverage to the lognormal surface area distribution of 22 Ag/α-Al2O3 samples using a single exponential decay function. Fully chlorided small Ag particles exhibit low rates of epoxidation and only large Ag particles which exhibit sub-monolayer Cl coverages catalyze ethylene epoxidation with high ethylene oxide (EO) rates and selectivity. The Ag particle size dependence of Cl coverages plausibly explains why ≥ 100 nm Ag particles are typical in promoted Ag/α-Al2O3 ethylene epoxidation catalysts.

Influence of residual chlorine on Ru/TiO2 active sites during CO2 methanation

Titania-supported ruthenium (Ru/TiO2) is an established catalyst for the hydrogenation of carbon dioxide to methane (Sabatier reaction). Chlorine contamination, owed to the RuCl3 precursor, is demonstrated to have a detrimental impact on methanation activity. After calcination and reduction the catalyst contains residual chlorine, shown by XPS. An aqueous ammonia wash removes Cl without leaching Ru. The washed catalysts exhibit improvements in CH4 site-time yields. Low Ru loading catalysts encounter the greatest activity enhancements after washing (∼4.5–fold). DFT calculations indicate that chlorine and CO2 directly compete for adsorption on Ru step sites, with Cl impeding the adsorption of CO2 at under-coordinated sites and at higher Cl coverages. H2-chemisorption/TPR show that Cl removal lowers the onset of low temperature H2 dissociation on Ru. DRIFTS provide evidence that the removal of Cl facilitates low temperature dissociative binding of CO2, indicated by the formation of surface bound linear CO species.

Unconventional Electrochemical Behaviors of Cu Underpotential Deposition in a Chloride-Based Deep Eutectic Solvent: High Underpotential Shift and Low Coverage.

The (5 × 5) Moiré pattern resulting from coadsorption of Cu atoms and chloride ions on the Au(111) electrode is one of the most classical structures for underpotential deposition (UPD) in electrochemical surface science. Although two models have been proposed to describe the pattern, the details of the structure remain ambiguous and controversial, leading to a question that remains to be answered. In this work, we investigate the UPD behaviors of Cu on the Au(111) electrode in a chloride-based deep eutectic solvent ethaline by in situ scanning tunneling microscopy (STM). Benefiting from the properties of the ultraconcentrated electrolyte, we directly image not only Cu but also Cl adlayers by finely tuning tunneling conditions. The structure is unambiguously determined for both Cu and Cl adlayers, where an incommensurate Cu layer is adsorbed on the Au(111) surface with a Cu coverage of 0.64, while the Cl coverage is 0.32 (only half of the expected value); i.e., the atomic arrangement of the observed (5 × 5) Moiré pattern in ethaline matches neither of the models proposed in the literature. Meanwhile, STM results confirm the origin of the cathodic peak in the cyclic voltammogram, which indicates that the underpotential shift of Cu UPD in ethaline indeed increases by ca. 0.40 V compared to its counterpart in a sulfuric acid solution, resulting in a significant deviation from the linear relation between the underpotential shift and the difference in work functions proposed in the literature. The unconventional electrochemical behaviors of Cu UPD reveal the specialty of both the bulk and the interface in the chloride-based deep eutectic solvent.

Studies of Bis-(Sodium-Sulfopropyl)-Disulfide and 3-Mercapto-1-Propanesulfonate on/into the Copper Electrodeposited Layer by Time-of-Flight Secondary-Ion Mass Spectrometry

Interactions of functional additives SPS (bis-(sodium-sulfopropyl)-disulfide), MPS (3-Mercapto-1-Propanesulfonate), and Cl accumulated and incorporated on/into a copper electrodeposited layer were studied using time-of-flight secondary-ion mass spectrometry (TOF-SIMS) in combination with cyclic voltammetry measurements (CV). It was shown that the Cl and MPS surface coverage is dependent on the applied overpotential and concentration of Cl, SPS, or MPS in the solution. Detailed discussion on the mechanism of yielding CH2SO3-, C3H5SO3-, CuSC3H6SO3-, and CuS- fragments and their assignment to the gauche or trans conformation was proposed. The mechanism of the process of incorporation and re-adsorption of MPS on/into a copper surface under electrochemical conditions without and with chloride ions and its impact on electrochemical properties was proposed. Moreover, it was shown that the presence of chloride ions, the ratio gauche/trans of MPS molecules, as well as the ratio chloride/thiols demonstrate a high impact on the accelerating abilities. Comparative studies conducted under open circuit potential conditions on the nitinol and copper substrate allowed for the identification of specific reactions/interactions of MPS, or SPS and Cl ions on the nitinol and copper surface.

Adsorption and Co‐adsorption of Chlorine and Water‐Chlorine Complexes on Au(111) Surfaces: First‐Principles DFT Study

AbstractThe adsorption of atomic chlorine (Cl) on perfect Au(111) surfaces has been investigated by employing extensive density functional theory calculations, including the Cl coverage range from 1/9 to 1 monolayer (ML) and the effect of water on the Cl adsorption. The structural, energetic, and electronic properties are calculated and compared with previously reported experimental studies towards adsorption of Cl on metal surfaces. We found that there is a significant difference in energy and electronic structures for adsorption of Cl at the top site, compared with adsorption of Cl on other sites of the Au surface. Surface coverage lower than 1/3 ML is enough for the adsorption of Cl atoms at an fcc hollow site, whereas the Cl adopts a mixed adsorption sites at higher coverage reaching 3/4 ML. The negatively charged Cl− ion increase the work function of the Au(111) surface at all coverage, and the Au−Cl interaction is dominated by a strong coupling between the d‐band of Au and the p state of Cl atoms. The appearance of water molecules makes Cl more stable for adsorbing on the top site of Au (111) through hydrogen bonding interaction, and also promotes interfacial charge transfer between Cl and Au(111). We emphasize here is that the Au−Cl stretching vibrational frequency at the top site is similar to the observed values in the experiment, while the Au−Cl stretching frequency redshifts as the coverage increases for Cl adsorped at the fcc site. The presence of water would cause a significant redshift of the Au−Cl stretching vibrational frequency because of the stretched bond length and the decrease of the covalent bonding component. These changes motivate us to rethink of the Au−Cl vibration frequency and the corresponding interfacial structure.

Nanosystem’s density functional theory study of the chlorine adsorption on the Fe(100) surface

Abstract This contribution investigates chlorine (Cl) interaction with the Fe(100) surface, with a focus on governing adsorption energies and geometrical features at the nanoscale using the density functional theory (DFT) approach. The Cl/Fe(100) system can be considered as a building block to create nanosystems with specific and desired electronic, material, mechanical, or environmental properties. We report adsorption energies, surface relaxations. and buckling distances for Cl adsorbed as a function of Cl coverage. The computational DFT framework employs a vdW-DF functional with coverages varying from 0.25 to 1 ML. Adsorption at a bridge site with coverage of 0.5 ML appears to be the most preferred site, with an adsorption energy of −4.44 eV. For all coverages, Cl adsorption at the bridge and hollow sites incurs slightly higher adsorption energies than adsorption at the top (T) site. The potential energy surface (PES) for the dissociation of a Cl molecule over the Fe(100) surface was calculated. Dissociative adsorption of the Cl molecule on Fe(100) ensues via a modest activation barrier of only 0.58 eV in a noticeably exothermic reaction of 2.94 eV. In agreement with experimental observations, the work function decreases upon Cl addition in reference to the clean iron surface. The electronic interaction between Cl and the Fe(100) surface was examined by calculating the differential charge density distribution of the most stable structure (B-0.5 ML). The vdW-DF interactions increase the adsorption energies and reduce the equilibrium distances when compared with the corresponding results from plain DFT.

Functions of MnOx in NaCl Aqueous Solution for Artificial Photosynthesis.

SummaryPhotoelectrochemical water splitting has been intensively investigated as artificial photosynthesis technology to convert solar energy into chemical energy. The use of seawater and salted water has advantages for minimum environmental burden; however, the oxidation of Cl− ion to hypochlorous acid (HClO), which has toxicity and heavy corrosiveness, should occur at the anode, along with the oxygen evolution. Here, O2 and HClO production in aqueous solution containing Cl− on photoanodes modified with various metal oxides was investigated. The modification of MnOx resulted in the promotion of the O2 evolution reaction (OER) specifically without HClO production over a wide range of conditions. The results will contribute not only to the practical application of artificial photosynthesis using salted water but also to the elucidation of substantial function of manganese as the element for OER center in natural photosynthesis.

Evidence for anti-synergism between ion-assisted etching and in-plasma photoassisted etching of silicon in a high-density chlorine plasma

Etching of p-Si in 60 mTorr 10%Cl2/90%Ar Faraday-shielded inductively coupled high density plasmas was investigated under both ion-assisted etching (IAE) and photoassisted etching (PAE) conditions. Real-time etching rates and after-etching Si surface chemical compositions were obtained by laser interferometry and vacuum-transfer x-ray photoelectron spectroscopy (XPS), respectively. Precisely controlled ion energy distributions (IEDs) were generated by applying pulsed negative DC bias on the conductive sample stage. Above a 36 eV threshold at a total flow rate of 250 SCCM, the IAE rate increased with the square root of the ion energy. In contrast to the DC bias, etching under RF bias did not exhibit a threshold ion energy because of the wide IED. XPS spectra revealed that the surface layer under PAE conditions had a significantly lower chlorine content, composed of only SiCl. Under IAE conditions, however, silicon dangling bonds (Si•), SiCl2, and SiCl3 were found on the surface, in addition to SiCl, with a relative abundance of SiCl > SiCl2 > SiCl3. The absence of higher chlorides and Si• under PAE conditions suggested that vacuum ultraviolet photons and above threshold-energy ions interact with the surface very differently. By varying the duty cycle of the pulsed DC bias, it was found that the IAE rate scaled with the energetic ion dose, but only for low duty cycles. For higher duty cycles, the apparent IAE yield fell off with an increasing Cl coverage on the surface, as the duty cycle went up, which pointed to a negative synergy (antisynergism) between PAE and IAE as the explanation. This antisynergism was further supported by the observed decrease of the total etching rate with an increasing period of the pulsed DC bias. A plausible mechanism is that increasing the pulsing period causes more near-surface damage, creating more recombination centers that lead to a higher loss rate of electron-hole pairs through recombination, thereby reducing the PAE rate.

Density Functional Theory Study of the Initial Stages of Cl-Induced Degradation of α-Cr2O3 Passive Film

The ion exchange and point defect models are two prominent models describing the role of anions, such as chlorides, in the degradation of passive oxide films. Here the thermodynamic feasibility of critical steps of Cl-induced degradation of a hydroxylated α-Cr2O3 (0001) surface, as proposed by these two models, are studied. Both models begin with Cl substitution of surface OH and H2O, which becomes less favorable with increasing Cl coverage. The initial stages of Cl-induced breakdown of the α-Cr2O3 depend on Cl coverage and the presence of O vacancy near the surface as follows: (1) neither Cl insertion (supporting the ion exchange model) nor Cr vacancy formation (supporting the point defect model) is feasible at low Cl coverages except in the presence of O vacancies near the surface, where Cl insertion is thermodynamically feasible even at low coverages, (2) in the absence of O vacancies, Cr vacancy formation becomes feasible from 10/12 ML onwards whereas Cl insertion by exchange with subsurface OH only becomes feasible at full coverage. This implies that at higher coverages Cl-induced degradation first initiatesthrough a vacancy formation mechanism, but both insertion and vacancy formation would be feasible at full coverage.

Kinetics of chlorine deposition and removal over promoted silver catalysts during ethylene epoxidation

High partial oxidation selectivity in ethylene oxidation over Ag catalysts is achieved in part via the presence of adsorbed chlorine promoters. Chlorine coverage during ethylene oxidation is maintained by co-feeding organic chlorides in ppm levels and alkane moderators in concentrations of ~0.5 mol%. The relative efficacy of organic chlorides in Cl deposition and of alkanes in Cl removal is typically estimated using empirical correlations. Apparent kinetics of Cl removal were measured by feeding ethyl chloride and propane during ethylene oxidation catalysis over a highly-promoted Ag/α-Al2O3 catalyst and quantifying the rate of chloropropane formation during steady state reaction. Propane oxychlorination forms 1- and 2-chloropropane in a ~1:2 ratio, as expected given the lower bond dissociation enthalpy of methylene than methyl CH bonds in propane, and at total rates on the order of 10-4 mol C3H7Cl (mol Agsurf)-1 s−1. Concurrent ethylene oxidation rates of order 10-1 mol (mol Agsurf)-1 s−1 provide evidence that Cl moderation occurs in a distinct catalytic cycle over promoted Ag catalysts. Cl removal has a supralinear dependence on propane pressure (PC3H8) and an inverse dependence on ethyl chloride pressure (PC2H5Cl) at high PC2H5Cl/PC3H8, while Cl removal is positive order in pressures of both C2H5Cl and C3H8 at low PC2H5Cl/PC3H8, as expected for surfaces that are increasingly covered in Cl as PC2H5Cl/PC3H8 increases. Chloropropane formation rates were positive order in dioxygen pressure for all PC2H5Cl/PC3H8 ratios. Taken together, the kinetics of Cl removal require both rate determining deposition of Cl from an alkyl chloride and kinetically relevant removal of Cl by an alkane. We develop a kinetic model that describes these trends and allows for quantification of rate and equilibrium parameters, and provide evidence for the elementary steps enumerated by measuring a kinetic isotope effect and assessing the fate of alkyl fragments from chloroethane and chloropropane in gas-phase batch reactions. Rate and equilibrium constants regressed from this kinetic model are used to develop an isotherm for Cl coverage as a function of the ratio of organic chloride promoter to alkane moderator pressures. This isotherm quantitatively predicts Cl coverages measured in situ after steady state reactions with feedstreams containing both ethyl chloride-ethane and ethyl chloride-propane mixtures. Measured reaction orders for chloropropane formation with respect to ethylene and carbon dioxide pressure reflect surface ethylene and carbon dioxide coverages that are lower for sites that form chloropropane than for sites that form ethylene oxide, suggesting that site ensembles for Cl deposition and removal vary from those involved in selective oxidation of ethylene.

Kinetic Modeling of Ethylene Epoxidation Kinetics as a Function of Chlorine Coverage over a Highly Promoted Silver Catalyst

Kinetic modeling of previously reported data demonstrate that chlorine exhibits an electrostatic influence, rather than solely ensemble site blocking, to improve the selectivity of ethylene epoxidation over promoted silver catalysts. Kinetic models that neglect the impact of surface chlorine content on either oxygen dissociation or ethylene adsorption enthalpies are unable to describe observed changes in the apparent reaction orders of ethylene consumption. Models that employ solely ensemble effects to adjust product selectivity with chlorine content falsely predict a simultaneous maximum in ethylene oxide (EO) and carbon dioxide synthesis rates with respect to chlorine coverage at 10–15% of a monolayer of chlorine. Incorporating electronic effects of chlorine on the transition states of the selectivity-determining steps into the model, however, properly describes the measured rate trends with surface chlorine content. Specifically, the model reflects the maxima observed in ethylene oxide synthesis rate with the concomitant monotonic decrease in combustion rates with increasing chloride content. Slight improvements to model predictions by combining ensemble and electronic effects on EO selectivity with increasing Cl coverage suggest that both effects may be relevant in combination but that ensemble effects alone cannot describe the observed behavior.

Molecular characteristics governing chlorine deposition and removal on promoted Ag catalysts during ethylene epoxidation

The selectivity of Ag ethylene epoxidation catalysts correlates with surface coverage of the promoter chlorine. Cl coverage, in turn, is a function of co-fed organic chloride and alkane moderator pressures. Organic molecules can be classified based on their deprotonation enthalpy, as those that are inert, those capable of selective oxidation, or those that act as gas-phase acids. The influence of alkane moderator and alkyl chloride promoter identities on Cl coverage was assessed by titration of Cl by ethane in a recirculating gas-phase batch reactor immediately following steady-state reactions with various alkane and alkyl chlorides included in ethylene oxidation feedstreams. The faculty of organic molecules for Cl removal correlates with the weakest CH bond dissociation enthalpy, such that molecules with CH bonds susceptible to homolysis remove Cl more effectively than those with stronger CH bonds. Cl deposition likely occurs via heterolytic cleavage of CCl bonds in alkyl chlorides, such that molecules with weak CCl bonds deposit multiple monolayers of Cl. Cl coverage affects ethylene epoxidation rates identically regardless of the organic chloride or the aliphatic hydrocarbon employed, demonstrating that these molecules only impact catalysis by moderating Cl coverage. These findings implicate the concurrent prevalence of homolytic and heterolytic pathways during ethylene epoxidation on promoted Ag/α-Al2O3 catalysts and provide context to examine additional roles of surface oxygen beyond involvement in oxidation pathways.

Mechanistic Insights into the Initial Stages of Chloride Enhanced Depassivation of Chromium Oxide Film, a Density Functional Theory Study

The formation of a passive film of metal oxides, such as chromium (e.g. Cr2O3) has been shown to play a key role in the corrosion resistance of stainless steels. Understanding the surface chemistry of these oxide surfaces, in different conditions, provides an essential insight into the stability and functionality of the protective layers. Aggressive ions, such as chlorides, are often part of the solution surrounding stainless steel, but these ions can lead to a breakdown of the oxide film and loss of protection for the metal surface. Here, we used density functional theory to investigate the interactions of Cl and adsorbed water with α-Cr2O3 (0001) passive film to better understand these breakdown mechanisms. Previous studies have suggested the Cr passive film (Cr2O3) is terminated by a mixture of hydroxyls and adsorbed water in aqueous solution, so mixed hydroxylated/water terminated surfaces are included in our study. The chloride (Cl) interaction was modeled by Cl substitution with hydroxyls and subsurface insertion of Cl via exchange with oxygen (O). These have been proposed to be the initial steps of Cl induced breakdown of the passive film. The effect of Cl surface concentration on substitution and insertion was also calculated from 1/6ML to a monolayer coverage. Competitive adsorption of Cl by substitution with OH and H2O is more favorable at the low Cl coverages but becomes less favorable at higher coverages while the insertion of Cl is unfavorable at lower coverages but becomes favorable at monolayer coverage. This implies that Cl can penetrate into the film at high Cl concentrations suggesting a Cl concentration dependent depassivation.

Bifunctional oxygen electrocatalyst derived from photochlorinated graphene for rechargeable solid-state Zn-air battery

Photochlorination method was used to engineer the structural defects and covalent CCl bonds in graphene. Cl coverage of 18 atom% was successfully confirmed by X-ray photoelectron spectroscopy, and D, G and 2D peaks changes upon irradiation were monitored by Raman. The mobility of chlorinated graphene field-effect transistor decreased to 66.1 cm2/(V s) in comparison to pristine graphene of 730.4 cm2/(V s). Raman indicated that the defects and CCl bonds are sensitively associated with the irradiation time, thus the structural defects and composition can be engineered accordingly. Given the creation of active sites, namely the polarization of carbon, as well as fast ionic and electronic transport, chlorinated graphene has demanded features for electrocatalytic reactions. As a proof of concept, chlorinated vertically-oriented graphene grown on carbon cloth (CC@VG) served as excellent bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) free-standing electrodes, showing overpotential of 405 mV for OER, Tafel slope of 56 mV, which has better durability than that of commercial Ir/C. Meanwhile, the ORR reduction peak appeared at 0.776 V vs. RHE, more positive than that of carbon cloth (0.60 V). All-solid-state Zn-air battery (ZABs) using chlorinated CC@VG as bifunctional air-cathode showed stable discharge voltage of 1.28 V at current density of 2 mA/cm2, power density of 45.8 mW/cm2 at 80 mA/cm2. More than 108 discharge/charge cycles (20 min per cycle) was obtained at current density of 2 mA/cm2, and round-trip efficiency decreased from 57.4% to 50.0%. Present work developed a universal chlorination method to endow carbonaceous materials with abundant defects and polarized carbon as active site as efficient bifunctional electrocatalysts, and opened a new avenue for developing promising air-cathodes for rechargeable solid-state ZABs.

Moderation of chlorine coverage and ethylene epoxidation kinetics via ethane oxychlorination over promoted Ag/α-Al2O3

Optimal ethylene oxide selectivity during steady-state ethylene oxidation over Cs and Re-promoted Ag/α-Al2O3 catalysts is attained only if co-fed alkyl chlorides deposit chlorine adatoms and co-fed alkanes moderate surface chlorine coverages. Surface chlorine coverage under reaction conditions was measured in chemical transient studies immediately after reaching steady-state by removing reactants in flowing helium and recirculating ethane and oxygen in a gas-phase batch reactor; this resulted in removal of chlorine from the catalyst as ethyl chloride and enabled assessment of the total number of moles of chlorine desorbed per surface silver atom, which varied from 0.10 to 0.45 depending on the process conditions employed. Batch recirculation experiments demonstrated that oxygen is required to remove chlorine from silver catalysts, suggesting molecular events akin to those involved in oxychlorination of alkanes moderate chlorine coverage during steady-state catalysis. The chlorine content was largely unperturbed by changes in ethylene (20–40 mol%), carbon dioxide (1–5%), or oxygen (2–7%) concentrations during steady-state catalysis. Increasing temperature resulted in a monotonic decrease in chlorine coverage. A power law model accurately describes trends in chlorine coverage as a function of steady-state ethane (0.1–15 mol%) or ethyl chloride (0.5–6.3 ppmv) concentrations. The carbon selectivity for ethylene oxide increases with chlorine coverage, in agreement with expectations from steady-state measurement of rates and selectivity as a function of co-fed alkyl chloride promoters. These results explicate the critical role of oxygen-containing species in moderating Cl coverage during ethylene epoxidation catalysis, include a model that describes chlorine coverage across a broad concentration range of alkyl chloride promoter and alkane moderators, and enable correlation between kinetic parameters measured as a function of process conditions and the predicted chlorine coverage.

First principles thermodynamic studies for recycling spent nuclear fuels using electrorefining with a molten salt electrolyte

Using first principles DFT (density functional theory), we have examined the thermochemical mechanism of electrorefining spent uranium (U) from a LiCl–KCl molten salt on a tungsten (W) surface. We calculated 197 different U/W(110) surfaces to identify the most thermodynamically and electrochemically stable structures as a function of U and Cl coverages. The results indicate that local structures of the double-layer interface between the W(110) surface and the LiCl–KCl salt are the key factors governing the electrorefining performance. The results also provide important thermodynamic properties for the design of efficient recycling systems for spent nuclear fuels, such as pyroprocessing technologies, and may be applicable as well to general electrochemical applications involving strong redox reactions of transition metals exposed to non-aqueous solutions.

Impact of Chlorine Functionalization on High-Mobility Chemical Vapor Deposition Grown Graphene

We systematically investigated plasma-based chlorination of graphene and compared its properties before and after such treatment. X-ray photoelectron spectroscopy revealed that a high Cl coverage of 45.3% (close to C2Cl), together with a high mobility of 1535 cm(2)/(V s), was achieved. The C:Cl ratio n (CnCl) can be effectively tuned by controlling the dc bias and treatment time in the plasma chamber. Chlorinated graphene field-effect transistors were fabricated, and subsequent Hall-effect measurements showed that the hole carrier concentration in the chlorinated graphene can be increased roughly by a factor of 3. Raman spectra indicated that the bonding type between Cl and graphene depends sensitively on the dc bias applied in the plasma chamber during chlorination and can therefore be engineered into different reaction regimes, such as ionic bonding, covalent bonding, and defect creation. Micro-Raman mapping showed that the plasma-based chlorination process is a uniform process on the micrometer scale.

Do observations on surface coverage-reactivity correlations always describe the true catalytic process? A case study on ceria

In situ (operando) investigations aim at establishing structure-function and/or coverage-reactivity correlations. Herein, we investigated the gas-phase HCl oxidation (4HCl+O2→2Cl2+2H2O) over ceria. Despite its remarkable performance, under low oxygen over-stoichiometry, this oxide is prone to a certain extent to subsurface/bulk chlorination, which leads to deactivation. In situ Prompt Gamma Activation Analysis (PGAA) studies evidenced that the chlorination rate is independent of the pre-chlorination degree but increases at lower oxygen over-stoichiometry, while dechlorination is effective in oxygen-rich feeds, and its rate is higher for a more extensively pre-chlorinated ceria. Even bulk CeCl3 could be transformed into CeO2 under oxygen excess. Electron Paramagnetic Resonance experiments strongly suggested that oxygen activation is inhibited by a high surface chlorination degree. The coverages of most abundant surface intermediates, OH and Cl, were monitored by in situ infrared spectroscopy and PGAA under various conditions. Higher temperature and p(O2) led to enhanced OH coverage, reduced Cl coverage, and increased reactivity. Variation of p(HCl) gave rise to opposite correlations, while raising p(Cl2) did not induce any measurable increase in the Cl coverage, despite the strong inhibition of the reaction rate. The results indicate that only a small fraction of surface sites is actively involved in the reaction, and most of the surface species probed in the in situ observation are spectators. Therefore, when performing in situ steady-state experiments, a large set of variables should be considered to obtain accurate conclusions.