Complex Surface Reactions Research Articles

S‐Scheme MnO2/Co3O4 Sugar‐Gourd Nanohybrids with Abundant Oxygen Vacancies for Efficient Visible‐Light‐Driven CO2 Reduction

AbstractConverting CO2 to carbon‐based fuels using solar energy via photocatalysis is a promising approach to boost carbon neutrality. However, the solar‐to‐chemical conversion efficiency is hampered by interconnected multiple factors including insufficient light absorption, low separation efficiency of photogenerated carriers as well as complex and sluggish surface reaction kinetics. Herein, we incorporate MnO2 nanowires and Co3O4 hollow polyhedrons with abundant oxygen vacancies (VO) into MnO2/Co3O4 sugar‐gourd nanohybrids for boosting CO2 photoreduction. The MnO2/Co3O4 nanohybrids not only display strong absorption in the visible–near infrared region, but also facilitate the separation of photogenerated carriers in terms of S‐scheme transfer pathway, supplying abundant electrons for CO2 reduction reaction. Furthermore, the presence of VO enhances the separation efficiency of photogenerated carriers and promotes the chemical adsorption to CO2 molecules. In addition, the interfacial electronic interaction between MnO2 and Co3O4 also contributes to the chemical adsorption and activation to CO2. Owing to the synergy of S‐scheme transfer pathway and VO, the MnO2/Co3O4 hybrids exhibit greatly enhanced photocatalytic activity towards CO2 reduction under the irradiation of visible light in comparison with bare MnO2 and Co3O4, delivering a CO evolution rate of 15.9 umol g−1 h−1 with a 100 % selectivity.

(Invited) Advances in Modeling Ion Transport in Micropores and Charging Behavior of Porous Electrodes: Merging Molecular Insights with Machine Learning for Energy Storage

Electric double layer (EDL) charging is a complex process coupling electro-osmotic flow, surface reactions, and ion concentration gradients. The charging dynamics depends on the properties of electrolyte and electrode structure, as well as initial and boundary conditions. While molecular dynamics (MD) simulations and first-principles calculations can encompass all factors, their computational demands are substantial even for simple electrochemical systems such as electrolytes near a planar surface. This presentation explores recent advancements in modeling ion transport in micropores and the charging behavior of porous electrodes with different theoretical and simulation methods. Illustrative examples will be discussed on the integration of molecular modeling and machine-learning techniques to enhance our understanding of complex electrochemical processes and improve the design of energy storage devices.

Formation of Pre-PCTA/DT Intermediates from 2-Chlorothiophenol on Silica Clusters: A Quantum Mechanical Study.

Silica (SiO2), accounting for the main component of fly ash, plays a vital role in the heterogeneous formation of polychlorinated thianthrenes/dibenzothiophenes (PCTA/DTs) in high-temperature industrial processes. Silica clusters, as the basic units of silica, provide reasonable models to understand the general trends of complex surface reactions. Chlorothiophenols (CTPs) are the most crucial precursors for PCTA/DT formation. By employing density functional theory, this study examined the formation of 2-chlorothiophenolate from 2-CTP adsorbed on the dehydrated silica cluster ((SiO2)3) and the hydroxylated silica cluster ((SiO2)3O2H4). Additionally, this study investigated the formation of pre-PCTA/DTs, the crucial intermediates involved in PCTA/DT formation, from the coupling of two adsorbed 2-chlorothiophenolates via the Langmuir-Hinshelwood (L-H) mechanism and the coupling of adsorbed 2-chlorothiophenolate with gas-phase 2-CTP via the Eley-Rideal (E-R) mechanism on silica clusters. Moreover, the rate constants for the main elementary steps were calculated over the temperature range of 600-1200 K. Our study demonstrates that the 2-CTP is more likely to adsorb on the termination of the dehydrated silica cluster, which exhibits more effective catalysis in the formation of 2-chlorothiophenolate compared with the hydroxylated silica cluster. Moreover, the E-R mechanism mainly contributes to the formation of pre-PCTAs, whereas the L-H mechanism is prone to the formation of pre-PCDTs on dehydrated and hydroxylated silica clusters. Silica can act as a relatively mild catalyst in facilitating the heterogeneous formation of pre-PCTA/DTs from 2-CTP. This research provides new insights into the surface-mediated generation of PCTA/DTs, further providing theoretical foundations to reduce dioxin emission and establish dioxin control strategies.

Quantifying Ozone-Dependent Emissions of Volatile Organic Compounds from the Human Body.

Ozone reactions on human body surfaces produce volatile organic compounds (VOCs) that influence indoor air quality. However, the dependence of VOC emissions on the ozone concentration has received limited attention. In this study, we conducted 36 sets of single-person chamber experiments with three volunteers exposed to ozone concentrations ranging from 0 to 32 ppb. Emission fluxes from human body surfaces were measured for 11 targeted skin-oil oxidation products. For the majority of these products, the emission fluxes linearly correlated with ozone concentration, indicating a constant surface yield (moles of VOC emitted per mole of ozone deposited). However, for the second-generation oxidation product 4-oxopentanal, a higher surface yield was observed at higher ozone concentrations. Furthermore, many VOCs have substantial emissions in the absence of ozone. Overall, these results suggest that the complex surface reactions and mass transfer processes involved in ozone-dependent VOC emissions from the human body can be represented using a simplified parametrization based on surface yield and baseline emission flux. Values of these two parameters were quantified for targeted products and estimated for other semiquantified VOC signals, facilitating the inclusion of ozone/skin oil chemistry in indoor air quality models and providing new insights on skin oil chemistry.

3D modeling of feature-scale fluorocarbon plasma etching in silica

Fluorocarbon dry etching of vertical silica-based structures is essential to the fabrication of advanced complementary metal-oxide-semiconductor and dynamic random access memory devices. However, the development of etching technology is challenged by the lack of understanding of complex surface reaction mechanisms and by the intricacy of etchant flux distribution on the feature-scale. To study these effects, we present a three-dimensional, TCAD-compatible, feature-scale modeling methodology. The methodology combines a level-set topography engine, Langmuir kinetics surface reaction modeling, and a combination of reactant flux evaluation schemes. We calibrate and evaluate our model to a novel, highly selective, etching process of a mathrm {SiO_2} via and a textrm{Ru} hardmask by mathrm {CF_4/C_4F_8}. We adapt our surface reaction model to the novel stack of materials, and we are able to accurately reproduce the etch rates, topography, and critical dimensions of the reported experiments. Our methodology is therefore able to prototype and study novel etching processes and can be integrated into process-aware three-dimensional device simulation workflows.

Real-space imaging of a phenyl group migration reaction on metal surfaces

The explorations to extend present chemical synthetic methods are of great importance to simplify synthetic routes of chemical species. Additionally, understanding the chemical reaction mechanisms is critical to achieve controllable synthesis for applications. Here, we report the on-surface visualization and identification of a phenyl group migration reaction of 1,4-dimethyl-2,3,5,6-tetraphenyl benzene (DMTPB) precursor on Au(111), Cu(111) and Ag(110) substrates. With the combination of bond-resolved scanning tunneling microscopy (BR-STM), noncontact atomic force microscopy (nc-AFM) and density functional theory (DFT) calculations, the phenyl group migration reaction of DMTPB precursor is observed, forming various polycyclic aromatic hydrocarbons on the substrates. DFT calculations reveal that the multiple-step migrations are facilitated by the hydrogen radical attack, inducing cleavage of phenyl groups and subsequent rearomatization of the intermediates. This study provides insights into complex surface reaction mechanisms at the single molecule level, which may guide the design of chemical species.

Coupling mesoscale transport to catalytic surface reactions in a hybrid model.

In heterogeneous catalysis, reactivity and selectivity are not only influenced by chemical processes occurring on catalytic surfaces but also by physical transport phenomena in the bulk fluid and fluid near the reactive surfaces. Because these processes take place at a large range of time and length scales, it is a challenge to model catalytic reactors, especially when dealing with complex surface reactions that cannot be reduced to simple mean-field boundary conditions. As a particle-based mesoscale method, Stochastic Rotation Dynamics (SRD) is well suited for studying problems that include both microscale effects on surfaces and transport phenomena in fluids. In this work, we demonstrate how to simulate heterogeneous catalytic reactors by coupling an SRD fluid with a catalytic surface on which complex surface reactions are explicitly modeled. We provide a theoretical background for modeling different stages of heterogeneous surface reactions. After validating the simulation method for surface reactions with mean-field assumptions, we apply the method to non-mean-field reactions in which surface species interact with each other through a Monte Carlo scheme, leading to island formation on the catalytic surface. We show the potential of the method by simulating a more complex three-step reaction mechanism with reactant dissociation.

Surface reactions between LiHMDS, TMA and TMP leading to deposition of amorphous lithium phosphate

Complex surface reactions between TMA and TMP result in an amorphous, polymerized lithium phosphate.

Benchmarking an Embedded Adaptive Sampling Configuration Interaction Method for Surface Reactions: H2 Desorption from and CH4 Dissociation on Cu(111).

Embedded (emb-) correlated wavefunction (CW) theory enables accurate assessments of both ground- and excited-state reaction mechanisms involved in heterogeneous catalysis. Embedded multireference second-order perturbation theory (emb-MRPT2) based on reference wavefunctions generated via embedded complete active space self-consistent field (emb-CASSCF) theory is currently state-of-the-art. However, the factorial scaling of CASSCF limits the size of active space and the complexity of systems that can be studied. Here, we assess the efficacy of an alternative CW method, adaptive sampling configuration interaction (ASCI)-which enables large active spaces to be used-for studying surface reactions. We couple ASCI with density functional embedding theory (DFET) and benchmark its performance for two reactions: H2 desorption from and CH4 dissociation on the Cu(111) surface. Unlike embedded complete active space second-order perturbation theory (emb-CASPT2) that accurately reproduces a measured H2 desorption barrier, embedded ASCI, using a very large active space (though one that still comprises a small portion of the full set of orbitals) fails to do so. Adding an extra correlation term from embedded Møller-Plesset second-order perturbation theory (emb-MP2) improves the desorption barrier and endothermicity predictions. Thus, the inaccuracy of embedded ASCI comes from the missing dynamic correlation from the many other electrons and orbitals not included in the active space. For CH4 dissociation, again embedded ASCI overestimates the dissociation barrier compared to emb-CASPT2 predictions. Adding dynamic correlation from emb-MP2 helps correct the barrier. However, this composite approach suffers from double counting of correlation within embedded ASCI followed by emb-MP2 calculations. We therefore conclude that the state-of-the-art emb-MRPT2 based on reference wavefunctions generated via emb-CASSCF remains the method of choice for studying surface reactions. emb-ASCI is useful when large active spaces beyond the limit of emb-CASSCF are essential, such as to study complex surface reactions with significant multiconfigurational character (static correlation) but weak dynamic correlation.

Real-Space Imaging of a Single-Molecule Monoradical Reaction.

Organic radicals consisting of light elements exhibit a low spin-orbit coupling and weak hyperfine interactions with a long spin coherence length, which are crucial for future applications in molecular spintronics. However, the synthesis and characterization of these organic radicals have been a formidable challenge due to their chemical instability arising from unpaired electrons. Here, we report a direct imaging of the surface chemical transformation of an organic monoradical synthesized via the monodehydrogenation of a chemically designed precursor. Bond-resolved scanning tunneling microscopy unambiguously resolves various products formed through a complex structural dissociation and rearrangement of organic monoradicals. Density functional theory calculations reveal detailed reaction pathways from the monoradical to different cyclized products. Our study provides unprecedented insights into complex surface reaction mechanisms of organic radical reactions at the single molecule level, which may guide the design of stable organic radicals for future quantum technology applications.

Formation and behavior of negative ions in low pressure aniline-containing RF plasmas

This paper is focused on the formation mechanisms and the general behavior of negative ions in low pressure radio-frequency (RF) plasmas operated in a mixture of argon and aniline vapor. This type of plasma is mostly used for the synthesis of polyaniline, one of the most studied conductive polymers. Experiments based on mass spectroscopy measurements reveal the necessity to have a thin layer of plasma synthesized polyaniline on the electrodes to produce negative ions through complex surface reactions. In addition, thin-films deposited using this type of discharge are analyzed by means of Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS). The material analysis gives a first indication about the possible contribution of negative ions to polyaniline deposition.

Deposition of SiOxCyHz Protective Coatings on Polymer Substrates in an Industrial-Scale PECVD Reactor

The deposition of protective coatings on aluminised polymer substrates by a plasma enhanced chemical vapour deposition PECVD technique in a plasma reactor with a volume of 5 m3 was studied. HMDSO was used as a precursor. Plasma was sustained in a capacitively coupled radiofrequency (RF) discharge powered by an RF generator operating at 40 kHz and having an adjustable output power up to 8 kW. Gaseous plasma was characterised by residual gas mass spectrometry and optical emission spectroscopy. Polymer samples with an average roughness of approximately 5 nm were mounted into the plasma reactor and subjected to a protocol for activation, metallisation and deposition of the protective coating. After depositing the protective coating, the samples were characterised by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). The combination of various techniques for plasma and coating characterisation provided insight into the complex gas-phase and surface reactions upon deposition of the protective coatings in the industrial-size plasma reactor.

XPS investigation on the reactivity of surface imine groups with TFAA

The chemical reaction of imine groups with vapors of trifluoroacetic anhydride (TFAA) was investigated in detail with X‐ray photoelectron spectroscopy (XPS) for the potential application in chemical derivatization (CD) studies of plasma treated surfaces. Imine groups were at first prepared by converting surface amine groups of a polymer precursor using a common vapor phase derivatization reaction with fluorine tagged aldehydes and ketones. The originally low yield for the imine forming reaction of approx. 50%, performed under standard conditions was dramatically enhanced up to 100% by an own developed procedure using a catalyst. This step allowed to obtain a consistent quantification and interpretation of the complex surface reaction products from different types of imine groups derivatized by TFAA.

Incorporation of Si during vapor phase epitaxy of III-V compounds: Evidence of an enthalpy-entropy compensation effect

The incorporation of dopants in III-V compound semiconductor epilayers during chemical vapor deposition involves complex homogeneous and surface reaction kinetics and is expected to be an activated process. In particular, silicon is an element of choice for n-type doping of various III-V compound epilayers for which there is a wealth of data involving different growth and doping precursors. Kinetic arguments such as the role of multi-excitation entropy or quasi-equilibration at the growth interface during incorporation predict an enthalpy-entropy compensation effect (EECE), which is observed from compiled data for more than 14 orders of magnitude of the prefactor describing the activated incorporation rate. For this particular observation of the EECE, an explanation involving data pre-selection imposed by restrictive growth conditions may also be invoked.

Examining Electrical Properties of Solid Electrolyte Interface on Silicon Electrodes

The complex surface reactions contribute to the formation of solid-electrolyte interphases (SEI), and cause the irreversible capacity loss of silicon (Si) electrodes. The chemical and mechanical instability of the interphases finally leads to performance degradation. A microscopic characterization of the electrical properties at the interface is expected to gain information to understand the physical and chemical evolution caused by the formation of SEI, and provide the guidance for improvements in performance. The microscopic electrical characterization techniques, including Kelvin probe force microscopy (KPFM) and scanning spreading resistance microscopy (SSRM), have been applied to silicon electrodes. The nanoelectrical probes have been developed in one atomic force microscopy (AFM)-based platform, and the complementary electrical characterizations can be performed on the same sample area in nm-scale. Based on the contact potential difference method, KPFM maps the work function (WF) on electrode surfaces, which is expected to largely change with lithiation and delithiation. WF is sensitive to Li-ion distribution and diffusion, which can be highly inhomogeneous in nm-scale and can be assessed and evaluated from the KPFM mapping. Local conductivity/resistivity of Li-ion electrodes has been studied by SSRM. Based on the contact mode of AFM, SSRM maps of local resistivity in 10-nm resolutions have been acquired by applying a voltage between the probe and a sample and measuring the current through the probe. Ex-situ measurements along the Lithiation/delithiation process have been performed to understand the formation of SEI and the impact of electrochemical reaction on the electrical properties of SEI. Figure 1 shows the SSRM resistance changes before and after lithiation of Si electrode. The inhomogeneity with grain and domain sizes of 200-500 nm is observed at the surface of Si electrode. Surface modification by using atomic layer deposition and molecular layer deposition has also been used to modify the surface chemistry of silicon electrodes. This presentation will discuss the evolution in nm-scale resistance of SEI during electrochemical reactions, and the impact of surface coating materials on the electrical properties of SEI. Figure 1

Adhesion of Shewanella oneidensis MR-1 to Goethite: A Two-Dimensional Correlation Spectroscopic Study.

Bacterial adhesion to mineral surfaces is an important but underappreciated process. To decipher the molecular level process and mechanism, the adhesion of Shewanella oneidensis MR-1 cells to goethite was investigated using flow-cell attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy coupled with two-dimensional correlation spectroscopy (2D-COS) analysis. The FTIR results indicate that bacterial phosphate-moieties play an important role in the formation of mono- and bidentate inner-sphere complexes, whereas carboxylic groups on cell surface only have a minor contribution to its adhesion. The 2D-COS analysis in short-term (0-120 min) and long-term (2-18 h) stages reveal that the adhesion process was in the following sequence: change in H-bonds of proteins on cell surfaces > formation of monodentate inner-sphere surface complexes > formation of outer-sphere surface complexes > transformation of protein secondary structure on cell surfaces > formation of additional bridging bidentate surface complexes. In addition, the adhesion of MR-1 cells on goethite was pH dependent due to pH impacts on the cell structure and the interface charge. The in situ ATR-FTIR integrated with 2D-COS analysis highlights its great potential in exploring complex surface reactions with microbes involved. These results improve our understanding of microbe-mineral interactions at the molecular level and have significant implications for a series of biogeochemical processes.

In-Situ Analysis of Co-Electrolysis Species Via Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

Syngas production through co-electrolysis of steam and carbon dioxide has been shown as an effective method of CO2 utilisation, however little is known about the complex surface reaction mechanisms involved. Further understanding is needed in order to optimise SOC operation parameters and inform materials development. Currently many investigative techniques are ex-situ, some of which are even destructive: e.g. SEM imagining of the microstructure. To fully understand reaction intermediates, characterise electrochemical performance, and develop novel materials and microstructures for co-electrolysis SOCs; in-situ analysis methods are required. The EPSRC funded research programme, 4CU aims to enhance the fundamental understanding of CO2 and its role in co-electrolysis by developing a suite of in-situ investigative methods for high temperature SOC operation. Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) we can monitor both adsorbed and gas phase species in-situ. Variation of potential and temperature conditions allow for visualization of surface mechanisms associated with single atmosphere fuel cell operation. This, along with dual atmosphere measurements, will be discussed. This work illustrates the applicability of the new technique for SOCs operated in a range of atmospheres using Au, Pt and Ag electrodes. The comparison of potential dependent DRIFTS combined with electrochemical measurements provides an important insight into the roles of oxygen ion and proton transport within the systems studied. Figure 1

In situ spectroscopy of complex surface reactions on supported Pd-Zn, Pd-Ga, and Pd(Pt)-Cu nanoparticles.

It is well accepted that catalytically active surfaces frequently adapt to the reaction environment (gas composition, temperature) and that relevant "active phases" may only be created and observed during the ongoing reaction. Clearly, this requires the application of in situ spectroscopy to monitor catalysts at work. While changes in structure and composition may already occur for monometallic single crystal surfaces, such changes are typically more severe for oxide supported nanoparticles, in particular when they are composed of two metals. The metals may form ordered intermetallic compounds (e.g. PdZn on ZnO, Pd2Ga on Ga2O3) or disordered substitutional alloys (e.g. PdCu, PtCu on hydrotalcite). We discuss the formation and stability of bimetallic nanoparticles, focusing on the effect of atomic and electronic structure on catalytic selectivity for methanol steam reforming (MSR) and hydrodechlorination of trichloroethylene. Emphasis is placed on the in situ characterization of functioning catalysts, mainly by (polarization modulated) infrared spectroscopy, ambient pressure X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and X-ray diffraction. In the present contribution, we pursue a two-fold, fundamental and applied, approach investigating technologically applied catalysts as well as model catalysts, which provides comprehensive and complementary information of the relevant surface processes at the atomic or molecular level. Comparison to results of theoretical simulations yields further insight. Several key aspects were identified that control the nanoparticle functionality: (i) alloying (IMC formation) leads to site isolation of specific (e.g. Pd) atoms but also yields very specific electronic structure due to the (e.g. Zn or Ga or Cu) neighboring atoms; (i) for intermetallic PdZn, the thickness of the surface alloy, and its resulting valence band structure and corrugation, turned out to be critical for MSR selectivity; (ii) the limited stability of phases, such as Pd2Ga under MSR conditions, also limits selectivity; (iii) favorably bimetallic catalysts act bifunctional, such as activating methanol AND water or decomposing trichlorothylene AND activating hydrogen; (iv) bifunctionality is achieved either by the two metals or by one metal and the metal-oxide interface; (v) intimate contact between the two interacting sites is required (that cannot be realized by two monometallic nanoparticles being just located close by). The current studies illustrate how rather simple bimetallic nanoparticles may exhibit intriguing diversity and flexibility, exceeding by far the properties of the individual metals. It is also demonstrated how complex reactions can be elucidated with the help of in situ spectroscopy, in particular when complementary methods with varying surface sensitivity are applied.

Bipolaron Formation Induced by Oxygen Vacancy at Rutile TiO2(110) Surfaces

The formation of polarons in the presence of oxygen vacancies at the rutile TiO2 surfaces is a widely studied topic due to its importance in complex surface reactions. In this paper, we have studied polaron formation both near and infinitely far away from the oxygen vacancy on the (110) rutile TiO2 surface by ab initio density functional theory and screened hybrid functionals. We conclude that a polaron prefers to stay near the oxygen vacancy due to electrostatic attraction between the positively charged oxygen vacancy and polaron. Although two polarons introduced by an oxygen vacancy repel each other, the oxygen vacancy effectively binds two polarons, forming a bipolaron. Moreover, our calculated vacancy formation energies elucidate the conditions under which the polarons are likely to be formed.

Virtual Probes of Mineral-Water Interfaces: The More Flops, the Better!

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