Elementary Mechanisms Research Articles

Analytic solution to the rate law for a fundamental autocatalytic reaction mechanism operating in the “efficient” regime

AbstractIntegrated rate laws in analytic form are scarce in the kinetics literature in cases where a reaction mechanism involves an autocatalytic step. As an example, we have taken up the challenge of integrating the rate law for a mechanism where an initial autocatalytic step competes with a subsequent step in which the catalyst decays unimolecularly to form the product. In addition to its chemical application, this mechanism forms the basis for theories of metabolism and chemical evolution. Even though it is one of the most elementary mechanisms possible, an exact analytic solution to the governing rate law has never been obtained in closed form. Of greatest chemical interest is the case where the autocatalytic step is much faster than the unimolecular step. This situation has the effect of enhancing the yield of the product and is known as the efficient regime. We derive a closed form analytical solution to the rate equation which is accurate in this limit. We use our analytic solution to obtain a formula for the induction time during which the rate of formation of product is accelerating.

Heterogeneous chemical reactions—A cornerstone in emission reduction of local pollutants and greenhouse gases

The current state and challenges of advanced experimental and modeling methods for a better understanding of heterogeneous chemical reactions are discussed using examples from developing and future technologies in the area of emission reduction of local pollutants and greenhouse gases. In situ and operando experimental techniques using laser and X-ray absorption spectroscopy, for instance, are able to resolve spatial and temporal concentration and temperature profiles in the near-wall gas phase, the interphase and inside the solid bulk. They have been exploited for a better understanding of the interaction of chemical reactions and transport processes. The experimental elucidation of chemical conversion on the microscopic scale leads to elementary step-like surface reaction mechanisms. The microkinetic description of gas-surface reactions is still challenging due to the complex influence of the modification of the solid material itself on the microscopic scale during the chemical reaction, which is caused by intrinsic materials’ modifications due to adsorbed species and temperature variations. Furthermore, transient inlet and boundary conditions on the reactor scale have a strong impact on the material and reaction rate. In addition to thermochemical reactions, an additional complexity comes into play with electrochemical ones. This paper will discuss heterogeneous chemical reactions in the light of emerging technologies such as emission control of natural gas and hydrogen fueled engines, use of CO2 in chemical (methanation, dry reforming) and steel industry (off-gas reforming), hydrogen production by pyrolysis of methane, small-scale ammonia synthesis and use, and recyclable carbon-free energy carriers. Hence, this article will also reveal a new playground and the potential of methods, know-how, and skills of the combustion community to significantly contribute to the solution of climate-change relevant challenges.

Numerical investigation of the reaction kinetics of dry reforming of methane over the yolk–shell and single-atom alloy catalysts

Dry reforming of methane (DRM) over Ni-based supported catalysts has been widely studied due to Ni's low cost and high activity. Despite their excellent performance, side reactions such as methane decomposition and Boudouard reaction can cause severe coke formation leading to catalyst deactivation. Our recent work reported that both yolk–shell morphology and Pt-Ni interaction in single-atom-alloy (SAA) structures could maintain the DRM activity. However, the effect of morphology and Pt-Ni interaction could not be elucidated. In this work, we studied the reaction kinetics of DRM by proposing a detailed elementary reaction mechanism with 12 surface species and 17 elementary steps over the Ptx-NiCe@SiO2 and Pt0.25-NiCe/SiO2WI catalysts. Due to the different DRM activity over the wet-impregnated and yolk–shell structures, we examined multiple reaction models to explain the effect of morphology and Pt-Ni interaction on the DRM activity. Our results show that the reverse Boudouard reaction and dissociative adsorption of CO2 and CH4 are the rate-limiting steps. The desorption of CO* and H* is also critical to products yield and selectivity. Compared with Pt0.25-NiCe/SiO2WI catalyst, the C* removal followed by the fast CO* desorption is more favored on Pt0.25-NiCe@SiO2 SAA, suggesting the reverse Boudouard reaction prevents the catalyst deactivation by coking. The high O* and low H* coverages are observed on Pt0.25-NiCe@SiO2 SAA due to the confined yolk–shell morphology and enhanced Pt-Ni interaction in SAA structures, respectively. Both these effects can lead to facile C* removal in the Pt0.25-NiCe@SiO2 SAA catalyst leading to a stable DRM activity.

Three Regimes in the Tribo‐Oxidation of High Purity Copper at Temperatures of up to 150 °C

Surface oxidation of high‐purity copper is accelerated under tribological loading. Tribo‐oxide formation at room temperature is associated with diffusion processes along defects, such as dislocations or grain boundaries. Herein, the additional influence of temperature on the tribo‐oxidation of copper is investigated. Dry, reciprocating sliding tests are performed with a variation of the sample temperature between 21 and 150 °C. Microstructural changes are monitored and analyzed with state‐of‐the‐art electron microscopy techniques. Oxide layer formation through thermal oxidation is observed for 150 °C, but not for lower temperatures. As the temperature increases from room temperature up to 100 °C, a significantly stronger tribo‐oxidation into deeper material layers and an increase in the amount of formed pores and oxides is detected. Up to 75 °C, diffusional processes of oxygen along grain boundaries and dislocation pipes are identified. Starting at 100 °C, CuO is detected. Hence, tribological loading significantly alters the CuO formation in comparison with static oxidation. Along with the CuO formation at temperatures ≥90 °C, the oxide layer thickness decreases while the friction coefficient increases. The observations broaden the understanding of the elementary mechanisms of tribo‐oxidation in copper. Eventually, this will allow to systematically customize surfaces showing tribo‐oxidation for specific tribological applications.

An elementary mechanism for simultaneously modeling discrete decisions and decision times

An elementary mechanism for simultaneously modeling discrete decisions and decision times

Ultrafast Dynamics of Photochemical Nitrile Imine Formation

AbstractThe chemical reactivity of nitrile imines is of great utility in organic synthesis with applications rapidly expanding into the materials and life sciences. Yet, our understanding of the electronic and molecular structures of nitrile imines remains incomplete and the elementary mechanism of their photoinduced generation is entirely unknown. Here, femtosecond infrared spectroscopy after 266 nm‐excitation of 2,5‐diphenyltetrazole has been carried out to temporally resolve the formation and structural relaxation dynamics of the nascent diphenylnitrile imine in liquid solution under ambient conditions. The infrared‐spectroscopic evolution is interpreted by an initial sequence of intersystem crossings within 250 fs followed by the cleavage of N2 with formation of a structurally relaxed nitrile imine on the adiabatic ground‐state singlet surface within a few tens of picoseconds. The infrared spectrum supports the notion of a “floppy” nitrile imine molecule whose equilibrium character ranges from fully propargylic to fully allenic in the room temperature liquid solution.

Ultrafast Dynamics of Photochemical Nitrile Imine Formation.

The chemical reactivity of nitrile imines is of great utility in organic synthesis with applications rapidly expanding into the materials and life sciences. Yet, our understanding of the electronic and molecular structures of nitrile imines remains incomplete and the elementary mechanism of their photoinduced generation is entirely unknown. Here, femtosecond infrared spectroscopy after 266 nm‐excitation of 2,5‐diphenyltetrazole has been carried out to temporally resolve the formation and structural relaxation dynamics of the nascent diphenylnitrile imine in liquid solution under ambient conditions. The infrared‐spectroscopic evolution is interpreted by an initial sequence of intersystem crossings within 250 fs followed by the cleavage of N2 with formation of a structurally relaxed nitrile imine on the adiabatic ground‐state singlet surface within a few tens of picoseconds. The infrared spectrum supports the notion of a “floppy” nitrile imine molecule whose equilibrium character ranges from fully propargylic to fully allenic in the room temperature liquid solution.

Potential TRIP/TWIP coupled effects in equiatomic CrCoNi medium-entropy alloy

High-entropy alloys (HEA) and medium-entropy alloys (MEA), containing multiple principal elements typically with equiatomic or near-equiatomic ratios, have drawn considerable attention due to their unique and promising mechanical profiles, such as high tensile strength, superior ductility and exceptional fracture toughness. However, the elementary mechanisms controlling the deformation and fracture of this new class of alloys still need complementary analyses. In the present study, the nucleation and growth mechanisms of deformation bands in the medium-entropy CrCoNi alloy were investigated by atomic-resolution scanning transmission electron microscopy (STEM). It was revealed that planar dislocation slip is the dominant deformation mode in the early stages of deformation. With increasing strain, both deformation twins and hexagonal close packed (HCP) lamella simultaneously appear. Careful analysis of the dislocations involved in these processes confirms that two different mechanisms are responsible for the nucleation of deformation bands in the CrCoNi alloy: the three-layer mechanism proposed by Mahajan et al. and the transformation from HCP phase to twin. Activation of multiple slip systems at larger deformation levels leads to the activation of deviation-based mechanisms, which contribute to the twin growth. It was also observed that many HCP bands remain and overlap with nanoscale twins, leading to short range HCP-twin stackings, which contribute to the high work hardening rate of this alloy.

Inflammatory Mediators in Atherosclerotic Vascular Remodeling.

Atherosclerotic vascular disease remains the most common cause of ischemia, myocardial infarction, and stroke. Vascular function is determined by structural and functional properties of the arterial vessel wall, which consists of three layers, namely the adventitia, media, and intima. Key cells in shaping the vascular wall architecture and warranting proper vessel function are vascular smooth muscle cells in the arterial media and endothelial cells lining the intima. Pathological alterations of this vessel wall architecture called vascular remodeling can lead to insufficient vascular function and subsequent ischemia and organ damage. One major pathomechanism driving this detrimental vascular remodeling is atherosclerosis, which is initiated by endothelial dysfunction allowing the accumulation of intimal lipids and leukocytes. Inflammatory mediators such as cytokines, chemokines, and modified lipids further drive vascular remodeling ultimately leading to thrombus formation and/or vessel occlusion which can cause major cardiovascular events. Although it is clear that vascular wall remodeling is an elementary mechanism of atherosclerotic vascular disease, the diverse underlying pathomechanisms and its consequences are still insufficiently understood.

Elementary mechanism for the electrocatalytic reduction of nitrobenzene on late-transition-metal surfaces from density functional theory

Reduction of nitrobenzene to aniline is important in the production of industrial chemicals and treatment of wastewater. The electrocatalytic mechanism of nitrobenzene reduction across late-transition-metal electrocatalysts is not well established, making rational electrocatalyst design challenging. Density functional theory (DFT) methods are used to identify elementary steps for nitrobenzene reduction and determine trade-offs that dictate optimizing electrocatalysts. Overall reduction activity is optimized based on a trade-off between activity for the initial reduction of the NO 2 -phenyl∗ and the reduction of surface-bound hydroxide (OH∗) species. The binding of O serves as a descriptor to predict the energetics of both steps, and the optimal electrocatalyst will have an intermediate O∗ binding strength. DFT results predict that, of close-packed late-transition-metal surfaces, the Cu (111), Ir (111), Pd (111), and Pt (111) metal surfaces will most effectively balance these trade-offs. Bimetallics that could offer high electrocatalytic activity for nitroaromatic reduction are suggested. • The first elementary reduction of R-NO 2 limits the rate of nitrobenzene reduction • Strong O binding poisons catalysts otherwise active for nitrobenzene reduction • Intermediate O binding leads to an optimal nitrobenzene reduction electrocatalyst The reduction of nitroaromatics is important in both the production of fine chemicals and the treatment of wastewater. Electrocatalytic reduction could offer an energy-efficient and scalable approach for these applications. The development of active and selective electrocatalysts is facilitated by elementary mechanism determination. We applied density functional theory to determine the rate-limiting steps in electrocatalytic nitrobenzene reduction over late-transition metals. A “volcano” trade-off dictated by the metals’ affinity for binding O was developed and used to predict single-metal and bimetallic surfaces that would optimize nitrobenzene electroreduction activity. Rate-limiting steps in electrochemical nitrobenzene reduction on late-transition-metal surfaces are determined using density functional theory calculations. A rationale for electrocatalyst optimization is presented and used to suggest optimal nitroaromatic reduction electrocatalysts.

Understanding the Structure and Energy Transfer Process of Undoped Ultrathin Emitting Nanolayers Within Interface Exciplexes

Organic light-emitting diodes (OLEDs) have great potential for display, lighting, and near-infrared (NIR) applications due to their outstanding advantages such as high efficiency, low power consumption, and flexibility. Recently, it has been found that the ultrathin emitting nanolayer technology plays a key role in OLEDs with simplified structures through the undoped fabricated process, and exciplex-forming hosts can enhance the efficiency and stability of OLEDs. However, the elementary structure and mechanism of the energy transfer process of ultrathin emitting nanolayers within interface exciplexes are still unclear. Therefore, it is imminently needed to explore the origin of ultrathin emitting nanolayers and their energy process within exciplexes. Herein, the mechanism of films growing to set ultrathin emitting nanolayers (<1 nm) and their energy transfer process within interface exciplexes are reviewed and researched. The UEML phosphorescence dye plays a key role in determining the lifetime of excitons between exciplex and non-exciplex interfaces. The exciplex between TCTA and Bphen has longer lifetime decay than the non-exciplex between TCTA and TAPC, facilitating exciton harvesting. The findings will be beneficial not only to the further development of OLEDs but also to other related organic optoelectronic technologies.

Uncovering the existence of anti-backstress associated with dislocations accumulated at grain boundaries during plastic deformation

The influence of the grain orientation on the long-range internal stress associated with the accumulation of geometrically necessary dislocations (GNDs) during plastic deformation is numerically investigated. GNDs stored at grain boundaries (GBs) do not systematically generate a backstress inside grains. Surprisingly, long-range anti-backstress promoting dislocation dynamics and plastic strain inside grains arises at certain GBs interfaces from the accumulation of GNDs. This discovery on one important elementary mechanism affecting the macroscopic mechanical behavior of polycrystals provides guidelines to improve the physical content of current crystal plasticity models.

Expanded Reactor Engineering Calculations for the Oxidative Coupling of Methane

The catalytic activation of CH4 by limited amounts of O2 produces a mixture of synthesis gas (CO, H2) and light hydrocarbons (C2Hx), the relative amounts of each depending on catalyst type and process conditions. Using an elementary reaction mechanism for the oxidative coupling of methane (OCM) on a La2O3/CeO2 catalyst derived from the literature, this study replaces the activating O2 with moist H2O2 vapor to reduce synthesis gas production while improving C2Hx yields and selectivities. As the H2O2 content of the activating oxidant rises, more of the CH4 conversion occurs in the gas phase instead of with the catalytic surface. In a packed bed reactor (PBR), the use of H2O2 allows the PBR “light-off” to occur using a lower feed temperature. In exchange for a small decline in CH4 conversion, C2Hx selectivity increases while synthesis gas production drops. In a continuous stirred tank reactor (CSTR), H2O2 improves C2Hx over synthesis gas across a wider range of feed temperatures than is possible with the PBR. This suggests the CSTR will likely reduce OCM preheating requirements.

A meso-scale model of clay matrix: the role of hydration transitions in geomechanical behavior.

While much progress has been made on the modeling of swelling clays at the molecular scale in recent decades, up-scaling to the macroscopic scale remains a challenge, in particular because the mesoscopic scale (between a few nanometers and a few hundreds of nanometers) is still poorly understood. In this article, we propose a new 2D granular model of clay at the mesoscale. This model is adapted to the modeling of a dense clay matrix representing geomechanical conditions (up to pressures of 10-100 MPa). Some salient features of this model with respect to the existing literature are: (1) its ability to capture hydration transitions occurring at small basal spacings (essential to model complex hydro-mechanical behaviors such as drying shrinkage), (2) the flexibility of the clay layers that becomes important at pressures exceeding 1 MPa, and (3) the control of the inter-layer shear strength critical to model plasticity. The model calibration is purely bottom-up, based on molecular modeling results only. The case of Na-montmorillonite (Na-Mnt) is investigated in detail, regarding isotropic compression (elasticity and plasticity), yield surface and desiccation. The behavior of the granular model appears well consistent with what is known experimentally for pure Na-Mnt, and offers valuable insight into meso-scale processes that could not be reached so far (role of hydration transition, layer flexibility, and impact of loading history). This granular model is a first step toward quantitative up-scaling of molecular modeling of swelling clay for geomechanical applications.

Nickel-Catalyzed Enantioselective Reductive Conjugate Arylation and Heteroarylation via an Elementary Mechanism of 1,4-Addition

Nickel-Catalyzed Enantioselective Reductive Conjugate Arylation and Heteroarylation via an Elementary Mechanism of 1,4-Addition

Insights into the mechanism of ethanol conversion into 1,3-butadiene on Zr-β zeolite

1,3-butadiene is an important monomer in the production of rubber, resin and engineering plastics. In recent years, ethanol to 1,3-butadiene (ETB) has been widely studied. Zr-β molecular sieves have been paid more attention to because of their relatively good catalytic properties. However, there is still a lack of specific understanding of the microscopic properties of Zr-β closed and open sites and their synergistic effects in the catalytic process of ETB. In this work, density functional theory (DFT) calculations were used to study the structural properties of open and closed site on Zr-β zeolite and the mechanism of ETB reaction. The results show that the key reaction species are adsorbed more strongly at Zr-β open site, and the proton transfer step is the highest barrier step (Ea = 1.79 eV) for the ETB process at closed site, which is similar to the process on the zirconium oxide surface. At open site, based on the cycle mechanism that Zr-OH participates in the proton transfer step of the ETB process proposed in this study, Zr-OH assists the proton transfer, which reduces the energy barrier of the proton transfer step. As a result, Meerwein-Ponndorf-Verley (MPV) reaction at open site is the rate-limiting step (Ea = 0.78 eV). So Zr-β open site exhibits relatively better catalytic performance than closed site and zirconia. For the conclusion that the number of Zr-β open site is positively correlated with catalytic performance, the explanation of the elementary reaction mechanism has been put forward.

Optical pump assisted broadband terahertz frequency comb

A broadband terahertz (THz) frequency comb assisted by an optical pump in THz quantum cascade lasers (QCLs) is investigated theoretically and numerically through a Maxwell–Bloch model combined with the coupled wave theory. When an optical pump is injected into the laser cavity with dispersion, the intrinsic four-wave-mixing nonlinear process becomes not only an important elementary phase-locking mechanism during the mode proliferating process, but also the bandwidth of the frequency comb is increased and the power is amplified through the nonlinear parametric process. The relative shift between the frequency of the optical pump and the zero-dispersion frequency of THz QCLs tremendously affects the conversion efficiency of the nonlinear parametric process. The simulation results show that appropriately optical pumping could assist in generating the broadband THz frequency comb with over 1 THz and more than 80 lines, which may open many potential applications in designing and optimizing high resolution THz spectroscopy sources.

Sound emission and annihilations in a programmable quantum vortex collider.

In quantum fluids, the quantization of circulation forbids the diffusion of a vortex swirling flow seen in classical viscous fluids. Yet, accelerating quantum vortices may lose their energy into acoustic radiations1,2, similar to the way electric charges decelerate on emitting photons. The dissipation of vortex energy underlies central problems in quantum hydrodynamics3, such as the decay of quantum turbulence, highly relevant to systems as varied as neutron stars, superfluid helium and atomic condensates4,5. A deep understanding of the elementary mechanisms behind irreversible vortex dynamics has been a goal for decades3,6, but it is complicated by the shortage of conclusive experimental signatures7. Here we address this challenge by realizing a programmable vortex collider in a planar, homogeneous atomic Fermi superfluid with tunable inter-particle interactions. We create on-demand vortex configurations and monitor their evolution, taking advantage of the accessible time and length scales of ultracold Fermi gases8,9. Engineering collisions within and between vortex-antivortex pairs allows us to decouple relaxation of the vortex energy due to sound emission and that due to interactions with normal fluid (that is, mutual friction). We directly visualize how the annihilation of vortex dipoles radiates a sound pulse. Further, our few-vortex experiments extending across different superfluid regimes reveal non-universal dissipative dynamics, suggesting that fermionic quasiparticles localized inside the vortex core contribute significantly to dissipation, thereby opening the route to exploring new pathways for quantum turbulence decay, vortex by vortex.

Role of oxygen diffusion in the dislocation reduction of epitaxial AlN on sapphire during high-temperature annealing

Recovery of epitaxial AlN films on sapphire at high temperatures is now an established process to produce pseudo-substrates with high crystalline perfection, which can be used to grow epitaxial structures for UV-light-emitting devices. To elucidate the elementary mechanisms taking place during the thermal treatment of MOVPE-grown films, we studied as-grown and annealed samples combining transmission electron microscopy techniques and secondary ion mass spectrometry (SIMS). By using SIMS, we find a temperature-dependent increase in the overall oxygen content of the films, which cannot be explained quantitatively with either simple bulk or pure pipe-diffusion from the sapphire substrate. Instead, we propose a lateral outdiffusion from the dislocation cores to explain qualitatively and quantitatively the presence of observed oxygen concentration plateaus. Based on the formation enthalpy of various atomic defects and complexes found in literature, we conclude that the di-oxygen/aluminum vacancy complex (VAl–2ON) is the dominant point defect controlling the annealing process. The formation of this defect at high temperatures promotes a dislocation core climb process, which causes the annihilation/fusion of the threading dislocation segments.

An empirical test of a fundamental Harrod-Kaldor business cycle model

Going back to the roots of modern business cycle modelling, the paper advances a macroeconomic model of a most elementary (and now largely forgotten) cycle mechanism within a growth framework. Essentially, it combines a nonlinear Harrodian instability for a business sentiment index with Harrod’s and Kaldor’s old idea that firms tend to reduce investment if they have already built up high capacities relative to their assessment of the normal market potential. For greater realism, a previous version is augmented by a delay in the capital growth rate and by in general three random shock processes. This makes it possible to estimate the model on quarterly US data of utilization and a certain capital ratio. The focus is on an inquiry into the model’s goodness-of-fit, for which the method of simulated moments is a fruitful technique. Here, 14 fairly ambitious summary statistics that the model should try to reproduce allow a detailed diagnosis of its merits and demerits. The results are sufficiently encouraging and form a sound basis for stepwise extensions of the model in future work, or for refinements in the estimation procedure.