Low Temperature Catalytic Water Gas Shift in an Electric Field

Unveiling the Mechanism of Plasma-Catalytic Low-Temperature Water-Gas Shift Reaction over Cu/γ-Al2O3 Catalysts

The water-gas shift (WGS) reaction is a crucial process for hydrogen production. Unfortunately, achieving high reaction rates and yields for the WGS reaction at low temperatures remains a challenge due to kinetic limitations. Here, nonthermal plasma coupled to Cu/γ-Al2O3 catalysts was employed to enable the WGS reaction at considerably lower temperatures (up to 140 °C). For comparison, thermal-catalytic WGS reactions using the same catalysts were conducted at 140-300 °C. The best performance (72.1% CO conversion and 67.4% H2 yield) was achieved using an 8 wt % Cu/γ-Al2O3 catalyst in plasma catalysis at ∼140 °C, with 8.74 MJ mol-1 energy consumption and 8.5% H2 fuel production efficiency. Notably, conventional thermal catalysis proved to be ineffective at such low temperatures. Density functional theory calculations, coupled with in situ diffuse reflectance infrared Fourier transform spectroscopy, revealed that the plasma-generated OH radicals significantly enhanced the WGS reaction by influencing both the redox and carboxyl reaction pathways.

Kinetics and mechanism study of dyes degradation in electric field-promoting catalytic wet air oxidation process

Wet air oxidation (WAO) is a clean and eco-friendly technology for dyes removal, but the high operating temperature and pressure limit its practical application. In the present work, an electric field-promoting (EF-promoting) catalytic WAO process is developed to degrade dyes under room condition. The oxidation kinetics of four different types of dyes and their degradation pathways are studied. A kinetic model is constructed by including the exogenous electric field into the Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism framework, and quantitative structure-activity relationship (QSAR) analysis is conducted to correlate the kinetic parameters to the physicochemical properties of the dyes. A negative linear relationship is found between the adsorption equilibrium constants of the dyes and their first ionization energies, and their surface reaction rate constants are positively linearly associated to E_sum (E_LUMO + E_HOMO). The degradation pathways of the different dyes are proposed according to the degradation intermediates and the activities of the atoms within the dye molecules. The heteroatoms N and S, and the atom C connecting the aromatic rings are identified as the susceptible sites upon the electrophilic attack of O₂. Bond cleavage at these sites gives rise to aromatic fragments which are eventually mineralized via carboxyl acids. The results of this work is helpful for guiding the design and operation of the EF-promoting catalytic WAO process into the treatment of various dye wastewaters.

Enhanced Effect of an External Electric Field on NH3BH3 Dehydrogenation: an AIMD Study for Thermolysis

How to improve the dehydrogenation properties of ammonia borane (AB, NH₃BH₃) is always a challenge for its practical application in hydrogen storage. In this study, we reveal the enhanced effect of an external electric field (E _ext) on AB dehydrogenation by means of the ab initio molecular dynamics method. The molecular rotation induced by an electrostatic force can facilitate the formation of the H-N···B-H framework, which would aggregate into poly-BN species and further suppress the generation of the volatile byproducts. Meanwhile, the dihydrogen bond (N-H^δ+···^δ-H-B) is favorably formed under E _ext, and the interaction between relevant H atoms is enhanced, leading to a faster H₂ liberation. Correspondingly, the apparent activation energy for AB dissociation is greatly reduced from 18.42 to around 15 kcal·mol^-1 with the application of an electric field, while that for H₂ formation decreases from 20.4 to about 16 kcal·mol^-1. In the whole process, the cleavage of the B-H bond is more favorable than that of the N-H bond, no matter whether the application of E _ext. Our results give a deep insight into a positive effect of an electric field on AB dehydrogenation, which would provide an important inspiration for hydrogen storage in industry applications.

Enhanced activity of catalysts on substrates with surface protonic current in an electrical field - a review

It has over the last few years been reported that the application of a DC electric field and resulting current over a bed of certain catalyst-support systems enhances catalytic activity for several reactions involving hydrogen-containing reactants, and the effect has been attributed to surface protonic conductivity on the porous ceramic support (typically ZrO2, CeO2, SrZrO3). Models for the nature of the interaction between the protonic current, the catalyst particle (typically Ru, Ni, Co, Fe), and adsorbed reactants such as NH3 and CH4 have developed as experimental evidence has emerged. Here, we summarize the electrical enhancement and how it enhances yield and lowers reaction temperatures of industrially important chemical processes. We also review the nature of the relevant catalysts, support materials, as well as essentials and recent progress in surface protonics. It is easily suspected that the effect is merely an increase in local vs. nominal set temperature due to the ohmic heating of the electrical field and current. We address this and add data from recent studies of ours that indicate that the heating effect is minor, and that the novel catalytic effect of a surface protonic current must have additional causes.

Reaction mechanism of low-temperature catalysis by surface protonics in an electric field

The process of combining heterogeneous catalysts and direct current (DC) electric fields can achieve high catalytic activities, even under mild conditions (<500 K) with relatively low electrical energy consumption. Hydrogen production by steam reforming of methane, aromatics and alcohol, dehydrogenation of methylcyclohexane, dry reforming of methane, and ammonia synthesis are known to proceed at low temperatures in an electric field. In situ/operando analyses are conducted using IR, Raman, X-ray absorption fine structure, electrochemical impedance spectroscopy, and isotopic kinetic analyses to elucidate the reaction mechanism for these reactions at low temperatures. The results show that surface proton hopping by a DC electric field, called surface protonics, is important for these reactions at low temperatures because of the higher surface adsorbate concentrations at lower temperatures.

Fractionation of stable oxygen and clumped isotopes during acid digestion of calcite in the presence of an external direct current electric field

RATIONALE

Phosphoric acid digestion of carbonate generates CO₂ for stable oxygen and clumped isotope analysis using a gas source isotope ratio mass spectrometer. The initial step of the digestion reaction is protonation of calcite while the product CO₂ equilibrates with the system allowing further exchange of isotopes to various extents depending on the nature of the acid digestion methods. An external electric field is introduced in the break seal method to demonstrate the role of the protonation reaction and the post-digestion isotopic exchanges in the final isotopic composition of product CO₂ .

METHODS

An acid digestion experiment following the break seal method was conducted at a constant temperature of 25 ± 0.5°C in the presence of a uniform external electric field of 0.5 kV/cm within a specially fabricated corona chamber. Replicate samples of a calcite powder of a reference standard (MAR J1) were reacted for 24 h in the presence and absence of an external electric field for varying exposure times (6 to 24 h) and the evolved CO₂ was analyzed using a dual-inlet MAT 253 isotope ratio mass spectrometer.

RESULTS

The CO₂ yield from the phosphoric acid digestion of MAR J1 calcite was 20% lower during the reaction in presence of an electric field for an exposure time of 24 h, while the corresponding δ¹⁸ O value and Δ₄₇ composition were 0.3‰ and 0.1‰ less, respectively, than without any electric field.

CONCLUSIONS

We documented the systematic control of oxygen and clumped isotope ratios in CO₂ evolved from the carbonate acid digestion reaction with varying exposure time to the external electric field. We provide a new method involving use of an external electric field to manipulate the isotopic fractionation during the acid digestion reaction of calcite. The experimental observation enabled theoretical understanding of the reaction mechanism of carbonate with phosphoric acid which will be useful for stable and clumped isotope studies.

Platinum Based Catalysts in the Water Gas Shift Reaction: Recent Advances

The water gas shift (WGS) is an equilibrium exothermic reaction, whose corresponding industrial process is normally carried out in two adiabatic stages, to overcome the thermodynamic and kinetic limitations. The high temperature stage makes use of iron/chromium-based catalysts, while the low temperature stage employs copper/zinc-based catalysts. Nevertheless, both these systems have several problems, mainly dealing with safety issues and process efficiency. Accordingly, in the last decade abundant researches have been focused on the study of alternative catalytic systems. The best performances have been obtained with noble metal-based catalysts, among which, platinum-based formulations showed a good compromise between performance and ease of preparation. These catalytic systems are extremely attractive, as they have numerous advantages, including the feasibility of intermediate temperature (250–400 °C) applications, the absence of pyrophoricity, and the high activity even at low loadings. The particle size plays a crucial role in determining their catalytic activity, enhancing the performance of the nanometric catalytic systems: the best activity and stability was reported for particle sizes < 1.7 nm. Moreover the optimal Pt loading seems to be located near 1 wt%, as well as the optimal Pt coverage was identified in 0.25 ML. Kinetics and mechanisms studies highlighted the low energy activation of Pt/Mo2C-based catalytic systems (Ea of 38 kJ·mol−1), the associative mechanism is the most encountered on the investigated studies. This review focuses on a selection of recent published articles, related to the preparation and use of unstructured platinum-based catalysts in water gas shift reaction, and is organized in five main sections: comparative studies, kinetics, reaction mechanisms, sour WGS and electrochemical promotion. Each section is divided in paragraphs, at the end of the section a summary and a summary table are provided.

Enhanced methane activation on diluted metal–metal ensembles under an electric field: breakthrough in alloy catalysis

Synergy between an electric field and Pd–Zn alloy allows improved catalytic activities in the steam reforming of methane.

Non-Thermal Plasma Activation of Gold-Based Catalysts for Low-Temperature Water-Gas Shift Catalysis

Non-thermal plasma activation has been used to enable low-temperature water-gas shift over a Au/CeZrO4 catalyst. The activity obtained was comparable with that attained by heating the catalyst to 180 °C providing an opportunity for the hydrogen production to be obtained under conditions where the thermodynamic limitations are minimal. Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), structural changes associated with the gold nanoparticles in the catalyst have been observed which are not found under thermal activation indicating a weakening of the Au-CO bond and a change in the mechanism of deactivation.