Boosting Electrocatalytic Ethylene Epoxidation by Single Atom Modulation

The objective of this work was to investigate the effects of two different oxidizing agents [nitrous oxide (N2O) and air], compared to the reported pure oxygen (O2), on the ethylene epoxidation performance in a low-temperature parallel plate dielectric barrier discharge (DBD) system with two frosted glass plates as the dielectric barrier material under ambient temperature and atmospheric pressure. A separate feed technique, with the ethylene (C2H4) feed position fraction of 0.5, was used to reduce all the undesirable reactions and to maximize the ethylene oxide (EO) production (selectivity and yield). The N2O system with a N2O/C2H4 feed molar ratio of 0.17:1 exhibited the highest ethylene epoxidation performance (80.1% EO selectivity and 21.9% EO yield) with a very low carbon monoxide selectivity and the lowest power consumption, as compared to pure O2 and air as oxidant systems at their respective optimal feed molar ratios of 0.2:1 and 0.1:1, respectively. Interestingly, the EO yield from the DBD system ...

In this paper, the epoxidation of ethylene over different catalysts—namely Ag/(low-surface-area, LSA)α-Al2O3, Ag/(high-surface-area, HSA)γ-Al2O3, and Au–Ag/(HSA)γ-Al2O3—in a low-temperature corona discharge system was investigated. In a comparison among the studied catalysts, the Ag/(LSA)α-Al2O3 catalyst was found to offer the highest selectivity for ethylene oxide, as well as the lowest selectivity for carbon dioxide and carbon monoxide. The selectivity for ethylene oxide increased with increasing applied voltage, while the selectivity for ethylene oxide remained unchanged when the frequency was varied in the range of 300–500 Hz. Nevertheless, the selectivity for ethylene oxide decreased with increasing frequency beyond 500 Hz. The optimum Ag loading on (LSA)α-Al2O3 was found to be 12.5 wt.%, at which a maximum ethylene oxide selectivity of 12.9% was obtained at the optimum applied voltage and input frequency of 15 kV and 500 Hz, respectively. Under these optimum conditions, the power consumption was found to be 12.6 × 10−16 W s/molecule of ethylene oxide produced. In addition, a low oxygen-to-ethylene molar ratio and a low feed flow rate were also experimentally found to be beneficial for the ethylene epoxidation.

Effect of catalyst preparation and storage on chemical looping epoxidation of ethylene

Particle size dependence of ethylene epoxidation rates on Ag/α-Al2O3 catalysts: Why particle size distributions matter

Ethylene oxide (EO), a valuable chemical feedstock in producing many industrial chemicals, which is industrially produced by the partial oxidation of ethylene, so-called ethylene epoxidation, has been of great interest in many global research studies. In this work, the epoxidation of ethylene under a low-temperature dielectric barrier discharge (DBD) was feasibly investigated to find the best operating conditions. It was experimentally found that the EO yield decreased with increasing O2/C2H4 feed molar ratio, feed flow rate, input frequency, and electrode gap distance, while it increased with increasing applied voltage up to 19 kV. The highest EO yield of 5.6% was obtained when an input frequency of 500 Hz and an applied voltage of 19 kV were used, with an O2/C2H4 feed molar ratio of 1:1, a feed flow rate of 50 cm3/min, and an electrode gap distance of 10 mm. Under these best conditions, the power consumption was found to be as low as 6.07 × 10−16 Ws/molecule of EO produced.

Kinetic hydrogen isotope effects in ethylene oxidation on silver catalysts

The indirect electro‐epoxidation of ethylene (C2H4), produced from CO2 electroreduction (CO2R), holds immense promise for CO2 upcycling to valuable ethylene oxide (EO). However, this process currently has a mediocre Faradaic efficiency (FE) due to sluggish formation and rapid dissociation of active species, as well as reductive deactivation of Cu‐based electrocatalysts during the conversion of C2H4 to EO and CO2 to C2H4, respectively. Herein, we report a bromine‐induced dual‐enhancement strategy designed to concurrently promote both C2H4‐to‐EO and CO2‐to‐C2H4 conversions, thereby improving EO generation, using single‐atom Pt on N‐doped CNTs (Pt1/NCNT) and Br−‐bearing porous Cu2O as anode and cathode electrocatalysts, respectively. Physicochemical characterizations including synchrotron X‐ray absorption, operando infrared spectroscopy, and quasi in situ Raman spectroscopy/electron paramagnetic resonance with theoretical calculations reveal that the favorable Br2/HBrO generation over Pt1/NCNT with optimal intermediate binding facilitates C2H4‐to‐EO conversion with a high FE of 92.2 %, and concomitantly, the Br− with strong nucleophilicity protects active Cu+ species in Cu2O effectively for improved CO2‐to‐C2H4 conversion with a FE of 66.9 % at 800 mA cm−2, superior to those in the traditional chloride‐mediated case. Consequently, a single‐pass FE as high as 41.1 % for CO2‐to‐EO conversion can be achieved in a tandem system.

The indirect electro-epoxidation of ethylene (C2 H4 ), produced from CO2 electroreduction (CO2 R), holds immense promise for CO2 upcycling to valuable ethylene oxide (EO). However, this process currently has a mediocre Faradaic efficiency (FE) due to sluggish formation and rapid dissociation of active species, as well as reductive deactivation of Cu-based electrocatalysts during the conversion of C2 H4 to EO and CO2 to C2 H4 , respectively. Herein, we report a bromine-induced dual-enhancement strategy designed to concurrently promote both C2 H4 -to-EO and CO2 -to-C2 H4 conversions, thereby improving EO generation, using single-atom Pt on N-doped CNTs (Pt1 /NCNT) and Br- -bearing porous Cu2 O as anode and cathode electrocatalysts, respectively. Physicochemical characterizations including synchrotron X-ray absorption, operando infrared spectroscopy, and quasi in situ Raman spectroscopy/electron paramagnetic resonance with theoretical calculations reveal that the favorable Br2 /HBrO generation over Pt1 /NCNT with optimal intermediate binding facilitates C2 H4 -to-EO conversion with a high FE of 92.2 %, and concomitantly, the Br- with strong nucleophilicity protects active Cu+ species in Cu2 O effectively for improved CO2 -to-C2 H4 conversion with a FE of 66.9 % at 800 mA cm-2 , superior to those in the traditional chloride-mediated case. Consequently, a single-pass FE as high as 41.1 % for CO2 -to-EO conversion can be achieved in a tandem system.

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

Ethylene epoxidation is catalyzed by α-alumina supported silver catalysts. The influence of silver particle size has been a topic of debate, and was typically investigated without the industrially essential chloride promoter. We studied the catalyst behavior in the presence of chloride. Transient behavior was observed in the first tens of hours on stream, not as a result of particle growth, but due to the gradual change in the nature of the active silver site in the presence of chloride. Different strategies were used to tune the particle size: either varying the silver loading or varying the decomposition atmosphere. Increasing the particle size from 13 to 50 nm by changing the Ag loading from 2 to 15 wt% increased the selectivity from 35 to 80%. However, increasing the 15 wt% Ag particle size from 48 to 184 nm by varying the heat treatment led to a decrease in selectivity from 80 to 50%. Changing the Ag particle size with both strategies also changes the Ag : Al2O3 surface ratio. The ethylene oxide selectivity is actually correlated to the Ag : Al2O3 surface ratio, rather than to the particle size in this size range. This can be explained by its influence on the probability of a formed ethylene oxide molecule to subsequently further react over support surface groups.

Epoxidation of ethylene over silver catalysts supported on α-alumina crystal carriers

Ethylene epoxidation on Ag catalysts supported on non-porous, microporous and mesoporous silicates

The chemical looping epoxidation (CLE) of ethylene was performed over an Ag catalyst supported on strontium ferrite perovskite (SrFeO3-δ). CLE consists of a reduction step in ethylene where oxygen is transferred from the support to the Ag catalyst to form ethylene oxide (EO), and a separate regeneration step, in which the support is reoxidised in air. The effect of altering the reduction and oxidation times was investigated, analysing changes in the conversion of ethylene and selectivity to EO. Experiments were conducted at 270 °C in a packed bed of Ag(15 wt.%)/SrFeO3-δ with a gas hourly space velocity of 9600 h−1 and a total operation time > 40 h. When the time of the reduction step was increased from 1 to 3 min, selectivity to EO only decreased by 0.4%, demonstrating that CLE can run with prolonged reduction times while maintaining high selectivity. Increased duration of the reoxidation step resulted in both selectivity and conversion increasing, but when varying the oxidation time from 10 to 15 min, the overall improvement in the performance was minimal; thus, CLE can feasibly operate at shortened oxidation times. By increasing the pressure during the oxidation step to 2.5 bar, the duration of the reoxidation step was further shortened to 5 min without impacting the CLE performance. With 1.5 min reduction and 5 min reoxidation steps, a CLE installation producing EO in a pseudo-steady manner would require 4 packed bed reactors operating in parallel. The role of Ag2O in the CLE process was also investigated, demonstrating that the oxide was not selective towards EO.

Designing and constructing high-density single-atom catalysts (SACs) is vital for electrochemical hydrogen evolution to meet the demand for fundamental research and practical applications of electrocatalysis. However, it is challenging to synthesize atomically dispersed electrocatalysts with high density and high performance. Herein, an integrated g-C3N4-C-TiO2 heterostructural nanosphere with oxygen-rich vacancies is constructed by a multicomponent assembly-calcination strategy. Abundant single Ru atoms (12.4 wt %) are then anchored via the occupation of partial oxygen vacancies, forming a unique Ru/g-C3N4-C-TiO2 heterostructure. A reasonable configuration is developed including single Ru atoms bonded with two oxygens and two nitrogens and coupled with jacent oxygen vacancies on the g-C3N4-C-TiO2 nanosphere. Density functional theory calculations reveal that the remaining oxygen vacancies are beneficial for water dissociation, while single Ru atoms facilitate hydrogen adsorption. As expected, the result exhibits high electrocatalytic activity, delivering overpotentials of 112 and 107 mV at 10 mA cm-2, Tafel slopes of 83 and 65 mV dec-1 in H2SO4 and KOH, and a turnover frequency of 0.28 H2 s-1 at -100 mV toward the hydrogen evolution reaction (HER). Benefiting from the outstanding electrocatalytic performance, such a unique heterostructure with dense single Ru sites and oxygen vacancies could serve as a prominent alternative HER catalyst for renewable energy applications.

Epoxidation of ethylene by anion radicals of α-oxygen on the surface of FeZSM-5 zeolite