Articles you may be interested inDielectric response of transformer oil based ferrofluid in low frequency range J. Appl. Phys. 114, 034313 (2013); 10.1063/1.4816012 Effect of nanoparticle polarization on relative permittivity of transformer oil-based nanofluids Iron oxide nanoparticles fabricated by electric explosion of wire: focus on magnetic nanofluids AIP Advances 2, 022154 (2012); 10.1063/1.4730405Effect of electron shallow trap on breakdown performance of transformer oil-based nanofluids Transformer oil-based nanofluids with conductive nanoparticle suspensions defy conventional wisdom as past experimental work showed that such nanofluids have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. This paradoxical superior electrical breakdown performance compared to that of pure oil is due to the electron charging of the nanoparticles to convert fast electrons from field ionization to slow negatively charged nanoparticle charge carriers with effective mobility reduction by a factor of about 1 ϫ 10 5 . The charging dynamics of a nanoparticle in transformer oil with both infinite and finite conductivities shows that this electron trapping is the cause of the decrease in positive streamer velocity, resulting in higher electrical breakdown strength. Analysis derives the electric field in the vicinity of the nanoparticles, electron trajectories on electric field lines that charge nanoparticles, and expressions for the charging characteristics of the nanoparticles as a function of time and dielectric permittivity and conductivity of nanoparticles and the surrounding transformer oil. This charged nanoparticle model is used with a comprehensive electrodynamic analysis for the charge generation, recombination, and transport of positive and negative ions, electrons, and charged nanoparticles between a positive high voltage sharp needle electrode and a large spherical ground electrode. Case studies show that transformer oil molecular ionization without nanoparticles cause an electric field and space charge wave to propagate between electrodes, generating heat that can cause transformer oil to vaporize, creating the positive streamer. With nanoparticles as electron scavengers, the speed of the streamer is reduced, offering improved high voltage equipment performance and reliability.

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Transformer oil-based nanofluids with conductive nanoparticle suspensions have been experimentally shown to have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. A comprehensive electrodynamic analysis of the processes which take place in electrically stressed transformer oil-based nanofluids has been developed and a model is presented for streamer formation in transformer oil-based nanofluids. Through the use of numerical simulation methods the model demonstrates that conductive nanoparticles act as electron scavengers in electrically stressed transformer oil-based nanofluids converting fast electrons to slow charged particles. Due to the low mobility of these nanoparticles the development of a net space charge zone at the streamer tip is hindered suppressing the propagating electric field wave that is needed to continue electric field dependent molecular ionization and ultimately streamer propagation further into the liquid. A general expression for the charging dynamics of a nanoparticle in transformer oil with infinite conductivity is derived to show that the trapping of fast electrons onto slow conducting nanoparticles is the cause of the decrease in positive streamer velocity.

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Part of tetrahedral framework aluminum in a protonic mordenite (HMOR) will convert geometry to distorted tetrahedral and octahedral coordination. Highfield 27 Al NMR data show that more framework Al atoms at T 3 and T 4 sites change geometry to nonframework structures than others. These nonframework Al species preferentially reside in the side pockets, which will decrease the accessibility of acid sites in the 8membered ring (MR) channel, impairing the dimethyl ether (DME) carbonylation reaction. The arisen octahedrally coordinated Al species are framework-associated, which can be reverted into the zeolite framework. Herein, we find that a facile treatment with pyridine could force the octahedral coordination Al back into a tetrahedral environment, which could increase the number of available active sites and enhance the diffusion of DME, thus improving the reactivity (4 times) of the DME carbonylation reaction and prolonging the lifetime of catalysts.

Controlling the location of aluminum atoms in a zeolite framework is critical for understanding structure–performance relationships of catalytic reaction systems and tailoring catalyst design. Herein, we report a strategy to preferentially relocate mordenite (MOR) framework Al atoms into the desired T3 sites by low‐pressure SiCl4 treatment (LPST). High‐field 27Al NMR was used to identify the exact location of framework Al for the MOR samples. The results indicate that 73 % of the framework Al atoms were at the T3 sites after LPST under optimal conditions, which leads to controllably generating and intensifying active sites in MOR zeolite for the dimethyl ether (DME) carbonylation reaction with higher methyl acetate (MA) selectivity and much longer lifetime (25 times). Further research reveals that the Al relocation mechanism involves simultaneous extraction, migration, and reinsertion of Al atoms from and into the parent MOR framework. This unique method is potentially applicable to other zeolites to control Al location.

Catalysis research always pursues more efficient catalysts and realizes selectivity-controlled conversion. The local environment of acid sites in a zeolite is regarded as the vital reason for its catalytic selectivity in many chemical reactions. Herein, we have demonstrated that the acid sites in the 12-membered-ring (12-MR) channels of mordenite zeolite could be selectively covered by a trimethylchlorosilane (TMCS) silylation treatment, which could significantly improve the dimethyl ether (DME) carbonylation performance. Detailed mechanism studies by in situ DRIFT, 1H MAS NMR, and FTIR spectra analyses indicate that the TMCS species replace the Brønsted H+ in the bridging hydroxyl groups when chloro groups are rapidly hydrolyzed by the protons, accompanied by the formation of siloxane bonds. Due to the space limitation, the silylation reaction mainly occurred in the 12-MR channels and created most of the remaining acid sites (80%) in the 8-MR channels of the MOR zeolite, which gave better selectivity and a much longer lifetime in the DME carbonylation reaction. This work realizes a conceptual pathway to selectively control the distribution of acid sites within different confinement positions of zeolites to improve the catalytic selectivity of catalysts.