On the basis of the first-principles calculation, single transition-metal (TM) atoms of first and second transition series are anchored on the β 12 phase of the boron monolayer (BM) electrocatalyst for the sustainable production of ammonia (NH 3 ) by reducing nitrogen (N 2 ). We have found a new type of electrocatalyst, i.e., V atom support on the β 12 -BM (V/β 12 -BM), which has a low onset potential (0.28 V), low cost, high stability, high selectivity, and high efficiency under mild conditions. The difference charge density and local density of state further demonstrated that there is an "acceptance−donation" interaction between TM atom and N 2 , and the ionization of 1π orbital of N 2 can greatly elongate the N−N bond length, leading to the activity enhancement of N 2 . We also point out that the bonding orbital (1π) and antibonding orbital (1π*) of N 2 play a crucial role in increasing the activity of N 2 . Further, the climbing nudged elastic band method and molecular dynamics simulation indicate that V/β 12 -BM has high dynamic and thermodynamic stabilities. Moreover, V/β 12 -BM can also effectively suppress the hydrogen evolution reaction during the whole N 2 reduction reaction process. So, we propose V/β 12 -BM as an excellent and promising catalyst for N 2 reduction to NH 3 at ambient conditions.
The influence of thermal treatment temperature on the flowability and wax deposition characteristics of Changqing waxy crude oil was researched in detail through pour point/rheological tests, a cylindrical Couette wax deposition experimental device, DSC analyses, asphaltenes stability tests, and microscopic observations. It is found that the flowability of the crude oil can be greatly improved through increasing the thermal treatment temperature. Meanwhile, the wax deposition rate of the crude oil can be outstandingly inhibited with the increase of thermal treatment temperature from 50 to 70 °C. Moreover, the flow regime can also influence the wax deposition characteristics. Under cold flow regime (22 °C/12 °C), the structure of formed wax deposits is homogeneous while the aging of the wax deposits is not obvious. Under hot flow regime (30 °C/20 °C), the aging of the wax deposits is obvious, but a heterogeneous two-layer structure exists in the formed wax deposits at the thermal treatment temperatures 50 and 60 °C, with a hard/thin bottom layer and a relatively soft/thick surface layer. With the increase of thermal treatment temperature to 70 °C, the two-layer wax deposit structure cannot be identified due to the extremely thin wax deposit thickness. Increasing the thermal treatment temperature can disperse and activate the asphaltenes in the crude oil; the activated asphaltenes have stronger interactions with waxes and then decrease the WAT of the oil and change the precipitated wax crystals’ morphology further, hence dramatically improving the crude oil flowability and inhibiting wax deposition. Under hot flow regime, the two-layer wax deposit structure is mainly triggered through the diffusion of wax molecules and asphaltenes from the bulk oil into the wax deposits. Under cold flow regime, the asphaltenes have already coprecipitated with wax molecules into big wax flocs, which is difficult to diffuse from the bulk oil into the already existed wax deposits. Meanwhile, the aggravated flowability of bulk oil under cold flow regime further hinders the diffusion of wax molecules into the interior of the wax deposit. Therefore, the two-layer wax deposit structure cannot be found under cold flow regime.
Asphaltenes are natural pour point depressants, and the effect of asphaltene polarity on the low-temperature rheology of waxy oils has been well studied. In this paper, the influence of asphaltene polarity on the wax precipitation and deposition characteristics of synthetic waxy oils was investigated through a differential scanning calorimeter (DSC), a rheometer, a polarizing microscope, and a Couette wax deposition experimental device. It was found that the asphaltenes with lower polarity have higher H/C ratio and can disperse into smaller associating particles in the synthetic waxy oils, which is beneficial for the cocrystallization interaction between asphaltenes and wax molecules, and also provide more nucleating sites for wax molecules. With the decrease of asphaltene polarity, the wax crystals of synthetic waxy oils are gradually changed from long needle-like to small needle-like or nearly spherical, which is not conducive to the overlap and insertion of wax crystals, thus greatly improving the rheological properties of synthetic waxy oils. The addition of asphaltenes with lower polarity can significantly reduce the wax deposition rate but increase the aging rate; that is, a thinner and harder wax deposit tends to form with the decrease of asphaltene polarity. Meanwhile, the formed wax deposits become heterogeneous along the radial direction after the asphaltenes addition; the wax contents of the wax deposits increase gradually from the bottom layer to the surface layer. On the one hand, with the decrease of asphaltene polarity, the low-temperature structural strength of synthetic waxy oils is continuously decreased. Therefore, the initial wax deposit layers require higher solid wax contents to resist shearing of pipe flow. On the other hand, the lower viscosity and smaller aspect ratio of wax crystals are also beneficial for the diffusion of wax molecules in the bulk oil and wax deposit, which increases the aging rate of wax deposits. Therefore, with the decrease of asphaltene polarity, the difference of wax content between the surface and bottom wax deposits in the formed heterogeneous wax deposit structure becomes larger and larger.
It is important for environmental protection to search for catalysts with excellent performance and cost-effective to reduce SO2 by CO. In this work, using first-principles calculation, we have studied the catalytic performance of Au5 Mn (M = Ni, Pd, Pt, Cu, Ag, Au; n = 1, 0, −1) clusters, and showed that, by giving a negative charge to the Au5 M (M = Cu, Ag, Au, Pd) clusters, we could improve the selectivity of SO2 and avoid effectively catalyst CO poisoning simultaneously. At the same time, the catalytic reaction rate for the reduction of SO2 by CO with Au5 M − (M = Cu, Ag, Au, Pd) clusters is greatly improved when the Au5 M clusters are charged. These advantages can be well explained by the charge transfer between the clusters and adsorbed molecules, which means that we can effectively control the performance of the catalyst. The equilibrium structures of Au5 Mn (M = Ni, Pd, Pt, Cu, Ag, Au; n = 1, 0, −1) clusters without or with adsorbed SO2 or CO molecule are also discussed, and the most stable geometrical structures of Au5 Mn -ML (ML = SO2, CO, SO, and COS) can be explained very well by the match of orbitals symmetry and density of electron cloud through their frontier molecular orbitals. Considering the catalyst cost (Cu is much cheaper than Ag and Au), selectivity of SO2, and effectively avoiding the catalyst CO poisoning, we propose that Au5Cu− is an ideal catalyst for getting rid of SO2 and CO simultaneously.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.