The search for ideal model systems to investigate the role of different parameters in heterojunction composites for enhanced photocatalysis is a high‐priority target. Herein, a series of ...heterojunction composites, namely Nax‐C3N4/Pt@UiO‐66, being composed of UiO‐66 and Na‐doped g‐C3N4 with adjustable light absorbance and band structures, have been prepared with different Na contents, which exhibit a volcano curve towards photocatalytic H2 production. Benefiting from the interplay of the two critical factors between light harvesting ability and electron transfer efficiency, the optimized Na0.02‐C3N4/Pt@UiO‐66 shows excellent photocatalytic H2 production, far surpassing its corresponding single counterparts.
The heterojunction composites, Nax‐C3N4/Pt@MOF, have been fabricated for photocatalytic H2 production. The optimized activity can be achieved by rationally tuning the light harvesting and band structures of Nax‐C3N4. The results indicate that light response and electron transfer are key factors in heterojunction composites for photocatalysis.
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•Controllable synthesis of MnxFe3-xO4 microspheres for the FTO reaction.•Mn could reduce the size of MnxFe3-xO4 and carbide content during the reaction.•Mn doping expands ferrite ...lattice spacing and stabilizes MnxFe1-xO and ε-Fe2C.
A series of uniform microspheres of spinel-type Mn-doped ferrites (MnxFe3-xO4) with different Fe/Mn ratios were employed for the Fischer-Tropsch-to-olefins reaction (FTO). The highest selectivity of 54.4% and the space-time-yield (STY) of 73.0g·kgcat−1·h−1 to lower olefins (C2-4=) were achieved over an Fe-Mn catalyst with 13 wt% Mn. A panoramic view of structure evolution of MnxFe3-xO4 was revealed using multiple in/ex situ techniques. We demonstrated that the expansion of lattice spacing induced by Mn exerted remarkable effects on the stabilization of MnxFe1-xO and ε-Fe2C during activation and reaction. Moreover, Mn endows Fe catalysts with strong surface basicity and consequently enhances the adsorption and dissociation of CO; while weakening the H availability and readsorption of olefinic intermediates. In addition, the impeded carburization of MnxFe1-xO and the reduction in particle size of catalysts jointly contribute to the volcano curve in the catalytic activity. The insights presented could guide the rational design of high performance catalysts via heteroatom doping.
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•Descriptors for oxidative coupling of methane were investigated.•Density functional theory described the actual catalysis results.•Optimum methyl radical adsorption energy was found ...for good catalysts.•The roles of surface oxygen species were revealed by surface characterizations.
Catalytic descriptors were studied to design optimum catalysts for the oxidative coupling of methane (OCM) by combining density functional theory (DFT) calculations and actual reaction experiments. SrTiO3 perovskite catalysts, selected for OCM, were modified using metal dopants, and their electronic structures were calculated using the DFT method. The CH3 adsorption energy Eads(CH3) and the oxygen vacancy formation energy Ef(vac) exhibited volcano-type correlations with the C2+ selectivity and O2-consumption for the formation of COx, respectively. The optimum catalytic activity, represented by the C2+ selectivity, was obtained for Eads(CH3) = −2.0 to −1.5 eV, indicating that overly strong adsorption of methyl radicals (or easily dissociated CH bonds of methane) and relatively insufficient oxygen supplementation to the catalyst surface improve deep oxidation to CO and CO2. Praseodymium (Pr)- and neodymium (Nd)-doped SrTiO3 catalysts confirm the DFT-predicted optimum electronic structure of the OCM catalysts.
The development of denitrification catalysts which can reduce nitrate and nitrite to dinitrogen is critical for sustaining the nitrogen cycle. However, regulating the selectivity has proven to be a ...challenge, due to the difficulty of controlling complex multielectron/proton reactions. Here we report that utilizing sequential proton–electron transfer (SPET) pathways is a viable strategy to enhance the selectivity of electrochemical reactions. The selectivity of an oxo-molybdenum sulfide electrocatalyst toward nitrite reduction to dinitrogen exhibited a volcano-type pH dependence with a maximum at pH 5. The pH-dependent formation of the intermediate species (distorted Mo(V) oxo species) identified using operando electron paramagnetic resonance (EPR) and Raman spectroscopy was in accord with a mathematical prediction that the pK a of the reaction intermediates determines the pH-dependence of the SPET-derived product. By utilizing this acute pH dependence, we achieved a Faradaic efficiency of 13.5% for nitrite reduction to dinitrogen, which is the highest value reported to date under neutral conditions.
Here, the authors performed density functional theory calculations to study the catalytic performance of the nitric oxide reduction reaction (NORR) via a series of transition metal borides (MBenes). ...This work screened the M2B2 type MBenes from the IVB to V transition metals from the periodic table and systematically probed the catalytic activity and selectivity for the NORR process. It has been reported that Fe2B2, Mn2B2, and Rh2B2 can be high‐performance catalysts for converting NO to NH3 with smaller limiting potentials than other MBenes, and Nb2B2 and Hf2B2 have low limiting potentials of −0.11 V and −0.17 V for the NO production of NH3. The binding energy of ΔG*N can be a good descriptor of catalytic performance and is determined by the volcano plot of the rate‐determining step. The reaction mechanisms for NO reduction to NH3, N2, and N2O have been studied in detail, atomic *N can interact with another *N or one *NO molecule to form N2 and N2O via two successive hydrogenations. In this regard, *NO hydrogenation to *NOH has a lower formation energy than *HNO, and the MBenes have high selectivity for promoting the NORR and suppressing the hydrogen evolution reaction competition process.
Selective nitric oxide electroreduction to ammonia is an efficient pathway to solve the nitrogen oxide air pollution problem; it reveals that highly efficient NORR toward NH3 can be achieved on Ru2B2, Mo2B2, and Cr2B2 MBenes with low limiting potentials. In terms of selectivity, the highly limiting potentials prevent the formation of N2, H2, and N2O.
Computational catalyst screening has the potential to significantly accelerate heterogeneous catalyst discovery. Typically, this involves developing microkinetic reactor models that are based on ...parameters obtained from density functional theory and transition-state theory. To reduce the large computational cost involved in computing various adsorption and transition-state energies of all possible surface states on a large number of catalyst models, linear scaling relations for surface intermediates and transition states have been developed that only depend on a few, typically one or two descriptors, such as the carbon atom adsorption energy. As a result, only the descriptor values have to be computed for various active site models to generate volcano curves in activity or selectivity. Unfortunately, for more complex chemistries the predictability of linear scaling relations is unknown. Also, the selection of descriptors is essentially a trial and error process. Here, using a database of adsorption energies of the surface species involved in the decarboxylation and decarbonylation of propionic acid over eight monometalic transition-metal catalyst surfaces (Ni, Pt, Pd, Ru, Rh, Re, Cu, Ag), we tested if nonlinear machine learning (ML) models can outperform the linear scaling relations in prediction accuracy when predicting the adsorption energy for various species on a metal surface based on data from the rest of the metal surfaces. We found linear scaling relations to hold well for predictions across metals with a mean-absolute error of 0.12 eV, and ML methods being unable to outperform linear scaling relations when the training dataset contains a complete set of energies for all of the species on various metal surfaces. Only when the training dataset is incomplete, namely, contains a random subset of species’ energies for each metal, a currently unlikely scenario for catalyst screening, do kernel-based ML models significantly outperform linear scaling relations. We also found that simple coordinate-free species descriptors, such as bond counts, achieve as good results as sophisticated coordinate-based descriptors. Finally, we propose an approach for automatic discovery of appropriate metal descriptors using principal component analysis.
Extending available body space loading active species and controllably tailoring the d‐band center to Fermi level of catalysts are of paramount importance but extremely challenging for the ...enhancement of electrocatalytic performance. Herein, a melamine‐bridged self‐construction strategy is proposed to in situ embed Co‐based bimetallic nanoparticles in the body of N‐doped porous carbon spheres (CoM‐e‐PNC), and achieve the controllable tailoring of the d‐band center position by alloying of Co and another transition metal M (M = Ni, Fe, Mn, and Cu). The enrichment and exposure of the active sites in the body interior of porous carbon spheres, and the best balance between the adsorption of OH species and the desorption of O2 induced by optimizing the d‐band center position, collectively enhance the oxygen evolution reaction (OER) performance. Meanwhile, the relationship of d‐band center position and OER activity is found to exhibit the volcano curve rule, where the CoNi‐e‐PNC catalyst shows optimal OER performance with an overpotential of 0.24 V at 10 mA cm−2 in alkaline media, outperforming those of the ever‐reported CoNi‐based catalysts. Besides, CoNi‐e‐PNC catalyst also demonstrates high OER stability with slight current decrease after 100 h.
The enrichment and exposure of active sites in the surface and body interior of porous N‐doped carbon spheres, and the best balance between the adsorption of OH species and the desorption of O2 induced by the optimizing d‐band center position, collectively enhance the oxygen evolution reaction activity and stability.
The realization of a hydrogen economy would be facilitated by the discovery of a water-splitting electrocatalyst that is efficient, stable under operating conditions, and composed of earth-abundant ...elements. Density functional theory simulations within a simple thermodynamic model of the more difficult half-reaction, the anodic oxygen evolution reaction (OER), with a single-walled carbon nanotube as a model catalyst, show that the presence of 0.3–1% nitrogen reduces the required OER overpotential significantly compared to the pristine nanotube. We performed an extensive exploration of systems and active sites with various nitrogen functionalities (graphitic, pyridinic, or pyrrolic) obtained by introducing nitrogen and simple lattice defects (atomic substitutions, vacancies, or Stone–Wales rotations). A number of nitrogen functionalities (graphitic, oxidized pyridinic, and Stone–Wales pyrrolic nitrogen systems) yielded similar low overpotentials near the top of the OER volcano predicted by the scaling relation, which was seen to be closely observed by these systems. The OER mechanism considered was the four-step single-site water nucleophilic attack mechanism. In the active systems, the second or third step, the formation of attached oxo or peroxo moieties, was the potential-determining step of the reaction. The nanotube radius and chirality effects were examined by considering OER in the limit of large radius by studying the analogous graphene-based model systems. They exhibited trends similar to those of the nanotube-based systems but often with reduced reactivity due to weaker attachment of the OER intermediate moieties.
We propose a theory for the drying of liquid droplets of surfactant solutions. We show that the added surfactant hinders droplet receding and facilitates droplet spreading, causing a complex behavior ...of the contact line of an evaporating droplet: the contact line first recedes, then advances, and finally recedes again. We also show that the surfactant can change the deposition pattern from mountain-like to volcano-like and then to coffee-ring-like. Specially, when the contact line motion undergoes a clear receding–advancing transition, a two-ring pattern is formed. The mechanism of the two-ring formation is different from the stick–slip mechanism proposed previously and may be tested experimentally.
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