Molten-Salt Electrochemical-Assisted Synthesis of the CeO2-OV@GC Composite-Supported Pt Clusters with a Pt-O-Ce Structure for the Oxygen Reduction Reaction.

Highly active and robust Pt-based electrocatalysts for an oxygen reduction reaction (ORR) are of crucial significance for the development of proton exchange membrane fuel cells (PEMFCs). Herein, the high-loading and well-dispersive Pt clusters on graphitic carbon-supported CeO2 with abundant oxygen vacancies (PtAC/CeO2-OV@GC) were successfully fabricated by a molten-salt electrochemical-assisted method. The bonding of Pt with the highly electronegative O induces charge redistribution through the Pt-O-Ce structure, thus reducing the adsorption energies of oxygen-containing species. Such a PtAC/CeO2-OV@GC electrocatalyst exhibits a greatly enhanced ORR performance with a mass activity of 0.41 ± 0.02 A·mg-1Pt at 0.9 V versus a reversible hydrogen electrode, which is 2.7 times the value of a commercial Pt/C catalyst and shows negligible activity decay after 20000 cycles of accelerated degradation tests. It is anticipated that this work will provide enlightening guidance on the controllable synthesis and rational design of high-performance Pt-based electrocatalysts for PEMFCs.

Reactivity of surface oxygen vacancy sites and frustrated Lewis acid-base pairs of In2O3 catalysts in CO2 hydrogenation.

The effects of oxygen vacancy (VO) formation energy and surface frustrated Lewis acid-base pairs (SFLPs) on the CO2 hydrogenation activity of In2O3 catalysts were studied using density functional theory calculations. The VO formation energy of 2.8-3.3 eV was found to favor HCOO formation, whereas the presence of SFLPs is conducive to CO formation.

Molten-Salt Electrochemical Deoxidation Synthesis of Platinum-Neodymium Nanoalloy Catalysts for Oxygen Reduction Reaction.

Platinum-rare earth metal (Pt-RE) nanoalloys are regarded as a potential high performance oxygen reduction reaction (ORR) catalyst. However, wet chemical synthesis of the nanoalloys is a crucial challenge because of the extremely high oxygen affinity of RE elements and the significantly different standard reduction potentials between Pt and RE. Here, this paper presents a molten-salt electrochemical synthetic strategy for the compositional-controlled preparation of platinum-neodymium (Pt-Nd) nanoalloy catalysts. Carbon-supported platinum-neodymium (Ptx Nd/C) nanoalloys, with distinct compositions of Pt5 Nd and Pt2 Nd, are obtained through molten-salt electrochemical deoxidation of platinum and neodymium oxide (Pt-Nd2 O3 ) precursors supported on carbon. The Ptx Nd/C nanoalloys, especially the Pt5 Nd/C exhibit a mass activity of 0.40 A mg-1 Pt and a specific activity of 1.41 mA cm-2 Pt at 0.9 V versus RHE, which are 3.1 and 7.1 times higher, respectively, than that of commercial Pt/C catalyst. More significantly, the Pt5 Nd/C catalyst is remarkably stable after undergoing 20 000 accelerated durability cycles. Furthermore, the density-functional-theory (DFT) calculations confirm that the ORR catalytic performance of Ptx Nd/C nanoalloys is enhanced by compressive strain effect of Pt overlayer, causing a suitable weakened binding energies of O* Δ E O ∗ $\Delta {E}_{{{\rm{O}}}^*}$ and Δ E OH ∗ $\Delta {E}_{{\rm{OH}}^*}$ .

Computational screening of single-atom doped In2O3 catalysts for the reverse water gas shift reaction.

The reverse water gas shift (RWGS) reaction is an important method for converting carbon dioxide (CO2) into valuable chemicals and fuels by hydrogenation. In this paper, the catalytic activity of single-atom metal-doped (M = Pt, Ir, Pd, Rh, Cu, Ni) indium oxide (c-In2O3) catalysts in the cubic phase for the RWGS reaction was investigated using density functional theory (DFT) calculations. This was achieved by identifying metal sites, screening oxygen vacancies, followed by further calculating the energy barriers for the direct and indirect dissociation pathways of the RWGS reaction. Our results show that the single-atom dopant in the indium oxide lattice promotes the creation of oxygen vacancies on the In2O3 surface, thereby facilitating the adsorption and activation of CO2 by the oxide surface and initiating the subsequent RWGS reaction. Furthermore, we find that the oxygen vacancy (OV) formation energy on the surface of the single-atom metal doped c-In2O3(111) surface can be used as a descriptor for CO2 adsorption, and the higher the OV formation energy, the more stable the CO2 adsorption structure is. The Cu/In2O3 structure has relatively high energy barriers for both direct (1.92 eV) and indirect dissociation (2.09 eV) in the RWGS reaction, indicating its low RWGS reactivity. In contrast, the Ir/In2O3 and Rh/In2O3 structures are more conducive to the direct dissociation of CO2 into CO, which may serve as more efficient RWGS catalysts. Furthermore, microkinetic simulations show that single atom metal doping to In2O3 enhances CO2 conversion, especially under high reaction temperatures, where the formation of oxygen vacancies is the limiting factor for CO2 reactivity on the M/In2O3 (M = Cu, Ir, Rh) models. Among these three single-atom catalysts, the Ir/In2O3 model was predicted to have the best CO2 reactivity at reaction temperatures above 573 K.

DFT-based microkinetic studies on methanol synthesis from CO2 hydrogenation over In2O3 and Zr-In2O3 catalysts.

Density functional theory (DFT) calculations and microkinetic simulations were performed to study the structure-performance relationship of In2O3 and Zr-doped In2O3 catalysts for methanol synthesis, focusing on the In2O3(110) and Zr-doped In2O3(110) surfaces. These surfaces are expected to follow the oxygen vacancy-based mechanism via the HCOO route for CO2 hydronation to methanol. Our DFT calcualtions show that the Zr-In2O3(110) surface is more favorable for CO2 adsorption than the In2O3(110) surface, and although the energy barriers are not lowered, most intermediates in the HCOO route are stablized with the introduction of the Zr dopant. Microkinetic simulations suggest that the CH3OH formation rate is improved by ∼10 times and CH3OH selectivity increased significantly from 10% on In2O3(110) to 100% on the Zr1-In2O3(110) catalyst model at 550 K. We find that the higher CH3OH formation rate and CH3OH selectivity on the Zr1-In2O3(110) surface than those on the In2O3(110) surface can be attributed to the slightly increased OV formation energy and the stablization of the reaction intermediates, whereas the much lower CH3OH formation rate on the Zr3-In2O3(110) surface is due to the much higher OV formation energy and the over binding of the H2O at the OV site.

Highly Selective Photoelectroreduction of Carbon Dioxide to Ethanol over Graphene/Silicon Carbide Composites.

Using sunlight to produce valuable chemicals and fuels from carbon dioxide (CO2 ), i.e., artificial photosynthesis (AP) is a promising strategy to achieve solar energy storage and a negative carbon cycle. However, selective synthesis of C2 compounds with a high CO2 conversion rate remains challenging for current AP technologies. We performed CO2 photoelectroreduction over a graphene/silicon carbide (SiC) catalyst under simulated solar irradiation with ethanol (C2 H5 OH) selectivity of>99 % and a CO2 conversion rate of up to 17.1 mmol gcat -1 h-1 with sustained performance. Experimental and theoretical investigations indicated an optimal interfacial layer to facilitate the transfer of photogenerated electrons from the SiC substrate to the few-layer graphene overlayer, which also favored an efficient CO2 to C2 H5 OH conversion pathway.

Water interaction with B-site (B = Al, Zr, Nb, and W) doped SrFeO3-δ-based perovskite surfaces for thermochemical water splitting applications.

Density functional theory (DFT) calculations were performed to study the interaction of water with the SrO and FeO2 terminations of the SrFeO3-δ (001) surface, where the effects of the metal dopants (Al, Zr, Nb, and W), surface oxygen vacancies, and oxygen ion migration were investigated. Our calculations showed that the metal dopants benefited the molecular and dissociative adsorptions of H2O on both the perfect and oxygen-vacancy-containing surfaces. The surface oxygen vacancies were predicted to promote the dissociative adsorption of H2O and the formation of H2. For all structures studied, H2 release was found to be always an overall endothermic process, except for the W-doped structure which will become exothermic at high temperature. On the oxygen-vacancy-containing surface, H2 generation was predicted to be easier at the SrO termination than the FeO2 termination. Furthermore, we also investigated the oxygen ion migration mechanism on all surface structures, predicted the behaviour of oxygen migration and the effect of oxygen vacancy defects. Our results showed that Al doping facilitated not only the formation of surface oxygen vacancies, but also oxygen migration from the surface to the subsurface, in contrast to the Zr, Nb and W-doped structures. This study provided significant insights into the interaction of water with the surfaces of doped SrFeO3-δ perovskite materials for thermochemical water splitting applications.

Tandem Catalysis for Selective Oxidation of Methane to Oxygenates Using Oxygen over PdCu/Zeolite.

Selective oxidation of methane to oxygenates with O2 under mild conditions remains a great challenge. Here we report a ZSM-5 (Z-5) supported PdCu bimetallic catalyst (PdCu/Z-5) for methane conversion to oxygenates by reacting with O2 in the presence of H2 at low temperature (120 °C). Benefiting from the co-existence of PdO nanoparticles and Cu single atoms via tandem catalysis, the PdCu/Z-5 catalyst exhibited a high oxygenates yield of 1178 mmol g-1 Pd h-1 (mmol of oxygenates per gram Pd per hour) and at the same time high oxygenates selectivity of up to 95 %. Control experiments and mechanistic studies revealed that PdO nanoparticles promoted the in situ generation of H2 O2 from O2 and H2 , while Cu single atoms not only accelerated the activation of H2 O2 for the generation of abundant hydroxyl radicals (⋅OH) from H2 O2 decomposition, but also enabled the homolytic cleavage of CH4 by ⋅OH to methyl radicals (⋅CH3 ). Subsequently, the ⋅OH reacted quickly with the ⋅CH3 to form CH3 OH with high selectivity.

A DFT-based microkinetic study on methanol synthesis from CO2 hydrogenation over the In2O3 catalyst.

In this work, we performed density functional theory (DFT)-based microkinetic simulations to elucidate the reaction mechanism of methanol synthesis on two of the most stable facets of the cubic In2O3 (c-In2O3) catalyst, namely the (111) and (110) surfaces. Our DFT calculations show that for both surfaces, it is difficult for the H atom adsorbed at the remaining surface O atom around the O vacancy (Ov) active site to migrate to an O adsorbed at the Ov due to the very high energy barrier involved. In addition, we also find that the C-O bond in the bt-CO2* chemisorption structure can directly break to form CO with a lower energy barrier than that in its hydrogenation to the COOH* intermediate in the COOH route. However, our microkinetic simulations suggest that for both surfaces, CO2 deoxygenation to form CO in both pathways, namely the COOH and CO-O routes, are kinetically slower than methanol formation under typical steady state conditions assuming a CO2 conversion of 10% and a CO selectivity of 1%. Although these results agree with previous experimental observations at relatively low reaction temperature, where methanol formation dominates, they cannot explain the predominant formation of CO at relatively high reaction temperature. We tentatively attribute this to the simplicity of our microkinetic model as well as possible structural changes of the catalyst at relatively high reaction temperature. Furthermore, although the rate-determining step (RDS) from the degree of rate control (DRC) analysis is usually consistent with that judged from the DFT calculated energy barriers, for CO2 hydrogenation to methanol over the (111) surface, our DRC analysis suggests homolytic H2 dissociation to be the rate-controlling step, which is not apparent from the DFT-calculated energy barriers. This indicates that CO2 conversion and methanol selectivity over the (111) surface can be further enhanced if homolytic H2 dissociation can be accelerated for instance by introducing transition metal dopants as already shown by some experimental observations.

Catalytic cycle of the partial oxidation of methane to methanol over Cu-ZSM-5 revealed using DFT calculations.

Density functional theory (DFT) calculations were performed to investigate the catalytic cycle of methane conversion to methanol over both [Cu2(O2)]2+ and [Cu2(µ-O)]2+ active sites in the Cu-ZSM-5 catalyst. The [Cu2(O2)]2+ site is found to be active for the partial oxidation of methane to methanol, and although it has a higher energy barrier in the methane activation step, it involves a very low energy barrier in the methanol formation step (36.3 kJ mol-1) as well as a lower methanol desorption energy (52.5 kJ mol-1). As the [Cu2(O2)]2+ active site is also thermodynamically stable, it may play an important role during methane conversion to methanol. Furthermore, the methane activation step follows the homolytic route and the heterolytic route for the [Cu2(O2)]2+ and [Cu2(µ-O)]2+ active sites, respectively, whereas the methanol formation step follows the direct radical rebound mechanism and the indirect rebound mechanism, respectively. Our calculations further indicate that the electronic properties of the reactive O atoms in the active site influence their reactivity toward methane oxidation. More specifically, the higher the spin density and the more negative the charge of the reactive O atom at the active site are, the lower the energy barrier for methane activation will be; and the more negative the charge of the hydroxyl group in the reaction intermediate during the partial oxidation of methane to methanol is, the higher energy barrier of the methanol formation step will be in the triplet state. Furthermore, we used a larger cluster model to predict the mechanism of the methane activation step and the effect of atomic charge of the O atom at the [Cu2(µ-O)]2+ and [Cu2(O2)]2+ active sites on the energy barriers of partial oxidation of methane to methanol, and the conclusions drawn employing the larger cluster model are consistent with those drawn using the smaller double-5T-ring cluster model. In addition, different from the traditional mechanism for methane activation at [Cu2(O2)]2+, which consists of two transition states, we find that the partial oxidation of methane at [Cu2(O2)]2+ can also occur via a single step by direct insertion of one of the O atoms at the active site into the C-H bond of methane.

First-principles studies of oxygen ion migration behavior for different valence B-site ion doped SrFeO3-δ ceramic membranes.

Density functional theory calculations were performed to investigate the structural, electronic, and oxygen ion migration properties of B-site ion doped SrFeO3-δ perovskite (B = Al, Zr, Nb, and W) materials, which were used as oxygen transport membranes (OTMs) for pure oxygen output and catalytic reactions. The results of our calculations indicate that the Fe-O bond length increased and the M-O bond length decreased with the doping of Zr, Nb, and W. And the doping of Al caused the valence state of Fe ions to increase. The states near the Fermi level were mainly contributed by Fe atoms and O atoms. The strength of the Fe-O bond gradually weakened with the increase in the valence of the doped ions. Through studying the oxygen vacancy defect and the mechanism of oxygen ion migration, it was found that the doping of Al promoted the migration of oxygen ions, while the doping of Zr, Nb, and W limited the migration of oxygen ions. This study provides important insights into the behavior of oxygen ion migration in doped SrFeO3-δ perovskite materials.

Novel enhanced GFP-positive congenic inbred strain establishment and application of tumor-bearing nude mouse model.

Transgenic GFP gene mice are widely used. Given the unique advantages of immunodeficient animals in the field of oncology research, we aim to establish a nude mouse inbred strain that stably expresses enhanced GFP (EGFP) for use in transplanted tumor microenvironment (TME) research. Female C57BL/6-Tg(CAG-EGFP) mice were backcrossed with male BALB/c nude mice for 11 generations. The genotype and phenotype of novel inbred strain Foxn1nu .B6-Tg(CAG-EGFP) were identified by biochemical loci detection, skin transplantation and flow cytometry. PCR and fluorescence spectrophotometry were performed to evaluate the relative expression of EGFP in different parts of the brain. Red fluorescence protein (RFP) gene was stably transfected into human glioma stem cells (GSC), SU3, which were then transplanted intracerebrally or ectopically into Foxn1nu .B6-Tg(CAG-EGFP) mice. Cell co-expression of EGFP and RFP in transplanted tissues was further analyzed with the Live Cell Imaging System (Cell'R, Olympus) and FISH. The inbred strain Foxn1nu .B6-Tg(CAG-EGFP) shows different levels of EGFP expression in brain tissue. The hematological and immune cells of the inbred strain mice were close to those of nude mice. EGFP was stably expressed in multiple sites of Foxn1nu .B6-Tg(CAG-EGFP) mice, including brain tissue. With the dual-fluorescence tracing transplanted tumor model, we found that SU3 induced host cell malignant transformation in TME, and tumor/host cell fusion. In conclusion, EGFP is differentially and widely expressed in brain tissue of Foxn1nu .B6-Tg(CAG-EGFP), which is an ideal model for TME investigation. With Foxn1nu .B6-Tg(CAG-EGFP) mice, our research demonstrated that host cell malignant transformation and tumor/host cell fusion play an important role in tumor progression.

First principles investigation of dissociative adsorption of H2 during CO2 hydrogenation over cubic and hexagonal In2O3 catalysts.

Dissociative adsorption of molecular hydrogen (H2) and migration of the adsorbed H adatom as a hydride or a proton on two of the most stable facets of the In2O3 catalyst in the cubic (c-In2O3) and hexagonal (h-In2O3) phases for CO2 hydrogenation to methanol were investigated by extensive density functional theory (DFT) calculations. Due to the relatively small variation in the oxygen vacancy (Ov) formation energy with respect to surface Ov coverage of these In2O3 surfaces, especially c-In2O3(110) and h-In2O3(012), no limit on the Ov coverage is expected to exist under the typical reaction conditions. Thus, we consider three scenarios for the dissociative adsorption of H2 and the migration of the H adsorbate, namely on the perfect (stoichiometric) In2O3 surfaces, on the partially reduced In2O3 surfaces with a single Ov site, and on the fully reduced In2O3 surfaces. Our DFT calculations show that the oppositely charged In and O pair sites on the perfect and partially reduced In2O3 surfaces facilitate the heterolytic dissociation of H2, leading to the formation of the anionic hydride at the In site crucial for CO2 hydrogenation to methanol. Our comparative studies on the four In2O3 surfaces suggest that the h-In2O3(104) surface is superior to the other three surfaces due to the facile formation of the Ov sites at low coverage and the favorable formation of the hydride adsorbate at the In site from H2 dissociation. These results indicate that this surface might be preferred for CO2 hydrogenation to methanol.

Mechanistic studies on millerite chlorination with ammonium chloride.

Millerite (NiS) is the main source for metallurgical production of nickel worldwide. To improve the extraction rate of nickel, chlorination is usually carried out, as the resulting nickel chloride (NiCl2) can easily dissolve in water and be separated. Although molecular chlorine (Cl2) can be used as the chlorination reagent, greener reagents such as ammonium chloride (NH4Cl) are preferable from a process design perspective. However, the efficiency of NH4Cl as a chlorination reagent must be further improved for this process to be viable for industrial applications, and mechanistic understanding is imperative to this end. Here, we performed extensive density functional theory (DFT) calculations to elucidate the chlorination mechanism of NiS by exploring three possible pathways. We first considered the direct chlorination of NiS by Cl2, which was suggested to form by the reaction between NH4Cl and SO3 catalyzed by a metal oxide. Alternatively, NH4Cl was found to react favorably with the partially or fully oxidized NiS surface in the presence of oxygen (O2). During the oxidation of NiS, sulfur dioxide (SO2) may form. Furthermore, sulfur or oxygen vacancy was predicted to form during the chlorination of NiS or NiO with NH4Cl. Based on the available experimental evidence and our computational results, three possible mechanisms for the chlorination of NiS using NH4Cl as the chlorination reagent in the presence of O2 were proposed.

Mechanistic insights into higher alcohol synthesis from syngas on Rh/Cu single-atom alloy catalysts.

Low cost Cu-based catalysts are attractive options in catalyzing higher alcohol synthesis (HAS) from syngas. Introducing isolated Rh single atoms into the surfaces of these Cu catalysts has the potential to dramatically improve the performance of these Cu-based catalysts. In this work, extensive density functional theory (DFT) calculations were performed with periodic slab models to systematically investigate the possibility of using Rh/Cu single-atom alloys (SAAs) as HAS catalysts. The mechanism of ethanol synthesis from syngas on the representative Rh/Cu(111) and Rh/Cu(100) surfaces was elucidated. All possible formation pathways of the C1 and C2 fragments leading to the ethanol main product, as well as the methane and methanol by-products were considered. Our calculations show that for ethanol formation, the C-C bond coupling is easier over the Rh/Cu SAA catalysts than pure Cu catalysts, suggesting that Rh/Cu SAA catalysts are more favorable for the formation of higher alcohols.

Correction: Understanding of binding energy calibration in XPS of lanthanum oxide by in situ treatment.

Correction for 'Understanding of binding energy calibration in XPS of lanthanum oxide by in situ treatment' by Jerry Pui Ho Li et al., Phys. Chem. Chem. Phys., 2019, 21, 22351-22358.

Gamma-Ray Irradiation to Accelerate Crystallization of Mesoporous Zeolites.

Gamma-ray (γ-ray) irradiation was introduced into zeolite synthesis. The crystallization process of zeolite NaA, NaY, Silicalite-1, and ZSM-5 were greatly accelerated. The crystallization time of NaA zeolite was significantly decreased to 18 h under γ-ray irradiation at 20 °C, while more than 102 h was needed for the conventional process. Unexpectedly, more mesopores were created during this process, and thus the adsorption capacity of CO2 increased by 6-fold compared to the NaA prepared without γ-ray irradiation. Solid experimental evidence and density function theory (DFT) calculations demonstrated that hydroxyl free radicals (OH*) generated by γ-rays accelerated the crystallization of zeolite NaA. Besides NaA, mesoporous ZSM-5 with MFI topology was also successfully synthesized under γ-ray irradiation, which possessed excellent catalytic performance for methanol conversion, suggesting the universality of this new synthetic strategy for various zeolites.

Understanding of binding energy calibration in XPS of lanthanum oxide by in situ treatment.

Rare earth oxides have seen increased usage over the years in batteries and catalysts. Due to their unique electronic properties, they are the subject of fundamental and practical interest. However, the complexity in their electronic structures makes unambiguous characterization, such as X-ray photoelectron spectroscopy (XPS), very challenging. Lanthanum oxide (La2O3) has attracted special attention as a promising catalyst for the oxidative coupling of methane (OCM) reaction. In this work, a new and reliable way of XPS calibration is developed by applying various in situ preparations for a nanorod La2O3 catalyst to intentionally form different lanthanum compounds, followed by XPS characterization and corroboration with first principles calculations. To form different compounds, five sample treatments were performed including heating in vacuum and treatment with O2, CH4, CO2, and H2O, which are all relevant to OCM reaction conditions. Adventitious carbon or lattice oxygen, as conventional calibration standard species for energy scale, is only suitable for one or few in situ prepared surfaces. Our results also clearly demonstrate the vital difference between performing the ex situ analysis after exposure of the sample to the atmosphere and the in situ analysis. By carefully comparing the spectra of various photoemission peaks of different compounds, we conclude that the binding energy of 102.2 eV for the La 4d7/2 peak can be used as the internal calibration standard for all considered samples. Furthermore, different oxygen species were unambiguously identified by matching the oxygen 1s binding energies from the in situ measurements and first principles predictions.

Direct Production of Higher Oxygenates by Syngas Conversion over a Multifunctional Catalyst.

Selective synthesis of higher oxygenates (linear α-alcohols and α-aldehydes, C 2 + OH) from syngas is highly attractive but remains challenging owing to the low C 2 + OH selectivity and low catalytic stability. Herein we introduce a multifunctional catalyst composed of CoMn and CuZnAlZr oxides that dramatically increased the oxygenates selectivity to 58.1 wt %, where more than 92.0 wt % of the produced oxygenates are C 2 + OH. Notably, the total selectivity to value-added chemicals including oxygenates and olefins reached 80.6 wt % at CO conversion of 29.0 % with high stability. The appropriate component proximity can effectively suppress the formation of the undesired C1 products, and the selectively propulsion of reaction network by synergetic effect of different components contributes to the enhanced selectivity to higher oxygenates. This work provides an alternative strategy for the rational design of new catalysts for direct conversion of syngas into higher oxygenates with co-production of olefins.

Guest-regulated chirality switching of planar chiral pseudo[1]catenanes.

pseudo[1]Catenane 3 is in a self-included conformation in chloroform, but in dichloromethane, it exists in an equilibrium between the self-included conformational state and a de-threading one. The planar chirality inversion of 3 can be triggered by the host-guest complexation of 3 with adiponitrile G, but the extent of such chiral switching depends on the length of self-included bis(pyrazin-2-yloxy)alkane chains in 3 - longer chains are more favored than shorter ones in the inversion.