A stable low-temperature H2-production catalyst by crowding Pt on α-MoC.

The water-gas shift (WGS) reaction is an industrially important source of pure hydrogen (H2) at the expense of carbon monoxide and water1,2. This reaction is of interest for fuel-cell applications, but requires WGS catalysts that are durable and highly active at low temperatures3. Here we demonstrate that the structure (Pt1-Ptn)/α-MoC, where isolated platinum atoms (Pt1) and subnanometre platinum clusters (Ptn) are stabilized on α-molybdenum carbide (α-MoC), catalyses the WGS reaction even at 313 kelvin, with a hydrogen-production pathway involving direct carbon monoxide dissociation identified. We find that it is critical to crowd the α-MoC surface with Pt1 and Ptn species, which prevents oxidation of the support that would cause catalyst deactivation, as seen with gold/α-MoC (ref. 4), and gives our system high stability and a high metal-normalized turnover number of 4,300,000 moles of hydrogen per mole of platinum. We anticipate that the strategy demonstrated here will be pivotal for the design of highly active and stable catalysts for effective activation of important molecules such as water and carbon monoxide for energy production.

A manganese-based catalyst system for general oxidation of unactivated olefins, alkanes, and alcohols.

Non-noble metal-based catalyst systems consisting of inexpensive manganese salts, picolinic acid and various heterocycles enable epoxidation of the challenging (terminal) unactivated olefins, selective C-H oxidation of unactivated alkanes, and O-H oxidation of secondary alcohols with aqueous hydrogen peroxide. In the presence of the in situ generated optimal manganese catalyst, epoxides are generated with up to 81% yield from alkenes and ketone products with up to 51% yield from unactivated alkanes. This convenient protocol allows the formation of the desired products under ambient conditions (room temperature, 1 bar) by employing only a slight excess of hydrogen peroxide with 2,3-butadione as a sub-stoichiometric additive.

Initial stage of carbonization of iron during hydrocarbons dissociation: a molecular dynamics study.

The carbonization of iron is a very important early phenomenon in the field of heterogeneous catalysis and the petrochemical industry, but the mechanism is still controversial. In this work, the carbonization mechanism and carbonization structure of iron nanoparticles by different carbon sources (CH4, C2H6, C2H4, C2H2) were systematically investigated using the reactive molecular dynamics method. The results show that saturated alkanes are dehydrogenated while adsorbed, but unsaturated olefins and alkynes undergo bond-breaking while adsorbed. The C-H bond is more likely to break than the C-C bond. Hydrocarbons with high carbon content have a strong ability to carbonize Fe nanoparticles under the same conditions. For C2H4 and C2H2, the C atoms generated from dissociation form a large number of long carbon chains intertwined with branched chains and multiple carbon rings. The C2 species formed by C2H2 after complete dehydrogenation diffuse rapidly to the interior of the nanoparticles, releasing the surface active sites and accelerating the carbonization process. Carbon-rich iron carbides (FeCx) with different Fe/C ratios were obtained by carbonization with different carbon sources. In addition, the Fe(110) surface exhibits the strongest carburizing ability. These findings provide systematic insights into the initial stages of metal Fe carburization.

Low-temperature hydrogen production from water and methanol using Pt/α-MoC catalysts.

Polymer electrolyte membrane fuel cells (PEMFCs) running on hydrogen are attractive alternative power supplies for a range of applications, with in situ release of the required hydrogen from a stable liquid offering one way of ensuring its safe storage and transportation before use. The use of methanol is particularly interesting in this regard, because it is inexpensive and can reform itself with water to release hydrogen with a high gravimetric density of 18.8 per cent by weight. But traditional reforming of methanol steam operates at relatively high temperatures (200-350 degrees Celsius), so the focus for vehicle and portable PEMFC applications has been on aqueous-phase reforming of methanol (APRM). This method requires less energy, and the simpler and more compact device design allows direct integration into PEMFC stacks. There remains, however, the need for an efficient APRM catalyst. Here we report that platinum (Pt) atomically dispersed on α-molybdenum carbide (α-MoC) enables low-temperature (150-190 degrees Celsius), base-free hydrogen production through APRM, with an average turnover frequency reaching 18,046 moles of hydrogen per mole of platinum per hour. We attribute this exceptional hydrogen production-which far exceeds that of previously reported low-temperature APRM catalysts-to the outstanding ability of α-MoC to induce water dissociation, and to the fact that platinum and α-MoC act in synergy to activate methanol and then to reform it.

Atomically Dispersed Ni/α-MoC Catalyst for Hydrogen Production from Methanol/Water.

Methanol-water reforming is a promising solution for H2 production/transportation in stationary and mobile hydrogen applications. Developing inexpensive catalysts with sufficiently high activity, selectivity, and stability remains challenging. In this paper, nickel-supported over face-centered cubic (fcc) phase α-MoC has been discovered to exhibit extraordinary hydrogen production activity in the aqueous-phase methanol reforming reaction. Under optimized condition, the hydrogen production rate of 2% Ni/α-MoC is about 6 times higher than that of conventional noble metal 2% Pt/Al2O3 catalyst. We demonstrate that Ni is atomically dispersed over α-MoC via carbon bridge bonds, forming a Ni1-Cx motif on the carbide surface. Such Ni1-Cx motifs can effectively stabilize the isolated Ni1 sites over the α-MoC substrate, rendering maximized active site density and high structural stability. In addition, the synergy between Ni1-Cx motif and α-MoC produces an active interfacial structure for water dissociation, methanol activation, and successive reforming processes with compatible activity.

Simple mechanisms of CH4 reforming with CO2 and H2O on a supported Ni/ZrO2 catalyst.

To understand the metal-support interaction of oxide supported transition metal catalysts, we computed the reaction mechanisms of dry and steam reforming of methane on a tetragonal ZrO2(101) supported Ni catalyst. Based on the limited number of active sites on the surface, an irregular and non-ideal Ni13 cluster on ZrO2(101) is identified as a catalyst. A simple reaction mechanism is proposed, and the first direct dissociation step of CO2, CH4 and H2O is the most difficult based on the computed Gibbs free energies and no surface CHXO and CHXOH intermediates are involved, different from that on the flat Ni(111) surface. Analysis of other supported nickel catalysts shows that not only the support but also the size and shape of the metal clusters play an important role in the reaction mechanisms and kinetics.

Piezo1 mediates neuron oxygen-glucose deprivation/reoxygenation injury via Ca2+/calpain signaling.

OBJECTIVE: We investigated whether Piezo1 could regulate oxygen-glucose deprivation/reoxygenation injury of neurons through Ca2+/calpain signaling. METHODS: Piezo1 expression in rat brain cortex and PC12 cells were confirmed by immunohistochemistry, immunofluorescence and Western blotting. The effects of Yoda1 and GsMTx4 on OGD/R-induced decrease in cell viability, increase in cell apoptosis and activation of downstreaming Ca2+/calpain signaling were investigated. Furthermore, calpain signaling was inhibited by PD151746 to see whether Ca2+/calpain signaling participated in the neurotoxic effects of Piezo1 activation. RESULTS: Piezo1 expression was increased in rat cerebral cortex after ischemia/reperfusion and in PC12 cells after OGD/R. Activation of Piezo1 by Yoda1 enhanced OGD/R-induced cell viability inhibition, apoptosis, increase intracellular calcium levels and enhanced calpain activity while GsMTx4 showed the opposite effects. The effects of Piezo1 activation on cell viability and apoptosis were reversed by PD151746. CONCLUSION: Piezo1 could regulate neuron oxygen-glucose deprivation/reoxygenation injury via activation of Ca2+/calpain signaling.

Suppression by Pt of CO adsorption and dissociation and methane formation on Fe5C2(100) surfaces.

To understand the chemical origin of platinum promotion effects on iron based Fischer-Tropsch synthesis catalysts, the effects of Pt on CO adsorption and dissociation as well as surface carbon hydrogenation on the Fe5C2(100) facet with different surface C* contents have been studied using the spin-polarized density functional theory method. CO dissociation initiating from diverse sites was calculated through both direct and H-assisted pathways via the CHO intermediate. On the perfect (100) surface, CO can hardly dissociate, and the surface carbon can be facially hydrogenated to CH4. On the C*-defect and C*-free (100) surfaces, CO can strongly adsorb on the C* vacant sites and direct dissociation is favored to occur. The activity is higher with the decrease of the surface carbon content. When platinum atoms are added on the surfaces, the C*-vacancies have a higher activity for CO dissociation than the new sites generated by Pt adsorption. However, both the CO dissociation and the surface carbon consumption through CH4 formation are hindered. The evolution of surface carbon is predicted to be suppressed by the addition of Pt on the Fe5C2(100) surface.

Development of a reactive force field for the Fe-C interaction to investigate the carburization of iron.

The approach of molecular dynamics with Reactive Force Field (ReaxFF) is a promising way to investigate the carburization of iron which is pivotal in the preparation of desired iron-based materials and catalysts. However, it is a challenge to develop a reliable ReaxFF to describe the Fe-C interaction, especially when it involves bond rearrangement. In this work, we develop an exclusive set of Reactive Force Field (ReaxFF) parameters, denoted RPOIC-2017, to describe the diffusion behavior of carbon atoms in the α-Fe system. It inherited some partial parameters in 2012 (ReaxFF-2012) which are suitable for hydrogen adsorption and dissociation. This set of parameters is trained against data from first-principles calculations, including the equations of state of α-Fe, the crystal constant of Fe3C and Fe4C, a variety of periodic surface structures with varying carbon coverages, as well as the barriers of carbon diffusion in the α-Fe bulk and on diverse surfaces. The success in predicting the carbon diffusion coefficient and the diffusion barrier using the developed RPOIC-2017 potential demonstrates that the performance is superior to that of the traditional MEAM potential. The new ReaxFF for the Fe-C interaction developed in this work is not only essential for the design of novel iron based materials, but could also help understand atomic arrangements and the interfacial structure of iron carbides.

Structures of seven molybdenum surfaces and their coverage dependent hydrogen adsorption.

Systematic density functional theory calculations and ab initio atomistic thermodynamics were applied to investigate the stability of seven metallic Mo surfaces [(110), (211), (111), (321), (310), (210) (100)] and their coverage dependent hydrogen adsorption. Only dissociative hydrogen adsorption is favored on these surfaces up to more than one monolayer saturation coverage. The computed hydrogen desorption temperatures on Mo(100) at 500 K and on Mo(110) at 410 K are in agreement with the available temperature-programmed desorption results. Under the consideration of H2 as the reduction reagent in Mo catalyst preparation, the computed surface morphology of Mo single crystal shows only exposed (110), (211) and (100) at high temperature; and the estimated surface proportion order of (110) > (211) > (100) agrees very well with the X-ray diffraction detected intensity order of (110) > (211) > (100). Surface reconstruction upon hydrogen adsorption has also been discussed.

How far away are iron carbide clusters from the bulk?

Combining the basin hopping structure searching algorithm and density functional theory, the iron carbide clusters, FexCy (x ≤ 8 and y ≤ 8), and clusters with various stoichiometries (Fe2nCn, Fe3nCn, FenC2n, FenC3n and FenC4n (n = 1-7), Fe5nC2n, and Fe4nCn (n = 1-5)) are predicted. The stable structures of iron rich carbide clusters are composed of C-C dimers or single C atoms on the surface of the clusters, which are remarkably different from their corresponding bulk structures, where the carbon atoms are atomically distributed in the iron matrix. The most stable carbon rich clusters are highly diverse in topology (bowl, basket, plane, shoe, necklace, etc.) with long carbon chains. The Bader charge analysis shows that the size effect on iron carbide clusters is an electronic tuning. Large carbon-rich clusters appear even under low carbon chemical potentials, whereas small iron-rich clusters are only energetically stable in high carbon chemical potentials, which indicates that changing the carbon chemical potential can tune the morphology (size and stoichiometry) of the iron carbide clusters. These results may help us understand the catalytic activity of iron and iron carbides in reactions such as the Fischer-Tropsch synthesis and the carbon nanotube formation process.

Highly Tunable Selectivity for Syngas-Derived Alkenes over Zinc and Sodium-Modulated Fe5 C2 Catalyst.

Zn- and Na-modulated Fe catalysts were fabricated by a simple coprecipitation/washing method. Zn greatly changed the size of iron species, serving as the structural promoter, while the existence of Na on the surface of the Fe catalyst alters the electronic structure, making the catalyst very active for CO activation. Most importantly, the electronic structure of the catalyst surface suppresses the hydrogenation of double bonds and promotes desorption of products, which renders the catalyst unexpectedly reactive toward alkenes-especially C5+ alkenes (with more than 50% selectivity in hydrocarbons)-while lowering the selectivity for undesired products. This study enriches C1 chemistry and the design of highly selective new catalysts for high-value chemicals.

Coverage dependent adsorption and co-adsorption of CO and H2 on the CdI2-antitype metallic Mo2C(001) surface.

The adsorption and co-adsorption of CO and H2 at different coverage on the CdI2-antitype metallic Mo2C(001) surface termination have been systematically computed at the level of periodic density functional theory. Only molecular CO adsorption is possible and the monolayer coverage (1 ML) can have 16CO adsorbed at the top sites. Dissociative H2 adsorption is favored thermodynamically and the monolayer coverage (1 ML) can have 16H adsorbed at the hollow sites. Since CO has much stronger adsorption energy than H2, pre-adsorption of CO is possible. CO pre-adsorption strongly affects atomic hydrogen co-adsorption at a high CO/H2 ratio, while hardly affects that at a low CO/H2 ratio. Under ultra-high vacuum conditions (200 K, 10(-12) atm and CO/H2 = 1/1), the most stable adsorbed surface state has CO/H2 = 15/1. Comparison among the metallic terminations of the CdI2-antitype Mo2C(001), eclipsed Mo2C(001) and orthorhombic Mo2C(100) surfaces shows their different CO and hydrogen adsorption as well as activation properties, which reveals that the CdI2-antitype Mo2C(001) surface is least active. These differences come from their surface bonding properties; the CdI2-antitype Mo2C(001) surface is saturated and less metallic, while the eclipsed Mo2C(001) and orthorhombic Mo2C(100) surfaces are unsaturated and more metallic.

High coverage adsorption and co-adsorption of CO and H2 on Ru(0001) from DFT and thermodynamics.

The adsorption and co-adsorption of CO and H2 at different coverages on p(4 × 4) Ru(0001) have been computed using periodic density functional theory (GGA-RPBE) and atomistic thermodynamics. Only molecular CO adsorption is possible and the saturation coverage is 0.75 ML (nCO = 12) with CO molecules co-adsorbed at different sites and has a hexagonal adsorption pattern as found by low energy electron diffraction. Only dissociative H2 adsorption is possible and the saturation coverage is 1 ML (nH = 16) with H atoms at face-centered cubic sites. The computed CO and H2 desorption patterns and temperatures agree reasonably with the experiments under ultrahigh vacuum conditions. For CO and H2 co-adsorption (nCO + mH2; n = 1-6 and m = 7, 6, 5, 5, 3, 1), CO pre-coverage affects H adsorption strongly, and each pre-adsorbed CO molecule blocks 2H adsorption sites and H2 does not adsorb on the surface with CO pre-coverage larger than 0.44 ML (nCO = 7); all these are in full agreement with the experiments under ultrahigh vacuum conditions. Our results provide the basis for exploring the mechanisms of catalytic conversion of synthesis gas.

Coverage dependent water dissociative adsorption on Fe(110) from DFT computation.

Using density functional theory calculations and ab initio atomistic thermodynamics, H2O adsorption and dissociation on the Fe(110) p(4 × 4) surface at different coverages have been computed. At the lowest coverage, the adsorbed H2O, OH, O and H species can migrate easily on the surface. For (H2O)n adsorption, H2O molecules donating H atoms for H-bonding adsorb more strongly than those accepting H atoms for H-bonding. Monomeric H2O dissociation is favored both thermodynamically and kinetically. On nO pre-covered Fe(110) surfaces (n = 1-8), H2O dissociation is accessible for nO + H2O (n = 1-7) both kinetically and thermodynamically, while H2O desorption instead of dissociation occurs at n = 8. With the increased number of surface O atoms, H2 dissociative adsorption energies vary in a narrow range for n = 1-4 and decrease for n = 5-7, while at n = 8, the surface does not adsorb H2. At low OH coverage (n = 2, 4), OH groups are perpendicularly adsorbed without H-bonding, while for n≥ 6, adsorbed OH groups are linearly arranged and stabilized by H-bonding. The maximal OH coverage (n = 12) is 0.75 ML and the reasonable O coverage (n = 7) is 0.44 ML, in line with the experiment. The calculated desorption temperatures of H2O and H2 agree well with the available experimental data. These results provide fundamental insights into water-involved reactions catalyzed by iron and interaction mechanisms of water interaction with metal surfaces.

Structures and energies of Cu clusters on Fe and Fe3C surfaces from density functional theory computation.

Spin-polarized density functional theory computations have been carried out to study the stable adsorption configurations of Cun (n = 1-7, 13) on Fe and Fe3C surfaces for understanding the initial stages of copper promotion in catalysis. At low coverage, two-dimensional aggregation is more preferred over dispersion and three-dimensional aggregation on the Fe(110) and Fe(100) surfaces as well as the metallic Fe3C(010) surfaces, while dispersion is more favorable over aggregation on the Fe(111) surface. On the Fe3C(001) and Fe3C(100) surfaces with exposed iron and carbon atoms, the adsorbed Cu atoms prefer dispersion at low coverage, while aggregation along the iron regions at high coverage. On the iron surfaces, the adsorption energies of Cun (n = 2-7) are highest on Fe(111), medium on Fe(100) and lowest on Fe(110). On the Fe3C surfaces, the adsorption energies of Cun (n = 1-3) are highest on Fe3C(100), medium on Fe3C(010) and lowest on Fe3C(001), while, for n = 4-7 and 13, Fe3C(010) has stronger adsorption than Fe3C(100). On the basis of their differences in electronegativity, the adsorbed Cu atoms can oxidize the metallic Fe(110), Fe(100) and Fe3C(010) surfaces and become negatively charged. On the Fe3C(001) and Fe3C(100) surfaces with exposed iron and carbon atoms, the adsorbed Cu atoms interacting with surface carbon atoms are oxidized and positively charged. Unlike the most stable Fe(110) and Fe3C(001) surfaces, where the Fe(110) surface has stronger Cu affinity than the Fe3C(001) surface, which is in agreement with the experimental finding, the less and least stable Fe3C(010) and Fe3C(100) surfaces have stronger Cu affinities than the Fe(110) and Fe(100) surfaces. Since less stable facets are not preferably formed thermodynamically, it is crucial to prepare such surfaces to explore Cu adsorption and promotion, and this provides challenges to surface sciences.

Bird's-Eye View of the Activity Distribution on a Catalyst Surface via a Machine Learning-Driven Adequate Sampling Algorithm.

Rational design of catalysts relies on a deep understanding of the active centers. The structure and activity distribution of active centers on a surface are two of the central issues in catalysis and important targets of theoretical and experimental investigations. Herein, we report a machine learning-driven adequate sampling (MLAS) framework for obtaining a statistical understanding of the chemical environment near catalyst sites. Combined strategies were implemented to achieve highly efficient sampling, including the decomposition of degrees of freedom, stratified sampling, Gaussian process regression, and well-designed constraint optimization. The MLAS framework was applied to the rate-determining step in NH3 synthesis, namely the N2 activation process. We calculated the produced population function, PA, which provides a comprehensive and intuitive understanding of active centers. The MLAS framework can be broadly applied to other more complicated catalyst materials and reaction networks.

Stabilized ε-Fe2C catalyst with Mn tuning to suppress C1 byproduct selectivity for high-temperature olefin synthesis.

Accurately controlling the product selectivity in syngas conversion, especially increasing the olefin selectivity while minimizing C1 byproducts, remains a significant challenge. Epsilon Fe2C is deemed a promising candidate catalyst due to its inherently low CO2 selectivity, but its use is hindered by its poor high-temperature stability. Herein, we report the successful synthesis of highly stable ε-Fe2C through a N-induced strategy utilizing pyrolysis of Prussian blue analogs (PBAs). This catalyst, with precisely controlled Mn promoter, not only achieved an olefin selectivity of up to 70.2% but also minimized the selectivity of C1 byproducts to 19.0%, including 11.9% CO2 and 7.1% CH4. The superior performance of our ε-Fe2C-xMn catalysts, particularly in minimizing CO2 formation, is largely attributed to the interface of dispersed MnO cluster and ε-Fe2C, which crucially limits CO to CO2 conversion. Here, we enhance the carbon efficiency and economic viability of the olefin production process while maintaining high catalytic activity.

Energies and spin states of FeS(0/-), FeS2(0/-), Fe2S2(0/-), Fe3S4(0/-), and Fe4S4(0/-) clusters.

The structures and energies of the electronic ground states of the FeS(0/-), FeS2(0/-), Fe2S2(0/-), Fe3S4(0/-), and Fe4S4(0/-) neutral and anionic clusters have been computed systematically with nine computational methods in combination with seven basis sets. The computed adiabatic electronic affinities (AEA) have been compared with available experimental data. Most reasonable agreements between theory and experiment have been found for both hybrid B3LYP and B3PW91 functionals in conjugation with 6-311+G* and QZVP basis sets. Detailed comparisons between the available experimental and computed AEA data at the B3LYP/6-311+G* level identified the electronic ground state of (5)Δ for FeS, (4)Δ for FeS(-), (5)B2 for FeS2, (6)A1 for FeS2(-), (1)A1 for Fe2S2, (8)A' for Fe2S2(-), (5)A'' for Fe3S4, (6)A'' for Fe3S4(-), (1)A1 for Fe4S4, and (1)A2 for Fe4S4(-). In addition, Fe2S2, Fe3S4, Fe3S4(-), Fe4S4, and Fe4S4(-) are antiferromagnetic at the B3LYP/6-311+G* level. The magnetic properties are discussed on the basis of natural bond orbital analysis.

Copyrolysis of Waste Cartons and Polyolefin Plastics under Microwave Heating and Characterization of the Products.

Municipal organic solid waste contains many recoverable resources, including biomass materials and plastics. The high oxygen content and strong acidity of bio-oil limit its application in the energy field, and the oil quality is mainly improved by copyrolysis of biomass with plastics. Therefore, in this paper, a copyrolysis method was utilized to treat solid waste, namely, common waste cartons and waste plastic bottles (polypropylene (PP) and polyethylene (PE)) as raw materials. The products were analyzed by Fourier transform infrared (FT-IR) spectroscopy, elemental analysis, GC, and GC/MS to investigate the reaction pattern of the copyrolysis. The results show that the addition of plastics can reduce the residue content by about 3%, and the copyrolysis at 450 °C can increase the liquid yield by 3.78%. Compared with single waste carton pyrolysis, no new product appeared in the copyrolysis liquid products but the oxygen content of the liquid decreased from 65% to less than 8%. The content of CO2 and CO in the copyrolysis gas product is 5-15% higher than the theoretical value; the O content of the solid products increased by about 5%. This indicates that waste plastics can promote the formation of l-glucose and small molecules aldehydes and ketones by providing H radicals and reduce the oxygen content in liquids. Thus, copyrolysis improves the reaction depth and product quality of waste cartons, which provides a certain theoretical reference for the industrial application of solid waste copyrolysis.