Mn-promoted MoS2 catalysts for CO2 hydrogenation: enhanced methanol selectivity due to MoS2/MnOx interfaces

Considering the alarming scenario of climate change, CO2 hydrogenation to methanol is considered a key process for phasing out fossil fuels by means of CO2 utilization. In this context, MoS2 catalysts have recently shown to be promising catalysts for this reaction, especially in the presence of abundant basal-plane sulfur vacancies and due to synergistic mechanisms with other phases. In this work, Mn-promoted MoS2 prepared by a hydrothermal method presents considerable selectivity for CO2 hydrogenation to methanol in comparison with pure MoS2 and other promoters such as K and Co. Interestingly, if CO is used as a carbon source for the reaction, methanol production is remarkably lower, which suggests the absence of a CO intermediate during CO2 hydrogenation to methanol. After optimization of synthesis parameters, a methanol selectivity of 64% is achieved at a CO2 conversion of 2.8% under 180 °C. According to material characterization by X-ray Diffraction and X-ray Absorption, the Mn promoter is present mainly in the form of MnO and MnCO3 phases, with the latter undergoing convertion to MnO upon H2 pretreatment. However, following exposure to reaction conditions, X-ray photoelectron spectroscopy suggests that higher oxidation states of Mn may be present at the surface, suggesting that the improved catalytic activity for CO2 hydrogenation to methanol arises from a synergy between MoS2 and MnOx at the catalyst surface.

Group 10 Metal Allyl Amidinates: A Family of Readily Accessible and Stable Molecular Precursors to Generate Supported Nanoparticles

The synthesis of well-defined materials as model systems for catalysis and related fields is an important pillar in the understanding of catalytic processes at a molecular level. Various approaches employing organometallic precursors have been developed and established to make monodispersed supported nanoparticles, nanocrystals, and films. Using rational design principles, a new family of precursors based on group 10 metals suitable for the generation of small and monodispersed nanoparticles on metal oxides has been developed. Particle formation on SiO2 and Al2O3 supports is demonstrated, as well as the potential in the synthesis of bimetallic catalyst materials, exemplified by a PdGa/SiO2 system capable of hydrogenation of CO2 to methanol. In addition to surface organometallic chemistry (SOMC), it is envisioned that these precursors could also be employed in related applications, such as atomic layer deposition, due to their inherent volatility and relative thermal stability.

Active Site Descriptors from 95Mo NMR Signatures of Silica-Supported Mo-Based Olefin Metathesis Catalysts

The olefin metathesis activity of silica-supported molybdenum oxides depends strongly on metal loading and preparation conditions, indicating that the nature and/or amounts of the active sites vary across compositionally similar catalysts. This is illustrated by comparing Mo-based (pre)catalysts prepared by impregnation (2.5-15.6 wt % Mo) and a model material (2.3 wt % Mo) synthesized via surface organometallic chemistry (SOMC). Analyses of FTIR, UV-vis, and Mo K-edge X-ray absorption spectra show that these (pre)catalysts are composed predominantly of similar isolated Mo dioxo sites. However, they exhibit different reaction properties in both liquid and gas-phase olefin metathesis with the SOMC-derived catalyst outperforming a classical catalyst of a similar Mo loading by ×1.5-2.0. Notably, solid-state ⁹⁵Mo NMR analyses leveraging state-of-the-art high-field (28.2 T) measurement conditions resolve four distinct surface Mo dioxo sites with distributions that depend on the (pre)catalyst preparation methods. The intensity of a specific deshielded ⁹⁵Mo NMR signal, which is most prominent in the SOMC-derived catalyst, is linked to reducibility and catalytic activity. First-principles calculations show that ⁹⁵Mo NMR parameters directly manifest the local strain and coordination environment: acute (SiO-Mo(O)₂-OSi) angles and low coordination numbers at Mo lead to highly deshielded ⁹⁵Mo chemical shifts and small quadrupolar coupling constants, respectively. Natural chemical shift analyses relate the ⁹⁵Mo NMR signature of strained species to low LUMO energies, which is consistent with their high reducibility and corresponding reactivity. The ⁹⁵Mo chemical shifts of supported Mo dioxo sites are thus linked to their specific electronic structures, providing a powerful descriptor for their propensity toward reduction and formation of active sites.

Solution-Processed Cu2S Nanostructures for Solar Hydrogen Production

Cu₂S is a promising solar energy conversion material due to its suitable optical properties, high elemental earth abundance, and nontoxicity. In addition to the challenge of multiple stable secondary phases, the short minority carrier diffusion length poses an obstacle to its practical application. This work addresses the issue by synthesizing nanostructured Cu₂S thin films, which enables increased charge carrier collection. A simple solution-processing method involving the preparation of CuCl and CuCl₂ molecular inks in a thiol-amine solvent mixture followed by spin coating and low-temperature annealing was used to obtain phase-pure nanostructured (nanoplate and nanoparticle) Cu₂S thin films. The photocathode based on the nanoplate Cu₂S (FTO/Au/Cu₂S/CdS/TiO₂/RuO _x ) reveals enhanced charge carrier collection and improved photoelectrochemical water-splitting performance compared to the photocathode based on the non-nanostructured Cu₂S thin film reported previously. A photocurrent density of 3.0 mA cm^-2 at -0.2 versus a reversible hydrogen electrode (V _RHE) with only 100 nm thickness of a nanoplate Cu₂S layer and an onset potential of 0.43 V _RHE were obtained. This work provides a simple, cost-effective, and high-throughput method to prepare phase-pure nanostructured Cu₂S thin films for scalable solar hydrogen production.

Ultrahigh Mass Activity Pt Entities Consisting of Pt Single atoms, Clusters, and Nanoparticles for Improved Hydrogen Evolution Reaction

Platinum is one of the best-performing catalysts for the hydrogen evolution reaction (HER). However, high cost and scarcity severely hinder the large-scale application of Pt electrocatalysts. Constructing highly dispersed ultrasmall Platinum entities is thereby a very effective strategy to increase Pt utilization and mass activities, and reduce costs. Herein, highly dispersed Pt entities composed of a mixture of Pt single atoms, clusters, and nanoparticles are synthesized on mesoporous N-doped carbon nanospheres. The presence of Pt single atoms, clusters, and nanoparticles is demonstrated by combining among others aberration-corrected annular dark-field scanning transmission electron microscopy, X-ray absorption spectroscopy, and electrochemical CO stripping. The best catalyst exhibits excellent geometric and Pt HER mass activity, respectively ≈4 and 26 times higher than that of a commercial Pt/C reference and a Pt catalyst supported on nonporous N-doped carbon nanofibers with similar Pt loadings. Noteworthily, after optimization of the geometrical Pt electrode loading, the best catalyst exhibits ultrahigh Pt and catalyst mass activities (56 ± 3 A mg^-1 _Pt and 11.7 ± 0.6 A mg^-1 _Cat at -50 mV vs. reversible hydrogen electrode), which are respectively ≈1.5 and 58 times higher than the highest Pt and catalyst mass activities for Pt single-atom and cluster-based catalysts reported so far.

Selective ligand exchange synthesis of Au16(2-PET)14 from Au15(SG)13

Replacement of protecting ligands of gold nanoclusters by ligand exchange has become an established post-synthetic tool for selectively modifying the nanoclusters' properties. Several Au nanoclusters are known to additionally undergo size transformations upon ligand exchange, enabling access to cluster structures that are difficult to obtain by direct synthesis. This work reports on the selective size transformation of Au₁₅(SG)₁₃ (SG: glutathione) nanoclusters to Au₁₆(2-PET)₁₄ (2-PET: 2-phenylethanethiol) nanoclusters through a two-phase ligand exchange process at room temperature. Among several parameters evaluated, the addition of a large excess of exchange thiol (2-PET) to the organic phase was identified as the key factor for the structure conversion. After exchange, the nature of the clusters was determined by UV-vis, electrospray ionization-time of flight mass spectrometry, attenuated total reflection-Fourier transform infrared, and extended x-ray absorption fine-structure spectroscopy. The obtained Au₁₆(2-PET)₁₄ clusters proved to be exceptionally stable in solution, showing only slightly diminished UV-vis absorption features after 3 days, even when exposed to an excess of thiol ligands.

The Dynamic Structure of Au38(SR)24 Nanoclusters Supported on CeO2 upon Pretreatment and CO Oxidation

Atomically precise thiolate protected Au nanoclusters Au38(SC2H4Ph)24 on CeO2 were used for in-situ (operando) extended X-ray absorption fine structure/diffuse reflectance infrared fourier transform spectroscopy and ex situ scanning transmission electron microscopy-high-angle annular dark-field imaging/X-ray photoelectron spectroscopy studies monitoring cluster structure changes induced by activation (ligand removal) and CO oxidation. Oxidative pretreatment at 150 °C "collapsed" the clusters' ligand shell, oxidizing the hydrocarbon backbone, but the S remaining on Au acted as poison. Oxidation at 250 °C produced bare Au surfaces by removing S which migrated to the support (forming Au+-S), leading to highest activity. During reaction, structural changes occurred via CO-induced Au and O-induced S migration to the support. The results reveal the dynamics of nanocluster catalysts and the underlying cluster chemistry.

Ligand Migration from Cluster to Support: A Crucial Factor for Catalysis by Thiolate-protected Gold Clusters

Thiolate protected metal clusters are valuable precursors for the design of tailored nanosized catalysts. Their performance can be tuned precisely at atomic level, e. g. by the configuration/type of ligands or by partial/complete removal of the ligand shell through controlled pre-treatment steps. However, the interaction between the ligand shell and the oxide support, as well as ligand removal by oxidative pre-treatment, are still poorly understood. Typically, it was assumed that the thiolate ligands are simply converted into SO2, CO2 and H2O. Herein, we report the first detailed observation of sulfur ligand migration from Au to the oxide support upon deposition and oxidative pre-treatment, employing mainly S K-edge XANES. Consequently, thiolate ligand migration not only produces clean Au cluster surfaces but also the surrounding oxide support is modified by sulfur-containing species, with pronounced effects on catalytic properties.

On the mechanism of rapid metal exchange between thiolate-protected gold and gold/silver clusters: a time-resolved in situ XAFS study

The fast metal exchange reaction between Au₃₈ and Ag_xAu_38-x nanoclusters in solution at -20 °C has been studied by in situ X-ray absorption spectroscopy (time resolved quick XAFS) in transmission mode. A cell was designed for this purpose consisting of a cooling system, remote injection and mixing devices. The capability of the set-up is demonstrated for second and minute time scale measurements of the metal exchange reaction upon mixing Au₃₈/toluene and Ag_xAu_38-x/toluene solutions at both Ag K-edge and Au L₃-edge. It has been proposed that the exchange of gold and silver atoms between the clusters occurs via the SR(-M-SR)_n (n = 1, 2; M = Au, Ag) staple units in the surface of the reacting clusters during their collision. However, at no point during the reaction (before, during, after) evidence is found for cationic silver atoms within the staples. This means that either the exchange occurs directly between the cores of the involved clusters or the residence time of the silver atoms in the staples is very short in a mechanism involving the metal exchange within the staples.

Analysis of the amino acid indospicine in biological samples by high performance liquid chromatography

Indospicine is a hepatotoxic amino acid that accumulates in the meat of horses that consume the legume Indigofera linnaei. A method to determine indospicine concentration in biological samples using an amino acid analyser has been reported, but the analysis time is long and therefore not suited to the analysis of large numbers of samples. A rapid and reliable method was developed for the analysis of indospicine in horsemeat and serum using High Performance Liquid Chromatography. Horsemeat and serum were extracted with either water or 0.01 N hydrochloric acid, respectively, and deproteinized by ultrafiltration. Precolumn derivatization of samples with phenylisothiocyanate was followed by separation of indospicine from other amino acids on a Pico-Tag C 18 column and UV detection at 254 nm. The calibration curves for indospicine in horsemeat extract were linear over the concentration range 0.4 microg ml(-1) to 20 microg ml(-1), while for indospicine in serum, the linear range was from 0.17 microg ml(-1) to 16.67 microg ml(-1). The mean recovery of indospicine in horsemeat extract was 87.2 +/- 6.8% and in serum was 97.3 +/- 9.9%. Analysis time for indospicine in horsemeat samples was 31 min and in serum samples was 36 min.

Batimastat (BB-94) inhibits matrix metalloproteinases of equine laminitis

A method for culturing explants of lamellar hoof was developed to investigate the process of lamellar separation that occurs in laminitis. Explants, consisting of hoof wall, dermal and epidermal lamellae and the adjacent sub-lamellar connective tissue remained intact when cultured in tissue culture medium for 2 days. However, when cultured in the presence of the matrix metalloproteinase (MMP) activator aminophenylmercuric acetate (APMA), the lamellae separated when tension was applied by pulling the hoof wall in an opposite direction to the connective tissue. The separation occurred between the epidermal basal cells and the basement membrane therefore mimicking the lesion of laminitis. Electrophoresis of culture medium from control hoof explants into gradient polyacrylamide gels co-polymerised with gelatin revealed that the explants had produced 2 gelatinases of molecular weight 92 and 72 kDa corresponding to EqMMP-9 and EqMMP-2 respectively. Minor bands of lower molecular weight were the active forms of these enzymes. The zymograms of culture medium from APMA treated explants revealed an increase in the amount of active MMPs. Equine polymorphs cultured for 2 days produced only EqMMP-9. Lamellar explant medium from horses with acute laminitis contained increased amounts of zymogen and active EqMMP-2 and EqMMP-9 particularly in explants from the fore hooves. Zymography of homogenates of normal lamellar hoof tissue revealed only EqMMP-2 and a minor active band. However, homogenates of lamellar tissue from horses with laminitis showed that EqMMP-9 was present as well as increased EqMMP-2 in both zymogen and active forms. Addition of the MMP inhibitor batimastat (BB-94) to the culture medium of APMA treated explants prevented lamellar separation. BB-94 incubated with polyacrylamide strips containing the MMPs from laminitis affected lamellar explants inhibited enzymatic activity at a concentration of 1 mmol/l. It is concluded that activation of MMPs may be responsible for the lamellar separation seen in laminitis and that MMP inhibitors may be useful clinically for preventing this process.

Decreased glucose metabolism causes separation of hoof lamellae in vitro: a trigger for laminitis?

Explants of horses' hooves remained intact for up to 8 days when incubated in Dulbecco's modified Eagle medium (D-MEM) containing 25 mmol/l glucose but separated within 36 h when incubated in saline. The separation occurred between the basal epidermal cells and their basement membrane which is characteristic of the hoof separation that occurs in laminitis. Separation of hoof explants was prevented by addition of glucose to saline and was induced by adding 2-deoxyglucose or aminophenylmercuric acetate to D-MEM. Glucose consumption by the hoof explants was inhibited by 2-deoxyglucose and aminophenylmercuric acetate. The explants consumed relatively large amounts of glucose during the first 2 days of incubation and then little over the next 6 days. Despite the reduced glucose consumption, the hoof explants did not separate over 8 days of incubation. The results indicated that the integrity of the hoof explants was initially dependent on consumption of glucose and provide a possible explanation for the development of laminitis caused by conditions such as carbohydrate overload, acute inflammatory conditions, corticosteroid therapy and hyperlipidaemia. It would be expected that these conditions would induce a major hormonally-mediated metabolic shift away from glucose consumption by many peripheral tissues. It is suggested, therefore, that if the metabolic change occurred faster than the hoof tissue could adapt to an alternative energy substrate, then hoof separation and laminitis would occur.

Effects of the naturally occurring arginine analogues indospicine and canavanine on nitric oxide mediated functions in aortic endothelium and peritoneal macrophages

The ability of the naturally occurring non-protein toxic amino acids indospicine and canavanine to inhibit nitric oxide synthesis was tested in isolated rat aorta and cultured rat peritoneal macrophages. Both compounds inhibited acetylcholine induced relaxation of rat aorta contracted with noradrenaline, a process mediated by nitric oxide generated in vascular endothelium. Nitric oxide is generated in vascular endothelium from arginine by a constitutive nitric oxide synthase. Indospicine and canavanine also increased superoxide mediated reduction of cytochrome c by phorbol myristate acetate stimulated rat peritoneal macrophages. The increase in superoxide under these conditions was due to decreased nitric oxide synthesis. Macrophage synthesis of nitric oxide is mediated by an inducible form of nitric oxide synthase. It is concluded that indospicine and canavanine are inhibitors of constitutive and inducible nitric oxide synthases and it is suggested that the toxicity associated with these compounds could be related to this activity.

The differential effect on two hybrid proteins of deletion mutations within the hydrophobic region of the Escherichia coli OmpA signal peptide

Oligonucleotide-directed site-specific mutagenesis was used to systematically shorten the hydrophobic region within the signal peptide of the Escherichia coli outer membrane protein OmpA. DNA encoding the wild type and mutant OmpA signal peptides were then fused in frame to DNA encoding the mature regions of Staphylococcus aureus nuclease A and TEM beta-lactamase. The ability of these signal peptides to direct processing of the resulting hybrid proteins was dependent on both their length and the protein to which they were fused. Deletion of two or more residues progressively slowed processing of pro-OmpA-nuclease. By contrast, pro-OmpA-beta-lactamase was less sensitive to the length of the hydrophobic region than to the nature of the deleted residue(s). Deletion of an Ala residue tended to reduce processing efficiency of pro-OmpA-beta-lactamase, while deletion of an Ile residue, together with the Ala residue, resulted in improvement. The loss of either 3 or 4 residues abolished processing of both hybrids. These data indicate that both the length as well as the identity of residues in the hydrophobic region are important. The relative importance of these two factors depends on the mature region of the protein being secreted.

Effect of amino acid substitutions at the signal peptide cleavage site of the Escherichia coli major outer membrane lipoprotein

The requirement for the glycine residue at the COOH terminus of the signal peptide of the precursor of the major Escherichia coli outer membrane lipoprotein was examined. Using oligonucleotide-directed site-specific mutagenesis, this residue was replaced by residues of increasing side chain size. Substitution by serine had no effect on the modification or processing of the prolipoprotein. Substitution by valine or leucine resulted in the accumulation of the unmodified precursor, whereas threonine substitution resulted in slow lipid modification and no detectable processing of the lipid modified precursor. The results indicate that serine is the upper limit on size for the residue at the cleavage site. Larger residues at this position prevent the action of both the glyceride transferase and signal peptidase II enzymes, indicating that the cleavage site residue plays a role in events prior to proteolytic cleavage. The upper limit on size of the cleavage site residue is similar to that found for exported proteins cleaved by signal peptidase I, as well as eucaryotic exported proteins. The possibility that the cleavage site residue may have a role other than active site recognition by the signal peptidase is discussed.

A functional prolipoprotein signal peptide with a deletion of four amino acid residues from the hydrophobic region

The deletion of several codons within the signal sequence coding region of the Escherichia coli lipoprotein gene has been accomplished by oligonucleotide-directed site-specific mutagenesis. The deletion of the Leu-13 residue in a mutant in which two glycine residues had previously been deleted from the hydrophobic region (Inouye, S., Vlasuk, G., Hsiung, H., and Inouye, M. (1984) J. Biol. Chem. 259, 3729-3733) was found to cause the accumulation of the unmodified form of the protein in the cytoplasm and cytoplasmic membrane. This mutation also caused a cessation in cell growth within 15 min after synthesis of the mutant protein was induced. A deletion of the Val-7 residue was capable of suppressing the effect of the Leu-13 deletion when both are present. However, by itself the Val-7 deletion appeared to have little effect on the glycine mutant. The ability of the signal sequence to mediate the secretion of the protein after the deletion of 4 residues from the hydrophobic region demonstrates a surprising degree of flexibility in the length of this region. The deletion mutations were also found to have an unusual effect on the rate of synthesis of lipoprotein.

A relationship between hepatic metabolism of reduced lantadene A and its toxicity in rats and sheep

The metabolism of the cholestatic triterpene acid reduced lantadene A has been studied in susceptible and resistant rats and in sheep which are susceptible to intoxication. Sheep and susceptible female rats produced a similar major metabolite and rats produced a second metabolite which was a glucuronide. These metabolites were also observed in extracts of bile canalicular membranes prepared from intoxicated rats. Resistant male and female rats produced a similar major metabolite which was different to those synthesized by susceptible animals. It is concluded that in rats and sheep there is a correlation between the type of metabolites produced in the liver and the susceptibility to intoxication by reduced lantadene A.

Role of primary structure and disulfide bond formation in beta-lactamase secretion

Plasmid pBR322-encoded beta-lactamase was shown to contain a single disulfide bond, which caused the protein to migrate faster in sodium dodecyl sulfate-polyacrylamide gels than the fully reduced form. A similar difference in mobility of the in vitro synthesized precursor before and after reduction indicates that it also contained a disulfide bond. Formation of the disulfide bond in vivo, however, occurred concomitant with processing. In vivo accumulation of the precursor by inhibition of secretion did not allow disulfide bond formation to occur. This result is consistent with post-translational translocation of the precursor. Synthesis of a fragment of beta-lactamase lacking the carboxy terminus was obtained by insertion of a foreign DNA segment into the PstI site of bla. Processing and secretion of the protein did not appear to be greatly affected, indicating that the carboxy terminus is not required for secretion.