Self-supported Catalysts Research Articles

Self-magnetic-attracted NixFe(1−x)@NixFe(1−x)O nanoparticles on nickel foam as highly active and stable electrocatalysts towards alkaline oxygen evolution reaction

A facile self-magnetic-attracted approach was developed for highly active and stable NixFe(1−x)@NixFe(1−x)O/NF electrocatalysts towards alkaline oxygen evolution reaction. Firstly, a low-cost and scalable synthesis method was developed to synthesis 4–5 nm hydrophilic NixFe(1−x)@NixFe(1−x)O core–shell nanocrystals with superparamagnetism. Then, these NixFe(1−x)@NixFe(1−x)O nanoparticles (NPs) could be easily supported on nickel foam without any binders or additives. Optimized by the composition effect, the Ni0.7Fe0.3@Ni0.7Fe0.3O/NF exhibits excellent activity for oxygen evolution reaction (OER), requires only 215 mV at 10 mA·cm−2 and 260 mV at 100 mA·cm−2, with a Tafel slope of 47.4 mV·dec−1 in 1.0 M KOH. Moreover, the underlying mechanism was carefully studied by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectra (XANES) analysis and density functional theory (DFT) calculations. Due to the self-magnetic attraction, the catalyst shows outstanding stability throughout the electrocatalysis, surpassing than most self-supported catalysts. This work provides a new strategy for the construction of highly active and stable OER electrocatalysts, the nearly monodisperse magnetic NixFe(1−x)@NixFe(1−x)O NPs also serve an ideal building block for fundamental research of nickel-iron based catalyst.

Atom Probe Tomography for Catalysis Applications: A Review

Atom probe tomography is a well-established analytical instrument for imaging the 3D structure and composition of materials with high mass resolution, sub-nanometer spatial resolution and ppm elemental sensitivity. Thanks to recent hardware developments in Atom Probe Tomography (APT), combined with progress on site-specific focused ion beam (FIB)-based sample preparation methods and improved data treatment software, complex materials can now be routinely investigated. From model samples to complex, usable porous structures, there is currently a growing interest in the analysis of catalytic materials. APT is able to probe the end state of atomic-scale processes, providing information needed to improve the synthesis of catalysts and to unravel structure/composition/reactivity relationships. This review focuses on the study of catalytic materials with increasing complexity (tip-sample, unsupported and supported nanoparticles, powders, self-supported catalysts and zeolites), as well as sample preparation methods developed to obtain suitable specimens for APT experiments.

Post modification of zinc based coordination polymer to prepare Zn‐Mo‐ICP nanoparticles as efficient self‐supported catalyst for olefin epoxidation

Preparation, characterization, and catalytic properties of bimetallic coordination polymer constructed from 2‐aminoterephthalic acid as linker, zinc cations as node, and cis‐dioxo molybdenum units as catalytic active sites are reported via two pathways. Molybdenum centers were placed in N,O positions created by condensation reaction of 2‐aminoterephthalic acid with salicylaldehyde while zinc cations coordinated via carboxylic acid groups of linker to achieve infinite chains of metalo‐ligand. The obtained coordination polymer was fully characterized and its catalytic properties in the epoxidation of olefins with tert‐butyl hydroperoxide (TBHP) described. In comparison with previously reported heterogenized molybdenum catalysts, this new coordination polymer exhibited good conversion as well as high selectivity in the epoxidation of olefins. The catalyst is stable under ambient conditions and could be reused as active catalyst for at least five times.

Self-assembled growth of Pd–Ni sub-microcages as a highly active and durable electrocatalyst

A bottom-up synthesis of Pd35Ni1 sub-microcages was reported as a self-supported catalyst for methanol electro-oxidation, showing excellent activity and durability.

Redox route to ultrathin metal sulfides nanosheet arrays-anchored MnO2 nanoparticles as self-supported electrocatalysts for efficient water splitting

The efficient and sustainable production of high-purity hydrogen gas through electrochemical water splitting calls for robust and bifunctional catalysts to accelerate the two half reactions of water splitting. Herein, we in-situ craft ultrathin CoNi-sulfides nanosheet (∼2.2 nm in thickness) arrays-anchored MnO2 nanoparticles (∼3.2 nm in diameter) (denoted U-CoNi-S-NSA/MnO2) via a spontaneous redox process between CoNi-sulfides nanosheet and MnO4− anions at ambient temperature. The hierarchical U-CoNi-S-NSA/MnO2 nanocomposites are then directly employed as self-supported catalysts for the two half reactions of water splitting, showing excellent activity with small overpotentials of 170 mV for oxygen evolution reaction and 67 mV for hydrogen evolution reaction to achieve 10 mA cm−2, respectively. Moreover, an efficient water electrolyzer through using U-CoNi-S-NSA/MnO2 as both anodic and cathodic catalysts is fabricated, which achieves current density of 10 mA cm-2 at a small voltage of 1.51 V over a long-time operation of 20 h. This outstanding performance is markedly superior than that of precious Pt/C//RuO2 counterpart (1.61 V). Therefore, the as-synthesized hierarchical nanocomposites are promising candidates for cheap and efficient water splitting.

Co-Ag alloy protected by nitrogen doped carbon as highly efficient and chemoselective catalysts for the hydrogenation of halogenated nitrobenzenes

The design of lower-cost alternative heterogeneous catalysts for the hydrogenation of halogenated nitrobenzenes using green method to synthesize the corresponding anilines is highly desirable. In this study, Ag was incorporated into the Co-MOFs during the growing process (Co-Ag(n)-MOFs), and then followed the carbothermal reduction process without any additional procedures, we synthesized a series of Co-Ag(n)@NCs. The self-supported catalysts exhibited excellent and stable catalytic performances for the chemoselective hydrogenation of halogenated nitrobenzenes without obvious dehalogenation. The Co-Ag bimetallic alloy nanoparticles were well-dispersed and protected from aggregation and leaching by the porous nitrogen doped carbon. Besides, either hydrazine hydrate (N2H4·H2O, generating byproducts N2 and H2O) or H2 could be used as green reducing agent with excellent selectivity towards synthesizing the corresponding anilines. And when the Co/Ag content ratio was approximate 1:1, the Co-Ag(1:1)@NC showed the best catalytic performance. Moreover, the Co-Ag(1:1)@NC could be efficiently recovered by using an external magnetic force and reused without obvious decrease of catalytic activity. Thus, such highly efficient, inexpensive, stable and magnetically recyclable catalysts could show great potentials in practical applications for many important reactions.

Nano-structured platinum group metal-free catalysts and their integration in fuel cell electrode architectures

The novel platinum group metal-free (PGM-free) catalyst for the oxygen reduction reaction (ORR) is synthesized by a modified sacrificial support method (SSM). The catalyst chemical/surface composition is studied by X-ray photoelectron spectroscopy, and the morphology of the material is observed using both HR-SEM and HR-TEM, demonstrating the open-frame, self-supported catalysts. This new catalyst’s electrochemical performance is evaluated by polarization curves and has behaviour comparable to the state-of-the-art PGM-free catalysts. Meso-structure imaging shows pores on the order of 100nm, the mean size of the individual silica particles in the sacrificial support. For the first time, PGM-free catalyst layer (CL) morphology in a membrane electrode assembly (MEA) is studied in detail by combined nano- and micro X-ray computed tomography (CT) and interpretational modelling. The highly inhomogeneous, high-tortuosity, through-thickness structure of the CL is observed with micro-CT. The nano-CT method for these thick PGM-free electrodes is not sufficient to capture the full through-thickness morphology of these electrodes. Water retention curves suggest water pooling at the MEA components’ interfaces and significant dependence of capilary pressure and saturation on through-thickness location. This study is the first of its kind to identify morphology-dependent transport losses in the thick PGM-free electrodes using scale-bridging between meso-, micro-, and macro.

Ordered mesoporous copper oxide nanostructures as highly active and stable catalysts for aqueous click reactions

We demonstrate that ordered mesoporous CuO with gyroid mesostructures can be used as highly efficient, stable, and recyclable self-supported catalysts for click reactions in aqueous media.

Isomorphic MOFs functionalized by free-standing acylamide and organic groups serving as self-supported catalysts for the CO2 cycloaddition reaction

MOFs exhibited different catalytic activities depending on the organic groups decorated on the pore walls, which displayed a synergetic catalytic effect.

Robust Iridium Coordination Polymers: Highly Selective, Efficient, and Recyclable Catalysts for Oxidative Conversion of Glycerol to Potassium Lactate with Dihydrogen Liberation

Along with the rapid expansion of the biodiesel industry to deal with the world energy crisis, inexpensive glycerol is also produced in large scale as the main byproduct in biodiesel production via transesterification. Much attention has been paid to the development of environmentally benign technologies for the transformation of glycerol to valuable DL-lactic acid and its derivatives. Herein, a series of NHC-Ir coordination polymers were readily synthesized via reaction of some structurally rigid bis-benzimidazolium salts with iridium precursors under alkaline conditions and were successfully applied as robust self-supported catalysts in the oxidative dehydrogenation of glycerol to potassium lactate with dihydrogen liberation. Extremely high activity and selectivity were attained in open air under the mild reaction conditions even with ppm-level loadings of the catalysts, which were readily recovered after reaction by simple filtration and reused for up to 31 runs without obvious loss of activity or selectivity. Probably owing to the effective suppression of inactive binuclear iridium species in a homogeneously catalyzed reaction, the catalysts assembled via self-supported strategy exhibited high selectivity and productivity for potassium lactate, with up to 1.24 × 105 turnover numbers (TON) being attained even in large-scale reactions of neat glycerol at an elevated temperature. The high catalytic activity, recyclability, and scalability of the robust self-supported catalysts highlight their potential toward the development of practical technologies for transformation of glycerol to value-added chemicals.

Noble-metal-free Fe-N/C catalyst for highly efficient oxygen reduction reaction under both alkaline and acidic conditions.

In this work, we report the synthesis and assessment of a new non-precious-metal oxygen reduction reaction (ORR) catalyst from pyrolysis of an iron-coordinated complex which manifests superior activity in both alkaline and acidic media. 11,11'-bis(dipyrido[3,2-a:2',3'-c]phenazinyl) (bidppz) was selected as a ligand for the formation of a nitrogen-rich iron-coordinated coordination polymer (Fe-bidppz) which forms a self-supporting catalyst containing high densities of nitrogen and iron doping by pyrolysis. The catalyst pyrolyzed at 800 °C (Fe-N/C-800) shows the highest ORR activity with onset and half-wave potentials of 923 and 809 mV in 0.1 M KOH, respectively, which are comparable to those of Pt/C (half-wave potential 818 mV vs RHE) at the same catalyst loading. Besides, the Fe-N/C-800 catalyst has an excellent ORR activity with onset and half-wave potentials only 38 and 59 mV less than those of the Pt/C catalyst in 0.1 M HClO4. The optimal Fe-N/C-800 catalyst displays much greater durability and tolerance of methanol than Pt/C. We propose that the Fe-N/C-800 catalyst has a considerably high density of surface active sites because Fe-N/C-800 possesses excellent ORR activity while its specific surface area is not so high. Electrochemical measurements show that the Fe-N/C-800 catalyst in KOH and HClO4 follows the effective four-electron-transfer pathway.

Experimental and Theoretical Properties of MoS<sub>2+x</sub> Nanoplatelets

The synthesis and the catalysis in the HDS of DBT reaction of nanostructured self-supported catalyst containing MoS2+x nanoplatelets have been investigated. Enhancement of higher activity observed in sulfide catalyst sample (d) with respect to the ex situ and in situ references is more closely related to the morphology change of particles (nanoplatelets). In this work, we suggest that certain structures present in model catalysts maybe related to low dimensional structures and present a theoretical study of two MoS2 clusters (one made of 34 atoms/cluster and the second one made of 41 atoms/cluster), to these clusters seven sulfur atoms were randomly located at the surface of the sulfur layer, in order to simulate certain structures resembling arrow shaped nanoplatelets that were found in a High Resolution TEM analysis performed in some MoS2 samples. Additionally, one of the goals is to enquire about the electronic properties presented in such structures when the clusters terminated as Moor S-edge and if it could be correlated to the catalyst behavior of these compounds. To the 34 atoms/cluster Mo-edge yielded metallic behavior while the second cluster the 41 atoms/ cluster S-edge yielded a semiconductor behavior with a forbidden energy gap Eg of the order of @ 3.6 eV between the Valence and Conductions bands respectively. Moreover, to the same clusters enunciated formerly, when the sulfur atoms were located at the surface of the S-layer, for the first cluster (34 atoms/cluster) yielded a more metallic behavior, while the second one (41 atoms/cluster) yielded an isolator behavior. Our results agree with the experimental and theoretical results presented by several groups in different laboratories arriving to the conclusion that the S-Mo-S Mo-edge arrow heads structures could be responsible to the enhancement of the catalytic activity on the MoS2 studied samples.

Self-supported Chiral Catalysts for Heterogeneous Asymmetric Catalysis

A conceptually new strategy for chiral catalyst immobilization, self-supported catalysts, in heterogeneous asymmetric catalysis is highlighted in the present article. Various homochiral metal-organic polymers with diverse structures have been designed and readily prepared through coordination assembly of modular polytypic/polyfunctional ligands and metal ions without using any supports. These polymers have been successfully employed as chiral catalysts in a variety of heterogeneous asymmetric ractions, including hydrogenation, epoxidation, sulfoxidation, carbonyl-ene reactions, diethylzinc addition, and Michael addition. The self-supported heterogeneous chiral catalysts showed activity and enanioselectivity comparable or even superior to those obtained with their corresponding homogeneous counterparts, and could be readily recovered and reused several times without significant loss of activity or enantioslectivity.

A Study on Applications of N-Substituted Main-Chain NHC-Palladium Polymers as Recyclable Self-Supported Catalysts for the Suzuki–Miyaura Coupling of Aryl Chlorides in Water

The preparation and characterization of a number of main-chain organometallic polymers (NHC-Pd MCOP) with different N-alkyl substituted groups such as benzyl (3a), n-hexyl (3b), and n-dodecyl (3c) are described. Among these polymers, 3c bearing the more lipophilic group n-dodecyl was found to be a more reactive and recoverable catalytic system in the Suzuki-Miyaura cross-coupling reaction of chloroarenes, including both deactivated and hindered aryl chlorides with different types of arylboronic acids under aqueous conditions. While the catalysts seem to be highly recyclable, on the contrary, we have provided much compelling evidence, such as kinetic monitoring, poisoning experiments, and average molecular weight determination before and after catalysis, that shows that the described organometallic polymers might be indeed the source of production of active soluble Pd species in the form of either Pd nanoparticles or fragmented NHC-Pd complexes. Our studies showed that in order to assess whether the catalysts are functioning in a heterogeneous pathway or they are simply a source of production of active Pd species, it is crucial to devise a suitable and highly efficient poison that could capture essentially soluble catalytic species. In this regard, we interestingly found that among a variety of well-known catalyst poisons such as Hg(0), SBA-15-PrSH, and cross-linked poly(4-vinylpyridine) (PVP), only PVP could efficiently quench catalysis, thus providing clear evidence of the formation soluble Pd species in our protocol. In addition, several experiments such as bright-field microscopy, dynamic light scattering (DLS) of the reaction mixture, and kinetic monitoring of the reaction at an early stage confirm not only that the described organometallic polymers could be a source of production of trace amounts of Pd nanoparticles but the capsular structures of these lipophilic polymers in water provides a means of entrapment of nanoclusters in a hydrophobic region, thus accelerating the reaction in pure water in the absence of any co-organic solvent.

Recent developments in solar water-splitting photocatalysis

Abstract

Au/Pt and Au/Pt3Ni nanowires as self-supported electrocatalysts with high activity and durability for oxygen reduction

Novel Au/Pt and Au/Pt(3)Ni nanostructures consisting of Pt and Pt(3)Ni alloy nanodendrites grown on Au nanowires were synthesized, which exhibited high electrocatalytic activity and durability toward oxygen reduction when used as self-supported catalysts.

Pd2L2 metallacycles as molecular containers for small molecules

M(2)L(2) type metallacyclic complexes, [Pd(2)(L1)(2)Cl(4)]·1.5CH(2)Cl(2) (1), [Pd(2)(L1)(2)Cl(4)]·2CHCl(3) (2), [Pd(2)(L2)(2)Cl(4)]·2CH(2)Cl(2)·2CH(3)CN (3), [Pd(2)(L2)(2)Cl(4)]·2CHCl(3)·2CH(3)CN (4) and [Pd(2)(L3)(2)Cl(4)]·CH(2)Cl(2)·2CH(3)CN (5), have been prepared from three semi-rigid benzimidazol or benzotriazol ligands, 1,4-bis(benzimidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L1), 1,4-bis(5,6-dimethylbenzimidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L2) and 1,4-bis(benzotriazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L3). All the complexes were structurally characterized by single-crystal X-ray diffraction and the phase purity was confirmed by powder X-ray diffraction (PXRD) measurements. The solution structure of representative complex 1 was studied by (1)H NMR titration and ESI mass spectroscopy. The thermal stability and guest-exchange properties of 1, 3 and 4 were investigated, revealing that the Pd(2)L(2) metallacycles can act as a selective receptor for CH(2)Cl(2) or CHCl(3) guest molecules. The catalytic activity of 1 in Suzuki-Miyaura coupling reaction was also studied and 1 could be recycled at least 5 times under heterogeneous conditions, indicative of a potential self-supported catalyst.

Effects of external donors and hydrogen concentration on oligomer formation and chain end distribution in propylene polymerization with Ziegler‐Natta catalysts

AbstractThe effect of type and concentration of external donor and hydrogen concentration on oligomer formation and chain end distribution were studied. Bulk polymerization of propylene was carried out with two different Ziegler‐Natta catalysts at 70 °C, one a novel self‐supported catalyst (A) and the other a conventional MgCl2‐supported catalyst (B) with triethyl aluminum as cocatalyst. The external donors used were dicyclopentyl dimethoxy silane (DCP) and cyclohexylmethyl dimethoxy silane (CHM). The oligomer amount was shown to be strongly dependent on the molecular weight of the polymer. Catalyst A gave approximately 50 % lower oligomer content than catalyst B due to narrower molecular weight distribution in case of catalyst A. More n‐Bu‐terminated chain ends were found for catalyst A indicating more frequent 2,1 insertions. Catalyst A also gave more vinylidene‐terminated oligomers, suggesting that chain transfer to monomer, responsible for the vinylidene chain ends, was a more important chain termination mechanism for this catalyst, especially at low hydrogen concentration. Low site selectivity, due to low external donor concentration or use of a weak external donor (CHM), was also found to increase formation of vinylidene‐terminated oligomers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 351–358, 2010

Development of a Continuous‐Flow System for Asymmetric Hydrogenation Using Self‐Supported Chiral Catalysts

Well-designed, self-assembled, metal-organic frameworks were constructed by simple mixing of multitopic MonoPhos-based ligands (3; MonoPhos=chiral, monodentate phosphoramidites based on the 1,1'-bi-2-naphthol platform) and [Rh(cod)(2)]BF(4) (cod=cycloocta-1,5-diene). This self-supporting strategy allowed for simple and efficient catalyst immobilization without the use of extra added support, giving well-characterized, insoluble (in toluene) polymeric materials (4). The resulting self-supported catalysts (4) showed outstanding catalytic performance for the asymmetric hydrogenation of a number of alpha-dehydroamino acids (5) and 2-aryl enamides (7) with enantiomeric excess (ee) ranges of 94-98 % and 90-98 %, respectively. The linker moiety in 4 influenced the reactivity significantly, albeit with slight impact on the enantioselectivity. Acquisition of reaction profiles under steady-state conditions showed 4 h and 4 i to have the highest reactivity (turnover frequency (TOF)=95 and 97 h(-1) at 2 atm, respectively), whereas appropriate substrate/catalyst matching was needed for optimum chiral induction. The former was recycled 10 times without loss in ee (95-96 %), although a drop in TOF of approximately 20 % per cycle was observed. The estimation of effective catalytic sites in self-supported catalyst 4 e was also carried out by isolation and hydrogenation of catalyst-substrate complex, showing about 37 % of the Rh(I) centers in the self-supported catalyst 4 e are accessible to substrate 5 c in the catalysis. A continuous flow reaction system using an activated C/4 h mixture as stationary-phase catalyst for the asymmetric hydrogenation of 5 b was developed and run continuously for a total of 144 h with >99 % conversion and 96-97 % enantioselectivity. The total Rh leaching in the product solution is 1.7 % of that in original catalyst 4 h.

Synthesis of hard mesoporous macro-spheres with silicate and aluminosilicate compositions

This study reports the synthesis mechanism and the influence of different variables in the preparation of mesoporous macro-spheres having silicate and aluminosilicate compositions. The spheres possess sizes in the range 200–1000 μm with a narrow particle size distribution and a significant mechanical resistance (hard macro-spheres). In addition, the presence of a highly regular mesoporosity (around 3 nm) and of a high surface area (normally about 1000 m2/g) make these materials as very interesting self-supported adsorbents or catalysts that could be directly applied without no binder, which is a great advantage compared to materials in powder form. The synthesis of the hard mesoporous spheres takes place in a biphasic system, generated by the use of tetrabutoxysilane as silica source, which is reacted with water and NaOH leading to the formation of small primary particles which initially do not present any mesoscopic ordering. However, after about 5 h of synthesis, the presence of cetyltrimethylammonium chloride, as surfactant, gives rise to the detection of a mesostructured silica material. Subsequently, after 9 h of synthesis the small mesostructured particles, with sizes in the micrometer range, acquire a spherical shape. Finally, the reorganization and fusion of these particles cause the formation of the silica macrospheres, with a particle size of several hundreds of micrometers. When the synthesis is carried in the presence of an aluminium source, the aluminosilicate materials so obtained exhibit smaller surface area and pore volume and greater particle size than the pure silica ones.