Co2 FeGe Heusler Alloy Nanoparticle Catalysts for Propyne Hydrogenation and Ammonia Decomposition.

Heusler alloys (X2 YZ) can be a candidate for new catalysts as well as other intermetallic compounds. We previously found good catalytic properties of Co2 FeGe for selective hydrogenation of alkynes and developed nanoparticles of Co2 FeGe supported on SiO2 . However, the average diameter of the nanoparticles was 23 nm, which is not small enough compared to those of state-of-the-art nanoparticle catalysts. In this study, we developed SiO2 -supported Co2 FeGe nanoparticles of <10 nm in diameter. A catalytic test for selective hydrogenation of propyne indicated a partial formation of sites with low selectivity including excess Co atoms. For ammonia decomposition, enhancement of turnover frequency was achieved by reducing the particle size.

Synthesis of Polynorbornadiene within the Pores of Activated Carbons: Effects on EDLC and Hydrogen Adsorption Performances.

Norbornadiene (NBD) is adsorbed on activated carbon (AC), and the adsorbed NBD is polymerized within the pores of AC. Two kinds of ACs─AC-2 with only micropores of ∼2 nm and AC-4 with not only micropores but also mesopores below 4 nm─are examined to study the effects of the hybridized polynorbornadiene (PNBD) on the electric double-layer capacitor and hydrogen adsorption performance. Various measurements are performed to determine the form of the hybridized PNBD inside the pores of AC. Scanning and transmittance electron microscopy observations of the AC/PNBD hybrids confirm that PNBD is hybridized inside the pores of AC, and there is little PNBD on the surface of AC particles. The nitrogen adsorption/desorption measurement for the hybrids of AC-4 reveals that PNBD is not hybridized preferentially inside micropores rather than mesopores irrespective of the amount of PNBD. In addition, both micropore and mesopore volumes decrease at a constant rate with increasing amounts of PNBD. These results suggest that PNBD is hybridized not as a layer but as an agglomerate for both ACs, and the agglomerate delocalizes over the whole AC pores, which is supported by the results of electrochemical measurements and hydrogen adsorption behavior of the hybrids.

Synthesis of Co2FeGe Heusler alloy nanoparticles and catalysis for selective hydrogenation of propyne.

Although intermetallic compounds are attracting attention of catalysis researchers, ternary intermetallic catalysts have scarcely been investigated due the difficulty of synthesizing supported nanoparticles. In this study, we successfully synthesized SiO2 supported Co2FeGe Heuslar alloy nanoparticles. This catalyst exhibited high catalytic performance for selective hydrogenation of propyne by nano-sizing.

High catalytic performance of Al-Pd-(Ru, Fe) icosahedral approximants for acetylene semi-hydrogenation.

Three cubic crystalline icosahedral approximants (C phase: Al72.0Pd16.4Fe11.6, P40 phase: Al72.0Pd16.4Ru11.6, P20 phase: Al70.0Pd22.3Ru7.7) exhibit high ethylene selectivity of over 90% for hydrogenating acetylene at 150 °C. Moreover, the powdered P20 also demonstrates a high catalytic performance under an industry-like ethylene feed containing 0.5% acetylene as an impurity. Overall, icosahedral approximants in the Al-Pd-(Ru, Fe) systems are promising as a novel class of alloy catalysts.

Au3Si4- and Au4Si4: Electronically Equivalent but Different Polarity Superatoms.

A series of AuxSi4- cluster anions (x = 1-4) were generated most abundantly by laser ablation of a Au4Si alloy target. Photoelectron spectroscopy and density functional theory (DFT) calculation of AuxSi4- (x = 1-4) revealed that Au3Si4- can be viewed as an electronically closed superatom and is composed of a Si4 unit whose three adjacent edges of a single facet are bridged by three Au atoms. Such phase-segregated structure is facilitated by aurophilic interaction between the three Au atoms and results in a large permanent dipole moment (4.43 D). DFT calculations on an electronically equivalent superatom Au4Si4 predicted a new structure in which the uncoordinated Si atom of Au3Si4- is bonded by Au+. This Au4Si4 is much more stable than a cubic structure previously reported and has a large HOMO-LUMO gap (1.68 eV) and a small permanent dipole moment (0.41 D).

Ability of hydrogen storage CeNi5-x Ga x and Mg2Ni alloys to hydrogenate acetylene.

Hydrogen storage properties and reactivity for hydrogenation of acetylene in a series of CeNi5-x Ga x (x = 0, 0.5, 0.75, 1, 1.25, 1.5) alloys and Mg2Ni were determined and compared. The structure of CeNi5 (CaCu5 type) was maintained up to CeNi3.5Ga1.5 when Ni was replaced by Ga. The replacement facilitated hydrogenation absorption by creating larger interstitial spaces through expansion of the lattice, allowing CeNi4.25Ga0.75 to absorb the greatest proportion of hydrogen atoms among the alloys under the same conditions. The results showed that the absorbed hydrogen in CeNi3.75Ga1.25 improved reactivity. In contrast, Mg2Ni formed a hydride upon hydrogenation of acetylene and thus possessed much lower activity. The difference of the activity of absorbed hydrogen between CeNi5-x Ga x and Mg2Ni was confirmed from transient response tests under reaction gases alternately containing He and H2.

Enhanced catalytic activity of inhomogeneous Rh-based solid-solution alloy nanoparticles.

Catalytic Rh-based alloy nanoparticles (NPs) with inhomogeneous solid-solution structures were prepared from homogeneous solid-solution alloy NPs. Compared with homogeneous alloy NPs, these inhomogeneous alloy NPs exhibited enhanced catalytic activity and superior catalytic durability. Homogeneous solid-solution alloy NPs consisting of Rh and other immiscible noble metals were synthesized by laser-induced nucleation method in metallic ion solutions. STEM elemental mapping and EDS composition analysis of the particles clearly demonstrated that all the constituents were uniformly dispersed within the NPs. Moreover, the compositions of the alloys were nearly identical to the initial feeding ratios of metallic ions in the mixed solutions, strongly indicating the formation of equimolar solid-solution alloy NPs over the entire composition range. Although the catalytic stability of these Rh-based homogeneous alloy NPs during CO oxidation was improved, their catalytic activity was comparable to that of pure metal catalysts, owing to the uniform local structure at the atomic level. However, the catalytic activity of the alloy NPs was enhanced by heat treatment, which introduced inhomogeneity in the atomic distribution within the NPs. The enhanced activity was due to dissimilar interfaces in the inhomogeneous solid-solution alloy NPs.

Catalytic Properties of Heusler Alloys for Steam Reforming of Methanol.

Intermetallic compounds have attracted research attention in catalysis because of their unique catalytic properties. Recently, a group of intermetallic compounds, referred to as Heusler alloys (X2YZ), has been investigated as new catalysts. In this study, catalytic properties of 14 Heusler alloys with X = Fe, Co, Ni, or Cu; Y = Ti, Mn, or Fe; Z = Al, Si, Ga, Ge, or Sn for the steam reforming of methanol were examined. Co2TiAl and Ni2TiAl alloys exhibited relatively high H2 production rates because of the formation of fine particles via the selective oxidation of Ti. X2MnZ alloys exhibited high CO2 selectivity because of a water-gas shift reaction catalyzed by using MnO that was formed during the reaction. Crystal phases, surface microstructures, and surface compositions of most alloys were changed because of the reaction, and the formation of fine particles possibly assisted in the observation of catalytic activity. Heusler alloys can be beneficial as catalyst precursors by the selection of appropriate elemental sets depending on target reactions.

Catalysis-tunable Heusler alloys in selective hydrogenation of alkynes: A new potential for old materials.

Heusler alloys (X 2 YZ) are well-established intermetallic compound materials in various fields because their function can be precisely adjusted by elemental substitution (e.g., X 2 YZ 1-x Z' x ). Although intermetallic compound catalysts started attracting attention recently, catalysis researchers are not familiar with Heusler alloys. We report their potential as novel catalysts focusing on the selective hydrogenation of alkynes. We found that Co2MnGe and Co2FeGe alloys have great alkene selectivity. Mutual substitution of Mn and Fe (Co2Mn x Fe1-x Ge) enhanced the reaction rate without changing selectivity. The substitution of Ga for Ge decreased the selectivity but increased the reaction rate monotonically with Ga composition. Elucidation of these mechanisms revealed that the fine tuning of catalytic properties is possible in Heusler alloys by separately using ligand and ensemble effects of elemental substitution.

Probing Single Pt Atoms in Complex Intermetallic Al13Fe4.

The atomic structure of a 0.2 atom % Pt-doped complex metallic alloy, monoclinic Al13Fe4, was investigated using a single crystal prepared by the Czochralski method. High-angle annular dark-field scanning transmission electron microscopy showed that the Pt atoms were dispersed as single atoms and substituted at Fe sites in Al13Fe4. Single-crystal X-ray structural analysis revealed that the Pt atoms preferentially substitute at Fe(1). Unlike those that have been reported, Pt single atoms in the surface layers showed lower activity and selectivity than those of Al2Pt and bulk Pt for propyne hydrogenation, indicating that the active state of a given single-atom Pt site is strongly dominated by the bonding to surrounding Al atoms.

Fast Oxidation of Porous Cu Induced by Nano-Twinning.

The fcc lattice of porous Cu prepared by dealloying Al2Cu with HCl aqueous solution exhibits a high density of twinning defects with an average domain size of about 3 nm along the ⟨111⟩ directions. The high density of twinning was verified by X-ray diffraction and qualitatively interpreted by a structural model showing the 5% probability of twinning defect formation. Most of the twinning defects disappeared after annealing at 873 K for 24 h. Twinned Cu reveals much faster oxidation rate in comparison to that without (or with much fewer) twinning defects, as shown by X-ray diffraction and hydrogen differential scanning calorimetry. Using ab initio DFT calculations, we demonstrate that twinning defects in porous Cu are able to form nucleation centers for the growth of Cu2O. The geometry of the V-shaped edges on the twinned {211} surfaces is favorable for formation of the basic structural elements of Cu2O. The fast oxidation of porous Cu prepared by dealloying can thus be explained by the fast formation of the Cu2O nucleation centers and their high density.

Correction to "Heusler Alloys: A Group of Novel Catalysts".

[This corrects the article DOI: 10.1021/acsomega.6b00299.].

Understanding the catalytic activity of nanoporous gold: Role of twinning in fcc lattice.

Nanoporous gold (NPG) prepared by de-alloying Al2Au exhibits correlation between the high catalytic reactivity towards CO oxidation and the density of twinning defects in the fcc lattice of NPG. It was also discovered that on the internal surface of NPG, quite common twinning defects can create close-packed rows of six-coordinated catalytically active Au atoms denoted as W-chains. In this work, using density functional theory methods, we investigate energy conditions for formation, thermal stability, and chemical reactivity of these active sites. The possibility of dioxygen chemisorption on various surface sites is studied in detail. A contribution from the dispersion interactions is also considered. The calculated surface density of the active six-coordinated atoms in NPG comparable with that of supported gold nanoparticle catalysts, exothermic chemisorption of dioxygen, and the energy profiles of reaction pathways for CO oxidation indicate that the six-coordinated sites created by twinning can significantly contribute to the catalytic activity of NPG.

Heusler Alloys: A Group of Novel Catalysts.

In this study, we investigated the catalytic properties of various Heusler alloys for the hydrogenation of propyne and the oxidation of carbon monoxide. For propyne hydrogenation, Co2FeGe alloy showed a higher activity than that of elemental Co, where neither Fe nor Ge showed any activity. This clearly indicates an alloying effect. For the oxidation of carbon monoxide, although most alloys showed a significant change in catalytic activity during measurement due to an irreversible oxidation of the alloy, Co2TiSn alloy showed a very small change. The results indicate that the catalytic activity and stability of a Heusler alloy can be tuned by employing an appropriate set of elements.

Twinning in fcc lattice creates low-coordinated catalytically active sites in porous gold.

We describe a new mechanism for creation of catalytically active sites in porous gold. Samples of porous gold prepared by de-alloying Al2Au exhibit a clear correlation between the catalytic reactivity towards CO oxidation and structural defects in the fcc lattice of Au. We have found that on the stepped {211} surfaces quite common twin boundary defects in the bulk structure of porous gold can form long close-packed rows of atoms with the coordination number CN = 6. DFT calculations confirm that on these low-coordinated Au sites dioxygen chemisorbs and CO oxidation can proceed via the Langmuir-Hinshelwood mechanism with the activation energy of 37 kJ/mol or via the CO-OO intermediate with the energy barrier of 19 kJ/mol. The existence of the twins in porous gold is stabilized by the surface energy.

Insights into the dominant factors of porous gold for CO oxidation.

Three different porous Au catalysts that exhibit high catalytic activity for CO oxidation were prepared by the leaching of Al from an intermetallic compound, Al2Au, with 10 wt. %-NaOH, HNO3, or HCl aqueous solutions. The catalysts were investigated using Brunauer-Emmett-Teller measurements, synchrotron X-ray powder diffraction, hard X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy (TEM). Broad diffraction peaks generated during the leaching process correlated with high activity for all the porous Au catalysts. CO oxidation catalyzed by porous Au leached with NaOH and HNO3 is considered to be dominated by different mechanisms at low (< 320 K) and high (> 370 K) temperatures. Activity in the low-temperature region is mainly attributed to the perimeter interface between residual Al species (AlOx) and porous Au, whereas activity in the high-temperature region results from a high density of lattice defects such as twins and dislocations, which were evident from diffraction peak broadening and were observed with high-resolution TEM in the porous Au leached with NaOH. It is proposed that atoms located at lattice defects on the surfaces of porous Au are the active sites for catalytic reactions.

Optical properties of Group X-XII intermetallic compounds studied by HR-EELS.

Electronic structure of d orbital states in transition metals is a key factor for their physical properties and chemical functions. Copper and intermetallic compound PdZn have good catalysis function for the methanol steam reforming reaction. Tsai et al. showed that from results of XPS measurements the d electronic structure of PdZn was similar with that of copper, and the catalysis function should be related to the d electron states [1]. This similarity of d electronic states leads to another view point of the mechanism for coloring the intermetallic compounds. It is well-known that the characteristic red color of copper is caused by interband transition from the d electrons. Therefore, PdZn and Group X-XII intermetallic compounds are expected to be colored and the optical properties should depend on the d electronic states. In this study, the relations between optical properties and d electron states of Group X-XII intermetallic compounds were investigated by using high energy-resolution electron energy-loss spectroscopy (HR-EELS) based on transmission electron microscopy (TEM). From the relation between optical properties and d electronic states, the mechanism of colored intermetallic compounds will be discussed.Figure shows the optical reflectivity of NiZn, PdZn and PtZn, which were derived from EELS spectra by Kramers-Kronig analysis. Intensity drops (arrows) of the reflectivity were observed in visible energy region. These are caused by the interband transitions from d electronic states. The energy positions of the reflectivity drops have tendency of shifting to higher energy side with increasing atomic number of Group X elements (Ni → Pd → Pt). This indicates that the transition energies of d electrons become larger with the atomic number of the elements. First principle calculations (WIEN2k) confirmed that the interband transitions of d electronic states were excitations from bonding d states to hybrid states of anti-bonding s, p, and d states of Group X elements. The bonding anti-bonding energy split increase with the atomic numbers because of increasing crossover of wave function. This implies the intermetallic compounds should be colored and the color should be changed gradually depending on the atomic number of Group X elements.

Activation of Al-Cu-Fe quasicrystalline surface: fabrication of a fine nanocomposite layer with high catalytic performance.

A fine layered nanocomposite with a total thickness of about 200 nm was formed on the surface of an Al63Cu25Fe12 quasicrystal (QC). The nanocomposite was found to exhibit high catalytic performance for steam reforming of methanol. The nanocomposite was formed by a self-assembly process, by leaching the Al-Cu-Fe QC using a 5 wt% Na2CO3 aqueous solution followed by calcination in air at 873 K. The quasiperiodic nature of theQC played an important role in the formation of such a structure. Its high catalytic activity originated from the presence of highly dispersed copper and iron species, which also suppressed the sintering of nanoparticles.

Effect of electronic structures on catalytic properties of CuNi alloy and Pd in MeOH-related reactions.

We investigated the catalytic properties of a CuNi solid solution and Pd for methanol-related reactions and associated valence electronic structures. Calculations and X-ray photoelectron spectroscopy measurements revealed that the CuNi alloy has a similar valence electronic structure to Pd and hence they exhibited similar CO selectivities in steam reforming of methanol and decomposition of methanol. Samples prepared by various processes were found to have similar CO selectivities. We conjecture that alloying of Cu and Ni dramatically alters the valence electronic structures, making it similar to that of Pd so that the alloy exhibits similar catalytic properties to Pd. First-principles slab calculations of surface electronic structures support this conjecture.