Bimetallic Aggregates Research Articles

Ni–Fe, Co–Fe, and Co–Ni nanocomposites based on carbon nanotubes in the reaction of CO2 methanation

For effective carbon dioxide (CO2) methanation, bimetallic Co80Ni20/CNTs, Ni80Fe20/CNTs, and Co93Fe07/CNTs nanocomposite catalysts, where subscript numbers indicate the metal content (wt.%), have been prepared with wet impregnation method, further thermal decomposition, and chemical reduction stages using carbon nanotubes (CNTs) as a carrier. The catalysts were characterized using TG, SEM-EDX, argon physisorption, and X-ray powder diffraction methods. The pristine nanotubes demonstrated a slight activity in CO2 conversion due to the presence of small quantity of Ni. Among the tested nanocomposites the bimetallic Ni80Fe20/CNTs catalyst showed improved catalytic activity compared to Co80Ni20/CNTs and Co93Fe07/CNTs. The SEM-EDX analysis showed not uniform distribution of bimetallic aggregates on carbon nanotubes matrix for Ni80Fe20/CNTs and Co93Fe07/CNTs. However, this fact did not affect to catalytic properties of mentioned nanocomposites. Ni80Fe20/CNTs exhibited high activity in the CO2 methanation with 88% of CH4 yield at 350 °С unlike Co93Fe07/CNTs with 64% of CH4 yield.

Preparation and characterization of Ni–Co/SiO2 nanocomposite catalysts for CO2 methanation

For effective CO2 methanation, monometallic Co(10)/SiO2 and Ni(10)/SiO2 and bimetallic Ni(5)Co(5)/SiO2 and Ni(2)Co(8)/SiO2 nanocomposite (NC) catalysts, where numbers between brackets indicate the metal content (wt%), have been prepared with solvate-stimulated modification, thermal decomposition, and chemical reduction stages using nanosilica A-300 as a carrier. The catalysts were characterized using SEM–EDX, nitrogen physisorption, and X-ray powder diffraction. The samples before and after the reduction with hydrogen have a similar mesoporous texture. The bimetallic Ni–Co/SiO2 NC catalysts showed improved catalytic activity compared to Ni/SiO2. The formers are characterized by the smallest bimetallic crystallites of ca. 12 nm in size. Activity in the methanation of the Ni–Co/SiO2 NC catalysts depends on the cobalt content resulting in a prominent increase in CO2 conversion and methane yield at 250–350 °C. The SEM–EDX analysis showed unusual flower-like nanostructures and proved the random distribution of Ni/Co bimetallic aggregates in the nanosilica matrix at the nanoscale level. Both Ni(2)Co(8)/SiO2 and Ni(5)Co(5)/SiO2 exhibited high activity in the CO2 methanation with 81–88% CH4 yield at 450 °C. The results of thermo-programmed desorption mass spectroscopy revealed that the catalytic reaction is passed through the formation of O-containing intermediates bound to the active Ni–Co and nanosilica sites.

Steric Control over Supramolecular Polymer Formation in trans-1,2-Bis(4-pyridyl)ethylene Adducts of Zinc Xanthates: Implications for Luminescence

Steric control over supramolecular aggregation is demonstrated for a series of adducts formed between Zn(S2COR)2 and trans-1,2-bis(4-pyridyl)ethylene, whereby supramolecular zigzag polymers are found when R is small, that is, Et (1) and n-Bu (2), but only bimetallic aggregates could be formed when R = Cy (3). This results in different coordination geometries: four-coordinate N2S2 zinc in 1 and 2, and five-coordinate NS4 zinc in 3, a feature which greatly influences photophysical responses in the solid state. When excited in the UV region, {Zn(S2COEt)2L}∞ (1) and {[Zn(S2COCy)2]2L} (3) produce broad luminescence in the visible region. Five-coordinate 3 produces more broad and intense luminescence than four-coordinate 1. The configuration interaction singles (CIS) and post-Hartree−Fock (HF) calculations for 1 and 3 indicate the A-band excited state is responsible for the observed luminescence which is strongly associated with charge transfer from S and Zn.

Influence of Alumina-Supported M–Pt Catalysts in DeNOx Reactions with M = Pd or Ir or Ru

To better understand N2 formation in the selective catalytic reduction of NOx by propene under oxidative atmosphere, we studied the catalytic behavior of bimetallics. Surface segregation phenomena may provoke geometric and electronic modifications of the catalytic active sites. To explain our results we invoked: (i) the facile way to oxidize the bimetallic aggregates, (ii) the presence of atoms with low coordination number able to be oxidized first and (iii) an eventual formation of like “vic-diNOx species” on such bimetallics leading to N2 formation.

Co/Ni/Mg/Al Layered Double Hydroxides as Precursors of Catalysts for the Hydrogenation of Nitriles: Hydrogenation of Acetonitrile

Layered double hydroxides (LDHs) with a hydrotalcite-like structure and containing Ni2+/Co2+/Mg2+/Al3+ cations in different amounts were prepared and activated in various conditions. Depending on the chemical composition and the calcination temperature, mixed-oxide and spinel-like phases of complex compositions are obtained. They lead to well-dispersed bimetallic phases of high metal loadings upon reduction. Temperature-programmed reduction by H2 showed that the introduction of Mg decreases the reducibility of metals and that most of the Ni and Co are together in bimetallic aggregates. These catalysts were tested in the gas-phase hydrogenation of acetonitrile between 350 and 450 K and with a H2/CH3CN molar ratio of ca. 33. The main product is ethylamine (MEA); secondary products are N-ethyl,ethylimine at low conversion, diethylamine and triethylamine at high conversion. The Ni-free catalyst is three orders of magnitude less active than the Ni-containing samples. The by-products are formed by condensation between “imine-” and “amine-like” adsorbed species on metal and acid sites (bifunctional mechanism) and on the metal sites alone as well. The tuned addition of Mg (Mg/(Mg+Ni+Co)≈0.25) lowers the surface acidity and the bifunctionalized formation of by-products consequently. A net increase in MEA selectivity is further reached thanks to the formation of bimetallic NiCo phases. It is proposed that by-product formation on the metal surface occurs by condensation at Ni0 sites between multibonded adsorbed species, which could be of the acimidoyl and aminomethylcarbene types. The first role of Co is the dilution of the Ni surface in small ensembles less prone to accomodate neighboring multibonded species. The IR spectroscopy of adsorbed CO provided evidences of the dilution of Ni by Co in bimetallic NiCo particles. A catalyst obtained from the Co/Ni/Mg/Al (0.27/0.26/0.22/0.25) LDH, calcined at 393 K and then reduced at 893 K, exhibits the highest selectivity to ethylamine, 98.2% at 10% CH3CN conversion.

Bimetallic aggregates at surfaces: spectromicroscopy study of Pt–Ag interaction on graphite

The formation of bimetallic Pt/Ag alloy in an amorphous Ag film supported on highly oriented pyrolitic graphite has been investigated with scanning photoemission and scanning tunneling microscopy. This spatially resolved study reveals that alloying strongly depends on the morphology of the deposited metal. Whereas little metal intermixing is found in small Ag clusters (50 Å), efficient alloying is observed in much larger heterogeneous structures. Subsequent annealing results in a decrease of the exposed alloy component, due to segregation of the two metal species.

Study on an Assembly System Including Tetrathiovanadate. Syntheses and Structural Characterizations of V2Cu2S4 Cubane-Like Clusters and VS4Cu4 Bimetallic Aggregates

Study on an Assembly System Including Tetrathiovanadate. Syntheses and Structural Characterizations of V2Cu2S4 Cubane-Like Clusters and VS4Cu4 Bimetallic Aggregates

Alkane conversion and topological segregation in PtM/Al 2O 3 catalysts

The site distribution of the two components at the surface of small bimetallic aggregates has been investigated by using the hydrogenolysis of model alkanes. The catalysts were supported on γ-alumina and consisted of platinum particles ( size 1–2 nm ), modified by Sn, Pb, Ge, Al or Zn. They were prepared by using the surface reaction of platinum hydride with organometallic compounds. On platinum catalysts, the conversion of n-hexane shows a low sensitivity to changes of the surface topology, corresponding either to a modification of the particle size or to the addition of a small amount of the modifier. At variance, the conversions of 2,2,3,3-tetramethylbutane (TeMB) and methylcyclopentane (MCP) are highly sensitive to the topology of the metallic surface. TeMB conversion yields large amounts of 2,2,3-trimethylbutane and methane on small particles, which have essentially sites of low coordination (corners and edges), whereas isobutane is selectively formed on large particles, which show more sites of high coordination (facets, planes ). The addition of a small amount of Sn, Zn or Al (M/Pt < 0.2, at./at. ) favors the formation of isobutane. Therefore, Sn, Zn and Al inhibit selectively the reactions catalyzed by the sites of low coordination, and this demonstrates that there is a topological segregation of these elements to these sites. Further tin addition shirts the catalytic properties of platinum towards those of smaller platinum particles, showing the dilution of the platinum surface by tin.

Ru L-edge x-ray absorption studies of the formation of Ru–Cu bimetallic aggregates on Cu(100)

X-ray absorption measurements at the Ru L3 edge have been used to study the adsorption of Ru3(CO)12 on Cu(100) surfaces at submonolayer and monolayer coverages and the subsequent formation of Ru–Cu bimetallic aggregates at these surfaces. The analysis of the Ru L3 edge x-ray absorption near edge structure (XANES) reveals (a) the surface Ru–Cu bimetallic aggregates are three dimensional clusters of which the Ru atoms are in the ‘‘bulk’’ and the surface of the cluster is covered with Cu atoms, and (b) relative to pure Ru metal, Ru in the bimetallic aggregates gains d character. The chemical properties of these bimetallic surfaces and the effect of the cluster size on their electronic properties revealed by XANES as well as LEED and Auger are also reported. These results are compared with previous results of Ru3(CO)12/Cu(111). The implications of these observations are discussed.

Photoemission studies of Ru3(CO)12 adsorption and formation of Cu–Ru bimetallics on Cu(111)

Photoemission spectra of Ru3(CO)12 adsorbed on Cu(111) and its decomposition products induced by electron irradiation and annealing are reported. It is found that electron bombardment and subsequent annealing of Ru3(CO)12/Cu(111) lead to the formation of Cu–Ru bimetallic aggregates which exhibit similar features as those reported previously on polycrystalline Cu, that is that Ru–Cu aggregates are formed of which Ru atoms are at the core and the surface is wetted with Cu and there exist distinct interface states. The mechanism for the formation of the bimetallic Cu–Ru aggregates on Cu(111) is almost certainly one in which two-dimensional Ru aggregation occurs on the surface upon initial annealing (∼225 °C) followed by three-dimensional aggregation and wetting of the Ru cluster surface by Cu after further annealing at higher temperatures (≳450 °C). The interaction between Ru surface intermediates and Cu(111) before Cu wetting occurs is also examined with photoemission and SW–Xα calculations.

Topological segregation in small bimetallic particles

The spatial distribution of the two components at the surface of small RhM/Al2O3 (M=Sn, Pb, Ge) bimetallic aggregates has been investigated by using hydrogenolysis of model alkanes, IR spectroscopy of adsorbed CO and quantum chemical calculations. The addition to a Rh aggregate of a small amount of Sn or Pb inhibits selectively the sites of low coordination, and this demonstrates that there is a topological segregation to these sites; Ge atoms appear to be randomly distributed at the surface. Quantum chemical calculations of the relative stabilities of Rh12M model clusters support these conclusions.

Site segregation in small rhodium bimetallic aggregates: a combined catalytic and quantum chemical study

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSite segregation in small rhodium bimetallic aggregates: a combined catalytic and quantum chemical studyBernard Coq, Annick Goursot, Touria Tazi, Francois Figueras, and Dennis R. SalahubCite this: J. Am. Chem. Soc. 1991, 113, 5, 1485–1492Publication Date (Print):February 1, 1991Publication History Published online1 May 2002Published inissue 1 February 1991https://doi.org/10.1021/ja00005a002RIGHTS & PERMISSIONSArticle Views131Altmetric-Citations41LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

The influence of a second metal component (Cu, Sn, Fe) on Pd/SiO2 activity in the hydrogenation of 2,4-dinitrotoluene

The catalytic activity of palladium in the hydrogenation of 2,4-dinitrotoluene is reduced by the addition of Cu or Sn. Upon addition of Fe, the activity increases. The formation of highly dispersed Pd-Fe bimetallic aggregates is suggested, where the Fe atoms, contiguous to Pd, act as promoters activating of the N=O bond.

Alumina-supported bimetallics of palladium alloyed with germanium, tin, lead, or antimony from organometallic precursors: I. Preparation and characterization

Bimetallic PdSn, PdSb, PdPb, and PdGe on alumina catalysts with a low metal content have been prepared using either chloride or Organometallic precursors. For the catalysts obtained from chloride precursors no interaction was observed between the two metals, and the catalysts behaved like pure Pd Al 2O 3 . In contrast, the reactions of (C 4H 9) 4Sn, (C 4H 9) 4Pb, (C 4H 9) 4Ge, or (C 4H 9) 3Sb in n-heptane solution with reduced Pd Al 2O 3 catalyst yielded a supported alloy. The interaction between metallic palladium and the organic modifier is highly selective and leads to the formation of a well-tailored bimetallic catalyst. When these final solids are reduced at 573 or 773 K, the second metal locates preferentially at the outer layer of the bimetallic aggregates. After reduction at 773 K large metallic aggregates are obtained (particle size ≅ 15 nm), and the formation of β-palladium hydride, which can be formed with pure palladium catalysts, is suppressed by the addition of a small amount of the second metal. The specific activity of the palladium surface atoms for isoprene hydrogenation is then lowered, and the selectivity increased.

Ru L-edge X-ray absorption studies of the formation of Ru-Cu bimetallics on Cu(111) and Cu(100) surfaces

X-ray absorption at the Ru L3 has been used to study the adsorption of Ru3(CO)12 on Cu(100) surface and the subsequent formation of Ru-Cu bimetallic aggregates. The analysis of the Ru L3 edge reveals that Ru gains appreciable d character upon the formation of the bimetallics.

Influence of the support on the catalytic properties of bimetallic Ru-Cu samples

Propane hydrogenolysis has been studied on a series of Ru-Cu catalysts supported on SiO 2, Al 2O 3 and MgO. On SiO 2 the hydrogenolysis activity of Ru decreased continuously on addition of copper over the entire range of composition. On A1 2O 3 and MgO however, it was found that on increasing the amount of Cu the catalytic activity initially decreases, reaching a minimum at intermediate Ru/Cu ratios, and then increases at higher Cu contents. For each support, the less active samples were also found to be the most selective towards ethane formation. The lower catalytic activity and the higher selectivity observed on the bimetallic samples has been attributed to a Ru-Cu interaction, the degree of which depends on the nature of the support. It is suggested that the relative amount of bimetallic aggregates present on the reduced samples is strongly dependent either on the total surface area of the support, which influences the rate of formation and growth of the metallic particles, and/or on the acid-base characteristic of the oxide, which would determine the degree of hydrolysis of the catalyst precursor.

Bimetallic Ru-Au catalysts: Effect of the support

Ru-Au catalysts supported on SiO 2 were characterized by using H 2 and O 2 chemisorption, wide-angle X-ray scattering, diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. Catalytic activity was measured for the hydrogenolysis of propane and ethane. The hydrogenolysis activity of ruthenium decreased by two orders of magnitude with addition of gold. This suggested that Ru and Au did not exist as separate particles but formed bimetallic aggregates. Chemisorption and XPS experiments showed a surface composition similar to the bulk. A comparison was made with a previously studied Ru-Au-on-MgO system, on which an enrichment of Ru on the surface of bimetallic Ru-Au clusters was discovered. It is suggested that the strength of the metal-support interaction can affect the surface composition of multimetallic supported systems.

The State of the Metal Phase in Supported Platinum-Containing Bimetallic Catalysts

The catalytic reforming of crude oils has been considerably improved during the last ten years by the addition of a second metal, generally rhenium or iridium, to the small amounts of platinum present in alumina supported catalysts. A most important question, from a scientific point of view, is to differentiate between the various possible states of the metal phase, which may range from a mixture of platinum and the second metal particles to homogeneous bimetallic aggregates. This differentiation is extremely difficult because of the very small size of the metallic particles. It is reported that hydrogen-oxygen titration provides evidence of the presence of small particles of pure rhenium, iridium or ruthenium in platinum bimetallic alumina supported catalysts.