Formation Of Bimetallic Particles Research Articles

Synthesis and Characteristics of Ag–Pd Nanoparticles: Inhibition of Palladium Surface Catalytic Activity by Silver

Palladium nanoparticles catalyze the reduction of Ag+ ions with hydrogen on their surface, thereby leading to the formation of bimetallic particles with the “core–shell” structure. The catalytic reaction of methylviologen reduction with hydrogen at ratio [Ag] : [Pd] > 1 is suppressed by silver atoms located on the surface of palladium nanoparticles. At [Ag] : [Pd] ≤ 1, the reduction takes place; however, it is preceded by an induction period, and, the higher the silver content on the nanoparticle surface, the longer the induction period. The rate constant of the catalytic reduction, which begins after the end of the induction period, decreases with increasing silver content on the surface of palladium nanoparticles.

Using Xps Data for Determining Spatial Distribution of Metals in Bimetallic Particles Supported on a Flat Surface

Expressions are obtained for the intensities of photoemission lines of silver and gold in bimetallic particles formed during sequential deposition of metals on the flat surface of the support. Two cases are considered: bimetallic particles with a non-uniform distribution of metals (silver core covered with a gold shell) and alloy particles of uniform composition. The particle shapes are assumed to be hemispherical. Expressions were obtained for the photoemission lines of gold Au4f, Au3d3/2 and silver Ag3d as well as for the Auger line of silver AgMVV recorded using radiation AlKα (hν = 1486.6 eV) and AgLα (hν = 2983.4 eV). Gold shells on the surface of silver particles are shown to diminish the intensity ratio of AgMVV line to Ag3d line, and this effect becomes stronger with the increasing ratio γ of gold atoms to silver atoms. In the case of alloy particles Ag–Au, the intensity ratio of these lines also decreases with increasing γ, though to a much lesser extent. The intensity ratios of gold Au4f and Au3d3/2 lines to the silver Ag3d line are also considered. The obtained equations can be used in XPS studies to consider formation of bimetallic particles in model planar systems and the changes occurring with the particles as a result of heat treatment in a vacuum or in a reaction medium.

Energy characteristics of the electrical explosion of two intertwined wires made of dissimilar metals

In this article, we present the results of the research into the characteristics of the conditions of heating and explosive destruction of Al-Cu, Fe-Ti, Fe-Cu, and Fe-Pb wires under a pulse of current with the density of 107 A/cm2. It has been shown that the energy that is deposited into the wire may depend on the relation between the thermophysical parameters and specific electric resistivity of the metals. It has been determined that under a pulse of current, the wires may explode synchronously or non-synchronously. During a synchronous explosion of wires, a single voltage pulse is generated. In the case of non-synchronous explosion, the wires explode in a succession, thus generating two voltage pulses. We suggested a dimensionless parameter that allows for predicting whether an electrical explosion of two wires of dissimilar metals is synchronous or non-synchronous. According to the research findings, non-synchronous nature of wire explosion may impact the formation of bimetallic particles through the explosion of two intertwined wires made of dissimilar metals.

Effective hydrodeoxygenation of lignin-derived phenols using bimetallic RuRe catalysts: Effect of carbon supports

We have previously shown that an activated carbon-supported ruthenium catalyst promoted with ReOx (RuRe/AC) is highly active for the hydrodeoxygenation (HDO) of lignin-derived phenols (e.g., guaiacol). In this work, we have investigated the effect of carbon supports on the structure and HDO activity of bimetallic RuRe particles using three different carbon supports, i.e., activated carbon (AC), carbon black (Vulcan carbon, VC), multi-walled carbon nanotube (MWCNT). The MWCNT- and VC-supported catalysts show remarkably enhanced activity and hydrocarbon selectivity for the HDO of a range of phenolic molecules (i.e., guaiacol, eugenol, benzyl phenyl ether) compared to RuRe/AC. STEM-EDS and XPS analyses reveal that bimetallic RuRe particles are more common than monometallic Ru or Re particles in the VC- and MWCNT-supported catalysts, and hexavalent rhenium species are more easily reduced to tetravalent rhenium during the HDO reactions in these catalysts, suggesting that Ru and Re in close proximity are required for the efficient hydrogenolysis of phenols. The formation of bimetallic particles on the AC surface is likely hindered by high microporosity and high surface oxygen functionalities, both of which restrict the mobility of Re and Ru for assembly.

Low-Temperature Water–Gas Shift Reaction over Au Supported on Anatase in the Presence of Copper: EXAFS/XANES Analysis of Gold–Copper Ion Mixtures on TiO2

Cu-modified Au/TiO2 (anatase, 200 m2/g) has been prepared by the incipient-wetness technique with introduction of the modifier either before or after Au loading. Such catalysts gave higher catalytic activities for the water–gas shift (WGS) reaction when compared to the unmodified catalysts mainly because of the existence of a synergetic interaction between Cu and Au, as the activities of both Cu/TiO2 and Au/TiO2 are lower than that of the bimetallic system. The WGS activities of both the as-prepared Au/TiO2 and a Au–Cuc/TiO2 catalyst were found to be high and stable. The presence of nitrates on Cuc–Au/TiO2 was found to be detrimental to the activity of Au on TiO2, as a result of the poisoning of Au and enhanced Au agglomeration by NO2 formed during reaction. The activities of Au/TiO2 catalysts modified with Cu-containing acetate counterions were found to decrease during the first 30 min on stream, reaching a constant value of (45 ± 2)%. However, when the poisoning by the acetate anion was eliminated through calcination, the activity increased compared to those of the Au/TiO2 catalysts. An increase in Cu loading was found to lower the activity of Au as a result of the loss of the interaction between the Au and the support interface. The XANES spectra of uncalcined as-prepared catalysts exhibited a pre-edge feature (due to the 1s-to-3d transition) characteristic of Au in a tetrahedral environment. The water–gas-shift-treated sample showed a reduction in the intensity of the white line, indicating a reduction of Au (a partial reduction of Au3+ to Au+/Au0). The XANES spectrum of calcined Cuc–Au/TiO2 (0.44 wt % Cu/3.74 wt % Au) confirmed that Cu exists as ions (Cu+/Cu2+) before and during the WGS reaction. The Au catalyst was not stable and continuously deactivated with time, and the white-line region showed a decrease in peak intensity during use. An increase in gold activity on Cu2+-modified TiO2 was achieved because at least 90% of the gold was reduced to the zero-valent state whereas most of the stabilizing metal (Cu2+) remained in the ionic state. We concluded that the addition of a stabilizing metal in ionic form during the preparation of the Au–Cuc/TiO2 system has marked effects in improving the initial dispersion of the reduced gold and also in helping to maintain a highly dispersed state of gold during use. The atomic ratio of stabilizing metal ions to gold metal plays a crucial role in maximizing the dispersion/stabilizing effects. The formation of bimetallic particles was not detected by EXAFS data analysis. The observed effects can be interpreted as a mutual influence of gold and copper ions and reduced species of gold and copper because of their competion for ion-exchange sites. It is unclear at present how this is achieved. However, EXAFS data analysis of both metals suggests that it is not due to a direct interaction between Au and Cu. In contrast to the results for the WGS reaction, copper was found to show no promotional effect on low-temperature CO oxidation or on preferential CO oxidation (PROX) in excess hydrogen.

Dechlorination of Dichloromethane by Hydrotreatment with Bimetallic Pd-Pt/C Catalyst

A bimetallic Pd-Pt catalyst (0.5 and 0.9% w/w of Pd and Pt, respectively) supported on activated carbon has been prepared and tested for hydrodechlorination of dichloromethane (DCM). Different reduction temperatures were used during the catalyst synthesis, and the catalyst reduced at 250 °C was tested under varying reaction conditions, which included H2/DCM molar ratio, inlet DCM concentration, reaction temperature and space-time, always at 1 atm pressure. Formation of bimetallic particles was evidenced by TEM. Complete dechlorination was achieved at 250 °C, 2.5 kg h mol−1 space-time and H2/DCM = 50.

Controlled preparation and characterization of Pt-Rh/Al2O3 bimetallic catalysts for reactions in reducing conditions

Pt-Rh bimetallic catalysts on alumina support were prepared according to three different techniques, one leading to random deposition of the two metals at the support’s surface, the two others, namely the organometallic grafting and the colloidal route, favoring the formation of bimetallic particles with strong Pt-Rh interactions. Catalysts were characterized by ICP, H2 chemisorption, TPR, FTIR of chemisorbed CO and NO-CO. Then they were evaluated in cyclohexane dehydrogenation and methylcyclopentane hydrogenolysis reactions. Strong interaction between Pt and Rh was demonstrated for catalysts prepared by controlled preparation method, which leads to a strong synergetic effect for methylcyclopentane ring-opening, in terms of activity and selectivity, whereas these interactions do not play a role in cyclohexane dehydrogenation.

Pd–Cu catalysts from acetate complexes in liquid-phase diphenylacetylene hydrogenation

Properties of Pd–Cu/Al2O3 catalysts prepared using PdCu(CH3CO2)4 acetate heteronuclear complexes as precursors in the liquid-phase diphenylacetylene (DPA) hydrogenation have been studied. It has been established that the reaction over the Pd–Cu/Al2O3 catalyst proceeds more selectively than over the commercial Lindlar catalyst; in addition, high activity is achieved at a substantially lower palladium content. The maximum selectivity of DPA hydrogenation is observed with the catalyst reduced in a hydrogen atmosphere without any intermediate calcination that can result in the destruction of the bimetallic acetate complex. FTIR spectroscopy data for adsorbed CO show that the high selectivity of hydrogenation is due to the formation of homogeneous Pd–Cu particles and to the absence of monometallic palladium particles. This can be explained by the retention of the initial complex structure at all of the catalyst preparation stages until the formation of bimetallic particles during hydrogenation.

Effect of PdIn bimetallic particle formation on CO2 reduction over the Pd–In/SiO2 catalyst

A Pd–In/SiO2 catalyst was prepared by the wet impregnation method and used for CO2 reduction at atmospheric pressure. The catalyst was shown to have 100% selectivity toward CO with no detectable CH4 formation. In order to understand the high CO selectivity of Pd–In/SiO2, a detailed catalyst characterization in combination with the DFT study was conducted and compared with that over the Pd/SiO2 catalyst. The XRD and TEM analyses indicate the formation of the PdIn bimetallic nanoparticles on Pd–In/SiO2 after H2 reduction. The unique catalytic property of Pd–In/SiO2 was attributed to the formation of the bimetallic particles. Characterizations including H2 chemisorption and DRIFT and DFT modeling show that the formation of PdIn bimetallic nanoparticles reduces the activity toward H2 dissociative adsorption, weakens CO binding on the surface and increases the barrier of CO hydrogenation, suppressing the further hydrogenation beyond CO.

Synergetic effect in bimetallic Au–Ag/TiO2 catalysts for CO oxidation: New insights from in situ characterization

In a former study, we showed that bimetallic Au–Ag nanoparticles supported on TiO2 activated under H2 at 550°C present the highest activity and a high stability in the reaction of CO oxidation due to a compromise between the particle size and the bimetallic character of the nanoparticles. This paper focuses on the chemical phenomena occurring during the preparation and on the evolution of the Au and Ag species during the activation of the catalysts leading to the formation of bimetallic particles. To characterize the samples, three in situ techniques were used: in situ UV–visible, DRIFTS coupled with CO adsorption and XPS after thermal treatment in adjacent treatment chamber. UV–visible characterization proves that during the preparation of Au–Ag/TiO2 by sequential deposition–precipitation (silver first and then gold), silver reduced during the first step suffers a reoxidation due to the chlorides added during the deposition of gold and the formation of AgCl. The UV–visible study during in situ reduction indicates that the bimetallic particles gradually form as the reduction temperatures increases. The evolution of the gold and silver species present at the surface of the metal particles studied by XPS and CO-DRIFTS reveals an increasing proportion of silver atoms in the bimetallic particles during activation under hydrogen while chemical analysis indicates a gradual elimination of the chlorides with temperature. We conclude that the optimum temperature of activation of 550°C for the reaction of CO oxidation is the result of a competition between these positive and negative effects.

Role of the preparation method on properties of Pd/Cu-MCM-41 hydrodechlorinating catalysts

The effect of the preparation method on properties of Pd/Cu MCM-41 catalysts tested in the hydrodechlorination of CF3OCFClCF2Cl to CF3OCFCF2 was studied. MCM-41 samples were synthesized using sodium silicate as silica source and cetyltrimethylammonium bromide as template. The incorporation of Pd and Cu active species has been performed by impregnation, inclusion in the MCM-41 synthesis procedure and using pre-formed Pd/Cu nanoparticles. The effect of the removal of the template was also studied using calcination and ionic exchange procedures. The hexagonal structure of MCM-41, large specific surface area and pore volume were preserved after inclusion of active species both as cations during the synthesis of MCM-41 or as Pd/Cu preformed nanoparticles. PXRD and HRTEM evidenced the formation of bimetallic particles in all the prepared catalysts; however their particle size and composition varied with the synthesis procedure. The template removal by ionic exchange avoided the sintering of copper species observed in calcined sample. The use of preformed nanoparticles led to small and well dispersed bimetallic species in the MCM-41 structure; however, also in this case, the template removal by calcination provoked both the increase of the particle size and the oxidation of metals. Catalytic performances in the hydrodechlorination of CF3OCFClCF2Cl, namely the dehalogenating activity and product selectivities, were dependent on catalysts properties.

Gas phase hydrogenation of p-chloronitrobenzene over Pd–Ni/Al2O3

The gas phase (1atm; T=393K) hydrogenation of p-chloronitrobenzene (p-CNB) has been examined over alumina supported mono-(Pd or Ni) and bi-metallic (Pd:Ni=1:3, 1:1 and 3:1) catalysts prepared by (co-)impregnation. The catalytic action of physical mixtures (Pd/Al2O3+Ni/Al2O3) was also considered. The catalysts have been characterised by temperature programmed reduction (TPR), H2 chemisorption, powder XRD, HRTEM–EDX and XPS measurements. TPR analysis has provided evidence of a more facile reduction of Pd and Ni in Pd–Ni/Al2O3 relative to Pd/Al2O3 and Ni/Al2O3. TEM–EDX and XPS measurements show Pd surface enrichment in Pd–Ni/Al2O3 with evidence of modification to Pd electronic character. Ni/Al2O3 promoted exclusive -NO2 group reduction to generate p-chloroaniline (p-CAN). Pd/Al2O3 delivered a higher specific p-CNB consumption rate, yielding undesired nitrobenzene and aniline via coupled hydrodechlorination and hydrogenation. The Pd/Al2O3+Ni/Al2O3 combination generated an overall rate and product distribution equivalent to Pd/Al2O3, i.e. the catalytic response was governed by Pd in the physical mixture. In contrast, reaction over Pd–Ni/Al2O3 resulted in exclusive production of the target p-CAN. This is consistent with lower chlorobenzene hydrodechlorination activity recorded for Pd–Ni/Al2O3 (relative to Pd/Al2O3). Reaction rate was sensitive to composition where Pd:Ni=1:1 delivered the highest p-CAN yield. We attribute the catalytic response observed for Pd–Ni/Al2O3 to bimetallic particle formation as demonstrated by TPR, XRD and XPS analyses.

Development of Bi-Metallic Fe—Bi Nanocomposites: Synthesis and Characterization

We investigate the formation of bi-metallic particles in the Fe-Bi system, well known as totally immiscible in the bulk, using a large combination of structural and element-sensitive techniques, well-adapted to the nanoscale. The synthesis approach makes use of the kinetics of decomposition of the different precursors to achieve a controlled sequential growth of the different elements. Different ligands have also been used in order to limit the size and ensure dispersion of the synthesized particles. Our results give evidence for the presence of body-centered cubic ferromagnetic iron nanograins together with larger bismuth crystallites. Interestingly, while the iron particles remain very small, the resistance to oxidation of the Fe-Bi nanocomposites highly depends on the stabilizing ligand used in the synthesis. The presence of both metals, Fe and Bi, in a single cluster has been clearly revealed in the oxidation resistant composite synthesized using the HMDS ligand.

Effect of iron oxide on isobutane dehydrogenation over Pt/Fe2O3-Al2O3 catalyst

The effect of adding Fe2O3 to an Al2O3-supported Pt catalyst on isobutane dehydrogenation was studied. Catalytic activity, catalyst stability, and selectivity to isobutene were improved significantly by adding a small amount of Fe2O3 to Pt/Al2O3. The catalysts were characterized by CO and NH3 chemisorption measurements, Fourier transformed-infrared (FT-IR) analysis of adsorbed CO, and thermogravimatric-differential thermal analysis (TG-DTA) of the used catalyst. FT-IR analysis of the adsorbed CO showed that the electron density of Pt was increased by the formation of bimetallic particles. CO chemisorption suggested the formation of Pt-Fe bimetallic particles partially covered with Fe species. Furthermore, the number of strong acid sites on the Al2O3 surface was decreased by adding Fe2O3. Analysis of the used catalysts suggested that the addition of Fe2O3 inhibited sintering of supported Pt particles. The addition of Sn to Pt/Fe2O3-Al2O3 further improved the dehydrogenation activity of the catalyst. This catalyst showed better catalytic activity than that of Pt-Sn/Zn-Al-O, which has been employed in a commercial process and has been reported as an excellent catalyst in dehydrogenation reactions. We claim that we have developed a novel dehydrogenation catalyst system, Pt-Sn/Fe2O3-Al2O3, which has better catalytic properties than the reported commercial catalyst.

In situ spectroscopic characterization of some LaNi1-xCoxO3 perovskite catalysts active for CH4 reforming reactions

ABSTRACTLanthana-supported Ni and Co catalysts were investigated by “operando” techniques (XAS and APPES) for methane reforming reactions. The samples were prepared by the “solid phase crystallization” method (spc), where the precursors La(Ni1-xCox)O3 contains homogeneously distributed metals (Ni, Co) in the crystal structure (perovskite), which, on further reduction, result in the formation of catalytic system Ni1-xCox/La2O3. The monometallic samples (NiLaO3, CoLaO3) have been compared with a bimetallic system of an intermediate composition Ni0.5Co0.5LaO3. This “operando” study has allowed us to obtain important conclusions about the bimetallic particles and the metal-support interactions. The data revealed the formation of bimetallic particles (NiCo); on these ones, the Ni avoids the Co oxidation during the reaction. However, this protection does not induce an improvement in the activity, which presents an intermediate behaviour between Ni/La2O3 and Co/La2O3. These bimetallic particles form a pseudo-alloy with the surface enriched in cobalt (under reduced conditions), resulting nearly in a core-shell structure (Ni@Co).

Reaction of thiophene with mono- and bimetallic Ni–Co particles supported on γ-Al 2O 3 and HDS activities of obtained sulfides

Reaction between thiophene and M/γ-Al 2O 3 (20 wt.%, M = Ni, Co, 2Ni–Co and 2Co–Ni) was studied by thermal gravimetric analysis (TGA) and by sulfidation in a fixed bed microreactor. The samples were prepared by incipient wetness impregnation followed by drying and reduction in hydrogen flow at 450 °C. Formation of bimetallic particles in samples containing both Ni and Co was confirmed by HRTEM with EDX analysis. The reaction with thiophene (20 or 40 mbar) was done at 360 °C in hydrogen atmosphere. TGA of the reaction with thiophene revealed that the initial rate of sulfidation drops sharply when going from Ni/γ-Al 2O 3 to Co/γ-Al 2O 3 (by a factor of 40 under used conditions). It follows from the sulfidation in a fixed bed reactor that this difference can be attributed to a poor catalytic activity of metallic Co in thiophene hydrodesulfurization (HDS) and consequently much lower amount of produced H 2S in comparison with Ni. Upon sulfidation the HDS activity of Ni-rich particles progressively decreases, while, surprisingly, that of Co-rich particles is enhanced so that after a complete sulfidation (confirmed by XRD and sulfur analysis) the catalytic activity of the obtained sulfides in thiophene HDS increases in the order: 2Ni–Co < Ni < 2Co–Ni < Co.

Novel mesoporous aluminosilicate supported palladium-rhodium catalysts for diesel upgrading

Abstract Bimetallic PdRh (with Pd/Rh mole ratio = 2/1) catalysts supported on a mesoporous aluminosilicate have been prepared and three methods of metal incorporation in the support have been compared: direct incorporation into the synthesis gel, impregnation and ion-exchange. Physico-chemical characterisations (nitrogen adsorption–desorption, hydrogen chemisorption, transmission electron microscopy, X-ray photoelectron and extended X-ray absorption fine structure spectroscopies), as well as simulated coking and regeneration tests are described. The dispersion, metal particle size and the formation of bimetallic particles within the material depend on the method of metal incorporation. Direct incorporation and ion-exchange methods lead to co-existence of PdRh intermetallic aggregates and segregated Pd; on the other hand, the impregnation method leads to the formation of very small and well dispersed PdRh alloy particles, with Pd preferentially located on the alloy particle surface.

Co-Pt bimetallic catalysts for the selective oxidation of carbon monoxide in hydrogen-containing mixtures

The performance of a Co-Pt powder and of Co-Pt catalysts supported on γ-Al2O3 and on the graphite-like carbon material Sibunit in selective CO oxidation in hydrogen-containing mixtures is considered. Fine particles of metal-metal solid solutions and intermetallides were obtained by the decomposition of a Co- and Pt-containing double complex salt in a hydrogen atmosphere at ∼400°C. As compared to their Pt and Co monometallic counterparts, the bimetallic catalysts are more active and allow the CO concentration in hydrogen-containing mixtures to be reduced from 1 to 10−3 vol %. This effect is likely due to the formation of bimetallic particles of a Co-Pt solid solution on the support surface.

Steam reforming of ethanol at moderate temperature: Multifactorial design analysis of Ni/La 2O 3-Al 2O 3, and Fe- and Mn-promoted Co/ZnO catalysts

Novel Co (10%) catalysts supported on ZnO and promoted with Fe and Mn (1%) were synthesized and characterized by high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS). Their catalytic activity for steam reforming of ethanol was compared with that of Ni catalysts supported on La 2O 3-Al 2O 3. Experiments at 400 and 500 °C, steam to carbon ratios of 2 and 4, and a wide interval of contact time were analyzed following a multifactorial experimental design. At 500 °C and a steam to carbon molar ratio of 4, complete conversion of ethanol was achieved above a contact time of 200 g min mol −1 for all catalysts. The ratio of selectivity between hydrogen and methane was around 23 mol H2/mol CH4 in the Co catalysts, while it approached the thermodynamic equilibrium (5.7 mol H2/mol CH4) in the Ni catalysts. The Co catalysts do not promote methane-forming reactions like ethanol cracking and acetaldehyde decarbonilation, nor do they facilitate the reverse methane steam reforming reaction. The catalytic behavior of cobalt is enhanced by promotion with iron or manganese through the formation of bimetallic particles, which facilitates cobalt reducibility. This suggests that Co-Mn/ZnO and Co-Fe/ZnO catalysts have a good potential for their use for ethanol reforming at moderate temperature.

Synthesis and characterization of bimetallic Ni–Cu particles

Bimetallic Ni–Cu particles were synthesized from either suspensions of nickel carbonate and copper carbonate, and solutions of nickel nitrate and copper nitrate in ethylene glycol which acts both as solvent and reducing agent. The nature and composition of the powders depend on both the reaction temperature and time, and the reactants. Using the carbonates, bimetallic Ni–Cu powders composed of a nickel-rich and a copper-rich solid solution were obtained after 39 h at 140°C. Increasing the reaction temperature to 190°C gives a Ni–Cu powder composed of a copper-rich solid solution and nickel. Particles obtained under these conditions, however, are polydisperse. The nitrate solution gave bimetallic Ni–Cu particles with a narrow size distribution of about 140 nm after 4 h of reaction at 196°C. These particles are made of a copper core and a nickel shell. The mechanism of bimetallic particle formation is controlled by the solubility of the reactants, the formation of intermediate metal glycolates and Cu 2O, and the required reduction temperature.