Bimetallic Catalysts General Introduction:
The economy of a chemical manufacturing process heavily depend upon the rate of reaction, some chemical reactions are very fast and difficult to control, and some reactions are very slow, even difficult to observe the reaction proceeding. The real interest of a chemist to pace the reaction rate and minimize the cost of any chemical manufacturing process (1). The rate of reaction depend upon temperature, usually high temperature increases the reaction rate, but sometimes reaction does not proceed at acceptable speed even at high temperature. These type of reactions are accelerated by using catalysts. Catalysts are used in a large number of chemical reaction in manufacturing and process industry (2). Mostly catalysts are used in the highly porous form and most of the reaction proceed on the surface of catalysts. The reactant molecules diffuse into the pores of catalyst where the chemical reaction occurs and product also diffuse back out of these porous catalysts particles (3). In 1960, Exxon Research and Engineering Company started the study on the bimetallic catalyst activity and in 1980 John H. Sinfelt introduced the term bimetallic cluster for a dispersed supported metallic catalysts of silica and alumina (4). In last few decades, many scientists investigated the relation between metallic electronic configuration and catalysts activity. By this research bimetallic catalyst become a major category of heterogeneous catalysts (5). In the start, most of the research were done on bimetallic catalysts of nickel-chromium, Ruthenium-copper, Osmium-Copper, Platinum-Iridium, and Platinum- Ruthenium and these catalyst mainly investigated in hydrogenation, dehydrogenation, and isomerization process. In these processes, the valance electronic configuration of metallic catalysts plays an important role to define the catalytic activity. In last few decade the library of bimetallic catalyst expanded very significantly, many combinations from GROUP VIII and GROUP IB are examined (6). And now it becomes a very fast growing field of process and manufacturing industrial chemistry for energy conversion (7).
Bimetallic catalysts are now very important, they are used in a large number of chemical reactions including reduction of oxygen in fuel cell technology (8). David Martin et al. investigated the effect of bimetallic catalysts on the production of biofuel from biomass, and they concluded that bimetallic catalysts offer high activity, modified selectivity and improved stability for the conversion of biomass (9). J. Hutchings et al. investigated the different type of application of bimetallic catalysts in the chemical industry (10). Wei et al. studied the role of bimetallic catalysts for production of hydrogen and find that selectivity toward hydrogen can be controlled by using different combinations of bimetallic catalysts (11). Bimetallic catalysts of Ru-Cu used for the catalysis of the hydrogenolysis of glycerol, selectivity of reaction and yield of product also controlled by the different ratio of both metals in the catalyst (12). For hydrogenation of organic compounds, bimetallic catalysts of Ru-Sn are used to change the selectivity from hydrogenation of to while nano-metallic catalysts of Pt or Pd favour the hydrogenation of (13-15).
From the practical view, bimetallic catalysts have an edge on monometallic catalysts, it is due to change in electronic configuration, surface composition, and oxidation state. That is why bimetallic catalysts have greater potential than monometallic catalysts in catalytic chemistry (16-18).
M Mohl et al. and Huang et al. investigated the properties of bimetallic Nano particles and concluded that both physical and chemical properties of nanoparticles influenced from both metals (19, 20). Addition of any other metal will result in completely different properties (21). Different combination of metals are extremely useful to improve Plasmonic coupling for different sensing applications (22). Monometallic particles show very uniform optical properties due to surface resonance but when the composition is changed the optical properties also become unique and tunable. Change in the composition also greatly effect on the local electric field (23, 24). The absorption, dispersion, and reflection of light depend on the morphology and composition of metallic nanoparticles (25). The nanoparticles of gold and silver powerfully absorb and scattered the light when they are smaller than the wavelength of visible light. Same when the structure of bimetallic nanoparticles are changed it also diverse the optical properties, even they have the same composition (26).
The bimetallic nanoparticles of silver and gold also shows antibacterial properties against both Gram positive and Gram negative bacteria, according to the study of Ramakritinan et al. same bimetallic nanoparticles with 1:3 silver and gold also inhibit the pathogenic bacteria (27).
Structures of Bimetallic Nanocatalysts:
Bimetallic catalysts usually available in six different structures: Crown-jewel structure, Hollow structure, Heterostructure, Core—shell structure, Alloyed structure, and porous structure (28).
Crown-Jewel means that atoms of one element are controllably assembled on the surface of other metal at special positions. This structure is used for expensive metals, atoms of expensive metals jeweled at the crown of inexpensive metal. Effective use of precious metal and improved catalytic activity are two major advantages of this type of catalysts. Synthesis of a crown-jewel structure is not an easy task, it is carefully handled at the atomic level. Chemical vapour deposition method is commonly used for such structure. Sykes et al. prepared Pd-Cu bimetallic catalyst by using this approach (29). Solution state method can also be used for the production of crown-jewel structure but this method is more difficult to CVD approach, Toshima et al. used this method for the production of Au-Pd nanocatalysts (30).
Hollow structure also getting more attention these days due to its high surface to volume ratio, he interior hollow structure enclosed the active components and it also provides space for reactions (31, 32). Hollow structure shows different catalytic properties from solid catalysts, and they also have low density, which resulted in the saving of materials and lower the cost. Template –mediated approach is used for the production hollow structure nanoparticles, this approach further divided in hard templating, soft templating and sacrificial templating (33).
Pt-based hetero-nanocatalyst are used in fuel cell for their better catalytic activity in proton exchange membrane (34), according to the work of Xia et al. heterostructure of Pd-Pt are 2.5 times more active than the equivalent mass of Pt. they are produced by using heterogeneous seeded growth (35). Core-shell structure made from active metal shell, with a metal support, they are most promising with high catalytic efficiency (36). Most of the chemical reaction take place on the surface of catalysts, the interior atoms are usually wasted, for this reason, the interior side of the core-shell structure is made from with inexpensive metal. One-pot co-reduction or Seed-mediated approach is used to produce core-shell structure (37). In alloyed structure both metal are in homogeneous distribution in particles. Wet chemical synthesis approach with rigidly controlled reaction kinetic is used for the production of alloyed structure (38). Xu et al used co-reduction approach for the synthesis of Ni-Fe alloyed structure nanoparticles (39). Surface area is an important factor in chemical catalytic reactions, in recent year, researchers invented a new structure with high surface area (40). Porous catalysts offer high surface area, then another type of structures. Raney Nickel allow is the best example of porous bimetallic catalysts.
There are different factors that affect the catalytic activity of bimetallic nanoparticles.
Effect of size:
Haruta et al. Studied the factor of size on the catalytic activity and selectivity of Nanocatalysts. The total surface area of metal particles is inversely proportional to the square of the diameter of nanoparticles, it means the surface area will increase with decreasing size of particles. All the chemical reactions occur on the surface of the catalyst so with decreasing size of particles resulted in high catalytic activity (41, 42). For some chemical reaction, the rate of reaction increased with the certain size of catalyst, further decrement in the size of nano-catalysts resulted in a slower reaction. In photochemical hydrogen generation by using nanocatalysts of Pt, the critical size of catalysts is 3 nm, above and below size will slow down the chemical reaction (43). The size also effects on the selectivity of catalysis. According to Lopez et al. particle size is a determining factor of catalyst performance (44).
Compared to monometallic catalysts, bimetallic catalysts have more flexibility in design for activity and selectivity of the catalyst. As in Platinum-based nano-catalysts, Pt has the more active electronic configuration for oxygen reduction in the fuel cell. To increase the selectivity of catalysts, a fraction of Pt is reduced by using some other metal. This will change the electronic state of Pt, which effect on the distance between Pt-Pt bonds and on coordination number (45).
Interparticle distance effects:
In order to obtain stability of catalysts in a chemical reaction, well-defined material composition and with tunable interparticle distance are required. According to the work of Roldan’s initial particles size is not only important, their distribution on the substrate is also important to improve the lifetime of bimetallic nanocatalysts (46, 47).
In order to take advantage of bimetallic nanoparticles, besides other factors, the addition of other metal need to be fully understood the change the electronic configuration which creates ligand and strain effects. There are several ways to increase the catalytic activity of bimetallic catalysts, included charge transfer phenomena which will change the binding energy and decrease the barrier for specific chemical reactions (48). It also provides a shield from catalysts poisoning and catalytic deactivation (49, 50). Change in composition leads to several benefits, it decreases the poisoning effect, and it opens the new reaction pathway which will lead to a distinct selectivity. It also produces a synergistic effect which changes the electronic configuration and increases the catalytic activity. Alloy and bimetallic nanocatalysts also provide better thermal stability in the chemical reaction (47).
Bimetallic Catalysts Preparation Methods and Characterization
The impregnation is most common and widely used method to produce supported bimetallic catalysts. In this technique support is contacted with a solution and then this solution is dried, some organic can be used to impregnation of water solution. There is two type of impregnation techniques are used: incipient wetness and wet impregnation. This classification is based on volume of solution with respect to the volume of support, in incipient wetness, the solution volume is less than the pore volume of the support and in wet impregnation method volume of solution high than the pore volume (51). Pretreatment conditions like temperature, time of heating, final temperature and supporting material controlled the characteristics of the catalyst. During the calcination of the solution, there are chances of chemical reaction between solution and metal support, with the change conditions also lead a different metal support interaction.
Deposition-precipitation approach is used, when solution generates an insoluble form of supported active phase, and this active phase deposit on the solution connected to support. This approach is very suitable for the production of precursors of highly active supported gold catalysts (52). According to Prati et al. usually hydroxide or carbonates are produced in this method and they deposit on the support (53).
Mostly metal ions are soluble in acidic water solution and they precipitate in the form their hydroxide, the precipitation of hydroxide leads to generate base metal oxides. These hydroxides are decomposed to anhydrous oxide by using calcination. The co-precipitation of base metal cations produced mixed oxide in solid-solution form. From precipitating hydrotalcite of bivalent cations produces Co-Precipitation of bivalent cations in the form of hydroxy-carbonate, hydroxyl-chloride or hydroxyl nitrate (54). The basic precipitation usually resulted in the contamination of the precipitate in final product, complex washing procedure is applied to limit this contamination.
In Sol- immobilization a collide dispersion of a chemical precursor, that is deposited. The aging time of few minutes to a week allows immobilizing the precipitate of the Sol. High viscous Sol is allowed to deposit on a thicker layer with cracks. The immobilization occurred due to polymerization process, and solvent is removed by evaporation, (55) initially this method used to prepare silica gel and other silica based materials, but now this method also used for gelation of many other metal oxides. This technique is usually operated at low temperature, which lower the production cost of high yield of bimetallic catalysts (56).
Due to inherently high surface energy gained from the method of production, nanoparticles are usually not stable and in non-equilibrium structure. And complete characterization of bimetallic nanoparticles gives chemical and morphological information, which necessary to correlate non-equilibrium state with catalytic activity. TEM: Transmission Electron Microscopy is a most common technique used for the structural characterization of bimetallic nanoparticles (57, 58). Usually, high-resolution transmission electron microscopy is used to determine the lattice spacing and this information is used to calculate the chemical composition of nanoparticles. Crystallographic planes are determined by measuring the scattering of the electron beam and this also tells about phase and crystallinity of bimetallic nanoparticles. There is another technique named as scanning transmission electron microscopy by energy dispersive spectroscopy used to calculate the chemical composition of particles. In this method, narrow beam of electron focused on a little spot on the sample, the energy of x-ray emitted from nanoparticles is mapped and electron energy losses are measured in the form of spectrum. This measurement used to correlate the image with quantitative data (59). Wu Zhou et al. used this technique to characterize the supported metal oxide, and find the active side of catalyst, and it also helps him to construct a meaningful structure of bimetallic catalysts to enhance the efficiency (60). Akita et al. study the morphology of gold nanoparticles by using electron microscopy (61).
X-ray diffraction is also a powerful tools used to find the crystallographic structure and chemical composition and it also highlight the amorphous state of a material (62). XR consist of three parts, a beam source, a goniometer and a detector. According to Bragg’s law, X-rays diffracted from material with a specific angle, this diffraction is used to determine the composition of material. This information is also used to find the size of crystal by using Scherrer equation. This technique also can be used to distinguish the random and ordered alloy. Usually different alloy have different packing structures like FCC and BCC, both give different diffraction patterns (63). Results of XRD are compared with database patterns of International Center for Diffraction Data. Rietveld analysis is used for precise measurement and positioning of atoms in complex structures. XRD can be used in severe condition, like high temperature and this can also be used during chemical reaction (64).
UV-vis method also used to characterize the electronic structure of bimetallic nanoparticles, it reveals the electronic absorption in UV and visible region (65, 66). Electron conductor absorbs near the UV and visible region and semiconductor absorbs in the UV and visible region. This method is generally used in solid powder and it give information about electronic properties in coordination and oxidation state.
Vibrational spectroscopy by using infrared spectroscopy and Raman spectroscopy gives information about structure of both amorphous and crystalline bimetallic nanoparticles (67, 68). But mostly Raman spectroscopy is used due to its adaptability to in situ work. Recently Bao et al. study the M-CO bonding by using this technique, and they identify the active surface of nanocatalysts inside carbon tubes (69). Somorjai et al. studied the structural properties of nanoparticles of Rh and Pt by using vibrational spectroscopy (70).
X-ray photoelectron spectroscopy is used to find the surface properties of metals in bimetallic catalysts. Photoelectric effects allowed on the surface of solid nanoparticles, the electron belonging to the nanoparticles become exited and escaped into the vacuum (71). By using the energy of photoelectron the bonding energy of electrons are calculated. This binding energy is dependent on oxidation stated so it also gives the information about oxidation state (67). Now this technique also used under a small pressure of gas.
X-ray fluorescence is used to determine the elemental composition, x-rays are emitted during the jump of an electron from higher to a lower level. These x-rays generated by accelerating the electron in metal by using high potential charge. The primary X-rays have absorbed the material and it cause the movement of an electron from one level to another level and also emit secondary x-rays. The analysis of these secondary x-rays provides information about the chemical composition. It also used to identify the catalytic poisoning by determining the element deposition, such as the deposition of chlorine, sulfur. The main advantage of this technique that it does required any kind of sample preparation.
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