Nanomaterial-based catalysts are usually heterogeneous catalysts broken up into metal nanoparticles in order to enhance the catalytic process. Metal nanoparticles have high surface area, which can increase catalytic activity. Nanoparticle catalysts can be easily separated and recycled. [1] [2] [3] They are typically used under mild conditions to prevent decomposition of the nanoparticles. [4]
Functionalized metal nanoparticles are more stable toward solvents compared to non-functionalized metal nanoparticles. [5] [6] In liquids, the metal nanoparticles can be affected by van der Waals force. Particle aggregation can sometimes decrease catalytic activity by lowering the surface area. [7] Nanoparticles can also be functionalized with polymers or oligomers to sterically stabilize the nanoparticles by providing a protective layer that prevents the nanoparticles from interacting with each other. [8] Alloys of two metals, called bimetallic nanoparticles, are used to create synergistic effects on catalysis between the two metals. [9]
Nanoparticle catalysts are active for the hydrogenolysis of C-Cl bonds such as polychlorinated biphenyls. [5] [6] Another reaction is hydrogenation of halogenated aromatic amines is also important for the synthesis of herbicides and pesticides as well as diesel fuel. [5] In organic chemistry, hydrogenation of a C-Cl bond with deuterium is used to selectively label the aromatic ring for use in experiments dealing with the kinetic isotope effect. Buil et al. created rhodium complexes that generated rhodium nanoparticles. These nanoparticles catalyzed the dehalogenation of aromatic compounds as well as the hydrogenation of benzene to cyclohexane. [6] Polymer-stabilized nanoparticles can also be used for the hydrogenation of cinnamaldehyde and citronellal. [5] [7] [10] [9] Yu et al. found that the ruthenium nanocatalysts are more selective in the hydrogenation of citronellal compared to the traditional catalysts used. [9]
The Reduction of gold, cobalt, nickel, palladium, or platinum organometallic complexes with silanes produces metal nanoparticle that catalyze the hydrosilylation reaction. [11] BINAP-functionalized palladium nanoparticles and gold nanoparticles have been used for the hydrosilylaytion of styrene under mild conditions; they were found to be more catalytically active and more stable than non-nanoparticle Pd-BINAP complexes. [11] [12] The reaction may also be catalyzed by a nanoparticle that consists of two metals. [5] [13]
An oxidation reaction to form adipic acid is shown in figure 3 and it can be catalyzed by cobalt nanoparticles. [5] This is used in an industrial scale to produce the nylon 6,6 polymer. Other examples of oxidation reactions that are catalyzed by metallic nanoparticles include the oxidation of cyclooctane, the oxidation of ethene, and glucose oxidation. [5]
Metallic nanoparticles can catalyze C–C coupling reactions such as the hydroformylation of olefins, [5] the synthesis of vitamin E and the Heck coupling and Suzuki coupling reactions. [5]
Palladium nanoparticles were found to efficiently catalyze Heck coupling reactions. It was found that increased electronegativity of the ligands on the palladium nanoparticles increased their catalytic activity. [5] [14]
The compound Pd2(dba)3 is a source of Pd(0), which is the catalytically active source of palladium used for many reactions, including cross coupling reactions. [4] Pd2(dba)3 was thought to be a homogeneous catalytic precursor, but recent articles suggest that palladium nanoparticles are formed, making it a heterogeneous catalytic precursor. [4]
Iron oxide and cobalt nanoparticles can be loaded onto various surface active materials like alumina to convert gases such as carbon monoxide and hydrogen into liquid hydrocarbon fuels using the Fischer-Tropsch process. [15] [16]
Much research on nanomaterial-based catalysts has to do with maximizing the effectiveness of the catalyst coating in fuel cells. Platinum is currently the most common catalyst for this application, however, it is expensive and rare, so a lot of research has been going into maximizing the catalytic properties of other metals by shrinking them to nanoparticles in the hope that someday they will be an efficient and economic alternative to platinum. Gold nanoparticles also exhibit catalytic properties, despite the fact that bulk gold is unreactive.
Yttrium stabilized zirconium nanoparticles were found to increase the efficiency and reliability of a solid oxide fuel cell. [17] [18] Nanomaterial ruthenium/platinum catalysts could potentially be used to catalyze the purification of hydrogen for hydrogen storage. [19] Palladium nanoparticles can be functionalized with organometallic ligands to catalyze the oxidation of CO and NO to control air pollution in the environment. [17] Carbon nanotube supported catalysts can be used as a cathode catalytic support for fuel cells and metal nanoparticles have been used to catalyze the growth of carbon nanotubes. [17] Platinum-cobalt bimetallic nanoparticles combined with carbon nanotubes are promising candidates for direct methanol fuel cells since they produce a higher stable current electrode. [17]
In magnetic chemistry, nanoparticles can be used for catalyst support for medicinal use.
Besides conventional catalysis, nanomaterials have been explored for mimicking natural enzymes. The nanomaterials with enzyme mimicking activities are termed as nanozymes. [20] Many nanomaterials have been used to mimic varieties of natural enzymes, such as oxidase, peroxidase, catalase, SOD, nuclease, etc. The nanozymes have found wide applications in many areas, from biosensing and bioimaging to therapeutics and water treatment.
Nanocatalysts are of wide interest in fuel cells and electrolyzers, where the catalyst strongly affects efficiency.
In fuel cells, nanoporous materials are widely used to make cathodes. Porous nanoparticles of platinum have good activity in nanocatalysis but are less stable and their lifetime is short. [21]
One drawback to the use of nanoparticles is their tendency to agglomerate. The problem can be mitigated with the correct catalyst support. Nanoparticles are optimal structures to be used as nanosensors because they can be tuned to detect specific molecules. Examples of Pd nanoparticles electrodeposited on multi-walled carbon nanotubes have shown good activity towards catalysis of cross-coupling reactions. [22]
Nanowires are very interesting for electrocatalytic purpose because they are easier to produce and the control over their characteristics in the production process is quite precise. Also, nanowires can increase faradaic efficiency due to their spatial extent and thus to greater availability of reactants on the active surface. [23]
The nanostructures involved in electrocatalysis processes can be made up of different materials. Through the use of nanostructured materials, electrocatalysts can achieve good physical-chemical stability, high activity, good conductivity and low cost. Metallic nanomaterials are commonly made up of transition metals (mostly iron, cobalt, nickel, palladium, platinum). Multi-metal nanomaterials show new properties due to the characteristics of each metal. The advantages are the increase in activity, selectivity and stability and the cost reduction. Metals can be combined in different ways such as in the core-shell bimetallic structure: the cheapest metal forms the core and the most active one (typically a noble metal) constitutes the shell. By adopting this design, the use of rare and expensive metals can be reduced down to 20%. [24]
One of the future challenges is to find new stable materials, with good activity and especially low cost. Metallic glasses, polymeric carbon nitride (PCN) and materials derived from metal-organic frameworks (MOF) are just a few examples of materials with electrocatalytic properties on which research is currently investing. [25] [26] [27]
Many of the photocatalytic systems can benefit from the coupling with a noble metal; the first Fujishima-Honda cell made use of a co-catalyst plate as well. For instance, the essential design of a disperse photocatalytic reactor for water splitting is that of a water sol in which the dispersed phase is made up of semiconductor quantum dots each coupled to a metallic co-catalyst: the QD converts the incoming electromagnetic radiation into an exciton whilst the co-catalyst acts as an electron scavenger and lowers the overpotential of the electrochemical reaction. [28]
Some techniques that can be used to characterize functionalized nanomaterial catalysts include X-ray photoelectron spectroscopy, transmission electron microscopy, circular dichroism spectroscopy, nuclear magnetic resonance spectroscopy, UV-visible spectroscopy and related experiments.
Catalysis is the increase in rate of a chemical reaction due to an added substance known as a catalyst. Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quickly, very small amounts of catalyst often suffice; mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst.
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and sometimes broadened to include metalloids like boron, silicon, and selenium, as well. Aside from bonds to organyl fragments or molecules, bonds to 'inorganic' carbon, like carbon monoxide, cyanide, or carbide, are generally considered to be organometallic as well. Some related compounds such as transition metal hydrides and metal phosphine complexes are often included in discussions of organometallic compounds, though strictly speaking, they are not necessarily organometallic. The related but distinct term "metalorganic compound" refers to metal-containing compounds lacking direct metal-carbon bonds but which contain organic ligands. Metal β-diketonates, alkoxides, dialkylamides, and metal phosphine complexes are representative members of this class. The field of organometallic chemistry combines aspects of traditional inorganic and organic chemistry.
Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to a molecule, often an alkene. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures. Hydrogenation reduces double and triple bonds in hydrocarbons.
The Suzuki reaction is an organic reaction, classified as a cross-coupling reaction, where the coupling partners are a boronic acid and an organohalide and the catalyst is a palladium(0) complex. It was first published in 1979 by Akira Suzuki, and he shared the 2010 Nobel Prize in Chemistry with Richard F. Heck and Ei-ichi Negishi for their contribution to the discovery and development of palladium-catalyzed cross-couplings in organic synthesis. This reaction is also known as the Suzuki–Miyaura reaction or simply as the Suzuki coupling. It is widely used to synthesize polyolefins, styrenes, and substituted biphenyls. Several reviews have been published describing advancements and the development of the Suzuki reaction. The general scheme for the Suzuki reaction is shown below, where a carbon-carbon single bond is formed by coupling a halide (R1-X) with an organoboron species (R2-BY2) using a palladium catalyst and a base. The organoboron species is usually synthesized by hydroboration or carboboration, allowing for rapid generation of molecular complexity.
The Sonogashira reaction is a cross-coupling reaction used in organic synthesis to form carbon–carbon bonds. It employs a palladium catalyst as well as copper co-catalyst to form a carbon–carbon bond between a terminal alkyne and an aryl or vinyl halide.
The Negishi coupling is a widely employed transition metal catalyzed cross-coupling reaction. The reaction couples organic halides or triflates with organozinc compounds, forming carbon-carbon bonds (C-C) in the process. A palladium (0) species is generally utilized as the metal catalyst, though nickel is sometimes used. A variety of nickel catalysts in either Ni0 or NiII oxidation state can be employed in Negishi cross couplings such as Ni(PPh3)4, Ni(acac)2, Ni(COD)2 etc.
In organic chemistry, carbon–hydrogen bond functionalization is a type of organic reaction in which a carbon–hydrogen bond is cleaved and replaced with a C−X bond. The term usually implies that a transition metal is involved in the C−H cleavage process. Reactions classified by the term typically involve the hydrocarbon first to react with a metal catalyst to create an organometallic complex in which the hydrocarbon is coordinated to the inner-sphere of a metal, either via an intermediate "alkane or arene complex" or as a transition state leading to a "M−C" intermediate. The intermediate of this first step can then undergo subsequent reactions to produce the functionalized product. Important to this definition is the requirement that during the C−H cleavage event, the hydrocarbyl species remains associated in the inner-sphere and under the influence of "M".
In organic chemistry, the Kumada coupling is a type of cross coupling reaction, useful for generating carbon–carbon bonds by the reaction of a Grignard reagent and an organic halide. The procedure uses transition metal catalysts, typically nickel or palladium, to couple a combination of two alkyl, aryl or vinyl groups. The groups of Robert Corriu and Makoto Kumada reported the reaction independently in 1972.
Platinum nanoparticles are usually in the form of a suspension or colloid of nanoparticles of platinum in a fluid, usually water. A colloid is technically defined as a stable dispersion of particles in a fluid medium.
An electrocatalyst is a catalyst that participates in electrochemical reactions. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinized electrode. Homogeneous electrocatalysts, which are soluble, assist in transferring electrons between the electrode and reactants, and/or facilitate an intermediate chemical transformation described by an overall half reaction. Major challenges in electrocatalysts focus on fuel cells.
In organic chemistry, a cross-coupling reaction is a reaction where two different fragments are joined. Cross-couplings are a subset of the more general coupling reactions. Often cross-coupling reactions require metal catalysts. One important reaction type is this:
In chemistry, a catalyst support is the material, usually a solid with a high surface area, to which a catalyst is affixed. The activity of heterogeneous catalysts is mainly promoted by atoms present at the accessible surface of the material. Consequently, great effort is made to maximize the specific surface area of a catalyst. One popular method for increasing surface area involves distributing the catalyst over the surface of the support. The support may be inert or participate in the catalytic reactions. Typical supports include various kinds of activated carbon, alumina, and silica.
Carbon nanotube supported catalyst is a novel supported catalyst, using carbon nanotubes as the support instead of the conventional alumina or silicon support. The exceptional physical properties of carbon nanotubes (CNTs) such as large specific surface areas, excellent electron conductivity incorporated with the good chemical inertness, and relatively high oxidation stability makes it a promising support material for heterogeneous catalysis.
Graphitic carbon nitride (g-C3N4) is a family of carbon nitride compounds with a general formula near to C3N4 (albeit typically with non-zero amounts of hydrogen) and two major substructures based on heptazine and poly(triazine imide) units which, depending on reaction conditions, exhibit different degrees of condensation, properties and reactivities.
Decarboxylative cross coupling reactions are chemical reactions in which a carboxylic acid is reacted with an organic halide to form a new carbon-carbon bond, concomitant with loss of CO2. Aryl and alkyl halides participate. Metal catalyst, base, and oxidant are required.
The dehydrogenative coupling of silanes is a reaction type for the formation of Si-Si bonds. Although never commercialized, the reaction has been demonstrated for the synthesis of certain disilanes as well as polysilanes. These reactions generally require catalysts.
Dehydrogenation of amine-boranes or dehydrocoupling of amine-boranes is a chemical process in main group and organometallic chemistry wherein dihydrogen is released by the coupling of two or more amine-borane adducts. This process is of due to the potential of using amine-boranes for hydrogen storage.
In organometallic chemistry, palladium-NHC complexes are a family of organopalladium compounds in which palladium forms a coordination complex with N-heterocyclic carbenes (NHCs). They have been investigated for applications in homogeneous catalysis, particularly cross-coupling reactions.
Karen Ila Goldberg is an American chemist, currently the Vagelos Professor of Energy Research at University of Pennsylvania. Goldberg is most known for her work in inorganic and organometallic chemistry. Her most recent research focuses on catalysis, particularly on developing catalysts for oxidation, as well as the synthesis and activation of molecular oxygen. In 2018, Goldberg was elected to the National Academy of Sciences.
Heterogeneous metal catalyzed cross-coupling is a subset of metal catalyzed cross-coupling in which a heterogeneous metal catalyst is employed. Generally heterogeneous cross-coupling catalysts consist of a metal dispersed on an inorganic surface or bound to a polymeric support with ligands. Heterogeneous catalysts provide potential benefits over homogeneous catalysts in chemical processes in which cross-coupling is commonly employed—particularly in the fine chemical industry—including recyclability and lower metal contamination of reaction products. However, for cross-coupling reactions, heterogeneous metal catalysts can suffer from pitfalls such as poor turnover and poor substrate scope, which have limited their utility in cross-coupling reactions to date relative to homogeneous catalysts. Heterogeneous metal catalyzed cross-couplings, as with homogeneous metal catalyzed ones, most commonly use Pd as the cross-coupling metal.