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Advanced Oxidation Processes for Wastewater Treatment
Published in Sreedevi Upadhyayula, Amita Chaudhary, Advanced Materials and Technologies for Wastewater Treatment, 2021
Gunjan Deshmukh, Haresh Manyar
Transition metals have been widely studied as homogeneous catalytic systems in AOPs. The activity of homogeneous catalysts is well known, and these catalysts are mainly involved in Fenton's process, photo-Fenton's process and electrochemical oxidations integrated with O3 and UV. The most commonly studied transition metals involved in the above mentioned AOPs are Fe, Zn, Mn, Ti, Cr, Cu, Co, Ni, Cd, and Pb salts in their possible multivalent forms [5]. Among these elements, Fe is the most extensively studied element in Fenton and photo-Fenton processes to generate hydroxyl radicals. Further, Fe salts are widely used in catalytic ozonolysis processes, which help to enhance the activity of overall process [6]. An ideal homogeneous photocatalyst should be effective in a wide range of pH, and it was observed that the degradation efficiency of pollutants is higher at acidic pH with Fe as a catalyst compared to a catalyst-free process [7]. Further, it can also be noted that higher Fe catalyst concentration leads to Fe3+ ion formation, which increases the scavenging effect and decreases the efficiency of the catalyst. There are disadvantages of homogeneous catalysts, which can be listed as narrow pH range, short life span of the catalyst, difficult isolation and recovery cost, scavenging effect, no reusability, formation of by-products, and use of large excess of oxidants. To overcome these barriers, heterogeneous catalysts are advantageous in current science for water purification.
Structural Design for Molecular Catalysts
Published in Qingmin Ji, Harald Fuchs, Soft Matters for Catalysts, 2019
Qingmin Ji, Qin Tang, Jonathan P. Hill, Katsuhiko Ariga
Homogeneous catalysis and organometallic chemistry have stimulated and supported each other since the early days of hydroformylation (1938), olefin polymerization (1953), and acetaldehyde synthesis (1959) [10]. Nevertheless, heterogeneous catalysis dominates the industrial processes. Heterogeneous catalysis using the surfaces of metals or ionic platform materials may suffer from a limited number of active sites in catalysis. By contrast, homogeneous catalysis allows exquisite control on reactivity and selectivity by relatively readily analysis and facile tuning on active sites. Organometallic catalysts are said to be able to act as a bridge between heterogeneous and homogeneous catalysts. They bring together the benefits of heterogeneous catalysis (i.e., recyclability and easy removal from the reaction mixture) and homogeneous catalysis with intimate control over the transformations in the metal-ligand system.
Synthesis Of 1,4-Cis Polybutadiene By The Heterogenized Dithiosystem On Base Of Nanosize Montmorillonite
Published in A. K. Haghi, Lionello Pogliani, Devrim Balköse, Omari V. Mukbaniani, Andrew G. Mercader, Applied Chemistry and Chemical Engineering, 2017
Fizuli A. Nasirov, nazil F. Janibayov, sevda R. Rafiyeva, gulara N. Hasanova
The processes of oligomerization, polymerization of olefins and dienes processed with application of the homogeneous catalytic systems of Ziegler-Natta type. The homogeneous catalysis has a number of advantages concluding in high selectivity, large reaction proceeding rate, etc. But simultaneously it has a number of essential defects: single use of the catalysts, application of a large quantity of solvent, necessity of washing of purposeful products by water from catalysts owing to what a large number of quantities of waste waters containing ions of heavy metals, deteriorating ecological situation of production in formed. In addition, it is extremely difficult to create a continuous technology or production, to carry out a process in the gas phase, etc.
The influence of Pd4Cr6@(NH2)M-MOF(Cr) catalyst on components of formic acid dehydrogenation gaseous products
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2023
Ruthenium-based, iridium-based, iron-based, and copper-based homogeneous catalysts all present good catalytic activities. However, homogeneous catalysts are difficult to be reused, which restricts their industrial application. The reaction temperature of heterogeneous catalysts such as non-precious copper oxide-cerium oxide (Pechenkin et al. 2019), molybdenum carbide (Cao, Wang, and Ma 2018), molybdenum (Wang, Li, and Zheng 2018), and metal nickel (Bing, Liu, and Yi 2019) is high, and the content of carbon monoxide in the products is far beyond the tolerance range of precious metal electrodes of fuel cells (Sung et al. 2013). Some researchers have developed catalysts of Pd nanoparticles supported on a variety of carbon carriers (such as carbon nanospheres, porous carbon, biomass carbon, carbon nanotubes, etc.) (Duan et al. 2022; Garnica et al. 2020; Yao et al. 2020; Zhong et al. 2019). Other researchers have developed carbon supported bimetallic catalysts (such as Pd-Cr, Pd-Ag, etc.) (Gao, Wang, and Wang 2019; Wan, Zhou, and Xu 2022; Wang, Chi, and Gao 2019). Through theoretical calculations, domestic and foreign scientific researchers have also studied mechanisms of formic acid on Pt (100) and Pt (111) (Bhandari et al. 2020), Pt13/N-GNS (Feng and Wang 2021), Pt-TiO2 (Rezaei and Chermahini 2020), and Ni (100) (Rafiee and Bashiri 2019).
Study on obtaining high performance diesel/biodiesel fuel by using heterogeneous catalysts
Published in Petroleum Science and Technology, 2019
Liming Sun, Mengmeng Li, Xiaoou Ma, Zhenyuan Ma, Changfeng Ma, Ping Li
At present, the synthesis of alkyl esters is commonly carried out by using homogeneous catalysts (Dias, Alvim-Ferraz, and Almeida 2008; Miao, Li, and Yao 2009; Brito et al. 2012; Soriano, Venditti, and Argyropoulos 2009), due to their wide availability and low cost. However, homogeneous catalysis can lead to difficult separation of products, side reactions, costly neutralization, equipment corrosion and environmental pollution. So, this technique is unsatisfactory when considering cost and environmental issues. In order to overcome these drawbacks associated with homogeneous catalysis, heterogeneous catalysts are prepared and used in the synthesis of alkyl esters. This catalyst possesses good catalytic activity and is quite effective in facilitating the conversion of alkyl esters.
New perspectives on the nature and imaging of active site in small metallic particles: I. Geometric effects
Published in Chemical Engineering Communications, 2021
It is also important to understand some important distinctions between the traditional liquid phase processes, employing homogeneous catalysts, and the vapor phase processes, based on a solid catalyst and gaseous reactants. In homogeneous catalysis, the reactions take place within the liquid phase, with a homogeneous catalyst which is dissolved or dispersed in the liquid phase (in molar quantities) at the start of the reaction. As the liquid phase is continuous and isotropic, the concentration of the catalyst (say, µmol/lit) is homogeneous and uniform, and it is easy to accurately count the number of active sites or centers. However, in heterogeneous catalysis, i.e., in the case of supported metallic particles (nm-sized clusters on inorganic supports, typically oxidic) or in cases of free-standing particles or single crystals, further additional complications arise: The active centers (atomic sites) are usually located on different crystal planes and orientations (say, (111), (211), (100), (110), and other high-index planes, in some cases), and in different configurations, such as at corners, steps, edges, and other planar junctions or features, i.e., the planar discontinuity between 2 different crystal face surface terminations. The vastly different surface energetics of these atomic sites (primarily the coordination number and the electronic character – or, electronic density of states, d-band model) govern the key features and characteristics of activated chemisorption processes – such as heat of chemisorption, activated state complexes, and heat of desorption – which govern the surface reactivity (or, TOF). The TOF value is thus an “average” value, for a given particle size (with different contributions from different atomic sites, indirectly factored in the calculation). The relative proportion of these atomic sites will also change, even quite significantly, as a function of particle size. Even the adsorption modes of a reactant molecule, say, CO, can be different. For the oft-studied case of CO on Rh, CO can bind to Rh sites in three different ways: (a) linear CO, (b) bridged CO, and (c) gem dicarbonyl, Rh(CO)2. Finally, it is also important to note for metallic particles that are very small, <3 nm in size, and because these particles are non-equilibrium crystal structures that incorporate a significant lattice strain (as much as 10%, as measured by lattice parameter), it is not possible to definitely assign a particle shape as the crystal habits are yet not well-developed. For small metallic particles, there is also the prevalence of metal-support interactions (MSI), which usually manifest in a charge transfer (flow of electrons) from support to metal (in case of reducible oxidic supports, with ionic vacancies and interstitial species) or from metal to support (typically for non-reducible). These effects are usually termed as Schwab effect of the first type or second type. For this reason, the metal-support interface is a charged one, typically with excess electronic charge, and also a structural motif that is a locus of primary reaction zone, when compared to bulk areas away from it.