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Biodiesel is known as the best candidate for the replacement of nonrenewable biodiesel. Currently, it has been produced from transesterification and esterification of triglycerides and fatty acids, respectively. However, the production of biodiesel from waste oils surrounded by numbers of obstacle such as saponification problem, complex purification process, high-cost production, and dual-step processes has made them the least choice feedstock. Recently, an advanced scientific finding in the catalysis technology has been reported for the reduction of free fatty acid (FFA) from waste oils. Photocatalysis is a light-induced catalytic process that involves oxidization and/or reduction of organic molecules via redox reactions activated through the electron-hole pair generated on the surface of semiconductors upon light irradiation. The esterification of FFA with ethanol is carried out according to the Langmuir Hinshelwood reaction path: when a photocatalyst is irradiated by ultraviolet light, the photogenerated electrons (e−) in the valence band are excited and transferred to the conduction band, and the same number of holes (h+) will be produced. The methanol adsorbed on the catalyst surface reacts with photogenerated hole (h+) to generate H+ and radical methanol. At the same time, fatty acid molecules adsorbed on the surface of the catalyst is reduced to form free radical by accepting the photogenerated electron (e−) on the surface. Then, the reaction between radicals and protons will react to form methyl ester and water. In this chapter, we successfully studied, analyzed, and discussed the classical technique of biodiesel production, and fundamental, theories, concept, and potential of advanced photocatalysis technology in the biodiesel production.

Developing new high efficiency and stable catalysts for the degradation of organic compounds in effluents, under solar light has been a great challenge. In this regard, this work describes the synthesis of a α-Fe2O3/Nb2O5 mixed oxide by the Pechini method. The results of XRD and SEM/EDS confirmed the presence of Fe and Nb in the materials synthesized. The mixture of semiconductors oxides resulted in the material of greater specific surface area. The catalyst efficiency was verified through the photodegradation of methylene blue (MB) by the heterogeneous photo-Fenton reaction, under different experimental conditions using a 24 factorial experimental design with the central point in triplicate. The variables that most influenced the system response were the concentrations of MB, of the catalyst and of H2O2. The mechanism proposed followed Langmuir-Hinshelwood kinetics, with the greatest color removal percentage of 85%, under the optimal experimental conditions found. The mixed oxide exhibited greater efficiency when compared to the pure oxides synthesized since the synergic effect resulting from the mixture between Fe and Nb oxides led to the hematite activation. The catalyst remained stable even after five consecutive reaction cycles, unlike Nb2O5, suggesting its potential application in the environmental pollutants treatment.

Char structural changes occurring during gasification are normally only related to conversion and few data is published on the effect of carbon dioxide partial pressure on the extent of pore development. In this study, the char–CO2 reactivity and consequential pore development of different Highveld coal chars were investigated over a wide pressure range. Reactivity experiments were performed in a fixed bed reactor under reaction controlled conditions. The initial reaction rate was found to be solely a function of temperature and CO2 partial pressure and was well described by the Langmuir Hinshelwood rate equation. Surface analyses were performed on fresh and partially converted chars to quantify the effect of CO2 partial pressure on pore development. It was found that pores development more rapidly with conversion if the CO2 partial pressure is increased. This outcome has fundamental implications for the interpretations of gasification reaction kinetics. The term describing the total amount of active sites in the Langmuir Hinshelwood equation might not be constant at all partial pressures, and the possible impact of that is shown in this work. More work is required to further understand these effects and incorporate them appropriately into high pressure rate equations.

On-surface synthesis is an alternative approach for the manufacture of polymers, currently under development, which enables creating materials not accessible by conventional wet chemistry. Molecules are deposited on a metal surface acting as support and catalyst for reactions, interlinking the monomers into polymers, enabling the formation of many fascinating materials. A major challenge with this approach is that the on-surface reactions are difficult to predict. Therefore, the quest for finding new materials with on-surface synthesis has been, to a large extent, based on trial-and-error experimentation. Last years’ computer simulations, in terms of electronic structure theory and transition state theory, have played an important role in elucidating the reaction mechanisms of reactions relevant to on-surface polymerization, bringing us closer to the appreciation needed to anticipate how molecules react on surface. Here, the author highlights some of the theoretical work that has been performed to gain knowledge about reactions on surfaces aided by computer simulations, and particularly focuses on the on-surface Ullmann coupling, the surface-chemistry of terminal alkynes and an electrocyclic ring-closure reaction. The discussion will be centered both around the prevailing theoretical understanding as well as what knowledge we are currently lacking, and what we may expect from computer simulations during forthcoming years.

Publisher Summary Catalytic mechanisms in heterogeneous catalysis with two steps can be exemplified by a Mars-van Krevelen oxidation mechanism. Such a redox mechanism takes into account possible reduction of the oxide, which occurs at a rate dependent on organic reactant and takes place in the absence of gas phase oxidants. Isotope labeling studies confirmed that oxygen in product comes from lattice oxygen, not gas phase species directly. In the case of heterogeneous catalysis, equations for the reaction rates are exactly the same. Another option in analysis kinetics of complex reactions is to separate activity from selectivity and to eliminate time as a parameter in the system of differential equations. The concept of ideal surfaces is often too simplistic to describe adsorption and kinetics, since adsorbed molecules are not equal in the ability to bind chemisorbed molecules. The multicentered nature of adsorption on catalyst's surfaces indicates that the number of adsorption sites on which a molecule (especially in organic catalysis) is adsorbed is not equal to unity as is frequently (and tacitly) assumed. Moreover, in some reactions there could be a different number of adsorption sites for different substances. This results in a competition for sites between participating molecules. Intermediates which are involved in heterogeneous catalysis could have ionic character, which require an extension of the general treatment of complex reactions.

This work studied the degradation and adsorption of dimethylphthalate (DMP) using various TiO-based photocatalysts: TiO Aeroxide P25, Kronos vlp7000, Hombikat UV-100, Kemira 650 and a synthesized photocatalyst named SG750. As the photocatalysts with mixed anatase and rutile phases, P25 and SG750, showed greater activity than those of pure phase, an in-depth study was undertaken of these two catalysts in the adsorption and degradation of DMP. The degradation results were fitted with a high degree of correlation to the Langmuir-Hinshelwood model and those for adsorption to the Freundlich model. The Freundlich constants showed good correlation with FTIR observations of the DMP-P25 and DMP-SG750 interactions. These two photocatalysts were additionally modified by photodeposition with Pt and Au (0.5–1.5 wt%) to study the effect of these metals on degradation and mineralization kinetics., E. Pulido Melián would like to thank the University of Las Palmas de Gran Canaria for her postdoctoral contract.

A first ever, comprehensive overview is presented of the thermodynamics and reaction kinetics principles focusing on gas-exchange and gas–solid reactions involved in the technically important gaseous surface engineering processes ‘carburising’, ‘carbonitriding’, ‘nitriding’ and ‘nitrocarburising’. The most important gas-exchange and gas–solid reactions is described, thereby providing a consistent set of equilibrium constants for the reactions. The mechanisms and the kinetics of gas-exchange and gas–solid reactions are dealt with in a fundamental way. The available reaction kinetic data are provided for the technically important gaseous and mass transfer reactions. On this basis (future) modelling of the processes mentioned becomes possible. A set of extended examples of model calculation has been incorporated. Finally a discussion on phase and potential diagrams for the binary Fe-C and Fe-N systems and on phase relations in the ternary Fe-C-N system has been included.

Catalysts are substances or materials that increase the reaction rates while emerged unaltered after reaction. Solid catalysts are particularly important because of their ease of separation from reaction mixture and stability. In other occasions, solid substrates are employed to produce a desired product. Reactions involving solid surface are thus common. Reactions occurring on solid surface start with reactants or catalyst from the fluid phase collide and associate with active centers (or sites) on the surface, i.e. adsorption. Adsorption can be described as Langmuir isotherm if uniform surface activity (or ideal surface) is assumed. Nonideal surfaces can be modeled by a distribution of interaction energy or multiple adsorption layers. Exland and UniLan are two models based on exponential and uniform distributions of active sites with adsorption energy. The adsorption isotherms exhibit different dependence on the bulk adsorbate concentration: Types I through V. LHHW kinetics is derived from Langmuir adsorption isotherm. PSSH is more general and a better approximation than FES as PSSH can be traced back to the overall effect of all the fluxes on the reaction network or pathway. However, the final rate expressions are quite similar when the rate constants are lumped together. Both ExLan and UniLan isotherms can also be employed to derive at reaction rate expressions. The rate expression can also appear to be of power-law form if Freundlich or Temkin approximation were applied to UniLan isotherms. The multilayer adsorption can also be applied to describe reactions on nonideal surfaces as steric interactions and interaction potential differences can be effectively modeled by multilayers. Rate expressions for non catalytic reactions involving solids can be derived in the same manner as LHHW. The participation of the surface active centers in actual reactions can affect the site balance, either active center renewal or depletion occurs. For catalytic surfaces, the activity can decrease with increase duration of service. The surface activity decline can be treated the same way as the concentration of a reacting component in the mixture.

Clinoptilolite, a natural zeolite, has been investigated as the basic raw material to synthesize a catalyst for methanol dehydration to dimethyl ether and a novel kinetic model, based on Langmiure-Hinshelwood method, was presented for the reaction with the optimized catalyst and the kinetic parameters were determined as functions of temperature.The modified clinoptilolite zeolite subject of this study might be of high industrial interest because of the relative lower activation energy (ca. 60 kJ mol −1 ) compared to other reported zeolitic and nonzeolitic catalysts.

Publisher Summary This chapter provides information of surface reactions. Different surfaces cause a substance to react in different ways. A surface reaction required five consecutive steps—diffusion of the reactant molecules to the surface; adsorption of the gases on the surface; reaction of the adsorbed reactants on the surface; desorption of the products formed; and diffusion of the desorbed products into the main body of the gas phase. A fundamental understanding of the great diversity of chemistry occurring at surfaces requires familiarity with the molecule–surface interaction. Two main types of adsorption may be distinguished. One is relatively weak and arises mainly through van der Waals interactions; this is called physical adsorption, or physisorption. Physical adsorption equilibria are usually established very rapidly, except when limited by gaseous diffusion. The other type of adsorption is of a much stronger nature. When the molecules are bound to a surface, chemical bonds can be established between the adsorbed molecule and the surface in the so-called chemisorption.

The reactions of methane and O2, H2O, CO2 comprise a multiple reaction system. In modeling by a Langmuir-Hinshelwood model, the judicious choice of reactions is important. In modeling by microkinetics, many kinetic parameters are needed and one should use information from many independent sources.

The aldolic condensations of benzaldehyde on acetone and acetophenone have been investigated on hydrotalcites, KF and KNO 3 supported on alumina, La 2 O 3 , X zeolites containing Cs and Mg clusters. A Langmuir Hinshelwood kinetic mechanism with competitive adsorption of the reactants is observed. Changing the polarity of the solvent induces large differences of rate. Hydrated hydrotalcite is the best catalyst at 273 K, reaching 70% conversion with 90% selectivity to aldol. Supported KF catalyses these reactions at 273 with lower selectivity. The different solid bases have been compared at a reaction temperature of 423 K: KF treated at 723 K is the most active, reaching 90% selectivity in chalcone at 80% conversion.

The partial oxidation of methane to syn-gas was investigated at conditions as close as possible to those industrially relevant: pressures up to 800 kPa and temperatures above 1200 K. Both the kinetics of the gas-phase reactions and the catalytic reactions, using a Rh/α-Al 2 O 3 catalyst, were studied. The gas-phase reactions were modelled successfully with a reaction network consisting of 40 elementary radical reactions. The Rh catalyst showed two active states: a low activity oxidized state and a high activity reduced state. Simulations with the gas-phase model indicated that gas-phase reactions are important in the catalytic oxidation reactions. In addition, quench of radicals on the inert, the catalyst pellets and the reactor wall was found to suppress the reactant conversions. Detailed kinetic modelling of the reactions on the catalyst in the oxidized state requires the simultaneous treatment of catalytic reactions, (surface initiated) gas-phase reactions and quench reactions.

Carbonyl sulphide hydrolysis using two alumina catalysts (surface areas 150 m2g- 1 and 300m2g- 1) has been studied. Previous studies have focused upon concentration ranges for the reactant ([COS] typically > 1000 ppm) and temperatures (typically > 100 °C) that are much higher than those encountered commercially. In the study presented here, the concentration of carbonyl sulphide is 150 ppm, reactor pressure (125 – 187 kPa) and temperatures in the range of 30 – 250 °C. At the higher temperature (250 °C) the data collected are in agreement with previous studies and the reaction follows Langmuir Hinshelwood kinetics with the surface hydrolysis of a thiocarbonate being the rate determining step. Data obtained at lower temperature (30 – 60 °C) indicates that the rate of COS hydrolysis decreases monotonically with increasing [H2O]. These data are consistent with Langmuir Hinshelwood kinetics where the products are not adsorbed and either (a) the adsorption of COS is rate determining or (b) the surface reaction of adsorbed COS and an intermediate derived from H2O is rate determining. The current experimental data cannot differentiate between these possibilities. Using present test conditions it is apparent that the surface area of the alumina catalyst is an important design parameter, a feature not observed in previous high temperature studies.

The solid state hydrogenation, deuteration and alkylation of 4-tert-butylphenol have been studied under mild conditions : at room temperature and one bar gas pressure. These catalytic organic solid-gas reactions are completely heterogeneous. Experimental kinetic studies show that the reaction rates depend on the interface nature. Theoretical results are in good agreement with a diffusional process and allow to depicte catalytic solid-gas reactions by a three steps mechanism.

Publisher Summary This chapter describes the effect of ash components on the NO reduction and formation in the presence and absence of air. The reason for the low concentrations of NOx in the flue gas is the internal reduction or decomposition reactions of NO with radicals such as –CHN and –NH and reactions on surfaces, above all on char, coal, and limestone. The reduction of NO is, thus, both homogeneous and heterogeneous. In a study described in the chapter, the reaction of NO over a mixture of sand and oxides showed a few percent reductions in the absence of oxygen even at a space velocity of 10000 h−1. The effect was most pronounced with Fe2O3 in the bed. With oxygen and nitrogen present, a net production was measured. The reduction of 5000 ppm NH3 over activated carbon was not much influenced by the presence of nitrogen and 1% O2, whether it is mixed with sand or other oxides. The fly ash was less reactive, with regard to the reduction of NO, than activated carbon, which may be attributed to a lower specific surface area. The presence of oxygen and nitrogen decreased the reduction at 800°C and 900°C.

Summary The synthesis of paratolunitrile (PTN) and terephtalonitrile (TPN) by reaction of paraxylene with nitrogen monoxide was studied over a series of aerogel chromium oxide alumina catalysts. The stabilization of the active phase was interpreted on the basis of Cr 2 O 3 support interactions. Kinetic studies show that the reaction follows a “redox” mechanism for the formation of PTN and a Langmuir Hinshelwood mechanism for the production of TPN.

Publisher Summary This chapter presents an account of the main results of the studies of the kinetics of some industrial heterogeneous catalytic reactions. The phenomenon of catalysis is a subject of chemical kinetics. The accumulation of data on the kinetics of concrete catalytic reactions favors the progress of the theory of catalysis. The chapter discusses mass transfer in heterogeneous catalysis. Purely physical phenomena––namely, the diffusion of reactants to the catalyst surface (in the majority of cases, to the inner surface of pores), the diffusion of products from this surface, and the transfer of heat evolved or absorbed in the course of the reaction––are indispensable components of the heterogeneous catalystic process. A purely experimental criterion of the kinetic region––namely, independence of the experimentally observed reaction rate on the size of catalyst grains––is used.

Publisher Summary This chapter illustrates the studies that have been made on the various electron transfer reactions, occurring between metal ions (of the same element) in homogeneous solution. These reactions include the types known as exchange reactions, mutual oxidation-reduction reactions, catalyzed substitutions and isomerizations, and electron transfer reactions. The isotopic method is the most widely used experimental technique for the study of exchange reactions. A suitable radioactive tracer is used to label one or other of the oxidation states of the metal ion; the extent of the exchange reaction is then followed by the analysis of the amount of radioactivity present in one or both of the oxidation states after mixing reactants, quenching the reaction and separating the reactants at various times. Depending on the exchange rate, either a rapid mixing and quenching technique or the more usual slower technique is employed. Exchange reactions can be sometimes investigated by the techniques of polarimetry, nuclear magnetic resonance, and electron spin resonance.

Publisher Summary A wide variety of methods are used to study the kinetics of heterogeneous reaction. This chapter focuses on those methods that are, particularly developed for the study of such reactions and discusses the study of kinetics by conventional measurements, for example, chemical analysis and pressure changes. The chapter discusses solid–gas interface. There are two distinguishable types of adsorption at the gas–solid interface––physical adsorption and chemisorptions. Measurement of weight changes is a very direct method of obtaining adsorption kinetics and is also used to follow the course of other heterogeneous reactions. It is, particularly suitable for adsorption measurements with small surface areas at relatively high pressures, where the volumetric method is insensitive. The solid–liquid interface and the solid–solid interface are also reviewed. There are certain essential differences between solid state reactions and reactions involving gaseous or liquid phases. In the latter case, the kinetic motion of the reactant molecules ensures that they are available to one another for reaction under conditions that can be defined by statistical laws. Solid state reactions occur among regular crystal lattices, in which the kinetic motion is very restricted and depends on the presence of lattice defects.

Publisher Summary This chapter discusses the catalysis and reaction kinetics at liquid interfaces. Interfacial reactions differ from those in bulk in the uniform and controlled accessibility and orientation of the reactant molecules. In addition, there is a possibility of a dramatic intensification of electric effects. The fundamental energy of activation of a chemical reaction may also be modified, though this has been substantiated only at solid interfaces so far. These reactions have some unique features—for example, with suitable control of the experimental conditions, not only the rate but also the position of equilibrium and the constitution of the products may be modified by the presence of the liquid interface. The practically complete inhibition of a reaction through a slight change in the molecular packing in the interface is demonstrated, providing a stimulating model for the similar high specificity in biological systems. The electric potentials at air–water and oil–water surfaces can easily be measured, clarifying the remarkable charge effects observed for ionic reactions at electrically charged surfaces. The rapidly increasing practical importance of reactions at liquid interfaces is seen from the common industrial use of reactions in emulsions, including the processes of soap manufacture and polymerization of synthetic rubber.