Advanced Catalytic Materials

By Ashutosh Tiwari

The subject of advanced materials in catalysisbrings together recent advancements in materials synthesis and technologies to the design of novel and smart catalysts used in the field of catalysis. Nanomaterials in general show an important role in chemical processing as adsorbents, catalysts, catalyst supports and membranes, and form the basis of cutting-edge technology because of their unique structural and surface properties.

Advanced Catalytic Materials is written by a distinguished group of contributors and the chapters provide comprehensive coverage of the current literature, up-to-date overviews of all aspects of advanced materials in catalysis, and present the skills needed for designing and synthesizing advanced materials. The book also showcases many topics concerning the fast-developing area of materials for catalysis and their emerging applications.

The book is divided into three parts:  Nanocatalysts – Architecture and Design; Organic and Inorganic Catalytic Transformations; and Functional Catalysis: Fundamentals and Applications. Specifically, the chapters discuss the following subjects:

• Environmental applications of multifunctional nanocomposite catalytic materials
• Transformation of nanostructured functional precursors using soft chemistry
• Graphenes in heterogeneous catalysis
• Gold nanoparticles-graphene composites material for catalytic application
• Hydrogen generation from chemical hydrides
• Ring-opening polymerization of poly(lactic acid)
• Catalytic performance of metal alkoxides
• Cycloaddition of CO2 and epoxides over reusable solid catalysts
• Biomass derived fine chemicals using catalytic metal bio-composites
• Homoleptic metal carbonyls in organic transformation
• Zeolites: smart materials for novel, efficient, and versatile catalysis
• Optimizing zeolitic catalysis for environmental remediation

• Language: English
• Publisher: Wiley
• Release date: May 6, 2015
• ISBN: 9781118998953

Book preview

Preface

The subject of advanced materials in catalysis brings together recent advancements in materials synthesis and technologies to the design of novel and smart catalysts used in the field of catalysis. Nanomaterials in general play an important role in chemical processing as adsorbents, catalysts, catalyst supports, and membranes and form the basis of cutting-edge technology because of their unique structural and surface properties. Advanced Catalytic Materials is written by a distinguished group of contributors, and the chapters provide comprehensive coverage of the current literature, up-to-date overviews of all aspects of advanced materials in catalysis, and presents the skills needed for designing and synthesizing advanced materials. The book also showcases many topics concerning the fast-developing area of materials for catalysis and their emerging applications.

The goal of this volume is to assemble recent advances in material syntheses and technologies in the design of novel and smart catalysts used in a wide range of applications. Catalysis covers diverse fields of chemistry and chemical engineering and plays a vital role in chemical processes. Over the past several decades, a large variety of catalysts has been synthesized and studied ranging from macromolecules to mesoporous silica to nanocatalysts. Advanced catalytic materials offers detailed chapters on the current syntheses of various types of catalysts and their wide range of applications. The design of materials with specific functional and effective properties is of great interest and enormous potential in their application in biomedical sciences and drug delivery. The remarkable growth in synthetic methods for new advanced materials during the last decade has led to the development of new approaches based on the state-of-the-art nanotechnology and is still receiving significant attention.

This book is written by a distinguished group of contributors suitable for a diverse readership by science and engineering scholars from different backgrounds, interests, and expertise in both academia and industry. It provides comprehensive coverage of the current literature, an up-to-date overview of all aspects of advanced materials in catalysis, and the skill required in designing and synthesizing advanced materials as catalysts. However, the scope of this book is much broader and includes topics concerning the growing area of materials for catalysis and their applications. The book is divided into two parts. Part 1: Nanocatalysts – Architecture and Design and Part 2: Organic and Inorganic Catalytic Transformations.

Chapter 1 discloses the environmental applications of multifunctional nanocomposite catalytic materials, the preparation of various combinations of materials with two or more distinct catalytic functionalities and application of these in three different cases which are relevant to environmental and sustainable catalysis viz. (i) coupling NOx storage systems with urea hydrolysis and selective catalytic reduction (SCR) catalysts, (ii) constructing a material that would act as a four-way catalyst, and (iii) studies on the generation of a material to promote selective oxidation of an organic molecule using H2O2 synthesized in situ from H2/O2 mixtures. This is an example of an attempt to couple two heterogeneously catalyzed reactions (in an atom efficient and clean manner) to replace other reactions that would be considered environmentally troublesome.

Chapter 2 highlights the state of the art with cost-effective synthesis of ultra-pure functional nanomaterials from single source molecular precursors, to understand the structural–functional surface properties relationship based on chemical–thermal stability of metal ions as well as Lewis acidic behavior of the metal ion in the coordinated state.

Nanoscience demands efficient synthetic methods for materials with controlled particle properties by tuning the preparative chemistry and has led to several methods adopted for hierarchical inorganic materials for potential applications. Aided by the soft chemical approach, highly stabilized crystalline and monodispersed nanomaterials may be synthesized on bench scale and may subsequently be scaled up for higher production level with an important facet of the molecules-to-materials approach. By tuning the desired functional properties of precise size and shape, this may offer exciting possibilities to fabricate new nanodevices with reproducible results based on structural–performance–activity relationships with high reliability.

Active carbon has played a key role in heterogeneous catalysis as a support for precious novel nanoparticles (NPs), such as palladium, platinum, and gold. In the 1990s, novel allotropic forms of carbon displayed a better-defined structure of active carbons which became commercially available. This triggered an interest for comparing active carbons with carbon allotropes until finally controls have been developed to show that these carbons have intrinsic active sites. Chapter 3 covers the state-of-the-art use of graphene either as a carbon catalyst or as a support of metal NPs. Considering the relatively short time that has elapsed since graphenes and related materials have become available in sufficient quantities for evaluation of their catalytic properties, it is easy to foresee that in the near future there should be a remarkable growth in this area. Novel preparation methods will make larger quantities of known doped graphenes available for evaluation as catalysts or supports for virtually any catalytic reactions. The design and modification of graphene supports are the key concept in increasing the interaction with the metal. The target in this area is to show the advantages in terms of optimal use of the support, fine tuning of the catalytic activity of the metal, and stability of the graphene-based supported catalyst with respect to any other support including metal oxides. Considering the features of graphene as a one-atom-thick surface, and combining the possibility to imprint the active site or support the metal NPs, in a few years use of graphene could lead to a drastic change in the panorama of catalysis optimizing the use of noble and critical metals and reducing the dependency of catalysis on these inorganic elements.

As catalysis, Au-NPs appear to be particularly important and efficient in organic reactions. They offer a most favorable combination of activity and selectivity in various catalytic reactions, viz., electro-catalysis, redox catalysis, carbon–carbon bond formation, and photocatalytic reactions. Moreover, recent literature reports that Au-NPs deposited on graphene nanosheets exhibit unprecedented catalytic activity for CO oxidation, reduction of nitro-aniline, and Suzuki–Miyaura coupling reactions of chlorobenzene with arylboronic acid. Chapter 4 mainly focuses on the synthesis and characterization of Au-NPs on graphene nanosheets and its catalytic activity toward synthesis and transformation. Au-NPs–graphene composite materials prove to be promising owing to their wide range of applications, viz., semiconductors, catalysis, photocatalysis, sensing platforms, surface-enhanced Raman scattering (SERS), electronics, and optics.

Chapter 5 gives insights into the synthesis of novel catalysts and their morphologies for the highest possible hydrogen generation kinetics. It further demonstrates that morphology can be tailored to achieve high-performance hydrogen generation. Ruthenium (Ru), platinum (Pt), nickel (Ni), palladium (Pd), cobalt (Co), Ni–B, Co–B, Co–P, Ni–Co–B, carbon nanotubes (CNT), and graphene are examples of these catalysts. Moreover, platinum supported on carbon (Pt/C), which is extensively utilized in proton exchange membrane fuel cells (PEMFCs), is also appropriate for hydrogen gas generation. Precious metal catalysts are costly, whereas metal and alloy catalysts from iron, nickel, and cobalt are less expensive. Therefore, replacement of precious metal catalysts with inexpensive materials for hydrogen generation would reduce the cost significantly.

A search for environmentally benign and sustainable material that could replace the commonly used petroleum-based materials will lead to less pollution to our environment. Polylactic acid (PLA) has recently attracted much interest as a replacement for conventional oil-based materials due to their biorenewability, biodegradability, and biocompatibility. Although several methods for synthesis of PLAs exist, the most convenient and promising route is the ring-opening polymerization (ROP) of lactide, in which the break of ring strain is the driving force. Using ROP makes it possible to control the chemistry of polymerization accurately, and thus, the properties such as molecular weight, molecular weight distribution, and architecture of the resulting polymer can be varied to suite the application. The method also provides the possibility to achieve desired end groups and copolymerization of various monomers, depending on the type of catalyst utilized. ROP has been carried out by solution polymerization, bulk polymerization, melt polymerization, and suspension polymerization. Chapter 6 emphasizes polymerization kinetics and the control exhibited by the different types of aluminum initiators/catalysts.

Metal alkoxides have a well-established role in catalytic reactions. In Chapter 7, a brief review on the history, characteristics and synthetic routes for preparing metal alkoxides are illustrated. The catalytic processes performed by these catalysts include polymerization of different olefin oxides and cyclic esters, asymmetric reduction of aldehydes and ketones, oxidation of sulfides and olefins, and a variety of asymmetric reactions. The remainder of the chapter discusses characteristics of these catalytic systems. Other challenges separate from the metal alkoxide catalysis involve development of catalytic protocols in solvent-free or in green solvent conditions, viz., H2O or liquid CO2. The second challenge is recovery of catalyst without loss of its activity. Supporting metal alkoxide onto inorganic solids, especially magnetic ones, may effectively solve the later problem.

Chapter 8 addresses the synthesis of cyclic carbonates between CO2 and epoxides and alkenes/arenes using reusable solid catalysts. The state-of-the-art reaction performance using reusable solid catalysts is highlighted relative to reaction mechanisms which are categorized into three groups, viz., (i) inorganic materials (layered mixed oxides, metal oxides, micro/mesostructured inorganic materials, and clays), (ii) organic materials (polymers, resins, and ionic liquids), and (iii) organic–inorganic hybrids composites (Metal Organic Frameworks (MOFs) and organic-functionalized inorganic materials). Finally, future perspectives of the synthesis of cyclic carbonates from CO2 and epoxides are given. Compared to homogeneous catalysts, heterogeneous catalysts have the advantages of typically being superior in stability and reusability, thereby facilitating process intensification. However, most heterogeneous catalysts have drawbacks such as limited catalytic activity and the necessity of solvents and/or cocatalysts. For this reason, development of new heterogeneous catalysts with industrial relevance is a great challenge. Detailed understanding of the reaction mechanisms over different catalysts in the cycloaddition of CO2 and epoxides with olefins will have significant impact on the rational design of catalysts and process engineering. Application of in situ spectroscopic characterization techniques and advanced data analysis are necessary to identify fundamental reaction steps, possibly leading to an in-depth understanding of the reaction and active sites. This information will be important for establishing catalyst structure versus catalytic activity/selectivity relationships. It has been clearly shown that effective catalysts have a dual feature, viz., combining the Lewis acid nature to activate the epoxide with a nucleophile to open the ring of the epoxide and also Lewis or Bronsted base nature to activate the CO2.

Chapter 9 presents an overview of architectures adopted for the catalytic/biocatalytic composites widely used in applications, viz., biomass valorization or the fine chemical industry. Information presented will update the reader with the most recent examples of construction designs and concepts considered for the synthesis of such composites whose catalytic properties result from the introduction of catalytic functionalities and vary from inorganic metal species (e.g., Ru, Ir, Pd, or Rh) to well-organized biochemical structures like enzymes (e.g., lipase, peroxidase, and β-galactosidase) or even whole cells.

Chapter 10 briefly discusses the role of homoleptic metal carbonyls in organic transformations. Metal carbonyls belong to a unique class of organometallic compounds where carbon monoxide is bonded to the metal atom through the carbon end. They enjoy their relevance in the synthesis of various complex and cluster compounds as well as acting as an agent in organic transformations and occasionally catalyze some unique chemical transformations. A fair effort has been made to accommodate organic reactions developed in the past few decades to illustrate how these may be employed to overcome difficulties experienced in conventional organic synthesis which requires the adoption of multistep syntheses. Discussion is confined to groups 6 and 8 transition metal carbonyls with a limited focus on some other metal carbonyls within the scope of the book.

Zeolites are smart materials that provide very attractive insights into the field of catalysis. Chapter 11 covers the fundamentals of zeolite materials science and their application as catalysts and includes the background and history of evolution of zeolites in the field of catalysis. Zeolites are solid acids, and the chemical nature, density, strength, and location of the acid sites are discussed. Shape-selective catalysis, which is a unique feature of zeolites, is also briefly addressed. The chapter summarizes their syntheses, application in organic transformations, medical application, disease control, and wastewater treatment.

Chapter 12 discusses the effects of chemical and physical properties of zeolites as they affect the catalytic efficacy and applications in environmental remediation. Heterogeneous catalysts, which reflect the majorly, have been extensively used in various technologies for several decades. Use of solid catalysts, especially for environmental remediation technologies, requires adapting the characteristics of the solid with respect to those generally used in conventional catalytic applications. Studies of different catalytic activities of some zeolites on selected organic pollutants demonstrated that optimization of zeolitic working conditions in purification of contaminated waters is paramount. Lastly, when the available zeolite is not suitable for a desired reaction, chemical modifications of the zeolite to display the required chemical and physical characteristics is an option. These different properties have a profound influence on the size, type, and nature of the molecules they adsorb. Due to their unique properties, zeolites have a great potential as effective sorbent materials for a large number of environmental treatment applications, such as water softening, ammonia removal from municipal sewage, fertilizer factory wastewaters, fish-breeding ponds, swimming pools, removal of heavy metals from natural waters, acid mine drainage treatment, industrial wastewater treatment, removal of phosphate, removal of dissolved organic compounds and dyes, oil spillages treatment, separation of solid impurities, radioactive wastewater purification, seawater desalination, permeable reactive barriers PRB, and many others.

The book is written for readers from diverse backgrounds across chemistry, physics, materials science and engineering, environmental and chemical engineering, and biotechnology. It offers a comprehensive view of cutting-edge research on advanced catalytic materials of a range of technological significance.

Editors

Ashutosh Tiwari, PhD, DSc

Salam Titinchi, PhD

January 12, 2015

Part I

NANOCATALYSTS – ARCHITECTURE AND DESIGN

Chapter 1

Environmental Applications of Multifunctional Nanocomposite Catalytic Materials: Issues with Catalyst Combinations

James A. Sullivan*, Orla Keane, Petrica Dulgheru and Niamh O’Callaghan

UCD School of Chemistry and Chemical Biology, Belfield, Dublin, Ireland

*Corresponding author: [e-mail]

Abstract

The combination of two or more catalytically active heterogeneous catalysts into a composite material which can perform more than one catalytic function allows improvements in situations where single catalytic beds might be used in place of a series of catalysed processes. This chapter describes attempts to combine catalytic materials (or processes) of environmental and green chemistry interest which would normally be separated. The specific combinations of catalysts and processes discussed are (a) NOx storage and reduction with NH3-selective catalytic reduction, (b) particulate matter combustion with NOx trapping materials and (c) H2O2 synthesis with selective epoxidation catalysts. In each case study, we present an introduction to the specific field detailing the current state of the art and discussing why these reactions, catalysts and processes are of interest in environmental and sustainable chemistry. Then we present a synopsis of our efforts to generate combined materials and processes and the various materials, process control and kinetics issues that arise during each of these combinations.

Keywords: Heterogeneous catalysts, multifunctional materials, nanocomposite materials, process integration, environmental and sustainable catalysis

1.1 Introduction

The use, where possible of catalysed processes is one of the tenets of green chemistry [1] and most reactions or processes of environmental importance utilise some catalytic step. The combination of two or more catalytically active heterogeneous catalysts into a composite material which can perform more than one catalytic function would allow improvements in several situations where single catalytic beds could be used in place of a series of catalysed processes.

Successful combinations of such catalysts and processes would allow

(a) nominally sequential process to be carried out in a single step in processes analogous to the desirable 1-pot reaction sought after by our synthetic chemistry colleagues,

(b) the combination of two materials that each have a catalytic function which operates under a specific process condition in order to generate a material that can operate under a range of conditions,

(c) combinations of catalytically active materials (each with specific catalytic functionalities) that operate within a complex process stream with each individual component carrying out a specific function.

This approach would lead to savings in the number of catalytic steps required in multistep processes and also perhaps to synergies between the catalysed processes.

1.1.1 The Three Way Catalyst

The most successful example of this approach is the development of the three way catalyst [2]. These automotive emissions aftertreatment systems are exceptionally effective, being able to selectively remove pollutants (some of which are initially only at ppm levels within the engine’s exit gas stream) from gasoline engine exhausts. The catalysts consist of a range of components, each with a specific purpose within the process stream, be it either to catalyse some reaction or to allow the overall composite catalyst to operate under non-optimum conditions. In order for them to operate most effectively significant changes to the processes generating the reactant stream in which they operated had to be developed.

Modern three way catalysts are nanocomposite materials containing catalytic and non-catalytic components which allow the materials to efficiently carry out a range of functions (CO oxidation, CxHy oxidation and NOx reduction) under a wide range of temperature conditions (250–800°C) in a process stream of variable composition (varying continuously between net oxidising and net reducing character).

Legislation has driven the development of these systems in order to decrease the emission of pollutants such as CO (a poisonous gas), volatile organic compounds (VOC) and NOx (implicated in smog formation) from vehicle exhausts [3].

1.1.2 Operation and Composition of the TWC

The operation and composition of modern TWCs has been comprehensively reviewed in a range of publications [4–7]. Here we will give a brief summary of their main features emphasising the combinations of materials used, the modifications to the catalyst and the modifications to the process stream required in order to allow them to effectively operate.

The catalyst reacts species which can be oxidised within the exhaust (i.e. reductants such as CO and CxHy) with those that can be reduced (i.e. oxidants such as NOx and O2). Since the exhaust gas is a product of the combustion process in the engine, the relative levels of reductants and oxidants within the exhaust stream can be controlled by defining the reductant: oxidant ratio entering the combustion chamber, i.e. the air-to-fuel (A/F) ratio. A range of pollutants (CO, CxHy and NOx) are generated during every combustion cycle and the concentrations of these within the exhaust gas vary as a function of A/F [4].

If an engine operates stoichiometrically (with an A/F ratio of 14.7:1 for gasoline combustion) then there is sufficient air in the fuel–air mixture to completely combust the hydrocarbons within. The exhaust gas produced will be neither oxidising nor reducing ([CO] + [CxHy] = [NOx] + [O2]). If it operates under fuel-rich conditions (A/F < 14.7) there will be insufficient O2 present in the reaction stream to react with the fuel (and the exhaust gas will be net reducing, [CO] + [CxHy] > [NOx] + [O2]). Finally, if it operates in a fuel lean mode (A/F > 14.7) there will be an excess of O2 in the combustion chamber and the exhaust gas produced will be of a net oxidising nature ([CO] + [CxHy] < [NOx] +[O2]).

The three components that need to be removed from these streams (NOx, CO and CxHy) can only all be removed if the balance between oxidants and reductants in the exhaust mixture is maintained.

Under fuel-rich conditions there will be insufficient O2 and NOx to oxidise the CO and CxHy (to CO2 and CO2 + H2O) and therefore the catalyst (which will exist in a reduced state) will not remove all the CO and CxHy from the exhaust gas mixture. Under fuel lean conditions there will be insufficient CO and CxHy to reduce the NOx and O2 within the mixture (to N2 and H2O, respectively), the catalyst will be fully oxidised and NOx will be emitted.

In its simplest form, the catalyst itself contains various combinations of nanoparticulate Rh, Pd, Pt dispersed on an Al2O3 support, which itself is deposited on a cordierite ceramic monolith. Pt (or Pd) catalyses oxidation of CO and CxHy (unburned hydrocarbons) to CO2 and H2O, while the Rh component catalyses the reduction of NOx to N2. Other additives include stabilised CeO2 materials (predominantly ceria zirconia solid solutions) whose function includes dampening fluctuations in the A/F ratio allowing the catalyst to operate under conditions outside the optimum (see in the following) [8–10]. In order to allow the technology to operate as it should, there needs to precise control of the gas stream within which it operates. This requirement necessitates a far stricter control of the engine management systems of the vehicles than had been previously necessary. The development of sophisticated engine management systems which controlled the A/F inlet ratio to the combustion chamber was required [11].

1.1.3 Process Control to Allow the TWC Operate

Since propelling a vehicle is a dynamic situation (depending on driving conditions), the A/F ratio of the mixtures entering the combustion chamber constantly changes as a vehicle speeds up, cruises at a motorway velocity, slows down, idles in traffic, etc. Control of the A/F ratio entering the engine is not a straightforward task. Recall, the A/F ratio determines the nature of the exhaust stream, i.e. the absolute and relative concentrations of pollutants.

Efforts to more precisely control this involved the development of an exceptionally sophisticated engine management system involving continuous electronically controlled modification of the engine in A/F ratio driven from the readings of a sensor placed in the exhaust pipe.

The sensor (a λ sensor) is designed to deliver measurements of the [O2] in the exhaust gas. P(O2) in the exhaust can be taken as a direct measure of the A/F ratio in the combustion chamber’s inlet. The sensor is essentially a yttrium-stabilised zirconia electrode whose potential depends on P(O2). This electrode (which is composed of the same material as the electrolyte in a solid oxide fuel cell) transports O2 as O²– ions generating an electrical signal whose strength is proportional to P(O2). The signal is sent to the fuel injection system which increases or decreases the A/F ratio as desired in order to keep the mixture stoichiometric [12].

The implementation of this λ sensor-driven engine management system decreased fluctuations in the inlet A/F ratio by orders of magnitude but, by definition A/F variations about the stoichiometric point still take place.

As discussed earlier, the effects of these will be to render the catalyst inoperative in either the oxidation (if A/F < 14.7) or the reduction (if A/F > 14.7) modes resulting in the periodic emission of either CO and VOC or NOx, respectively.

1.1.4 Changes to Catalyst Formulations Allowing Oscillating A/F Ratios

To counter this and dampen the effects of these excursions to fuel-lean or fuel-rich atmospheres catalyst manufacturers have altered the composition of the catalyst adding CeO2 (or more recently a solid solution of CeO2/ZrO2) to the composite material.

As well as providing some catalytic function (it promotes the water gas shift reaction – CO + H2O → CO2 + H2) which aids in de-pollution, its major role within the catalyst is to briefly allow the catalyst to promote all three desired reactions as the A/F ratio oscillates either side of the stoichiometric point.

CeO2 provides O2 for oxidation reactions during excursions to fuel-rich periods (through the 2CeO2→ Ce2O3 + O2 reaction) and also removes O2 from the gas phase during excursions to fuel lean periods (through the reverse Ce2O3 + O2→ 2CeO2 reaction). In this way the catalyst can continue (for so long as CeO2 remains available) to carry out oxidation of CO and VOC during fuel-rich periods and also (for so long as Ce2O3 remains) catalyse the reduction of NO to N2 during fuel lean periods. Obviously the extents to which these functions can apply depend on the stoichiometric amount of CeO2 (or ceria zirconia solid solution) present in the catalyst. The functions of CeO2 containing additives in TWC operation have been extensively reviewed and [8–10]. In relation to this chapter we feel it is a useful example of a catalyst additive which is designed to allow the catalyst to operate within a changing process stream.

In conclusion, in order to get the TWC (a composite catalyst containing different catalytic functionalities with distinct catalytic roles) to operate effectively (to remove NO, CO and VOC), changes needed to be made to the process stream (via the λ sensor) and to the catalyst formulation (via the addition of a ceria component).

The limitations in the TWC, which are discussed in the next section, have led to several proposed next generation catalysts and catalysed processes and we have, in the first two case studies covered in this chapter, looked at combining some of these different catalysts and processes.

1.1.5 Problems with TWC Technology

Considering the harsh environment in which it operates, and the stringent demands placed on it, the TWC is a triumph of catalytic chemistry coupled with mechanical and electronic engineering. However, there are problems with it that have led to significant ongoing work to try to improve the systems.

The first set of problems will relate to any aftertreatment system placed in a vehicle exhaust and relate to deactivation through attritional wear and tear, loss of active species through dissolution in the support, sintering or even evaporation from the catalyst [13].

A second set of problems relates to cold-start issues, i.e. the catalyst does not become active until it reaches a particular temperature (>250°C) and in general – since the materials are heated by the exhaust gasses from room temperature following the engine ignition, this takes a not-inconsiderable time. Catalyst companies are looking at catalyst additives such as hydrocarbon traps and specific engine cycles to generate exotherms on the catalysts as well as providing stand-alone heating systems to attempt to combat this issue [14].

The major problem with TWC technology, however, is the intrinsic fuel loss that the vehicle must suffer in order to provide sufficient reductants to reduce formed NOx. Vehicle engines are run under stoichiometric conditions in order that the exit gas should contain sufficient reductants to be able to reduce the O2 and NOx present in the exhaust. By definition this means that a certain proportion of the fuel added to the tank before a vehicle drives will never be used to propel the vehicle, but rather will be used to reduce NOx. This has a direct impact on both fuel economy (L/100 km) and CO2 emissions (g/km). Given that (a) the fuel is generally derived from a depleting natural resource [15] and is inherently valuable and (b) CO2 is implicated in the enhanced greenhouse effect [16] and therefore is thought to contribute to global warming, this is not a desirable situation.

Setting combustion mixtures at A/F ratios higher than 14.7 (fuel lean conditions) ensures that significantly more fuel is combusted to CO2 within the combustion chamber, and therefore the energy released is used to drive the vehicle. This results in greater fuel economy and decreased CO2 emissions.

Diesel-powered and lean-burn gasoline vehicles operate with higher-than-stoichiometric A/F ratios and these vehicles indeed have greater fuel efficiencies and lower CO2 emissions than traditional gasoline vehicles and therefore their use is environmentally preferable to stoichiometrically operated gasoline-powered vehicles. However, NOx emissions from these vehicles remain an issue due to the lack or reductants (or large excess of oxidants) in the exhaust gas over the TWC. Proposed catalytic aftertreatment systems that are designed to remove this lean NOx are discussed in Section 1.2, and our efforts to develop composite multifunctional materials through the combination of such aftertreatments are discussed in Section 1.3.

A final problem with TWC technology is that it is only designed to deal with the emission of three of the pollutants that are emitted from gasoline exhausts, i.e. CO, VOC and NOx. There are a range of other pollutants which are either not removed from emissions or dealt with in different ways, e.g. SOx is controlled through the use of low-sulphur fuel, CO2 is essentially ignored (indeed, it is a product of the operating TWC and the primary product of combustion), and it has no mechanism for dealing with the emission of particulate matter (PM). These features are unsurprising given that these emissions were not legislated against at the time of the TWC’s evolution.

The components of PM as well as currently methodologies for controlling their emission are discussed in Section 1.4 while our work in combining one particular PM combustion strategy with a lean NOx removal strategy is described in Section 1.5.

Finally, in Sections 1.6 and 1.7, we will expand from the theme of combining catalysts to generate multifunctional materials for use in automotive aftertreatments to the use of nanocomposite multifunctional materials for promoting reactions of green chemistry significance, i.e. using selective epoxidation as a case study we will look at current epoxidation processes, including the issues therein, and then look at the use of multifunctional materials in a tandem catalysis approach to greener selective oxidations.

1.2 Proposed Solutions to the Lean-Burn NOx emission Problems

The emission of lean NOx is therefore a problem in relation to lean-burn gasoline engines. For reasons of fuel economy the combustion of fossil fuels under lean conditions is also carried out in fossil fuel burning electricity generating power stations. And, for reasons related to the particular 4-stroke cycle of a diesel engine, these also run with a large excess of O2. Therefore, removal of NOx from these emissions faces similar issues to that of lean-burn gasoline vehicles [17]. Two different technologies are proposed to solve these issues and the operation of these – and the materials involved will be discussed in the next sections.

1.2.1 NH3-SCR

The removal of NOx emissions from stationary electricity generating stations is a far more tractable problem than NOx removal from mobile combustion power sources. There are two reasons for this.

Firstly, fuel combustion under these conditions is a relatively static process. Once operational, the A/F ratio (and therefore the combustion temperature, and the concentration of the emission’s components) are constant. This differs from the dynamic situation seen when combustion is carried out in a vehicle’s internal combustion engine and different concentrations of various pollutants continuously emerge.

Secondly, this combustion generally takes place in a large-scale power plant with the capacity and space to safely construct and operate reductant reservoirs which contain NH3 (or NH3 precursors) that can be added to the exhaust stream in order to reduce NOx over a suitable heterogeneous catalyst. Since the [NOx] emitted from these power sources is constant the [reductant] that must be added to reduce this is also constant, making process control relatively straightforward.

The addition of such reservoirs to the aftertreatment system of a vehicle would be expensive both to install and to transport (adding weight to the vehicle). Also, since the [NOx] from a vehicle’s engine is variable the [reductant] would also be variable and therefore such a system would require a much more sophisticated process control system than that needed for the stationary power source.

The use of NH3 as a reductant (rather than the VOC and CO used in the TWC) is interesting [18–22]. This molecule will (with the correct catalyst) selectively react with NOx in an exhaust stream. In the case of the TWC the redox reactions are unselective, i.e. NO and O2 would react with VOC and CO and in order for full reaction to take place their relative concentrations had to be equivalent (see above). In the case of a selective reduction the added reductant (NH3) will selectively react with (and reduce) the NOx component of the exhaust gas rather than the O2 component, NO + NH3 + xsO2→ N2 + H2O + xsO2.

Therefore – using a suitable catalytic material – added NH3 will selectively react with NOx rather than O2, notwithstanding the fact that in these exhausts the NOx concentrations can be as low as 500 ppm and the O2 concentrations can be as high as 10% (~3 orders of magnitude higher).

This reaction on first inspection seems ideal for the deNOx of stationary power source and indeed NH3 is incredibly selective as a reductant for the desired reaction. However, it should be noted that at higher temperatures the reaction does become unselective. This presents problems for several reasons.

Firstly, the provision of NH3 reductant is an expense that adds to the cost of power generation. Any unselective use of the NH3 (in reacting with O2 rather than NOx) adds to this expense, since more NH3 will be needed to reduce NOx.

Secondly, the product of unselective NH3 oxidation reaction may itself be NOx (indeed, the oxidation of NH3 to NO is the first step in the industrial preparation of HNO3). Therefore, an unselective reaction renders the entire selective catalytic reduction (SCR)-NH3 deNOx process an expensive method of increasing the NOx concentration of the emitted gas, NH3 + O2→ N2 OR NO.

For this reason, the successful application of SCR-NH3 technology requires extremely sophisticated nanocomposite catalytic materials.

A range of selective nanocomposite catalysts are suitable and these are all characterised by their ability to promote redox reactions (they will have surface sites that can shuttle between two different oxidation states) and also their ability to adsorb and activate basic NH3 (they will have surface acidic sites).

The two most prevalent materials used in the NH3-SCR reaction are TiO2-supported V2O5 catalysts [23–25] and ion-exchanged zeolite species (Cu or Fe are most prevalent) [26–29]. Both are related by the presence of surface active redox and acidic sites and the materials are described below.

1.2.1.1 TiO2-Supported V2O5 Catalysts

The redox and acidic sites on these materials are related to one another. The redox sties arise from an interconversion between surface V=O species (V+5) and surface V-OH species (V+3). These V-OH surface species also act as acidic sites with the OH groups acting as Bronsted acid sites within the reaction.

As well as offering all the advantages in terms of surface dispersion and prevention of sintering, and offering mechanical and chemical stability, TiO2 has another unusual function in this reaction. The lattice spacings in V2O5 and anatase phase TiO2 are very close to one another and the assembly of V2O5 species onto TiO2 surfaces results in the V=O group of the V2O5 moieties being oriented perpendicular to the surface and directed into the reaction medium [30]. This in turn makes them easier to reduce (generating the required redox active and acidic surface sites) and more active than bulk V2O5 in similar reactions.

V2O5/TiO2 catalytic materials are also used in a range of selective oxidation reactions for similar reasons [31, 32], i.e. the ease with which vanadium can shuttle between +3 and +5 oxidation states. In these applications, the V2O5 content of the composite material is of the order of 10%, whereas for the SCR reaction, it is generally < 1%.

The mechanism of the deNOx reaction has been extensively studied [33, 34] over these materials.

Normally, for industrial applications, tungsten is added to the catalytic material. This serves a range of functions including preventing sintering of the V2O5, retarding the phase change of TiO2 from anatase to the lower surface area rutile polymorph and decreasing the activity of the material for the unselective NH3 + O2 reaction [35].

1.2.1.2 Ion-Exchanged Zeolites in NH3-SCR

Zeolites are microporous aluminosilicate materials that are generally synthesised through condensation of Al- or Si-containing precursors in the presence of a template molecule [36]. The presence of these template molecules directs the secondary structure of the condensing SiO4 or AlO4− tetrahedra. The choice of template molecule during the condensation step of their synthesis directs the secondary (and hence final) structure of the solid phase material and judicious choice of this template yields microporous materials with a range of 1D, 2D or intersecting channels of molecular size and there are a range of naturally occurring and synthetic zeolites which differ according to their channel structure. For this reason, they find use as molecular sieves in chromatography.

Chemically, synthetic zeolites can also be tuned through selection of (a) the Al/Si ratio in the preparation mixture (each Al atom will generate an AlO4− tetrahedron which will require a cation for charge balance) and (b) the type of counter ion, e.g. Na+, K+, Li+, H+, NH4+, Cu²+ and Fe³+. These counter ions do not form part of the extended zeolitic lattice and can easily be exchanged out of a zeolite through a straightforward ion-exchange treatment. The higher the concentration of Al within the zeolite the higher the concentration of counter ions required and hence the higher ion-exchange capacity of the materials. The ion-exchange capacity of zeolites has led to their applications as feed and soil supplements, water hardness treatments in detergent formulations and in waste water treatment (including radioactive clean up).

A combination of ion-exchange and molecular sieve properties – as well as their chemical stability has led to their use as heterogeneous catalysts. If the counter ion in a zeolite is a proton this means the materials can essentially behave as solid phase Bronsted acids will promote all the various acid-catalysed reactions in organic chemistry (e.g. cracking, isomerisation, esterifications and aldol reactions) and they have a long history of application as acidic catalysts in the oil refining industries (where the molecular sieving properties are also important) [37].

Zeolites, following a high-temperature treatment (which results in dehydroxylation and the condensation of H2O) can also be converted into Lewis acids, where a Si atom carrying a positive charge can act as an electron acceptor.

If the counter ion is di- or tri-valent, then these can counter 2 (or 3) AlO4− tetrahedra within the lattice. This situation also leads to the possibility of redox active sites within the zeolite where individual ions can shuttle between two different oxidation states and hence catalyse a redox reaction. Such redox pairs are proposed to operate in Fe- and Cu-exchanged zeolites where Fe(III)/Fe(II) and Cu(II)/Cu(I) redox couples are proposed to be important [38–41].

There has been significant work on the mechanism of the deNOx [42, 43] and the related fast deNOx reaction [44–46] over Fe-containing zeolites and it is considered that the presence of NO2 within the reaction mixture is essential for the fast deNOx process. Subsequent to this there has been much work on adding NO oxidation catalysts to the base Fe zeolite catalyst in order to promote NO2 formation in order to allow the fast deNOx process. Fe zeolites play a major role in the nanocomposite materials discussed in Section 1.3.

1.2.1.3 SCR-Urea Reactions

While NH3 is an exceptionally useful material for this reaction, being unusually selective for the deNOx reaction in the presence of large excesses of O2 and a suitable SCR catalyst, it is clearly not