Catalysis for Alternative Energy Generation

Catalysis plays a key role to address the challenge of sustainable energy and alternative methods to produce energy with respect to using fossil fuels. This field of research and development has given a new impetus to research on catalysis in areas such as producing biofuels, development of advanced electrodes for a number of applications (from new-generation photovoltaic cells to fuel cells), production of renewable H₂ and in a longer-term perspective solar fuels. However, the discussion on the technical aspects on the development of catalysts in these areas should be complemented with considerations on the general economic and social context and related constrains which determine the choice of the research priorities. This introductory chapter was mainly focused on these aspects.

Lignocellulosic biomass resources are abundant worldwide and have the potential to displace petroleum in the production of liquid fuels for the transportation sector of our society. Bioethanol, the dominant biofuel used today, suffers from low energy density and high solubility in water, properties that are undesirable for transportation fuels. The production, from lignocellulosic sources, of liquid hydrocarbon fuels that are chemically similar to those currently used in the transportation sector is a promising alternative to overcome the limitations of bioethanol. The transformation of highly functionalized biomass into oxygen-free liquid fuels can be carried out by gasification, pyrolysis, and aqueous-phase processing, as outlined in this chapter, with particular emphasis on the catalytic aspects of these processes.

Environmental concerns and sustainability issues demand the production of energy carriers from renewable resources using, if possible, technologies and infrastructure developed for fossil fuels. Biogas, a product of waste biomass anaerobic digestion, is a promising raw material for this purpose. As it consists mainly of CH₄ and CO₂, the most suitable process for its utilization is the dry reforming of methane (DRM) to synthesis gas and then to liquid energy carriers via the Fischer–Tropsch technology. This chapter reviews the chemistry of DRM and the catalytic systems developed for this process, with emphasis on the most important issue, namely, catalyst deactivation due to accumulation of carbonaceous deposits.

The use of hydrogen for fuel cell application represents one of the most environmental friendly processes for the production of electric energy for automotives in the near future.

The currently increasing interest in catalytic reactions of methanol, CH₃OH, is—in addition to its customary role as an important base chemical and feedstock for value-added molecules—due to its potential as a chemical storage molecule for hydrogen. For mobile applications, e.g. in the transportation sector, hydrogen can be produced onboard from methanol by the methanol steam reforming reaction and used as a fuel for a downstream polymer electrolyte fuel cell (PEMFC). Methanol is industrially produced from natural gas- or coal-derived syngas, but can in principle also be synthesized from CO₂ by hydrogenation. Methanol might thus play a key role in the transition toward a future energy scenario, which has to be more and more independent from fossil sources. This chapter focuses mainly on the challenges of catalyst development for methanol steam reforming. It is divided into two parts treating different families of catalytic materials: the widely studied Cu-based catalysts, and intermetallic compounds.

Biodiesel, which is an alternative renewable fuel, is defined as mono alkyl ester of long-chain fatty acids and has properties comparable to those of fossil-based diesel. Biodiesel can be produced from vegetable oils or animal fats. The most common method used to produce biodiesel is a reversible chemical reaction called transesterification. This reaction takes place either in the presence of catalysts at lower temperature and pressure or in the absence of catalysts at higher temperature and pressure in supercritical state. Catalyzed transesterification reaction is preferred in biodiesel production because of the moderate reaction conditions. Homogeneous base catalysis can be used in transesterification when fresh vegetable oil is used as a feedstock due to its low cost, high catalytic activity, and feasibility to operate at low temperatures. Homogeneous acid catalysis is a better choice when the feedstock contains higher amounts of free fatty acids (FFAs). Heterogeneous base and acid catalysis are preferred due to their easy separation from biodiesel, hence reducing number of product purification steps. However, heterogeneous catalysis is still under development and has a promising future in biodiesel industries. In this chapter, various acid- and base-catalyzed esterification and transesterification reactions are discussed, and recent trend in catalyst development is highlighted. It is recommended that a proper selection of catalyst is made in a transesterification reaction, depending largely on the type of feedstock.

The combination of dwindling oil reserves and growing concerns over carbon dioxide emissions and associated climate change is driving the urgent development of routes to utilise renewable feedstocks as sustainable sources of fuel and chemicals. Catalysis has a rich history of facilitating energy-efficient selective molecular transformations and contributes to 90% of chemical manufacturing processes and to more than 20% of all industrial products. In a post-petroleum era, catalysis will be central to overcoming the engineering and scientific barriers to economically feasible routes to biofuels and chemicals. This chapter will highlight some of the recent developments in heterogeneous catalytic technology for the synthesis of fuels and chemicals from renewable resources, derived from plant and aquatic oil sources as well as lignocellulosic feedstocks. Particular attention will be paid to the challenges faced when developing new catalysts and importance of considering the design of pore architectures and effect of tuning surface polarity to improve catalyst compatibility with highly polar bio-based substrates.

The combustion of methane has been investigated for production of heat and for removal of unburnt fuel. Achievement of complete methane oxidation at lower temperatures has been desired in every application. In this chapter, studies for methane combustion over catalyst are summarized from a perspective of catalyst materials. The development of catalysts with high activity at low temperatures and long-term durability under reaction conditions is required. Therefore, this chapter deals with the studies on low-temperature catalytic combustion of methane, while the hexaaluminate-related compounds for high-temperature combustion are described. In the case of low-temperature combustion, a large number of previous researches are classified into four kinds of catalysts: Pd, Pt, CeO₂–ZrO₂ mixed oxide, and perovskite-type oxide. The catalytic activities, durabilities, reaction mechanism, and degradation phenomena, the influence of support material, the effect of addition of other components, and so on are discussed.

Polymer electrolyte membrane fuel cells (PEMFCs), which convert the chemical energy stored in the fuel hydrogen directly and efficiently into electrical energy and water, have the potential to eliminate our fossil energy dependency and emissions, when the hydrogen is derived from renewable energy sources such as solar, wind, biomass, among other possibilities. PEMFCs are being developed as electrical power sources for vehicular, stationary, and portable power applications. In spite of tremendous R&D efforts in the advancements of PEMFC technology, the commercialization is still a long way to go due to the prohibitively high cost of platinum-based catalysts used in the electrodes. However, attempts were made to reduce the quantity of platinum-based catalyst and to extract the maximum activity from a given quantity of platinum in various ways including the development of supported system, employing binary or ternary Pt-based or non-Pt alloy systems, and finding alternate catalysts of various kinds with no platinum in them. In this chapter, we set to examine various logistics and underpinning science in PEMFC catalyst development in one frame analysis, and further, we propose future directions to push the frontiers ahead in order to realize PEMFC commercialization in aspects of both anode and in cathode catalysts of PEMFC.

The direct methanol fuel cell (DMFC) is a particular case of a low-temperature proton exchange membrane (PEM) fuel cell (FC). A DMFC utilizes CH₃OH as anode fuel and O₂ as cathode fuel. Depending on the application, a DMFC is typically operated in the range of 40–80°C. DMFCs are very attractive due to the high energy density of CH₃OH, thus making them lightweight devices. In fact, DMFCs can have 15 times the energy density of a Li-ion battery. Other advantages are that DMFCs can be refueled on the fly within seconds, and CH₃OH is an inexpensive and readily available fuel. Furthermore, CH₃OH is a liquid, thus facilitating its distribution, and it can be taken on airplanes in designated cartridges. The impact of the eventual successful commercialization of DMFCs is estimated to be large and expands into the microelectronics industry. However, significant obstacles need to be overcome before DMFCs can be truly considered to be a viable technology. Some of these challenges are related to the anode catalyst such as lowering the cost of the catalyst used by lowering the amount of the noble metal component, as well as extending the lifetime of both the anode and cathode catalysts. A number of reviews describing the technical aspects of DMFCs as an entire device are available (Scott et al. J Power Sources 79:43–59, 1999; Lamm and Müller System design for transport applications. In: Vielstich et al. (ed) Handbook of fuel cells fundamentals technology and applications, Wiley, New York, 2003; Narayanan et al. DMFC system design for portable applications. In: Vielstich et al. (ed) Handbook of fuel cells fundamentals technology and applications, Wiley, New York, 2003; Gottesfeld Design concepts and durability challenges for mini fuel cells. In: Vielstich et al. (ed) Handbook of fuel cells fundamentals technology and applications, Wiley, New York, 2009). Therefore, these aspects are not covered in this chapter, which instead focuses on the catalysis aspects of the electrochemical CH₃OH oxidation reaction. However, cross-references to proton electrolyte fuel cells (PEMFCs) and related reactions are given where appropriate.

Inorganic colloids and especially metal nanoparticles (NPs) have been in the focus of interest for a long time. Their valuable characteristics due to their small size, such as their unique electron structure and extremely large specific surface area, open the way for their practical utilization. By virtue of their high activity and selectivity, they have become widely known as novel type catalysts. Various methods are developed for their preparation, from which colloidal chemical routes became more and more widespread. In this study, some colloidal methods for preparation of metal (Pd, Rh, Au, Ag) NPs and NP-based catalysts are presented. The effects of various polymer molecules, clay lamellae, and reducing agents on the kinetic of NPs formation were investigated. The formation of NPs was followed by transmission electron microscopy (TEM), UV–Vis spectroscopy, isothermal titration calorimetry (ITC), and dynamic light scattering (DLS). NPs were also prepared on clay mineral surface. Interlamellar space of clay minerals is capable of stabilizing colloid particles. Influence of the NPs into the original lamellar structures was examined by X-ray diffraction and small-angle X-ray scattering. The surface oxidation state of the particles sitting on the support in the metal-containing catalysts was determined by XPS.

Titania-based heterogeneous photocatalysis has been extensively studied at both solid–liquid and solid–gas interfaces. Numerous efforts have been directed to improving the photocatalytic activity of TiO₂ in both the UV and the visible wavelength ranges. To enhance the photoactivity, a number of techniques for the doping of TiO₂ with various elements have been developed. This chapter presents a brief discussion of three promising materials: phosphate-, nitrogen-, and silver-modified TiO₂. Phosphate- and silver-modified TiO₂ exhibited very high photocatalytic activity under UV irradiation, while the nitrogen-doped TiO₂ had visible light activity. The modified TiO₂ derivates were prepared by simple chemical methods and studied by various surface and structural investigation techniques. The effects of the dopant concentration on the structure and photocatalytic activity are discussed.

Various research groups have focused on the development of catalysts for selective oxidation of hydrocarbons. Generally interesting results have been obtained. Roughly selective photocatalytic oxidation studies can be divided into gas-phase processes and liquid-phase processes, and both will be addressed, using oxidation of propane and cyclohexane, respectively, as examples.

Amongst the renewable energies the solar energy is supposed to become the dominating one in the remote future. Process intensification is necessary in order to exploit unevenly distributed solar and other renewable energy sources. This article aims to highlight the role of heterogeneous catalysts in the novel catalytic processes designed for alternative energy generation. It is essential to develop catalysts and technologies for storing and transporting energy produced from various renewable sources. Storage in batteries or in chemical forms, for example in hydrogen, is under investigations. Due to robustness of the present infrastructure built for the exploitation of fossil resources, a part of the studies is devoted to develop catalytic processes for production of liquid fuels of biomass origin or using CO₂ as carbon source. New catalysts are complex multicomponent systems with hierarchically organized 3D-nanostructures. Characterization and modeling of such systems requires the development of in situ characterization tools.