Hydrogen Production Technologies

The book is organized in three parts.  Part I shows how the catalytic and electrochemical principles involve hydrogen production technologies. Part II is devoted to biohydrogen production and introduces gasification and fast pyrolysis biomass, dark fermentation, microbial electrolysis and power production from algae. The last part of the book is concerned with the photo hydrogen generation technologies. Recent developments in the area of semiconductor-based nanomaterials, specifically semiconductor oxides, nitrides and metal-free semiconductors based nanomaterials for photocatalytic hydrogen production are extensively discussed in this part.

• Language: English
• Publisher: Wiley
• Release date: Mar 20, 2017
• ISBN: 9781119283669

Book preview

Preface

Energy is one of the most important issues for humankind. Increasing energy demand, regional limitations and serious environmental effects of conventional energy sources have brought about the need for new, clean and sustainable energy. This book series has been planned as a presentation of the basics in the areas of renewable energy and storage as well as the cutting-edge new technologies for these applications. Hydrogen Production Technologies is the first volume of the series due to the undeniable importance of hydrogen as a clean energy carrier. Hydrogen has been gaining more attention in both transportation and stationary power applications. Fuel cell-powered cars are on the roads and the automotive industry is demanding feasible and efficient technologies to produce hydrogen. There are various ways to produce hydrogen in a safe and cost-effective manner. This volume covers the new technologies used to obtain hydrogen more efficiently via catalytic, electrochemical, bio- and photohydrogen production and as such is a valuable component in the research area of hydrogen production. The principles and methods described herein lead to reasonable mitigation of the great majority of problems associated with hydrogen production technologies. The book is edited to be useful as a text for university students at both introductory and advanced graduate levels and as a reference text for researchers in universities and industry. The chapters are written by distinguished authors who have extensive experience in their fields. Besides researchers in the engineering area, those in the energy, materials science and chemical engineering fields have been focusing on new materials and production technologies in order to generate hydrogen in an efficient and cost-effective way. Hence a multidisciplinary approach is taken to covering the topics of this book. Readers will absolutely have a chance to compare the fundamental production techniques and learn about the pros and cons of these technologies.

The book is organized into three parts. Part I shows the catalytic and electrochemical principles involved in hydrogen production technologies. It should be clear from this part that the fundamentals and modern status of water electrolysis, ammonia decomposition, methane reforming, steam reforming of hydrocarbons and biethanol, hydrolysis of ammonia borane and also SO2 electrolyzer are of great importance. Therefore, their various aspects are discussed such as catalyst development, thermodynamics and kinetics of reaction mechanisms, reactor and mathematical modeling, novel membrane structures, and advanced nanoparticles. Part II is devoted to biohydrogen production. This part is designed to be a good introduction to gasification and fast pyrolysis of biomass, dark fermentation, microbial electrolysis and power production from algae. It specifically presents various catalytic formulations as well as reactor designs to overcome catalytic deactivation due to coking. In addition to gasification of wood, dried sewage sludge, and plastic waste, newly developed staged gasifiers with fewer impurities are discussed. Moreover, there is a discussion of dark fermentation using sulphate-reducing bacteria from the genus Desulfovibrio utilized in hydrogen production. Part II also addresses hydrogen production from electrochemically active bacteria (EAB) by decomposing organic compound into hydrogen in microbial electrolysis cells (MECs). Lastly, highly efficient harvesting of energy from algae in the forms of hydrogen and enhanced process integration reducing exergy destruction are demonstrated. The last part of the book is concerned with photohydrogen generation. Recent developments in the area of semiconductor-based nanomaterials, specifically semiconductor oxides, nitrides and metal-free semiconductor-based nanomaterials for photocatalytic hydrogen production are extensively discussed. Moreover, Part III also includes pristine and doped TiO2 nanostructures for fast hydrogen production during photocatalytic water splitting. Finally, an earth abundant catalyst for water splitting is presented as a very promising narrow band gap visible-light photocatalyst.

Since the findings range over many useful topics specifically discussed in the book, readers from diverse fields such as chemistry, physics, materials science and engineering, mechanical and chemical engineering and also energy-focused engineering programs can benefit from this comprehensive review of the hydrogen production technologies.

Series Editors

Mehmet Sankır, PhD and Nurdan Demirci Sankır, PhD

Department of Materials Science and Nanotechnology Engineering

TOBB University of Economics and Technology

Ankara, Turkey

January 1, 2017

Part I

CATALYTIC AND ELECTROCHEMICAL HYDROGEN PRODUCTION

Chapter 1

Hydrogen Production from Oxygenated Hydrocarbons: Review of Catalyst Development, Reaction Mechanism and Reactor Modeling

Mohanned Mohamedali, Amr Henni and Hussameldin Ibrahim*

Clean Energy Technologies Research Institute (CETRi), Process Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Canada

*Corresponding author: [e-mail]

Abstract

Hydrogen is viewed as a clean and efficient fuel for future energy generation, with an enormous amount of research being pursued to study the various routes for the production, storage, and application of hydrogen fuel. To date, diverse approaches have been employed for the production of hydrogen-rich fuel through catalytic processes using nonrenewable materials as well as sustainable feedstocks. This review of the recent literature, is intended to provide an outlook on the catalyst development, reaction mechanism and reactor modeling studies of hydrogen production using catalytic steam reforming of oxygenated hydrocarbons with focus on methanol, ethanol, and glycerol feedstocks. Various attempts to optimize the catalyst performance, including the utilization of various noble and transition active metals as well as oxide support materials, are extensively discussed. Tremendous effort has been dedicated to develop a reaction mechanism for the reforming of oxygenated hydrocarbons, with no consensus to date on the exact reaction pathway due to the complex nature of the reforming process. This review provides insights into the fundamental understanding of the reaction mechanism and the contribution of the active metals and support on the observed kinetics. Moreover, the previous literature on the modeling and simulation of the hydrogen production process is also reviewed.

Keywords: Hydrogen production, oxygenated hydrocarbons, catalyst development, reaction kinetics, reaction mechanism, reactor modeling

1.1 Introduction

The global reliance on fossil fuels as the main energy source for power generation, transportation, and as a feedstock for chemical industries is widely increasing with the discoveries of new fossil fuel reserves and the technological advancement in their production and application. According to the recent annual energy outlook released in 2014 by the International Energy Agency (IEA), fossil fuels are projected to supply more than 80% of the world total energy by 2040. However, fossil fuel-based energy generation has increased the concentration of greenhouse gas emissions to an alarming level of 400 ppm in 2013 [1]. The continued increasing levels of anthropogenic greenhouse gases in the atmosphere will ultimately cause further weather changes, resulting in severe impacts on life on earth; therefore, combating climate change requires sustainable development of green technologies and policies to mitigate climate change. In accordance with the Paris Climate Conference (COP21) of 2015, several countries have pledged to reduce their emission levels to possibly achieve a 2 °C scenario (2DS) and cut the emissions to 60% by 2100, corresponding to cumulative CO2 emissions of 1000 GtCO2. In order to achieve such objectives a portfolio of low-carbon technologies has to be deployed to reach the 2DS, consisting of energy efficiency, fuel switching, and renewable energies. According to the 2016 energy technology perspective report issued by the IEA, the contribution toward the reduction of the cumulative CO2 emissions in the 2DS over the period 2013 to 2050 is estimated to be 38% from electricity efficiency, 12% for carbon capture and sequestration (CCS), and around 32% should come from the deployment of renewable energy sources. To establish clean energy for the future, the development of low carbon energy supply is urgently required. Among the possible alternatives, hydrogen has the potential to provide an ideal energy carrier that can meet the increasing global demand for energy and efficiently replace the existing fossil fuels [2, 3]. Hydrogen can provide an energy of 122 kJ/g, which is almost three times higher than hydrocarbon fuels [4], and is projected to contribute 34% of the total renewable resources in 2050 [5]. The application of hydrogen in the transportation and power generation sectors is receiving growing interest from both the technological and the policy-making aspects [6–8]. The contribution of hydrogen as a fuel for the transportation sector is mainly driven by the great achievements in fuel cell technology and the development of internal combustion engines that uses hydrogen fuel [9–12]. Fuel cell-based engines have three times higher efficiency than conventional gasoline engines due to the excellent characteristics of hydrogen as an energy carrier [13], in addition to the outstanding performance of hydrogen as a transportation fuel [14]. Hydrogen fuel being a gas at normal temperature and pressure, as compared to liquid hydrocarbon fuels, presents a major challenge for safe storage and transportation [15, 16]. Traditional storage schemes require energy-intensive techniques and have great safety concerns; however, the latest developments in the methods and technologies of the materials used for hydrogen storage are promising for realizing the hydrogen economy. Several review papers have described the current status and future trends in hydrogen storage materials [15, 17, 18]. Hydrogen can be produced from various energy sources using different processes, which could be categorized into renewable and nonrenewable resources. Hydrogen production from fossil fuel derivatives, such as methane and coal through gasification and thermocatalytic processes, is considered the major source for nonrenewable hydrogen production, representing more than 95% of the hydrogen produced to date [19]. In addition to being nonrenewable, hydrogen produced from fossil fuel resources contributes to global warming by releasing CO2 during the production process. On the other hand, biomass is considered as a sustainable route for hydrogen production with less net CO2 produced due to the fact that the CO2 released from the conversion of biomass has already been naturally captured from the atmosphere. In addition to the most widely used thermochemical technology, other methods, such as the electrolysis of water, have also been used for hydrogen production, with a major drawback of being highly energy intensive and having a low efficiency of around 25% [20, 21]. Other technologies, such as the photobiological techniques, are also reported based on the photosynthetic stimulation of some types of bacteria to release hydrogen; however, the sluggish release rate of hydrogen is considered a major challenge for these technologies [22–24]. Several review papers are available that give a detailed overview of the different hydrogen generation technologies [14, 25, 26]. Dincer et al. [27] followed a comparative assessment approach to evaluate several hydrogen production schemes such as natural gas reforming, electrolysis, coal and biomass gasification. The assessment criteria included environmental, economic and social impacts of these various methods. It was concluded that for the case of Turkey, biomass gasification has the best energy efficiency, whereas electrolysis methods were found to be less attractive when the hydrogen cost is considered.

This chapter aims at reviewing the sustainable and environmentally friendly hydrogen production from the steam reforming of oxygenated hydrocarbons, with a special focus on methanol, ethanol and glycerol, to recapitulate the state of the art in this field, and summarize the research conducted in the past five years (2012 to 2016) in order to get deep insights into the promising future for these technologies. The literature pertaining to the catalyst development for the steam reforming process, reaction mechanism, reactor modeling and simulations is thoroughly reviewed following a comparative analysis approach whenever possible.

1.2 Catalyst Development for the Steam Reforming Process

The catalyst development is considered the heart of sustainable hydrogen production through the steam reforming of oxygenated hydrocarbons. The hydrogen production rate, purity, and the selectivity of the reforming process are significantly impacted by the characteristics of the catalyst used. This crucial role of the catalyst has been highlighted by the numerous research projects conducted over the past years to understand the fundamentals of the catalytic process, and to develop highly efficient catalysts that can increase the overall conversion, improve hydrogen yield and prolong their lifetime [28, 29]. There are certain catalytic traits that need to exist for an efficient catalyst to be used in the steam reforming hydrogen production. These characteristics are prominently dependent on the nature of the oxygenated hydrocarbon feed (i.e., methanol, ethanol or glycerol) as well as the feed purity (i.e., crude versus pure) [30]. However, there are general requirements for catalytic surfaces such as: (1) the activity for C-C bond cleavage to produce CO, CO2, and CH4, (2) steam reforming of intermediates to produce hydrogen, and (3) the ability to produce free oxygen while preventing coke formation as well as C-O bond creation [31, 32]. Based on the contribution in the catalytic reforming reaction, there are three distinct parts of the catalyst: the active metal, the support, and the metal-support interactions. Control of the interaction between the metal and support is essential to improve the dispersion of the active sites and consequently achieve a better reaction rate and hydrogen yield. It was found that it is not only the nature of the individual support and metal sites that affects the reforming reaction but rather the interface that plays a vital role as reported recently [33]. In the following section we will thoroughly review and summarize the work that been performed over the past five years in the development of active metals and support materials for the catalytic transformation of oxygenated hydrocarbons to hydrogen. As stated earlier, this review chapter will focus on methanol, ethanol and glycerol as models for the oxygenated hydrocarbon feed; thus, accordingly, this section will be discussed in light of these three contexts.

1.2.1 Catalyst Development for the Steam Reforming of Methanol (SRM)

A very good review paper by Sá et al. [29] has been published which summarizes the development on catalysts used for the SRM process reported before 2010. In this section we will mainly present the latest work conducted after 2010 to provide the most recent perspective in order to keep up to date with the rapid progress in the research related to the catalyst development for the SRM process. The most common catalyst for SRM is Cu-based catalyst. Tremendous effort has been dedicated to understanding the catalytic reforming over Cu-based catalysts and to prepare efficient catalysts with high dispersion, high surface area, and small particle sizes. Several approaches are available to accomplish these objectives such as investigating novel synthesis methods [34], using promoters [33, 35], utilizing active support materials and the optimization of the operating conditions for higher hydrogen yield and improved catalyst stability [36, 37]. Table 1.1 summarizes the recent literature pertaining to the heterogeneous catalyst development for SRM process using Cu-based catalysts. Researchers in this field have been focusing on improving certain characteristics of the Cu-based catalysts such as the particle size, support surface area, and Cu dispersion. To achieve these objectives several approaches were used, including the optimization of the synthesis method, using support promoting materials, and the utilization of novel non-oxide supports. Cu supported on ZnO has gained considerable attention in the literature owing to its high activity in SRM [38]. The ZnO support provides the required surface area to disperse the Cu metals and prevent its agglomeration, and most importantly increase the reducibility of Cu by acting as a withdrawing agent for H atoms [37, 39]. A recent study suggested that increasing the surface area of the ZnO support by varying its calcination temperature can significantly improve the Cu dispersion, whereas the reducibility of the ZnO support could be controlled by changing the Zn precursor gel [39]. The selectivity of the Cu/ZnO catalyst prepared using highly polar precursor solution of Zn acetate as opposed to Zn nitrate was proven to be even higher than the commercial Cu/ZnO/Al2O3 catalyst [39], which was attributed to the increase in the catalyst reducibility. The effects of the support pretreatments, such as calcination conditions [40] and anodic oxidation [41], have also been studied in the literature. Nakajima et al. [42] have also proposed a new approach for the fabrication of Cu/ZnO catalyst by preparing ZnO nanowires on quartz substrates and then using UV laser to grow Cu on the surface of ZnO nanowires. The careful control of the ZnO nanowires length has shown an improved selectivity toward hydrogen production. The major limitation of Cu/ZnO catalysts is the catalyst deactivation and low selectivity due to the side reaction leading to CO formation [37, 43]; therefore, this has motivated the use of stabilizing oxides such as Al2O3 to form the commercial Cu/ZnO/Al2O3 catalyst [44, 45]. There is no general consensus to date on the exact role of Al and Zn as stability and structural promoters in Cu/ZnO/Al2O3 catalyst, but the formation of Cu-Al spinel and the oxidation state of Cu were evident, which contributed at different stages of the SRM process [46, 47]. Great attention has been paid to investigate the effects of synthesis methods in order to optimize the performance of Cu/ZnO/Al2O3 catalysts as listed in Table 1.1. Due to the complexity associated with the conventional co-precipitation methods [48], novel approaches have been suggested to overcome these limitations such as the utilization of microwave irradiation methods [49] and the gas dynamic spray [50]. The microwave-assisted combustion method has increased the surface area, homogeneity, and produced smaller particle sizes in shorter times as compared to conventional ovens, consequently leading to methanol conversion of 100% maintained for 20 hours at 240 °C due to the increased Cu dispersion [49]. However, the Cu/ZnO/Al2O3 catalysts are prone to deactivation, especially at temperatures higher than 300 °C [51–54], which propelled the efforts to use support materials that are stable at high reforming temperatures such as ZrO2 supports [55–60]. The stability of Cu/ZnO/ZrO2 in the SRM at high temperatures of 400 °C and 500 °C was further improved by the co-precipitation of the catalyst on ZrO2 substrate to enhance the Cu dispersion even though the Cu amount of the unsupported Cu/ZnO/ZrO2 catalyst was double that of the supported material [58]. This was in agreement with a work done by the same group to promote the stability of Cu/ZnO/ZrO2 catalyst using In2O3 and a combination of In2O3-Y2O3 before coating them on ZrO2 substrate [57]. The addition of In2O3 was found to reduce the deactivation rate at the cost of initial methanol conversion, which was then improved when Y2O3 was incorporated into the catalyst. The optimized Cu/ZnO/ZrO2/Y2O3/In2O3 catalyst with atomic ratio of 1.0/1.0/0.33/0.11/0.076 was deposited on the amorphous ZrO2 support using a precipitation impregnation approach, achieving three times higher conversion than the unsupported catalyst due to the increase of Cu surface activity. The positive effect of In2O3 as a stabilizing agent for the SRM over Cu/ZnO catalyst was proven in a previous study, which was merely due to the enhancement of Cu dispersion [59]. As most of the research is dedicated to improve the dispersion of Cu on ZrO2 supports, Mayr and coworkers [56] have prepared an inverse catalyst by the deposition of ZrO2 and partially hydroxylated ZrOxHy on Cu. Due to the agglomeration of ZrO2 on Cu, the catalyst activity diminished with time; however, the ZrOxHy/Cu catalyst showed considerably increased selectivity toward hydrogen due to the presence of Zr⁰, which was detected by the high-resolution electron energy loss spectroscopy (HREELS). The use of CeO2 supports has gained significant attention recently, mainly due to its strong interaction with transition metals, including Cu, the high redox ability, and its high oxygen mobility [61–63]. The variation of Cu loading in Cu/CeO2 and Cu/ZnO/CeO2 catalysts was studied by many researchers to understand the effect of CeO2 on the Cu oxidation state as well as the hydrogen selectivity [63–66], and it was concluded that a 5% nominal Cu loading showed the highest activity and stability due to the increased reducibility of Cu+2 to the active Cu⁰ state. The promotion of the Cu-based catalyst using a combination of various oxide support materials has been the focus of several research groups in the past decade such as ZnO/Al2O3/ZrO2 [34, 67, 68], CeO2/Al2O3 [69], ZnO/CeO2/Cr2O3/Al2O3 [70], CeO2/ZrO2 [71, 72], ZnO/ZrO2 [73], and ZnO/CeO2/ZrO2 [74]. The objective is to study the metal-support interactions in order to design effective catalyst utilizing synergism between these materials. There was a consensus that ZrO2 improves the dispersion and reducibility of the catalyst and in the presence of ZnO it enhanced the Cu-ZnO interactions and led to the formation of CuO state [68]. The high oxygen mobility of CeO2 supports rendered them as excellent storage for oxygen, leading to the suppression of CO formation and further improving the catalyst durability by burning off the carbonaceous residues [71]. Also, the interaction between CeO2 and ZrO2 has a high impact on the oxidation state of Cu during the SRM process, forming different Cu+/Cu⁰ ratios [72]. In addition to the mostly used Cu-based catalysts for SRM processes, the use of Ni as a catalyst promoter has recently been investigated [75–78]. The promotion effect of Ni on improving the catalytic activity and selectivity was ascribed to the formation of Ni layer on the support surface acting as anchoring sites for the active metal, leading to enhanced dispersion and surface activity, as reported for Ni-Zn catalyst supported on Al2O3 [75]. Nevertheless, Ni-based catalysts are also vulnerable to deactivation by sintering and coke formation, therefore the careful selection of the support is very essential to obtain a stable SRM catalyst such as the support surface area, and basicity [79, 80]. Metals from group 8 to 10 have been thoroughly studied by various research groups as promising candidates in the SRM process, especially the Pd-based catalysts [81–89]. The selection of the appropriate support for Pd-based catalyst is detrimental to its catalytic performance. The deployment of highly reducible ZnO support has been extensively studied in the literature, and was ascribed mainly to the formation of Pd-Zn alloys detected by XRD and XPS analysis techniques [84]. Therefore, tuning this Pd-Zn interaction through the variation of the ZnO support characteristics was the focus of several research studies. Eblagon et al. [82] investigated the effects of support calcination atmosphere (air, N2, H2, and O2) on the low temperature SRM process at 180 °C. The highly reducing calcination atmosphere in the presence of H2 showed the highest activity, which was attributed to the appearance of surface defects on the ZnO acting as oxygen suppliers and providing active sites for water adsorption during the SRM reaction. Additionally, these oxygen vacancies present on the ZnO surface contributed to enhancing the selectivity by suppressing the formation of CO. In a separate study, the effects of Zn/Pd ratio of the intermetallic unsupported Zn100-xPdx was investigated using in-situ XPS in the SRM process at 360 °C [90]. It was concluded that the Zn-rich catalyst showed higher selectivity and methanol conversion as compared to the Pd-rich catalysts due to the formation of oxidized Zn state and ZnPd intermetallic structure in the case of Zn-rich catalyst that was not detected for the Pd-rich samples. This was further confirmed by the work of Halevi et al. [86] on the surface composition and oxidation state of the unsupported PdZn catalysts. Another approach to enhance the performance of the PdZn alloys is through the coating of the PdZn catalyst on oxide-based supports using washcoating methods [83] or co-precipitation techniques [85] to improve their stability by enhancing the dispersion of the PdZn particles. Other intermetallic alloys were also reported in the literature such as Pd-Ga [81, 88–91] and Pd-In [87] catalysts.

Table 1.1 Summery of SRM reaction over various metal-oxide supported catalyst.

The use of nanoparticles of Au and Pt supported on oxides has gained tremendous attention in the past decade. A detailed review pertaining to the catalyst structure, physicochemical properties and applications of Au- and Pt-based supported catalysts in reforming reactions have been reported recently [92]. The optimization of synthesis parameters, metal loading, and the reaction parameters has been reported for the Au/CeO2 catalyst using the deposition-precipitation method [93, 94]. However, the catalysts prepared using this method lost activity after 168 hours due to coke deposition and metal sintering at the reforming conditions (200 to 300 °C). To overcome these limitations, a mixed oxide of CeO2-ZrO2 [95], CeO2-Fe2O3 [96, 97], and CeO2/CuO [93, 98] were suggested. The Zr-based supports were found to increase the initial activity due to the increased Au-support interactions, while the Fe2O3-based catalyst at Ce:Fe of 1:1 and 3 wt% Au loading showed the highest activity due to the coexistence of Fe2O3 and Ce1-xFexO2 phases in the reaction media [97], in agreement with previous findings [99]. On the other hand, the addition of CuO to the Au/CeO2 catalyst expressed high activity and selectivity attributed to the synergism between Au and Cu species [97].

The combination of two metals in the catalyst structure to create bimetallic catalysts have demonstrated excellent performance in the SRM process due to the contribution of the two metals at different stages of the SRM reaction. The bimetallic system of Cu-Ni supported on ZrO2 support has gained considerable attention in the past few years to obtain Ni-Cu alloys [100–103]. The catalytic activity of these bimetallic catalysts is controlled by the particle size, shape and distribution of the Cu and Ni on the surface of ZrO2 support. According to Perez and coworkers [100], the Cucore-Nishell formulation measured by TEM images exhibited the highest methanol conversion, confirming the findings by Lytkina et al. [101]. Also, there was a consensus that the support structure has a major impact on the bimetallic catalyst activity while the selectivity is mainly controlled by the dominant metal. The successive impregnation synthesis scheme is found to be more efficient than the co-precipitation method, due to the structural impacts of the synthesis method. Successive incorporation of metals onto the ZrO2 supports affects the distribution of Cu and Ni in the shell and core side of the support and favors the formation of the most active Cucore-Nishell nanoparticles as opposed to the simultaneous impregnation method which favors the formation of different Cu-Ni alloys [102]. These findings highlight the importance of the synthesis method and the catalyst pretreatment, as was observed by Huang et al. [104] for the Cu-Ni catalyst deposited on Fe2O3 support using solid-state reaction method. The effect of the catalyst reduction environment and conditions on the properties of the catalyst were significant due to the change in dispersion and surface area [105]. Other bimetallic combinations were also investigated such as Au-Cu/CeO2ZrO2 [106, 107], Pd-Ag [108], and M-Cu/ZnO/Al2O3 (where M is Pd, Pt, Ru, and Rh) [109]. The synergism of Au and Cu metals supported on Ce0.75Zr0.25O2 prepared using co-precipitation method showed 100% methanol conversion at 350 °C and was strongly dependent on the pH during the synthesis step [106]. A detailed analysis of the individual effects of Cu:Au ratio, calcination temperature, and metal loadings is reported by the same group elsewhere [107].

1.2.2 Catalyst Development for the Steam Reforming of Ethanol (SRE)

Steam reforming of ethanol is considered one of the promising sustainable routes for the synthesis of hydrogen fuel, driven by the abundant resources of ethanol from the fermentation of biomass [119], low toxicity (opposed to methanol) [120], hydrogen content, safe handling and storage characteristics of ethanol. These excellent features render ethanol as efficient promising feedstock for hydrogen production using the steam reforming process. Different heterogeneous catalytic systems have shown high ethanol conversion, high hydrogen yield, and durable catalyst performance. All these metal-support systems have the ability to rupture the C-C bond in ethanol in the presence of steam at high temperatures to release H2, CO2, CO, and other by-products. The catalyst systems developed for SRE could be categorized into noble metals (Rh, Pt, Pd) and transition metals (Ni, Co, Cu) supported on various oxide-based substrates. Hou et al. [31] have presented a descent review article for the catalysts used in SRE process following the trend of similar previous review papers [121–124]. Amongst the noble metals-based catalysts, Rh-based systems demonstrated high activity, selectivity and stability for SRE; hence, they have been intensively studied in the literature from both theoretical and experimental perspectives to gain deep insights into their outstanding merits for SRE [125]. Table 1.2 summarizes the most recent literature pertaining to noble metal-based catalysts featuring Rh as the major catalyst. The Rh-support interaction was found to be the main reason for its high activity and stability, for instance, due to the formation of Rh-O-Ce in the case of CeO2 support deemed responsible for prolonging the catalyst lifetime by creating anchorage sites to shift the equilibrium toward gasification of carbonaceous residues [126]. Despite the complete conversion on most of the Rh-based catalysts, the amount of coke deposited on Al2O3 was much higher than the CeO2-based catalysts due to the high oxygen storage capacity of CeO2, leading to several modification schemes by using mixed support systems to allow for an extra dimension to control the performance of the catalyst, as reported in Table 1.2. Nevertheless, ZrO2 supports were mainly added to enhance the catalyst thermal stability.

Table 1.2 Summary of SRE over noble metal catalysts.

1.2.2.1 Co-Based Catalysts for SRE

Transition metals such as Ni and Co have been proposed to replace the costly noble metals, while preserving the high activity and H2 selectivity. Table 1.3 summarizes the recent literature for SRE over Co-based catalysts supported on single and mixed oxides. Detailed characterization of Co-based catalysts confirms the formation of the oxide phase Co3O4, as demonstrated by the case of Al2O3 and CeO2 supports. A comparison study was presented by Maia et al. [140] which attributed the high selectivity in Co/Al2O3 to the high surface area of the alumina-based support, which, however, suffered a severe deactivation due to coke formation. This issue was overcome either by using oxidative SRE to burn off the carbon or by modifying the support with CeO2 owing to its excellent oxygen storage capacity. It is worth mentioning that addition of oxygen had an adverse effect in the case of Co/CeO2 catalyst due to the oxidation of the active phase as a result of the extra oxygen. The impact of the accessible oxygen on preventing the coke formation was studied by Yu et al. [141] on Co/CeO2 catalysts prepared by impregnation and hydrothermal ultrasonic-assisted methods. Further improvement in Co/CeO2 catalysts could be realized by adding La2O3, which was found to interact with the CeO2 support leading to better H2 selectivity attributed to the increased dispersion, beside longer lifetime due to the hindrance of graphitic carbon formation [142], as illustrated in Figure 1.1.

Graphic

Figure 1.1 Effects of La2O3 on the performance of Co/CeO2 in SRE. (Adapted from [142])

Table 1.3 Summary of SRE over Co-based catalysts.

1.2.2.2 Ni-Based Catalysts for SRE

Ni-based catalysts are among the most used systems for SRE, due to the high activity and role of Ni in the C-C bond cleavage, which is already available at a commercial scale level for the reforming of natural gas [159]. However, the greatest drawback of Ni-based catalysts is the fast deactivation by metal sintering and coke formation. Table 1.4 recapitulates the strategies deployed in an attempt to improve the catalyst lifetime by using mixed support systems, optimizing the SRE operating conditions, synthesis methods, and the catalyst regeneration methods along with other strategies. It is worth noting that a direct comparison between the catalyst systems listed in Table 1.4 is not reasonable due to the fact that the effects of the operating conditions, reactor setup and the long-term performances are not taken into account. However, these tables can provide a general guide to understanding the typical values associated with each catalyst system constrained by the given conditions. The use of SiO2 as a support for Ni metals is reported by several researchers [160]. Wu et al. have investigated the effect of the synthesis conditions in a sol-gel method by varying the acidity of the precursor solution and the calcination atmosphere [160–162]. The low solution acidity in the synthesis step favored the high activity in SRE while calcination under N2 atmosphere showed a great impact on the H2 selectivity and catalyst stability, which was ascribed to the high Ni dispersion in addition to the formation of fine Ni particles as compared to larger particles when calcined in air, as was evident from the XRD analysis. The effect of the NiO particle size was further investigated by Bej et al. [161], who successfully prepared nano-size NiO supported on SiO2, that shown enhanced catalytic stability, especially when using a steam to ethanol ratio of 8. Moreover, using the flame pyrolysis synthesis method as compared to the conventional precipitation-impregnation method has been proven to form nanostructured NiO with high dispersion, leading to more metal-support interaction and hence improving the catalyst activity and thermal stability [162]. The use of Al2O3 as a support for Ni-based catalyst has been widely investigated due to its high surface area and low cost. However, it is highly susceptible to metal sintering and coke formation originating from the formation of ethylene as carbon precursor. Several attempts were reported to improve the Ni/Al2O3 stability. For instance, using ionic liquid (1-hexadecyl-3-methylimidazoliumchloride) as surfactant in the single-step evaporation-induced self-assembly method has been shown to influence the surface area of the final catalyst, leading to high Ni dispersion and consequently high H2 yield [163]. In addition to the synthesis method, the pretreatment of the as-prepared Ni/Al2O3 catalyst using dielectric barrier discharge (DBD) is proven to promote the catalyst activity and stability using the metal-atom collisions to reduce the catalyst forming smaller particles [164]. A review paper is available elsewhere that addresses the challenges and opportunities of using plasma in ethanol reforming [165]. The most effective approach to improve the catalyst stability of Ni/Al2O3 catalysts is by mixing the Al2O3 support with La2O3, ZrO2, and CeO2 that have higher oxygen mobility. Lanthanum-based promoters mainly work by enhancing the gasification of coke off the catalyst as well as enhancing the Ni dispersion and number of NiO [166]. A detailed explanation of the evolution of coke formation in Ni/Al2O3 modified with La2O3 is given by Montero et al. [167], indicating the formation of filamentous coke at early stages of reaction and non-filamentous coke at the last stages, as shown in Figure 1.2. The promoting effect of La2O3 support was further increased by adding CeO2 forming triplet oxide support (Al2O3-La2O3-CeO2) [168], which is attributed to the Ce-Ni interactions as elucidated in details in a recent study [169].

Graphic

Figure 1.2 Coke formation at different stages of SRE over Ni/Al2O3-La2O3. (Adapted from [167])

Table 1.4 Summary of SRE over Ni-based catalysts.

The use of ZrO2 as a catalytic promoter in the Ni/Al2O3 was also shown to enhance the catalyst activity and stability in the SRE. Optimization of the Zr/Al ratio in the support was performed by Han et al. [170], and it was found that the reducibility of the catalyst increased by increasing the Zr/Al ratio while the acidity was reduced. In addition to the support structure, the same group also studied the effect of Ni loading and it was concluded that the high Ni surface area is detrimental to the catalyst stability. Very high Ni loading leads to forming clusters which agglomerate during the SRE and get easily deactivated, therefore a Ni loading of 15 wt% was found to be optimal for the SRE process [171].

Apart from the conventional metal-support catalyst systems that are prepared mainly by the impregnation of the active metal phase on the support materials, several researchers have proposed the use of perovskite-based oxides which inherently contain the active metals present as finely dispersed entities on the surface of the oxide. The careful substitution of the different components in the perovskite structure can lead to the synthesis of different catalysts with varying properties where each part plays a particular role toward enhancing the catalytic performance in SRE. Table 1.5 shows the work done using perovskite-type oxides. Using Co as an active metal in perovskite-type catalysts in conjunction with La and Ce to form La0.6Sr0.4CoO3 was studied by Morales and Segarra [201]. The improved catalytic activity was ascribed to the high dispersion of Co nanoparticles in the perovskite framework, while the presence of La in the matrix enhanced the catalyst stability in a similar manner as illustrated in Figure 1.1. The replacement of Sr in the matrix with Mg, Ca, and Ce was investigated by Ma et al. [202]. The superior performance of Sr-based perovskite was confirmed, while Ce-substituted samples showed higher stability. Similarly for the Ni-based perovskite-type catalysts, the catalyst activity and selectivity was high due to the formation of shell-core structures with the perovskite phase forming the shell layer and the active metal in the core side [203]. This was supported by another study for La1-xCaxAl1-yNiyO3 catalyst, however the presence of Ca hindered the incorporation of Ni into the perovskite matrix [204].

Table 1.5 Summary of SRE over perovskite-based catalysts.

One of the promising approaches to address the catalyst stability in the Co- and Ni-based catalysts is to use transition metal promoters, such as Mn, Mg, Ba, Ca, K, and Fe, as reported in Table 1.6. Potassium (K) has been used to promote the catalytic performance of Co-based catalysts supported on ZnO-Al2O3 [209], ZrO2 [210], and CeO2 [211]. It was concluded that the presence of K in the catalyst helped to reduce the coke formation by suppressing the ethylene formation reaction. The impact of the K promoters is highly pronounced for smaller Co crystal size, while its effects were minimal for larger Co crystals. It was also found that the positive effects of Ca and Mg promoters are dependent on the nature of the active metal used, as discussed in a recent work [212]. The Mg and Ca incorporated into Co/SBA-15 catalyst have increased its reduction temperature, thus not enough Co⁰ species were available to reform the C2 intermediate compounds. On the contrary, the addition of the same promoters (Mg and Ca) to Ni/SBA-15 resulted in mild reduction temperatures; hence a complete ethanol conversion was achieved at around 91% H2 selectivity. This observation could be attributed to the synthesis method of the modified Co-based catalysts. The impregnation of both Mg and Co into the as-prepared Al2O3 support showed less activity in the SRE as compared to the direct synthesis of Mg-Co-Al solid solution, due to the abundance of Co⁰ species in the latter method [213], which is supported by a recent study [214].

Table 1.6 Promoting of metal supported catalyst using transition metals.

1.2.2.3 Bimetallic-Based Catalysts for SRE

In the previous sections we have discussed the monometallic catalyst systems for SRE reaction, and several strategies were proposed to improve the catalytic performance of these materials which were mainly focused around the use of mixed support systems. The active metal loading was also optimized to achieve higher loading without being susceptible to metal clustering and segregation. Another promising strategy that has been thoroughly investigated in the literature is the bimetallic catalyst systems by incorporating two active sites into single and/or mixed support materials, as reported in Table 1.7. These include a combination of noble-noble, active-active, and noble-active metals. Chiou et al. have studied the Ni-Co bimetallic system supported on Ce0.5Zr0.5O2 using co-impregnation method. This catalyst, however, suffered from severe deactivation after only 6 hours of operation due to having a high facile reduction temperature of 375 °C as well as the tendency for clustering [220]. A similar observation was reported for Ni-Co bimetallic catalyst prepared on Ca-modified Al2O3 support. It was noted that the catalytic activity was reduced with decreasing the Ni loading in the bimetallic system [221]. Replacing Co with Cu to form Ni-Cu bimetallic catalyst supported on CeO2 and sodium-modified Nb2O5 [222] and on SiO2 [223] is reported. It is concluded that the support and the Cu-Ni interface play a crucial role in the observed catalytic activity. The presence of individual Cu and Ni phases favored the SRE while the mixed alloy Cu-Ni had less SRE activity [223]. Similarly, the support reducibility and oxygen mobility has a detrimental impact on the catalytic performance, as was observed for the case of CeO2 as compared to the Nb2O5. Bimetallic systems based on noble metal combinations have also gained considerable attention in the past decade. Cobo et al. [224] have reported the SRE over Rh-Pt supported on La2O3 and have obtained an optimum steam to ethanol ratio and optimum reaction temperature to obtain complete ethanol conversion (steam/ethanol = 7, and T = 600 °C). In a similar work by Divins and Llorca [225], it was concluded that the support has a significant role in the oxidation state of the bimetallic Rh-Pd system. The CeO2 supported Rh-Pd catalyst was found to be more oxidized than the unsupported Rh-Pd system due to the interaction with the CeO2 support. The bimetallic Ni-Pt supported on CeO2 was evaluated for the low temperature SRE by several researchers [226–228]. The catalytic activity of these various systems is reported in Table 1.7. The addition of Pt was found to improve the catalyst lifetime by minimizing the attack of Ni by carbon and prevent the formation of NiC. Moreover, the fast kinetics of coke removal by Pt due to the high hydrogenation of the carbon attached to the surface resulted in less carbon diffusion into Ni metals, as illustrated in Figure 1.3.

Graphic

Figure 1.3 Inhibition of coke formation in Ni-Pt bimetallic catalyst. (Adapted from [226])

Table 1.7 Summary of bimetallic catalyst systems for SRE process.

1.2.3 Catalyst Development for the Steam Reforming of Glycerol (SRG)

The use of glycerol as a potential candidate for hydrogen production is driven by the sharply increasing biodiesel production using transesterification to produce glycerol as a major by-product [247]. This rapidly growing biodiesel production industry has resulted in the abundant availability of glycerol at very low prices of around $1/lb for pure glycerol and less than $0.3/lb for crude glycerol [248]. Ayoub and Abdullah have provided a review highlighting the existing and future strategies for the utilization of glycerol in the sustainable energy sector [248]. The steam reforming of glycerol represents the most acceptable route for the deployment of glycerol as a feedstock in hydrogen production, mainly due to the fact that steam reforming technology is already available at industrial scale for methane steam reforming, thus no great infrastructure changes are required. Moreover, reforming of 1 mole glycerol gives a stoichiometric 7 mole theoretical hydrogen as compared to 4, 3, and 6 moles in methane, methanol, and ethanol steam reforming respectively. Similar to SRM and SRE, SRG is an endothermic reaction with an equilibrium threshold for hydrogen production; therefore, the SRG reaction equilibrium has been the focus of several investigations [249–251]. In order to shift the reaction equilibrium toward more hydrogen production several approaches could be used such as optimizing the reaction temperature, glycerol concentration and the reaction pressure [252–254]. The catalyst system employed can also enhance the glycerol conversion by favoring certain reactions and suppressing other unwanted pathways, ultimately increasing hydrogen yield. In this section we aim at addressing the SRG challenges through the careful selection of heterogeneous catalyst for high glycerol conversion, high hydrogen yield, long lifetime and low cost. Due to the extensive research and high publication volume in this field, we noticed that several review papers addressing the SRG challenges have been reported over the past few years [255–257]. This reflects the fast growing interest in SRG as a potential route for sustainable hydrogen production. The most studied catalysts are Ni, Co, Pt, and Ru supported on various oxide- and non-oxide-based supports. Table 1.8 contains a summary of the recent Ni-based catalyst systems reported from 2012 to date. Papageridis et al. have compared the catalytic activity of three active metals, Ni, Co and Cu, supported on Al2O3 at metal loadings of 8 wt% to understand their physicochemical properties and the effect of reaction temperature on their catalytic performance [258]. The Ni/Al2O3-based catalyst showed higher catalytic activity and H2 selectivity as compared to Co- and Cu-based catalysts, which could be ascribed to the superior activity of Ni in C-C bond cleavage; however, the coke formation on Ni-based catalyst was much higher than Cu and Co catalysts, which was proportionally correlated to the catalyst acidity as measured by the NH3-TPD analysis. To address the stability issue of Ni-based catalysts, the use of alkaline earth promoters have proven to offer a significant improvement in the catalyst lifetime [249, 259–262]. It was found that the incorporation of 10 wt% MgO and CaO into a 7 wt% Ni supported on SBA-15 considerably enhanced the Ni-SBA-15 interactions, especially CaO, which showed a conversion of 98%, with more than 50% of the formed gaseous products being H2. Most importantly, this has reduced the coke formation by about 25% at 600 °C [259]. This observation could be related to the nature of the coke formed during the reaction (nanofibers), as observed by Calles and coworkers [259]; however, a similar study of MgO addition to Ni/Al2O3 has attributed this behavior to the high basicity of the MgO modified supports [260]. Nevertheless, the optimization of Ni loading in the presence of MgO modifiers is also crucial to the catalytic activity, as the NiO content increases at higher Ni loadings, leading to increasing the catalyst reducibility and hence the SRG activity; on the other hand, this might increase the NiO particle size, consequently reducing the glycerol conversion [261]. The strong impact of the nature of the support on the catalytic activity has been reported in many articles due to the difference in surface area, support basicity, oxygen mobility and thermal stability [263, 264]. The mixed oxide approach has recently been shown to give improved catalyst attributes toward SRG reaction [265, 266]. The superior activity of Al2O3 as compared to La2O3, ZrO2, SiO2, and MgO is confirmed by Zamzuri et al. [263], as can be seen in Figure 1.4; therefore, Al2O3 is normally considered as the main support in the mixed oxide strategy. The influence of B2O3 and La2O3 on the characteristics of Ni/Al2O3 was studied by Kousi et al. [265]. The catalytic performance of the unmodified samples were dependent on the surface acidity and support calcination temperature, while the presence of La2O3 improved the glycerol conversion toward gaseous products, especially at low temperatures, in contrast to B2O3, which favored the formation of liquid products. The design of the oxide ratios in the mixed oxide catalyst is essential to the control of the Ni dispersion and oxidation state during the reaction; therefore, a systematic analysis of mixed oxide systems is always required [267, 268].

Graphic

Figure 1.4 Comparison between different oxide supports for SRG over Ni. (Adapted from [263])

Table 1.8 Summary of SRG over Ni-based catalysts.