Heterogeneous Catalysis for Sustainable Energy E-Book

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• Sprache: Englisch

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Veröffentlichungsjahr: 2022

Beschreibung

Explore the state-of-the-art in heterogeneous catalysis In Heterogeneous Catalysis for Sustainable Energy, a team of distinguished researchers delivers a comprehensive and cutting-edge treatment of recent advancements in energy-related catalytic reactions and processes in the field of heterogeneous catalysis. The book includes extensive coverage of the hydrogen economy, methane activation, methanol-to-hydrocarbons, carbon dioxide conversion, and biomass conversion methane activation. The authors explore different aspects of the technology, like reaction mechanisms, catalyst synthesis, and the commercial status of the reactions. The book also includes: * A thorough introduction to the hydrogen economy, including hydrogen production, the reforming of oxygen-containing chemicals, and advances in Fischer-Tropsch Synthesis * Comprehensive explorations of methane activation, including steam and dry reforming of methane and methane activation over zeolite catalysts * Practical discussions of alkane activation, including cracking of hydrocarbons to light olefins and catalytic dehydrogenation of light alkanes In-depth examinations of zeolite catalysis and carbon dioxide as C1 building block Perfect for catalytic, physical, and surface chemists, Heterogeneous Catalysis for Sustainable Energy also belongs in the libraries of materials scientists with an interest in energy-related reactions and processes in the field of heterogeneous catalysis.

Leseprobe

Heterogeneous Catalysis for Sustainable Energy

Edited by Landong Li and Justin S. J. Hargreaves

Editors

Prof. Landong Li

Nankai University

School of Materials Science and Engineering

38# Tongyan Road

Haihe Education Park

Jinnan District

300350 Tianjin

China

Prof. Justin S. J. Hargreaves

University of Glasgow

School of Chemistry

Joseph Black Building

G12 8QQ Glasgow

United Kingdom

Cover Image: © skegbydave/Getty Images

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Preface

Energy can be defined as the ability to do work. Specifically, energy exists in various forms such as chemical energy, thermal energy, and radiant energy. Moreover, the controllable transformation of one form to another is of great significance for modern civilization. With the development of human society, the demand for energy has been increasing generally each year, and energy and its interconversion have become a highly topical issue. Nowadays, there are growing concerns not only about accessible energy reserves but also about the energy infrastructure necessary to maintain sustainable development of society. The concept of sustainable energy, which represents energy that can meet current demand and not cause harmful repercussions for the future, is increasingly recognized. Sustainable energy depends upon not only its origin but also how it is utilized.

Catalysis is a process in which a catalyst can change, generally increase, the rate of a chemical reaction without being consumed in the process. It has had huge impact on society, and the rapid development of human society is always accompanied by notable breakthroughs in catalysis, for example, the well‐known catalytic ammonia synthesis, catalytic polymerization, and catalytic cracking processes. Catalysis has an indispensable role in industrial production. It is involved in about 90% of chemical processes and the manufacture of over 60% of industrial products.

Catalysis is closely related to energy. In a general catalytic process, a chemical reaction can proceed much faster than that in the absence of a catalyst, and therefore, less energy may be consumed. More intuitively, catalysis can directly participate in the energy transformation processes, for example, converting thermal energy to chemical energy via thermal catalysis, converting photo energy to chemical energy via photocatalysis, and converting electrical energy to chemical energy via electrocatalysis. A key issue in energy transformation is efficiency, and catalysis undoubtedly plays an essential role in promoting the energy efficiency for more sustainable development. It is well known that the energy structure for society is undergoing distinct changes: i.e. the proportion of traditional energy sources like petroleum and coal applied is gradually declining, the proportion of greener energy is gradually increasing, and, correspondingly, the proportion of sustainable energy is gradually increasing. However, society presently must rely on the traditional fossil energy sources of petroleum (∼32%), coal (∼27%), and natural gas (∼24%). Improvements in the transformation of fossil energy or resources via catalysis can make a significant contribution to modern requirements. On the other hand, the direct generation of sustainable energy from renewable resources via the development of routes involving heterogeneous catalysis might provide a possible solution to the global energy challenges.

Under such circumstances, it is meaningful and desirable to summarize current research progress in heterogeneous catalysis for sustainable energy. Related to this, this book provides a summary relating to the fundamental science of heterogeneous catalysis and sustainable energy, and it is timely to encourage further research activity in this highly topical research field. In this book, two different aspects are summarized, namely, i) the more efficient transformation of fossil resources using heterogeneous catalysis as a tool and ii) the exploitation of renewable energy through heterogeneous catalysis. Specifically, aspects related to hydrogen energy, methane activation, alkane activation, zeolite catalysis, the application of carbon dioxide as building block, and biomass conversion have been included. The authors of the various chapters included in this book are all active researchers in the field of heterogeneous catalysis and sustainable energy. Authors from around the world, including China, the United Kingdom, Australia, Mexico, and Singapore, have contributed.

Nankai University andGlasgow UnversityOctober, 2021

By

Landong Li and Justin S. J. Hargreaves

Part IHydrogen Economy

1Catalytic Hydrogen Production

Xingyuan Gao and Sibudjing Kawi

National University of Singapore, Department of Chemical and Biomolecular Engineering, 4 Engineering Drive 4, 117585 Singapore, Singapore

1.1 Introduction

Currently and in the near future, fossil fuels are/will be the major source of hydrogen production [1, 2]. As shown in Figure 1.1, almost all the industrial manufacturing processes for hydrogen rely on fossil fuels directly or indirectly [1]. Among the fossil fuels like coal, heavy hydrocarbons, and natural gas, methane is mostly studied due to its high abundance in the world, such as in the United States and Malaysia [4–8]. On the other hand, around 48% of hydrogen gas is generated from natural gas [9–11]. Moreover, methane is easy to handle and has a high H/C ratio [12]. Therefore, we focus on the thermocatalytic conversion of methane first in this part, followed by the conversion of other fossil fuels to produce hydrogen gas.

1.1.1 Thermocatalytic Decomposition of Methane

Only hydrogen gas and carbon are produced in the thermal decomposition of hydrocarbons. This CO‐free process is promising for industry [13, 14]. As the major component of hydrocarbons, methane has drawn much interest in the recent years in the production of hydrogen gas via thermal decomposition. The reaction mechanisms are shown below [9]. Initially, methane is chemisorbed on the exposed face of catalyst crystals; secondly, CH bonds are broken in gaseous methane molecules to produce methyl groups and hydrogen atoms, followed by stepwise dissociations to generate CHx and more hydrogen atoms; thirdly, two hydrogen atoms combine to generate hydrogen molecules, which are released in gaseous form; fourthly, atomic carbon aggregates and diffuses onto the surface of catalyst particles; fifthly, nucleation and growth of carbon filaments occur in the trailing face of catalyst particles.

Among several methane cracking technologies like plasma, pyrolytic, thermocatalytic, and photocatalytic routes, we focus on thermocatalytic decomposition due to the simultaneous formation of carbon nanofibers or carbon nanotubes that can be potentially used in various applications [14, 15]. Because of the very inactive CH bond in methane molecules, the activation energy is high, and the reaction is strongly endothermic [16]. Therefore, catalysts are necessary to lower the reaction temperatures and promote the kinetics, including metal‐ and carbon‐based catalysts [1].

Figure 1.1 The major sources of the industrial hydrogen manufacturing.

Source: Reproduced with permission: © 2007, International Energy Agency [3].

1.1.1.1 Metal Catalysts

Since the 1960s, transition metals (Ni, Fe, Co) have been extensively studied and show good catalytic performance in methane decomposition, which occurs at 500–800 °C, much lower than 1200 °C required without a catalyst [17–20]. However, the industrialization of this catalytic system is impeded due to the carbon deposits covering the active sites, leading to rapid deactivation [16, 21]. Modifications have been made to improve the reactivity and stability of transition metal‐based catalysts using other transition metals and rare earth metals [22–26].

Bayat et al. [22] studied the Ni–Fe alloy derived from the reduction of the spinel NiFe2O4 phase. Below 650 °C, the addition of Fe inhibited the encapsulation of carbon by facilitating the carbon diffusion. However, the active sites become fewer with increasing Fe content due to the lower degree of reducibility. To offset the negative effect of Fe, Bayat et al. [23] doped Cu into the Ni–Fe alloy to enhance the methane adsorption and Ni dispersion on alumina. The optimal ratio of Ni/Fe/Cu was 5 : 1 : 1.

Instead of adding Fe, Lua and Wang [24] doped Co into Ni–Cu to form a tri‐metallic alloy. Since Co possesses a high melting point, the quasi‐liquid phenomenon occurring between 650 and 775 °C was effectively inhibited, leading to enhanced stability. However, phase separation may be an issue with the further addition of Co. Following this work, a series of catalysts comprising Co and W in different ratios were developed [25]. When Co/W equaled 4 : 1, hydrogen gas and multiwall carbon nanotubes were simultaneously produced, showing the highest conversion of methane. It was found that non‐interacted Co3O4 was responsible for the superior catalytic performance.

Besides doping of transition metals, a series of rare earth metals were added to Ni to form bimetallic catalysts. Among the additives La, Sr, Nd, Pr, Y, and Sm, Ni‐La exhibited the highest activity and lowest solid carbon formation due to their good thermal stability and small Ni particle size [26] (Table 1.1).

Table 1.1 Summary of metal catalysts in methane decomposition.

Catalyst

Conditions

Findings

References

Ni–Fe/Al

2

O

3

700 °C for 3 h; 30 vol% CH

4

and 70 vol% N

2

.

Fe inhibited the encapsulation of carbon by facilitating the carbon diffusion

[22]

Ni–Fe–Cu/Al

2

O

3

700 °C for 3 h; 30 vol% CH

4

and 70 vol% N

2

.

Cu enhanced methane adsorption and improved the reducibility and nickel dispersion

[23]

Ni–Co–Cu

500–850 °C; 20 vol% CH

4

and 80 vol% N

2

.

The high melting point of Co inhibited the quasi‐liquid phenomenon, leading to an enhanced stability

[24]

Co–W/MgO

700 °C; CH

4

at a flow rate of 50 sccm.

When Co/W equaled to 4 : 1; non‐interacted Co

3

O

4

was responsible for the highest conversion of methane

[25]

Ni–La–Si

300∼750 °C; CH

4

at a flow rate of 10 ml min

−1

The high activity and low solid carbon formation were attributed to the good thermal stability and small Ni particle size

[26]

1.1.1.2 Carbon Catalysts

Due to the low cost, resistance to sulfur, and temperature, various carbon materials have been studied as alternatives to transition metals in methane decomposition [1, 16], including active carbon particles [27–30], ordered mesoporous carbons [31, 32], carbon black particles [33], and commercial carbon materials [34]. However, gradual deactivation occurred on carbon catalysts resulting from the coverage of inactive turbostratic carbon. To alleviate this issue, Muradov et al. [35] prolonged methane decomposition by generating active carbon aerosols continuously in a non‐thermal plasma device. Besides, Dufour et al. [36] discovered that the addition of small amounts of oxidizing agents like oxygen, CO2, and steam in the feedstock could effectively enhance the sustainability. Furthermore, the gaseous form of carbon particles derived from partial gasification could inhibit the deactivation of catalyst in a fluid state and cyclic process between the reactor and heater [37].

1.1.2 Partial Oxidation of Methane

Partial oxidation of methane (POM) has drawn much attention recently due to the compactness, good response time, and lower sensitivity to the type of fuels. In the presence of oxygen, POM is considered as a fast and highly exothermic reaction to produce syngas as shown in Eq. 1.1[38]:

Figure 1.2 Schematic representation of Ni/CeO2 and CeO2 surface having surface defects with under coordinated oxygen atoms.

Source: Reproduced with permission from Pal et al. [40]; © 2015, American Chemical Society.

(1.1)

The reaction can occur at a very high temperature without catalysts. However, the use of catalysts can lower the reaction temperature greatly that saves the energy input. The commonly studied catalysts for POM include transition metals, noble metals, and perovskites as shown in Table 1.2.

Amongst the transition metals, Ni with different supports is widely applied in POM. Pantaleo et al. [39] compared the catalytic performance of CeO2 and La2O3 single oxide supports and CeO2–La2O3 mixed oxide supports prepared by wet impregnation and coprecipitation. Interestingly, coke only deposited on the single oxide supported catalysts. The enhanced anti‐coking property of mixed oxide supported catalyst was attributed to the formation of a series of Ni–La2O3 species with different oxidation states of Ni. Besides, in another study regarding Ni/CeO2[40], the surface and point defects with undercoordinated oxygen atoms in CeO2 originating from the formation of O–Ni–O–Ce superstructures promoted the activation of CH bonds (Figure 1.2). In addition to Ni‐based catalysts, Co/ZrO2 exhibited a very high conversion of methane and selectivity to hydrogen gas, out‐performing many other catalysts [41].

Noble metals are also used in POM. A comparison among Pt, Pd, and bimetallic catalysts was conducted by Abbasi et al. [42]. It was found that Pd performed the best in this comparison, followed by the mixture and Pt alone. On the other hand, supports can also affect the performance of Rh‐based catalysts [43]. Due to the oxygen spillover from Ce0.5Zr0.5O2, Rh was easily reoxidized and lost active sites; however, this spillover effect could be alleviated by Al2O3.

Perovskite structures present superior anti‐coking properties due to the reaction of carbon deposits and oxygen species derived from the structure [60]. Sr0.8Ni0.2ZrO3 exhibited a highly stable conversion of methane at 900 °C under a reducing atmosphere [44]. Similarly, LaGa0.65Mg0.15Ni0.2O3 achieved 81% conversion of methane and 100% selectivity to hydrogen gas at 900 °C. This excellent catalytic property may be attributed to the existence of La2O3 and La2O2CO3 besides the perovskite structure [45].

Table 1.2 Summary of catalysts in partial oxidation of methane.

Catalyst

Conditions

Findings

References.

Ni/CeO

2

–La

2

O

3

700 °C; WHSV = 60 000; O/C = 0.5

Over 90% methane conversion; the formation of a series of Ni–La

2

O

3

species with different oxidation states of Ni inhibited cokes

[39]

Ni/CeO

2

750 °C; WHSV = 50 000; O/C = 0.5

Over 85% methane conversion and 65% H

2

selectivity; the surface and point defects originated from the formation of O–Ni–O–Ce structures activated the CH bonds

[40]

Co/ZrO

2

800 °C; WHSV = 60 000; O/C = 0.5

Co/ZrO

2

exhibited 100% conversion of methane and 98.1% selectivity to hydrogen gas

[41]

Pd/γ‐Al

2

O

3

650 °C; WHSV = 38 400; O/C = 2

Pd performed the best (nearly 100%), followed by Pt–Pd and Pt alone

[42]

Rh/Al

2

O

3

and Rh/Ce

0.5

Zr

0.5

O

2

600 °C; WHSV = 252 000; O/C = 2

Rh was easily reoxidized by oxygen spillover in Ce

0.5

Zr

0.5

O

2

; this spillover effect could be alleviated by Al

2

O

3

, maintaining 60% methane conversion for 10 h

[43]

Sr

0.8

Ni

0.2

ZrO

3

900 °C; WHSV = 66 000; O/C = 0.5

Over 94% methane conversion; a highly stable conversion of methane at 900 °C under reducing environment

[44]

LaGa

0.65

Mg

0.15

Ni

0.20

O

3−

δ

900 °C; WHSV = 3300; O/C = 0.5

81.2% methane conversion and 100% H

2

selectivity were attributed to the existence of La

2

O

3

and La

2

O

2

CO

3

besides the perovskite structure

[45]

Pt–NiO/Al

2

O

3

800 °C; WHSV = 7200; O/C = 0.5

91.8% methane conversion and 98.4% H

2

selectivity; Ni reduction was promoted by Pt

[46]

Co/Al

2

O

3

850 °C; WHSV = 60 000; O/C = 0.5

95% methane conversion and 93.6% H

2

selectivity were attributed to the formation of Co

3

O

4

as the major phase after 500 °C calcination

[47]

Ni/12CaO·7Al

2

O

3

800 °C; WHSV = 30 000; O/C = 0.5

Over 90% methane conversion and 95% H

2

selectivity were attributed to the active oxygen ions and high dispersion of Ni.

[48]

Co/MgO

850 °C; WHSV = 20 000; O/C = 0.5

95% methane conversion, little coke formation, and sintering were attributed to small crystals embedded in the support derived from CoO–MgO solid solution

[49]

Ni/CeO

2

/Al

2

O

3

800 °C; WHSV = 152 432; O/C = 0.5

Low loading of CeO

2

(1%) generated a highly dispersed CeO

2

particle, enhancing the reducibility and obtaining 80.3% methane conversion with less carbon deposition

[50]

Ni–Cr/Al

2

O

3

700 °C; WHSV = 195 000; O/C = 0.5

85% methane conversion and enhanced stability were realized by the more dispersed Ni particles and surface basicity with addition of Cr

[51]

Ni–Rh/Al

2

O

3

–MgO

750 °C; WHSV = 354 044; O/C = 0.5

93% methane conversion and 95% H

2

selectivity; Rh prevented the oxidation of Ni

[52]

Rh/CeO

2

700 °C; WHSV = 60 000; O/C = 0.5

The Rh ions in the surface lattice of CeO

2

were active in POM and obtained 95.2% methane conversion and 92.9% H

2

selectivity

[53]

Ni/ZrO

2

@SiO

2

core shell

750 °C; WHSV = 50 000; O/C = 0.5

Over 90% methane conversion and 75% H

2

selectivity were obtained with strong coke resistance due to the high oxygen storage capacity and steric hindrance

[54]

Ni/zeolite catalysts

750 °C; WHSV = 90 000; O/C = 0.5

100% methane conversion with strong anti‐deactivation ability was attributed to less surface acidity and higher thermal stability

[55]

Ni/TiO

2

800 °C; WHSV = 4800; O/C = 0.5

86.3% methane conversion and 99.7% H

2

selectivity were obtained, but serious deactivation was observed, resulting from the NiO and NiTiO

3

formation

[56]

LaCoO

3

/γ‐Al

2

O

3

800 °C; WHSV = 899 550; O/C = 0.25

Over 35% methane conversion and 40% H

2

selectivity; excellent stability was caused by highly dispersed Co and carbon removal by La

2

O

3

[57]

La

0.08

Sr

0.92

Fe

0.20

Ti

0.80

O

3

900 °C; WHSV = 30 000; O/C = 0.5

Over 50% methane conversion and 60% H

2

selectivity; high oxygen vacancy concentration was responsible for the high activity

[58]

La

0.5

Sr

0.5

CoO

3

850 °C; WHSV = 30 000; O/C = 0.5

Over 70% methane conversion and 75% H

2

selectivity were realized with highly dispersed Co particles in the La

2

O

3

and SrO matrix

[59]

WHSV, weight hour space velocity (unit: ml h−1  gcat−1); O/C, O2‐to‐carbon ratio.

1.1.3 Catalytic Reforming of Methane

The popular syngas production methods consist of steam reforming of methane (SRM), oxidative steam reforming of methane (OSRM), and dry reforming of methane (DRM). The following will respectively introduce the reaction mechanism, issues to be overcome, and catalytic systems. Kinetic modeling will also be included.

1.1.3.1 Steam Reforming of Methane (SRM)

SRM possesses many advantages, including high hydrogen yield and low cost to obtain hydrogen gas [61, 62]. The reaction equation is shown as below:

(1.2)

Due to the endothermic nature of this reaction, a high reaction temperature is preferred to generate a high yield of H2[63]. However, a simultaneous water–gas shift (WGS) reaction occurs, and the CO conversion is inhibited at high temperatures since WGS reaction is exothermic according to Eq. 1.3[64]:

(1.3)

To solve this issue in industry, a two‐reactor system has been adopted to achieve both a high conversion of methane and a high yield of H2. In detail, the reactants, methane and steam, are passed through the first reactor operated at 300–450 °C where the reaction is accelerated kinetically and more methane is converted thermodynamically according to Eq. 1.2 in spite of the low conversion of CO according to Eq. 1.3. Afterward, the intermediate products are continuously fed to the second low‐temperature reactor (175–250 °C) with a high ratio of steam to convert more CO to form CO2 and H2 [65, 66].

To further purify or enhance the yield of hydrogen gas in the final products, CO2 and H2 is required to be removed in situ respectively by sorbents and selective membranes as shown in Figure 1.3[67].

Figure 1.3 Representation of H2 and CO2 removal by H2‐selective membranes and using CO2 sorbents.

Source: Reproduced with permission from Ji et al. [67]: © 2018, The Royal Society of Chemistry.

Figure 1.4 Simplified block diagram of a modern SMR plant with major CO2 containing streams. SMR, steam methane reforming; WGS, water–gas shift reactor; PSA, pressure swing adsorption unit.

Source: Reproduced with permission from Muradov [1]: © 2017, Elsevier.

Specifically for CO2 sorbents, different from the hot potassium carbonate or amine scrubber used about two to three decades ago, pressure swing adsorption (PSA), a physical adsorption technology, is widely adopted in modern SRM plants, achieving an ultrahigh purity of 99.999% for H2 (shown in Figure 1.4) [1]. In this process, CO2 is not selectively separated from other gases, but used together with CH4 and CO to provide heat for the reformer with CO2 as an exhaust vented out of the reactor system in the end [68].

Besides the design of the reactor system, SRM catalysts should possess the follow stringent features: high catalytic stability, high conversion of methane, superior mechanical strength, excellent thermal stability, low pressure drop, and good heat transfer [69]. Ni‐based catalysts are commonly studied due to their high conversion and selectivity. However, carbon deposition hinders their large‐scale application in industry. Additives and support materials have been used to alleviate this issue, including rare earth metals, noble metals, and perovskites as summarized in Table 1.3. For example, with the addition of Sn into Ni/YSZ, the activation energy of CH4 increased, and the binding of carbon to the low‐coordinated Ni sites was weakened, thus inhibiting the nucleation of carbon at Ni sites [78]. Similarly, Cu can potentially block the sites for carbon formation when forming Ni–Cu alloy, achieving 98% methane conversion and 99% H2 selectivity [75]. When perovskite oxides were used as the supports or precursors, the catalytic stability could be enhanced, and coke formation could be diminished due to their high thermal stability under a wide range of oxygen partial pressures and also the formation of finely dispersed metal nanoparticles [60]. For example, La0.8Ce0.2Fe0.7Ni0.3O3 exhibited stable catalytic performance regardless of the S/C ratio and only 0.2 wt% coke formed after a 20 hour test. This superior performance was attributed to the oxygen vacancy, which promoted the dissociation of steam and coke gasification [79]. Similarly, the lattice oxygen provided by LaAlO3 and SrTiO3 were located near the Ni surface, which migrated easily to the CHx fragments and oxidized the CHx [80].

In addition to the modifications mentioned above, preparation methods can also affect the physicochemical properties of Ni‐based catalysts. For example, dielectric barrier discharge (DBD) was adopted together with the impregnation method in the preparation of Ni/SiO2 catalysts, forming a smaller Ni particle size than that prepared with the conventional impregnation method [74]. For core–shell Ni–Al2O3/silicalite zeolite catalysts, repeated calcinations at elevated temperatures generated NiAl2O4 spinel phase in the core, the catalytic performance of which was 10% higher than that prepared with traditional method [74]. Regarding Ni/CeO2 catalysts, a hierarchical structure was formed with a template synthesis process, where the nanoporous and interwoven ceramic fiber template was loaded with NiO nanoparticles and supported on the CeO2 scaffold. After thermal treatment, the microstructure of the nanocatalyst increased the conversion of methane up to 98% at 800 °C, which remained stable for five hours [81].

Besides Ni‐based catalysts, noble metals with different supports have been tested in SRM. With ZrO2 and Al2O3 as mixed support materials, Pd–Rh metal foams presented excellent catalytic stability over 200 hours with little coke formation at 800 °C [70]. Ru also proved to be highly active and selective in SRM. By using γ‐Al2O3 as the support, the catalyst outperformed commercial Ni/Al2O3 by 2 orders of magnitude for methane conversion. However, when the Ru loading was lower than 0.15 wt%, the oxidation of sub‐nanometer Ru clusters caused a rapid decline in the conversion [71]. Despite the size limitations, Ru‐based catalysts could maintain stable perfomance with a low S/C ratio (=1), while Ni/α‐Al2O3 catalyst deactivated very rapidly when S/C = 2 [71]. Another noble metal, Pd, was found to be sensitive to the type of support material. Compared with Al2O3, La2O3 promoted the formation of Pd0[Pdδ+OxLa] species due to the metal–support interaction (MSI). La2O3 could also enhance the interaction between Pt and Al2O3 because of the improved thermal stability of Al2O3[72].

Table 1.3 Summary of catalysts in steam reforming of methane.

Catalyst

Conditions

Findings

References

Pd–Rh/metal foam

300 °C; GHSV = 2000 h

−1

; S/C = 2.5.

96.7% CH

4

conversion for 200 h was attributed to the absence of pore diffusion limitations, stable structure, and small loading of active metals

[70]

Ru/γ‐Al

2

O

3

600 °C; WHSV = 750; S/C = 1.

75% methane conversion and 68% H

2

selectivity; when the Ru loading was lower than 0.15 wt%, the oxidation of sub‐nanometer Ru clusters caused the fast drop of the conversions

[71]

Pd/La

2

O

3

–Al

2

O

3

510 °C; S/C = 3.

La

2

O

3

promoted the formation of Pd

°

[Pd

δ

+

O

x

La] species and enhanced the interaction between Pt and Al

2

O

3

[72]

Ni‐nano‐CaO/Al

2

O

3

600 °C; WHSV = 2 700; S/C = 4.

86% methane conversion, 92% H

2

selectivity, and enhanced stability were attributed to high‐temperature pretreatment and formation of Ca

12

Al

14

O

33

[73]

Ni/SiO

2

800 °C; WHSV = 24 000; S/C = 0.5.

Enhanced methane conversion was obtained by the smaller catalyst size prepared by dielectric barrier discharge (DBD) plasma

[74]

Ni–Cu/Al

2

O

3

500 °C; WHSV = 2 000; S/C = 3.

Cu could possibly block the sites for carbon formation when forming Ni–Cu alloy, realizing over 85% methane conversion and 97% H

2

selectivity

[75]

Ni/Ce

1−

x

Gd

x

O

2

700 °C; WHSV = 560 000; S/C = 3.

75% methane conversion; Ga enhanced MSI so metal sintering was inhibited and carbon deposits were more reactive

[76]

Ru/MgO–Nb

2

O

5

700 °C; WHSV = 20 000; S/C = 4.

Over 95% methane conversion, 72% H

2

selectivity; tetragonal Nb

2

O

5

and metallic Ru were formed

in situ

from amorphous niobic acid and Ru

4+

during the reaction

[77]

Sn/Ni/YSZ

740 and 850 °C; S/C = 1.

The binding of carbon to the low‐coordinated Ni sites was weakened with Sn addition, thus inhibiting the nucleation of carbon at Ni sites

[78]

La

0.8

Ce

0.2

Fe

0.7

Ni

0.3

O

3

500–800 °C; WHSV = 108 000; S/C = 3.

Only 0.2 wt% coke formation after a 20 h test, which was attributed to the oxygen vacancy, promoting the dissociation of steam and coke gasification

[79]

Ni/LaAlO

3

and Ni/SrTiO

3

800 °C; W/F = 1.58 g h mol

−1

. S/C = 2.

91.7% and 88.4% methane conversions with longer stabilities were achieved due to the lattice oxygen in perovskites, promoting the oxidation of CH

x

species

[80]

WHSV, weight hour space velocity (unit: ml h−1  gcat−1); S/C, steam‐to‐carbon ratio.

1.1.3.2 Oxidative Steam Reforming of Methane (OSRM)

OSRM is a combination of POM and SRM, which is also called autothermal reforming of methane. OSRM has the advantages of the heat generated from POM and hydrogen gas produced by SRM [82]. Compared with SRM, less methane is burnt, and thus the carbon efficiency is higher for OSRM [83]. Another merit of OSRM is that the process can be stopped and started rapidly, and the amount of hydrogen gas produced is large [38]. Owing to these advantages, OSRM is widely applied in the syngas production units in Fischer–Tropsch plants [84]. However, to reduce the carbon in the spent catalysts, an expensive process needs to run to separate C2+ from methane [85]. Also, considering the high temperature of the burner, the high cost of operation is another concern [86]. To run the reaction properly, the S/C and O2/fuel ratios need to be controlled carefully to prevent the by‐product formation, optimize the reaction temperature, and tune the gas compositions [82, 87, 88].

Due to the good activation of CC bonds and low cost, Ni‐based catalysts are widely studied for this reaction [38]. The MgAl2O4 spinel material has been widely applied as the support material for Ni [84]. Besides, Ce–ZrO2 mixed oxides have been popularly applied because of their high oxygen storage capacity (OSC). Together with the metal surface area of Ni, cubic phase Ce0.75Zr0.25O2‐supported Ni catalyst exhibited high catalytic performance [89]. In detail, mobile oxygen species formed through a redox cycle inhibited coke deposition on the Ni active sites. As shown in Figure 1.5, a three‐layer model was proposed comprising a layer of Ni–Ce–Zr–Ox in the middle of free Ni metal and Ce–ZrO2 support. The lattice oxygen consumed was replenished by the O2[90].

Figure 1.5 Schematic of synthesis gas production over Ni/Ce–ZrO2 catalyst.

Source: Reproduced with permission from Roh et al. [90]: © 2001, Springer.

Several promoters were added to the above catalyst and Ag was shown to be better than Fe, Pt, and Pd due to its greater redox property. Compared with 0.1 wt%, higher loading of Ag (1 wt%) presented a higher reactivity [91]. Ce–ZrO2 can also promote the catalytic properties of noble metals. The high reducibility and OSC enhanced the reactivity of Rh catalyst. Compared with mixed phase of Ce0.5Zr0.5O2, single cubic phase led to superior performance [92]. Similar effect was exerted on Pt catalyst, where the carbon deposits were continuously removed at the metal–support interface due to the OSC of the Ce0.5Zr0.5O2 support [93]. When Ce–ZrO2 was adopted as the support, the addition of Al2O3 could enhance the metal dispersion and reduce the size of particles. Besides the carbon inhibition effect, Zr in the support could prevent the formation of inactive NiAl2O4 [94, 95].

Different opinions existed that considering the high reaction temperature, deactivation or poisoning of the catalysts in OSRM may not be as serious as the catalysts in steam reforming. Therefore, instead of focusing on the catalyst design, however, more attention should be paid to the optimization of pellet shape and minimizing the pressure drop [84].

1.1.3.3 CO2/Dry Reforming of Methane

Among the two greenhouse gases – methane and CO2 – though the concentration of methane is much lower than that of CO2, methane's global warming potential is 28–36 times higher than CO2