Water gas shift reaction for hydrogen production and carbon dioxide capture: A review

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Water gas shift reaction for hydrogen production and carbon dioxide capture: A review Wei-Hsin Chena,b,c,d, , Chia-Yang Chena ⁎

a

Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan Department of Chemical and Materials Engineering, College of Engineering, Tunghai University, Taichung 407, Taiwan c Department of Mechanical Engineering, National Chin-Yi University of Technology, Taichung 411, Taiwan d Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan b

HIGHLIGHTS

GRAPHICAL ABSTRACT

is an important reaction for H • WGSR production and CO capture. comprehensive review of the re• Asearch progress in the WGSR is given. thermodynamic and • State-of-the-art kinetic characteristics of the WGSR are 2

2

underlined.

behaviors in certain special • WGSR environments are emphasized. in membrane reactors for • WGSR carbon capture and H production is addressed.

2

ARTICLE INFO

ABSTRACT

Keywords: Water gas shift reaction Hydrogen production Carbon capture and storage Thermodynamics and kinetics Catalyst Palladium-based membrane

The water gas shift reaction is an important and commonly employed reaction in the industry. In the water gas shift reaction, hydrogen is produced from water or steam while carbon monoxide is converted into carbon dioxide. Over the years, on account of the progress in hydrogen energy and carbon capture and storage for developing alternative fuels and mitigating the atmospheric greenhouse effect, the water gas shift reaction has become a crucial route to simultaneously reach the requirements of hydrogen production and carbon dioxide enrichment, thereby enhancing CO2 capture. This article provides a comprehensive review of the research progress in the water gas shift reaction, with particular attention paid to the thermodynamic and kinetic characteristics. The performance of the water gas shift reaction highly depends on the adopted catalysts whose progress in recent years is extensively reviewed. The behaviors of the water gas shift reaction in special environments are also illustrated, several cases have the ability to proceed with water gas shift reaction without any catalyst. The utilization of several separation technologies on the water gas shift reaction such as carbon capture and storage and membrane reactors for purifying hydrogen and enriching carbon dioxide will be addressed as well. Reviewing past studies suggests that separating hydrogen and carbon dioxide in the product gas from the water gas shift reaction can not only increase efficiency but also enhance the usability for further application. The CO conversion is beyond the thermodynamic limitation after applying membrane for the water gas shift reaction.

⁎

Corresponding author at: Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan. E-mail addresses: [email protected], [email protected] (W.-H. Chen).

https://doi.org/10.1016/j.apenergy.2019.114078 Received 28 June 2019; Received in revised form 12 October 2019; Accepted 2 November 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Wei-Hsin Chen and Chia-Yang Chen, Applied Energy, https://doi.org/10.1016/j.apenergy.2019.114078

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Nomenclature ANNs CMA CPOM CCSU DME Higee HTC HTSR

Kp LTSR MC MWNTs PEMFCs RPB SMR TOF TPR WGSR

artificial neural networks calcium magnesium acetate catalytic partial oxidation of methane carbon capture, storage, and utilization dimethyl ether high gravity high-temperature catalyst high-temperature shift reaction

1. Introduction

equilibrium constant low-temperature shift reaction methane combustion multiwalled carbon nanotubes proton exchange membrane fuel cells rotating packed bed steam methane reforming turnover frequency temperature programmed reduction water gas shift reaction

bioethanol, biodiesel, biogas, bio-oil, biobutanol, etc.) [5,6] have received a great deal of attention. However, as described earlier, fossil fuels still play the most important role in energy supply. Therefore, developing methods for reducing CO2 emissions is an issue of considerable concern. In recent years, California has initiated the S.B. 100 law, which is aimed at shifting the reliance on energy from fossil fuels to renewable energy. The goal of this law is to ensure electricity is generated by 100% clean energy to reduce greenhouse gas emissions by 2045. More importantly, in 2016, the Paris Agreement, aimed at controlling the enhancement of ambient temperature to no higher than 2 °C, has been signed by 171 countries. Carbon capture, storage, and utilization (CCSU) are considered as effective ways to abate CO2 emissions from anthropogenic activity [7]. To date, three different capture techniques, namely post-combustion, pre-combustion, and oxy-fuel combustion, have been developed [8]. In pre-combustion, fuels react with insufficient oxidants (i.e., oxygen, air, or steam) to produce product gases in which synthesis gas or syngas, the gas mixture of carbon monoxide and hydrogen, is the main gaseous species. In general, the CO concentration in the product gas is high; in response, the water gas shift reaction (WGSR) can be carried out to convert CO in the product gas into CO2. Meanwhile, hydrogen can be further increased from the steam in the reaction. Due to the enrichment of CO2 and H2 in the processed gas, it is easier to capture and separate CO2 from the product stream, while H2 can be used as a fuel in gas turbine combined-cycle plants. Furthermore, to intensify H2 production and separate CO2, utilizing a membrane or a sorbent has the potential to enhance the WGSR, according to Le Chatelier’s principle. After separating CO2 and H2, the purified H2 has a higher energy density and is suitable as a fuel or feedstock for chemical production.

Energy consumption is an essential and crucial factor in the progress of civilization and industrialization; it is also a pivotal driving force for economic growth and better living standards in modern society. In meeting the demand of rising living standards and the growth of the world’s population, global energy consumption has risen rapidly since the Industrial Revolution. To date, fossil fuels have been the most important fuels for energy production and still constitute around 81% of all primary energy production in the world, with oil, coal and natural gas accounting for 31.7%, 28.1%, and 21.6%, respectively [1]. Most fossil fuels are consumed through combustion, thereby emitting large amounts of air pollutants, such as carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxides (SOx), particulate matter (PM), and unburned hydrocarbons (UHCs), into the atmosphere. In addition to air pollution, a huge amount of carbon dioxide (CO2), one of the primary by-products from fuel combustion, is also released into the atmosphere. Though CO2 is not an air pollutant, it strengthens the atmospheric greenhouse effect, which in turn causes global warming and climate change. In fact, the atmospheric CO2 concentration has risen from 260 to 270 ppm in the pre-industrial period [2] to 411.97 ppm in March 2019 [3], representing an approximately 57% increase in the concentration. Considering the increase in CO2, reducing anthropogenic CO2 emissions has become the most important issue facing the global community and the most severe challenge for environmental sustainability. To mitigate the atmospheric greenhouse effect, developments in renewable energy (e.g., wind and solar power, geothermal energy, marine energy, etc.) [4] and carbon-neutral fuels such as biofuels (e.g.,

Fig. 1. Hydrogen applications. 2

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Over the last several decades, hydrogen has been considered a promising substitute for fossil fuels used in heat and power generation. This arises from the fact that hydrogen is a non-carbon fuel and the most abundant element in the universe [9]. Unlike electricity generated from solar and wind power, which is difficult to store, hydrogen is a potential energy carrier that can be applied in the transportation sector. By virtue of the much higher power density or heating value per unit mass of hydrogen when compared to other gas or liquid fuels [10], it has been employed in liquid rockets and space shuttles as an important propellant for many decades. Other than the high energy density, when hydrogen is burned, water is the only by-product with no CO2 emitted, realizing its clean characteristic. Based on the aforesaid benefits, Toyota, Honda, Hyundai, and other motor companies have recently launched sales of hydrogen fuel-cell vehicles [11], resulting in a foreseeable future hydrogen economy. Recently, Toyota has announced it will introduce over 100 fuel-cell buses ahead of the Tokyo 2020 Olympic and Paralympic Games in Tokyo, and a hydrogen-fuel-cellbattery hybrid bus project is progressing in Foshan, China, with the goal of manufacturing 300 buses and the installing 20 hydrogen refueling stations by 2020 [12]. The applications of hydrogen in the industry are also shown in Fig. 1 [13,14]. Though hydrogen is the most abundant element on earth, most of the hydrogen in nature is bonded with other elements, such as carbon and oxygen, and is stored in compounds such as water, fossil fuels, biomass, and hydrocarbons. Therefore, a number of routes, consisting of thermochemical, electrochemical, photobiological, and photochemical methods, have been developed for extracting hydrogen from these compounds [15]. Among these methods, the thermochemical method plays a prominent role in commercial hydrogen production when compared to the others. Currently, steam methane reforming (SMR) [16] is the most commonly adopted technique to produce hydrogen. Other thermochemical methods, including partial oxidation or catalytic partial oxidation [17], autothermal reforming [18], methane thermocatalytic decomposition [19], dry reforming [20], gasification [21], and WGSR [22] can also be employed to produce hydrogen. Within these thermochemical methods, the WGSR is likely to play an important role in the prospective hydrogen economy and CO2 reduction. This arises from the fact that hydrogen production and CO2

enrichment can be simultaneously accomplished from this chemical reaction. To provide a comprehensive overview of the WGSR, recent developments and progress in hydrogen production and CO2 reduction from the WGSR will be reviewed. The fundamental characteristics of the WGSR, especially from the thermodynamic and kinetic points of view, will also be addressed. The reaction behaviors of the WGSR in certain environments will also be illustrated. Furthermore, it is of importance that the thermodynamic limitation could be overcome by applying a membrane reactor for WGSR. The review on principle of membrane reactor and different typologies of membrane reactors would be introduced. 2. Water gas shift reaction The WGSR is a redox-type reaction and is expressed as

CO + H2 O

CO2 + H2

Ho298

K

=

41.1 kJ mol

1

(1)

The above equation indicates that the chemical reaction pertains to a reversible and moderately exothermic reaction. The WGSR will be triggered when carbon monoxide and steam co-exist in an environment and the energy barrier of the chemical reaction is overcome. Because oxygen contained in H2O is transferred to CO in the WGSR, the latter is then transformed into CO2 and water becomes the source of H2 production. The WGSR was first discovered by an Italian physicist Felice Fontana in 1780 [23]. Thereafter, the reaction was employed in industry for the production of syngas, as a part of the Haber-Bosch process of ammonia manufacture [24]. In fact, the WGSR remains a commonly encountered reaction in the industry. For example, it occurs in the reactions of steam reforming [25,26], partial oxidation [27,28], autothermal reforming [29,30], gasification [31,32], methanol and dimethyl ether (DME) syntheses [33,34], the Fischer–Tropsch process [35], the ironmaking process in a blast furnace [36,37], and so forth. The WGSR can be used to reduce the CO concentration in a gas stream. For example, the performance of proton exchange membrane fuel cells (PEMFCs) degrades when CO is present due to its poisoning effect on the electrodes of the fuel cells [38,39]. Once the fuel processing is implemented, the WGSR can be subsequently carried out to abate the CO concentration to a level of 0.1–0.3% [40]. Under such a

Fig. 2. Classification of the water gas shift reaction. 3

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situation, it is easier to remove CO using palladium or other membranes to avoid poisoning the electrodes of the fuel cells [39]. Moreover, not only is the CO concentration reduced from the WGSR, but also the efficiency of the fuel cells can be intensified due to the increase of hydrogen production. Meanwhile, as CO is shifted to CO2 through the WGSR, the absorbent can more easily capture CO2 in a stream with higher CO2 concentration, thereby achieving CO2 removal and improving the hydrogen concentration for further application.

Table 2 Equilibrium constants at various temperatures. Vielstich, Lamm [46]

3. Methodology To process the conversion of CO and H2O to CO2 and H2, respectively, depending on whether catalysts are used, the WGSR can be classified into catalytic WGSR and non-catalytic WGSR, as shown in Fig. 2. As the name implies, the catalytic WGSR means that the chemical reaction is driven in the presence of catalysts. A variety of catalysts have been developed to trigger WGSR. The non-catalytic WGSR can be elected in certain environments such as supercritical water and plasma systems. The reaction behavior of the WGSR is highly related to temperature and is an equilibrium-limited reaction. Therefore, an understanding of the thermodynamic characteristics of the chemical reaction is conducive to the design of the reactor and its operation. In practice, catalysts are essential in the WGSR for commercial applications in the industry nowadays. This implies, in turn, that the chemical kinetics is an important issue when the WGSR is concerned. Consequently, in the following discussion, the thermodynamics and kinetics of the WGSR will be illustrated. In addition, information regarding a variety of catalysts developed for driving the WGSR will be elaborated, and its reaction characteristics under certain environments will be addressed. In addition, separation technologies such as CCSU and membrane reactor for processing WGSR are also presented. By applying separation, the reaction would step toward the right side of the EGSR, stemming from Le Chatelier's principle. The membrane with high H2 permselectivity has ability to get purified hydrogen in the downstream which is conducive to further H2 application. The advanced hybrid system combining both CCSU and membrane reactor is originated to obtain ultra-high hydrogen purity as well as reaction performance. As a result, these technologies including the introduction of the developed reactors will be reviewed as well.

Twigg [45]

Temperature (°C)

Kp

Temperature (°C)

Kp

93.3 148.9 204.4 260.0 315.6 371.1 426.7 482.2 537.8 593.3 648.9 704.4

4523.0 783.6 206.8 72.75 31.44 15.89 9.03 5.61 3.749 2.653 1.966 1.512

200 250 300 350 400 450 500 550

210.83 83.956 38.833 20.303 11.723 7.3369 4.9035 3.4586

The equilibrium constant (Kp) for the gas phase reaction is defined as

KP =

PCO2 PH2 XCO2 X H2 (1 + 1 = P PCO PH2 O XCO X H2 O total

1 1)

=

XCO2 X H2 XCO X H2 O

(2)

where Pi , X, and Ptotal denote the partial pressure of species i, mole fraction, and total pressure, respectively. The equilibrium constant of the WGSR is a function of temperature, and can be obtained thermodynamically from the following formula [44],

KP = exp

S0 R

H0 (Patm ) RT

N (v'' v' ) j=1 j j

(3)

where S 0 , H 0 , R, T, Patm, N, and vj represent the change of total entropy on the standard state, the change of total enthalpy on standard state, the universal gas constant (=8.314 J K−1 mol−1), temperature (K), atmospheric pressure, number of species, and stoichiometric coefficient, respectively. The values of S 0 and H 0 for CO, H2O, H2, and CO2 are tabulated in Table 1. Table 2 lists the values of the equilibrium constant at various temperatures provided in [45,46]. As can be seen in the table, the value is on the order of 103 when the reaction temperature is less than 100 °C. Once the temperature is as high as 500 °C, the constant lowers to a level of around 5. This clearly illustrates that the equilibrium constant declines rapidly with increasing temperature and that the forward reaction in the WGSR is thermodynamically favored at low temperatures. This can be explained by the exothermicity of the WGSR, as expressed in Eq. (1). To provide a straightforward way for predicting the equilibrium constant, Moe [47] established a simple correlation of the equilibrium constant and the reaction temperature, which is expressed by

4. Thermodynamics 4.1. Equilibrium constant Thermodynamically speaking, the gas species concentrations at the equilibrium state are determined by the equilibrium constant. In some studies [41–43], the chemical kinetics of the WGSR have been established in terms of the equilibrium constant. This reveals that the equilibrium constant is an important factor when the WGSR is investigated.

KP = exp

4577.8 T

4.33

(4)

Table 1 The theoretical formation enthalpy and entropy of the WGSR components at different temperatures [143]. Temperature (K)

CO Hf*

298 400 500 600 700 800 900 1000

−110541 −110121 −110017 −110156 −110477 −110924 −111450 −112022

H2O S

**

197.543 206.125 212.719 218.204 222.953 227.162 230.957 234.421

Hf −241845 −242858 −243822 −244753 −245638 −246461 −247209 −247879

CO2 S 188.72 198.673 206.413 212.92 218.61 223.693 228.321 232.597

H2

Hf

S

−393546 −393617 −393712 −393844 −394013 −394213 −394433 −394659

* J/mol. ** J/mol/K. 4

213.685 225.225 234.814 243.199 250.663 257.408 263.559 269.215

Hf 0 0 0 0 0 0 0 0

WGSR S 130.574 139.106 145.628 150.968 155.487 159.44 162.942 166.114

H −41160 −40638 −39873 −38935 −37898 −36828 −35774 −34758

Kp S −42.004 −40.467 −38.69 −36.957 −35.413 −34.007 −32.777 −31.687

Eq. (3) 104907.9 1559.963 139.5156 28.7879 9.511159 4.249139 2.313062 1.446485

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where T is the temperature (K). Smith et al. [40] also provided a correlation, which is given by

being Fe-Cr-based catalysts and Cu-Zn-based catalysts, respectively [49]. On the one hand, the Arrhenius Law reveals that increasing the reaction temperature can facilitate the reaction rate; on the other hand, according to thermodynamics or Le Chatelier’s principle, a lower reaction temperature is conducive to superior CO conversion or hydrogen yield as a consequence of the exothermic reaction involved. In general, the HTSR is governed by chemical kinetics, whereas the LTSR is dominated by thermodynamic equilibrium [44]. For this reason, to increase CO conversion, hydrogen production and CO2 enrichment in the influent, the WGSR is frequently executed in two stages [43]. In the first stage, the HTSR is activated at higher temperatures to accelerate the chemical reaction and CO conversion in a short time. Subsequently, in the second stage, the LTSR is triggered at lower temperatures for further pushing the forward reaction of the WGSR to enrich CO2 and H2 in the effluent. Because the thermodynamic aspect of the WGSR was already discussed in the previous section, this section focuses on its kinetics. Two insightful kinetics review papers have been published by Mendes et al. [24] and Smith et al. [40] in 2010. Consequently, the kinetics part in the present study will mainly concentrate on studies after 2010 or papers that were not mentioned in the aforementioned papers.

KP = exp

49170 5693.5 + + 1.077ln(T ) + 5.44 × 10 4T T2 T

× 10 7T 2

13.148

1.125 (5)

The equilibrium constants at various temperatures according to Eq. (3) (Table 1), the data in Table 2 [45,46], and the correlations in Eqs. (4) and (5) [40,47] are plotted in Fig. 3. In general, the equilibrium constants correlate well by Eqs. (3) and (4) when the temperature is between 400 K and 900 K. Eq. (3) underestimates the constant for temperatures less than 400 K, whereas Eq. (4) overestimates it once the temperature exceeds 900 K. 4.2. Equilibrium concentration For the WGSR with 1 mol of CO and the initial steam-to-CO molar ratio (i.e., S/C ratio) of y in the reactants, when the converted fraction of CO is x, Eq. (1) can be expressed as

KP =

( )( ) = ( )( ) (1 x y+1

x y+1

1 x y+1

y x y+1

x2 x )(y

x)

5.1. High-temperature shift catalyst

(6)

Past studies have indicated that high-temperature shift catalysts include Fe-Cr-based and Ni-based catalysts. On account of the limitations of commercialized high-temperature shift catalysts, some improvements have been developed, such as replacing part of the metals with some modifier or doping some alkalis. Several alternative choices of high-temperature shift catalysts were also discovered, and are introduced in the following. Two commercial Fe-Cr catalysts (named HTC1 and HTC2) were tested by Park et al. [50], where various inlet gas mixtures were used to simulate real conditions. Because the two catalysts have a similar proportion of Fe-Cr, identical trends were found in different situations. The maximum reaction rate was attained under the condition of the highest inlet CO concentration. Thereafter, the reaction rate decreased with a diminishing CO concentration and enhancing CO2 and H2 concentrations. The rate expression (Table 3) suggested that the kinetics were greatly related to the CO concentration (reaction order ≈ 1 for both cases). HTC1 had more negative exponents of CO2 and H2, revealing that HTC2 was more appropriate for a CO2-rich environment. Alternatively, H2O showed no influence on the overall reaction rate in

Accordingly, CO conversion can be attained when the temperature and S/C ratio are given. The three-dimensional distributions of CO conversion and the reaction heat of the WGSR are shown in Fig. 4, where the reaction temperature and S/C ratio range from 200 to 1000 °C and from 1 to 8, respectively, and the pressure is 1 atm. The calculations are carried out using the commercial software HSC Chemistry 7.0. For the reaction temperature of 200 °C, the CO conversion is between 93.87% (at S/C = 1) and 99.94% (at S/C = 8), as shown in Fig. 4a. It follows that over 93% of the CO can be converted at this temperature. As described earlier, the WGSR is a reversible and moderately exothermic reaction at the standard state (i.e., 1 atm and 25 °C). In light of Le Chatelier’s principle, a low reaction temperature is conducive to the forward reaction, thereby facilitating the CO conversion [43]. In contrast, when the reaction temperature reaches 1000 °C, the CO conversion is in the range of 43.50–83.50%, depending on the S/C ratio. The reaction heat increases when the reaction temperature or the S/C ratio increases, as shown in Fig. 4b. Though a higher S/C ratio intensifies the CO conversion, especially at high temperatures, more energy is required to drive the WGSR. With a low temperature such as 200 °C, little or no energy is required to trigger the chemical reaction. In reality, however, the reaction rate at a low temperature may be slow, thereby requiring a large reactor to achieve the equilibrium state. Conversely, although a higher temperature is not optimal for the forward reaction of the WGSR, the reaction rate is high. The temperature of the produced raw syngas in an IGCC power plant may be as high as 1400 °C [48]. If the WGSR is employed at high temperatures, the thermodynamic loss of the syngas can be reduced. 5. Kinetics Conceptually, when CO and H2O co-exist in a system, the WGSR should be triggered. However, in practice, the reaction will not proceed if the reaction temperature is low due to the reaction’s energy barrier. Consequently, catalysts are required to reduce the activation energy and overcome the energy barrier of the WGSR. Based on the adopted catalysts or reaction temperatures, the WGSR is frequently categorized into two different reactions; one is the high-temperature shift reaction (HTSR) and the other is the low-temperature shift reaction (LTSR). The temperature regimes for the HTSR and LTSR are approximately 350–500 °C and 150–250 °C, with the most commonly adopted catalysts

Fig. 3. Distributions of the WGSR equilibrium constant. 5

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the HTC1 case, while it had a positive effect on the reaction rate of HTC2. Though Fe-Cr-based catalysts are popularly used for HTSR, the concern of chromium pollution in the environment has hindered its development. Other alternative metals are thus applied in HTSR. Cerium (Ce) and Cobalt (Co) have been found to be a good choice for Cr-free HTSC. The promotional effects and the optimal blending ratio were investigated by Damma et al. [51]. A Fe/Ce/Co ratio of 10:1:1.5 was experimentally verified as the best blending ratio, yielding the highest reaction rate (62.6 μmol/g/s) compared to the other blending ratios (from 30.7 to 52.2 μmol/g/s). The possible reasons included the largest surface area, lattice disorder, and reducibility of this ratio, all of which benefited what is considered the rate-determining step of the WGSR: the dissociation of water onto the catalyst. Further study regarding the effects of doping rare earth elements (La, Y, and Ce) onto CoMo/MgAl catalysts was uncovered by Mi et al. [52] and in the aforementioned study [51]; Ce was recognized as the best additive with Co, which together improved the catalyst activity by the largest magnitude. The fixed-bed reactor test showed that the WGSR rates over CoMo/MgAlCe obtained the highest reaction rate (2.52 mmol/g/s), which was superior to the other two modified catalysts (1.94 mmol/g/s for CoMo/MgAlLa and 1.74 mmol/g/s for CoMo/ MgAlY) and even higher than the unmodified CoMo/MgAl (~0.9 mmol/g/s) and commercial catalyst CoMo/MgAl2O4 (~0.55 mmol/g/s). The characteristics of Ni-based catalysts with high CO conversion have attracted much attention. Nonetheless, the unwanted side reaction of methane formation reduces the hydrogen yield, thereby restricting the applications of the Ni-based catalysts [53]. Saw et al. [54] unveiled the effect of bimetallic Ni-Cu catalysts on HTSR. With the least methane formation and a high WGSR reaction rate, the Ni-Cu ratio of 1 was found to be the optimal proportion. In addition, the Ni-Cu alloy was found to increase the CO adsorption strength at high temperatures, while preventing CO dissociation and hydrogenation into methane during HTSR. Their kinetics study revealed that the reaction rate was dominated by the CO concentration (reaction order = 1.0) rather than other chemicals (reaction order = −0.3 to 0.2). Potassium (K) doping of various amounts onto ceria-supported

(a) CO conversion

100 90

CO conversion (%)

80

99 98 95 90 85 80 75 70 65 60 55 50 45

70 60 50 40 30 20 10

98

9 8 99 95

90

80

80

75 Ste 6 7070 am 4 /CO 0 5 r at i 2 1000 o

80 75 55 800

T

99 95 90 200 400

600

0

re ( er at u em p

C)

(b)

∆H

350

300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 -20

300

∆ H (kJ/ mol CO)

250 200 150 100 50 0 -50 -100 -150 1000

200 180 120 80 140160 140 160 10080 604040 40 40 20 0 6 800 0 0 600 4 400 Temp 2 atio r 200 eratu O /C 0 120

8

m Stea

re ( C )

Fig. 4. Three-dimensional distributions of (a) CO conversion and (b) heat reaction of the WGSR. Table 3 The empirical power-law rate expression of the HTSR for different catalysts. Catalyst

Temperature (°C)

2 Fe-Cr commercial catalyst

450

Rate expression

Ref

HTC1*:

r = 102.845 ± 0.03exp HTC2:

(

r = 10 0.659 ± 0.0125exp (PH2)

5%Ni/5%Cu/CeO2

375

Ni/5% K/CeO2

400

La0.7Ce0.2FeO3

550–600

(

=

88 ± 2.18 RT

0.36 ± 0.043 (P

) (P

0.31 ± 0.056 (P 0.156 ± 0.078 CO2)

CO)

0.9 ± 0.041 (P

H2 O )

H2)

0.09 ± 0.0007 (1

0.95 (P

( ) (P

0.19 (P 0.58 (P 0.15 (P ) 0.51 (1 H2 O ) CO2) H2

CO)

H2 O )

0.16 (P 0.34 (P ) 0.19 (1 CO2) H2

)

Ea = 41.3 kJ/mole with product gas and 37.4 kJ/mole without product gas

r = Aexp 550 °C:

Ea RT

600 °C:

CO)

85.6 RT

) (P

0.81 (P 0.22 (P ) 0.04 (1 CO2) H2

(

85.6 RT

) (P

0.86 (P 0.18 (P ) 0.047 (1 CO2) H2

CO)

CO)

( ) (P ) r = Aexp ( ) (P ) (P r = 100.866exp 54 RT

85.6 RT

CO

)

[54] [55] [57]

(

550–600 °C:

)

)

( ) (P

r = 4.10exp

450

)

[50]

(PCO)1.0 ± 0.031 (PCO2)

) where Kp is in equation (2).

Ea RT

r = 5.77exp

10% Cu/Ce (30% La)Ox

(

0.05 ± 0.0006 (1

[CO2 ][H2 ] [CO][H2 O] Kp

r = Aexp

111 ± 2.63 RT

CO

0.8

0.73 (P 0.23 (P ) 0.046 (1 CO2) H2

0.2 H2 O ) (PCO2)

Ea did not change after 20 h usage

* High temperature catalyst. 6

0.3 (P ) 0.3 (1 H2

)

) )

)

[59]

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Table 4 The kinetic rate expression of the LTSR for different catalysts. Catalyst

Temperature (°C)

Rate expression

ref

CuO/ZnO/Al2O3

180–300

180–300 °C:

[61]

r= 4.785 ± 2.187exp

(

34.983 ± 0.014 RT

0.573 ± 0.0035 (1

(PH2) 180–200 °C:

r = Aexp

(

67.4 RT

) (P

0.854 ± 0.005 (P 1.990 ± 0.058 (P 1.926 ± 0.005 CO ) H2 O ) CO2)

)

) (P

0.76 (P 1.7 1.9 (P ) 0.54 (1 CO ) H2 O ) (PCO2) H2

Cu-Fe-Cr Cu-Fe-Mn CuO/ZnO/Al2O3

200–300

Cu0.3Fe0.7Ox Cu0.3Fe0.7Al0.1Ox

250

Pt, Pd, Rh/CeO2 with EF

150, 200, 250

Pt-Na-MWNTs 1.5 wt% Pt/CeO2

200–280 275

Pt/FeOx

250

rco =

120,180

Where Xco = CO conversion (%) Fco = molar flow rate of CO (mol/h) mPt = mass of Pt (g) Pt in single atom (loading = 0.05%): Apparent activation energy: 33 kJ/mole Reaction rate: 16.36 mol CO/gcatal/h Pt in single atom (loading = 1.15%): Apparent activation energy: 53 kJ/mole Reaction rate: 6.93 mol CO/gcatal/h Pt in nanoparticle (loading = 1.31%): Apparent activation energy: 61 kJ/mole Reaction rate: 0.69 mol CO/gcatal/h r= [Xc (d) R c + Xp (d) Rp + Xs (d) Rs ]

Au/TiO2 Au/Al2O3

)

Cu-Fe-Cr: Apparent activation energy: 198.5 ± 3 kJ/mole Pre-exponential Frequency Factor: 6.5 ± 5.5× 106 Reaction rate: ~118 mmol/g/hr Cu-Fe-Mn: Apparent activation energy: 132.8 ± 1.22 kJ/mole Pre-exponential Frequency Factor: 5.56 ± 0.08× 108 Reaction rate: ~125 mmol/g/hr CuO/ZnO/Al2O3: Apparent activation energy: 110.1 ± 4 kJ/mole Pre-exponential Frequency Factor: 4.0 ± 0.25× 108 Reaction rate: ~119 mmol/g/hr The apparent activation energy and pre-exponential frequency factor were obtained from reduction kinetic data by using TPR analysis. The reaction rate concerned CO conversion conducted in a fixed-bed reactor at the temperature of 300 °C.

rco =

Xco F where W

Xco = CO conversion (%)

F = whole flow rate of steam (mol/s) W = mass of catalyst (g) Cu0.3Fe0.7Ox: Apparent activation energy: 47.0 kJ/mole Reaction rate (initial): 10 μmol/g/s Reaction rate (25 h operation): ~5 μmol/g/s Cu0.3Fe0.7Al0.1Ox: Apparent activation energy: 44.9 kJ/mole Reaction rate (initial): 15 μmol/g/s Reaction rate (25 h operation): ~14 μmol/g/s Pd/CeO2: Apparent activation energy (kJ/mole): 26.4 (without EF), 8.3 (with EF) Pre-exponential Frequency Factor: 13.4 (without EF), 11.5 (with EF) Pt/CeO2: Apparent activation energy (kJ/mole): 53.6 (without EF), 3.8 (with EF) Pre-exponential Frequency Factor: 19.9 (without EF), 10.6 (with EF) Rh/CeO2: Apparent activation energy (kJ/mole): 43.0 (without EF), 6.2 (with EF) Pre-exponential Frequency Factor: 16.9 (without EF), 11.3 (with EF) Apparent activation energy = 65 ± 5 kJ/mol (Product gas free), 105 ± 10 kJ/mol (Product gas) Nano-rods support structure: Apparent activation energy: 47.9 kJ/mole Pre-exponential Frequency Factor: 1.1× 108 Reaction rate: 189.8 mol CO/kgcatal/h Nano-cubes support structure: Apparent activation energy: 46.1 kJ/mole Pre-exponential Frequency Factor: 1.8× 107 Reaction rate: 43.1 mol CO/kgcatal/h Nano-particles support structure: Apparent activation energy: 49.4 kJ/mole Pre-exponential Frequency Factor: 1.7× 107 Reaction rate: 49.9 mol CO/kgcatal/h X co fco mPt

xc(d), xp(d), xs(d) = the fractions of corner, perimeter, and surface sites, respectively. Rc, Rp, Rs = turnover rates

7

[62]

[63]

[64]

[65] [67]

[68]

[69]

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nickel catalysts (Ni/xK/CeO2) was investigated by Ang et al. [55]. They discovered that the presence of K strengthened the prevention of methanation due to the interaction between Ni and K, as well as the CO adsorption on Ni through the formation of bridging carbonyls. Potassium had the potential to enhance water dissociation, leading to the formation of OH groups. The OH groups can further dissociate into adsorbed oxygen, which could react with adsorbed CO on Ni. The optimum loading of K was found to be 5 wt%; excessive K could block the active sites, thereby decreasing the performance. Their conducted kinetics, with coefficient determination (R2 = 0.91), is shown the Table 3. From the reaction rate expression, the rate orders of CO, H2O, CO2, and H2 were 0.19, 0.58, −0.15, and −0.51, respectively, revealing the dominant impact of the H-species over the overall reaction rate. In addition to Ni, Popa et al. [56] surveyed the feasibility of replacing Cr with Al. However, the Fe-Al-Cu catalyst had a poor CO conversion rate in all tested conditions when compared to the Fe-Cr-Cu reference catalyst. They also performed tests combining several elements to form Fe-Al-Cr-Cu catalysts. The results suggested that efficiency was greatly improved in terms of reaction rate and thermal stability. The optimal proportion was Fe 73 wt%, Al 15 wt%, Cr 8 wt%, and Cu 4 wt%, which gave the efficiency increment of approximately 73.8%. The reaction rate remained the highest under different temperatures and retention times over the optimized catalyst. Another problem for commercial Fe-Cr catalysts was its maximum operating temperature of around 470 °C, which restricts their applications in some high-temperature environments (e.g., coal gasification systems). Sun et al. [57] investigated the La-Ce-FeO3 perovskite-like catalyst, which has the potential to be operated in the temperature window of 550–600 °C. Their kinetics study indicated that the performance of the catalyst was comparable to the commercial Fe-Cr catalysts used at temperatures of 300–500 °C. Varying the temperature yielded a similar reaction order, which implied that the temperature difference was less important for kinetics than this catalyst. As shown in Table 3, the CO concentration had a significant impact on the overall reaction rate, as a consequence of higher reaction order, while the influence caused by water was negligible. The presence of CO2 would pull the reaction rate back, and the effect of hydrogen on the overall reaction rate was slight due to its low reaction order. It has been reported [58] that iron oxide-based catalysts might become poisoned by CO2, whereas Cu-Ce catalysts were very stable in the same environment. Qi et al. [59] tested 10 atom% Cu-Ce (30 atom% La) Ox catalysts for use in the HTSR. The activation energy of the catalyst used in this study was about 70 kJ/mol at 450 °C and did not change after 20 h of usage. According to the rate expression in this study (Table 3), it could be concluded that the reaction rate strongly depended on the CO content (reaction order = 0.8), while other gaseous species had only slight effects on the WGSR (reaction orders = 0.2 and −0.3). Another study doped Ti onto a Cu-Ce catalyst [60]. With Ti assistance, the performance was enhanced a lot because it improved the interaction between metal and support. The combination of Cu, Ce, and Ti was an order higher than the Cu-Ce catalyst regarding the reaction kinetics at the operating temperature of 450 °C. Moreover, the durability advantage of Cu-Ce catalyst didn’t change after adding Ti as CO conversion didn’t change for over 12 h.

Mendes et al. [61] conducted five kinetic rate expressions, including 3 microkinetic models (2 associative mechanisms + 1 redox mechanism) and 2 empirical models (Moe equation + power-law) to simulate the WGSR with a commercial CuO/ZnO/Al2O3 catalyst in a packed-bed reactor in the temperature range of 180–300 °C. They found that the power-law had the best fitness between simulation and experimental results. From Table 4, the reaction order of CO, H2O, CO2, and H2 was 0.85, 2.0, −1.9, and −0.57, respectively. It clearly showed that the concentrations of water and carbon dioxide dominated the reaction rate, in which the reaction orders were around 2. However, a lower reaction order for all species was found for a narrow temperature range (180–200 °C) while the apparent activation energy increased from 35 to 67.4 kJ/mol. Hossain et al. [62] evaluated the reduction kinetics of Cu-Fe-Mn, Cu-Fe-Cr, and commercial CuO-ZnO/Al2O3 catalysts in a fixed-bed reactor using temperature programmed reduction (TPR) to perform their reducibility. The reducibility presented a significant relationship with the activity of the metal oxide catalysts. They reported that the additive Mn significantly enhanced the active Cu species. Among several kinetic models, the second-order power law gave good fitness to the TPR data. The Cu-Fe-Mn catalyst displayed lower apparent activation energy when compared to the Cu-Fe-Cr catalyst; meanwhile, the highest preexponential factor was obtained by the Cu-Fe-Mn catalyst (Table 4). Similarly, the highest CO conversion rate in the reactor at 300 °C was found with the Cu-Fe-Mn case (~125 mmol/g/hr) while the Cu-Fe-Cr catalyst showed a lower reaction rate (~118 mmol/g/hr). Similar Cu-Fe catalysts were also investigated by Yan et al. [63]. The optimal blending ratio was found to be Cu0.3Fe0.7Ox, which had the highest Cu surface area and CO conversion. For the kinetic aspects, the apparent reaction orders of H2O and CO were 0.23 and 0.99, respectively, for the Cu0.3Fe0.7Ox catalyst. This implied that increasing the CO concentration in the inlet for this catalyst was more efficient for enhancing the WGSR kinetics compared to increasing the H2O. However, the authors discovered that the durability did not perform very well for this catalyst, and so an improver-Al component was thus doped into the catalyst. Although the Cu0.3Fe0.7Ox decreased the reaction rate from 10 to 5 μmol/g/s, the reaction rate remained high for Cu0.3Fe0.7Al0.1Ox after 25 h of operation (from 15 to 14 μmol/g/s). By virtue of the kinetic limitation at low temperatures, Sekine et al. [64] studied several catalytic hydrogen productions with electric field assistance with Pt, Pd, and Rh/CeO2 catalysts. The reactant conversion at 423 K could increase from 0.8–2.1% to 48.6–60.8% after applying the electric field, and the hydrogen formation rate increased from 0.6–7 to 274.7–308.4 μmol/min, thereby constituting a considerable improvement for the kinetics aspect. In addition, the apparent activation energy of the WGSR declined one order after the application of the electric field. The results showed that the presence of an electric field encouraged the catalytic activity and aided the entire reaction in both the conversion and kinetic phases. The improving effect of adding Na to Pt catalysts with supported multiwalled carbon nanotubes (MWNTs) on LTSR activity was uncovered by Zugic et al. [65]. Their results demonstrated that crude Pt/ MWNTs showed ~0% CO conversion under all the experimental conditions. On the other hand, Pt/MWNTs with Na promotion was comparable with supported highly-active Pt/metal oxides. With the Na addition, a stable Pt-OHx complex would form and change Pt into the active oxidation state. The WGSR rate varied with the changes of PtOHx concentration. A strong retardation effect by hydrogen in the inlet gas on the reaction was also found, which increases the apparent activation energy from 70 ± 5 kJ/mol to 105 ± 7 kJ/mol. From the mechanism point of view, unlike most papers that report two main mechanisms (redox and associate) for the WGSR, a new carboxyl mechanism was proposed by Chen et al. [66]. The mechanism showed that the WGSR catalyzed by Au/CeO2 would undergo the processes of (1) water dissociation at the interface between the metal and support, (2) OH combined with CO adsorbed on Au to form COOH,

5.2. Low-temperature shift catalyst In addition to commercialized low-temperature shift catalysts (LTSC), such as the Cu-Zn based catalyst, several candidates for being LTSC, including Au and Pt, have been surveyed in previous studies. The effects of support, structure, and size of catalyst on the performance of the catalysts have been studied as well. Details of the investigated results are described in the following. Moreover, some improvement methods, such as adding alkali metals and applying an electric field, are discussed and addressed as follows. 8

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Table 5 The rate expression of the WGSR for noble metal catalysts. Catalyst

Temperature (°C)

Rate expression

Rh/La2O3-SiO2

325–450

r=

280

Langmuir-Hinshelwood mechanism → rate limiting step = the surface reaction between adsorbed CO and water k = surface reaction rate constant Ki = adsorption equilibrium constant of species i Pi = partial pressure of species i Pd/Al2O3-0Zn*:

Pd/Al2O3-xZn x = different fraction

r = Aexp

82 ± 2 RT

) (P

0.38 ± 0.04 (P 0.44 ± 0.05 (P 0.06 ± 0.01 (P ) 0.36 ± 0.04 (1 H2 O ) CO2) H2

)

(

68 ± 3 RT

) (P

0.38 ± 0.04 (P 0.49 ± 0.05 (P 0.19 ± 0.01 (P ) 0.34 ± 0.04 (1 H2 O ) CO2) H2

)

)

Pd/Al2O3-19Zn:

r = Aexp

) (P

0.41 ± 0.04 (P 0.53 ± 0.05 (P 0.31 ± 0.01 (P ) 0.31 ± 0.04 (1 H2 O ) CO2) H2

(

65 ± 2 RT

) (P

0.1 (P

(

78 ± 1 RT

) (P

0.1 (P 1.0 0.2 (P ) 0.3 (1 H2 O ) (PCO2) H2

)

) (P

0.1 (P 1.0 0.2 (P ) 0.2 (1 H2 O ) (PCO2) H2

)

Pt/Al2O3-50Na:

r = Aexp

(

86 ± 2 RT

Pt/Al2O3-12Li:

r = Aexp

(

70 ± 2 RT

Pt/Al2O3-50Li:

r = Aexp

(

89 ± 1 RT

Pt/Al2O3-12 K:

r = Aexp

(

73 ± 1 RT

Pt/Al2O3-50 K:

r = Aexp 2.3% Au/TiO2 & 2.3%Au-(1Br:25Au)/TiO2

120

CO)

[73]

CO)

CO)

CO)

) (P

CO)

75 ± 3 RT

) (P

(

60 ± 3 RT

)

(

57 ± 3 RT

) (P

0.7 (P 0.2 0.3 (1 CO2 ) (PH2)

1.1 0.2 (P ) 0.2 (1 H2 O ) (PCO2) H2

CO)

)

)

0.1 (P 0.7 0.1 (P ) 0.3 (1 H2 O ) (PCO2) H2

CO)

) (P

H2 O )

0.2 (P 0.7 0.1 (P ) 0.3 (1 H2 O ) (PCO2) H2

) (P

(

2.3% Au/TiO2*:

r = Aexp

CO)

75 ± 2 RT

Pt/Al2O3-12Na:

r = Aexp

CO)

(

Pt/Al2O3-0Na*:

r = Aexp

[72]

(

Pd/Al2O3-10Zn:

250

[71]

kKCO K H2 O PCO P H2 O

( 1 + KCO PCO + K H2 O P H2 O )2

r = Aexp

Pt/Al2O3-xNa Pt/Al2O3-xLi Pt/Al2O3-xK x = different fraction

Ref.

0.1 (P

)

)

1.0 0.2 (P ) 0.3 (1 H2 O ) (PCO2) H2

) [75]

(PCO)0.75 ± 0.05 (PH2O)

0.3 ± 0.05 (P 0.1 ± 0.05 (P ) 0.2 ± 0.05 (1 CO2) H2

)

2.3% Au-(1Br:25Au)/TiO2:

r = Aexp Au/TiO2 Au/TiO2−x

200–240

Pt/TiO2

210–270

Pt/Mo2C Pt/CeO2 Pt/TiO2

240

CO)

0.85 ± 0.05 (P

H2 O)

0.35 ± 0.05 (P 0.05 ± 0.05 (P ) 0.15 ± 0.05 (1 CO2) H2

)

Au/TiO2, 200 °C: Apparent activation energy:51 kJ/mole TOF:0.74 1/s Au/TiO2−x, 200 °C: Apparent activation energy:45 kJ/mole TOF:1.43 1/s Rate:14.4 μmol/g/s Au/TiO2−x, 240 °C: Apparent activation energy:45 kJ/mole TOF:3.49 1/s Rate:41.1 μmol/g/s

r = 0.31exp

(

10.8 RT

) (P

r = Nsites × TOF ×

CO)

(

0.5 (P

MWPt NA × W

1.0 0.7 H2 O ) (PH2)

)

r = molCO/molPt/s Nsites = the active site density (sites/gcat) TOF = turnover frequency (1/s) MWPt = Pt atomic mass (78) NA = Avogadro’s number W = the Pt loading (gPt/gcat) Pt/Mo2C = 1.423 molCO/molPt/s Pt/CeO2 = 0.346 molCO/molPt/s Pt/TiO2 = 0.236 molCO/molPt/s

* The authors did not provide Power-law expressions, those displayed here were derived from the reaction orders and Ea provided in the paper.

9

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after which COOH donated its H to the new coming OH on the support to form CO2, and (3) hydrogen would form by two H atoms on Au. The results from this mechanism were very similar to the experimental values operated at the temperature of 450 K. They also found that the new mechanism was much quicker than those of the associate and redox mechanisms. Several researchers have focused on the structures or sizes of applied catalysts. Torrente-Murciano et al. [67] studied the effects of various Pt/CeO2 nanostructured supports on the WGSR catalytic reactivity. The results demonstrated that the nano-rods’ support provided the highest reactivity with the lowest methane selectivity compared to the nano-particles and nano-cubes. The WGSR started to proceed at 180 °C with the nano-rods, while the reaction was triggered at temperatures higher than 225 °C for the other two structures. The activation energy of all materials was located in the same region (46.1–49.4 kJ/mol); however, the pre-exponential factor of the nanorods (=1.1 × 108) showed an order higher than the other two (=1.7–1.8 × 107). This led to the highest reaction rate achieved by the rod structure (=189.8 mol CO/kgcatal/h) compared to the other two structures (=43.1–49.9 mol CO/kgcatal/h) at the temperature of 275 °C. A later study concerning the effect of Pt size on the WGSR was conducted by Chen et al. [68]. Two different size scales of Pt (nanoparticle and single atoms) were prepared with different doping amounts for the LTSR tests. The results illustrated that they underwent two different mechanisms: the catalyst using single-atom Pt facilitated the WGSR by the redox mechanism, while the associate mechanism occurred with the case of the catalyst using Pt nanoparticles. The activation energy of the WGSR was only 33 kJ/mol for the single-atom Pt catalyst but was double with the Pt nanoparticle catalyst. This might be the reason why the WGSR specific rate with the single-atom Pt catalyst was 16.36 mol/g/h, which was the highest among all other Pt-related catalysts with the lowest metal loading (0.05%). Shekhar et al. [69] conducted an experiment about the effects of size and support of Au catalysts on the reaction rate of the WGSR. From their data, the Au/TiO2 catalyst presented a reaction rate of about 20 times higher than that of Au/Al2O3. The reaction rate decreased with an increasing number average Au particle size. The correlations between

the particle size and the reaction rate for the Au/TiO2 and Au/Al2O3 catalysts were r = 0.22d−2.7 ± 0.1 and r = 0.007d−2.2 ± 0.2, respectively. The reaction orders for CO, H2, and CO2 were almost the same and independent of the supports. However, the reaction order of water changed from positive (alumina) to negative (titanium), which implied that the support had some influence on the water molecules' activation. 5.3. Noble metal catalyst Noble metal catalysts, which can be used to trigger both the HTSR and LTSR, have received much attention recently due to their superior kinetic performance [70]. Moreover, noble catalysts do not degrade easily under an ambient air environment, leading to convenient storage. The following research is introduced to illustrate several WGSRs energized by noble metal catalysts, including Rh, Pt, Pd, and Au. Investigations about doping certain additives and the supports will also be presented. Cornaglia et al. [71] prepared and tested two catalysts for the WGSR: Rh/La2O3 and Rh/La2O3-SiO2. Stability tests showed that Rh/ La2O3 deactivated over time, while the other presented very stable performance. Accordingly, Rh/La2O3-SiO2 was chosen to conduct the kinetics of the WGSR. The apparent activation energy test showed that the catalyst was even more active than a commercial high-temperature catalyst. The Langmuir-Hinshelwood mechanism, which states that the rate-determining step is the surface reaction of adsorbed CO and water, was used in their study to simulate the kinetics. Results indicated that the fitness between the model and experimental results was good. The noble metal catalyst Pd/Al2O3, with Zn addition at different fractions, was studied by Bollman et al. [72]. Adding Zn had the potential to bind with Pd to form a Pd-Zn alloy, enhancing the CO adsorption from 80% to 90%. The WGSR rate per mole of Pd increased after doping Zn because it hindered a few sites for the methanation reaction. In addition, Zn would react with the alumina support and reduce the interaction between Pd and alumina, which might deteriorate the WGSR. When it comes to reaction orders (Table 5), CO (0.31–0.41), water (0.44–0.62), and H2 (−0.26 to 0.36) showed no obvious trend with Zn addition, while CO2 displayed more negative

Fig. 5. Summary of the WGSR in various environments. 10

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values (−0.06 to 0.31). When more Zn was doped onto the Pd/Al2O3 catalyst, more carbonates appeared on the support and acted as spectators. In addition to Zn, adding alkali has the potential to accelerate the WGSR. Pazmino et al. [73] discovered that under the LTSR (200–250 °C), Na-promoted catalysts had a turnover frequency 107-fold higher than that of virgin Pt/Al2O3 and 4-fold higher than virgin Pt/ TiO2. The promotion rate order was Na > Li ~ K and the optimal proportion was Pt/Al2O3-30Na. After doping Na onto Pt/Al2O3, the reaction orders for water and H2 increased from 0.7 and −0.4 to 1 and −0.2, respectively, while the reaction orders of CO2 and CO decreased from 0 and 0.1 to −0.3 and −0.1, respectively. Similar trends were also discovered while adding Na to Pt/TiO2, reflecting that Na could create similar active sites on the support of TiO2. In contrast to adding helpful additives, doping halide to poison the catalyst was used to study the active sites for the CO oxidation and adsorption of Au [74]. Shekhar et al. [75] added bromine (Br) onto Au/ TiO2 to investigate the active sites and kinetics. Their results indicated that the WGSR reaction rate per total mole of Au was decreased by about 6 times based on plug-flow reactor experiments after adding 4% Br. Nevertheless, the apparent activation energy and reaction orders did not decrease markedly. It was noted that the reaction order of water showed a negative value, the possible reason for which was the competition between water and CO for the Au sites. Differing from the improvement by doping additives, Li et al., [76] modified the catalyst support of Au/TiO2. A hydrogen-etching technology was applied to change the traditional white TiO2 into black

TiO2−x. The plethora of stable surface oxygen vacancies, higher microstrain, and a large number of metallic Au0 species endowed the black TiO2−x with promising WGSR activity. For the kinetic aspects, under the same operating temperature of 200 °C, the turnover frequency (TOF) of Au/TiO2−x (1.43 1/s) was around twice that of Au/TiO2 (0.74 1/s). The activation energy diminished from an average of 51.1 (kJ/mol) to 45.4 (kJ/mol). Some articles have discussed the LTSR with the aid of Pt catalysts with different types of supports. Kalamaras et al. [77] discussed the kinetics of the LTSR (210–270 °C) catalyzed by Pt/TiO2 (0.5 wt% Pt). When the CO or H2O concentration increased in the feed stream, the overall reaction rate increased obviously. The opposite trend occurred when hydrogen was enhanced. Varying the inlet CO2 concentration had no impact on the reaction rate. The apparent activation energy in the power-law expression was 10.8 kcal/mol (~45.36 kJ/mol), where the reaction orders of CO, water, CO2, and H2 were 0.5, 1.0, 0, and −0.7, respectively. Another highlight of this study was that the WGSR was dominated by a redox mechanism rather than an associate mechanism, as evidenced by transient experiments; that is, adsorbed CO on Pt reacted with unstable oxygen to form CO2. The WGSR reaction mainly occurred around the interface between the metal and support, where the unstable oxygen was located. Schweitzer et al. [78] presented the WGSR over Pt with three different oxide supports, namely, Mo2C, CeO2, and TiO2. Among the catalysts, the Pt/Mo2C catalyst showed the highest WGSR rate (=1.423 mol CO/mol Pt/s at 240 °C) compared to the other candidates (0.346 mol CO/mol Pt/s for Pt/CeO2 and 0.236 mol CO/mol Pt/s for Pt/TiO2 at 260 °C). The rate expression

Fig. 6. Experimental apparatus for the water gas shift reaction in supercritical water [82]. Reprinted with permission from the reference. Copyright (2004) Elsevier. 11

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regarding the active site is described in Table 5. Rather than assuming an active site sat on the particle surface, the experimental data were fitted by the results, which suggested that active sites of the catalysts sat on the perimeter of the Pt particle. In other words, the rate-determining reaction occurred at the active sites around the Pt particle.

illustrated below. 6.1. Non-catalytic WGSR 6.1.1. Supercritical water Supercritical fluids can be used as a solvent extraction media and also as an environment for chemical reactions due to the unique solvent characteristics of these fluids. Supercritical conditions exist for water at temperatures above 374 °C (705 °F) and pressures above 22.1 MPa (3200 psia) [79]. Supercritical water properties are substantially different from those of room-temperature water and correlate strongly with the density of the water. The dielectric constant of water drops dramatically in the supercritical region, which makes water a non-polar solvent [6]. Under supercritical conditions, water behaves like a dense gas with high solubility of organics: complete miscibility in all proportions with high diffusivity, low viscosity, and low solubility and

6. WGSR in special environments As described earlier, the WGSR can be partitioned into non-catalytic and catalytic reactions (Fig. 5). The non-catalytic WGSR can be triggered in supercritical water and a steam plasma torch system. Seeing that no catalyst is used in the reaction, the non-catalytic WGSR can be thought of as the nonconventional WGSR. Alternatively, the catalytic or conventional WGSR can be excited by microwave-assisted heating, high-gravity, or a heat recirculation environment. Details of these phenomena of the WGSR in these special environments or reactors are

Fig. 7. Plasma torch system for conducting WGSR [89]. Reprinted with permission from the reference. Copyright (2014) American Chemical Society. 12

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dissociation of inorganics, particularly for ionic salts [79]. The solvation properties make supercritical water an excellent medium for the oxidation of organics inasmuch as organics and oxygen can be intimately mixed in a single and homogeneous phase, while inorganics can be readily removed from solution by precipitation. In the absence of catalysts, the WGSR reaction can be triggered when CO is immersed in supercritical water [79–82]. Water in the WGSR acts as a solvent and a reactant in the supercritical condition. For example, in a kinetics study by Araki et al. [81], it was found that the reaction proceeded according to the stoichiometric reaction as CO + H2O ⇌ CO2 + H2, the rate of which was first order on the concentration of CO. The rate-determining step of the WGSR was the formation process of formic acid, which subsequently dissociated to CO2 + H2 at a much higher reaction rate. An experimental study by Sato et al. [82] focused on the kinetics of the WGSR at 653–713 K, 10–30 MPa, and the CO/H2O ratio of 0.03 without a catalyst in supercritical water. Fig. 6 illustrated the experimental apparatus used in this study. The CO was sent into the reactor with a molten salt bath, while steam was produced by distilled water. After passing a gas–liquid separator, CO and CO2 were measured by the GC-TCD with helium carrier gas, while H2 was detected by another GC-TCD with argon carrier. Their results revealed that the first order rate constant for CO conversion at higher temperatures in supercritical water could be welldescribed by an Arrhenius equation, almost regardless of pressure. Demirel and Azcan [83] developed a thermodynamic model to predict the equilibrium composition of the WGSR in supercritical water. They found that the Gibbs free energy of reaction enhanced when the temperature increased, which meant that the WGSR favored lower temperatures. Due to the same reason, the equilibrium constant at 600 K was about 60 times that at 1400 K. Positive Gibbs free energy appeared at around 1100 K, which implied that the reverse WGSR would occur at this temperature in supercritical water. These results are consistent with the experimental data of Ge et al. [84]. The WGSR also commonly occurs in fuel reforming, such as partial oxidation and gasification in supercritical water. Ge et al. [85] established a kinetic model to describe the formation and consumption of gas components concerning the non-catalytic partial oxidation of coal in

supercritical water. In the initial stage (less than 0.9 min), most of the hydrogen came from fixed-carbon steam reforming, and the rate reduced along with the decreasing fixed carbon. Because CO was also the main product from the steam reforming of fixed carbon, based on Le Chatelier’s principle, the WGSR rate increased followed by increasing CO. As a result, the WGSR was the main hydrogen source at the late stage (longer than 0.9 min). However, the WGSR rate decreased when the CO concentration declined. The effect of the reverse WGSR on hydrogen consumption was negligible. Safari et al. [86] experimentally tested the gasification of biomasses in supercritical water media. They reported that wheat straw had the highest hydrogen yield and gasification efficiency due to its higher cellulose content and lower lignin content. The hydrogen yield increased over time with the maximum value occurring at 10 min (7.25 mmol hydrogen per one gram of wheat straw), after which it started to decrease. This reaction was caused by the methanation reaction. Although the WGSR happened simultaneously, methanation consumed 3 mol of hydrogen while the WGSR only produced 1 mol of hydrogen. Byrd et al. [87] investigated the behavior of hydrogen production using the supercritical water method combined with a Ru/Al2O3 catalyst. Their results indicated that the hydrogen yield from glycerol reforming decreased along with increasing residence time. Although the conversion of glycerol remained high (> 99%), methane formation by consuming hydrogen became dominant after an extended residence time. Otherwise, for the higher glycerol inlet concentration, the experimental CO yields were smaller than the predicted ones, while the H2 and CO2 yields showed higher values than the predicted ones. This implied that the WGSR reacted to near completion. Later research discovered that an acceleration of the WGSR could be attained by OH− assistance [88]. Based on this concept, most WGSR applications used an alkaline catalyst, which had the ability to generate more OH− during the WGSR. A theoretical pathway was also proposed, and the rate-determining step was posited to be the hydrogen generation step. However, the WGSR in a supercritical water circumstance possessed a much higher reaction rate than the traditional alkali-assisted WGSR. The solubility and hydrolysis limitations as well as the Fig. 8. The microwave-assisted heating system (A: CO; B: N2; C: mass flow controller; D: controller readout; E: gas mixer; F: water; G: pump; H: microwave reactor; I: reaction tube; J: thermocouple; K: mixing layer; L: catalyst layer; M: power controller; N: condenser; O: drier; P: gas chromatography; Q: gas analyzer; R: recorder) [95]. Reprinted with permission from the reference. Copyright (2012) Elsevier.

13

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OH− dissociation problem led to the inferior performance of alkaliassisted catalysts compared to a supercritical water environment.

reactor was over 6000 K, which meant that the chemical species were highly active and the WGSR could be activated. However, the straight flow of CO gas might influence the stability of the steam plasma. In response, the reverse vortex reactor was designed to improve the plasma stability and gas residence time. The H2 concentration in the product gas was at an average of 36 vol%, but the CO concentration was also high. Overall, this study indicated that steam plasma was an effective method for conducting the WGSR. Zhang et al. [90] investigated the glycerol decomposition and syngas production by DC arc plasma. The carbon conversion could

6.1.2. Steam plasma torch system In addition to supercritical water, the plasma system is another possible method to trigger WGSR. Ma et al. [89] employed a plasma torch system to carry out the reaction of steam and CO for hydrogen production. In this system, a cylindrical quartz reactor and a reverse vortex reactor were individually installed in the system to practice the WGSR (Fig. 7). The high-temperature region in the cylindrical quartz

(a) Outer ring Chamber radius (10 cm) Inner ring Inner radius (5.6 cm) θ (27 )

Catalyst bed

Outer radius (7.3 cm) Flow direction Seal Gas outlet

80

(b)

CO conversion (%)

75

70

65

60

Steam/CO=2 Steam/CO=4 Steam/CO=8

55

50

0

30

60

90

120

150

180

210

240

G number Fig. 9. (a) Top view of the designed rotating packed bed reactor and (b) CO conversion with respect to G number at three different steam/CO ratios [97]. Reprinted with permission from the reference. Copyright (2010) Elsevier. 14

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achieve 100% by providing sufficient input power (> 17 kW) with an energy conversion of 66%. The results indicated that the product gas stream consisted of approximately 38% CO and 56% H2 on an argonfree basis. Higher water content in the feed gas resulted in a larger amount of CO2 and H2, and decreasing CO concentration. This phenomenon implied that the WGSR happened with higher water content in the feedstock. Yoon et al. [91] also tested the gasification of glycerol with microwave plasma. The composition of the product gas was 57% H2 and 35% CO without any oxygen supply, which was very close to the prior results. The WGSR initially became severe, followed by an enhanced steam/fuel ratio.

environment in the reactor and is thus spoken of as “Higee” [96], which is an acronym for high gravity (G). RPBs are mainly utilized for promoting mass transfer processes between the gas phase and the liquid phase. Chen et al. [97] designed an elevated-temperature RPB to intensify the gas-phase WGSR, where a Cu-Zn catalyst bed was embedded in the reactor, and the rotor speed, reaction temperature, and S/C ratio were in the ranges of 0–1800 rpm, 250–350 °C, and 2–8, respectively. The configuration of the RPB is shown in Fig. 9a. A dimensionless parameter, the G number, was employed to describe the influence of the rotating speed of the catalyst bed on the performance of the WGSR. The G number was expressed as

6.2. Catalytic WGSR

G=

6.2.1. Microwave-assisted heating Microwave irradiation is another route that can be employed to trigger the WGSR. Microwaves have been widely used in industrial processes and household appliances, stemming from their advantages of minimizing heating time, saving space, and high energy efficiency. The heating mechanism with microwave irradiation is different from that of conventional heating. Unlike conventional conduction and convection where heat is delivered through superficial heat transfer, if the heated object pertains to dielectrics, microwave irradiation can penetrate into a catalyst bed and induce an interaction between the object and an applied electromagnetic field to create heat inside the material through a mechanism called dielectric heating [92]. In particular, when microwave-assisted heating is applied to activate WGSR, the reaction is characterized by microwave double-absorption [93] inasmuch as both the reactants and catalyst pellets can absorb microwaves and convert them into thermal energy, thereby achieving a rapid heating process. Chen et al. [94] first conducted a reactor with microwave-assisted heating to trigger the WGSR for hydrogen production, in which a Fe-Crbased catalyst was adopted. The microwave-assisted heating system is depicted in Fig. 8. With a mass controller, the flow rate of CO was controlled, while a pump was used for water injection. They were wellmixed at the top layer of a fixed bed reactor followed by a catalyst bed which triggered the WGSR. The fixed bed reactor just sat inside of a microwave oven with a power controller. The off-gas past a dryer first and then entered into a gas chromatography and a gas analyzer that analyzed H2, CO, and CO2. Their comparison of the chemical reaction with conventional heating and microwave-assisted heating suggested that the performance of the WGSR under microwave irradiation was better. However, cracks on the catalyst surface might occur at S/C = 1 as a result of less water to relieve the high thermal gradient caused by the microwave radiation on the catalyst bed. Thereafter, Chen et al. [95] developed a numerical model and kinetics to predict the WGSR with the aid of a Cu-Zn-based catalyst and compared the results with experimental measurement. The experiments indicated that the performance of the LTSR increased with increasing temperature, which was contrary to that with conventional heating. It follows that the LTSR with microwave heating was dominated by chemical kinetics rather than thermodynamic equilibrium. The numerical simulations suggested that the temperature distribution in the catalyst bed was nearly uniform, which was the consequence of the exothermic reaction. When the thermal behavior of the LTSR was examined, heat generation stemming from the microwave irradiation was larger than that from the chemical reaction.

2 2f 2 (ro + ri ) g

(7)

where f, r0, ri, and g denote the rotor speed (rps), the outer and inner radii of the catalyst bed (m), and the acceleration of gravity (=9.81 m s−2), respectively. Physically, the G number represents the ratio of centrifugal force to gravitational force. Chen et al. [97] suggested that when the rotor speed was 1800 rpm, the G number was 234. This implies that with the rotor speed of 1800 rpm, an average of 234 G of centrifugal force acted on the reactants when they passed through the catalyst bed. Fig. 9b displays the distributions of CO conversion with respect to the G number for the three S/C ratios. From the viewpoint of thermodynamics or Le Chatelier's principle, a higher S/C ratio advantages CO conversion. In other words, the higher the S/C ratio, the higher the H2 yield from the CO reaction. Fig. 9b shows that the WGSR in a high gravity environment also followed the rule of thermodynamic equilibrium. Alternatively, an increase in G influenced the CO conversion noticeably, especially in a regime with a relatively low G number. The CO conversion at S/C = 4 was raised from 57 to 73% as the rotor speed increased from 0 to 1800 rpm. In other words, 16% of the CO conversion was enhanced and the relative percentage improvement of the CO conversion was 28%(=(0.73 0.57)/0.57 × 100) . This demonstrates the tangible role played by the centrifugal force in intensifying the WGSR. Within the investigated ranges of the parameters, the CO conversion of the WGSR from the centrifugal force was increased up to 70% compared to that without rotation. 6.2.3. WGSR with heat recirculation When it comes to H2 production from natural gas, the catalytic partial oxidation of methane (CPOM) is one of the promising technologies. It has a lower cost than steam methane reforming and the thermal decomposition of methane because of the autothermal characteristic. CPOM involves a series of reactions containing methane combustion (MC), steam reforming, and dry reforming. The latter two reactions are endothermic reactions where MC is exothermic. In other words, MC is able to provide the heat needed to initiate the following two reactions. Based on the kinetic principle, maintaining a higher temperature can result in higher MC efficiency. Consequently, heat recirculation from waste heat can be used to not only reduce the cost but also improve the performance of CPOM. The Swiss-roll reactor was developed to recover the waste heat from combustion [98,99]. Unlike a traditional heat exchanger, the unique structure of the Swiss-roll reactor provides a platform for heat recirculation inside the reactor. The Swiss-roll reactor employs waste heat to preheat reactants immediately after heat is generated in the reactor. Kinetically, a higher temperature energizes the reactants and leads to a more complete reaction. Several Swiss-roll applications involving a microturbine engine and burner showed that their thermal efficiency, fuel economy, and air pollutants reduction were improved to a great extent after installing Swiss-roll reactors [100–102]. Chen et al. [103] tested the transient state of CPOM by using a Swiss-roll reactor. The simulation results indicated that increased gas hourly space velocity shortened the transient period of the CPOM reactions, yet it also lowered the methane conversion. This result

6.2.2. High gravity (Higee) environment The first rotating packed bed (RPB) reactor was invented by Ramshaw and Mallinson [96] to intensify the mass transfer between two fluid phases. Since then, this unique apparatus or reactor has been extensively used in absorption, desorption, stripping, oxidation, deaeration, and other chemical reactions [97]. In an RPB reactor, a packed bed is rotated at a given speed so that the mass transfer process takes place in a centrifugal field. This induces a high gravity 15

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stemmed from more reactants being injected into the reactor and shorter residence times for the reactants in the catalyst bed. The methane conversion with exergy recovery at steady state was larger than 80%, which was approximately 20% higher than conversion without heat recovery. Similar results were also discovered by Chen et al. [104]. Accordingly, heat recirculation greatly enhanced steam reforming and dry reforming, and resulted in an improvement in hydrogen yield and methane conversion of more than 10%. Chien et al. [105] experimentally studied methanol steam reforming using a Swiss-roll reactor. The experiments indicated that H2 was generated by high thermal efficiency (13–35%) in this reactor. However, high CO concentrations were also found in the exhaust. Under the high-temperature scenario, the CO concentration was enhanced along with reduced H2 concentration, as a consequence of the reverse WGSR. Based on the thermodynamic principle, the reverse WGSR would be triggered under high temperatures. Therefore, the WGSR has the potential to react inversely when applying heat recirculation, which might lead to lower H2 production efficiency. On the other hand, Chen et al. [106] investigated CPOM combined with a high temperature and/or low-temperature WGSR by adopting a suitable catalyst in a Swiss-roll reactor (Fig. 10). The results illustrated that higher O/C (oxygen to methane) ratios led to higher methane conversions, but decreased H2 selectivity because the dominant reaction was methane combustion. Moreover, a lower S/C (steam to methane) ratio resulted in a lower hydrogen yield, which is attributable to the lower performance of the WGSR. On the other hand, a higher S/C ratio also rendered a lower hydrogen yield inasmuch as the cooling effect of the stream influenced the CPOM behavior. Consequently, the optimal operation of hydrogen production was observed at O/C = 1.2 and S/C = 4–6. Four different combinations were evaluated and compared in this study, the results of which indicated that the three-step reaction (CPOM + HTSR + LTSR) showed a substantial increase in H2 yield (1.9–2.5 mol/mol CH4) compared to the single CPOM reaction (1.6–1.8 mol/mol CH4). Although the three-step reaction had the best performance, it was very close to the value given by CPOM + LTSR. This implied that more catalysts for the HTSR influenced the whole performance only slightly. Therefore, in practical operation, CPOM in

Fig. 11. Enhancement of the WGSR by membrane or CCS.

association with LTSR is recommended for H2 production in a Swiss-roll reactor. 6.2.4. Photocatalytic WGSR In commercialized operation, the WGSR is carried out under a temperature of around 350 °C, which increases its operation cost. The photocatalytic process is manipulated at room temperature and needs no energy, except for sunlight. This promising technology has the potential to greatly reduce the cost of WGSR operation. Some published research related to this process is described in the following. Millard et al. [107] proposed a detailed mechanism of Pd/TiO2 for methanol reforming and CO-injected WGSR for H2 production at the ambient temperature with a 400 W Xe arc lamp. The rate of the COinjected WGSR was about a quarter that for methanol reformation. For the WGSR, the increasing CO led to a rate increase. This rate reached a maximum value at 1 mL CO, after which no changes in the rate occurred despite further increases in CO, and registered approximately zero at the 10 mL CO condition. The reason for this might be that surplus CO may occupy the active sites of the Pd nanoparticles, thereby reducing the reaction rate for H2 production. The same applies to the Pd

Fig. 10. CPOM combined with the high and/or low-temperature WGSR by adopting a suitable catalyst [106]. Reprinted with permission from the reference. Copyright (2010) Elsevier. 16

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photocatalyst and light are essential for triggering the WGSR based on the results of a blank test. The results indicated that most of the catalysts showed negligible CO conversion, except for TiO2 or CeO2 with metal nanoparticles. Among all noble metals, Au provided the best performance for H2 production. The best ratio between metal and photocatalyst was Au/TiO2 (1%), showing 71% CO conversion and 10,506 µmol/gcat. However, the performance reduced greatly when the light source changed from solar simulator to LED visible light. Differing from the traditional TiO2 photocatalyst, CuOx/Al2O3 was used to promote the solar-driven WGSR through a 300 W Xe lamp and UV LED lamp [109]. Interestingly, the H2 evolution rate over CuOx/ Al2O3 (24.6 µmol/gcat/s) was even higher than with noble metal catalysts (6.3 µmol/gcat/s for Pt/Al2O3 and 3.5 µmol/gcat/s for Au/Al2O3) under the same blending ratio (2%). Moreover, the CO conversion was greater than 95% when employing the best blending ratio (19%), which was even better than most of the traditional Fe-based catalysts and Cubased catalysts. They also found that the WGSR with this catalyst was the same as the traditional thermal catalysts. They measured the oxygen vacancy occurring at the interface when the UV light was turned on, and vice versa. In addition to the photocatalytic property, this catalyst also featured a photothermal catalytic property. In other words, both photo-generated carriers and heat occurred after solar promotion. The synergistic effect endowed this catalyst with a 2-fold higher activity than the thermal catalyst.

Fig. 12. Picture of fixed-bed membrane reactor [129]. Reprinted with permission from the reference. Copyright (2017) Elsevier.

7. CCS using WGSR

loading effect. The optimal amount of Pd with active sites was observed at the Pd loading of 0.5 wt%. Then, the rate decreased greatly to zero at 2 wt%. The active sites were located on the interface between Pd and TiO2. The boundary length was reported to increase with weight loading, but extra loading had the effect of reducing the perimeter due to the particle agglomeration effect. Photocatalytic WGSR using visible or simulated solar light for roomtemperature H2 generation was conducted by Sastre et al. [108]. A

The sorption-enhanced WGSR is one of the most promising methods to remove carbon before/after combustion. It is also suitable for reducing carbon in syngas generated from coal-based fuels, leading to a higher hydrogen concentration in the product gas [110]. Van Selow et al. [111] conducted experiments regarding the sorption-enhanced WGSR by use of the sorbents of hydrotalcite-based material promoted by potassium. For the investigations of sorbents and CO2 only, the

Fig. 13. The ANNs model for the WGSR in a Pd membrane reactor [131]. Reprinted with permission from the reference. Copyright (2015) Elsevier. 17

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Fig. 14. The Pd tubular membrane reactor supported by porous stainless steel [132]. Reprinted with permission from the reference. Copyright (2012) Elsevier.

best choice for H2 production; nonetheless, the CO2 slip problem was also the maximum for CMA because the WGSR shifted to the right. For cyclic experiments, CMA offered the best stability after several cycles (CO2 absorption and regeneration by calcination). The other sorbents decreased the H2 generation over several cycles, especially limestone. At the end of the WGSR test, H2 production strayed far from the theoretical value. After the absorption, CaO would transform into CaCO3. CaO showed the ability to catalyze in the WGSR, while CaCO3 did not participate in the reaction. Consequently, the carbonation of CaO might reduce the catalysis in the WGSR, though it also helped the WGSR move rightward due to Le Chatelier's principle. Another novel method for H2 production, which includes the reaction of water-carbon, water–gas, and carbon capture reactions, was developed by Lin et al. [114] as follows

Fig. 15. A configuration of the multi-tubular membrane reactor [135]. Reprinted with permission from the reference. Copyright (2016) Elsevier.

C + H2 O

breakthrough capacity (1.4 mmol/g) was found and the main mechanism of decarbonization was reported to be the formation of MgCO3. The stability for CO2 adsorption showed that a carbon capture above 98% could be achieved in at least 4000 adsorption-desorption cycles. For the tests including HTSR catalysts (reactor at 400 °C) and sorbents, the feed gas composition was simulated as the syngas from the autothermal reforming of natural gas. After the feed gas started to be injected, only H2 was observed in the product gas and the phenomena remained for over 10 min. The CO conversion upgraded from 55% to 100% after applying the sorbents, following Le Chatelier's principle. The breakthrough capacity was similar to the previous test (1.3 mmol/ g), and the stability test showed that both the carbon capture ratio and CO conversion were above 98% for at least 550 cycles. Similar sorption-enhanced WGSR experiments were also conducted by Van Selow et al. [112], where the mixture of hydrotalcite-based sorbent and HTSR catalysts were packed into a 2-meter tall fixed-bed reactor. For a breakthrough test, a small amount of CO2 was found in the effluent after 7 min. This came from the temperature elevation caused by the exothermic reaction, which might dissociate some carbonates into CO2. The actual breakthrough of CO2 occurred 13 min after feeding. The breakthrough capacity was 1.35 mmol/g, while the total capacity was 10 mmol/g. Several thermocouples detected that the temperature rose as the reaction occurred through the reactor step by step. The CO conversion achieved 100% with CO2 sorbent. Nearly pure H2 was discovered in the downstream for few minutes and slightly decayed with the CO2 breakthrough. However, the temperature measured by the thermocouple suggested that the temperature remained high rather than went back to its original point after CO2 breakthrough, which implied that the WGSR did not stop after breakthrough. In addition to the hydrotalcite-based sorbent, four CaO-based CO2 sorbents, including two natural (limestone and dolomite) and two syntheses (calcium magnesium acetate, abbreviated by CMA, and CaO dispersed mayenite using a hydrolysis technique) were tested for the WGSR upgrade by Muller et al. [113]. It was found that CMA was the

CO + H2 O

CaO + CO2

CO + H2 , CO2 + H2 ,

CaCO3 ,

CaO + C + 2H2 O

Ho298K = 132 kJ mol Ho298K =

Ho298K

=

CaCO3 + 2H2 ,

(8)

1

41.1 kJ mol

178 kJ mol Ho298K =

(1)

1

(9)

1

87.5 kJ mol

1

(10) An autoclave with the supercritical water method displayed the H2 production and CO2 capture by mixing the feedstocks (different types of coals and organic wastes), CO2 sorbent (Ca(OH)2), and catalyst for the water-hydrocarbon reaction (NaOH). During the Taiheiyo coal test, 55% H2 (0.44 L/gcoal and 31% carbon conversion) with 31% CO2 were released in the condition of coal-only. Seventy-six percent (0.99 L/gcoal and 79.6% carbon conversion) and 85% (1.45 L/gcoal and 90% carbon conversion) H2 with non-detectable CO2 were found after adding Ca (OH)2 and Ca(OH)2 + NaOH. In the measured gas released from the CaCO3, the product gas consisted of 90% CO2, which provided strong evidence of carbon removal by Ca(OH)2. Otherwise, the H2 concentration increased from 39% to 89%, followed by the temperature increase from 873 K to 973 K. The effect of pressure was relatively less compared to the temperature. Different feedstocks had similar gas compositions, where the H2 composition was between 71% and 84%. It is worth noting that no sulfur and chlorine emissions were found in the product gas since Ca(OH)2 and NaOH had the ability to react with these two elements and form solid depositions. The other research conducted by Zou et al. [115] combined the WGSR and membrane system for CO2 enrichment and H2 production. Unlike most of the WGSR membrane reactors, which apply an H2-selective membrane (the details of which are introduced in the next section), this study used a CO2-selective membrane. With the assistance of the membrane, the CO concentration in the retentate side was even lower than the one without a membrane. This implied that the membrane caused a great influence for improving the WGSR. The CO concentration was lower than the limitation for PEMFCs, viz., 10 ppm, and the CO conversion was almost 100%. The product gas at the retentate side consisted of over 50% H2. High CO2 selectivity and permeability 18

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Fig. 16. The holder-free plate-type membrane reactor for WGSR [137]. Reprinted with permission from the reference. Copyright (2013) Elsevier.

were discovered, while the water content was in the sweep side, which was caused by the dilution effect where the water diluted the CO2 concentration in the sweep side. Accordingly, the high CO2 concentration gradient between the retentate and permeate sides remained. Differing from previous papers focusing on the CCS method, Carbo et al. [116] assessed the designs of conventional (2 sequential reactors with intermediate cooling) and advanced (4-staged reactors with split syngas and water quench) WGSR with pre-combustion CO2 capture systems. The advanced one achieved a CO2 capture ratio to 85% along with a 70% reduction in the steam requirement compared to the conventional one. As a result, based on similar CO2 capture ratios, the efficiency penalties of advanced systems were lower by 45% compared to conventional systems. The potential concern of the advanced system was the cost in that more reactors used more catalysts; however, this might be offset by the increased electric output.

separate and purify H2 from H2-rich gas mixtures. However, compared to the large-scale facilities that run such techniques, membrane separation [119] appears to be a more feasible process for small-scale and/or on-board H2 separation and purification for its consumption in low-temperature fuel cells. Membrane separation is characterized by its lower investment cost, no moving parts, low energy consumption, continuous operation, and easy operation and maintenance [120,121]. Over the years, palladium-based (Pd-based) membranes, including pure Pd and Pd-alloy membranes, have attracted a great deal of attention for their application in H2 separation and purification [122]. Pd-based membranes pertain to crystalline alloy membranes [123], and are characterized by high selectivity and affinity to H2. Under the operating temperatures of 300–400 °C, H2 would be adsorbed onto the surface of the Pd-based membrane; thereafter, H2 would degrade into hydrogen atoms, arising from the catalytic ability of the Pd. The atoms would permeate through the membrane. The hydrogen atoms would move toward the permeate side of the membrane owing to the concentration gradient. The atoms would recombine at the permeate side and H2 would desorb the membrane, accomplishing the hydrogen permeation process [124]. H2 permeates through the Pd-based membrane in a solution-diffusion mechanism [120], the process of which can be decomposed into seven steps: (1) movement of the incoming H2 molecule to the upstream surface of the membrane; (2) dissociation of chemisorbed H2 molecules into H2 ions (H+) and electrons (e−); (3) adsorption of surface H+ ions into the membrane bulk; (4) diffusion of

8. WGSR in membrane reactors It is well-known that H2 production is the first step in realizing an H2 economy. In most cases, H2-rich gases rather than pure H2 are produced from the thermochemical chemical reactions described earlier. Therefore, the separation process plays an essential role in H2 purification. Currently, the mature techniques of pressure swing adsorption [117] and cryogenic distillation [118] can be employed to 19

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Fig. 17. The fluidized Pd-Ag membrane reactor for WGSR [138]. Reprinted with permission from the reference. Copyright (2015) Elsevier.

H+ ions and electrons through the membrane via the lattice structure; (5) desorption of H+ ions from the membrane to the surface; (6) reassociation of the H+ ions and electrons into molecular H2; and (7) movement of the resulting H2 molecule away from the downstream surface of the membrane. Accordingly, H2 permeation through the membrane is related to not only physical transport processes but also chemical reactions. The combination of the WGSR in a Pd-based membrane reactor is considered a promising system to intensify CO conversion and H2 production. In a membrane reactor with elevated pressure of the inlet gas, H2 permeates through the membrane while the WGSR proceeds. In addition to elevated pressure, a sweep gas or vacuum could be introduced at the permeate side [125]. This not only increases the driving force but also decreases the partial pressure of the product gas to enhance the concentration gradient. The higher the sweep gas velocity and vacuum degree, the better the performance of hydrogen permeation [126,127]. Furthermore, on account of the continuous removal of H2 by the sweep gas velocity or in a vacuum environment, the reactants shift the reaction toward the formation of products by Le Chatelier’s principle (Fig. 11). This leads to higher CO conversion, even surpassing the thermodynamic equilibrium limitation [128]. In addition, adopting a Pd-based membrane not only improves the H2 concentration in the permeate side, but also enhances the CO2 concentration in the retentate side. Apart from increasing the H2 energy usage, enriched CO2 provides a straightforward path to carbon capture. The WGSR with a Pd-based membrane was simulated by Chen et al. [129]. The membrane reactor belonged to a fixed bed type because catalysts were packed in an area called the catalyst bed. The feed gas came into the retentate side and reacted with the catalyst in catalyst

Fig. 18. The integrated system combining both CCS and membrane for WGSR [140]. Reprinted with permission from the reference. Copyright (2015) Elsevier.

bed. After the reaction, hydrogen penetrated through the Pd membrane into the permeate side, and was carried away by a sweep gas (Fig. 12). The results indicated that a Pd-based membrane improved the CO conversion by 83% compared to without a membrane at high temperature (700 °C). This stemmed from the enhanced membrane permeation of H2 with increasing temperature; however, the CO conversion decreased due to this process thermodynamically disfavoring high temperature. The results also confirmed that the thermodynamic limit breakthrough of the CO conversion could be achieved with a Pd-based membrane. More specifically, the highest CO conversion could exceed the thermodynamic equilibrium by 61%. Sanz et al. [130] experimentally tested the WGSR before/after membrane installation under varied operating conditions. They discovered that the CO conversion and H2 recovery could achieve 99.0% and 99.5%, respectively under the optimal conditions. Furthermore, they built up a 2D model to simulate a fixed-bed membrane reactor. This model was capable of simulating the non-ideal flow and successfully predicted the membrane reactor. Different from the fixed-bed membrane reactor, the tubular membrane reactor was also commonly applied in water gas shift reaction. A parametric study on tubular Pd membrane reactor was conducted by [42]. A model considering physical, chemical, mathematical, and the boundary conditions were calculated by COMSOL multiphysics. The results illustrated that the optimal temperature was 450 °C for Fe-Cr 20

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Table 6 Summary of previous studies related to the WGSR in a membrane reactor. Inlet gas

S/C

Temp. (°C)

P (atm)

Catalyst

Membrane

GHSV (1/h)

HR (%)

XCO (%)

Ref.

CO:CO2:H2:H2O = 9.7:4.9:48.5:36.9

–

3.95–7.90

–

Pd75Ag25-

20,000–40,000

68

80

[135]

CO:CO2:H2:N2:others = 61.6:1.7:30.6:4.8:1.3

24.67

Fe/Cr/Cu

Pd

68.3

[144]

10.86

Cu/Zn/Al

81

80

[137]

CO:H2:CO2:H2O = 7.97:43.48:10.99:31.88

–

–

–

90

[136]

H2:CO:CO2 = 70:18:12

1 1-3 2 1-2 3 1 2.6 1

10 5-30 0.05

Fe/Cr

Pd/Ag Pd Pd

74,000 16,000–74,000 5000 5000–17,000 –

83.6

CO:H2:Ar = 60:36:4

1 1-4 2

400 360–400 400 180–400 400

99.5

99

[130]

1

Ni

SrCe0.9Eu0.1O3−δ

1550 4000–5500 –

92

~90

[133]

20 15 14 10

Fe/Cr/Cu Fe/Cr/Al2O3 Fe/Cr Fe/Cr/Al2O3

ZSM-5/silicalite Pd Pd/Ag Pd

48,000 – 2100 1596

90 100 81.2 75

98.5 98 98.2 52

[134] [42] [145] [129]

CO:H2O:Ar = 3:6:91 CO CO CO H2:CO:CO2 = 34.69:47.96:17.35

375 300–400 350 350–400 900 600–900 440 450 450 700

Fig. 19. The full eco-friendly CCS recycling cycle using microalgae as a media.

catalyst which is common-seeing in HTSR. Under the optimal condition, the CO conversion would achieve 98% coupled with 100% hydrogen recovery. The theoretical investigation on Pd tubular membrane reactor was also studied by [131]. Specifically, they used the artificial neural networks (ANNs) model to complete the theoretical investigation. Consisting of a lot of input neurons including membrane temperature, reaction temperature, GHSV, S/C ratio, nitrogen fraction in the feed gas, the flow rate of sweep gas, active membrane surface area, and the feed gas configuration, the calculation can be operated in a parallel. Three output neurons such as CO conversion, hydrogen recovery, and hydrogen composition and 20 hidden neurons were merged into the ANNs model (Fig. 13). The best operating conditions can be found by using a target value for the output neuron. The difference between the experimental data used in building the model and the output was lower than 3%, though few points were around 7%. The experimental validation was completed as the maximum difference was the hydrogen recovery (10.34%), while the error for CO conversion was only 0.34%. For the experimental work, Liguori et al. [132] used a Pd membrane on porous stainless steel support to test HTSR catalyzed by a commercial Fe-Cr catalyst. The catalyst pellets were sandwiched between two glass spheres layer so as to solidify the catalysts. After reacting in the catalyst bed, hydrogen would pass the tubular membrane reactor and then off the reactor following the direction of the permeate stream (Fig. 14). The best result could achieve 80% CO conversion and 70% H2 recovery with 97% purity of permeated H2 under the optimal conditions of S/C = 4:1, GHSV = 3450 1/h, and a reaction pressure of 11 bar. Other than Pd membranes, a new type SrCe0.9Eu0.1O3−δ

membrane was fabricated onto the support of Ni-SrCeO3−δ [133]. Due to the high proton conductivity of SrCeO3−δ and total electronic conductivity improvement with Eu adding, the membrane was suitable for H2 permeation and separation. The results reported that the performance of tubular membrane reactor overcome the thermodynamic limitation, and attained 92% total H2 yield with 32% pure permeated H2 yield. Apart from that, the zeolite (MFI type) membrane, a material possessed high thermal and chemical stability which is conducive to industrial usage, was manufactured with silicalite assistance [134]. The silicalite was used to shorten the pore size of zeolite so that this duallayer membrane had ability to differentiate hydrogen. The CO conversion was over 90% for the experiment using the Fe-Cr-Cu catalyst. Furthermore, a model was developed and well described the data from experiment. By using the established model, the optimal conditions were found: 550 °C, H/C = 2, GHSV = 60,000 1/h, and feed pressure = 20 atm. High CO conversion (> 95%) and H2 recovery (> 90%) were reached under the optimal conditions. Instead of the single tubular membrane reactor, the multi-tubular membrane reactor was applied for the WGSR which is illustrated in Fig. 15 [135]. Five tubular membranes were merged into a channel full of catalysts. Upon H2 generated, it was capable of transferring into the permeate side, while the unreacted feed gas would stay in the retentate side. The concentration polarization was clearly presented in this study. The profiles variation followed by different operating factors allowed the researcher to decrease the influence caused by polarization. Furthermore, the pressure and temperature effects were also well described where the permeability would enhance while both of the factors were 21

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elevated. Similar results were also discovered by Adrover et al. [136] where higher pressure and temperature resulted in higher CO conversion. Unlike most of the tubular and fixed-bed membrane reactors requiring a holder for catalyst, a plate-type membrane reactor was employed for the WGSR [137]. The catalyst was in disk-shaped and located onto the disk-type membrane. This design was appealing because of the delivering time from the catalyst bed to the membrane surface. As shown in Fig. 16, the catalyst bed directly adhered to the membrane with sealing o-rings. The results pointed out that the increasing pressure and declining GHSV were conducive to the performance of the platetype membrane reactor. The best results displayed 80% CO conversion and 81% H2 recovery with 100% H2 purity. To further increase the performance of the membrane reactor, the fluidized bed membrane reactor was innovated [138] because it was believed that the interaction between the fluidized catalyst and the feed gas would improve significantly. The feed stream injected in the bottom of the membrane reactor (Fig. 17). The stream had the ability to make catalysts fluidization, and that was advantageous for the mixing between the catalyst and reactants. In a 1 Nm3/h scale fluidized membrane reactor test [139], the one with membrane was superior to the single fluidized bed reactor in terms of CO conversion and H2 recovery. The H2 purity was also satisfied as it could be used for fuel cell directly (average concentration for CO was lower than 10 ppm). The durability test was also attractive as the bare variation of CO conversion and H2 recovery was found over 900 h. In recent years, the upgraded system merging CCS and membrane reactor into a unit for the WGSR was invented. Soria et al. [140] packed the adsorbent (K2CO3) for CO2 capture in the catalyst bed (Fig. 18). In other words, once the feed gas reacted with the catalyst and CO2 and H2 were produced, the CO2 would be held by the adsorbent and H2 would permeate the Pd-Ag membrane. Consequently, both of the product gases could be separated perfectly. The improvement caused by Le Chatelier's principle would advance. Under the optimal conditions, 100% CO conversion was attained together with 100% H2 recovery. In another study which experimentally investigated the performance of a sorption-enhanced membrane reactor and compared with a traditional fixed-bed reactor, a sorption-enhanced reactor, and a membrane reactor [141]. The results illustrated that the CO conversion also achieved 100% and it was beyond the thermodynamic limit. Moreover, the integrated system defeated all the other reactors independent of the operating conditions which was similar to the results from the numerical study [142]. A summary of the WGSR in membrane reactors is given in Table 6.

concentrated on CO2 sorbents. The main sorbent used for CCS is CaO which will transform into CaCO3 after reacting with CO2. Research concerning additional approaches to recycling CaCO3 could be performed. In addition, other CCS technologies have the potential to be executed in association with the WGSR. For example, unlike traditional CCSs that require a specific place to inject the captured CO2 underground, alternative CCS solutions include microalgae as the media to hold CO2 inside. Having CO2 as a nutrient, microalgae grow after performing photosynthesis. Propagating microalgae can be used as a biofuel, such as biodiesel or biochar (Fig. 19). Based on this concept, hopeful eco-friendly recycling from CCS for further application was proposed. However, experiments and assessments need to be completed. Be that as it may, the WGSR in membrane reactors is a promising technology, and many studies have reported that it breaks the thermodynamic limit of the WGSR, and achieves nearly 100% CO conversion. However, the main components of the commercial membrane are Pd or Pd-alloy, the cost of which inhibits the wide-adoption of the membrane. Hence, other anthropogenic or synthesized membranes need to be developed in the future so as to reduce the price of the membrane. Overall, the WGSR coupled with a membrane system creates many potential benefits for performance. 10. Conclusions On account of growing environmental concerns in many countries, using hydrogen as a clean and alternative fuel has received much attention recently. Hydrogen has a higher inherent energy density compared to fossil fuels, and water is the only by-product after combustion. The water gas shift reaction is an essential step in many industrial operations for hydrogen production and purification. To better understand the water gas shift reaction, this paper reviewed the reaction from both the thermodynamics and kinetics aspects for many applications. As a whole, the water gas shift reaction thermodynamically favors low temperatures due to its exothermal characterization, whereas it kinetically tends toward the high-temperature operation. The trade-off is that although the low temperature achieves more CO conversion, it needs a larger volume reactor, thereby increasing cost. On the other hand, high temperatures lead to a high reaction rate, yet it has poor conversion performance. Catalysts developed recently and used in both the high-temperature region (350–500 °C) and low-temperature region (150–250 °C) were also reviewed. The rate expressions for each catalyst tabulated in this study have provided insightful information for engineers and scientists. This paper discussed many cases regarding the water gas shift reaction in particular circumstances as well. Regardless of the process being non-catalytic (supercritical water and plasma) or catalytic (microwave-heating, Higee, heat recirculation, and photocatalytic), each has its unique characteristics. Carbon capture and storage and membrane systems for hydrogen purification and carbon dioxide enrichment were also reviewed in this study. According to Le Chatelier's principle, hydrogen production can be greatly enhanced and even precede the thermodynamic equilibrium with the assistance of a membrane reactor. In summary, all reviewed techniques have advantages and disadvantages. The information illustrated in this study relating to the water gas shift reaction provides useful insights into the development of hydrogen production and carbon emissions abatement to act as resources and for environmental sustainability.

9. Challenges Though the WGSR has been developed over a hundred years, there remain some challenges and problems to be overcome. For example, two main mechanisms, namely redox and association, were common in most of the WGSR papers. Several other mechanisms have also been proposed by a few research groups. However, which mechanism is the most important and dominant is still unknown. Moreover, because the suitability of the mechanism for every catalyst remains unknown, detailed investigations of the mechanisms should be conducted since many improvements can be made after understanding the basic concept. Insightful comprehension of some special environments is also needed. Several papers observed the behaviors of the WGSR in certain circumstances; nevertheless, the mechanisms were scarce. For instance, the results concerning the WGSR in Higee showed that the efficiency was higher than that without Higee, and the G number, S/C ratio, and centrifugal force all played important roles in Higee. However, why and how Higee led to improvements in the WGSR are still not well-known. Accordingly, relevant investigations should be conducted in the future. For CCS, the main technology related to the WGSR was

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 22

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Acknowledgments

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