CO2 valorisation via Reverse Water-Gas Shift reaction using advanced Cs

doped Fe-Cu/Al2O3 catalysts

L. Pastor-Pérez, F. Baibars, E. Le Sache, H. Arellano-García, S. Gu, T. R. Reina*

Department of Chemical and Process Engineering, University of Surrey, Guildford, GU2

7XH, United Kingdom

*Corresponding author: t.ramirezreina@surrey.ac.uk

Abstract

This paper evidences the viability of chemical recycling of CO2 via reverse water-gas shift

reaction using advanced heterogeneous catalysts. In particular, we have developed a

multicomponent Fe-Cu-Cs/Al2O3 catalyst able to reach high levels of CO2 conversions and

complete selectivity to CO at various reaction conditions (temperature and space velocities).

In addition, to the excellent activity, the novel-Cs doped catalyst is fairly stable for

continuous operation which suggests its viability for deeper studies in the reverse water-gas

shift reaction. The catalytic activity and selectivity of this new material have been carefully

compared to that of Fe/Al2O3, Fe-Cu/Al2O3 and Fe-Cs/Al2O3 in order to understand each

active component’s contribution to the catalyst’s performance. This comparison provides

some clues to explain the superiority of the multicomponent Fe-Cu-Cs/Al2O3 catalyst

Keywords: CO2 conversion, Reverse Water-Gas Shift, Iron catalysts, Caesium Promoter.

1

Introduction

Some of the world’s most critical issues include anthropogenic induced climate change,

depleting fuel resources and pollution. CO2 emissions are ever increasing and due to reliance

on fossil fuels and the complications of renewable energy sources, it is necessary to consider

the increasingly feasible alternative of capturing CO2 and converting it into fuels and

chemicals [1, 2]. The Fischer-Tropsch (FT) process has been proposed as a feasible solution

for CO2 conversion and is intended to be carried out in conjunction with the Reverse Water-

Gas Shift reaction (RWGS) [3, 4]. However, the feasibility of this alternative energy source is

reliant upon the development of a hydrogen source which is carbon neutral and economically

acceptable.

The RWGS reaction (Equation 1) is the reversible hydrogenation of CO2 to produce CO and

H2O. Due to the chemical stability of CO2, it is a relatively unreactive molecule and so the

reaction to convert it to the more reactive CO is energy intensive.

CO2 + H2 ↔ CO + H2O ΔHo298k = + 41 kJ mol-1 (1)

CO2 + 4H2 ↔ CH4 + 2H2O ΔHo298k = - 165 kJ mol-1 (2)

According to Le Châtelier's principle, as the reaction is endothermic, it is thermodynamically

favoured by higher temperatures. Increasing the H2/CO2 ratio maximises the CO2 conversion

and favours the RWGS reaction [5]. For this reason, when the reaction is carried out at lower

temperatures, the equilibrium will increasingly favour the WGS (reverse of Equation 1) and

methanation (Equation 2) reactions, as they are exothermic and the most prominent side

reactions under these conditions. However, this ratio and the temperature are limited to

ensure that the conditions applied during the experimental stages are economically beneficial

for industrial applications. From previous studies, it has been shown that there is no

significant effect of altering pressure on the reaction activity and position of the equilibrium

due to the stoichiometry of the reaction [4].

2

The RWGS reaction is a desired route for industrial applications, most commonly in

conjunction with the FT reaction to synthesise hydrocarbon fuels from syngas [3]. The

conditions used for the FT reaction lie in the range of 200 to 375 oC; lower temperatures for

long chain alkanes and higher temperatures for shorter. Therefore, an additional reason to

improve catalytic activity for the RWGS reaction at lower range temperatures is to reduce

heat requirements necessary for the FT process. However, naturally, the thermodynamics of

the RWGS reaction require higher temperatures to achieve acceptable levels of activity [6, 7].

Numerous studies have been carried out regarding catalysis of the RWGS reaction [8-10].

Iron based catalysts are often considered as one of the most successful active metals for

higher temperatures, due to its thermal stability and high oxygen mobility [11-13]. Typically

iron works in the high temperature range and for lower temperatures copper is often regarded

to be successful due to its enhanced adsorption of reaction intermediates at these lower

temperatures [14-16]. Also Cu-Fe combinations have been employed for WGS reactions

showing some advantages in terms of mechanical properties (i.e. sintering tolerance) and

synergistic electronic effects [17,18]

Alkali metals tend to be successful active promoters, as they intensify electrostatic

interactions, by promoting electron transfer with reacting molecules. This is known to

enhance CO2 adsorption capacity [19,20]. Potassium, in particular, is often regarded as a

successful promoter and has been considered in several studies, often with iron as the active

metal [4, 5]. Furthermore, it is generally accepted that the addition of alkali metal has an

important influence on the catalytic performances promoting the dispersion of active sites

[21,22]. Within the alkaline materials, Cs has been studied in a lesser extension as a catalysts

promoter and it is considered somehow an exotic dopant [23,24]. However, the biggest ionic

radius of Cs compared to Na or K makes it a more basic element (i.e. it is more prone to

donate electrons) and therefore could be an interesting dopant to promote CO2 activation

given the acidic nature of CO2.

Under these premises we have developed a novel Cs-doped CuO-Fe2O3 catalyst supported on

alumina for chemical CO2 recycling via the RWGS reaction. This advanced material has been

compared to Fe/Al2O3, Fe-Cu/Al2O3 and Fe-Cs/Al2O3 in order to elucidate each active

component’s chemical contribution to the catalytic behaviour. Furthermore several

characterization techniques were employed to gain insights of the chemical properties-

catalytic activity correlation.

3

Experimental

Catalysts synthesis

The catalysts were synthesised by wet impregnation in excess of solvent. For the Fe/Al2O3 the

necessary amount of iron nitrate (Aldrich, 99.95%) to obtain 15 wt.% Fe2O3 was dissolved in

ethanol. Then PURALOX SCFa-230 alumina support (Sasol, ≥ 99%) was impregnated

during 1 h in a rotary evaporator with the solution containing the metallic precursor. After

that, solvent was removed by evaporation and the resultant slurry was dried in at 110 oC for

24 h.

The same procedure was used for the synthesis of Fe-Cu/Al2O3, Fe-Cs/ Al2O3 and Fe-Cu-Cu/

Al2O3 catalysts by adding in the ethanol solution, in addition to the iron nitrate precursor, the

corresponding amount of copper nitrate (Aldrich, 99.95%) and/or caesium carbonate

(Aldrich, ≥ 99%) to obtain 10 wt.% CuO and/or 5 wt.% Cs2O respectively. All the samples

were calcined at 750 oC for 4 h.

Catalysts characterisation

X-Ray Diffraction analysis (XRD) analysis was carried out at room temperature on an X’Pert

Pro PANalytical diffractometer. The patterns were obtained using Cu Kα radiation (40 mA,

45 kV) with a 2ϴ-range of 10–90° and a step size of 0.05° with a step time of 160s.

X-Ray fluorescence (XRF) analysis was carried out on an EDAX Eagle III spectrophotometer

utilising rhodium as the radiation source.

Thermodynamic Simulation

ChemStations’ ChemCad software package was used to observe the thermodynamic limits of

RWGS reaction over a range of temperatures and H2/CO2 ratios. The Soave-Redlich-Kwong

equation of state was used in a Gibbs reactor. Material flows into the reactor are identical to

those intended to be used for experimentation. The results of these simulations are included

in the catalytic performance plots.

4

Catalytic behaviour

The activity tests were carried out in a vertical fixed bed reactor in the form of a 7 mm

diameter quartz tube. The catalyst was compacted onto quartz wool which was then centrally

positioned into the reactor. The reactor was then placed within a cylindrical ceramic furnace.

The Carbolite furnace was controlled by the Eurotherm 2416 temperature regulator and a

thermocouple installed within the furnace. The product stream was then analysed in an ABB

AO2020 Advanced Optima Process Gas Analyser.

Each catalyst was reduced for 1 h under a flow of 10 % H2 carried by N2 at 750 oC prior to

reactions. All reactions for the activity performance tests of each catalyst were evaluated

within a temperature range of 400 to 750 oC. The temperature was increased in 50 oC

segments which were held for 20 min with a heating rate between intervals of 15 oC min-1.

The flow of reactants was held at a constant weight hourly space velocity (WHSV) of 12,500

mL g-1 h-1 with a H2/CO2 ratio of 4:1 balanced in nitrogen. For the analysis of the effect of

space velocity on catalyst performance, the reaction conditions were tested at the same

temperatures and H2/CO2 ratio at space velocities of 6,250, 12,500 and 25,000 mL g -1 h-1. The

conditions for the stability tests were carried out at a space velocity of 12,500 mL g -1 h-1 with

a H2/CO2 ratio of 4:1 at 500 oC for 50 h.

According to previous studies and the ChemCad simulation of the RWGS reaction, the CO2

conversion can be improved by using higher temperatures (up to 1000 oC) and higher H2/CO2

ratios [25,26]. This is particularly true for copper containing catalysts which have been

reported to be activated by high hydrogen partial pressures [4]. Despite this, the reactions

have been carried out at the aforementioned conditions in order to demonstrate that the

developed catalyst can perform successfully under more economical conditions, which would

be beneficial for industrial application.

The parameters used for measuring catalytic activity of each sample in the investigation were

CO2 conversion (Equation 3), CO selectivity (Equation 4), CH4 selectivity (Equation 5) and

specific reaction rates. [27]

CO2 conversion (%) = ([CO2]In – [CO2]Out)/([CO2]In) × 100 (3)

CO selectivity (%) = ([CO]Out)/([CO2 ]In - [CO2]Out) × 100 (4)

5

CH4 selectivity (%) = ([CH4]Out)/([CO2 ]In - [CO2 ]Out)×100 (5)

where [CO2]Out and [CH4]Out are the concentrations of CO2 and CH4 in the outlet of the reactor

and [CO2]In in the CO2 concentration in the initial gas mixture. The error in CO2 conversion

and CO/CH4 selectivities for all the experiments is within ± 0.5%

Results and Discussion

Characterisation

Prior to reactions, the composition and structural properties of each fresh catalyst sample was

characterised by XRD, and XRF. Table 1 summarises the results of the actual composition of

each catalyst from an XRF analysis. As shown in the table the nominal values of the active

components (Cu and Fe) as well as the promoter (Cs) loading agrees very well with the

nominal composition which validate the synthesis procedure.

As for the chemical structure, the XRD patterns presented in Figure 1A reveals the crystalline

composition of the prepared materials. As shown in the figure the samples are mainly

composed by crystalline iron oxide particles with hematite Fe2O3 structure (JCPDS# 4-0784).

Furthermore, small peaks of face-centered cubic crystal structure of ferrite Fe3O4 (JCPDS#-

19-0629) appear in Fe/Al2O3 and Fe-Cs/Al2O3 samples. It seems that the ferrite phase appears

only for the copper free samples indicating some influence of the dopant (CuO) in the main

active phase structure (iron oxide). Some typical reflexions attributed to the primary support

γ-Al2O3 (JCPDS# 00-048-0367) are identified. Interestingly, no peaks related to copper or

caesium species were detected indicating a good dispersion and small particle size of both

promoters corroborating the successful catalysts preparation as intended from the XRF

results.

In order to obtain a more realistic idea of the samples composition during the RWGS

reaction, an X-Ray diffraction analysis was carried for the pre-reduced materials (Figure 1B).

As shown in the Figure, the reductive pre-treatment leads to full reduction of FeOx and CuOx

phases and new peaks corresponding to metallic Fe (JCPDS # 98-000-0064) and metallic Cu

6

(JCPDS # 85–1326). In view of this results metallic Fe and Cu can be proposed as the main

active phases of the catalysts during the reaction.

Catalytic behaviour

The catalytic activity in terms of CO2 conversion of each of the four catalysts are shown in

Figure 2. As predicted by the thermodynamic equilibrium and reported previously, the CO2

conversion is improved as temperature increases [4, 5]. In terms of CO2 conversion, the

catalyst performances are ranked as; Fe-Cu-Cs/Al2O3 > Fe-Cu/Al2O3 > Fe-Cs/Al2O3 >

Fe/Al2O3. As hypothesised, Fe-Cu-Cs/Al2O3 showed the highest level of performance, and

Fe/Al2O3 the least. The most significant improvements, compared to Fe/Al2O3 are seen at

lower temperatures, between 400 to 500 oC. Both bimetallic (Fe-Cu) and Cs-promoted

catalysts showed improved CO2 conversion. Furthermore, the addition of Cu (Fe-Cu/Al2O3

and Fe-Cu-Cs/Al2O3) favours the catalytic behaviour at higher temperatures. This is a very

positive result since the activity enhancement at low temperatures greatly favours heat

integration with a potential Fischer-Tropsch unit. Indeed, the addition of Cu in the catalysts

promotes the reaction by adding up active sites at the interface of the Cu-Fe catalysts since

metallic Cu and either Cu1+ or Cu2+ cations have been proposed as active species for this

reaction [28]. Furthermore, the Cu-Fe interaction can stabilise the Fe active phase, due to the

partial or complete oxidation of Cu, leading to a higher catalytic performance [29].

Overall, it seems that Cs promotion along with the addition of Cu result a very positive effect

for the CO2 conversion. Together with enhanced conversion, the selectivity is a key factor to

control in this reaction due to the strong competition with the CO2 methanation which

consumes much more hydrogen and limits syngas production. In this sense, Figure 3 shows

that both promoted Fe-Cs/Al2O3 and Fe-Cu-Cs/Al2O3 catalysts maintain a CO selectivity of ≥

99 % over the entire temperature range. The almost complete selectivity is an added value for

our catalysts and agrees fairly well with the results obtained by Lobo et al in a recent work

[5]. However, some differences in terms of selectivity are appreciated, i.e. Fe-Cu/Al2O3 and

Fe/ Al2O3 demonstrate an inferior performance until 550 oC and 600 oC respectively.

Exceeding 600 oC, all catalysts demonstrate equal performance in terms of CO selectivity.

Hence we can clearly conclude that the multicomponent Fe-Cu-Cs/Al2O3 is the most active

and selective material in the whole studied temperature range. The greater activity/selectivity

of the Cs doped sample at low temperature points that Cs enhances the electron transfer with

7

the CO2 reacting molecules favouring the CO2 conversion at lower temperature range and

also facilitating the pathway towards CO. Furthermore, the improvement seen in selectivity

of the caesium based catalysts, indicates that the caesium inhibit the methanation reaction

occurring on the iron and copper active phases at lower temperatures. Analogous results in

terms of supressed methanation were recently reported when alkaline metals such as K+ are

added to a Ni based catalysts for the WGS reaction [30].

In order to make our data comparable to further studies in literature, the specific reaction

rates for each catalyst have been calculated and the results are presented in Figure 4. This

Figure reflects clearly the effect of the different promoters. Normalizing the activity by gram

of catalysts evidences the poorest specific rate of the reference Fe/Al2O3 catalyst compared to

that exhibited upon addition of Cs and Cu. Interestingly, it seems that both promoters when

added separately have similar impact on the CO2 conversion but not on the CO selectivity

which is greatly enhanced by the presence of Cs. Indeed, Figure 4 shows clearly that the

selectivity improvement is exclusively associated to Cs. Nevertheless, the addition of both

promoters together in the multicomponent Fe-Cu-Cs/Al2O3 material greatly boost the specific

reaction rate indicating a strong Fe-Cu-Cs synergistic effect, besides improving the CO

selectivity. The normalisation of the activity in terms of mass of active phase (i.e. Fe and Cu)

slightly changes the scenario and highlights the superiority of the Fe-Cs/Al2O3 catalyst. In

fact, this clarifies that Cs is a very promising promoter because it remarkably boost the

activity and the selectivity simultaneously.

Overall, these data reveal the advantage of the advanced catalysts developed in this study

taking the edge over typical Fe based materials and revealing the possibility of improving the

catalysts design by the careful selection of promoters. In view of these results, the

multicomponent Fe-Cu-Cs/Al2O3 catalysts was selected for further studies since it combines

the catalytic promotion of Cs and the mechanical stability improvement of Cu as reported

elsewhere [17]

Effect of WHSV

One of the biggest limitation of the WGS and then of the RWGS is the low space velocity

needed to achieve high performance. This situation involves high reactor volumes for shift

converters what in turns add up substantial capital cost to the process. In this sense, the most

active sample was selected for a space velocity study. CO2 conversion was recorded at three

different space velocities in the same range of temperatures and the results are given in

8

Figure 5. As shown in the figure the best performance in terms of CO 2 conversion was

achieved for the lowest space velocity and also it seems that the CO2 conversion for space

velocities of 12,500 and 25,000 mL g-1 h-1 were slightly decreasing respectively. This is an

interesting result that open some options in terms of operational conditions of the reverse

shift converter. Indeed, 25,000 mL g-1 h-1 is a relatively high space velocity which can be

transferred in medium size shift converters.

Figure 6 shows CO selectivity at the previous space velocities. It can be seen that for 12,500

and 25,000 mL g-1 h-1 that the CO selectivity remained approximately constant at 100 %.

However, when the space velocity was reduced to 6,250 mL g-1 h-1 there was a slight

reduction in CO selectivity and a complementary increase in CH4 selectivity at low

temperatures. Therefore it can be concluded that the lower contact time favours the CH4

production. However this is mainly due to the temperature effect and the competition

between RWGS and methanation over this temperature window. In fact, it is well reported

that CO2 hydrogenation to produce CH4 is favoured at lower temperatures (200 oC-500 oC)

and a higher CO2 conversion is favoured at low space velocities [31,32]. Furthermore for the

CO2 hydrogenation, the CH4 selectivity starts to decrease at high temperatures (400 oC -500 oC) and on the contrary, CO selectivity begins to increase favouring the reverse water-gas

shift reaction.

Stability test

In the context of CO2 conversion technologies, it is clear that for stationary applications (i.e.

cement or steel manufacture) the potential application of a reverse shift converter to mitigate

CO2 emissions and upgrade the flue gas will require a continuous operation of such unit.

Therefore we have tested the long-term stability of our advanced Cs-doped catalysts as

pictured in Figure 7. The reaction conditions were selected to be sufficiently far from

equilibrium and then extract meaningful conclusions. The graph shows that CO2 conversion

remains approximately constant at 55 % over the 50 h time period indicating a great stability

for longer runs with a CO selectivity over 99% in good agreement with the catalytic

screening experiments presented in Figure 2. This result reveals the suitability of this

advanced catalyst for continuous operation allowing to achieve a good activity / selectivity /

stability compromise.

9

Herein further post reaction analysis support the excellent results found in terms of stability.

Figure 8 shows the XRD study of the spent Fe-Cu-Cs/Al2O3 catalysts after the long term run

along with the XRD patterns of all the studied samples after a single catalytic run. For the

post reaction samples (single runs) the XRD highlights the formation of graphitic carbon over

the Fe/Al2O3 sample indicating the susceptibility of this material to form coke [33,34]. This in

fact points the potential deactivation of this material for long term runs and makes more

relevant the introduction of dopants. Indeed, crystalline carbon deposits were not observed in

all the doped samples. Also for the Cu-doped materials a slight degree of Cu sintering was

observed after the reaction showing more pronounced Cu peaks. Such a phenomenon is more

evident for the Fe-Cu-Cs/Al2O3 sample after the long term run which exhibited a remarked

degree of Cu sintering. Cu mobility during the reaction seems to be an important factor which

results in an enhanced Cu sintering over Fe sintering thus leaving the main active phase (Fe

promoted with Cs) well dispersed and available to interact with the reactants.

Conclusions

A series of highly active iron-based catalysts for the reverse water-gas shift reaction has been

developed in this paper. All the samples are very active and especially highly selective

towards CO. The addition of Cu to the bare Fe/Al2O3 catalysts improves the activity and

selectivity due to the synergistic Cu-Fe effect and the enhanced resistance towards sintering.

However, the catalytic behaviour can be notably upgraded by adding Cs as promoter of the

Cu-Fe/Al2O3 catalysts. Indeed, the multicomponent Cs-Cu-Fe catalyst is the best sample

within the studied catalysts reaching excellent levels of CO2 conversion at low temperature

with full selectivity to carbon monoxide. It seems that Cs improves CO2 activation due to its

basic character, easing the electrons transfer from the catalyst to the reactants and facilitating

the CO2 adsorption on the catalysts surface. In addition Cs helps to suppress the methanation

reaction which is key to avoid hydrogen consumption in an undesired route. In fact, the

selectivity improvement is mainly associated to Cs as promoter. Along with its excellent

catalytic performance, this novel catalyst is rather stable for long-terms runs, an essential pre-

requisite for further scaling up.

Overall, the exceptional long term stability results and the significant activity/selectivity

achieved with the novel Cs- doped catalysts indicate the successful design of our engineered

catalytic materials which suggest its suitability for realistic applications in gas phase chemical

CO2 recycling.

10

Acknowledgments

The authors of this work would like to acknowledge the Department of Chemical and Process

Engineering at the University of Surrey for the access to technical equipment. Also financial

support for this work has been obtained from the EPSRC grant no EP/J020184/2.

References

[1] PBL Netherlands Environmental Assessment Agency, Trends in Global CO2 Emissions,

PBL 2016. http://edgar.jrc.ec.europa.eu/news_docs/jrc-2016-trends-in-global-co2-emissions-

2016-report-103425.pdf

[2] European Commission, Carbon Capture Utilisation and Storage, SETIS, 2016.

https://setis.ec.europa.eu/system/files/setis-magazine_11_ccus_final.pdf

[3] G. Centi, E. A. Quadrelli, S. Perathoner, Catalysis for CO2 conversion: a key technology

for rapid introduction of renewable energy in the value chain of chemical industries, Energy

Environ. Sci. 6 (2013) 1711- 1731.

[4] A. Y. Daza, N. J. Kuhn, CO2 conversion by reverse water gas shift catalysis: comparison

of catalysts, mechanisms and their consequences for CO2 conversion to liquid fuels, RSC

Adv. 6 (2016) 49675-49691.

[5] J. A. Loiland, M. J. Wulfers, N. S. Marinkovic, R. F. Lobo, Fe/γ-Al2O3 and Fe–K/γ-Al2O3

as reverse water gas shift catalysts, Cat. Sci. Tech. 6 (2016) 5267- 5279.

[6] P. M. Maitlis, A. Klerk, Greener Fischer-Tropsch Processes for Fuels and Feedstocks,

John Wiley and Sons, 2013.

[7] F. Bustamante, R. Enick, A. Cugini, R. Killmeyer, B. Howard, K. Rothenberger, M.

Ciocco, D. B. Morreale, A. Chattopadhyay and S. Shis, High-temperature kinetics of the

homogeneous reverse water–gas shift reaction, J. AIChE. 50 (2004) 1028-1041.

11

[8] D. H. Kim, S. W. Han, H. S. Yoon, Y. D. Kim, Reverse water-gas shift reaction catalyzed

by Fe nanoparticles with high catalytic activity and stability, J. Ind. Eng. Chem. 23 (2015)

67-71.

[9] X. Yang, X. Su, X. Chen, H. Duan, B. Liang, Q. Liu, X. Liu, Y. Ren, Y. Huang, T.

Zhang, Promotion effects of potassium on the activity and selectivity of Pt/zeolite catalysts

for reverse water gas shift reaction, Appl. Catal. B: Env. 216 (2017) 95-105.

[10] F.-m. Sun, C.-f. Yan, Z.-d. Wang, C.-q. Guo, S.-l. Huang, Ni/Ce–Zr–O catalyst for high

CO2 conversion during reverse water gas shift reaction (RWGS), Int. J. Hydrogen Energy. 40

(2015) 15985-15993.

[11] M. Wenzel, N. A. Dharanipragada, V. V. Galvita, H. Poelman, G. B. Marin, L. Rihko-

Struckmann and K. Sundmacher, CO production from CO2 via reverse water–gas shift

reaction performed in a chemical looping mode: Kinetics on modified iron oxide, J. CO2 Util.

17 (2016) 60-68.

[12] T. Bligaard, J. K. Nørskov, S. Dahl, J. Matthiesen, C. H. Christensen, J. Sehested, The

Brønsted–Evans–Polanyi relation and the volcano curve in heterogeneous catalysis, J. Catal.

224 (2004) 206-217.

[13] C. S. Chen, W. H. Cheng, S. S. Lin, Study of iron-promoted Cu/SiO2 catalyst on high

temperature reverse water gas shift reaction, Appl. Catal. A: Gen. 257 (2003) 97–106.

[14] C. S. Chen, W. H. Cheng and S. S. Lin, Mechanism of CO formation in reverse water-

gas shift reaction over Cu/Al2O3 catalyst, Catal. Lett. 68 (2000) 45-48.

[15] D. L. Jurkovic´, A. Pohar, V. D. Dasireddy, B. Likozar, Effect of Copper-based Catalyst

Support on Reverse Water-Gas Shift Reaction (RWGS) Activity for CO2 Reduction, J. Chem.

Eng. Tech. 40 (2017) 973-980.

[16] C. S. Chen, W. H. Cheng, S. S. Lin, Enhanced activity and stability of a Cu/SiO2 catalyst

for the reverse water gas shift reaction by an iron promoter, J. Chem. Comm. 18 (2001) 1770-

1771.

[17] X. Lin, R. Li, Y. Zhang, Y. Zhan, C.i Chen, Q. Zheng, J. Ma, The role of surface copper

species in Cu-Fe composite oxide catalysts for the water gas shift reaction, Int. J. Hydrogen

Energy 40 (2015) 1735-1741.

12

[18] Z. Bao, W. Ding, Q. Li, Effect of Fe/Cu ratio on the activity of Fe–Al–Cu catalysts for

water gas shift reaction under hydrogen-rich atmosphere, Int. J. Hydrogen Energy. 37 (2012)

951-955.

[19] P. S. S. Prasad, J. W. Bae, K. W. Jun, K. W. Lee, Fischer-Tropsch Synthesis by Carbon

Dioxide Hydrogenation on Fe-Based Catalysts, J. Cat. Surv. Asia. 12 (2008) 170-183.

[20] D. G. Miller, M. Moskovits, A study of the effects of potassium addition to supported

iron catalysts in the Fischer-Tropsch reaction, J. Phys. Chem. 92 (1988) 6081-6085.

[21] Y. Tong-Qi, Z. Zhao-Xia, X. Yong, Y. Shi-Zhi, Z. Jiu-Fang, L. Yong, L. Quan-Xin,

Higher alcohol synthesis from bio-syngas over Na-promoted CuCoMn catalyst, Acta Phys.

Chim. Sin. 27 (2011) 1493–1500.

[22] J. García-Aguilar, D. Cazorla-Amorós, A. Berenguer-Murcia, K- and Ca-promoted

ferrosilicates for the gas-phase epoxidation of propylene with O2, Appl. Catal. A: Gen. 538

(2017) 139-147.

[23] R. T. Figueiredo, M. S. Santos, H. M. C. Andrade, J. L. G.Fierro, Effect of alkali cations

on the CuZnOAl2O3 low temperature water gas-shift catalyst. Catal. Today. 172 (2011) 166-

170.

[24] S. J. Ahlers, M.-M. Pohl, J. Radnik, D. Linke, E. V. Kondratenko, Catalytic role and

location of Cs promoter in Cs–Au/TiO2 catalysts for propanol synthesis from CO2, C2H4 and

H2. Appl. Catal. B: Env. 176–177 (2015) 570-577.

[25] S. S. Hla, D. Park, G. Duffy, J. Edwards, D. Roberts, L. Morpeth, T. Nguyen, Kinetics of

high-temperature water-gas shift reaction over two iron-based commercial catalysts using

simulated coal-derived syn gases, J. Chem. Eng. 146 (2009) 148-154.

[26] F. Ghodoosi, K. Nikou, A. Shariati, Mathematical modelling of reverse water-gas shift

reaction in a fixed-bed reactor, J. Chem. Eng. Tech. 40 (2007) 598-607.

[27] Y. Liu. Z. Li. H. Xu. Y. Han, Reverse water–gas shift reaction over ceria nanocube

synthesized by hydrothermal method, Catal. Comm. 76 (2016) 1-6.

[28] M. Estrella, L. Barrio, G. Zhou, X. Wang, Q.Wang, W. Wen, J. C. Hanson, A. I.

Frenkel, J. A. Rodriguez, In Situ characterization of CuFe2O4 and Cu/Fe3O4 water-gas shift

catalysts, J. Phys. Chem. C.113 (2009) 14411-14417.

13

[29] N. Thouchprasitchai, A. Luengnaruemitchai, S. Pongstabodee, Water-gas shift reaction

over Cu–Zn, Cu–Fe, and Cu–Zn–Fe composite-oxide catalysts prepared by urea-nitrate

combustion, J. Ind. Eng. Chem. 19 (2013) 1483-1492.

[30] T. Maneerung, K. Hidajat, S. Kawi International, K-doped LaNiO3 perovskite for high-

temperature water-gas shift of reformate gas: Role of potassium on suppressing methanation,

Int. J. Hydrogen Energy. 42 (2017) 9840-9857.

[31] J. Kirchner, J. K. Anolleck, H. Lösch, S. Kureti, Methanation of CO2 on iron based

catalysts. Appl. Catal. B: Env. “in press” http://dx.doi.org/10.1016/j.apcatb.2017.06.025

[32] H. Kierzkowska-Pawlak, P. Tracz, W. Redzynia, J. Tyczkowski, Plasma deposited novel

nanocatalysts for CO2 hydrogenation to methane. J. CO2 Util. 17 (2017) 312-319.

[33] C. Pretschuh, C. Schwarzinger, A. A. Abdala, S. Vukusic, Characterization of

Conductive Nanographite Melamine Composites. J. Compos. Mater. 4 (2014) 61-71.

[34] X. Wang, T. Liu, Fabrication and Characterization of Ultrathin Graphene

Oxide/Poly(Vinyl Alcohol) Composite Films via Layer-by-Layer Assembly. J Macromol.

Sci. 50 (2011) 1098-1107.

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Figures and tables

Table 1: XRF characterisation results

15

CatalystsComposition (w/w %)

F

e2O3

CuO Cs2O

Fe/Al2O3 15.1 -- --

Fe-Cu/Al2O3 14.8 10.2 --

Fe-Cs/Al2O3 14.6 -- 4.2

Fe-Cu-Cs/

Al2O3 14.1 9.2 4.5

20 30 40 50 60 70

▽

▽

▽

▽

▽ ▽

▽

▽

▽Fe3O4▽

▲▲ □□□

□□□

□□□

□ □ □

▲▲▲

▲▲

▲

▲▲▲▲▲

▲▲

▲

▲▲▲▲▲

▲▲

▲

▲▲▲▲▲

▲▲

▲

Fe-Cu-Cs/Al2O3

Fe-Cs/Al2O3

Fe/Al2O3

Inte

nsity

(a.u

.)

2(deg)

Fe-Cu/Al2O3

□▽

Al2O3

Fe2O3

▲ (A)

10 20 30 40 50 60 70 80

Inte

nsity

(a.u

.)

Fe-Cs/Al2O3

Cu metallicAl2O3

Fe-Cu/Al2O3

Fe/Al2O3

Fe metallic (B)

2 (deg)

Fe-Cu-Cs/Al2O3

16

Figure 1. XRD analysis for all studied catalysts. A) Fresh samples B) pre-reduced materials

Figure 2. CO2 conversion for Fe/Al2O3, Fe-Cu/Al2O3, Fe-Cs/Al2O3 and Fe-Cu-Cs/Al2O3

Figure 3. CO selectivity and complimentary CH4 selectivity for Fe/Al2O3, Fe-Cu/Al2O3, Fe-Cs/Al2O3 and Fe-Cu-Cs/Al2O3

17

400 450 500 550 600 650 700 7500

20

40

60

80

100

CO

2 con

vers

ion

(%)

Temperature (OC)

Fe/Al2O3 Fe-Cu/Al2O3 Fe-Cs/Al2O3 Fe-Cu-Cs/Al2O3 Equilibrium

400 450 500 550 600 650 700 7500

20

40

60

80

100

CH4 selectivity

CO selectivity Fe/Al2O3

Fe-Cu/Al2O3

Fe-Cs/Al2O3

Fe-Cu-Cs/Al2O3

Fe/Al2O3

Fe-Cu/Al2O3

Fe-Cs/Al2O3

Fe-Cu-Cs/Al2O3

CO

& C

H4 s

elec

tivity

(%)

Temperature (oC)

50

60

70

80

90

100

Fe-Cs/Al2O3Fe-Cu/Al2O3 Fe-Cu-Cs/Al2O3Fe/Al2O3

0.0E+00

2.0E-04

4.0E-04

6.0E-04

8.0E-04

1.0E-03

1.2E-03

1.4E-03

1.6E-03

1.8E-03

2.0E-03

CO

sel

ectiv

ity (%

)

mol

CO

2 con

verte

d .g

-1.m

in-1

Figure 4. CO2 formation rate and CO selectivity at 400 °C for all catalysts. (Dashed bars: 5x

mol CO2 conv.g-1catatalyst.min-1; empty bars: mol CO2 conv.g-1active phase.min-1)

18

400 450 500 550 600 650 700 7500

20

40

60

80

100

CO

2 con

vers

ion

(%)

Temperature (oC)

6,250 12,500 25,000 equilibrium

Figure 5. CO2 conversion for Fe-Cu-Cs/Al2O3 at WHSV of 6,250, 12,500 and 25,000 mL gcat

-1 h-1.

400 450 500 550 600 650 700 7500

20

40

60

80

100

CO selectivity

CH4 selectivity

CO

& C

H4 s

elec

tivity

%

Temperature (oC)

6,250 12,500 25,000

6,250 12,500 25,000

Figure 6. CO selectivity for Fe-Cu-Cs/Al2O3 at W.H.S.V. of 6,250, 12,500 and 25,000 mL gcat

-1 h-1.

19

0 10 20 30 40 500

20

40

60

80

100

Fe-Cu-Cs/Al2O3

CO

2 co

nver

sion

(%)

Time (hours)

Figure 7. Stability test results for Fe-Cu-Cs/Al2O3 at 500 oC with a W.H.S.V of 12,500 mL gcat

-1 h-1 over 50 hours.

20

10 20 30 40 50 60 70 80

Graphite **

Fe/Al2O3

*

Graphite flakes

Fe-Cu/Al2O3

Cu metallicAl2O3

Fe metallic

Inte

nsity

(a.u

.)

Fe-Cs/Al2O3

Fe-Cu-Cs/Al2O3

2 (deg)

Fe-Cu-Cs/Al2O3 stability test

Figure 8. Post reaction X-Ray diffraction patterns of the studied samples.

21