Rational design of ceria-based nanocatalysts for CO2 hydrogenation

to value-added products

M. Konsolakis (1)*, M. Lykaki (1), S. Stefa (1), S.A.C. Carabineiro (2), G. Varvoutis (3),(4),

E. Papista (3), G. Marnellos (3),(4)

(1) School of Production Engineering and Management, Technical University of Crete, GR-

73100 Chania, Greece. Tel.: +30-28210-37682, e-mail: mkonsol@pem.tuc.gr (M. Konsolakis)

(2) Faculdade de Engenharia, Universidade do Porto, Porto, Portugal.

(3) Department of Mechanical Engineering, University of Western Macedonia, Greece

(4) Chemical Process & Energy Resources Institute, Centre for Research & Technology Hellas,

Greece

Abstract – Carbon dioxide hydrogenation is among the most promising CO2 valorization technologies,

contributing to the mitigation of CO2 emissions in the atmosphere as well as to the production of value-

added chemicals and/or fuels. Among the different investigated catalysts, ceria-based composites have

received particular attention. The unique redox and surface properties of CeO2 along with its combination

with transition metals can lead to an enhanced catalytic performance due to the synergistic metal-support

interactions. Research efforts have recently focused on the rational design of ceria-based catalytic materials

through the fine-tuning of different counterparts’ size and shape, or through structural/electronic

functionalization. In this work, we report on the impact of active phase nature (Cu, Co, Ni) in combination

with ceria nanoparticles of different morphology (nanorods (NR), nanocubes (NC)), on the

physicochemical characteristics and the CO2 hydrogenation activity/selectivity of ceria-based transition

metal catalysts (MOx/CeO2, M: Cu, Co, Ni). The results clearly revealed the significance of support

morphology on the textural, structural, redox properties of the various binary systems while the metal phase

incorporated into the ceria support mainly determined the CO2 hydrogenation activity/selectivity. Bare ceria

samples exhibited the worst hydrogenation performance with ~20% CO2 conversion at 400 oC, being,

however, highly selective to CO (~90%). The incorporation of transition metals to ceria drastically changed

both the conversion and selectivity performance. The Ni/CeO2 samples demonstrated an excellent

hydrogenation performance with complete CO2 conversion at temperatures higher than ca. 280 oC,

accompanied by 100% selectivity to CH4. Similarly, the Co/CeO2 samples are highly selective to CH4,

offering, however a lower conversion performance (90% at 400 oC). On the other hand, the Cu/CeO2

samples exhibited intermediate conversion performance (~55% at 400 oC), being, however, highly selective

to CO.

1. Introduction - The catalytic hydrogenation of carbon dioxide (CO2) into value-added products, such as

CH4, CO or methanol, is considered to be an efficient CO2 mitigation technology due to the rising

concentrations of CO2 in the atmosphere along with its contribution to climate change [1]. Among the

various materials investigated, cerium oxide or ceria (CeO2) has attracted much attention in the field of

heterogeneous catalysis as supporting carrier, due to its high oxygen mobility and unique redox properties

[2]. Besides bare ceria’s excellent redox properties, many studies have focused on the development of

highly efficient and low cost ceria-based catalysts, as the combination of various transition metals with

ceria can greatly enhance the catalytic activity/selectivity due to the peculiar metal-support synergistic

interactions [3–6]. In order to develop highly efficient ceria-based catalysts, research efforts have focused

on the rational design of catalytic materials through the appropriate selection of metal phase, the fine-tuning

of different counterparts’ size and shape or through structural/electronic functionalization. For instance, by

decreasing the particle size in the nanoscale, materials exhibit abundance in surface atoms and defect sites,

such as oxygen vacancies, greatly affecting the catalytic performance due to the electronic perturbations

M&Ns-19, Paris, 17-19 July 2019 Pag. 69

mailto:mkonsol@pem.tuc.gr

between the metal and support nanoparticles [3,6,7]. Moreover, by tailoring nanoparticles’ shape by means

of advanced synthetic routes, different crystal facets can be exposed leading to different oxygen storage

capacity (OSC) and oxygen mobility [2,8]. For instance, Ouyang et al. [9] have investigated ceria’s

morphological effects on the CuO/CeO2 mixed oxides for methanol synthesis by CO2 hydrogenation, with

copper-ceria nanorods exhibiting the highest CO2 conversion and methanol yield due to the strongest metal-

support interactions, as compared to nanocubes and nanospheres. Although there are several studies

regarding the hydrogenation of CO2 over ceria-based composites, it should be noted that CO2 hydrogenation

proceeds through a complex reaction pathway and there are many factors governing this process, such as

the metal-oxide interface and the formation of oxygen vacancies [10,11], the reaction intermediates [12,13],

the dispersion of the active phase or the catalysts’ basicity [13,14].

The aim of the present study is to investigate the effect of various parameters (active phase, size, shape) on

the physicochemical, structural, redox properties and consequently, the catalytic performance of ceria-

based nanocatalysts (MΟx/CeO2 where M: Cu, Co, Ni) in the hydrogenation of CO2. Two different ceria

nanostructures (nanorods (NR) and nanocubes (NC)) were prepared by the hydrothermal method and

consequently used as supports for the copper, cobalt and nickel oxide phases. The as-prepared samples

were characterized by several techniques, including N2 physisorption, XRD, TEM, H2-TPR and they were

catalytically evaluated in CO2 hydrogenation reaction.

2. Experimental - In this work, ceria-based transition metal catalysts were synthesized and evaluated for

the CO2 hydrogenation reaction. Firstly, the CeO2 supports were prepared by the hydrothermal method [15],

followed by the incorporation of the metal into the support through the wet impregnation method in order

to obtain a M/(M+Ce) atomic ratio of 0.2. The samples were subsequently dried at 90 oC for 12 h and

calcined at 500 oC for 2 h in air flow, with a ramp rate of 5 oC/min. The as-prepared samples are designated

as M/CeO2-NX (M: Cu, Co, Ni; NX: NR – nanorods, NC – nanocubes). Catalytic evaluation was conducted

in a U-shaped, fixed-bed quartz reactor (I.D. = 1 cm). The reactor was placed inside a furnace, equipped

with a thermocouple and a temperature controller. Prior to the experiments, catalysts were reduced at 400oC for 1 h under pure H2 flow (~ 50 cm3/min). In each experiment, the reactor was loaded with 200 mg of

catalyst, while the volumetric flow rate of the inlet stream was kept constant at 100 cm3/min and the

molecular feed ratio of CO2:H2 was equal to 1:9. All experiments were conducted at atmospheric pressure

in the temperature range of 200 - 500 oC with a heating rate of 1 oC/min. Effluents’ composition was

determined by GC analysis at intervals of approximately 25 oC. The outlet stream was heated continuously

at about 110 oC to avoid condensation of produced H2O.

3. Results and discussion

Catalysts characterization - The textural, structural and redox properties of bare ceria as well as of

M/CeO2 catalysts are presented in Table I. Bare ceria supports, i.e. CeO2-NR and CeO2-NC, exhibit a BET

surface area of 79 and 37 m2/g, respectively. The incorporation of the transition metals to ceria carriers

slightly decreases the BET area, without, however, affecting the order obtained for bare supports.

According to the XRD results (diffractograms not shown for brevity), the main peaks for all the samples

can be indexed to the (111), (200), (220), (311), (222), (400), (331) and (420) planes of a face-centered

cubic fluorite structure of ceria (Fm3m symmetry, no. 225) [16]. As for the Cu/CeO2 samples, peaks

corresponding to CuO crystal phases at 2θ = 35.3°, 38.2° and 62° are observed for all catalysts, indicating

heterodispersion or aggregation of copper species on the surface of ceria [17]. The Co/CeO2 samples exhibit

characteristic peaks of Co3O4 at 2θ ~ 36, 44 και 64ο [18], whereas Ni/CeO2 samples exhibit peaks at 2θ ~

37.2, 43.3 and 62.9ο corresponding to the (111), (200) and (220) planes of NiO phase, respectively [19,20].

According to the TPR results (Table I), the reduction profiles of bare CeO2 samples consist of two broad

peaks centred at 545-589 °C and 788-809 °C, which are attributed to ceria surface oxygen (Os) and bulk

oxygen (Ob) reduction, respectively [21]. In order to gain further insight into the impact of support

morphology as well as of the nature of the active phase on the redox properties of the as-prepared samples,

the H2 consumption in the low temperature range (100-600 oC), corresponding to the surface oxygen

reduction of active phase and ceria species, has been estimated (Table I). The CeO2-NR sample exhibits

higher values (0.59 mmol g-1) than CeO2-NC (0.41 mmol g-1), indicating its abundance in loosely-bound

oxygen species, which results in enhanced reducibility and oxygen mobility.

The Co/CeO2 samples exhibit two main reduction peaks at 318-335 oC (peak a) and 388-405 oC (peak b),

attributed to the subsequent reduction of Co3O4 to CoO and CoO to metallic Co [18]. As for the Cu/CeO2

M&Ns-19, Paris, 17-19 July 2019 Pag. 70

samples, the low-temperature peak (peak a), in the range of 181-194 °C, is attributed to the reduction of

finely dispersed CuOx species interacting strongly with the ceria surface. The peak at higher temperature

(peak b) is related to the formation of larger CuO clusters on the ceria surface [22]. It is obvious that the

reduction of Cu/CeO2 samples occurs at considerably lower temperature than those of bare ceria samples

due to the synergy between the two oxide phases that weakens the metal-oxygen bonds [22,23]. Taking

into account the hydrogen consumption for the Cu/CeO2 samples in the low temperature range, copper-

ceria nanorods exhibit the highest value (1.80 mmol g-1) in comparison with copper-ceria nanocubes (1.50

mmol g-1), indicating their superior reducibility, while following the same trend as bare ceria

nanostructures. The Ni/CeO2 samples exhibit two main peaks at 273-289 oC (peak a) and 335-354 oC (peak

b). The former peak can be attributed to the reduction of adsorbed oxygen species from the framework Ni-

O-Ce, since Ni2+ incorporation into the ceria lattice leads to charge imbalance and lattice distortion, which

in turn create oxygen vacancies on the surface [24]. Peak b is attributed to the well-dispersed NiO phase,

which interacts strongly with the CeO2 support, the so-called “Ni-O-Ce boundary” [25]. The peaks of

Ni/CeO2 shift to lower temperatures than those of bare ceria supports, indicating the strong interactions

between Ni and CeO2. The total hydrogen consumption, given in Table I, follows the order Co/CeO2 >

Cu/CeO2 > Ni/CeO2 > CeO2.

In order to gain further insight into the morphology of bare CeO2 and M/CeO2 samples, TEM analysis was

performed (Image 1). The NR samples (Image 1 a, c, e, g) clearly display ceria in the rod-like morphology

while the support’s nanocubic morphology is evident in the NC samples (Image 1 b, d, f, h). Apparently,

the incorporation of the active metal phase into the CeO2 lattice has no effect on the support morphology.

Table I. Textural, structural and redox properties of the as-prepared samples

Sample

BET

Analysis XRD Analysis TPR Analysis

SBET

(m2/g)

Average crystallite

diameter, DXRD (nm) H2 consumption

(mmol H2 g-1)a

Theoretical H2

(mmol H2 g-1)

Peak Temperature (°C)

CeO2 Co3O4/CuO/

NiO Os Peak Ob Peak

CeO2-NC 37 27 - 0.41 -

589 809

CeO2-NR 79 15 - 0.59 545 788

Peak a Peak b

Co/CeO2-NC 28 24 19 2.05 1.76

335 405

Co/CeO2-NR 72 14 16 2.37 318 388

Cu/CeO2-NC 34 19 52 1.50 1.34

194 228

Cu/CeO2-NR 75 11 43 1.80 181 217

Ni/CeO2-NC - 22 17 1.35 1.33

273 335

Ni/CeO2-NR - 14 23 1.82 289 354

a Estimated by the quantification of H2 uptake in the low temperature range (100–600 °C) of the TPR profiles. b Estimated by the subtraction of H2 amount required for metal oxide (MOx) reduction in MOx/CeO2 samples from the total H2 consumption.

M&Ns-19, Paris, 17-19 July 2019 Pag. 71

Image 1. TEM images of the samples: (a) CeO2-NR, (b) CeO2-NC, (c) Co/CeO2-NR, (d) Co/CeO2-NC,

(e) Cu/CeO2-NR, (f) Cu/CeO2-NC, (g) Ni/CeO2-NR, (h) Ni/CeO2-NC

Catalytic evaluation studies - In Image 2, the effect of temperature on CO2 conversion is presented for all

samples. The significant impact of different metal phases (Cu, Co, Ni) is evident. For instance at 400 oC,

the Ni/CeO2, Co/CeO2, Cu/CeO2 and CeO2 catalysts exhibit CO2 conversion values equal to 100%, 90%,

55% and 20%, respectively. These results clearly indicate the strong effect of metal phase, as well as the

synergistic interaction between ceria support and active metal. The extent of this effect is greater,

considering the selectivity to CH4 and CO, as indicated in Images 3a and 3b, respectively. It is evident that

the bare ceria supports and Cu/CeO2 samples are both highly selective to CO (> 85% and > 95%,

respectively). On the other hand, the addition of cobalt or nickel into CeO2 leads to a completely different

trend, since the selectivity towards CH4 for all the Co/CeO2 samples is approximately 97% at 400-500 oC

and practically 100% for Ni/CeO2 at 200-500 oC. It is worth mentioning that CO2 is converted almost

completely and exclusively into CH4 on the Co/CeO2 and Ni/CeO2 catalysts. It should be also mentioned

that the nickel catalysts supported on ceria nanoparticles are highly efficient in methane production at

temperatures as low as 280 oC under the reaction conditions employed, being superior or at least comparable

to the most active Ni-based catalysts reported in literature [10,26].

According to the present results, the catalytic activity is greatly enhanced by the addition of transition metals

into the bare ceria, while the nature of the metal phase determines products’ selectivity. Conversely, it can

be concluded that CO2 conversion and selectivity is not significantly affected by the support morphology

(NR and NC). In particular, a slightly better conversion performance is obtained for Ni/CeO2-NC and

Co/CeO2-NC samples, as compared to the corresponding NR samples, whereas the reverse order is

observed for bare CeO2 and Cu/CeO2 catalysts (Image 2). Taking, however, into account that the NC-based

samples exhibit almost the half BET surface area as compared to NR-based samples (Table Ι), it could be

argued that NC samples are more active, in terms of intrinsic reactivity (reaction rate per unit area). In a

similar manner, it was found that ceria nanocubes exhibited better CO2 conversion, as compared to nanorods

and nanooctahedra, for the reverse water-gas shift reaction, whereas the incorporation of nickel to ceria

nanocubes further improved the catalytic performance [27]. Moreover, Ru supported on ceria nanocubes

have shown the highest CO2 methanation activity in comparison with ceria nanorods and nanopolyhedrons

[28].

On the basis of the present findings, it should be mentioned that the impact of ceria morphology in

combination to the different active phases (Cu, Co, Ni) on the reaction mechanism and the intermediates

formed during the CO2 hydrogenation process ought to be further elucidated in order reliable structure-

property relationships to be established.

M&Ns-19, Paris, 17-19 July 2019 Pag. 72

Image 2. CO2 conversion as a function of temperature for bare CeO2 and M/CeO2 samples in the CO2 hydrogenation reaction.

(Volumetric flow rate = 100 cm3/min, GHSV = 20,000 h-1, H2:CO2 = 9:1, P = 1 atm).

Image 3. Selectivity to CH4 (a) and CO (b) as a function of temperature for bare CeO2 and M/CeO2 samples in the CO2 hydrogenation reaction.

(Volumetric flow rate = 100 cm3/min, GHSV = 20,000 h-1, H2:CO2 = 9:1, P = 1 atm).

4. Conclusions - Ceria nanostructures of different morphology (nanorods, nanocubes) were hydrothermally

synthesized and various transition metals (Cu, Co, Ni) were introduced to the ceria carriers by the wet

impregnation method. The as-prepared samples were assessed for the CO2 hydrogenation reaction, in order

to explore the effect of support morphology and metal phase on the catalytic activity and selectivity. Bare

ceria samples exhibited the worst hydrogenation performance (20% CO2 conversion at 400oC), being,

however, highly selective to CO. The incorporation of the different transition metals to ceria carriers

drastically modified both the conversion and selectivity performance. Specifically, the Co/CeO2 samples

exhibited high catalytic activity (> 90% CO2 conversion) and selectivity to CH4 (>95%) at 400-500 oC.

Interestingly, the Ni/CeO2 samples convert CO2 almost completely and exclusively to CH4 (100%

selectivity) even at temperatures as low as 280 oC. On the other hand, the addition of CuO led to a high

selectivity towards CO (> 90%), followed, however, by intermediate CO2 conversion values (~ 65% at 500oC). The present results clearly revealed that the combination of ceria nanoparticles with different transition

M&Ns-19, Paris, 17-19 July 2019 Pag. 73

metals can result to highly active and selective (either to CH4 or CO) catalysts for the CO2 hydrogenation

reaction. Further characterization studies are in progress in order the synergistic role of metal phase and

ceria support nanostructure on the reaction mechanism to be further elucidated.

Acknowledgements - This research has been co‐financed by the European Union and Greek national funds

through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call

RESEARCH – CREATE – INNOVATE (project code: T1EDK-00094).

5. References

[1] K. Chang, T. Wang, J.G. Chen, Appl. Catal. B Environ., 206, (2017) p. 704–711.

[2] M. Melchionna, P. Fornasiero, Mater. Today, 17, (2014) p. 349–357.

[3] M. Cargnello, V.V.T. Doan-Nguyen, T.R. Gordon, R.E. Diaz, E.A. Stach, R.J. Gorte, P. Fornasiero,

C.B. Murray, Science, 341, (2013) p. 771–3.

[4] G.N. Vayssilov, Y. Lykhach, A. Migani, T. Staudt, G.P. Petrova, N. Tsud, T. Skála, A. Bruix, F. Illas,

K.C. Prince, V. Matolín, K.M. Neyman, J. Libuda, Nat. Mater., 10, (2011) p. 310–5.

[5] M. Cargnello, P. Fornasiero, R.J. Gorte, Catal. Letters, 142, (2012) p. 1043–1048.

[6] M. Konsolakis, Appl. Catal. B Environ., 198, (2016) p. 49–66.

[7] S C. Sun, H. Li, L. Chen, Energy Environ. Sci., 5, (2012) p. 8475–8505.

[8] X. Yao, C. Tang, F. Gao, L. Dong, Catal. Sci. Technol., 4, (2014) p. 2814–2829.

[9] Β. Ouyang, W. Tan, B. Liu, Catal. Commun., 95, (2017) p. 36–39.

[10] S. Kattel, P. Liu, J.G. Chen, J. Am. Chem. Soc., 139, (2017) p. 9739–9754.

[11] S.-C. Yang, S.H. Pang, T.P. Sulmonetti, W.-N. Su, J.-F. Lee, B.-J. Hwang, C.W. Jones, ACS Catal.,

8, (2018) p. 12056–12066.

[12] L. Lin, S. Yao, Z. Liu, F. Zhang, N. Li, D. Vovchok, A. Martínez-Arias, R. Castañeda, J. Lin, S.D.

Senanayake, D. Su, D. Ma, J.A. Rodriquez, J. Phys. Chem. C, 122, (2018) p. 12934–12943.

[13] H. Muroyama, Y. Tsuda, T. Asakoshi, H. Masitah, T. Okanishi, T. Matsui, K. Eguchi, J. Catal., 343,

(2016) p. 178–184.

[14] T.A. Le, M.S. Kim, S.H. Lee, T.W. Kim, E.D. Park, Catal. Today, 293–294, (2017) p. 89–96.

[15] M. Lykaki, E. Pachatouridou, S.A.C. Carabineiro, E. Iliopoulou, C. Andriopoulou, N. Kallithrakas-

Kontos, S. Boghosian, M. Konsolakis, Appl. Catal. B Environ., 230, (2018) p. 18–28.

[16] V. Sharma, K.M. Eberhardt, R. Sharma, J.B. Adams, P.A. Crozier, Chem. Phys. Lett., 495, (2010) p.

280–286.

[17] P. Zhu, M. Liu, R. Zhou, Indian J. Chem., 51A, (2012) p. 1529–1537.

[18] J.-Y. Luo, M. Meng, X. Li, X.-G. Li, Y.-Q. Zha, T.-D. Hu, Y.-N. Xie, J. Zhang, J. Catal., 254, (2008)

p. 310–324.

[19] C. Tang, B. Sun, J. Sun, X. Hong, Y. Deng, F. Gao, L. Dong, Catal. Today, 281, (2017) p. 575–582.

[20] L. Bobrova, D. Andreev, E. Ivanov, N. Mezentseva, M. Simonov, L. Makarshin, A. Gribovskii, V.

Sadykov, Catalysts, 7, (2017) p. 310.

[21] S.-W. Yu, H.-H. Huang, C.-W. Tang, C.-B. Wang, Int. J. Hydrogen Energy, 39, (2014) p. 20700–

20711.

[22] A. Aboukaïs, M. Skaf, S. Hany, R. Cousin, S. Aouad, M. Labaki, E. Abi-Aad, Mater. Chem. Phys.,

177, (2016) p. 570–576.

[23] S. Jampa, K. Wangkawee, S. Tantisriyanurak, J. Changpradit, A.M. Jamieson, T. Chaisuwan, A.

Luengnaruemitchai, S. Wongkasemjit, Int. J. Hydrogen Energy, 42, (2016) p. 5537–5548.

[24] P. Zhao, F. Qin, Z. Huang, C. Sun, W. Shen, H. Xu, Catal. Sci. Technol., 8, (2018) p. 276–288.

[25] P. Maitarad, J. Han, D. Zhang, L. Shi, S. Namuangruk, T. Rungrotmongkol, J. Phys. Chem. C, 118,

(2014) p. 9612–9620.

[26] W. Li, H. Wang, X. Jiang, J. Zhu, Z. Liu, X. Guo, C. Song, RSC Adv., 8, (2018) p. 7651–7669.

[27] Y. Liu, Z. Li, H. Xu, Y. Han, Catal. Commun., 76, (2016) p. 1–6.

[28] F. Wang, C. Li, X. Zhang, M. Wei, D.G. Evans, X. Duan, J. Catal., 329, (2015) p. 177–186.

M&Ns-19, Paris, 17-19 July 2019 Pag. 74