Elsevier

Journal of Catalysis

Volume 381, January 2020, Pages 415-426
Journal of Catalysis

Investigating the dynamic structural changes on Cu/CeO2 catalysts observed during CO2 hydrogenation

https://doi.org/10.1016/j.jcat.2019.11.017Get rights and content

Highlights

  • Co-precipitation method produced finer CeO2 particles than deposition-precipitation.

  • Strong correlation between CeO2 crystallite size and lattice parameter profiles.

  • Metal-support interfacial area influenced the Cu state and product selectivity.

Abstract

CO2 hydrogenation is one route for partial CO2 abatement while producing valuable chemical products. Cu/CeO2 catalysts have attracted great interest as materials that facilitate the efficient hydrogenation of CO2 into methanol and CO. This paper investigates the effect of preparation methods on the structure, function and activity of Cu/CeO2 catalysts. The catalysts have been prepared by deposition–precipitation (DP) and the co-precipitation (CP) methods. The activity tests for the catalysts were conducted in a high-pressure packed bed reactor in the temperature range of 200–300 °C at 50 bar pressure. The DP catalyst showed higher methanol productivity and lower methanol selectivity compared its CP counterpart. The reasons for the higher performance of the DP catalyst and other structural and functional differences have been investigated through in-depth characterization using synchrotron radiated in-situ powder diffraction, DRIFTS, XPS, HR-TEM, TPR and N2O decomposition used to determine copper surface area. The higher methanol productivity in the DP catalyst was attributed to the higher Cu surface area and dispersion. Additionally, the in-situ synchrotron-based powder diffraction measurements provide fundamental insights on the interesting differences in the CeO2 structure i.e. the lattice parameter and the lattice microstrain during the reduction step that could be attributed to the size difference in the CeO2 support nanoparticles.

Introduction

Increasing anthropogenic greenhouse gas emissions, particularly CO2, is considered one of the principal causes for the climate change phenomena such as global warming and rise in mean sea levels. Currently, the CO2 concentration in atmosphere is around 408 ppm; and to restrict the average global temperature rise to 1.5 °C would require 80–95% decrease of global greenhouse gas emissions by 2050 compared to 1990 levels [1], [2]. One approach to achieve this target is to close the carbon-loop by utilizing CO2 as a feedstock for the synthesis of a variety of products such as fuels, polymers and cyclic carbonates. The hydrogenation route for CO2 utilization is particularly attractive and has received tremendous attention owing to its versatility for synthesizing several products including methanol, methane, carbon monoxide, dimethyl ether, formaldehyde and higher hydrocarbons. However, due to the chemical inertness of CO2, this still remains a challenging area which requires development of novel catalytic materials for enhanced product yields and selectivity.

Cu-based catalysts have been prevalent for the CO2 hydrogenation reactions particularly for the synthesis of methanol and CO [3], [4], [5], [6], [7], [8]. The activity of the Cu(1 1 1) for both CO and methanol synthesis was further enhanced when CeO2 (or ceria) was used as the support [9]. The Cu-CeO2 interface is efficient in activating CO2 and selectively stabilized the CO2 and OH reaction intermediates for the CO2 hydrogenation to methanol with an apparent activation energy lower than Cu(1 1 1) [9], [10], [11]. Numerous studies on Cu/CeO2 catalysts report the various aspects such as the effect of the structure of CeO2, nature of interaction between Cu and CeO2 to develop fundamental insights on structure-property relationships [3], [11], [12], [13], [14], [15], [16], [17]. The structure of CeO2- crystal plane, size, morphology influence the catalytic activity by providing suitable surface configuration for CO2 activation [18]. Moreover, the reduction of Ce4+ to Ce3+ results in varying activity due to strong metal-support interaction (SMSI) effect [17].

There have been several investigations on the structural evolution of CeO2 supported materials [13], [14], [19], [20], [21], [22], [23], [24]. The presence of the metal not only enhances the reducibility of the CeO2 support but also cause perturbation in the electronic properties of the metal and the metal-oxide interface possibly resulting in synergistic effects [3]. CeO2 when heated under reducing conditions such as H2 or CO undergoes lattice expansion caused due to the formation of Ce3+ [14], [24]. The degree of lattice expansion of CeO2 was found to vary as a function of the CeO2 morphology [13], [14]. Some of factors influencing the lattice expansion observed for the CeO2 samples of differing morphologies include reduction of CuO, crystal size, specific surface area of contact, presence of surface defects [14]. However, a few other studies on Cu/CeO2 catalysts suggest that the changes in the lattice parameter could be related to the incorporation of the Cu2+ into the CeO2 lattice leading to the formation of a mixed oxide of the form Ce1-xCuxO2 [21], [22], [23].

The defects and imperfections in ceria leads to interfaces with Cu that are active reaction sites [12], [18]. The nature of defects produced can be dependent on preparation method of the catalyst. Prasad and Rattan [25] reviewed the various preparation methods employed for the Cu/CeO2 catalysts. Wet impregnation methods are most commonly reported, however, precipitation based methods are generally preferred owing to higher Cu dispersion rates [26]. Some reports suggest the co-precipitation method was found to be suitable for water gas shift reactions due it its lower CeO2 size and reducibility [27], [28], [29]. Si et al. [29] determined the case of deposition–precipitation method, CeO2 must be nanoscale (<7nm) and oxygen defect rich to enable nucleation and stronger binding of copper oxide clusters. However, there is a very limited understanding on the role of preparation methods have on the nature of interaction between Cu and CeO2 in determining the activity of a specific reaction. The understanding of these aspects is limited to only a few systematic and comprehensive investigations [13].

In this publication, we aim to understand the influence of the preparation method of Cu/CeO2, particularly the role of CeO2, on the structure-activity relationships during CO2 hydrogenation. Through the use of in-situ synchrotron radiated powder diffraction measurements and DRIFTS we aim to understand the structural and the functional changes occurring during the catalyst reduction and reaction phases. The activity of the catalysts is linked to the structural and functional changes while reconciling the insights from the TPR, XPS and TEM analysis. Additionally, fundamental insights on the structural differences such as crystallite size, lattice parameter and microstrain on the CeO2 observed in the catalyst have been presented.

Section snippets

Catalyst preparation

The Cu/CeO2 catalysts were prepared by two methods the co-precipitation (CP) method [30] and the deposition–precipitation (DP) [4] of the active metal on the CeO2 support prepared by the sol-gel method. Throughout this manuscript DP and CP would be used to refer to the catalysts prepared by deposition-precipitation and co-precipitation method respectively.

Co-precipitation method 100 ml solutions of 0.2 M Cu(NO3). 2.5H2O (Sigma Aldrich, Purity: 98%) and 0.2 M Ce(NO3)3·6H2O (Sigma Aldrich, Purity

Catalyst activity

The catalytic reactions have been carried out at temperatures in the range of 200–300 °C at 50 bar pressure. From Fig. 1(a), the CP catalyst shows a lower yield per unit surface area of Cu, however, the selectivity is higher than the DP catalysts at all temperatures. The maximum yields of methanol are 1.75 mg/h.mCu2 for the DP and 1.62 mg/h.mCu2 for the CP catalyst. On the other hand, the methanol selectivity at 200 °C being 60% and 40% CP and DP catalysts respectively. The methanol selectivity

Discussion

The preparation method of the catalyst is known to influence the structural and the functional properties of the catalyst which ultimately influence the catalytic performance [26]. Specifically, influence of preparation method extends to the surface area, porosity, dispersion of active component, support characteristics such as the surface composition, size and morphology and therefore metal-support interface. In this study, the DP catalysts had slightly higher Cu surface area and dispersion

Conclusions

The influence of catalyst preparation method on Cu/CeO2 catalysts prepared by CP and DP was investigated by correlating the catalytic activity for CO2 hydrogenation to their redox, structural and functional properties. The DP catalyst had higher yield, but lower selectivity of methanol compared to CP catalyst which yielded 1.74 mg/h.m2Cu of methanol at a selectivity of 10% at 300 °C and 50 bar. Even with a lower surface area and similar dispersion levels (with the Cu surface area being only

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.

Acknowledgements

The authors would like to acknowledge the financial support received from Australian Research Council through the Industry Transformation Research Hub Grant IH130100016. The authors are grateful to their team members Dr. Anurag Parihar, Dr. M.A. Kibria, Dr. Srikanth Chakravartula, Dr. Priya Samudrala for their support and guidance during this research. This research was undertaken on the Powder Diffraction beamline at the Australian Synchrotron, part of ANSTO. The XPS analysis was performed in

References (58)

  • C. Binet et al.

    IR study of polycrystalline ceria properties in oxidised and reduced states

    Catal. Today

    (1999)
  • K. Yoshikawa et al.

    Synthesis and analysis of CO2 adsorbents based on cerium oxide

    J. CO2 Util.

    (2014)
  • T. Shido et al.

    Regulation of reaction intermediate by reactant in the water-gas shift reaction on CeO2, in relation to reactant-promoted mechanism

    J. Catal.

    (1992)
  • British Petroleum Company

    BP statistical review of world energy in

    (2018)
  • A. de Pee et al.

    Decarbonization of industrial sectors: the next frontier, in

    (2018)
  • J.A. Rodriguez et al.

    Hydrogenation of CO2 to Methanol: importance of metal-oxide and metal-carbide interfaces in the activation of CO2

    ACS Catal.

    (2015)
  • O. Martin et al.

    Zinc-rich copper catalysts promoted by gold for methanol synthesis

    ACS Catal.

    (2015)
  • L.C. Grabow et al.

    Mechanism of Methanol Synthesis on Cu through CO2 and CO Hydrogenation

    ACS Catal.

    (2011)
  • M.D. Porosoff et al.

    Catalytic reduction of CO2 by H2 for synthesis of CO, methanol and hydrocarbons: challenges and opportunities

    Energy Environ. Sci.

    (2016)
  • S. Natesakhawat et al.

    Active sites and structure-activity relationships of copper-based catalysts for carbon dioxide hydrogenation to methanol

    ACS Catal.

    (2012)
  • J. Graciani et al.

    Highly active copper-ceria and copper-ceria-titania catalysts for methanol synthesis from CO2

    Science

    (2014)
  • S.D. Senanayake et al.

    Hydrogenation of CO2 to methanol on CeOx/Cu(111) and ZnO/Cu(111) catalysts: role of the metal-oxide interface and importance of Ce3+ Sites

    J. Phys. Chem. C

    (2016)
  • S. Kattel et al.

    Tuning selectivity of CO2 Hydrogenation reactions at the metal/oxide interface

    J. Am. Chem. Soc.

    (2017)
  • L. Lin et al.

    In Situ Characterization of Cu/CeO2 nanocatalysts for CO2 hydrogenation: morphological effects of nanostructured ceria on the catalytic activity

    J. Phys. Chem. C

    (2018)
  • S.Y. Yao et al.

    Morphological effects of the nanostructured ceria support on the activity and stability of CuO/CeO2 catalysts for the water-gas shift reaction

    PCCP

    (2014)
  • M. Fernández-García et al.

    Nanostructured oxides in chemistry: characterization and properties

    Chem. Rev.

    (2004)
  • J.S. Elias et al.

    Structure, bonding, and catalytic activity of monodisperse, transition-metal-substituted CeO2 nanoparticles

    J. Am. Chem. Soc.

    (2014)
  • S.-C. Yang et al.

    Synergy between ceria oxygen vacancies and Cu nanoparticles facilitates the catalytic conversion of CO2 to CO under Mild conditions

    ACS Catal.

    (2018)
  • J.A. Rodriguez et al.

    Water-gas shift reaction on a highly active inverse CeOx/Cu(111) catalyst: unique role of ceria nanoparticles

    Angew. Chem. Int. Ed.

    (2009)
  • Cited by (0)

    View full text