Investigating the dynamic structural changes on Cu/CeO2 catalysts observed during CO2 hydrogenation
Graphical abstract
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 for the DP and 1.62 mg/h 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)
- et al.
Hydrogenation of CO2 to methanol over Pd–Cu/CeO2 catalysts
Mol. Catal.
(2017) The role of Copper-ceria interactions in catalysis science: recent theoretical and experimental advances
Appl. Catal. B
(2016)- et al.
Ceria-based catalysts for the production of H2 through the water-gas-shift reaction: time-resolved XRD and XAFS studies
Top. Catal.
(2008) - et al.
Low-temperature water–gas shift reaction over supported Cu catalysts
Renew. Energy
(2014) - et al.
Structure sensitivity of the low-temperature water-gas shift reaction on Cu–CeO2 catalysts
Catal. Today
(2012) - et al.
Ag addition to CuO-ZrO2 catalysts promotes methanol synthesis via CO2 hydrogenation
J. Catal.
(2017) - et al.
Mythen detector system
Nucl. Instrum. Methods Phys. Res., Sect. A
(2003) - et al.
TPR and TPD studies of CuO/CeO2 catalysts for low temperature CO oxidation
Appl. Catal. A
(1997) The Wagner plot and the Auger parameter as tools to separate initial- and final-state contributions in X-ray photoemission spectroscopy
Surf. Sci.
(2013)- et al.
Induced changes in ceria by thermal treatments under vacuum or hydrogen
J. Solid State Chem.
(1987)