Elsevier

Catalysis Today

Volume 355, 15 September 2020, Pages 737-745
Catalysis Today

Study of the effect of Gd-doping ceria on the performance of Pt/GdCeO2/Al2O3 catalysts for the dry reforming of methane

https://doi.org/10.1016/j.cattod.2019.04.079Get rights and content

Highlights

  • Pt/Al2O3 catalyst underwent severe deactivation for the dry reforming of methane.

  • The addition of CeGd mixed oxide to Pt/Al2O3 improved the stability of the catalyst.

  • The oxygen mobility of the mixed oxide inhibited the formation of carbon on Pt.

  • Catalyst with 36 wt% of CeGd and Ce/Gd ratio of 4 remained stable during reaction.

Abstract

This work studied the performance of Pt/Al2O3 and Pt/CexGd1-xO2/Al2O3 catalysts for the dry reforming of methane at 1073 K. Pt/Al2O3 catalyst underwent severe deactivation at the beginning of reaction, whereas the deactivation of Pt/Ce0.8Gd0.2/Al2O3 and Pt/Ce0.5Gd0.5/Al2O3 catalysts was less significant. Pt/36Ce0.8Gd0.2/Al2O3 catalyst remained quite stable during reaction. in situ XRD and XANES experiments showed the formation of

oxygen vacancies and Ce3+ species during the reduction of the mixed oxide, which promoted the mechanism of carbon removal from the Pt particle. In the absence of mixed oxide, carbon deposition led to the deactivation of Pt/Al2O3 catalyst. The cyclohexane dehydrogenation reaction revealed that Pt sintering is responsible for the deactivation of Pt/Ce0.8Gd0.2/Al2O3 and Pt/Ce0.5Gd0.5/Al2O3 catalysts. The stability of Pt/36Ce0.8Gd0.2/Al2O3 catalyst is likely due to the close contact between Pt and the CeGd mixed oxide because of the higher extent of alumina coverage by the mixed oxide. This promotes the mechanism of carbon removal from the surface of Pt particles and inhibits Pt particle size growth.

Introduction

With the discovery of large oil reserves located in the pre-salt area in Brazil, the oil companies have faced new issues. The challenge for the exploration of this area is related to the amount of associated gas produced and the high amount of CO2 content in this gas. Due to the restrictions of environmental laws, this gas cannot be vented or burned on the platform because it would release large amounts of CO2 into the atmosphere, contributing to the greenhouse effect and causing environmental damage. Furthermore, the costs associated with the separation of CO2 from natural gas stream are high. Therefore, it is important to find an alternative to the use of this associated gas to overcome these issues related to the oil exploration of the pre-salt reserves [1,2].

Like the associated gas, biogas, generated from the anaerobic digestion of biomass, also presents high levels of CO2, resulting in emissions into the atmosphere that also contribute to the increase of the greenhouse effect. As only a small part of this gas is used as a source of heat and energy through combustion processes, the development of alternative processes for the use of this gas would be of significant interest [2].

A promising alternative to the use of this gaseous mixture containing high CO2 content would be its conversion into synthesis gas (mixture of H2 and CO) by CO2 reforming of methane, the so-called dry reforming of methane (DRM). Synthesis gas can be converted into ammonia and methanol, as well as H2 and liquid fuels through Fischer-Tropsch synthesis [3]. In addition, the synthesis gas can also be used to directly feed the solid oxide fuel cells to generate energy [2].

The dry reforming of methane has been widely studied for the production of synthesis gas [[4], [5], [6], [7]]. Among the advantages of this reaction are the availability and low cost of the reagents and the use of two of the greenhouse gases as reagents [8]. One of the major challenges of this technology is related to the deactivation of the catalyst that occurs mainly due to carbon formation during the process at high temperature [[9], [10], [11]]. Therefore, the development of a catalyst resistant to carbon deposition during dry reforming of methane is crucial.

The carbon deposition on this reaction depends on the type of metal used. Some transition metals, such as Ru, Rh, Pt, Pd and Ni, are active for the CO2 reforming of methane and, according to the literature, the extent of carbon formation on these metals is related to their activity for CO2 dissociation [6]. For example, metals such as Rh and Ru have a high activity for CO2 dissociation, with no significant carbon formation during the reaction. However, the cost of these metals is very high. Pt and Pd metals are not as efficient at dissociating CO2 and, in this case, the use of a support is extremely important to avoid carbon deposition.

Several studies report the use of cerium oxide as catalyst support for the dry reforming of methane due to its redox properties and the high mobility of oxygen, which promotes the mechanism of carbon removal from the metal surface [2,6,12]. The addition of dopants promotes the oxygen mobility due to formation of a solid solution, improving the carbon removal [2,13,14]. Zirconia has been largely studied as ceria dopant on Ni-based catalysts for methane reforming reactions [7,13,15]. However, there are only a few papers investigating the effect of the type of the ceria dopant on the performance of catalyst for methane reforming reactions. Gaudillère et al. [16] compared the performance of Ni supported on ceria doped with Zr, Pr, and Gd for the combined reforming of methane at different temperatures. At 600 °C, the Gd-doped catalyst exhibited higher activity than the catalyst containing Zr. However, one of the main disadvantages of ceria and ceria-mixed oxides is the low surface area, which leads to low metal dispersion and then favors the deposition of carbon. One approach to increase the surface area of ceria and ceria-mixed oxides is to deposit them over a high-surface area oxide such as alumina [[17], [18], [19]]. Faria et al. [7] studied the performance of Ni/Al2O3 and Ni/CexZr1−xO2/Al2O3 (x = 0.5; 0.75; 1.0) catalysts for the DRM. Ni/Ce0.75Zr0.25O2/Al2O3 and Ni/Ce0.50Zr0.50O2/Al2O3 catalysts exhibited the lowest amount of carbon formed, which was attributed to the higher oxygen storage/release capacity of the ceria-zirconia mixed oxides. However, there are no works in the literature about the performance of Gd-doped ceria supported on alumina as catalyst support for the DRM.

Therefore, the aim of this work is to study the performance of Pt supported on cerium oxide doped with gadolinium deposited on alumina for DRM. The samples were characterized by in situ X-ray diffraction (XRD), in situ X-Ray absorption spectroscopy (XAS), dehydrogenation of cyclohexane reaction and cyclohexanol dehydration. The reasons for catalyst deactivation were investigated.

Section snippets

Catalysts preparation

The alumina was prepared by calcination of bohemite (Puralox, Condea) at 1073 K for 6 h in a muffle furnace. The samples were prepared with 24 wt.% of ceria mixed oxides in order to cover the alumina surface and Ce/Gd ratios of 4 and 1. One sample was synthesized with a larger amount of ceria mixed oxide (36 wt.%) and this was indicated in the nomenclature of the sample. For the supports, the alumina was co-impregnated with an aqueous solution of cerium (IV) ammonium nitrate and the dopant

Catalyst characterization

Table 1 lists the values of BET surface areas obtained for the supports and catalysts. A large decrease in the alumina area (163 m2/g) was observed with the addition of mixed oxides, which is likely due to the blockage of alumina pores by the deposition of doped cerium oxides [24,25]. The reduction of the surface area was more significant for the support with higher content of CeGd (36 wt.%). However, no significant changes were observed in the surface areas of the supports with the addition of

Conclusions

This work studied the deactivation of Pt/Al2O3 and Pt/CexGd1-xO2/Al2O3 catalysts during DRM reaction. Pt/Al2O3 catalyst strongly deactivated at the beginning of DRM reaction, whereas Pt/Ce0.8Gd0.2/Al2O3 and Pt/Ce0.5Gd0.5/Al2O3 underwent less severe loss of activity during 24 h of TOS. Pt/36Ce0.8Gd0.2/Al2O3 catalyst remained quite stable under this reaction conditions. TPO profile of used Pt/Al2O3 catalyst after DRM revealed the formation of carbon deposits on the metal particle and support. The

Acknowledgements

The authors acknowledge the scholarship and the financial support received from CNPq (314018/2017-4) and CAPES (001). The group thanks the LNLS for the assigned time at D08B-XAFS – 2 (20171082) and XRD1-D12A (20171087) beamlines.

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