Elsevier

Applied Catalysis A: General

Volume 572, 25 February 2019, Pages 61-70
Applied Catalysis A: General

Correlation between the physicochemical properties and catalytic performances of micro/mesoporous CoCeOx mixed oxides for propane combustion

https://doi.org/10.1016/j.apcata.2018.12.026Get rights and content

Highlights

  • Micro/mesoporous CoCeOx catalysts are prepared by double template combining sol-gel method.

  • Catalytic activity for propane total oxidation follows: Co1Ce1 > Co4Ce1 > Co3O4 > Co1Ce4 > CeO2.

  • A line correlation has been established between the rsurf and the sum of the surface Co3+ and Oads concentrations.

  • Langmuir–Hinshelwood model is proposed for propane combustion over the CoCeOx catalysts.

Abstract

A series of micro-mesoporous CoCeOx catalysts have been successfully prepared using a double template combining sol-gel method. The catalytic performance of novel CoCeOx catalysts are investigated and compared with pure Co3O4 and CeO2 catalysts for total oxidation of propane. It is found that the Co1Ce1 catalyst shows the highest catalytic activity (T50 = 217 °C) as well as a good reaction stability and water tolerance among the five catalysts. The Co3O4, CeO2 and CoCeOx catalysts are characterized using XRD, BET, Raman, XPS, H2-TPR, O2-TPD, HRTEM, HAADF-STEM and in situ DRIFTS. The results demonstrate that the larger BET surface area, smaller grain size, stronger reducibility and more active oxygen species of the Co1Ce1 catalyst are responsible for its outstanding catalytic performance among the five catalysts. Moreover, the synergistic effect between Co and Ce over CoCeOx catalysts are probably relevant to the formation of CoxCe1−xO2−σ solid solution. In addition, on the basis of the results of XPS, kinetic analysis and in situ DRIFTS, the surface Co3+ ions and active oxygen species are regarded as the major active sites of the CoCeOx catalysts for total oxidation of propane, and then a scheme of reaction model based on Langmuir-Hinshelwood mechanism is suggested at last. It can be expected that the micro-mesoporous CoCoOx catalysts are promising materials for VOCs removal, and the results in this research may also provide some new insights into the catalyst design and mechanism exploration for VOCs catalytic oxidation.

Introduction

Volatile organic compounds (VOCs) emissions from stationary and mobile sources often results in serious environmental problems like photochemical smog, ozone depletion, haze weather and greenhouse effect [1,2]. As one of light alkane VOCs, propane is commonly used as industrial chemicals and motor vehicle fuels. On the one hand, oxidative dehydrogenation of propane (ODHP) represents an economic and alternative process to produce propene, which is an important raw material in the modern chemical industry. On the other hand, as one of the principal components in liquefied petroleum gas (LPG), compressed natural gas (CNG) and liquefied natural gas (LNG), propane emission from the automotive exhaust source has been concerned in many countries. So its emission control is currently one of the most urgent and compelling problems for improvement of environment. Recently, catalytic oxidation is the common technology for both dehydrogenation of propane to propene and total combustion of propane to CO2. Compared with partial oxidation, propane total oxidation usually needs a higher energy and stronger active oxygen species to participate in reaction. It remains a great challenge to seek a suitable catalyst to achieve high conversion of propane at relative low temperatures.

Supported noble metals (Pd, Pt and Rh) exhibit excellent low-temperature efficiency for VOCs (toluene, formaldehyde, methane etc.) combustion. However, the high costs and easily sintering and aggregation restrict their extensive application for VOCs control [3,4]. Nowadays, a series of transition-metal-oxides catalysts have been developed and shown good catalytic performance for VOCs combustion. Bai et al. found that 3D-MnO2 with abundant surface-adsorbed oxygen species exhibited good catalytic properties for ethanol oxidation [5]. He et al. synthesized mesoporous CuCeOx catalysts via a simple self-precipitation protocol for toluene combustion. The T90 of toluene conversion over Cu0.3Ce0.7Ox reached 212 °C with the GHSV of 36,000 h−1 [6].

Among the various metal oxide catalysts, Co3O4 shows excellent catalytic performance for oxidation of CO and total oxidation of hydrocarbons [[7], [8], [9]]. In the Co3O4 spinel structure, one-eighth of the available tetrahedral sites are occupied by Co2+, and half of octahedral sites are occupied by Co3+. Co3+ sites with low Cosingle bondO bond energy and high capability to activate oxygen has been considered as the major active sites for CO oxidation over Co3O4 [10]. In addition, several paper found that the oxidation activity could also be influenced by morphology and particle size of Co3O4. Xie et al. prepared Co3O4 nanorods with a diameter of 10–20 nm and mainly exposed (110) planes, and found they could catalyse CO oxidation to CO2 at temperature as low as −77 °C [11]. Benjamin et al. reported that bulk cobalt oxide with small crystallite size could catalysis the complete conversion of C3H8 to CO2 down to 250 °C, but subjected to deactivation with time-on-stream [12]. Furthermore, abundant oxygen vacancies on catalysts surface are also believed as a precondition of high activity and stability for VOCs catalytic oxidation [13,14]. Therefore, introducing another metal element into Co3O4 is an common way to increase the amount of oxygen detects and improve catalytic performance [[15], [16], [17]].

Recently, CeO2 has been commonly used as main active ingredient or catalytic promotor for VOCs oxidation because of its non-toxic, high oxygen storage capacity (OSC) and remarkable oxygen mobility [6,18,19]. There are two potential advantages for incorporation CeO2 into Co3O4 catalyst for the enhancement of oxygen vacancies amount. On the one hand, the bond length of Cesingle bondO in cubic fluorite CeO2 (2.343 Å) is longer than that of Co2+–O (1.897 Å) and Co3+–O (1.971 Å) in Co3O4 spinel. So the oxygen vacancy on CeO2 may form more easily than Co3O4. On the other hand, Ce cation with a larger radius can distort the structure and generate Cesingle bondOsingle bondCo units, which can lead to chemical state change and new oxygen vacancy formation [20]. Therefore, the design of Cesingle bondCo composite catalyst is a good way for enhancing activity and stability of propane combustion. However, the product selectivity of propane oxidation should be further considered. Besides propane combustion, CeO2 has also been chosen as a support for the oxidative dehydrogenation of propane reaction [21,22]. Although the two reactions both need break the Csingle bondH bond of the propane further quantity, higher oxidation capacity and larger contact area of active oxygen species are essential for total oxidation than ODHP [23]. In short, it is expected that highly active Co-Ce catalyst should be metastable nanocrystalline oxide with plentiful cavity construction, high surface area, small grain size and remarkable redox property.

As is well known, preparation method can greatly affect the structure and properties of the target catalysts. Recently, a general method for the synthesis of mesoporous materials with thermally stable, crystalline and monomodal pore size had been developed by Suib et al. [24]. Preparation of such materials involved the use of inverse micelles, sol-gel chemistry, NO3 decomposition and calcination control. Various mesoporous metal oxides (Mn2O3, Co3O4, CuO etc.) with tunable porosity and crystallinity crystalline had been successfully synthesized with nitrates as precursors, P123 as the template and n-butanol as solvent [[25], [26], [27]]. So it is also a promising method for mixed traditional metal oxides preparation. On the other hand, hierarchical porous materials has attracted widespread interest in the potential application of catalysis, gas separations, energy storage because of highly porous, large surface area and abundant accessible space. Several researchers have found that dual-templating method is a feasible strategy for hierarchical material synthesis [28]. In general, anionic surfactants like CTAB, TTAB and SDS have been widely used for the synthesis of microporous materials, while nonionic surfactant like Pluronic P123 is beneficial to the formation of mesoporous materials [29,30]. Therefore, One way to get the hierarchical porous mixed oxides is adding anionic surfactants and multi-precursors to Suib’s micellar system.

Herein, micro-mesoporous CoCeOx catalysts are developed based on the modified Suib’s method by addition of SDS as a co-template. The synthesized CuCeOx oxides presents excellent catalytic activity, stability and water tolerance for total oxidation of propane. The physicochemical parameters, surface area, crystal structure, reducibility, and surface oxygen concentration of prepared catalysts are well correlated with their catalytic performance. Moreover, the potential active sites and reaction scheme for propane combustion are also proposed at last. The results in this work indicate that CoCeOx catalysts are efficient and promising porous materials for alkanes removal. What’s more, some new insights into the catalyst design and mechanism exploration may also provide referrals for VOCs catalytic oxidation.

Section snippets

Catalysts preparation

Micro-mesoporous CoCeOx catalysts were prepared by the modification of the Suib’s method for sodiumdodecylsulfate (SDS) introduction [24]. Typically, 2 g P123 (Sigma-Aldrich, Mn ˜ 5800), 0.1 g SDS (Beijing reagent, >98.5%) and the desired amount of Co(NO3)2·6H2O (Shanghai reagent, >98.5%) and Ce(NO3)6·4H2O (Sigma-Aldrich, 99.999%) with the total inorganic source (0.01 mol) were dissolved in the n-butanol firstly under magnetic stirring. Then 2 mL HCl (>36%) was added dropwise the above solution

Textural characteristics

The XRD patterns of the CoCeOx catalysts with varied Co/Ce ratios are displayed in Fig. 1. Pure CeO2 and Co1Ce4 catalysts only show a weak cubic fluorite structure (PDF-JCPDS 43-1002). No visible diffraction peaks of Co3O4 can be observed on Co1Ce4 catalyst but slight peak shift, which signifies the high dispersion of CoOx species in CeO2. It is considered that the Co ions can enter the ceria lattice because the radius of the Ce4+ cation with octahedral coordination (r = 1.01 Å) is larger than

Conclusions

A novel micro/mesoporous hybrid CoCeOx catalysts were prepared using a double template combining sol-gel method and showed a good performance for total oxidation of propane. The optimal molar ratio of Co/Ce is 1, and the activity of propane combustion follows the sequence: Co1Ce1 > Co4Ce1 > Co3O4 > Co1Ce4 > CeO2. Besides, the Co1Ce1 catalyst presents good CO2 selectivity, reaction stability and water resistance for total oxidation of propane. The excellent performance of Co1Ce1 is in connection

Competing financial interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (2016YFC0209203) and the Natural Science Foundation of Beijing (8182033).

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