Influence of Ce addition and Pt loading upon the catalytic properties of modified mesoporous PtTi-SBA-15 in total oxidation reactions
Graphical abstract
Introduction
Cerium oxide has been intensively studied, as catalyst or support, in various reactions such as the oxidation of CO [[1], [2], [3]], methane [[4], [5], [6], [7], [8], [9]] and other light hydrocarbons [[10], [11], [12]] thus having many practical applications for the pollution abatement [[12], [13], [14]]. Also, due to its unique property of storing and releasing oxygen provided by the redox couple Ce4+/Ce3+, ceria is a promoter and reducible support in automotive catalysts [6,15]. These properties also help the thermal resistance of the supports, dispersion of the supported metals, oxidation and reduction of supported noble metals and facilitate the decrease of coke formation on the catalyst surface [12,16]. Besides, cerium oxide can dramatically enhance the activity of some transition metal oxides [15,[17], [18], [19], [20]].
On the other side, mixing ceria-titania oxides led to effective catalytic systems with physicochemical and electronic properties unobserved in the individual components. Thus, ceria stabilizes the surface of titania [[20], [21], [22], [23], [24], [25]] which results in an enhanced thermal stability. Moreover, many reports on this mixed oxide evidenced an increase of the Ce3+ concentration relative to Ce4+ and more oxygen vacancies, i.e. active sites, as a result of a higher dispersion of ceria on the support [6,9,26]. As an effect, in many nanocomposite materials ceria have a mutual interaction with noble metals, like Pt, and other immobilized metal oxides promoting their catalytic activity and enhancing the stability [2,4,6,7,9,10,12,21]. Such a behavior is also related to a structure-sensitivity where the metal particle size influences catalytic activity. Thus, a higher activity was reported for the catalytic oxidation of methane on catalysts with larger Pd [7] or Pt [21] metal particle size. In our previous results Ti-KIT-6 and CeTi-KIT-6 were used as supports for Pt and activity of the catalysts in total oxidation of methane and CO was considered a result of metal-support interaction respectively, Pt/PtO molar ratio [21].
The effect of the dispersion of noble metals on the activity, selectivity and stability of the catalysts in the oxidation of the hydrocarbons is well documented [18]. Such reactions are limited by the C–H activation on the noble metal surface and the turnover rate increases with its dispersion. For the particular case of Pt, since its oxygen storage capacity is not influenced bydispersion, different Pt contents may affect the synergistic interaction [6,8] with the oxide support leading to different catalytic performances.
The catalytic combustion of methane and other volatile hydrocarbons present as pollutant impurities in air has been extensively studied in the last decades over noble metals and/or transition metallic oxides in view to design catalytic materials with high activity at lowest temperature [7,17]. Titania has been used as support for noble metals due to its reducible properties induced by a strong metal-support interaction (SMSI) effect [21,27]. The effect of titania addition to different mesoporous silica oxides containing supported palladium for the methane oxidation was found to depend on the type of the mesostructured material. Thus, addition of 10 wt% of TiO2 to silica SBA-15, combining the high surface area of the support and SO2 scavenger action of dispersed TiO2, improved the catalytic performance of the supported palladium catalysts compared to Pd-SBA-15, in terms of a better activity and better sulfur tolerance [28].
Based on this state, the effect of ceria and Pt loading on the activity in the total oxidation of CO and hydrocarbons was evaluated in the present work for a highly dispersed Ti-SBA-15 mesoporous support. Incorporation of the titania nanoparticles in the silica network was achieved by direct synthesis [21,29] following a modified synthesis protocol by using butyl alcohol as a co-solvent. Pt and Ce were immobilized on this support by impregnation of the precursors in two steps and different orders. These led to PtCeTi-SBA-15 and CePtTi-SBA-15 catalysts, respectively. The supports and active species properties were evaluated by different characterization techniques and correlated with catalytic activity.
Section snippets
Preparation of catalysts
Titanium-substituted mesoporous modified SBA-15 molecular sieve have been prepared by a co-solvent, ie butyl alcohol, method through direct synthesis (DS) [21] taking a BuOH/SiO2 molar ratio of 0.6. Tetraethyl-orthosilicate (TEOS) and tetrabutyl orthotitanate (TBOT) were used as a silica and titanium sources, respectively. In the first step, 2 g of amphiphilic triblock copolymer Pluronic 123 (from Aldrich) were added in 57 g water. The acidic pH was obtained with 4 g HCl 37 % solution. After 4
Characterization of materials
The ordered porous structure of samples was evidenced by X-Ray diffraction at low angles. Fig. 1 shows three well resolved diffraction peaks which can be indexed as the (100), (110) and (200) reflections associated with p6mm hexagonal symmetry. Comparing the diffraction patterns of samples from Figs. 1 and S1 it can be observed a preservation of the structure typical for a well ordered hexagonal arrangement of the mesopores in SBA-15 materials. The intensity of the peaks decreases after Pt and
Conclusions
PtCeTi-SBA-15 mesoporous catalysts with different Pt loadings Pt (0.25 %, 0.5 %, 1.0 %), were obtained via a three steps procedure. A new Ti-SBA-15 material was obtained in the first step to serve as a support for further successively immobilization. Thus, two types of catalysts were obtained: one with Pt immobilized on the supported Ti and Ce oxides and another with Ce deposited on the supported Pt and Ti species. Characterization results showed that ceria addition increased Pt dispersion,
Credit authorship contribution statement
Madalina Ciobanu: Conceptualization, Methodology, Materials preparation, Writing - Original Draft; Gabriela Petcu: Investigation, Validation, Formal analysis, Data Curation, Catalytic tests: Elena M. Anghel: Conceptualization, Raman investigation data, Writing: Florica Papa: TPR Investigation, Validation; Nicoleta G. Apostol: XPS characterization, Writing; Daniela C. Culita: N2 physorption investigation, Data validation; Irina Atkinson: XRD mesurements and their evaluation, Silviya Todorova:
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
The authors thanks to the research infrastructure developed by the project EU (ERDF) INFRANANOCHEM–No.19/01.03.2009, partially used for samples characterization. One of the authors (Nicoleta Apostol) acknowledges funding from the Romanian Ministry of Education and Research through the 75PCCDI/2018 Project.
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