Nature of support plays vital roles in H2O promoted CO oxidation over Pt catalysts
Graphical abstract
This work provides new insight into the mechanism of H2O promotion on CO oxidation, revealing hints for industrial flue gas purification.
Introduction
Carbon monoxide is a major atmospheric pollutant emitted from the incomplete combustion of fossil fuels in various industrial processes, such as steel production [1], [2]. Catalytic oxidation has been recognized as an effective and energy-saving strategy for removing CO from flue gases [3]. For sintering flue gas, the catalytic oxidation reactor can be suggested to installed between desulfurization and SCR denitrification procedures, challenging the catalytical activity in the sub 250 °C with percentage level of H2O and ppm level of SO2. In the past decades, numerous catalysts have been developed for CO oxidation. Non-noble metal components, including Cu, Mn, and Ce, showed extremely severe sensitivity to H2O and SO2 [4], [5]. Among noble metals, Au has attracted most attention due to its excellent CO oxidation activity [6], [7], [8], whereas the drawbacks of low thermal stability and poor sulfur resistance greatly restrain its applications in industrial plants [9], [10]. In contrast, the Pt catalyst shows outstanding CO oxidation performance in simulated industrial flue gas, and it has been recognized as the most promising candidate for industrial CO purifications [11], [12], [13].
Many reports revealed that H2O can improve CO oxidation on noble metals [14], [15], [16]. Therefore, the ultilization of H2O in practical flue gases to enhance the catalytic activity is of theoretical and practical importance. In studies on H2O promoted CO oxidation over Au catalysts, it was generally believed that H2O enhanced CO oxidation by continuously supplementing interfacial hydroxyl groups through H2O dissociation [17], [18]. Isotopic and kinetic studies showed *OOH as critical intermediate on nanosized novel metals at low oxygen level [19]. For Pt catalysts, H2O has been reported to promote CO oxidation by directly participating in CO2 formation via the Mars-van Krevelen (MvK) mechanism in single atom catalyst [20], which was proven by the isotope-labeling experiments [21]. The directly participating promotion mechanism of H2O is similar to that of the Au catalytic system, which well demonstrates the reaction process using a single atom catalyst model [16]. However, for nanoparticle catalysts, which are more practical considering industrial applications, systematic understanding of interactions between H2O and metal support sites needs further exploration.
In previous studies, much attention has been paid to dissociated OH from H2O, and the formation of COOH from OH and CO [16], both of which focused on the reactive behavior of H2O, while the role of support was neglected. It is noteworthy that the catalyst support plays vital roles in H2O migration and transformation: (1) H2O is first adsorbed on the support surface and then dissociates to OH and H, and the latter bonds with the lattice O to form the lattice OH. (2) The dissociated OH group is transferred from the support surface onto the Pt nanoparticle to react with adsorbed CO and form COOH, the oxygen vacancy left behind is filled by O2 to accelerate. In both processes, the nature of the support is crucial to the reaction path, which eventually determines the intrinsic and apparent results. Therefore, it is important to investigate H2O promoting effect by regulating the nature of the support, which will guide the design and optimization of CO oxidation catalysts for industrial applications.
Herein, we report the enhancement of activity in H2O promoted CO oxidation on Pt nanoparticle catalysts. Among conventional supports, anatase TiO2 is commonly used in environmental catalysis due to its chemical stability and resistance to poisoning [12], [13], [22], and SiO2 exhibits the advantages of large surface area, thermal and mechanical stability, and rich surface hydroxyl groups [23], [24], [25]. Accordingly, TiO2-SiO2 composite oxides were prepared as supports, and their physicochemical properties could be regulated by varying the composition of the supports [26]. The 18O-substituted CO2 from isotopic oxygen experiments directly confirmed the H218O anticipated pathway, and the C16O18O + C18O18O ratio could be controlled by changing the Ti/Si molar ratio. Density functional theory (DFT) calculations revealed that the generation of OH from H2O dissociation and the breakup of M−OH on the support surface was easier in TiO2 than in SiO2, which was consistent with the stronger H2O promoting effect of TiO2 and a higher substituted CO2 ratio.
Section snippets
Catalyst preparation
The TiO2/SiO2 composite supports were prepared using a co-hydrolyzation method [26]. Nitric acid (HNO3, 1 mol/L) was added to the tetrabutyl orthotitanate (Ti(OC4H9)4, Aladdin) in 60 °C water bath. After stirring for 1 h, tetraethyl orthosilicate (Si(OC2H5)4, Macklin) was added dropwise to the previous solution (molar ratio of Ti(OC4H9)4: Si(OC2H5)4 = 3:1, 1:1, 1:3, 1:5, 1:10). The mixture was kept stirring at 60 °C for 24 h to make sure both esters were fully hydrolyzed. Collected by multiple
Preparation and characterization of catalysts
TiO2, a series of TiO2-SiO2 composite supports with a gradient Ti-Si molar ratio, and SiO2 were synthesized by hydrolysis of tetrabutyl orthotitanate and tetraethyl orthosilicate. Correspondingly, a series of supported Pt catalysts (denoted as Pt/Ti, Pt/3Ti-1Si, Pt/1Ti-1Si, Pt/1Ti-3Si, Pt/1Ti-5Si, Pt/1Ti-10Si and Pt/Si, respectively) were prepared by the impregnation method. The loading amount of Pt was 0.1 wt% considering the cost in industrial applications (Table S1). Detailed procedures are
Conclusions
In this work, the promotion effect of H2O in CO oxidation was studied on Pt/TiO2-SiO2 catalysts. Pt/1Ti-3Si had the best oxidation activity with or without H2O, which was attributed to the synergistic effect due to the combination. The increase in activity was related to the dissociation of H2O on TiO2, which was greatly affected by the nature of the support regulated by composition variation. The physicochemical properties of Pt/TiO2-SiO2 were characterized, showing that the facial defects and
CRediT authorship contribution statement
Yutao Hu: Writing – original draft, Visualization, Conceptualization, Methodology, Data curation. Xiaolong Liu: Conceptualization, Writing – review & editing, Supervision, Project administration, Funding acquisition. Yang Zou: Methodology, Validation. Haijiao Xie: Software. Tingyu Zhu: Supervision, Project administration, Funding acquisition.
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
This work was supported by the National Natural Science Foundation of China (51938014, 52170118).
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These authors contributed equally to this work.