Hexagonal BN- and BNO-supported Au and Pt nanocatalysts in carbon monoxide oxidation and carbon dioxide hydrogenation reactions
Graphical abstract
Introduction
The on-going development of industrial technologies and associated growth in the production and consumption of energy resources have made a significant contribution to improving the quality of life, but this is accompanied with strong environmental pollution by CO and CO2 gases. The most effective ways to utilize and reduce their emissions are CO oxidation and CO2 hydrogenation based on heterogeneous catalysis. Depending on the processing conditions and the nature of catalyst, reduction of CO2 with hydrogen produces either CO, which is a valuable raw material for several chemical syntheses, or CH4. Noble metals are used in many industrial catalytic processes and they are able to dissociate molecular oxygen even at low temperatures and bind strongly to atomic oxygen and CO. These are important characteristics of effective catalysts [1], [2], [3].
Noble metal-based catalysts have shown high efficiency in CO oxidation reaction in the production of high-purity hydrogen for fuel cells via selective oxidation in the process of steam reforming, partial catalytic oxidation, and autothermal reforming of liquid fuels and hydrocarbons [4], [5]. Gold (Au) and platinum (Pt) nanoparticles (NPs) are well known as effective low temperature catalysts. Therefore, the production of dispersed supported noble metal NPs is of great interest for heterogeneous catalysis [6], [7], [8]. Au and Pt catalysts are used in chemical industry for processing hydrocarbons and polymer production [9]. Noble metal NPs-based catalysts have been commercially developed and used in systems for gas purification from CO, including gas masks [10].
The properties of various catalytic systems strongly depend on a support of catalytically active materials [11]. Such support can affect various chemical reactions while accelerating some and slowing down or even stopping the others. The support can change the dispersion of catalytically active NPs and their thermal stability by forming chemical bonds with the active material. This prevents nanoparticle agglomeration and changes the chemical properties of the active material through chemical bonds polarization. In recent years, hexagonal boron nitride (h-BN), which can be obtained in various morphologies with a high specific surface area, has been regarded as a promising support material. It has been shown that defects in h-BN structure (B and N vacancies, as well as N atoms substituted by B atoms) contribute to the dissociation of hydrogen molecules [12]. The chemisorption of CO molecules on h-BN defects can be accompanied by a charge transfer and can lead to narrowing of the band gap [13]. BN surface defects result in the polarization of electron density in the catalytically active particles, thereby improving their functional characteristics [14], [15]. Theoretical investigations indicate that metal atoms interact with h-BN surface defects; this prevents their aggregation [16]. Moreover, metal atoms embedded into h-BN surface effectively activate CO molecules. This leads us to consider that a metal/h-BN system could be a potentially effective catalyst for carbon monoxide oxidation.
Experimental and theoretical studies have also shown that the morphologies of catalyst and support both affect the reactivity of oxygen and the metal-support interactions. This is manifested in the catalyst activity. For example, it has been shown that the shape and size of catalyst NPs and nanostructured features of the support can significantly affect the oxygen activity, which is explained by different types of defects and the presence of isolated or clustered vacancies [17]. It has also been demonstrated that the adsorption of СО2 changes depending on the crystallographic orientation of the catalytically active NPs; this affects the desorption of the final products and conversion of CO [18].
Available data indicates that the usage of h-BN as a support can increase the catalyst activity compared to commonly used supports. Due to their layered structure, h-BN nanosheets enable a high-density attachment of metal NPs leading to an enhanced catalytic activity [19], [20], [21]. As a result of redistribution of electron density, defect-free h-BN surface possesses catalytic activity under interactions with metal NPs [22]. The presence of vacancy defects over h-BN surface can additionally enhance metal-support interaction via electron donor/acceptor mechanisms [23]. This can significantly improve the activity of a catalyst.
Theoretical studies have shown a high efficiency of metal nanoparticle catalysts supported on h-BN nanostructures in CO oxidation process [14], [24]. BN had usually been considered as a chemically inert material until its unusual behavior was discovered during catalysis. Catalytic activity of pure h-BN nanosheets was explained by the presence of oxygen-terminated “armchair” BN edges that act as catalytic active sites [25]. Due to high oxidation resistance of h-BN, it is difficult to form B-O active sites at the “armchair” edges during oxidative dehydrogenation. C-H activation to form B-O(H) active sites has been proposed to explain this mechanism [26].
A study of molecular adsorption energy on the surface of support and catalytically active phase made a great contribution to understanding the catalyst efficiency. The dissociations of small molecules onto graphene, h-BN, BN-doped graphene, and C-doped h-BN using density functional theory (DFT) were studied and a significant difference in dissociative adsorption energies of H2, methane, water, and methanol on the surface of these materials was documented [27]. Doping of pristine h-BN with carbon atoms increases the adsorption energies of molecules, wherein the reactivity toward non-polar adsorbates increases even more significantly.
In this study, Au and Pt NPs on the surface of two types of h-BN nanosheets (reduced and oxidized) were fabricated and thoroughly analyzed as promising catalysts in carbon monooxide oxidation and carbon dioxide hydrogenation reactions. Oxygen activation mechanisms and metal-support interactions are then discussed. The obtained materials are considered as universal catalysts in the CO oxidation and CO2 hydrogenation reactions.
Section snippets
Catalyst support fabrication
BN nanoparticles were produced via plasma-chemical synthesis using boron trichloride ("Plazmotherm", Russia). The nanoparticle size was 20–50 nm. The particles were organized in stacks made of 10–15 atomic BN layers. The specific surface area of BN nanoparticles measured by the BET method was 203 m2/g. The obtained material was purified from the possible secondary phases under annealing at 1500 °C for 2 h in vacuum. This type of BN nanoparticles is denoted as BNNPs. The half of annealed BNNPs
Structural characterization
FTIR spectra of BNNPs (annealed in vacuum at 1500 °C for 2 h) and BNNPsOx (annealed in air at 1100 °C for 2 min) samples are shown in Fig. 1a. The spectra reveal two main features: broad bands in a range of 781–808 cm−1, corresponding to out-of-plane B-N-B bending (R mode), and 1358–1379 cm−1, corresponding to in-plane B-N stretching vibrations (LO mode) [35]. In high-purity crystalline BN, stretching of h-BN network in tangential directions can lead to the appearance of additional adsorption
Discussion
Our experimental and theoretical results indicate that h-BN supported Pt NP catalysts exhibit high catalytic activity in CO oxidation and CO2 hydrogenation reactions. In addition, the oxidative state of BN support affects the catalytic activity. To understand the observed difference in the material catalytic behavior, several factors should be considered: (i) type of catalytically active centers and their adsorption capacities, (ii) nanoparticle size and distribution over the support surface,
Conclusions
Au/h-BN(O) and Pt/h-BN(O) nanohybrids have been fabricated using an impregnation method on plasma-chemical synthesized h-BN supports and analyzed as promising catalysts in carbon monooxide oxidation and carbon dioxide hydrogenation reactions. The important findings are:
- 1)
At the same metal loading doze of 4 wt%, nanoparticle dispersion in the Pt- and Au-based catalysts is different. In the Au-based materials, a broad particle size distribution is observed; the particle size is 2–14 nm (Au/h-BN)
CRediT authorship contribution statement
Andrey M. Kovalskii: Conceptualization, Methodology, Investigation, Formal analysis. Ilia N. Volkov: Conceptualization, Investigation, Visualization. Nikolay D. Evdokimenko: Investigation, Visualization. Olga P. Tkachenko: Investigation, Visualization. Denis V. Leybo: Investigation, Visualization. Ilya V. Chepkasov: Methodology, Validation. Zakhar I. Popov: Methodology, Validation. Andrei T. Matveev: Investigation, Visualization. Anton Manakhov: Data curation, Formal analysis. Elizaveta S.
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 gratefully acknowledge the financial support from the Russian Science Foundation (Agreement No. 20-79-10286) in the part of catalyst fabrication, characterization, and testing in CO2 hydrogenation reaction and the Ministry of Education and Science of the Russian Federation (Increase Competitiveness Program of NUST "MISiS" No. K2-2020-009) in the part of theoretical calculations. I.N.V. thanks the Russian Foundation for Basic Research (Agreement No. № 20-33-90070\20) in the part of
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