Beneficial role of the nitrogen-doped carbon nanotubes in the synthesis of the active palladium supported catalyst
Graphical abstract
Introduction
Presently, carbon nanomaterials are successfully used as supports for metal and oxide nanoparticles as well as for single metal atoms [[1], [2], [3], [4], [5]]. Doping of the carbon support makes it possible to control its chemical, electronic and mechanical properties [[6], [7], [8], [9], [10], [11]]. The introduction of a heteroatom into the carbon structure leads to the formation of specific metal adsorption sites [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]]. According to theoretical calculations, a higher adsorption energy is observed when Pt interacts with the foreign atoms residing on the external surface of a carbon material X-CNTs, where X = Be, B, C, O and N [17]. Therewith, the adsorption energy decreases with increasing the radius of a carbon nanotube in the following sequence of doping atoms: O > Be > B > N > pristine. In the case of nitrogen doped carbon nanotubes (N-CNTs), more advantageous in terms of energy is the interaction of Pt with the carbon atoms residing close to the nitrogen site as compared to the direct interaction with the doping atom. Similar conclusions were made from molecular dynamics calculations for Ru particles supported on N-CNTs [16]. Indeed, intercalation of nitrogen into the carbon structure induces changes in the electron-donor properties of adjacent carbon atoms [21]. Thus, experimental X-ray photoelectron spectroscopy (XPS) and EDX-mapping data revealed the presence of a strong interaction between nitrogen-containing sites and PtRu nanoparticles [18].
The effect of doping on the properties of metal‑carbon materials may be caused [8,24] by: i) changes in the rates of nucleation and growth of supported particles, which can produce a more narrow particle-size distribution and a decrease in their average size; ii) modification of the electronic structure of the metal and, accordingly, its catalytic behavior; and iii) changes of hydrophilic-hydrophobic properties of the material, which can affect the interaction with reagents during the catalytic process.
It is known that the use of nitrogen-doped carbon nanomaterials for stabilization of metallic particles makes it possible to improve functional properties (activity, stability and selectivity) of the catalysts in many reactions such as oxidation of organic compounds or CO, decomposition of formic acid, hydrogenation of acetylene or nitroarenes, etc. [10,18,22,23,[25], [26], [27], [28]]. Hydrogenation of aromatic nitrocompounds to amines is represented by important industrial reactions for which the advanced and efficient processes meeting the requirements of sustainable chemistry should be developed. Various functionalized amines are employed for the production of dyes, medicinal preparations, corrosion inhibitors, anti-knock additives to gasoline and motor fuels, etc. Nanoparticles of noble metals deposited on oxide and carbon supports are known to be highly active and selective in hydrogenation of nitrocompounds, thus lowering the content of metals and minimizing the detrimental action on the environment [[29], [30], [31], [32], [33]]. Therewith, the role of support, as in other reactions, includes not only stabilization of metal nanoparticles but also modification of their electronic properties. In this connection, the use of nitrogen-doped carbon supports in the synthesis of metal catalysts for hydrogenation of nitrocompounds is of research and applied interest. For example, it was shown in [34] that selectivity of the 1%Pt/N-CNTs catalyst in hydrogenation of nitrobenzene to aniline at 5 atm attains ~100% due to formation of the electron-rich chemical state of Pt caused by the interaction of platinum with the nitrogen sites of N-СNTs. The use of a Pd catalyst supported on N-СNTs also seems promising because palladium nanoparticles substantially change their catalytic properties in this reaction owing to the interaction with N-CNTs, as was demonstrated in [28,[35], [36], [37]]. It should be noted that very often this reaction is performed under high pressure although the use of ambient conditions is more favorable from a practical point of view.
Our work was aimed to show the beneficial role of the N-CNTs used as a support for the synthesis of active palladium catalyst for nitrobenzene hydrogenation reaction at atmospheric pressure. The substantially higher activity of the ~1.5 nm Pd particles compared to isolated palladium ions in Pd/N-CNT catalyst was discovered and referred to the metallic state of Pd nanoparticles stabilized by nitrogen species of N-CNTs.
Section snippets
Materials preparation
The synthesis of carbon nanotubes (СNTs) and N-CNTs was performed by a technique reported in [38], at a temperature of 700 °C using the 62%Fe-8%Ni-30%Al2O3 catalyst. The CNTs were synthesized by decomposition of 100% ethylene, while N-CNTs – by decomposition of a 40%С2H4/60%NH3 mixture. The synthesized CNTs and N-CNTs were treated with concentrated HCl during 7 days at room temperature, then boiled in 2 M HCl for 6 h and washed with distilled water until no chloride ions were detected in the
Properties of CNTs and N-СNTs
The CNTs are represented mostly by multiwalled carbon nanotubes with parallel walls, whereas the N-CNTs have a typical bamboo-like structure (Fig. 1).
According to XPS, both supports correspond to graphite-like sp2 carbon and the nitrogen incorporation into carbon structure is confirmed by the shift of the C1s peak from 284.4 eV (CNTs) to 284.7 eV (N-CNTs) and its broadening at the high energy side [38]. The washing of the tubes to remove the catalyst did not lead to the oxidation of carbon,
Discussion
In this work, CNTs and N-CNTs have been studied as Pd (0.2–2 wt%) catalyst supports for hydrogenation of nitrobenzene at atmospheric pressure. The 2%Pd/N-CNTs catalyst differing in the electronic state of palladium and particle size was found to be the most active in this reaction compared to 1%Pd/N-CNTs, 0.2%Pd/N-CNTs and nitrogen free catalysts. As we have already discussed in [44], the capacity of N-doped CNTs to stabilize ions on the surface of the support is limited to ~0.8 wt%, which may
Conclusions
A comparison of palladium catalysts supported on CNTs and N-СNTs by impregnation and reductive deposition techniques was made. In the case of N-СNTs, the formation of stable isolated palladium ions does not depend on the deposition method and is determined only by the presence of defects comprising carbon vacancies and pyridinic nitrogen centers in the graphite layer. The isolated palladium ions were found not to be encapsulated by a carbon shell, but nevertheless they showed low activity in
Acknowledgements
This work was supported by the Ministry of Science and Higher Education of the Russian Federation (project АААА-А17-117041710084-2). The HAADF-STEM study was supported by Spanish MINECO under the Maria de Maeztu Units of Excellence Program (MDM-2016-0618).
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