Conversion of Secondary C3-C4 Aliphatic Alcohols on Carbon Nanotubes Consolidated by Spark Plasma Sintering

Abstract

We analyze how the changes in the dimension of carbon nanomaterial (CNM) affect their catalytic conversion of secondary aliphatic alcohols. Carbon nanotubes (CNTs) consolidated by spark plasma sintering (SPS) were inactive in the conversion of secondary C3-C4 aliphatic alcohols because of the «healing» of defects in carbon structure during SPS. Gas-phase treatment of consolidated CNTs with HNO3 vapors led to their surface oxidation without destruction of the bulk structure of pellets. The oxygen content in consolidated CNTs determined by X-ray photoelectron spectroscopy increased from 11.3 to 14.9 at. % with increasing the oxidation time from 3 to 6 h. Despite the decrease in the specific surface area, the oxidized samples showed enhanced catalytic activity in alcohol conversion because of the increased number of oxygen radicals with unpaired electrons, which was established by electron paramagnetic resonance spectroscopy. We conclude that the structure of CNM determines the content and/or ratio of sp2 and sp3-hybridized carbon atoms in the material. The experimental and literature data demonstrated that sp3-hybridized carbon atoms on the surface are probably the preferable site for catalytic conversion of alcohols.

Composition and Electronic Structure of La2O3/CNFs@C Core-Shell Nanoparticles with Variable Oxygen Content

“…According to XPS data, the content of sp 3 -hybridized carbon atoms and carbon-oxygen bonds increased with the increase of oxidation duration (Figure 4c). We have also previously shown that an increase in the oxidation time of CNTs and CNFs, as well as consolidated and oxidized CNTs and CNFs, leads to an increase in oxygen groups and surface defectiveness according to Raman spectra [71] and, at the same time, the number of paramagnetic centers [25,28,29,31,48,49].…”

“…All observed spectra are well fitted with two Lorentzian lines with different widths corresponding to gland gh-factors and located at low-and high-applied fields, respectively (Figure 6b,d,f,h). Therefore, we consider two types of paramagnetic centers attributed to these lines [29,49]. Figure 6.…”

“…as consolidated and oxidized CNTs and CNFs, leads to an increase in oxygen groups and surface defectiveness according to Raman spectra [71] and, at the same time, the number of paramagnetic centers [25,28,29,31,48,49].…”

“…The CNTs [21,22], nitrogen-doped CNTs [23], boron-doped CNTs [24], CNFs [25][26][27], consolidate of CNTs and CNFs [28][29][30][31], GO [32][33][34], graphene [35], GO aerogels [36,37], GO quantum dots [38], carbon nanofibers [39], glass-like carbon [40], detonation diamonds and NDs [12], graphitepolystyrene composite [41], carbon films [42], natural coals [43], oxidized coals [44], carbon blacks [45], different carbon composites [46] and others [18] were investigated using the EPR. EPR signals contain information about possible paramagnetic centers of CNMs, their nature and the possible influence of oxygen-containing groups [28,[47][48][49]. The temperature dependencies of the intensity of the EPR spectra are used to identify the source of the observed magnetism [34].…”