ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2015, Vol. 89, No. 4, pp. 680–687. © Pleiades Publishing, Ltd., 2015. Original Russian Text © E.A. Tveritinova, Yu.N. Zhitnev, I.I. Kulakova, K.I. Maslakov, E.A. Nesterova, A.N. Kharlanov, A.S. Ivanov, S.V. Savilov, V.V. Lunin, 2015, published in Zhurnal Fizicheskoi Khimii, 2015, Vol. 89, No. 4, pp. 679–687.
PHYSICAL CHEMISTRY OF NANOCLUSTERS AND NANOMATERIALS
Effect of Structure and Surface Properties on the Catalytic Activity of Nanodiamond in the Conversion of 1,2-Dichloroethane E. A. Tveritinova, Yu. N. Zhitnev, I. I. Kulakova, K. I. Maslakov, E. A. Nesterova, A. N. Kharlanov, A. S. Ivanov, S. V. Savilov, and V. V. Lunin Department of Chemistry, Moscow State University, Moscow, 119991 Russia e-mail:
[email protected] Received May 29, 2014
Abstract—The catalytic activity of a detonation nanodiamond and its Ni-containing forms in the conversion of 1,2-dichloroethane is studied and compared with the activity of other carbon and nanocarbon materials: carbon nanotubes, “Dalan” synthetic diamond, and fluorinated graphite. The surface and structure of the carbon materials are characterized using XRD, diffuse reflectance IR spectroscopy, XPS, BET, and TPR. The catalytic properties of the materials are studied using the pulsed microcatalytic method. It is found that the synthetic diamond, the nanodiamond, and its Ni-containing forms are catalysts for dichloroethane conversion in a nitrogen atmosphere, where the main product is ethylene. It is noted that the catalytic activity of deactivated diamond catalysts is restored after hydrogen treatment. It is shown that the carbon structure of the nanodiamond and the “Dalan” synthetic diamond with hydrogen groups located on it plays a key role in the dichloroethane conversion. It is found that the nanodiamond acts simultaneously as a catalyst and an adsorbent of chlorine-containing products of dichloroethane conversion. Keywords: catalysis, detonation nanodiamond, surface properties, conversion of 1,2-dichloroethane. DOI: 10.1134/S0036024415040251
INTRODUCTION Carbon materials have long been used in catalysis as catalysts and supports of the active phase due to their unique properties, with which they compare favorably to inorganic materials. These properties include thermal stability, developed surfaces, and resistance to aggressive media. In recent years, researchers have focused on creating a new generation of carbon materials, e.g., detonation nanodiamond (ND), carbon nanotubes (CNTs), and nanofibers. The combination of an extremely strong regular structure and the presence of surface functional groups makes these materials promising for use in catalysis. According to [1], the fraction of surface atoms in ND is ~15% at an average ND particle diameter of 4.2 nm. Since almost all of the carbon atoms in ND are sp3 hybridized, an ND surface contains a large number of coordinatively unsaturated carbon atoms that are naturally saturated with functional groups. According to the authors of [2], this is responsible for the high reactivity of ND surfaces that allows us to compare them favorably with other forms of nanocarbon (i.e., sp2 carbon forms) in which the concentration of functional groups on the surface is considerably lower as a result of their structural features. The available literature data on the catalytic properties of ND mostly concern its use in supports of
metallic phases [3–9]. According to the authors of [6, 8], the surface functional groups of ND participate in the formation of active sites. ND, CNTs, and other carbon nanomaterials are catalysts of such processes as the oxidative dehydrogenation (ODH) of aromatic hydrocarbons and alkanes [10, 11, 13–16]. The authors of [10] largely attribute the high and stable catalytic activity in the ODH of ethylbenzene to the chemical structure of materials based on sp2 carbon. In a comparative study of the ODH of ethylbenzene over CNTs and ND [11], however, the same authors showed that despite differences between their structural and electronic properties, the two carbon nanomaterials exhibit the same activity, and the activation energies of the reaction over these catalysts are also identical. The authors therefore concluded that the active sites on sp2 and sp3 carbon surfaces were identical. The conversion of aliphatic alcohols over ND subjected to oxidation and reduction treatment was studied in [12]. It was found that ND exhibited catalytic activity, and the direction of conversion was strongly affected by the nature of a surface’s functional covering. The catalytic effects of CNTs on the ODH of light alkanes [13, 14] and ND on the dehydrogenation of aliphatic alcohols [12] were attributed to the presence
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of oxygen-containing surface groups. The studies showed that carbonyl groups act as Lewis basic sites and, due to their high electron density, are capable of abstracting hydrogen atoms from the С–Н bond of alkanes and the О–Н bond of aliphatic alcohols to produce the respective alkenes and ketones. According to [12], the carboxyl groups on an ND surface are responsible for the formation of olefins in the catalytic conversion of aliphatic alcohols. In most studies devoted to catalysis over carbon nanomaterials, the role of active sites is attributed to different surface functional groups, while the effect the chemical structure of a material has on catalysis is commonly ignored. Unlike the reaction over graphite and CNTs, the ODH of ethylbenzene over onion-like carbon containing no oxygen groups occurs with a certain period of induction. The authors of [15] attributed this to the formation of oxygen-containing quinoid groups, which are active sites of the ODH of ethylbenzene, during the reaction. In one study of the ODH of n-butane over ND [16], it was suggested that the catalytic function of the latter was associated with the complete rearrangement of its sp3 carbon structure into an sp2 carbon structure similar to that of fullerene. In addition to using ND as a catalyst for the conversion of hydrocarbons and alcohols, we consider it to be promising for dechlorination of organic substances. The available literature data on the dechlorination of dichloroethane (DCE) mostly concern studying this reaction with the participation of hydrogen over bimetallic catalysts [17–21], where Pt, Pd, Au, Ag, Ru, Ni, Cu, and Sn are used as the metallic phase and SiO2, Al2O3, and activated carbon are employed as supports. The authors of these studies believe that the role of the noble metals consists in enhancing the catalytic activity of the catalysts, and adding a second metal suppresses the secondary processes, contributing to a rise in ethylene selectivity. We could find no literature data on the conversion of chlorinated hydrocarbons over metal-free catalysts, particularly on the surfaces of carbon nanomaterials. In studies of hydrodechlorination processes [5, 22], ND was used for supports and functional groups were seen as sites of active metal phase adsorption. In [23], we found that ND-based catalysts were capable of retaining significant amounts of the active hydrogen that forms during the dissociative adsorption of molecular hydrogen. It was shown that the active hydrogen adsorbed on the surface of a Ni/ND catalyst is capable of hydrogenating acetylene in a nitrogen stream, while acetylene hydrogenation does not occur in either a hydrogen or a nitrogen stream over ND subjected to preliminary high-temperature hydrogen treatment of its surface. Analysis of the literature data shows that the effect of the structure of carbon nanomaterials and the nature of the surface groups on the catalytic properties of these materials is still poorly understood. In addiRUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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tion, it is still unclear whether ND catalyzes the dechlorination of organic substances. The aim of this work was therefore to identify the mechanism of the conversion of 1,2-dichloroethane over detonation ND and ND-based nickel catalysts, and to compare the findings to the mechanism of the catalytic action of other carbon nanomaterials with different structures of the carbon matrix. EXPERIMENTAL The following carbon materials were used as catalysts: detonation ND (Almaznyi Tsentr, St. Petersburg), CNTs (Department of Chemistry, Moscow State University), a “Dalan” synthetic diamond (Аsyn) (Chernogolovka, Moscow oblast), and fluorinated graphite (FG) (Kirovo-Chepetsk). Ni/ND nickel catalysts prepared by impregnating alcohol-wetted ND powder with an aqueous solution of nickel formate were also used. The decomposition of nickel formate deposited on ND was conducted under dynamic vacuum conditions according to the technique described in detail in [24]. The CNTs were synthesized via the pyrolysis of hexane vapors in nitrogen over Со0.03Mo0.1MgO catalyst at a temperature of 730°С. The catalyst was removed from the sample with hydrochloric acid, followed by repeated washing with distilled water. Nongraphite carbonaceous products were removed via calcination at 350–400°С. The resulting material was cylindrical multiwalled CNTs with diameters of 18– 25 nm. Reagent grade 1,2-DCE was used (Komponentreaktiv, Moscow). X-ray diffraction analyses of ND and CNTs were conducted on a DRON diffractometer equipped with a CоKα radiation source. The coherent scattering region was ~5 nm for the two carbon nanomaterials, as calculated using Scherrer’s equation. The IR spectra of the investigated carbon nanomaterials were recorded in the diffuse reflectance mode on an EQUINOX55/S instrument (Bruker). The specific surface areas (Ssp) of ND, CNTs, FG, and Аsyn were determined by means of BET using ASAP2010V2.00 and Quantachrom ASlWin.V.2.11 systems, from the lowtemperature nitrogen adsorption after preheating at 300°C in vacuum (10–5 Pa). The Ssp values were 284, 22, 160, and 250 m2/g for ND, Аsyn, CNTs, and FG, respectively. X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis Ultra DLD instrument equipped with an AlKα radiation source at transmission energies of 160 and 40 eV for individual lines. Temperature programmed reduction (TPR) of the catalysts was conducted in a quartz reactor under heating at a rate of 12 K/min in a stream of a gas mixture of 5% Н2 in Ar at a flow rate of 25 mL/min.
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I, arb. units
x, % 100
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Fig. 1. DCE conversion (x) in a nitrogen stream at 300°C over diamond nanomaterials: (1) ND, (2) ND after hydrogen regeneration at 450°C, (3) 2.5% Ni/ND, (4) 2.5% Ni/ND after hydrogen regeneration at 450°C, (5) Asyn, and (6) Asyn after hydrogen regeneration at 450°C; N is the pulse number.
DCE conversion was studied using the pulsed microcatalytic method [25]. A batch of the catalyst (~0.07 g) was loaded into a quartz reactor (length, 20 cm; internal diameter, 0.4 cm) between two fiberglass layers. The reactor was placed in a stainless-steel jacket equipped with inlets for injecting DCE and the carrier gas and heated in an electric furnace. The catalyst temperature was measured and controlled with an accuracy of ±1 K using a TRM-10 temperature controller. The volume of the injected DCE vapor sample was 1 mL. The reaction products were analyzed on a Chrom-5 chromatograph equipped with a flame ionization detector and a Porapak-S column. The carrier gas was either nitrogen or hydrogen, depending on the goal of the test; the rate of carrier gas flow was 30 mL/min. The temperature of the chromatographic column was 120°C. Hydrogen regeneration of the catalysts was conducted in the same reactor at 450°C for 3.5 h. The temperature of catalyst regeneration in a hydrogen stream was selected on the basis of TPR thermograms. The ratio between the basic and acid sites on the catalyst surface was determined using a test reaction of dehydrogenation–dehydration of 2propanol [26]. RESULTS AND DISCUSSION The carbon materials selected as catalysts differed in both their chemical structure and the nature and content of their surface functional groups. Catalytic activity in DCE conversion was exhibited only by ND and Аsyn. However, DCE conversion over Аsyn was considerably lower than over ND, in good agreement with
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t, °C
Fig. 2. Thermogram of hydrogen absorption by nanodiamond catalysts: (1) ND and (2) 2.5% Ni/ND.
the Ssp value of the latter. DCE conversion in a nitrogen stream over ND and Аsyn occurred with the formation of ethylene and a minor amount (
