**1. Introduction**

Due to a growing world production volume of such chlorine-substituted hydrocarbons as dichloroethane, vinylidene chloride, vinyl chloride, etc., the utilization of harmful organochlorine wastes is of great importance [1]. Most of these wastes come from the production of vinyl chloride [2], which is a starting monomer for the manufacturing of polyvinyl chloride—a highly demanded material in the modern polymer industry. Thus, the production of one ton of vinyl chloride is accompanied by the appearance of nearly 50 kg of organochlorine wastes, represented by a complex mixture of chlorinated derivatives of ethane and ethylene. All these substances are xenobiotics, i.e., they are foreign to the body or to an ecological system and therefore exert highly toxic effects on living forms, including humans [3–5].

Most conventional utilization approaches are not applicable for chlorine-containing wastes. For instance, the open burning of chlorinated substances leads to the formation of even more hazardous compounds [6,7]. The landfill of such wastes also causes ecological disasters [8]. Therefore, the catalytic processing of chlorine-substituted organics seems

**Citation:** Wang, C.; Bauman, Y.I.; Mishakov, I.V.; Stoyanovskii, V.O.; Shelepova, E.V.; Vedyagin, A.A. Scaling up the Process of Catalytic Decomposition of Chlorinated Hydrocarbons with the Formation of Carbon Nanostructures. *Processes* **2022**, *10*, 506. https://doi.org/ 10.3390/pr10030506

Academic Editors: Vincenzo Russo, Elio Santacesaria and Riccardo Tesser

Received: 23 January 2022 Accepted: 1 March 2022 Published: 3 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

to be the most prospective. In this case, two scenarios can be considered: hydrogenassisted dechlorination with the formation of corresponding unsubstituted hydrocarbons and complete decomposition with the formation of a solid carbon product [9]. The present research deals with the second scenario. As is known, the catalytic decomposition of hydrocarbons, including chlorine-substituted ones, proceeds over iron subgroup metals via the so-called carbide cycle mechanism [10]. Among these metals, nickel possesses higher resistance to chlorination, since the formation of metal chlorides completely deactivates the catalyst. The addition of odd hydrogen into the reaction mixture allows cleaning the nickel surface from the chemisorbed chlorine species, thus stipulating the pulse regime of the catalytic process. The solid carbon product of this process is represented by carbon nanofibers (CNF) with a unique segmented structure [11,12]. It should be noted that the potential of the practical application of CNFs intensively grows every year [13,14]. In recent years, such materials have been notably attractive for application in various electrochemical processes [15–20].

Despite the high resistance of Ni to the chlorine action, pure nickel catalysts undergo deactivation caused by the formation of amorphous carbon blocking the active surface of nickel particles. Doping nickel with other metals (Cr, Co, Cu, Mo, W, Pd, Pt) results in the enhanced activity and elongated stability of Ni-based catalysts [11,21,22]. It should be emphasized that both bulk alloys and specially prepared porous alloy systems can serve as precursors of self-organizing catalysts [11,23]. In this case, the initial alloy samples undergo rapid disintegration under the action of an aggressive chlorine-containing medium. This phenomenon is well-known in the literature under the name of metal dusting (MD) [24,25]. In industry, the MD process is negatively associated with the destruction of metal reactors and pipelines [26–29]. However, in recent decades, this phenomenon has been more often considered an alternative method of catalyst preparation for CNF production [30,31]. At the same time, the chemical aggressiveness of the reaction gas mixture restricts the application of metal reactors for the target decomposition of organochlorine wastes. Therefore, all heated elements of the installations used for this purpose are usually made of quartz.

Until now, the pilot-scale utilization of chlorine-containing compounds deals with their incineration or catalytic oxidation [32–36]. At the same time, their gas-phase conversion into nanostructured carbon is mainly utilized at the fundamental level. Therefore, the present work aims to scale up the process of catalytic decomposition of chlorinated hydrocarbons with the formation of the nanostructured carbon product. In general, the upscaling of any process proceeds through several stages, including the detailed analysis of kinetics and thermodynamics [37–39]. In materials science, when the target product is obtained using the chemical vapor deposition technique, the main challenge to be solved in upscaling is to maintain the uniformity and quality of the material [40].

In the present work, 1,2-dichloroethane (DCE) was used as a representative of the mentioned class of organochlorine compounds. The Ni-Pd alloy with palladium content of 5 wt% was chosen as a catalyst providing high enough efficiency [23,41]. Three different reactor types were examined. Attention was mainly paid to the characteristics of the solid carbon product being produced. The carbon deposits were studied by scanning and transmission electron microscopies, low-temperature adsorption of nitrogen, and Raman spectroscopy.
