Труды КНЦ вып.5 (ХИМИЯ И МАТЕРИАЛОВЕДЕНИЕ вып. 5/2015(31))

УДК541.135 SYNTHESIS OF HIGH ACTIVE CATALYTIC SYSTEMS BASED ON DOUBLE MOLYBDENUM CARBIDES V.S. Dolmatov1, S.A. Kuznetsov1, E.V. Rebrov2, J.C. Schouten3 11. V. Tananaev Institute o f Chemistry and Technology o f Rare Elements and Mineral Raw Materials o f the Kola Science Centre o f the RAS, Apatity, Russia 2University of Warwick, Warwick, UK 3Eindhoven University of Technology, Eindhoven, the Netherlands Abstract A new two-stage synthesis of double molybdenum and nickel carbides and high active and stable catalytic coatings of nickel- promoter molybdenum carbide in molten salts is developed. The first stage includes the formation of molybdenum-nickel alloys by an electrolytic method and currentless transfer in chloride melts. The second stage consists in the carbonization of the alloys in chloride-carbonate melt under various synthesis conditions. The stabilities of the nickel-promoter catalytic systems are studied, and their catalytic activities in the backwater-gas shift reaction are determined. Keywords: electrochemical synthesis, double molybdenum carbides, catalytic activity, water-gas shift reaction. The reforming of natural gas results in the formation of hydrogen with 10-12 vol % carbon monoxide. Since CO is a poison for the proton-exchange membrane of a fuel element, the water-gas shift reaction CO + H 2 O = CO 2 + H 2 (1) is used to decrease its concentration to 1 vol % and to form an additional hydrogen volume. Since the water-gas shift reaction (WGSR) is reversible and exothermic, a commercial Cu/ZnO/Al 2 O 3 catalyst is now used for WGSR [1]. This catalyst has the following disadvantages. First, it occupies 70-80% of the catalyst system volume of a fuel processor. Second, copper oxidation makes this catalyst dangerously explosive. The use of precious metal-based catalysts is too expensive, and this type of catalysts undergoes degradation at a temperature above 573 K. Molybdenum carbide is a promising catalytic system that can substitute for the well-known catalysts [2, 3]. The purpose of this work is to design the next generation of high-activity and stable Mo 2 C-based catalytic coatings for the water-gas shift reaction using electrochemical methods in molten salts. We are the first to apply two-stage electrochemical synthesis of double molybdenum and nickel carbides and nickel-promoter molybdenum carbides. Two-stage Electrochemical Synthesis of Double Carbides The salts were prepared as follows: they were mixed in the required quantities and loaded in a glassy carbon (SU-2000) crucible, which was placed in a hermetically closed retort made of a stainless steel. The latter was evacuated to a residual pressure o f 0.7 Pa, first at room temperature and then stepwise at 473, 673 and 873 K. The cell was heated using a programmable furnace. The temperature was measured using a Pt-Pt10Rh thermocouple. The retort was filled with high purity argon (U-grade: < 3 ppm H 2 O and < 2 ppm O 2 ), and the electrolyte was melted. The temperature was measured with a Termodat-17E3 temperature controller. Molybdenum plates located on current leads were immersed in a molten electrolyte through special holes in the retort. We used a bulk anode made from a metallic disperse nickel powder. During investigations, we chose the following two regimes of preparing molybdenum and nickel alloys: electrolysis at a cathodic current density of 5 mA-cm-2 in the NaCl-KCl-NiCb-Ni melt (anode is metallic nickel), at a temperature of 1123 K, process time of 1 h and currentless transfer in the NaCl-KCl-NiCb-Ni melt at the same temperature and time. The cyclic voltammetric curves were measured at a potential sweep rate varied from 5 10-3 to 2.0 V-s-1 in the temperature range 973-1123 K. Cyclic voltammograms were recorded on molybdenum and glassy carbon working electrodes 0.5-2.0 mm in diameter with respect to a platinum wire, which was used as a quasi-reference electrode. The glassy carbon crucible served as an auxiliary electrode. The prepared molybdenum and nickel alloys were carbonized under various conditions. Carbonization was performed by electrolysis in an equimolar mixture of sodium and potassium chlorides containing carbonate-ions (5 wt % Li2CO3) during cathodic polarization of a sample at a current density o f 5 mA-cm-2. The other process parameters, namely, the electrolysis time and temperature are given in table 1. After experiments the samples were washed in distilled water and alcohol. The currentless process can be described as a process whose driving force is represented by an alloy formation reaction [4]. When metallic nickel interacts with its salt (NiCl 2 ), nickel cations with a lower oxidation state are formed [5, 6]: Ni + Ni2+ ~ 2Ni+. (2) These cations diffuse through the melt and disproportionate on the surface of a molybdenum plate, 2Ni+ + Mo ~ Ni(Mo) + Ni2+. (3) The disproportionation is accompanied by the formation of an alloy and nickel cations with the oxidation state of +2. Ni2+cations again interact with metallic nickel, the process forms cycle, and the general reaction can be represented as Ni + Mo ~ Ni(Mo). (4) 220