CC BY
210
УДК 541.135
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 of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the Kola Science Centre of 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 back water-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 + H2O = CO2 + H2 (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/Al2O3 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 Mo2C-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 of 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 H2O and < 2 ppm O2), 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 of 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 (NiCl2), 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)
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As follows from XRD analysis, MoNi and MoNi4 alloys form on the surface of molybdenum plates during both currentless transfer and electrolysis. The alloy formation leads to the “loosening” of the molybdenum substrate surface, which increases the specific surface area of the samples during carbonization.
Table 1. Phase compositions of molybdenum-nickel alloys after carbonization
Alloy formation conditions Carbide formation conditions Phase composition exp #
О m О ._r cd 7 £ 7 ^ <N s у S iz currentless, 1 h 923 K, 0.5 h 973 K, 1 h 1023 K, 3 h 1123 K, 5h Mo,Ni,Ni3Mo3C, ^Mo0 25Ni0 75, ^MoC Mo, Ni, Mo2C, Ni3Mo3C Mo, Ni, Mo2C Mo2C, Mo, Ni, P-NiMoO4 A
electrolysis, ic = 5 mA-cm-2, 1 h 923 K, 0.5 h 973 K, 1h 1023 K, 3 h 1123 K, 5h Mo, Ni Mo, Ni, Mo2C Mo2C, Mo, Ni, p-NiMoO4 Mo, Ni, NiC, Mo2C B C
Fig. 1 shows the cyclic voltammograms recorded at various reverse potentials on a molybdenum electrode in the NaCl-KCl-Li2CO3 melt. These voltammetric curves have three cathodic waves (Rb R2, R3) and four electrooxidation peaks (Oxi, Oxf, Oxf\ Ox3). The height of wave Ri decreases monotonically with increasing polarization rate and almost vanishes at rate of 1.0 V-s-1. At the potential corresponding to wave R1, we performed potentiostatic electrolysis
Fig.l. Cyclic voltammograms on a molybdenum electrode in the NaCl-KCl-Li2C03 melt at various reverse potentials. The electrode area is 0.238 cm2, the polarization rate is 0.1 V-s'1. T = 1023 K. Concentration of Li2CO3: 2.37-10- molcm-3. The quasi-reference electrode: platinum
The electroreduction R1 current density is very low, which is likely to be caused by a low concentration of carbon-containing particles. Wave R1 can correspond to the reduction of carbon dioxide, since the solubility of CO2 in the NaCl-KCl melt at the given temperature is (6-8)^10-8 mol-cm-3 and the electrode process can be described by the following reaction
CO2 + 4e- + 2Mo ^ Mo2C + 2O2-. (5)
In the presence of a carbonate ion, the chemical reaction
CO32- ~ CO2 + O2- (6)
precedes reaction (5).
The use of reverse at the potentials corresponding to wave R1 (-0.77 V with respect to the platinum reference quasi-electrode) is accompanied by oxidation wave Ox1 corresponding to the dissolution of Mo2C. The reverse from the base of wave R2 (-0.850 V) does not cause a new oxidation wave, and the peak Ox1 height increases. This behavior means that only the Mo2C phase forms on the molybdenum electrode in the cathodic half-cycle at these conditions and waves Ox2 and Ox1 had the same potential which corresponds to the dissolution of Mo2C. A new anodic peak Oxf was observed in the anodic region when a more negative potential -0.887 V vs. Pt was applied corresponding to R2 wave. This peak can be assigned to the dissolution of the MoC phase. Therefore, the electrode processes corresponding to wave R2 can be described by the following reactions:
CO32- + 4e- + 2 Mo ^ Mo2C + 3 O2- (7)
CO32- + 4e- + Mo ^ MoC + 3 O2-. (8)
Waves R3 and Ox3 correspond to the discharge of alkali metal cations at the molybdenum cathode and the dissolution of alkali metals, respectively. Shoulder Ox4 on the voltammograms arises from the oxidation of oxide ions at a molybdenum surface, as it was confirmed by addition of Li2O to the melt.
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Fig. 2. XRD patterns of the coatings produced in series A, B, and C
Table 1 gives the products of the carbonization of molybdenum-nickel alloys synthesized under various conditions. The optimum carbonization conditions lead to the formation of Mo2C and double carbides rather than MoC, since it has a low catalytic activity. Fig.2 shows the XRD patterns of the coatings produced in series A, B, and C experiments, and fig.3 shows a micrograph of the surface of one sample from the A series.
Catalytic Activity of Double Molybdenum and Nickel Carbides and Nickel-Promoter Molybdenum Carbides
We performed three series of experiments to study the catalytic activity of double molybdenum and nickel carbides and nickel-promoter molybdenum carbides (table 1; series A, B, C).
We investigated the back water-gas shift reaction using a set of five 40x10x0.1-mm coated plates. The initial area of the set was approximately 40 cm2. This set was placed into a glass reactor through which gases of certain compositions passed. At the exit from the reactor, the gas compositions were subjected to on-line analysis with a Varian 3800 chromatograph equipped with a thermal conductivity detector.
The samples were preliminarily processed a flow of a gas mixture of hydrogen (50 vol %) and helium (50 vol %) upon gradual heating to 673 K at a rate of 1 K-min-1.
The catalytic activity and the reaction order were determined at atmospheric pressure. Carbon dioxide, hydrogen, and helium were used as inlet gases; their ratio was changed as a function of experimental conditions; and the total pressure in all experiments was constant (1105 Pa). A change in the atmospheric pressure was taken into account in experiments. The temperature inside the reactor was varied from 473 to 598 K. The hydrogen pressure was excessive, since the reaction is controlled by a carbon dioxide flow, and the CO2 partial pressure was changed from 300 to 1200 Pa.
We determined the catalytic activities of the samples of series A, B, and C. Table 2 presents the following data for determining the catalytic activities of the synthesized samples: conversion of carbon dioxide (XCO2), selectivity (S), and the yield of the products of the back WGSR (Y). We found that series A has the maximum catalytic activity.
Table 2. Temperature dependences of the CO2 conversion, the selectivity, and the yield of the products of the back water-gas shift reaction
T, K ^co, ScH4 Sco Sc н4 / Sc o ^СНд ^co
483 0.0564 0.334 0.675 0.49 0.01885 0.03809
493 0.0669 0.316 0.740 0.43 0.02114 0.04951
503 0.0823 0.328 0.760 0.43 0.02699 0.06254
513 0.0974 0.389 0.801 0.49 0.03787 0.07799
523 0.1283 0.371 0.660 0.56 0.04760 0.08467
Conversion is the ratio of the concentration of reacted CO2 to the initial CO2 concentration, i.e., the degree of
transformation of CO2 into the products of the reaction XC
^co2 —
r°
lco2
-CO?
r° uco2
where C0CO2 is the initial CO2 concentration and CCO2 is the final CO2 concentration.
Selectivity SCH4 or SCO is a dimensionless quantity, i.e., part of unity, where unity determines the carbon material balance: if 1 mol CO2 enters into the reaction, we have SCH4 + SCO = 1. The selectivity was calculated by the formulas:
Sch,. - 1
Ссн4
ScO — 1
cCO
z-* U z' " О U /'U z'
lC02-LC02 lco2-lco2
The products of the back water-gas shift reaction were found to be carbon monoxide, water, and methane. Thus, the back WGSR
CO2 + H2 = CO + H2O is accompanied by the formation of methane,
AH° = +41 kJ mol-1
(9)
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CO2 + 4H2 = CH4 + 2H2O AH° = -114 kJ mol-1 (10)
CO + 3H2 = CH4 + H2O AH° = -206 kJ mol-1 (11)
2CO + 2H2 = CH4 + CO2 AH° = -247 kJ mol-1 (12)
It was shown that the back WGSR is a first-order reaction, the activation energy in the Arrhenius equation
Eg
к = Ae rt
is Ea = 42 kJ-mol-1: the reaction constant is к = 4.5Ы011 s-1 (at 523 K), and the preexponential factor is 7.62G0-7 s-1.
The coatings of the nickel-promoter molybdenum carbides are stable at least for 30 h. After measuring the catalytic activity, the phase composition of these coatings is unchanged. We also found no changes in the morphology of the nickel-promoter molybdenum carbides after their catalytic activity measurements.
Fig.3. Micrograph of a molybdenum-nickel alloy produced by currentless transfer in the NaCl-KCl-NiCl2-Ni melt at 1123 Kfor 1 h followed by carbonization in the NaCl-KCl-Li2CO3 melt at ic = 5 mAcm,'2 and T = 1123 K for 5 h (series A)
The conversion of carbon dioxide on the synthesized catalysts is an order of magnitude higher than the conversion of CO2 on molybdenum carbide [7]. Since methane formation is an undesirable process in WGSR, it is necessary to check the probability of methane formation in the forward water-gas shift reaction. We assume that the synthesized coatings can also be active catalysts for the forward reaction.
Since metallic nickel is a catalyst for the formation of carbon due to the decomposition of methane and the disproportionation of CO, these processes can result in catalyst deactivation and the clogging of the proton-exchange membrane of a fuel element by elementary carbon,
CH4 = C(s) + 2H2. (13)
2CO = C(s) +CO2. (14)
In our case, however, we did not detect carbon formation during the back WGSR.
Fig.4. Molybdenum-cobalt alloy produced by currentless transfer in the NaCl-KCl-CoCl2 melt in contact with Co, T =
1123 K, т = 1 h, followed by carbonization in the NaCl-KCl- Li2CO3 melt, ic = 5 mAcm-(a); molybdenum-cobalt alloy produced by electrolysis in the same melt, ic = 5 mA cm'2, followed by carbonization ic = 5 mA cm'2, T = 1123 K, т = 5 h (b)
, T = 1023 K, т = 3 h T = 1123 K, т = 1 h,
Apparently, the use of double molybdenum and cobalt carbides and nickel-promoter molybdenum carbides in the forward and back water-gas shift reaction makes it possible to avoid methane formation. Therefore, we will study the catalytic activities of double Mo and Co carbides and nickel-promoter molybdenum carbides. The preliminary results of synthesizing these carbides demonstrate that their surface is much more developed as compared to the nickel-containing compositions (Fig.4). The products of carbonization of the molybdenum and cobalt alloys are carbides Co6Mo6C2, Co6Mo6C, Co3Mo3C, and cobalt-promoter Mo2C depending on the synthesis conditions.
Thus we proposed a new two-stage method for synthesizing double molybdenum and nickel carbides and nickel-promoter molybdenum carbide. It consists in electrochemical synthesis of molybdenum and nickel alloys in chloride melt followed by carbonization in chloride-carbonate melt.
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References
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3. A microstructured reactor/heat-exchanger for the water-gas shift reaction operated in the 533-673 K range / A.R. Dubrovskiy, E.V. Rebrov, S.A. Kuznetsov, J.C. Schouten // Catalysis Today. 2009. Vol. 147. P. 198-203.
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Information about the authors
Dolmatov Vladimir,
I.V.Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the KSC of the RAS, Apatity, Russia, Valdemarusss@gmail.com Kuznetsov Sergey,
Dr.Sc.(Chemistry), I.V.Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the KSC of the RAS, Apatity, Russia, kuznet@chemy.kolasc.net.ru Rebrov Evgeny,
Dr.Sc.(Chemistry), University of Warwick, Warwick, UK, e.rebrov@warwick.ac.uk Schouten Jacob Cornelis,
PhD, Eindhoven University of Technology, Eindhoven, the Netherlands, j.c.schouten@tue.nl УДК 541.135
КАТОДНЫЕ ПРОЦЕССЫ И ХИМИЧЕСКИЕ РЕАКЦИИ ПРИ ЭЛЕКТРОХИМИЧЕСКОМ СИНТЕЗЕ КАРБИДОВ ТАНТАЛА И КРЕМНИЯ В СОЛЕВЫХ РАСПЛАВАХ
В.С. Долматов, С.А. Кузнецов
Институт химии и технологии редких элементов и минерального сырья им. И.В. Тананаева Кольского научного центра РАН, Апатиты, Россия
Аннотация
Изучено электрохимическое поведение фторидных комплексов тантала в расплаве NaCl-KCl при их титровании карбонат-ионами. Определены условия совместного сосуществования в расплаве комплексов тантала и карбонатионов, при которых возможен электрохимический синтез карбида тантала. Установлены условия совместного электровосстановления комплексов кремния и карбонат-ионов с образованием карбида кремния в расплаве эквимолярной смеси хлоридов натрия и калия с добавкой фторида натрия.
Ключевые слова:
совместное электровосстановление, электролиз, фторидные комплексы тантала и кремния, карбонат -ионы, карбиды тантала и кремния, вольт-амперные кривые.
CATHODIC PROCESSES AND CHEMICAL REACTIONS IN THE ELECTROCHEMICAL SYNTHESIS OF TANTALUM AND SILICIUM CARBIDES IN MOLTEN SALTS
V.S. Dolmatov, S.A. Kuznetsov
I. V. Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the Kola Science Centre of the RAS, Apatity, Russia
Abstract
Electrochemical behavior of tantalum fluoride complexes in the NaCl-KCl melt has been studied under carbonate-ions titration. The conditions of tantalum complexes and carbonate-ions coexistence in the melt were determined, under which electrochemical synthesis of tantalum carbides was possible. Сonditions of joint silicon and carbon electroreduction with formation of silicon carbide in an equimolar sodium and potassium chlorides melt with addition of NaF were found.
Keywords:
joint electroreduction, electrolysis, fluoride complexes of tantalum and silicon, carbonate-ions, tantalum and silicon carbides, voltammograms.
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