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

° о 7L_л__ II ■(J-MojC а Мо о Ni * P-NiMo0 4 SeriesA ■ р-Мо2С *Mo о Ni * P-NiMoQ 4 Series С О 10 20 30 40 50 60 70 80 90 2 Theia. deg Fig. 2. XRD patterns o f the coatings produced in seriesA, B, and С 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 o f 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 o f 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 (1 1 0 5 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 CO2partial pressure was changed from 300 to 1200 Pa. We determined the catalytic activities of the samples o f 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 CO2conversion, the selectivity, and the yield of the products of the back water-gas shift reaction T, K ^ c o , ■^СНд ■^со 5 с H 4 / ^С О ^СНд ^ с о 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 o f the concentration of reacted CO2 to the initial CO2 concentration, i.e., the degree o f transformation of CO2 into the products of the reactionXCO2 rO _ r v co2 co2 co2 - r o uco2 where C0CO 2 is the initial CO 2 concentration and CCO 2 is the final CO 2 concentration. Selectivity SCH4 or SCOis 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: c _ CCH 4 c _ с CO 5 СH 4 r 0 r , 5 СО f' 0 r . 4 ссо 2 - с со 2 ссо 2 _ссо 2 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 AH° = +41 kJ mol-1 (9) is accompanied by the formation of methane, 222