Установлены оптимальные условия приготовления и предварительной обработки 9% Ni-Cu-Cr/2%Ce/(θ+α)-Al2O3 катализаторов селективного получения синтез-газа из метана. Показано, что оксиды Cu и Ni восстанавливаются водородом, образуя наночастицы Niо, Cuо и кластеры NiCu3,8 в виде поликристаллических пленок в условиях реакции при 1173 К.
Метаннан талғамды синтез-газ алуға арналған 9% Ni-Cu-Cr/2%Ce/(θ+α)-Al2O3 катализаторларын қолайлы дайындау және алдын-ала реакцияға дейін белсендіру жағдайлары анықталды. 1173 К реакция жағдайында Cu және Ni оксидтері сутегімен тотықсызданып, Niо, Cuо нанобөлшектерін және поликристаллды пленкалар түріндегі NiCu3,8 кластерлерін түзеді.
Optimum conditions for preparation and preliminary processing of the 9% Ni-Cu-Cr/2%Ce/(θ+α)-Al2O3 catalyst for selective production of synthesis-gas from methane were established. It was shown that Cu and Ni oxides are reduced by hydrogen, forming nano particles of Niо, Cuо, and their NiCu3,8 clusters as a loose polycrystalline film on Ni-Cu-Cr/2%Се/(θ+α)-Al2O3 catalyst during reaction at 1173 K.
Keywords: Methane, synthesis-gas, oxide, catalyst, nanoparticles
Production of CO and H2 by commercial steam reforming of natural gas for the subsequent synthesis of methanol is main method of chemical reprocessing of CH4. In modern practice the new process production of synthesis gas by direct oxidation of CH4 at deficiency of O2 and short contact time has been generated. Millisecond reactor which developed by Schmidt L.D. et al [1-3] has been used for that reaction. It was shown that high 80% selectivity by CO and H2 was achieved on reduced Pt-, Pd- and Rh catalysts supported on Al2O3. Later Ni, Co, Fe oxides, their mixtures, and perovskites have been used for selective catalytic oxidation (SCO) of CH4 at very short contact time [4,5]. Mixed Ni-Fe-, Ni-Ce- and Ni-Co-catalysts  (V = (0,9-1,5) × 105 h-1) have more high productivity. However data about use of the mixed Ni catalysts indicate on necessity of the further researches which would allow refusing from application of precious metals. In the present work, 9%NiCuCr/(θ+α)-Al2O3 catalyst modified by 2% Се has been used for selective oxidation of methane.
9% Ni-Cu-Cr/2%Ce/(θ+α)-Al2O3 catalyst was prepared by the moisture capacity impregnation method of the microspherical granules (θ+α)-Al2O3 modified by Ce with water solutions of nitrate salts of corresponding metals. The catalyst has been dried at 450-470 K for 4-5 h, heated at 873 K for 1-1,5 h, calcined on air in conditions of slow rise of temperature up to 1173 K and reduced by mixture Н2: Ar = 40 : 60 (vol.) at 1173 K for 1 h. SCO of CH4 was carried out in accordance with a procedure . Initial reaction mixture (СН4:О2:Ar = (1,4-2,2):(0,6-0,8):(97-98) (vol.%)) was supplied into flow reactor with V = 0,45-1,53•106 h-1. An Agilent 6890N gas chromatograph equipped with a thermal conductivity detector and flame ionization detector was employed for the on-line analysis of the products. The phase composition of the catalysts was determined on DRON-4-7 X-ray diffractometer with a Co anode (25kV; 25 mA; 2θ = 15o-80o). The morphology, size and chemical composition of particles were studied using an EM-125 K transmission electron microscope at a magnification of up to 100000 with the use of replica technique and electron microdiffraction. The temperature programmed desorption (TPD) of O2 and temperature programmed reduction (TPR), which were described in , were used for measuring the amount of O2 sorbed by catalysts and for characterizing this O2 and its ability to react with a reducing agent (H2). The TPD of H2 from the 9% Ni-Cu-Cr/2%Ce/(θ+α)-Al2O3 catalyst, which was preevacuated for 0,5 h and reduced, was performed by increasing of temperature at a linear rate 15 K/min from 393 K to a stabilization temperature (1173 K) in a flow of argon on a Setaram adsorption system (France) equipped with a thermal conductivity detector.
RESULTS AND DISCUSSION
Studying of the genesis of Ni-Cu-Cr/2%Ce/(θ+α)-Al2O3 catalyst for production of synthesis-gas was carried out with using of some physical and chemical methods. XRD  and TEM investigation demonstrated that the initial Ni-Cu-Cr-catalyst contains in the structure θ- and α-Al2O3, nano particles of Ni (30-50 Å, reflexes with d/n = 2,4 and 2,08 Å), and Cu oxides (30-50Å, reflexes 2,52, 2,33, 1,91, 1,79, 1,51 Å) or their mixes, СеО2 (intensive reflexes 3,13, 1,91 Å), and also Се6О11 (2,80, 1,96 Å) after heating at 873 K. At the further heating significant changes of phase structure of the catalyst take place: there is crystallization of СеО2 and transformation of θ-Al2O3 → α-Al2O3, the content and size of particles of Сu aluminates (30-50 Å) and Ni aluminates (100 Å) increases. After heating up to 1173 K intensity of СuО and NiO reflexes considerably decreases, there is an increase in the maintenance of X-ray amorphous oxide phases. After reduction by H2 at 1173 K the content of aluminates decreases in structure of catalyst, the particles of Ni (d/n = 2,06 and 1,67 Å), Cu (2,08, 1,81 Å), Се6О11 (50-60 Å), СеО2 and mixed phases СехCuyOz (2,24, 2,72 Å); translucent films and disperse fine-crystallized dense particles in the size ~ 40 Å are visible in electron-microscopic pictures. The structure of compound is described by formula NiCu3,8 (Figure 1). Cuо and Niо enter also into structure of films with thickening. Се6О11 is presented in pictures by translucent particles (50-60 Å). The catalyst which has worked 56 h in conditions of SCO without decrease of activity, consists of Се6О11, Cuо, Niо (50-100 Å) particles, and NiCu3,8 in the form of a polycrystalline film and larger dense units from disperse particles in the size 50 Å. Formations of carbon particles on the catalyst was not observed.
Figure 1. ТЕМ photos of 9%Ni-Cu-Cr/2%Ce/(θ+α)-Al2O3 catalyst reduced in Н2 at 1173K
Use of TPR and TPD of oxygen from Ni-Cu-Cr/2%Се/(θ+α)-Al2O3 catalyst allowed to study mechanisms of reduction and release of oxygen from oxides and to determine the ability of catalysts to adsorb О2 from a gas phase. TPR spectrum of catalyst heated at 873 K has 4 peaks of H2 absorption. According to published data [11, 12], these peaks correspond to reduction of CuO (Тмax = 523 K), mixed oxides NiO-CuO (Тмax = 573 K), NiO (Тмax = 673 K), СеО2 and formed Ni and Cu aluminates (Тмax = 1073 K) (Figure 2). After heating at 1473 K the peaks corresponding to reduction of NiО, CuO and mixed oxides disappear, and absorbed H2 is spent for reduction of СеО2 and reducing decomposition of Ni and Cu aluminates (spectrum 4). During ТPО there is an adsorption of O2 (Тмax = 523 K), and then formation of metal oxides (Тмax = 673 and 800 K). The catalyst heated at 1473 K with Ni and Cu aluminates (spectrum 4) is capable to sorb О2 at low temperatures (spectrum 4’) after reducing decomposition at temperatures up to 1223 K. It testifies that reduction of heated catalyst at 1173-1223 K promotes occurrence of Cuо and Niо on surface; at their oxidation oxides are formed. Reduction of oxides in ТPR regime is shown in the form of one peak at 1173 K (spectrum 5). At research of 9%NiCuCr/2%Се/(θ+α)-Al2O3 heated in O2 atmosphere at 873 K, it has been shown that there are inflections at 773, 923 K and a maximum at 1023-1270 K on TPD curve of O2 preliminary adsorbed at 673 K. Their occurrence is caused by desorption of the adsorbed O2 (670-870 K) and decomposition of copper oxide, mixed oxides, and partially Ni and Cu aluminates (above 1173 K). Activation energy of O2 desorption is 90,8 kJ/mol, activation energy of O2 release from mixed oxides - 142 ± 14,2 kJ/mol. After long heating of catalyst at 1473 K the first peak (desorption of the adsorbed O2) completely disappears and O2 release in the field of decomposition of Ni and Cu oxides and at stabilization temperature 1070 K sharply decreases in TPD spectrum. It explains the introduction of more part of metal oxides into interaction with the carrier with formation of Ni(Cu)Al2O4 aluminates as it has been shown by XRD and TEM methods. TPD spectrum of O2 again comes to an original form after reduction of catalyst by H2 at 1273 K and adsorptions of О2. These results completely correspond to those results which have been received at research by XRD, TPR and TPO methods.
TPD of H2 was carried out from 293 to 1373 K from heated at 1173 K 9%NiCuCr/2%Се/(θ+α)-Al2O3 after reduction at 673-1173 K. H2 desorption occurs in two regions: narrow (373-733 K) with Тмax = 443-483 K (corresponds to molecular adsorbed Н2ads), and wider (773-1373 K) which appears after Тred = 1073-1173 K with Тмax = 1060-1141 K and it is connected with desorption of Нads and dissolved in structure of catalyst (> 700 K). The total of desorbed H2 raises from 13,4 to 21,5 mol•10-5/g of catalyst with increase in reduction temperature from 673-873 to 1173 K; in I region oscillates from 6,78 to 7,85 mol•10-5/g of catalyst, and content of the most firmly combined H2 (> 873 K) reaches a maximum 8,57 mol•10-5/g of catalyst at Тred = 1173 K. Occurrence in TPD spectra of release region of the most strongly connected H2 (Тdes > 873 K) is caused by its dissolution in lattice of Niо and its alloy with copper which is formed only after reduction of catalyst at high temperatures (Н:Ме = 1,1-2,03). Thus, research of TPD of H2 has shown that Ni-Cu-Cr-catalyst is capable to sorb H2 not only on surface in the form of Н2ads, Нads, but also in the structure in form of strongly connected dissolved hydrogen.
Figure 2. TPR spectra (solid lines) of Ni-Cu-Cr/2%Ce/(θ+α)-Al2O3 catalyst on heated at (1) 873, (2) 973, (3) 1173, and (4) 1473 K and (1’, 3’, and 4’) the TPO spectra (dashed lines) of samples 1, 3, and 4, respectively, reduced at 1175-1225 K. (5) the TPR spectrum of the catalyst reduced at 1223 K and then reoxidized at 973 K
Studying of influence of some parameters of SCO of CH4 into synthesis-gas on reduced Ni-Cu-Cr/2%Ce/(θ+α)-Al2O3 catalyst have allowed to determine optimum conditions of SCO of CH4 in the diluted mixtures with O2 and Ar with formation of synthesis-gas: Т = 1173К, CH4:O2 = 2, ζ = 2,35-3,27 ms (V = 1,53-1,17∙106 h-1) with a degree of conversion αCH4 = 88-100% and selectivity by СО - 99,6-100%, H2 - 99-100 % with formation of CO2 traces (0,005%) (Figure 3). The catalyst has worked 56 hours without decrease of activity and formation of carbon on a surface. It specifies on prospect of its further research on stability in SCO with using of more concentrated CH4 mixtures with others alkanes both with O2, and in combined processes of SCO with steam conversion with use of porous block carriers from cordierite and metal alloys. SCO process proceeds selectively up to H2 and CO without formation of СO2. It specifies on carrying out of reaction by dissociative adsorption of CH4 on Niо, NiCu3,8 clusters up to carbon and atomic hydrogen which is dissolved in NiCu3,8 cluster. O2 activation for interaction with carbon can be carried out on Cuо, Niо, and also on particles of NiCu3,8 clusters according to TPO and TPD of oxygen. Probably, that there is a division of functions in activation of components on different parts of cluster: CH4 - on Ni atoms, О2 - on Cu atoms, and also on Се6О11. The reaction speed of oxidation of carbon particles exceeds speed of their formation, and accumulation of carbon does not occur at 100% balance on carbon in connection with excess of copper in structure of contact. Important value in the reaction mechanism of SCO gets ability of Ni-Cu-Cr catalyst to adsorb formed atomic hydrogen in the structure. It is known that yield of the dissolved hydrogen from Ni, Fe, Co and their alloys is carried out on sites of a surface with chemical adsorption of firmly combined hydrogen - for Ni-Cu-Cr catalyst with Тмax = 1123-1163К.
• at СН4:О2 = 2,0→Н2/СО = 2,0; at СН4:О2 = 2,60→Н2/СО = 1,84;
• at СН4:О2 = 2,8→Н2/СО = 1,81; at СН4:О2 = 3,75→Н2/СО = 1,65
Figure 3. Influence of СН4:О2 ratio on oxidation of СН4 at 1173 К on 9%NiCuCr/2%Ce/(θ+α)-Al2O3 after reduction at Т=1173К 1h, t=2¸2,4 ms (W = 1,81-1,5•106 h-1)
It was shown that nano particles of Ni and Cu oxides and their mixtures (d = 20-100 Å) are formed during genesis of Ni-Cu-Cr/2%Ce/(θ+α)-Al2O3 catalyst. These particles partially converted to larger mixed oxides of Ni with Cr, Ce with Cu, Cu with Cr, and Ni and Cu aluminates at high-temperature heating. Cu and Ni oxides are reduced to nano particles of Niо, Cuо, and their alloy NiCu3,8 (d = 2,08; 1,08; 1,27 Å) in the form of a friable polycrystalline film under influence of H2 at 1173K. There are also translucent particles of Ce6O11 (50-60 Å) and dense mixed phases on a surface of formed α-Al2O3. The specified phase composition of catalyst is kept in SCO into synthesis-gas. The fact of transformation of Ni and Cu aluminates into metal particles and formation of NiCu3,8 clusters under influence of H2 and CH4 are confirmed by TPR and TEМ data. The leading part in reduction decomposition of Ni(Cu)Al2O3 carries out, possibly, by copper which reduced and oxidized easier than nickel and other elements.
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