CC BY
19iSHS 2019
Moscow, Russia
SUPPORTED CATALYSTS OF DEEP OXIDATION AND HYDROGENATION BY SELF-PROPAGATING SURFACE SYNTHESIS
V. N. Borshch*" and I. M. Dement'eva"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, RAS,
Chernogolovka, Moscow, 142432 Russia e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10023
Supported catalysts are used in a wide variety of heterogeneous catalytic processes. Most of the work in modern catalytic literature is devoted to their development and research. Such interest is justified by the substantial advantages of this type of catalysts consisting of two components: the active phase (AP), which determines the catalytic properties, and the support, which is mainly responsible for the physical and mechanical properties of the catalyst (hardness, wear resistance, heat resistance). As a rule, the support has a well-developed surface and maintains the AP's highly dispersed state, which is virtually impossible to use in catalysis without fixation. Physicochemically, supports often increase the activity and selectivity of the AP both by influence on its electronic structure and the morphology of microparticles through chemical bonds and by a number of auxiliary reactions on a support. Moreover, a support improves the stability of AP particles by preventing their destruction or excessive growth because of migration of AP atoms. However, this separation of functions complicates the process of catalyst synthesis, especially the deposition and formation of AP on the surface of the support. As a rule, it is a multistage and energy-consuming technology. One of the approaches to significantly simplify this stage and reduce energy costs may be the process of self-propagating surface synthesis. The essence of this method is to initiate burning on the surface of the pores of a dry support impregnated with a mixture of solutions of an oxidizing agent (nitrates of catalytically active metals) and a reducing agent (fuel). AP of such catalysts is a mixture of metals and their oxides, the ratio of which depends on the composition of the impregnating solutions and the type of support. In the present work, nitrates of Mn, Co, and Ni were used as oxidants; urea, citric acid, sucrose, and sorbitol were used as fuel. The application was carried out on a wide range of supports, as with high Lewis acidity (y-AhO3), medium acidity (zeolites such as NaA, NaX and ZSM-5) and almost neutral (silica gel). In addition, deposition was carried out on ready-made Fe-Ni-Co-Mn polymetallic catalysts of deep oxidation (DOC), i.e. there was a modification of their surface. The content of AP on the supports varied within 515% wt in terms of metals. The deposition was limited in order to prevent excessively intense combustion and high process temperature, which could lead to AP sintering and loss of its activity. To stabilize the metal component of the AP, we proposed a method for processing the newly obtained catalysts with a solution of hydrogen peroxide in an inert atmosphere. The process consists in applying a thin protective film of oxygen on the surface of the AP during the decomposition of H2O2. This approach allows to obtain catalysts that are stable even in the process of deep oxidation at elevated temperatures.
Combustion was initiated by heating of sample in a quartz reactor in argon. Figure1 represents the typical thermograms of the process. When using urea as a fuel, there is a clearly marked onset of burning and a sharp temperature rise (Tmax = 305°C) in the visually observable combustion wave. In the case of sucrose, the combustion process is slow, the ignition is weak and starts at a lower temperature (Tignit = 48°C).
XV International Symposium on Self-Propagating High-Temperature Synthesis
(a) (b)
Fig.1. Thermograms of synthesis process of the catalyst 5%Co-5%Mn/y-AhO3 when using (a) urea and (b) sucrose as a fuel. Ignition temperatures are marked.
The XRD patterns of the catalysts obtained have a high level of noise, which indicates a highly defective, largely amorphous AP structure. The AP peaks, especially when its content is low, are masked by support peaks (Fig. 2).
30 35 40 45 50 55 60 65 70 75 80 85 90 95100
28, C
(a) (b)
Fig. 2. XRD patterns of the catalysts: (a) 5%Co-5%Mn/y-AhO3 obtained with sucrose as a fuel and (b) 5%Co-5%Cr/DOC obtained with urea as a fuel.
The AP obtained by this method has a high dispersion, however, in the case of using carriers with a large specific surface (more than 100 m2g-1), this dispersion turns out to be insufficient. After the formation of AP, the specific surface of the catalysts on these supports decreases. These conclusions can be made as a result of the analysis of Table 1.
Table 1. Specific surface of some supports and catalysts on these supports._
Zeolite 10%Co-NaA 5%Mn/ NaA
10%Co-5%Mn/ Zeolite y-Ah°3 y-AhO3 ZSM-5
5%Co-5%Mn/ ZSM-5
5%Co-5%Ni/ ZSM-5
Ssp, m2g-1 74.9
79.0
180.0
155.9
560
188
235
The surface of the obtained catalysts is coated with nanoformers, the shape of which is determined by the composition of AP. In turn, the size and shape of the pores are determined by the used carrier. This effect can be observed in Fig. 3.
SSHS2019
Moscow, Russia
(a) (b)
Fig. 3. SEM micrograph of AP of (a) catalyst 10%Co-10%Ni/silica gel and (b) catalyst 5%Mn/DOC.
All the catalysts obtained were tested in the processes of deep oxidation of CO and propane, and hydrogenation of CO2. The composition of the gas mixture in the process of deep oxidation: 0.2% vol propane, 0.7% vol CO, 2% vol O2, N2 up to 100%, gas hour space velocity, GHSV = 120,000 h-1. In the process of hydrogenation, the mixture consisted of the following gases: 2.7% vol CO2, 10.8% vol H2, He up to 100%, GHSV = 3000 h-1. The results are shown in Fig. 4. As can be seen in Fig. 4a, during the deep oxidation process, the catalyst showed high activity. CO oxidation begins already at a temperature of 100°C, and at 150°C its conversion exceeds 80%. Almost complete propane conversion is achieved at a temperature of 350°C. After 7 experiments, the drop of propane conversion at 350°C on this catalyst did not exceed 2%. The complete hydrogenation of CO2 on a cobalt-nickel catalyst supported by ZSM-5 occurs at 350°C with a selectivity of 100% for methane (Fig. 4b).
4.UU ¿JU JUU JJU H
T, °C Temperature, °C
(a) (b)
Fig. 4. (a) Conversion of CO and propane vs temperature on 10%oCo-5%oMn/y-AhO3 catalyst in deep oxidation process; (b) hydrogenation of CO2 on 10%Co-10%Ni/ZSM-5 catalyst.
Thus, self-propagating surface synthesis is promising method to produce catalysts with multicomponent AP on a wide range of supports, possessing high activity and stability in oxidizing and reducing processes.
