Chemical and texture promoters in Cu-Fe-Al oxide nanocomposite catalysts for combustion of solid fuel gasification products

https://doi.org/10.1016/j.apcata.2019.117364Get rights and content

Highlights

  • Cu-Fe-Al oxide nanocomposites are promising catalysts for catalytic combustion of solid fuels in a fluidized bed.

  • The mechanism of promotion of iron oxide by Cu and Al was elucidated.

  • Aluminum is a textural promoter stabilizing the high specific surface area of iron oxide based catalysts.

  • Copper is a chemical promoter which increases the catalytic activity in the CO oxidation.

Abstract

The Fe-Al and Cu-Fe-Al oxide nanocomposites active in the oxidation of CO were studied by XRD, TEM, EDX, Raman spectroscopy, TPR-H2 and differential dissolution techniques. The nanocomposites were prepared by fusion of aluminum, iron, and copper salts that leads to their inhomogeneity. The Al3+ cations partially dissolve in the Fe2O3 lattice, which leads to a significant decrease in size of iron oxide. Copper locates in the Al-rich agglomerates to form CuO nanoparticles and partially dissolves in alumina. The dissolution of copper in iron oxide and formation of a (Cu,Al,Fe)3O4 spinel was observed only at a high Cu loading. Hence, aluminum plays the role of a textural promoter, preventing sintering and stabilization of the high specific surface area of oxide. Copper does not act as a textural promoter, as it does not affect the crystal size of iron oxide. The addition of Cu increases the catalytic activity of iron oxide-based catalysts.

Introduction

Catalytic combustion of solid fuels in a fluidized bed is a promising technology of heat generation. Compared with traditional combustion methods using conventional flame combustion, layer burning, or even a fluidized bed of inert materials, this technology has several important advantages such as low process temperature (650−750 °C), high fuel utilization rate (up to 93 %), low concentrations of harmful substances, small overall dimensions, and low metal consumption of constructions [[1], [2], [3], [4]]. Moreover, this catalytic technology can provide effective combustion of low-grade fuels such as brown coal, peat, firewood, tar, and even various industrial wastes. This becomes possible because first the gasification of solid fuel occurs and only then the resulting gas consisting mainly of CO is oxidized on the catalyst granules. Currently, only the absence of inexpensive, highly efficient catalysts restricts the development of this technology. It is well known that supported noble-metal catalysts (Pt, Pd, Au, and Ag) are most active in the reactions of complete oxidation [5,6]. However, these catalysts are expensive and can be deactivated by sintering or poisoning; the catalysts also can be abraded under fluidization conditions leading to irreversible loss of noble metals. Transition metal oxides which also are high active in the complete oxidation can serve as a good alternative to the noble metals [7,8]. Moreover, copper chromate-based catalysts were successfully used in catalytic heat generators in which a solid fuel is burned in a fluidized catalyst bed [3,4]. However, Cr-based materials are toxic which limits their further application in industry; and new environmentally cleaner catalysts should be developed. The limiting stage of burning solid fuel in a fluidized bed is the gasification of the coke residue, the rate depends on the concentration of CO. The use of catalyst significantly increases the rate of CO oxidation and decreases the temperature of the combustion process. Therefore, the catalytic oxidation of CO was chosen as a test reaction.

Recently, it was shown that Fe2O3 demonstrates moderate activity in the oxidation of CO and the catalytic performance of the iron oxide(III)-based catalysts can be improved by Cu and Al promotion [9]. The most active catalyst was a Cu-Fe-Al oxide nanocomposite prepared by fusion of aluminum, iron, and copper salts. This preparation method provides synthesis of high loading catalysts containing 50–90 % Fe2O3 with a high specific surface area near 40−135 m2/g. Because these catalysts are inexpensive and nontoxic and the main reaction in the catalytic combustion of solid fuels in a fluidized bed is the oxidation of CO, they can be used in this technology. To further increase their catalytic activity, a mechanism of promotion of iron oxide by Cu and Al should be elucidated. For this purpose, we have studied chemistry, phase composition, and distribution of Fe, Al, and Cu cations in the Fe-Al and Cu-Fe-Al oxide nanocomposites using temperature-programmed reduction by H2 (TPR-H2), X-ray diffraction (XRD), high-resolution transmission electron microscopy (TEM) with energy dispersive X-ray analysis (EDX), Raman spectroscopy, and a differential dissolution (DD) technique [10].

Section snippets

Catalysts preparation

The catalysts were prepared as follows: the salts of the precursors (Cu(NO3)2·2H2O, Fe(NO3)3·9H2O, and Al(NO3)3·9H2O) were mixed in the required ratios, then the mixture was heated to give a homogeneous melt of crystal hydrate salts and was kept at a temperature near 200 °C until the water was completely removed; finally, the resulting dry precipitate was calcined at 400 °C for 1 h and then at 700 °C for 1 h in air. The chemical composition and notation of the catalysts are listed in Table 1.

Catalyst characterization

Results and discussion

Fig. 1 shows the temperature dependence of CO conversion for the Fe-Al-based nanocomposites. One can see that the addition of copper to oxide nanocomposites significantly increases the catalytic activity. There is a shift of the light-off curves toward low temperatures for Cu-containing catalysts. Moreover, the 5C78F17A catalyst with a copper oxide content of 5 % exhibits the best activity compared with other samples. The activation energy is 91, 80, and 58 kJ/mol for 82F18A, 5C78F17A, and

Conclusions

The structure of the Fe-Al and Cu-Fe-Al oxide nanocomposites active in the oxidation of CO was studied by temperature-programmed reduction by H2, X-ray diffraction, high-resolution transmission electron microscopy with energy dispersive X-ray analysis, Raman spectroscopy, and a differential dissolution technique. These nanocomposites were prepared by fusion of aluminum, iron, and copper salts that leads to their inhomogeneity due to formation of nanoparticles consisting of well-crystallized

CRediT authorship contribution statement

O.A. Bulavchenko: Writing - original draft, Investigation. A.A. Pochtar’: Investigation. E.Yu. Gerasimov: Investigation. A.V. Fedorov: Investigation, Methodology. Yu.A. Chesalov: Investigation. A.A. Saraev: Visualization, Project administration. V.A. Yakovlev: Conceptualization. V.V. Kaichev: Writing - review & editing.

Acknowledgements

This work was supported by the Russian Science Foundation, grant 17-73-20157. The studies are conducted using the equipment of the Center of Collective Use «National Center of Catalyst Research» at Boreskov Institute of Catalysis

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