High surface area chromia aerogel efficient catalyst and catalyst support for ethylacetate combustion

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Abstract

The effect of dispersion and ensembling mode of chromium oxide nanocrystals in bulk xerogels and aerogels on their performance as well as efficiency of high surface area chromia aerogel as catalysts support were investigated in complete ethylacetate (EA) oxidation. Two series of chromia catalysts with a structure of α-Cr2O3 (7–110 m2 g−1) and α-CrOOH (230–735 m2 g−1) were tested in EA complete oxidation as a model reaction for VOC combustion. For α-Cr2O3, the specific activity increased 3.3 times with decreasing the crystal diameter from ∼100 to 13 nm. For α-CrOOH materials, where the surface area was determined by the different packing mode of primary nanoparticles of the same size (3–5 nm), the similar specific rate constants were measured with all the tested samples. The activity of the chromia aerogel (α-CrOOH, 630 m2 g−1) was four times higher compared with 0.5% Pt/Al2O3 and 30 times higher relative to 30% Cr2O3/SiO2. Oxidative treatment (O2) at elevated temperatures converts both phases to CrO2, in case of α-Cr2O3—only in the crystals surface layer. In both reduced and oxidized states, high concentration of surface oxygen vacancies were detected in α-Cr2O3 and α-CrOOH catalysts. A redox cycle Cr(III)[]OHCr(IV)[O]O which determines the catalysts performance at the surface of both types of bulk chromia materials was proposed. Promotion of high surface area chromia aerogel with Pt, Au, Mn and Ce increased its activity in EA complete oxidation by a factor of 1.25–2.7. Addition of Pt, Mn to ceria-promoted chromia aerogel has a significant effect, yielding a high specific rate constant. Promotion with Ce- and Mn-additives improved the efficiency of redox cycle in Cr-aerogel and increased the concentration of surface oxygen vacancies.

Introduction

The emission of industrial volatile organic compounds (VOC) to the atmosphere is a subject of strict legislation due to increased concern about the photochemical smog, ground level ozone and toxic air emissions [1], [2], [3], [4]. Heterogeneous catalytic combustion of VOC in air with solid catalysts operating at 200–450 °C and short residence time (<1 s) is the most efficient method [3], [4], [5], [6]. It is fuel efficient and produces less NOx emission operating in smaller and lighter units with greater operational flexibility. The efficiency of catalytic VOC oxidation is determined by catalysts activity, selectivity and stability, so further catalysts improvement is required.

The commercial catalysts for VOC combustion contain noble metals, transition metal oxides (TMO) or their combinations stabilized in a high dispersion state at the surface of refractory oxides like highly porous alumina or silica [1], [6], [7], [8], [9], [10], [11], [12]. The noble metal catalysts are more active and tolerant to poisons [1], [6]. The most practical catalysts contain Pt or Pd alloyed with Ru, Rh or Ir that stabilizes the system to sintering at high temperatures [1]. Among the most active TMO in complete oxidation are oxides of Cr, Mn, V, Co and Cu [1], [12], [13]. They display lower activity compared with noble metals but are more efficient on cost basis.

Two issues have been raised in the ethylacetate EA combustion on supported chromia catalysts [12]: blocking of pores with active phase, which limits metal loading to ∼30 wt.%; and low efficiency of TMO redox cycle due to metal oxide–support interaction. Using pure nanostructured TMO with high surface area could resolve those issues. Chromia displays the highest specific activity in catalytic combustion compared with other TMO [12], [13] and could be prepared as mesoporous nanostructured oxide material with surface area >600 m2 g−1 [14], [15], higher than other TMO [16]. Chromia aerogel with surface area of 484–735 m2 g−1 has been prepared by homogeneous precipitation from aqueous chromium nitrate solution in presence of urea followed by supercritical drying of obtained gel. It yielded 3–5 nm CrOOH nanoparticles stable in air up to ∼400 °C [15]. No information about the performance of this high surface area chromia aerogel in any oxidation reaction is available in the literature.

EA is a good VOC model compound. Verykios et al. [8] investigated the oxidation activity of group VIII metals (Ni, Co, Ru, Pd and Pt) supported on γ-Al2O3, with a VOC mixture that included benzene and EA as model compounds of aromatics and oxygenates in VOC, respectively. EA proved to be a compound with lowest oxidation reactivity, 2 orders of magnitude lower than benzene and butanol oxidations. Esters (EA) are also the most recalcitrant compounds. The EA oxidation over Pt/Al2O3 was reported in several publications [8], [9], [17], [18]. At low EA conversion, acetic acid is the major product. Conversion of EA greater than 90% with selectivity to CO2 of 95% was obtained at >300 °C, at air space velocity of 30,000 h−1 [18] and >320 °C, at air space velocity of 48,000 h−1 [17]. Full conversion of EA to CO2 on 30 wt.% Cr2O3/SiO2 at 250 °C was obtained at air space velocity of 18,000 h−1 [12].

The objective of this study is to compare the performance of bulk chromium oxide-hydroxide, CrOOH (250–735 m2 g−1), in EA combustion with that of α-Cr2O3 (7–110 m2 g−1), 0.5 wt.% Pt/γ-Al2O3 and 30 wt.% Cr2O3/SiO2. The effects of Pt deposition as well as the promotion of high surface area chromia aerogel with Mn, Ce, Co, Cu and Au on the performance in EA combustion were also investigated. The bulk oxides were characterized by TPO–TPR XRD, X-ray photoelectron spectroscopy (XPS), and force magnetic resonance (FMR) in order to clarify the nature of their redox dynamics during EA combustion.

Section snippets

Bulk chromia catalysts

Three samples of microcrystalline α-Cr2O3 with different surface areas were prepared starting from chromium nitrate nonahydrate (CNN) Cr(NO3)3·9H2O, (Riedle de Haen). Cr-1 was obtained by thermal decomposition of CNN at 500 °C in vacuum (85 mbar) for 16 h. Samples Cr-2 and Cr-3 were prepared by precipitation from aqueous CNN (70 cm3, 0.071 M) after rapid addition (<5 min) of 150 cm3 of 1 M aqueous solution of ammonium hydroxide (Frutarom) at room temperature. The formed precipitate was separated by

Texture and structure of bulk chromia catalysts

The texture parameters of pure chromia bulk catalysts and supported reference samples as well as their phase compositions are listed in Table 1. Samples Cr-1 to Cr-3 according to XRD patterns (Fig. 1) represented a well-defined structure of hexagonal α-Cr2O3 (space group R3c, no. 167). The unit cell parameters and atomic positions calculated for these samples by means of the Rietveld software with Rwp factor of 0.10–0.12 were in good agreement with the known values of well-defined monocrystals

Conclusions

The effect of surface area on the performance in EA complete oxidation was measured for two series of bulk chromia catalysts with a structure of α-Cr2O3 (7–110 m2 g−1) and α-CrOOH (230–735 m2 g−1). For α-Cr2O3 where the surface area was determined by the crystal size, the specific rate constant per m2 of the material increased 3.3 times with decreasing the crystal size from ∼100 to 13 nm. In α-CrOOH materials, the surface area was determined by the packing mode of primary nanocrystals of the same

Acknowledgements

This study was supported by the Israel Science Foundation, Center of Excellence (Grant no. 8003). The authors gratefully acknowledge Dr. N. Frumin for conducting the XPS measurements, Dr. A. Erenburg for conducting the XRD measurements and Mr. V. Ezersky for providing the TEM micrographs.

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