Supported layered double hydroxide-related mixed oxides and their application in the total oxidation of volatile organic compounds

Applied Clay Science 53 (2011) 305–316

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y

Research Paper

Supported layered double hydroxide-related mixed oxides and their application in the total oxidation of volatile organic compounds František Kovanda a,⁎, Květa Jirátová b a b

Department of Solid State Chemistry, Institute of Chemical Technology, Prague, Technická 5, 166 28 Prague, Czech Republic Institute of Chemical Process Fundamentals of the AS CR, v.v.i., Rozvojová 135, 165 02Prague, Czech Republic

a r t i c l e

i n f o

Article history: Received 18 June 2010 Received in revised form 17 December 2010 Accepted 21 December 2010 Available online 1 January 2011 Keywords: Layered double hydroxides Hydrothermal reaction Mixed oxides Supported catalysts Ethanol total oxidation

a b s t r a c t Structured mixed oxide catalysts were prepared by the calcination of layered double hydroxides (LDHs) deposited on Al2O3/Al supports (anodized aluminum foil). The deposition of LDH precursors on the supports was carried out under hydrothermal conditions at 140 °C in aqueous solutions of Ni, Co, Cu, and Mn nitrates. MII-(Mn)-Al LDHs (MII = Ni, Co, Ni-Co, Ni-Cu, and Co-Cu) with only slight Mn contents were obtained. An increased pH of the solutions used for deposition enhanced the formation of LDH phases. After heating at 500 °C, spinel-like and/or NiO-like oxides were detected in the supported mixed oxides. Compared to the mixed oxides obtained by calcination of the coprecipitated LDH precursors, the supported mixed oxides exhibited worse reducibility and lower catalytic activity in the total oxidation of ethanol; both the formation of spinel-like phases and high structural ordering of the products deposited on the Al2O3/Al supports could explain the poor reducibility. Among the supported catalysts, the Ni-Cu-(Mn)-Al mixed oxide was the most active in the total oxidation of ethanol. Increasing the pH of solutions used during the hydrothermal deposition of the LDH precursors resulted in improved catalytic activity and selectivity of the supported mixed oxides. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Layered double hydroxides (LDHs), also known as hydrotalcitelike compounds or anionic clays, represent a class of synthetic layered materials with a chemical composition expressed by the general formula [MII1−xMIIIx(OH)2]x+[An−x/n⋅ yH2O]x−, where MII and MIII are divalent and trivalent metal cations, respectively, An− is an n-valent anion, and x usually has values between 0.20 and 0.33. The MII/MIII isomorphous substitution at the octahedral sites of the hydroxide sheets results in a net positive charge, which is neutralized by interlayers composed of anions and water molecules. The cation composition of the hydroxide layers, their charge density given by the MII/MIII molar ratio, and their interlayer anion composition can be tailored during LDH synthesis. The versatility in chemical composition and physico-chemical properties of synthetic LDHs offers a large variety of applications for these materials. They are used in polymer processing, adsorption and decontamination processes, pharmacy, and in the preparation of new materials based on LDH host structures intercalated with various organic and inorganic species. Recent advances in the synthesis and application of LDHs have been reviewed by Zhou (2010). Layered double hydroxides are also widely used in heterogeneous catalysis, mainly as precursors for the preparation of

⁎ Corresponding author. Tel.: +420 220444087; fax: +420 224311082. E-mail address: [email protected] (F. Kovanda). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.12.030

mixed oxide catalysts and catalyst supports (Cavani et al., 1991; Basile and Vaccari, 2001; Kannan, 2006; Kovanda et al., 2006a; Li and Duan, 2006; Takehira and Shishido, 2007, Zhang et al., 2008a). Volatile organic compounds (VOCs) in industrial gases are dangerous pollutants and represent a serious environmental problem. Some of them exhibit toxic, narcotic, or carcinogenic properties, and they can react with nitrogen oxides and oxygen to form harmful ozone: VOC + NOx + O2 + hν → O3 + other products. Therefore, the abatement of VOC emissions is very desirable. The concentration of VOCs in air can be reduced by applying catalytic total oxidation of these organic compounds to provide carbon dioxide and water as final products; this process is markedly energy-saving compared to the elimination of VOCs by thermal combustion. For this process, catalysts containing noble metals are currently used; they are highly active and stable but expensive (Spivey, 1987; Burch et al., 1996; Rymeš et al. 2002). Platinum is more active in the oxidation of saturated and aromatic hydrocarbons, while palladium is more efficient in the oxidation of unsaturated hydrocarbons, carbon monoxide, methane, and oxidation reactions in the presence of water vapor (Spivey and Butt, 1992; Basile et al., 2001; Okumura et al., 2003). Oxides of transition metals (in particular Cu, Mn, Cr, Co, and Ni) are a cheaper alternative to the noble metal catalysts; they are highly active, but they are more sensitive to deactivation (Spivey and Butt, 1992). Mixed oxide catalysts can be easily obtained through the controlled thermal decomposition of LDH precursors, which are usually prepared by coprecipitation of aqueous solutions of MII and MIII salts in alkaline

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media. The heating of LDHs at moderate temperatures gives finely dispersed mixed oxides of MII and MIII with large surface areas and good thermal stabilities. Various LDH-related mixed oxides have been reported as catalysts for the total oxidation of VOCs, with most using toluene or ethanol as representative compounds. The following mixed oxides have been studied: Cu-Al, Cu-Cr, and Zn-Cr (Bahranowski et al., 1999), Cu-Al and Cu-Mg-Al (Kovanda et al., 2001), Co-Mn-Al (Lamonier et al., 2007), Ni-Al (Mikulová et al., 2007), Co-Mg-Al (Gennequin et al., 2009), Co supported on calcined Mg-Al hydrotalcite (Gennequin et al., 2008), and Zn-Cu-Al and Mn-Cu-Al (Palacio et al., 2010). The Mncontaining mixed oxides showed high catalytic activity. The catalytic performance of the Co-Mn-Al mixed oxide catalyst was improved by modification with potassium (Jirátová et al., 2009). Mixed oxide catalysts obtained by thermal treatment of LDH precursors containing transition metal cations have also been used in the combustion of methane, including Ni-Mg-Al and Mg-Mn-Al (Jirátová et al., 2002), CuCo-X-Al where X = Fe, Mn, La, or Ce (Cheng et al., 2008), Cu-Mg-Al (Tanasoi et al., 2009), and Co-Mg-Al (Jiang et al., 2010). Grained mixed oxide catalysts are usually obtained by heating powdered LDH precursors formed into pellets or extrudates. A considerable part of the catalyst grain volume is often not fully utilized because of internal and external diffusion. Better utilization of active components, which are especially important in fast reactions, can be achieved through deposition on structured supports. Alumina is often utilized as a supporting material for the deposition of active components such as noble or transition metals. Alumina-supported catalysts are commonly prepared through the deposition of ionic precursors from aqueous solutions via impregnation or deposition– precipitation techniques followed by thermal activation. The formation of LDH phases was observed during the impregnation of γ-alumina with aqueous solutions containing divalent metal cations (Ni2+, Co2+, or Zn2+) at near-neutral pH (Paulhiac and Clause, 1993; d'Espinose de la Caillerie et al., 1995; Merlen et al., 1995). The impregnation procedure was carried out at ambient temperature in the presence of diluted ammonia, and thermally stable systems were obtained upon calcination due to a strong ion-support interaction. A dissolution–reprecipitation mechanism was proposed consisting of the adsorption of divalent metal cations on the alumina surface followed by alumina dissolution (a rate-limiting step promoted by adsorbed ions) and the precipitation of Al3+ cations released from the support, with divalent metal cations in solution. Simultaneous ion adsorption and alumina dissolution are expected; therefore, the amount of adsorbed cations is small, and the formation of the LDH phase is almost quantitative. In this process, alumina is not only the supporting material but also the source of Al3+. Layered double hydroxide phases were also formed during the impregnation of a γ-Al2O3 support with an aqueous solution of Cu and Ni nitrates; in this case, the alumina grains were previously doped with KOH (Marino et al., 2003). The preparation of a Ni-Al LDH precursor in the pores of spherical γ-Al2O3 particles was reported by Feng et al. (2009). The alumina support was impregnated with an aqueous solution of nickel nitrate and urea and then aged with residual solution in an autoclave at 120 °C. The Ni/Al2O3 catalyst was obtained after heating the aged material at 500 °C and subsequent reduction with hydrogen. A controlled formation of LDH crystals on Al-containing substrates was applied to obtain oriented LDH films, e.g., Zn-Al LDH on an Albearing glass substrate (Gao et al., 2006) or Ni-Al LDH on porous anodic alumina (Chen et al., 2006). Thin films growing directly from a substrate have better adhesion and mechanical stability compared with films obtained by colloidal-deposition techniques (e.g., spincoating or dip-coating). A monolayer of LDH microcrystals consisting of thin curved platelets oriented perpendicular to the substrate was observed by SEM. The microstructure of the LDH films was affected by temperature and time of crystallization, and hydrophobic surfaces can be obtained in this way (Chen et al., 2006; Yang et al., 2008; Zhang et al., 2008b). The mixed oxides prepared by thermal decomposition of

deposited LDHs have been shown to maintain the original morphology of the precursor films (Chen et al., 2008; Lü et al., 2008). Recently, direct growth of LDH films on various metal substrates was reported. Mg-Al LDH films on pure aluminum substrates were obtained because of urea hydrolysis in the presence of magnesium nitrate (Guo et al., 2009). Aluminum metal was used as both the substrate and the source of aluminum cations. The formation of oriented MII-Al LDH films (MII = Zn or Cu) was observed on the zinc or copper substrates (coated stainless steel) immersed together with aluminum foil in an aqueous solution of sodium carbonate and ammonia (Liu et al., 2008). An oriented Mg-Al LDH film formed on Mg-Zn-Al alloy in carbonated water (pH ~4.3) was also reported (Uan et al., 2010). Layered double hydroxide films deposited on metal substrates can also be prepared by electrochemical synthesis (Scavetta et al., 2007; Gupta et al., 2008; Yarger et al., 2008). In our recent work (Kovanda et al., 2009a), we studied the formation of LDH phases at the interface between aluminum oxide and diluted aqueous solutions of Co and Mn nitrates under hydrothermal conditions. The formation of an LDH phase was facilitated by increasing the reaction temperature and time; the surface hydration of the Al oxide and the subsequent formation of a boehmite-like phase took place before crystallization of the LDH. CoMn-Al LDH-related mixed oxides are promising catalysts for N2O decomposition (Obalová et al., 2005, 2009; Kovanda et al., 2006b) and the total oxidation of VOCs (Kovanda et al., 2006b; Jirátová et al., 2009). The deposition of such mixed oxides on supporting materials would be advantageous for industrial applications. The present work focused on the preparation of LDH precursors containing various transition metal cations (Co, Cu, Ni, Mn) deposited on an Al2O3/Al support obtained by anodic oxidation of aluminum foil. The formed products were characterized by chemical analysis, powder X-ray diffraction, and scanning electron microscopy. The reducibility of the calcined products and the catalytic activity of the deposited mixed oxides in the total oxidation of ethanol were also examined. 2. Experimental 2.1. Preparation of samples 2.1.1. Materials Cobalt, copper, manganese, nickel, aluminum, ammonium nitrates (Co(NO3)2∙6H2O, Cu(NO3)2⋅3H2O, Mn(NO3)2∙4H2O, Ni(NO3)2∙6H2O, Al(NO3)3⋅ 9H2O, and NH4NO3), aqueous ammonia (25 wt.%), and sulfuric acid (96 wt.%) were used as purchased. Distilled water was used for preparation of the solutions. The Al2O3/Al support was obtained by anodizing aluminum foil (purity N99.9 wt.%, thickness 0.1 mm) in H2SO4 solution (2.8 mol l− 1) for 60 min at room temperature and a current density of 30 mA cm−2. The aluminum foil (10 × 13 cm) was washed in ethanol and distilled water and placed between two cathodes (aluminum plates, purity N99.9 wt.%, thickness 3 mm); the distance between the anode and cathodes was 2 cm. The anodized foil was thoroughly washed in distilled water, diluted aqueous ammonia (5 wt.%), and again in distilled water, and then it was dried at 60 °C. 2.1.2. Deposition procedure The method reported by Chen et al. (2006) was adopted for LDH deposition. The Al2O3/Al support (4 × 12 cm) was vertically placed into aqueous solutions (75 ml) containing various divalent metal nitrates (total metal ion concentration of 0.1 mol l− 1) and ammonium nitrate (0.6 mol l− 1). The pH was 6.8 or 8.5 and was adjusted by adding diluted aqueous ammonia. In 100 ml Teflon-lined stainless steel bombs, the deposition was carried out under hydrothermal conditions at 140 °C for 65 h. The composition and molar ratios of metal cations in the solutions were set as follows: Ni-Mn (2:1), Co-Mn

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(2:1), Ni-Co-Mn, Ni-Cu-Mn, and Co-Cu-Mn (all 1:1:1). Next, the precursors of the supported catalysts were taken out, rinsed with distilled water and dried at 60 °C. The supported mixed oxide catalysts were obtained by calcination of the washed and dried samples at 500 °C for 4 h in air. 2.1.3. Preparation of coprecipitated samples The supported mixed oxide catalysts were compared with mixed oxides prepared by calcination of the coprecipitated LDHs. Ten LDH-type precursors containing different cations were prepared; the composition and molar ratios of cations in the solutions were as follows: Ni-Al (2:1), Ni-Mn (2:1), Co-Al (2:1), Co-Mn (2:1), Ni-Co-Al, Ni-Co-Mn, Ni-Cu-Al, NiCu-Mn, Co-Cu-Al, and Co-Cu-Mn (all 1:1:1). An aqueous solution (450 ml) of corresponding nitrates with a total metal ion concentration of 1.0 mol l− 1 was added at a flow rate of 7.5 ml min− 1 into a 1000 ml batch reactor containing 200 ml of distilled water. The flow rate of simultaneously added alkaline solution containing NaOH (3 mol l− 1) and Na2CO3 (0.5 mol l− 1) was controlled to maintain the reaction pH at 10.0 ± 0.1. Coprecipitation was carried out under vigorous stirring at 25 °C. The resulting suspension was stirred for 2 h at 25 °C. The obtained products were filtered off, washed thoroughly with distilled water, and dried at 60 °C. The dried LDH precursors were formed into pellets and then calcined at 500 °C for 4 h in air. The calcined pellets were crushed and sieved to obtain the fraction of particle size 0.160 to 0.315 mm, which was used in the temperature programmed reduction (TPR) and catalytic measurements.

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as prepared (crushed and sieved samples with particle sizes of 0.160 to 0.315 mm; 0.4 g was placed into the reactor). The supported catalysts (deposited on Al2O3/Al sheets) were cut into small pieces (approximately 2 × 2 mm), and 0.4 g (if not mentioned otherwise) was placed 1 into the reactor. The catalysts were examined at 20 l h− 1g− cat space velocity (GHSV) with an inlet concentration of ethanol in air of 1 g m− 3. The reaction products were analyzed using a Hewlett-Packard 6890 gas chromatograph equipped with a FID detector and a capillary column (HP-5 19091 J-413, 30 m × 0.32 mm× 0.25 mm with 5% phenyl methyl silicone). The T50 and T90 temperatures (the temperatures at which 50% and 90% conversion of ethanol were observed) were chosen as a measure of the catalyst activity. Selectivity in ethanol conversion was evaluated as the GC peak area of byproducts analyzed at the 95% conversion of ethanol. The accuracy of the conversion and selectivity determination was ±3%.

3. Results and discussion 3.1. Deposition of LDHs on Al2O3/Al support After hydrothermal deposition of solids on the Al2O3/Al supports, powder XRD patterns of the obtained samples showed wellcrystallized hydrotalcite-like LDH phases, together with two sharp diffraction lines corresponding to aluminum in the support (Fig. 1).

2.2. Characterization of the samples The content of transition metals in the deposited products was determined by atomic absorption spectroscopy (AAS). A sample (0.10 to 0.15 g) of the Al2O3/Al support after deposition was dissolved in 1.5 ml of hot hydrochloric acid (35 wt.%), and the obtained solution was diluted with distilled water to 25 ml. The concentrations of Co, Cu, Mn, and Ni were determined using a Spectr AA880 instrument (Varian). The content of metals was related to the weight increase of the dried support after deposition (considered as the approximate weight of the deposited product). The chemical composition of the coprecipitated samples was also determined by AAS after dissolution of the sample in hydrochloric acid. Powder X-ray diffraction (XRD) patterns were recorded using a Seifert XRD 3000P instrument with Co Kα radiation (λ = 0.179 nm, graphite monochromator, goniometer with Bragg-Brentano geometry) in the 2θ range of 10 to 80° with a step size of 0.05°. Qualitative analysis was performed with the HighScore software package (PANalaytical, The Netherlands, version 1.0d). Scanning electron micrographs (SEM) of the samples were taken with a Hitachi S-4700 scanning electron microscope. No conductive layer was applied for observing the samples to maintain all surface details at high resolution. A low accelerating voltage of about 2 kV was applied to avoid surface charging. Temperature programmed reduction measurements of the calcined samples (0.2 g) were performed with a H2/N2 mixture (10 mol. % H2), a flow rate of 50 ml min− 1, and a linear temperature increase of 20 °C min− 1 up to 1000 °C. Changes in the H2 concentration were detected with an Omnistar 300 mass spectrometer (Pfeiffer Vakuum). Reduction of the grained CuO (0.160 to 0.315 mm) was repeatedly performed to calculate the absolute values of the hydrogen consumed during reduction of the calcined samples. 2.3. Catalytic measurements The catalytic measurements were carried out in a fixed-bed glass reactor (5 mm i.d.) with an unsteady-state reaction temperature and a heating rate of 2.0 °C min− 1 in the range from 80 to 400 °C. The catalysts obtained by calcination of the coprecipitated LDH precursors were used

Fig. 1. Powder XRD patterns of the samples obtained after 65 h of deposition at 140 °C (pH 6.8); dried at 60 °C and calcined for 4 h at 500 °C in air; H — hydrotalcite-like phase, B — AlOOH (boehmite), ? — unidentified; O — NiO-like oxide, S — spinel-type mixed oxide, Al — aluminum (support).

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Boehmite (AlOOH) was also formed during hydrothermal treatment of the Al2O3/Al support because of surface Al oxide hydration. Recently, we reported the formation of a boehmite-like phase taking place before LDH crystallization on Al2O3/Al supports (Kovanda et al., 2009a). Chemical analysis of the dried samples indicated only slight incorporation of Mn in the deposited solid (Table 1); therefore, preferred formation of MII-Al LDHs was expected. The deposited samples were denoted as MII-(Mn)-Al (MII = Ni, Co, Ni-Co, Ni-Cu, and Co-Cu). In general, the metal cations in solutions used for hydrothermal deposition were precipitated at different pH values. The following pH values have been reported for the precipitation of divalent metal hydroxides in 0.01 M solutions: Cu 5, Ni 7, Co 7.5, and Mn 8.5 (Cavani et al., 1991). The pH of the solution can evidently affect the chemical composition of the deposited solid. For that reason, a series of supported samples was obtained by deposition in solutions containing Co and Mn nitrates at pH values varying from 6.5 to 8.75. An increasing pH of the solution enhanced deposition on the support and facilitated the incorporation of Co and Mn in the formed solid (Fig. 2). A marked increase in Mn and Co contents in the deposited samples was observed at solution pH values higher than 8. Incorporation of Mn cations to the detriment of smaller Al ions in the formed LDHs can explain the increasing LDH lattice parameter a in the samples obtained at various pH values (Fig. 3). The ionic radii of Al3+, Mn3+, and Mn2+ of 0.050, 0.066, and 0.080 nm, respectively, were reported by Cavani et al. (1991). A gradual increase in the lattice parameter a with an increasing Mn:Al molar ratio in the coprecipitated Co-Mn-Al LDHs was observed in our former study (Kovanda et al., 2006b). Scanning electron microscope images of the dried samples prepared by deposition on Al2O3/Al supports at various pH values are shown in Fig. 4. Relatively homogeneous layers consisting of thin curved platelets were formed at lower solution pH values (6.5 to 7). The LDH platelets crystallized with an orientation nearly perpendicular to the substrate. This result can explain the decreased intensity of the basal (003) and (006) diffraction lines (detected in the 2θ range from about 10° to 30°) compared to the non-basal ones in Fig. 1. The deposited layers obtained at higher solution pH values (7.5 to 8.5) also consisted of thin LDH platelets, but their packing was much more compact. The growth of additional LDH crystals on the primarily formed LDH layer was also observed (Fig. 4b). The close packing and additional growth of LDH crystals are consistent with the observed increase in weight of the deposited solid with increasing solution pH (Fig. 2). Despite the facilitated Mn incorporation into the deposited solid at higher solution pH, the Mn content determined in the obtained samples was very low. Only about 1 wt.% Mn (with respect to the deposited solid) was found in the sample after deposition in the solution of Co and

Fig. 2. LDH deposition on Al2O3/Al support in dependence on solution pH (Co:Mn molar ratio in the solution was 2:1, total metal ion concentration of 0.1 mol l− 1; 65 h of deposition at 140 °C): Increase in sample weight after hydrothermal treatment (upper); Co and Mn contents in the deposited solid (lower).

Mn nitrates, where the pH was 8.5. The Co:Mn molar ratio in the prepared samples was much higher than 2:1, which was the Co:Mn molar ratio in the nitrate solution used. Increasing the solution pH above

Table 1 Content of metal cations in the deposited solid after hydrothermal treatment of Al2O3/ Al supports in aqueous solutions of transition metal nitrates and the lattice parameter a of the formed LDH phases. Cations in solution (molar ratio)

Ni, Mn (2:1) Co, Mn (2:1) Ni, Co, Mn (1:1:1) Ni, Cu, Mn (1:1:1) Co, Cu, Mn (1:1:1) a)

Solution pH

6.8 8.5 6.8 8.5 6.8 8.5 6.8 8.5 6.8 8.5

Weight increasea)/ g g-1 0.529 0.570 0.356 0.640 0.443 0.599 0.400 0.457 0.247 0.375

Metal content in the deposited solid / wt.% Ni

Co

Cu

Mn

27.2 34.1 22.0 21.7 17.7 31.1 -

27.4 52.2 13.1 24.9 19.6 37.4

11.2 3.9 15.3 6.0

1.9 6.1 0.2 1.3 1.3 1.2 2.7 8.7 0.5 4.2

LDH lattice parameter a /10− 10 m 3.071 3.068 3.069 3.109 3.050 3.080 3.061 3.080 3.068 3.072

Difference in the sample weight before and after hydrothermal treatment related to the final sample weight.

Fig. 3. Lattice parameter a of the Co-(Mn)-Al LDHs formed on Al2O3/Al supports at various pH values in the solution (Co:Mn molar ratio in the solution was 2:1, total metal ion concentration of 0.1 mol l− 1; 65 h of deposition at 140 °C).

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was increased in the samples obtained at higher pH values. In these samples, a slightly increased LDH lattice parameter a was found (Table 1), likely due to enhanced incorporation of Mn cations into the hydroxide layers. The Mn content in all of the prepared samples was rather low (maximum of a few wt.%). Again, in all prepared samples, the deposited LDH layers consisted of thin curved platelets with orientations nearly perpendicular to the substrate, but their morphology varied depending on cation composition (Fig. 5). The samples obtained in solutions of higher pH were characteristic of either more close packing and additional growth of LDH crystals on the primarily formed LDH layers (Ni-(Mn)-Al, Ni-Co-(Mn)-Al and Ni-Cu-(Mn)-Al samples) or of layers consisting of larger LDH crystals (Co-Cu-(Mn)-Al sample). The supported mixed oxide catalysts were compared with the mixed oxides prepared by calcination of the coprecipitated LDHs. In the coprecipitated precursors, only hydrotalcite-like LDHs were found by XRD; the only exception was the Co-Mn sample, in which a phase with d ~ 0.665 nm was detected. This result was ascribed to an unidentified product, probably a hydroxide- and/or oxohydroxide-type material (Kovanda et al., 2006b). A trace amount of MnCO3 (rhodochrosite) was detected in the coprecipitated Ni-Mn precursor. In general, the coprecipitated LDHs containing Al (Ni-Al, Co-Al, Ni-Co-Al, Ni-Cu-Al, and Co-Cu-Al) exhibited a higher crystallinity compared to those of the Mncontaining analogues (Figs. 6 and 7). In particular, the Co-Cu-Mn and NiCu-Mn precursors were almost amorphous. During the coprecipitation, the majority of Mn2+ was oxidized to Mn3+, as determined from the mean valence of the metal cations in the coprecipitated LDHs containing manganese (Kovanda et al., 2003, 2006b). The LDH a and c lattice parameters of the coprecipitated samples are summarized in Table 2. The molar ratios of cations in the solid corresponded approximately to those in the nitrate solutions used for coprecipitation. 3.2. Mixed oxides obtained by calcination of LDH precursors

Fig. 4. SEM images of the Co-(Mn)-Al LDHs formed on Al2O3/Al supports; a — sample deposited at pH 6.8, b — sample deposited at pH 8.5, c — profile of the sample deposited at pH 6.8 after calcination at 500 °C.

8.5 caused a considerable precipitation of cations in the solutions and a drop of Co content in the solid (Fig. 2). An increased weight of the deposited solid was also observed in the samples obtained at higher pH in solutions containing other divalent metal cations (Table 1). The differences in pH, at which various divalent metal hydroxides are precipitated, influenced the relative content of cations. For example, a Ni-Co-(Mn)-Al sample with increased Ni concentration was obtained at pH 6.8, whereas lower Cu content was found in the Ni-Cu-(Mn)-Al and Co-Cu-(Mn)-Al samples prepared at pH 8.5. A substantial portion of Cu2+ cations in the solution was precipitated during the pH adjustment, which could cause a lower incorporation of Cu in the formed LDHs. The Mn content

Calcination of the coprecipitated precursors at 500 °C led to various MII-Al and MII-Mn mixed oxides. Thermal decomposition of the Co-Al and Co-Mn samples resulted in the formation of spinel-type phases (Fig. 6). The primary crystallization of a Co-rich Co3O4-type spinel followed by Mn incorporation into the spinel lattice was observed during heating of the Co-Mn precursor (Kovanda et al, 2006b). An analogous process could be expected during calcination of the Co-Al LDH precursor, i.e., Al incorporation into the lattice of a Co3O4-like oxide in an early stage of spinel formation. The lattice parameters of the Co3O4, Co2AlO4, and CoAl2O4 spinels were very close to each other and could not be distinguished by XRD. Calcination in air caused partial oxidation of both CoII to CoIII and MnIII to MnIV. MnIV-containing mixed oxides were found in the calcined Ni-Mn sample, including Ni6MnO8 with a murdochite-type structure and NiMnO3 with an ilmenite structure (Kovanda et al., 2003). Thermal decomposition of Ni-Al LDHs led to the formation of an Al-containing NiO-like oxide. The presence of considerable amounts of an Al-rich amorphous component was expected in the calcination product (Kovanda et al., 2009b). Spinel-type mixed oxides were also found by XRD in the calcination products obtained by heating of the coprecipitated ternary LDH precursors, in which Ni, Co, and Cu cations were combined with Al or Mn ions (Fig. 7). Only spinel-type phases were detected in the calcined NiCo-Al, Ni-Co-Mn, and Co-Cu-Mn samples. Diffraction lines corresponding to CuO (tenorite) were identified in the calcined Co-Cu-Al and Ni-Cu-Mn samples. Tenorite and spinels were previously found as the main crystalline phases in the calcined LDHs containing Cu (Grygar et al., 2004). The calcined Ni-Cu-Al sample exhibited a very different powder XRD pattern. The broad diffraction lines were ascribed to NiO-like oxides. Surprisingly, no distinct Cu-containing phase was detected in this sample. The powder XRD patterns of the supported mixed oxides obtained by heating of the LDH precursors deposited on Al2O3/Al supports are shown in Fig. 1. The products deposited at pH 6.8 contained very low

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Fig. 5. SEM images of the Cu-containing LDHs formed on Al2O3/Al supports at pH 6.8 and 8.5; a — Ni-Cu-(Mn)-Al, pH 6.8; b — Ni-Cu-(Mn)-Al, pH 8.5, c — Co-Cu-(Mn)-Al, pH 6.8; d — Co-Cu-(Mn)-Al, pH 8.5.

amounts of Mn. Therefore, the formation of MII-(Mn)-Al mixed oxides with low Mn contents was expected. Diffraction lines characteristic of NiO-like oxides were found in the powder XRD patterns of the Nicontaining samples Ni-(Mn)-Al, Ni-Co-(Mn)-Al, and Ni-Cu-(Mn)-Al. A spinel-type oxide with sharp diffraction lines was detected as the other crystalline phase in the supported Ni-Co-(Mn)-Al mixed oxide. Only spinel-type mixed oxides were identified in the supported Co(Mn)-Al and Co-Cu-(Mn)-Al samples, and the latter sample was rather amorphous. No distinct oxide phases containing Cu were detected in the supported Ni-Cu-(Mn)-Al and Co-Cu-(Mn)-Al mixed oxides despite the relatively high amount of Cu in these samples. Increasing the pH of the solution from 6.8 to 8.5 resulted in a slightly increased Mn content in the deposited solid, but no substantial change in phase composition of the calcined samples was observed (not shown here). The calcined products showed the same morphology as the LDH precursors formed on Al2O3/Al supports (Fig. 4c). 3.3. TPR results The catalytic activity of mixed oxides in oxidation reactions is related to the reducibility of the active components; TPR can detect and distinguish various forms of oxides in the catalysts. The TPR patterns of the supported mixed oxides prepared by calcination of the LDH precursors deposited on Al2O3/Al supports at solution pH values of 6.8 and 8.5 are shown in Fig. 8. The supported mixed oxide catalysts were compared with those obtained by heating the coprecipitated MII-Al and MII-Mn precursors. The mixed oxides prepared from the coprecipitated precursors are denoted as ‘cp’ in the subsequent text, e.g., Ni-Co-Al/cp sample.

The TPR pattern of the Ni-Al/cp mixed oxide (Fig. 8a) showed a broad reduction peak with a maximum at about 520 °C, which is in accord with previous data (Mikulová et al., 2007; Kovanda et al., 2009b). The peak was ascribed to the reduction of NiII to Ni0, but the reduction maximum was detected at much higher temperature compared to that measured with reference NiO (330 °C). The observed difference in temperatures of reduction maxima can be explained by partial incorporation of Al cations into the nickel oxide lattice and the presence of nickel aluminate-type phases likely formed upon heating of the Ni-Al LDH precursor (Trifiro et al., 1994; Jitianu et al., 2000; Benito et al., 2006). The Ni-Mn/cp mixed oxide was reduced at much lower temperature; a broad reduction peak was centered at about 350 °C, with a shoulder at 300 °C. The TPR pattern of the Ni-Mn/cp sample was similar to that reported for the reduction of MnOx and corresponded to MnIV → MnIII,IV → MnII, with maxima at 328 and 424 °C (Kapteijn et al., 1994a; Ferrandon et al., 1999; Stobbe et al., 1999). The shoulder at 300 °C could indicate the reduction of Ni in these oxides. It is necessary to note that the temperature of the reduction steps is dependent on surface area and crystallinity of the sample (Kapteijn et al., 1994b). Therefore, the presence of amorphous components can result in a decrease in the reduction temperature. The TPR patterns of the supported Ni-(Mn)-Al mixed oxides were similar to that of the NiAl/cp mixed oxide. This result is not surprising given that the manganese content in the supported samples was low (Table 1). There was a slight difference in the TPR patterns of the supported Ni-(Mn)-Al mixed oxides obtained from LDH precursors deposited on Al2O3/Al supports at solution pH values of 6.8 and 8.5. The latter showed a higher amount of reducible components, especially in the low temperature region, which is evidently connected with the increased content of Ni and mainly Mn in the sample.

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Fig. 6. Powder XRD patterns of the coprecipitated Ni-Mn, Ni-Al, Co-Mn, and Co-Al precursors and related mixed oxides obtained by calcination of the precursors at 500 °C in air; H — hydrotalcite-like phase, ⁎ — unidentified phase, O — NiO-like oxide, M — murdochite (Ni6MnO8), I — ilmenite (NiMnO3); S1 — Co-Al spinel, S2 — Co-Mn spinel.

Fig. 7. Powder XRD patterns of the coprecipitated MII-Mn and MII-Al precursors (MII = Co-Cu, Ni-Cu, and Ni-Co) and related mixed oxides obtained by calcination of the precursors at 500 °C in air; H — hydrotalcite-like phase, O — NiO-like oxide, T — tenorite (CuO), S — spinel-type mixed oxide.

The reduction of the Co-Al/cp mixed oxide (Fig. 8b) proceeded in two steps, with maxima at 390 and 690 °C. The first peak was likely associated with the reduction of Co3O4 (reduction of CoIII to CoII followed by reduction of CoII to Co0), while the second one was likely due to the reduction of Co in Co-Al spinel-type mixed oxides (Ribet et al., 1999; Todorova et al., 2010;). Three distinct reduction maxima at about 260, 350, and 525 °C were observed in the TPR pattern of the Co-Mn/cp mixed oxide. Based on our previously published results (Kovanda et al., 2006b; Jirátová et al., 2009), the low-temperature reduction peaks detected in the range from 200 to 400 °C were ascribed to the reduction of CoIII to CoII and the reduction of MnIV to MnIII oxides, though reduction of the reference Co3O4 showed a maximum reduction at slightly higher temperature (405 °C). It is highly likely that particles of cobalt oxides in the products obtained by calcination of the coprecipitated precursors were smaller and, therefore, more easily reducible than the commercial Co3O4 used as a reference sample. We observed an analogous effect with the reference MnO2 samples; the sample with a larger particle size was reduced at a higher temperature, and the difference between temperatures of corresponding reduction maxima was more than 200 °C (Jirátová et al., 2009). The occurrence of kinetic effects during the TPR measurements makes the assignment of TPR peaks extremely difficult. We suggest that the first reduction peak at 350 °C, with the distinct shoulder at 260 °C, was associated with the reduction of a finely dispersed Co3O4like oxide. This result can be explained by considering the particle size

effect: the smaller the particle size, the lower the observed reduction temperature. Such a tendency was found during the reduction of Co3O4, with various particle sizes supported on SiO2 (Okamoto et al., 1991). The high-temperature reduction peak at 400 to 700 °C in the TPR pattern of the Co-Mn/cp mixed oxide was attributed to the reduction of a Co-Mn mixed oxide (Das et al., 1995; Todorova et al., 2010). The Co-(Mn)-Al mixed oxide deposited on an Al2O3/Al support at a solution pH of 6.8 showed a maximum of high-temperature reduction at approximately the same temperature as the Co-Al/cp sample (~695 °C), but a new reduction peak with a maximum at about 595 °C

Table 2 Lattice parameters of the coprecipitated LDH precursors. Sample

a/10− 10 m

c/10− 10 m

Ni-Al Co-Al Ni-Co-Al Ni-Cu-Al Co-Cu-Al Ni-Mn Ni-Co-Mn Ni-Cu-Mn Co-Cu-Mn

3.022 3.066 3.048 3.044 3.069 3.078 3.089 n.d. n.d.

22.73 22.63 22.95 22.84 22.74 23.13 22.98 22.4 22.2

n.d. — not determined.

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a

b

c

d

e

Fig. 8. TPR patterns of mixed oxides deposited on Al2O3/Al supports (deposition of LDH precursors at pH 6.8 and 8.5): a — Ni-(Mn)-Al, b — Co-(Mn)-Al, c — Ni-Co-(Mn)-Al, d — Ni-Cu(Mn)-Al, e — Co-Cu-(Mn)-Al; cp — mixed oxides obtained by calcination of the coprecipitated precursors.

appeared (Fig. 8b). The low-temperature reduction peak was shifted to higher temperatures compared with the Co-Al/cp mixed oxide (the corresponding reduction maxima were observed at 445 and 405 °C,

respectively). Likely, mixed oxide phases with high structural ordering were formed upon heating of the deposited LDH precursors. As mentioned above, the LDHs deposited on the Al2O3/Al support

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consisted of relatively large and well developed platy crystals; the same morphology was also observed in SEM images of the calcined samples. It can be expected that the high structural ordering of the supported mixed oxides affects their reducibility. For example, the higher structural ordering and increased crystallite size of hydrothermally treated Ni-Al LDH precursors enhanced the stability of NiII species against reduction in calcination products (Benito et al., 2006; Kovanda et al., 2009b). An increased amount of reducible components and a shift of the low-temperature reduction peak to a lower temperature (~ 390 °C) was observed during reduction of the Co(Mn)-Al mixed oxide deposited at pH 8.5. The multicomponent mixed oxides exhibited more complex reduction behavior (Fig. 8c–e). The TPR pattern of the Ni-Co-Al/cp mixed oxide seemed to be a superposition of the main reduction peaks found in the TPR patterns of the Ni-Al/cp and Co-Al/cp samples. It showed two peaks, a low-temperature one at 340 °C and a broad high-temperature peak composed of at least two individual reduction peaks with maxima at 620 and 710 °C (Fig. 8 c). In the TPR pattern of the Ni-Co-Mn/cp mixed oxide, two distinct reduction peaks could be seen; the first one with a maximum at 300 °C together with a shoulder at 200 °C and a second one with a maximum at 450 °C. Both the Ni-CoAl/cp and Ni-Co-Mn/cp mixed oxides exhibited spinel-type crystal structures, but the substitution of Al for Mn resulted in easier reduction of the Ni-Co-Mn/cp sample and a shift of the reduction peaks to lower temperatures. The TPR patterns of the supported NiCo-(Mn)-Al mixed oxides were similar to that of the Ni-Co-Al/cp sample. The supported Ni-Co-(Mn)-Al mixed oxide obtained from the LDH precursor deposited at pH 8.5 had a low-temperature reduction maximum that was slightly shifted from about 335 to 305 °C. In the TPR patterns of the Ni-Cu-Al/cp and Co-Cu-Al/cp mixed oxides, a new sharp peak appeared at about 240 °C; it corresponded to the reduction of CuO to Cu. The broad reduction peaks observed in the TPR patterns of these samples at higher temperatures were ascribed to the reduction of spinel-type mixed oxides, e.g., Ni or Co aluminates. No CuO (tenorite) was detected in the powder XRD pattern of the NiCu-Al/cp sample (Fig. 7); an XRD-amorphous copper oxide was likely present. The TPR pattern of the Ni-Cu-Mn/cp mixed oxide (Fig. 8d) showed reduction maxima at about 180, 240, 300, and 370 °C. The peaks observed at low temperatures were ascribed to the reduction of copper oxide (CuO and an amorphous Cu-containing component). The reduction of the Co-Cu-Mn/cp mixed oxide (Fig. 8e) was similar to that of the Ni-Cu-Mn/cp sample; the reduction peaks observed in the low-temperature region (up to about 260 °C) were likely connected with the reduction of an amorphous copper oxide. Again, the TPR patterns of the Ni-Cu-(Mn)-Al mixed oxides deposited on Al2O3/Al supports were similar to that of the Ni-Cu-Al/cp sample. A low-temperature peak with decreased intensity corresponding to the reduction of copper oxide was found in the supported Ni-Cu-(Mn)-Al mixed oxide deposited at pH 8.5, likely due to a lower Cu content in this sample. The TPR pattern of the Co-Cu-(Mn)-Al mixed oxide deposited at pH 6.8 exhibited a broad, low-temperature reduction peak with a maximum at about 130 °C, which can be explained as the reduction of an amorphous Cu-containing component. The other reduction peaks were found at about 370 and 665 °C. In the TPR pattern of the supported Co-Cu-(Mn)-Al mixed oxide deposited at pH 8.5, the maxima of reduction peaks were shifted to lower temperatures, and no low-temperature peak connected with the reduction of copper oxide was found (Fig. 8e). 3.4. Catalytic activity and selectivity in the total oxidation of ethanol The best catalysts for the total oxidation of VOCs are those that oxidize them directly to CO2 and H2O without any stable reaction intermediates because some byproducts formed during the oxidation process can be more detrimental to the environment than the initial compounds. For example, acetaldehyde and/or acetic acid can appear during ethanol

313

oxidation (Bahranowski et al., 1999; Avgouropoulos et al., 2006). The prepared mixed oxides, the ones supported on Al2O3/Al supports, and those obtained from coprecipitated precursors were examined as catalysts in the total oxidation of ethanol. The catalytic reaction was carried out under unsteady-state conditions with a continuous temperature increase starting at 80 °C. Temperatures T50 and T90 were determined from the measured data of ethanol conversion vs. reaction temperature. In the case of very active catalysts, determination of the T50 and T90 temperatures was rather difficult due to simultaneous adsorption/desorption of the reactants and the catalytic reaction; the total reaction rate was then determined by the rate of the adsorption/ desorption processes. Adsorption of the reactants decreases with increasing reaction temperature. Therefore, the T50 and T90 temperatures of some catalysts exhibiting high catalytic activity (e.g., the Co-Cu-Mn/cp sample) were determined from the data obtained at higher ethanol conversions by taking into account an analogous course of the ethanol conversion vs. temperature dependence as those measured with other, less-active catalysts. The selectivity of the examined mixed oxide catalysts was determined from the concentration of all reaction byproducts continually detected during the catalytic measurements. The T50 and T90 temperatures documenting the catalytic activity of the examined mixed oxides are summarized in Table 3. Upon comparing the mixed oxides obtained by calcination of the coprecipitated precursors, the manganese-containing MII-Mn/cp samples were more active than their MII-Al/cp analogues. The highest catalytic activity in the total oxidation of ethanol was observed with Cu-containing Ni-Cu-Mn/ cp and Co-Cu-Mn/cp mixed oxides. The mixed oxides deposited on Al2O3/Al supports showed lower catalytic activity; the T50 temperatures measured with these catalysts were shifted to higher values, even in comparison with the corresponding MII-Al/cp samples. The supported mixed oxide catalysts contained only low amounts of Mn. Their lower catalytic activity compared with the MII-Al/cp mixed oxides was likely connected with the lower amount of easily reducible active components, as determined in the TPR measurements (Fig. 8). We consider that the high structural ordering of mixed oxide phases formed upon heating of the deposited LDH precursors resulted in poor reducibility and, consequently, lower catalytic activity of the supported mixed oxide catalysts. Compared to the MII-Al/cp samples, enrichment of the mixed oxides deposited on the Al2O3/Al supports with Al cannot be fully excluded (Kovanda et al., 2009a) because this outcome could contribute to the low reducibility of the supported mixed oxides. Increasing the pH of the solution used for deposition led to enhanced formation of LDH precursors on the Al2O3/Al supports as well as to an increase in the content of transition metal cations in the deposited solid. The catalytic activities of the supported Co-(Mn)-Al mixed oxides deposited at various solution pH values are compared in Table 4. A marked decrease in the T50 and T90 temperatures (from 272

Table 3 Surface area and catalytic activity of the prepared mixed oxides in the total oxidation of 1 ethanol (0.40 g of catalyst, concentration in air 1.0 g m− 3, GHSV 20 l h− 1g− cat , heating rate 2.0 °C min− 1); T50 and T90: temperatures, at which 50% and 90% conversion was achieved. Sample

SBET/m2 g− 1

T50/°C

T90/°C

Co-Al/cp Co-(Mn)-Al/pH 6.8 Co/Mn/cp Ni-Co-Al/cp Ni-Co-(Mn)-Al/pH 6.8 Ni-Co-Mn/cp Ni-Cu-Al/cp Ni-Cu-(Mn)-Al/pH 6.8 Ni-Cu-Mn/cp Co-Cu-Al/cp Co-Cu-(Mn)-Al/pH 6.8 Co-Cu-Mn/cp

82 28 44 130 84 63 150 37 54 96 70 62

205 242 162 169 218 145 192 204 125 153 268 100

280 284 248 204 262 178 220 228 154 194 307 145

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Table 4 Catalytic activity of the supported Co-(Mn)-Al mixed oxides deposited on Al2O3/Al supports at various pH values in the total oxidation of ethanol (0.16 g of catalyst, −1 1 concentration in air 1.0 g m− 3, GHSV 20 l h− 1g− ) and cat , heating rate 2.0 °C min amounts of consumed hydrogen during TPR measurements. Sample

Co-(Mn)-Al/pH 7.0 Co-(Mn)-Al/pH 7.5 Co-(Mn)-Al/pH 8.25 Co-(Mn)-Al/pH 8.75

H2 consumption/mmol H2 g− 1 (25–1000 °C)

(25–400 °C)

4.438 7.454 16.298 16.271

0.741 0.969 3.079 3.170

T50/°C

T90/°C

272 255 230 200

334 290 270 255

to 200 and from 334 to 255 °C, respectively), i.e., increased catalytic activity with increasing solution pH, was observed. A relationship between activity of the catalysts in the total oxidation of ethanol and the amount of reducible components (especially Mn) in the solid was evident (Table 4). Among all of the examined catalysts, the Co-Cu-Mn/cp mixed oxide was the most active in the total oxidation of ethanol (T50 and T90 temperatures of 100 and 145 °C, respectively, were found). The most active supported catalyst was Ni-Cu-(Mn)-Al, which showed T50 and T90 temperatures of 204 and 228 °C, respectively (Table 3). This catalyst contained rather high amounts of Mn in comparison with other supported mixed oxides deposited at pH 6.8. It was also reduced at relatively low temperatures, mainly due to the presence of amorphous copper oxide. The presence of transition metal cations that can attain variable oxidation states and the content of easily reducible components are especially important for oxidation-reduction processes. Acetaldehyde was the main byproduct found in the reaction off-gas during ethanol oxidation over all of the examined mixed oxide catalysts. The other potential byproducts, e.g., ethylene, acetic acid, or ethyl acetate, were detected only in negligible concentrations. As ethanol is oxidized more easily than acetaldehyde, the difference among the catalysts was more obvious at lower reaction temperatures, i.e., at a low conversion of ethanol. The acetaldehyde concentration in the reaction mixture (expressed as the peak area of acetaldehyde in the GC analysis) changed with the activity of the catalysts (Fig. 9). The most active MIIMn/cp mixed oxides showed a minimum acetaldehyde concentration in the off-gas. The highest acetaldehyde concentration was detected during ethanol oxidation over the mixed oxides deposited on Al2O3/Al supports. Comparing the supported Co-(Mn)-Al mixed oxides, the

Fig. 9. Formation of acetaldehyde during ethanol oxidation over mixed oxide catalysts deposited on Al2O3/Al supports (deposition of LDH precursors at pH 6.8) and mixed oxides obtained by calcination of the coprecipitated precursors.

increased pH in the solution used for deposition of the precursors improved both the activity and selectivity of the obtained catalysts. The lowest concentration of acetaldehyde was obtained during ethanol oxidation over the supported Co-(Mn)-Al mixed oxide deposited at a solution pH of 8.75; at the same time, the maximum acetaldehyde formation was observed at the lowest temperature (Fig. 10).

4. Conclusions Well-crystallized LDH precursors were obtained on Al2O3/Al supports during their reaction with aqueous solutions containing divalent metal nitrates (Ni, Co, Cu, Mn) under hydrothermal conditions. The formation of Al-containing MII-(Mn)-Al LDHs (MII = Ni, Co, Ni-Co, Ni-Cu, or Co-Cu) was expected because only slight incorporation of Mn cations into the deposited solid was found. The deposition of LDH phases was facilitated by increasing the pH of the solution as products with higher contents of transition metal cations were obtained. The layers of LDHs grown on the Al2O3/Al supports consisted of relatively large, welldeveloped thin platelet crystals oriented nearly perpendicular to the support. After heating at 500 °C, when mixed oxides were formed, the deposited solids showed the same morphology. The supported mixed oxide catalysts were compared with MII-Al and MII-Mn mixed oxides obtained by calcination of the coprecipitated LDH precursors. Spinel-type mixed oxides were found as the characteristic crystalline phase in the calcined products, namely those containing Co. Only a NiO-like oxide was detected in the calcined NiAl and Ni-Cu-Al precursors. Crystallization of CuO was detected after calcination of the coprecipitated Ni-Cu-Mn and Co-Cu-Al samples. Analogous crystalline phases were observed in the supported mixed oxides obtained from LDH precursors deposited on Al2O3/Al supports; a NiO-like oxide was found in the calcined samples containing Ni and spinel-type mixed oxides were formed during calcination of the Cocontaining samples. No distinct Cu-containing phases were detected in the supported Ni-Cu-(Mn)-Al and Co-Cu-(Mn)-Al mixed oxides. The LDH-related mixed oxides deposited on Al2O3/Al supports exhibited worse reducibility compared with those obtained by calcination of the coprecipitated precursors. This result can be explained by the formation of Al-containing spinel-type phases and higher structural ordering of the supported mixed oxides. A reduction peak in the low-temperature region (up to about 260 °C) was detected in the samples containing copper; this peak was assigned to the reduction of an amorphous copper oxide that was not detected by XRD. The mixed oxides deposited on Al2O3/Al supports at pH 8.5

Fig. 10. Formation of acetaldehyde during ethanol oxidation over supported Co-(Mn)Al mixed oxide catalysts deposited on Al2O3/Al support at various pH values.

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exhibited a slight shift of reduction maxima to lower temperatures, likely due to the increased Mn content in these samples. Compared to the mixed oxides obtained by calcination of the coprecipitated precursors, the supported mixed oxides showed lower catalytic activity in the total oxidation of ethanol, probably because of their poor reducibility. Acetaldehyde was the main reaction byproduct identified in the reaction mixture, while the other potential byproducts (acetic acid, ethyl acetate, and ethylene) were either not detected or were present in only negligible concentrations (ethylene). An increased solution pH during hydrothermal deposition of the LDH precursors resulted in improved catalytic activity and selectivity of the supported mixed oxides in the total oxidation of ethanol.

Acknowledgements This work was supported by the Czech Science Foundation (P106/ 10/1762 and 106/09/1664) and the Ministry of Education, Youth, and Sports of the Czech Republic (MSM 6046137302).

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