Total oxidation of ethanol over layered double hydroxide-related mixed oxide catalysts: Effect of cation composition

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Total oxidation of ethanol over layered double hydroxide-related mixed oxide catalysts: Effect of cation composition Kvˇeta Jirátová a,∗ , Frantiˇsek Kovanda b , Jana Ludvíková a , Jana Balabánová a , Jan Klempa a a b

Institute of Chemical Process Fundamentals of the CAS, v.v.i., Rozvojová 135, 165 02 Prague 6, Czech Republic Department of Solid State Chemistry, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czech Republic

a r t i c l e

i n f o

Article history: Received 30 July 2015 Received in revised form 14 October 2015 Accepted 27 October 2015 Available online xxx Keywords: VOC oxidation Transition metal oxides Layered double hydroxides Ethanol tab

a b s t r a c t Ethanol total oxidation over mixed oxide catalysts containing various transition metal cations was studied. The MII –MIII layered double hydroxide (LDH) precursors with MII /MIII molar ratio of 2 (MII = Cu, Co, Ni, Cu–Ni, Cu–Co, and Co–Ni; MIII = Mn or Al) were prepared by coprecipitation of nitrate solutions. The ternary mixed oxides containing Mn were more active than the binary Cu–Mn, Co–Mn, and Ni–Mn ones as well as the ternary Al-containing catalysts; the Cu–Ni–Mn mixed oxide was the most active. The catalytic activity increased with increasing amount of easily reducible components and amount of oxygen desorbed from catalysts surface at lower temperatures (up to 500 ◦ C). Main byproduct of the ethanol oxidation was acetaldehyde. Ethanol oxidation over Al-containing catalysts produced lower amounts of acetaldehyde than that over the catalysts comprising Mn; both the Cu–Ni–Mn and Co–Ni–Mn catalysts showed the lowest temperatures of acetaldehyde disappearance. © 2015 Published by Elsevier B.V.

1. Introduction Volatile organic compounds (VOCs) in industrial gases represent a serious environmental problem, as some of them exhibit toxic, narcotic, or carcinogenic properties. They can also react with nitrogen oxides and air oxygen to form harmful ozone: VOC + NOx + O2 + h → O3 + other products. Concentration of VOCs in air can be reduced applying the total oxidation to carbon dioxide and water as final products; catalytic process is markedly energy saving compared to elimination of VOCs by thermal combustion. The catalysts containing noble metals are currently used, as they are highly active and stable but expensive [1–3]. Platinum is more active in oxidation of saturated and aromatic hydrocarbons, while palladium is more efficient in oxidation of unsaturated hydrocarbons, carbon monoxide, methane, and in oxidation reactions in the presence of water vapor [4–6]. 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 more sensitive to deactivation [4]. For example, a mixture of Cu and Mn oxides was the first mixed oxide catalyst used for oxidation of VOC in submarines. Mixed oxides of transition metals can be easily obtained by calcination of layered double hydroxide (LDH) precursors. Such

∗ Corresponding author. E-mail address: [email protected] (K. Jirátová).

mixed oxide catalysts show fine dispersion of active components, large surface area and good thermal stability. There is a question, whether a synergic effect of transition metal cations present in the multicomponent LDH-related mixed oxide catalysts could increase their activity in oxidation of VOCs. In this work the LDH precursors containing divalent and trivalent transition metal cations in various combinations were prepared and properties of the related mixed oxide catalysts as well as their activity in the ethanol deep oxidation were examined, with the aim to enhance the activity and selectivity of the mixed oxide catalysts. Optimization of the catalysts composition could also minimize undesirable reaction byproducts formed during VOCs oxidation. Ethanol was chosen as a model volatile organic compound because ethanol is often used as a fuel for buses and cars (e.g., in Scandinavian countries and especially in South America). Therefore, the total oxidation of ethanol including the reaction intermediates is interesting and worth studying.

2. Experimental 2.1. Catalysts preparation The MII –MIII LDH precursors (MII = Cu–Ni, Cu–Co, or Co–Ni in molar ratio of 1:1, MIII = Mn or Al, MII /MIII molar ratio of 2) were prepared by coprecipitation. An aqueous solution (450 ml) of appropriate nitrates, i.e., Cu(NO3 )2 ·3H2 O, Co(NO3 )2 ·6H2 O, Ni(NO3 )2 .6H2 O, Mn(NO3 )2 ·4H2 O, and Al(NO3 )3 ·9H2 O (total metal

http://dx.doi.org/10.1016/j.cattod.2015.10.036 0920-5861/© 2015 Published by Elsevier B.V.

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ion concentration of 1.0 mol l−1 ) was added with flow rate of 7.5 ml min−1 into 1000 ml batch reactor containing 200 ml of distilled water. The flow rate of simultaneously added alkaline solution of 0.5 M Na2 CO3 and 3 M NaOH was controlled to maintain reaction pH 10.0 ± 0.1. Coprecipitation was carried out under vigorous stirring at 25 ◦ C. The resulting suspension was stirred 1 h at 25 ◦ C, the product was then filtered off, washed thoroughly with distilled water and dried overnight at 60 ◦ C in air. The dried and powdered product was formed into pellets and calcined at 500 ◦ C for 4 h in air. After cooling to room temperature, the calcined pellets were crushed and sieved to obtain the fraction of particle size 0.160–0.315 mm, which was used in further experiments. The obtained catalysts were denoted by acronyms with cation composition in the form MII –Mn or MII –Al, for example Cu–Ni–Mn or Co–Cu–Al. In addition to the ternary mixed oxide catalysts, the binary analogs Cu–Mn, Co–Mn and Ni–Mn (MII /Mn molar ratio of 2) were also prepared and examined for comparison.

2.2. Characterization of samples The content of metal cations in the prepared catalysts was determined by atomic absorption spectroscopy (AAS) using a SpectrAA880 instrument (Varian) after samples dissolution in hydrochloric acid. Powder X-ray diffraction (XRD) patterns were recorded using a Seifert XRD 3000P instrument with Co K␣ radiation ( = 0.179 nm, graphite monochromator, goniometer with the Bragg-Brentano geometry) in 2 range 10–80◦ , step size 0.05◦ . The qualitative analysis was performed with a HighScore software package (PANanalytical, The Netherlands, version 1.0d) Surface area and mesoporous structure of the catalysts were examined by adsorption/desorption of nitrogen at −196 ◦ C using Micromeritics ASAP 2010 instrument. The BET and BJH (Barrett, Joyner, Halenda) methods were used for evaluation of surface area and pore size distribution, respectively. Presence of micropores and their volume was determined by the t-plot method. Temperature programmed reduction (TPR) measurements were performed with 0.025 g of a calcined sample, stored in a closed bottle, with a H2 /N2 mixture (10 mol. % H2 ), flow rate 50 ml min−1 and linear temperature increase 20 ◦ C min−1 up to 1000 ◦ C. Changes in H2 concentration were detected with a katharometer. Reduction of the grained CuO (0.160–0.315 mm) was repeatedly performed to calculate absolute values of the hydrogen consumed during reduction of the calcined samples. Temperature-programmed desorption of NH3 (NH3 -TPD) was carried out to examine acidic properties of the catalysts surface. The measurements were accomplished with 0.050 g of a sample in the temperature range 20–900 ◦ C, with helium as a carrier gas and NH3 as adsorbing gas. Before the NH3 -TPD measurements the samples were heated in helium from 25 to 500 ◦ C with temperature ramp 20 ◦ C min−1 , then was the sample cooled in He to 25 ◦ C. The heating rate of 20 ◦ C min−1 was applied. Composition of gases evolved during the experiments was determined by a mass spectrometer (Balzers). The following mass contributions m/z were collected: 2-H2 , 18-H2 O, and 16-NH3 . The spectrometer was calibrated by dosing an amount (840 ␮l) of NH3 into the carrier gas (He) in every experiment. In the same set-up, temperature-programmed desorption of oxygen (O2 -TPD) from the oxide catalysts into helium was carried out. The catalysts (50 mg) calcined in air at 500 ◦ C for 4 h were cooled to 25 ◦ C, placed in the reactor vessel and changed He stream of 30 ml min−1 was introduced. The catalyst was then ramped to 900 ◦ C at a linear heating rate of 20 ◦ C min−1 . The analysis of O2 in the effluent gas was performed with a mass spectrometer (OmniStar QMS 200, Pfeiffer Vacuum) while monitoring the

m/z signal 32-O2 . The desorption peak areas corresponding to the adsorbed species were calibrated in separate experiments using pulses of oxygen from a calibrated volume. The TPR, NH3 -TPD and O2 -TPD experiments were evaluated using OriginPro 8.0 software with an accuracy of ±5%. 2.3. Catalytic measurements The catalytic reaction was carried out in a laboratory fixed-bed glass reactor (5 mm i.d.) in the temperature range from 80 to 400 ◦ C and the temperature ramp of 2 ◦ C min−1 . The catalyst (0.1–0.4 g of the sieved grains with particle size of 0.16–0.315 mm) was examined at space velocity (GHSV) of 20 and 80 m3 kg−1 h−1 . The inlet ethanol concentration in air was 1.2 g m−3 (equaled to 750 ppm). Reaction products were analyzed using a gas chromatograph Hewlett-Packard 6890 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). Conversions of ethanol at two values of GHSV were calculated according to the equation xEtOH = (co −c)/co , where co is inlet concentration of ethanol and c is ethanol outlet concentration at the actual reaction temperature. Temperatures T50 and T90 (the temperatures, at which 50% and 90% ethanol conversion were achieved) were chosen as a measure of the catalysts activity. Selectivity in ethanol conversion was evaluated as the amount of formed acetaldehyde (ppm) in the temperature range from 50 (or 100) to 400 ◦ C. The accuracy of the conversion and selectivity determination was ±3%. 3. Results and discussion 3.1. Chemical and phase composition Chemical analysis (AAS) of the prepared catalysts showed that molar ratios of cations in solids corresponded approximately to those in the nitrate solutions used for coprecipitation (Table 1). The ternary mixed oxide catalysts contained low amounts of residual sodium cations (<0.1 wt.%), which were not removed from the LDH precursors during washing. Higher Na contents (up to 1.3 wt.%) were found in the binary MII –Mn samples. Powder XRD patterns of the dried precursors showed diffraction peaks characteristic for hydrotalcite-like LDHs, no other phases were found (Fig. 1). The Al-containing LDHs exhibited a higher crystallinity compared to the samples containing Mn; the Cu–Co–Mn and Cu–Ni–Mn precursors were almost amorphous but the diffraction peaks characteristic for LDHs can be still distinguished. During the coprecipitation, the majority of Mn2+ was oxidized to Mn3+ , as it was determined from the mean valence of the metal cations in the coprecipitated LDHs containing manganese [8,9]. It is known that both CuII and MnIII show distortions in octahedral coordination due to the Jahn–Teller effect, which could make the formation of brucite–type sheets more difficult. Among the binary MII –Mn precursors, the hydrotalcite-like LDH was found only in the Ni–Mn sample, together with the trace amount of MnCO3 (rhodochrosite). In the Co–Mn sample, a phase with d ∼6.65 A˚ was detected, which was ascribed to an unidentified, probably hydroxide- and/or oxyhydroxide-type product [9]. The Cu–Mn precursor showed amorphous phase and only poorly crystalline CuO (tenorite) was detected in this sample (not shown here). Formation of oxide phases during heating of the coprecipitated binary Ni–Mn, Co–Mn, and Cu–Mn precursors was described in our former reports [8–10]. In the calcination product obtained by heating of the Ni–Mn LDH at 500 ◦ C in air the MnIV -containing mixed oxides were found, including Ni6 MnO8 with a murdochitetype structure and NiMnO3 isostructural with ilmenite [8]. Thermal decomposition of the Co–Mn precursor resulted in the formation

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Table 1 Chemical composition of the mixed oxide catalysts and their texture characterization. Catalyst

Metal content, wt.%

Co–Mn Ni–Mn Cu–Mn Cu–Ni–Mn Cu–Co–Mn Co–Ni–Mn Cu–Ni–Al Cu–Co–Al Co–Ni–Al

H

Cu

Ni

Co

Mn

Al

Na

0 0 52.4 25.8 25.1 0 28.2 28.5 0

0 32.8 0 24.0 0 24.6 25. 3 0 27.2

32.2 0 0 0 22.6 23.4 0 24.8 25.7

15.0 14.4 22.4 21.9 21.7 21.8 0 0 0

0 0 0 0 0 0 12.1 11.8 12.2

1.31 0.24 0.91 0.06 0.08 0.06 0.04 0.10 0.05

H

H

HH

H Ni-Co-Al

Ni-Cu-Al

Intensity

CBET

Vmicro , mm3 g−1

Vmeso , cm3 g−1

Rmax , nm

44 47 36 54 62 63 150 96 131

– 150 103 133 89 79 124 169 150

0.10 12.6 8.5 14 40 14 37 22 28

0.16 0.42 0.38 0.44 0.57 0.51 0.42 0.50 0.51

40 26 30 19 21 17 3.7 11 5

were also found 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. 1). Only spineltype phases were detected in the calcined Co–Ni–Al, Co–Ni–Mn, and Cu–Co–Mn samples. Diffraction lines corresponding to CuO (tenorite) were found, together with a spinel-type oxides in the calcined Cu–Co–Al and Cu–Ni–Mn samples. Spinel-type oxides were not found in the powder XRD pattern of the calcined Cu–Ni–Al sample. The broad diffraction lines were ascribed to a NiO-like oxide and no distinct Cu-containing phase was detected in this sample. In general, temperature of 500 ◦ C used for thermal decomposition of the prepared precursors is not very high and formation of nonstoichiometric and poorly ordered oxide phases in the obtained mixed oxide catalysts could be expected.

dried / 60 °C H

SBET , m2 g−1

250 cps Co-Cu-Al

Ni-Co-Mn

3.2. Texture characterization

Ni-Cu-Mn Co-Cu-Mn 10

20

30

40

50

S

S O

Intensity

Ni-Co-Al

S

S

O

S T T S

S

S

S

S

S

70

S

S S

O

Ni-Cu-Al Co-Cu-Al

S

T

S T

S

Ni-Co-Mn S

S

S

Ni-Cu-Mn

S

T

S

S

Co-Cu-Mn

S

S

S

S

S

80

S

calcined / 500 °C

250 cps

60

S

S

S

S

Surface areas of the ternary Mn-containing mixed oxides were around 60 m2 g−1 whereas surface areas of the samples containing Al were substantially larger (120–170 m2 g−1 ) as shown in Table 1. High value of adsorbed nitrogen at low relative pressures P/Po in adsorption isotherms indicated presence of micropores. Larger volume of micropores was detected in the Al-containing mixed oxides. High values of CBET constants of the BET equation (from 79 to 169) also confirmed the presence of micropores in the catalysts. After recalculation of the nitrogen adsorption data according to the three-parameter adsorption equation [7] the new BET constants showed lower values (from 8 to 20) slightly differing for the sets of Mn- and Al-containing catalysts. It indicates that surface properties of Mn- and Al-containing catalysts are not identical. Mean radius of mesopores of the Mn-containing catalysts was approximately three-times larger than that of the Al-containing ones excepting the Cu–Co–Al sample, which exhibited pore radius comparable with those of the Mn-containing catalysts (Fig. 2). Larger pore size of catalysts is advantageous as it makes the transport of reactants more efficient and, therefore, such catalysts could be more active in the catalytic reaction. 3.3. TPR and TPD results

10

20

30

40

50

60

70

80

2θ / ° (Co Kα) Fig. 1. Powder XRD profiles of the samples obtained by coprecipitation at pH 10 and 25 ◦ C; dried at 60 ◦ C and calcined 4 h at 500 ◦ C in air. H, hydrotalcite-like phase; O, NiO-like oxide; S, spinel-type mixed oxide; T, tenorite.

of spinel-type phases. Calcination in air caused partial oxidation of both CoII to CoIII and MnIII to MnIV . The primary crystallization of a Co-rich Co3 O4 -type spinel followed by Mn incorporation into the spinel lattice was observed during heating of the Co–Mn precursor [9]. Calcination of the Cu–Mn precursor led to formation of tenorite and MnIV -containing Cu–Mn spinel [10]. Spinel-type mixed oxides

TPR profiles of the examined catalysts are shown in Fig. 3, in which the TPR data obtained with Mn- or Al-containing ternary mixed oxide catalysts are compared with those measured with the binary MII –Mn samples. The Cu–Mn mixed oxide was reduced in one main peak centered around 220 ◦ C with a shoulder at about 180 ◦ C corresponding to the reduction of Cu oxides to Cu0 and Mn oxides to MnO. The Ni–Mn mixed oxide was also reduced at relatively low temperature; a broad reduction peak was centered at about 350 ◦ C with a shoulder at 300 ◦ C. Its TPR profile was similar to that reported for the reduction of MnOx and corresponded to the reduction of MnIV → MnIII,IV → MnII with the maxima at 328 a 424 ◦ C [11,12,13]. A shoulder at 300 ◦ C could be attributed to the

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4

1,2 5

1,0

dV/dlog r, cm3 g-1

2

0,8

1

3 6 4

0,6 0,4 0,2 0,0

1

10

100

r, nm Fig. 2. Pore size distributions of the ternary mixed oxide catalysts 1, Co–Cu–Mn; 2, Co–Ni–Mn; 3, Cu–Ni–Mn; 4, Cu–Co–Al; 5, Co–Ni–Al; 6, Cu–Ni–Al.

reduction of Ni oxides. In the TPR profile of the Co–Mn mixed oxide, three distinct reduction maxima at about 260, 350, and 525 ◦ C were observed. Based on our results reported formerly [9], the lowtemperature reduction peaks, detected in the range from 200 to 400 ◦ C, were ascribed to the reduction of CoIII to CoII and MnIV to MnIII , though the reduction profile of the reference Co3 O4 sample showed maximum reduction at slightly higher temperature (405 ◦ C). Very likely, particles of cobalt oxides in the calcined products were smaller and reducible more easily than the commercial Co3 O4 used as a reference. Occurrence of mass transport effects during TPR measurements makes the assignment of TPR peaks to reduction of individual oxide components extremely difficult. We suggest that the first reduction peak at 350 ◦ C with the distinct shoulder at 260 ◦ C may be associated with the reduction of a finely dispersed Co3 O4 -like oxide. The high-temperature reduction peak at 400–700 ◦ C in the TPR profile of the Co–Mn mixed oxide could be attributed to the reduction of Co–Mn mixed oxide. The ternary mixed oxides exhibited more complex reduction behavior than the binary ones. The TPR profile of the Co–Ni–Al mixed oxide showed two main reduction peaks: the lowtemperature peak at 340 ◦ C and a broad high-temperature peak composed of at least two individual reduction peaks with maxima at 620 and 710 ◦ C. When Al was substituted with Mn, two distinct reduction peaks can be seen in the TPR profile of the Co–Ni–Mn sample, the first one with maximum at 300 ◦ C (with a shoulder at 200 ◦ C) and the second peak with maximum at 450 ◦ C (Fig. 2c). Though both Co–Ni–Al and Co–Ni–Mn catalysts contained spineltype mixed oxides, substitution of Al cations by Mn ones resulted Cu-Ni-Al Cu-Ni-Mn Cu - Mn

0,05

0

200

400

600

Temperature, °C

800

1000

in easier reduction of the Co–Ni–Mn sample and in a shift of the reduction peaks to lower temperatures. In the TPR profiles of the Cu-containing ternary mixed oxides new sharp peak appeared at about 240 ◦ C; it was ascribed to the reduction of CuO to Cu0 . The TPR profile of the Cu–Ni–Mn mixed oxide showed the reduction maxima at about 180, 240, 300, and 370 ◦ C. The peaks observed at 180 ◦ C could be ascribed to the reduction of copper oxide (CuO and an amorphous Cu-containing component). Reduction of the Co–Cu–Mn sample was similar to that of the Cu–Ni–Mn mixed oxide. Despite the fact that no CuO diffraction lines were detected in the powder XRD pattern of the Co–Cu–Mn mixed oxide, the reduction peaks identified in the lowtemperature region (up to about 260 ◦ C) could be connected with the reduction of an amorphous copper oxide. The broad reduction peaks observed at higher temperatures in the TPR profiles of the Al-containing ternary mixed oxides can be ascribed to the reduction of spinel-type mixed oxides including aluminum, e.g., Ni or Co aluminates. The highest amounts of easily reducible components (detected in the range from 25 to 500 ◦ C) were found with the Co–Cu–Mn, Cu–Ni–Mn and Cu–Mn catalysts (Table 2). Acidic properties of the catalysts used in the VOCs total oxidation are also important, as they can influence the adsorption of the VOC on the catalyst surface and the subsequent pathway of ethanol conversion. For that reason we analyzed them using temperature programmed desorption of ammonia (NH3 TPD), which can give information about total amount as well as strength of the acid sites. Total concentration of acid sites can be evaluated from the amount of ammonia desorbed between 25 and 500 ◦ C per gram of the catalyst (Table 2). Strength of the acid sites can be determined by considering the strong acid sites being the sites retaining NH3 at temperatures higher than 300 ◦ C. Accordingly, the sites retaining NH3 at temperatures lower than 300 ◦ C can be considered as weak and/or moderate acid sites. All examined catalysts showed only negligible ammonia desorption at temperatures higher than 300 ◦ C and, therefore, the strong acid sites likely did not occur in the prepared mixed oxides. The total acidities measured for the catalysts followed the order of Co–Ni–Mn > Co–Ni–Al > Cu–Ni–Al > Cu–Co–Al ∼ Cu–Co–Mn ∼ Ni–Mn > Cu–Ni–Mn ∼ Co–Mn > Cu–Mn; however, the difference among them was not very big. The Al-containing ternary mixed oxides showed similar amount of acid sites (0.32–0.23 mmol/g) as the Mn-containing ones (0.30–0.24 mmol/g); the binary MII –Mn catalysts were less acidic (0.23–0.18 mmol/g). Tanaka and Ozaki [14] revealed formerly that acid-base properties of solids are determined by induction effect of metal ions. For example, they have obtained a linear correlation between isoelectric point of metal oxides and generalized electronegativity of metal

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

0,05

0

200

400

600

800

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

0,02

1000

Temperature, °C

0

200

400

600

800

1000

Temperature, °C

Fig. 3. TPR profiles of the mixed oxide catalysts.

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Table 2 Acidity, reducibility (H2 consumption in mmol/g), amount of oxygen desorbed from the mixed oxide catalysts, their activity in total oxidation of ethanol (expressed as temperatures T50VOC and/or T90VOC ) and selectivity toward acetaldehyde (AcA) formation. Catalyst

Electronegativity

NH3 a mmol/g

H2 a mmol/g

H2 b mmol/g

O2 -TPDa mmol/g

O2 -TPDb mmol/g

T90VOC c ◦ C

T50EtOH d ◦ C

T50VOC d ◦ C

AcAe ppm

Co–Mn Ni–Mn Cu–Mn Cu–Ni–Mn Cu–Co–Mn Co–Ni–Mn Cu–Ni–Al Cu–Co–Al Co–Ni–Al

8.90 8.82 7.93 8.5 8.5 8.93 8.58 8.59 9.02

– 0.23 0.18 0.20 0.23 0.32 0.26 0.24 0.30

5.0 8.1 10.6 12.7 11.6 9.9 5.3 6.3 1.9

12.7 10.8 12.5 13.0 12.0 12.8 7.5 6.9 9.6

0.14 0.06 0.03 0.14 0.06 0.14 0.04 0.06 0.03

0.28 0.38 0.19 0.38 0.20 0.20 0.17 0.05 0.12

– – – 154 173 181 220 194 204

121 118f 135 100 105 107 175 161 167

185 198f 186 162 167 169 194 190 197

64,258 66,916f 58,893 66,635 82,819 70,254 25,137 42,557 45,994

a b c d e f

25–500 ◦ C. (25–900 ◦ C). GHSV = 20 m3 kg−1 h−1 . GHSV = 80 m3 kg−1 h−1 . Total amount of acetaldehyde observed during experiment performed at GHSV = 80 m3 kg−1 h−1 . GHSV = 40 m3 kg−1 h−1 .

ions X(ion) calculated according to Sanderson [15] by the relation X(ion) = (1 + 2Z) X0 , where Z is the ion charge and X0 the Pauling electronegativity of a metal. Shibata et al. [16] have found a correlation between the highest acid strength of equimolar binary mixtures of metal oxides and the arithmetic mean of electronegativity of corresponding couple of metal ions. In order to examine the effect of different cation composition of the prepared mixed oxide catalysts on their acidity, we calculated the electronegativities of corresponding metal ions X(ion) under assumption of additive behavior and using molar fractions of the oxides as weights and summarized them in Table 2. Roughly linear dependence (y = 0.1173x − 0.7647, R2 = 0.70) of the catalysts total acidities, expressed as mmol NH3 g−1 desorbed in the temperature range from 25 to 500 ◦ C, with the calculated electronegativities was found. Easiness of oxygen desorption is another important feature of the catalysts active in oxidation–reduction reactions [17]. Oxygen adsorption properties of various metal oxides were investigated by TPD technique in [18]. At least four different states of adsorbed oxygen over nickel oxide were indicated by the appearance of TPD peaks with maxima at 30–40 (˛), 320–360 (ˇ), 420–450 (), and 520–550 ◦ C (ı), respectively. The corresponding adsorbed species were tentatively assigned to O2 (˛), O2 − (ˇ), and O− ( and ı) [19]. Similarly, Trawczynski et al. [20] found overlapping peaks of O2 desorption at 350, 524 and 780 ◦ C in the case of MnOx /Al2 O3 . Radhakrishnan et al. [21] studied O2 -TPD profiles of manganese catalysts (3 wt.% Mn) deposited on Al2 O3 , ZrO2 , TiO2 and SiO2 supports from acetate precursor. We measured the temperature programmed desorption of oxygen performed in helium; the data obtained over the prepared mixed oxide catalysts are summarized

3.4. Catalytic activity and selectivity The activity of the ternary mixed oxide catalysts was tested at conditions commonly used in our laboratory (GHSV=20 m3 kg−1 h−1 , linear ramp of temperature 2 ◦ C min−1 ). All ternary mixed oxide catalysts showed stable catalytic activity during repeated use. The Mn-containing ternary mixed oxides were more active than the analogous catalysts containing Al (Table 2). The Cu–Ni–Mn sample exhibited the highest activity among the tested ternary mixed oxide catalysts, as almost all organic compounds (90%) were transformed to CO2 and H2 O at T90VOC temperature of 154 ◦ C. With the other Mn-containing ternary mixed oxides (Cu–Co–Mn and Co–Ni–Mn samples) the same conversion (90%) was achieved at 173 and 181 ◦ C, respectively. The Al-containing ternary mixed oxide catalysts needed for achieving 90% conversion of all organic compounds substantially higher temperatures; T90VOC temperatures of 194, 204, and 220 ◦ C

Cu-Ni-Mn

1E-11

Cu-Mn Co-Mn

in Table 2 and the O2 -TPD profiles are shown in Fig. 4. The Mncontainining samples showed characteristic desorption peak of oxygen starting at about 400 ◦ C with maximum at around 550 ◦ C (binary MII –Mn catalysts) or 600 ◦ C (ternary catalysts). Amounts of oxygen desorbed from the Mn-containing ternary mixed oxides in the range 25–500 ◦ C were higher in comparison with those determined for the binary mixed oxide catalysts. Substitution of Mn by Al resulted in lower desorption of oxygen from the catalysts surface. The total amounts of oxygen desorbed in the temperature range 25–900 ◦ C showed an approximately linear correlation with the amounts of components reducible in the same temperature range.

Cu-Co-Mn

Cu-Co-Al Cu-Ni-Al

Co-Ni-Mn

Co-Ni-Al

1E-11

Ni-Mn

1E-11

Co-Mn

0

200

400

600

Temperature, °C

800

1000

0

200

400

600

800

1000

Temperature, °C

0

200

400

600

800

1000

Temperature, °C

Fig. 4. O2 -TPD profiles of the mixed oxide catalysts.

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Table 3 Most reliable ion radii of divalent and trivalent ions forming hydrotalcite-like compounds [25]. Radius, nm

M3+

Radius, nm

Cu Co Ni Mn

0.065 0.0745 0.069 0.083

Al Co Ni Mn

0.0535 0.0545 0.056 0.0645

were found with the Cu–Co–Al, Co–Ni–Al, and Cu–Ni–Al catalysts, respectively. The Mn-containing mixed oxides showed rather high activity in the total oxidation of ethanol and the precise determination of T50EtOH value (temperature, at which 50% ethanol conversion was achieved) from the conversion vs. temperature dependences measured at GHSV of 20 m3 kg−1 h−1 was very difficult or even impossible. Therefore, all prepared mixed oxide catalysts including the binary MII –Mn ones were tested again at the space velocity of 80 m3 kg−1 h−1 ; the corresponding T50EtOH values are summarized in Table 2. The Al-containing ternary mixed oxides revealed lower catalytic activity, with T50EtOH of nearly 70 ◦ C higher compared to their Mn-containing analogs. The lowest T50EtOH temperatures of about 100 ◦ C was found for the Cu–Ni–Mn catalysts; the Co–Cu–Mn and Co–Ni–Mn samples showed only slightly lower activity (T50EtOH of 105–110 ◦ C). The T50EtOH temperatures found for the binary MII –Mn mixed oxides were of about 20–30 ◦ C higher. The measured T50EtOH values indicated that partial substitution of Cu in the Cu–Mn mixed oxide by Ni or Co increased its catalytic activity. Similar finding of lower catalytic activity of single CuO or Mn2 O3 in comparison with Cu–Mn mixed oxides was reported earlier [22]. The catalysts examined in [22] were obtained from precipitated precursors and studied in the ethanol oxidation; ethanol conversion of 50% varied in the temperature range from 180 to 220 ◦ C at low GHSV (only 4 l h−1 g−1 ). As mentioned in [23], various pure manganese oxide catalysts oxidized ethanol from 50% in the temperature range from 200 to 240 ◦ C (GHSV = 53,050 h−1 , CVOC,in = 930 ppm). It is evident that ternary MII –Mn catalysts exhibit higher activity in ethanol oxidation than manganese oxides or Cu–Mn mixed oxide catalysts. It can be seen from Table 2 that the most active Cu–Ni–Mn and Cu–Co–Mn catalysts contained the highest amounts of easily reducible components (reduced in the temperature range 25–500 ◦ C). The less active catalysts showed lower amount of easily reducible components. Therefore, the obtained data confirmed relation between the catalysts activity in the ethanol total oxidation and their redox properties. Similar findings were reported by Morales et al. [24] on ethanol oxidation over Mn–Cu mixed oxides. Our data also showed that catalysts activity in ethanol oxidation depended on the amount of oxygen desorbed in the temperature range 25–500 ◦ C; temperature T50EtOH decreased with increasing amount of desorbed oxygen. The high catalytic activity of Mn-containing ternary mixed oxides in the ethanol total oxidation could be also explained by a worse structure ordering of the calcination products obtained by heating of the LDH precursors. In Table 3 ionic radii of some divalent and trivalent metal cations taken from [25] are presented. It can be seen that Ni2+ and Co2+ cations are bigger than Cu2+ one. Moreover, some of Ni2+ and Co2+ cations could be oxidized to trivalent ones during heating of the LDH precursors in air and their ionic radii then would be smaller than that of Mn3+ . These differences in ionic radii could cause lattice defects and change concentration of oxygen vacancies [26] in the Cu–Mn mixed oxide, when Cu is partly substituted by Ni or Co. Increasing concentration of lattice defects can be reflected in higher activity of the ternary mixed oxide catalysts. Temperatures T90VOC shown in Table 2 confirmed high activity of the ternary mixed oxide catalysts, activity of the Cu–Ni–Mn catalyst

Co-Mn Ni-Mn Cu-Mn

800

Acetaldehyde, ppm

M2+

1000

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

600

400

200

0 0

50

100

150

200

250

Temperature, °C Fig. 5. Formation of acetaldehyde during ethanol oxidation over the mixed oxide catalysts.

being slightly higher than that of the Cu–Co–Mn one; combination of Co, Ni, and Mn cations in the LDH-related mixed oxide led to less active catalyst. Selectivity of the deep oxidation catalysts is very important as reaction byproducts could be often more harmful than original volatile organic compounds. Concentrations of the harmful compounds at high conversion of VOCs are especially important. In the case of ethanol oxidation, such byproducts can be acetaldehyde, acetic acid, ethylene, ethyl acetate, acetone, diethyl ether and carbon monoxide. Formation of the byproducts depends on both chemical structure of the compound and the properties of the catalyst. Over the examined highly active ternary mixed oxide catalysts, the main byproduct found in the outlet gas mixtures was acetaldehyde; acetic acid, as well as other possible byproducts were either not observed or only in negligible concentrations. For that reason, the major path of ethanol oxidation appears to be the direct oxidation of acetaldehyde to CO2 , while its oxidation via the acetic acid should be a minor path [27]. Formation of carbon monoxide in the stage of incomplete ethanol oxidation was not observed in the presence of all examined catalysts excepting the Ni–Mn one (unless the reaction temperature reached 220 ◦ C). Carbon dioxide was the final reaction product in all cases. Changes in acetaldehyde concentration in the outlet gas mixtures in dependence on reaction temperature are demonstrated in Fig. 5. Both the Cu–Ni–Mn and Cu–Co–Mn catalysts showed the lowest temperatures of acetaldehyde disappearance. Total amounts of acetaldehyde formed during the catalytic tests (in temperature region 50–250 ◦ C) are summarized in Table 2. It can be seen that the Al-containing ternary mixed oxide catalysts produced lower amount of acetaldehyde than the catalysts containing manganese. Hence, the reaction rate of acetaldehyde formation from ethanol has to be lower, when the Al-containing catalysts are used. This statement was confirmed by the fact that dependence of the total amount of formed acetaldehyde on T50EtOH temperature was inversely proportional. Acidity of the catalysts evidently play a role in the reaction mechanism of ethanol oxidation as the amount of acetaldehyde formed decreased with increasing catalyst acidity. 4. Conclusions The LDH-related ternary MII –Mn mixed oxides (MII = Cu–Ni, Cu–Co, or Co–Ni) were more active in the total oxidation of ethanol than binary Cu–Mn, Co–Mn, and Ni–Mn mixed oxides; the ternary

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Cu–Ni–Mn and Cu–Co–Mn catalysts were found as the most active. Though the Mn-containing ternary mixed oxide catalysts exhibited lower specific surface area than their Al-containing analogs, they were more active. At the same time, the catalysts having larger mean pore size showed higher oxidation activity. Selectivity of the mixed oxide catalysts was not influenced noticeably by their cation composition; acetaldehyde was found as the main reaction byproduct. All Al-containing catalysts produced lower total amount of acetaldehyde than the Mn/containing ones. Both the most active Cu–Ni–Mn and Cu–Co–Mn catalysts showed the lowest temperatures of acetaldehyde disappearance. Partial substitution of Cu by Co or Ni cations in the Cu–Mn mixed oxide led to increasing in its oxidation activity. Combination of three transition metal cations, Cu, Co or Ni, and Mn, could increase concentration of lattice defects and change oxygen vacancy in the mixed oxide obtained from the coprecipitated LDH precursor, likely due to different ionic radii of the components. Activity of the examined mixed oxide catalysts was linearly dependent on the amount of easily reducible components (reduced in the range 25–500 ◦ C) and on the amount of oxygen adsorbed on the catalysts. Acknowledgements Authors thank the Czech Science Foundation for the financial ˇ support (project P106/14-13750S) and to both Mrs H. Snajdaufová for measurement of porous structure data and Mrs L. Soukupová for chemical analysis of the examined catalysts. References [1] J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165–2180. [2] R. Burch, P.K. Loader, F.J. Urbano, Catal. Today 27 (1996) 243–248.

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Please cite this article in press as: K. Jirátová, et al., Total oxidation of ethanol over layered double hydroxide-related mixed oxide catalysts: Effect of cation composition, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.10.036