Catal. Sustain. Energy 2017; 4: 17–24
Research Article E.V. Korneeva, T.Yu. Kardash, V.A. Rogov, E.A. Smal*, V.A. Sadykov
Catalytic steam reforming of ethanol over W-, V-, or Nb–modified Ni-Al-O hydrotalcite-type precursors DOI 10.1515/cse-2017-0004 Received April 5, 2017; accepted May 16, 2017
Abstract: 2:1 Ni/Al layered double hydroxides (LDH) doped by anions using ammonium salts (NH4)10[W12O41], NH4VO3 or (NH4)3[NbO(C2O4)3] have been prepared by co-precipitation, dried and calcined at 600оС, forming NiO-based solid solutions. Diffraction patterns are typical for the layered Ni-Al-O hydrotalcite-like structure. Anion incorporation into the interlayer space increases the interlayer distance for W- and Nb-containing anions but decreases it for VO3-1. Broad halos in the diffraction patterns indicate amorphous or strongly disordered phases containing the doping anions. H2 reduction of undoped Ni-Al-O (NA) and those doped by W (NAW) and Nb (NANb) occurred in one step, while that doped by V (NAV) was reduced in two steps. W doping increases the reduction temperature, but Nb doping decreases it. The hydrogen consumed increases in the row: NANb < NAW < NAV < NA. In the ethanol steam reforming reaction, modification by W and Nb anions results in ethanol conversion rates close to that of the unmodified sample, but V increases it nearly twofold. Keywords: layered double hydroxides, interlayer anion, ethanol steam reforming
1 Introduction Nickel-based catalysts are active and selective for a range of reactions in petroleum refining including hydro-treating [1], hydro-cracking [2] and hydro-processing [3]. Ethanol is *Corresponding author: E.A. Smal, Boreskov Institute of Catalysis, pr. Akademika Lavrentieva, 5, 630090, Novosibirsk, Russia E-mail:
[email protected] E.V. Korneeva, T.Yu. Kardash, V.A. Rogov, V.A. Sadykov, Boreskov Institute of Catalysis, pr. Akademika Lavrentieva, 5, 630090, Novosibirsk, Russia T.Yu. Kardash, V.A. Rogov, V.A. Sadykov, Novosibirsk State University, Pirogova str., 2, 630090, Novosibirsk, Russia Open Access. © 2017 E.V. Korneeva et al., published by De Gruyter Open. NonCommercial-NoDerivs 3.0 License.
a nontoxic liquid easily transported through the current infrastructure and is a potential hydrogen source. Nickelbased catalysts are also used for hydrogen production from ethanol (decomposition [4], steam reforming [5−8] and oxidative steam reforming [9]). Catalyst stability is the most important challenge for this process. Recently, catalyst deactivation during ethanol conversion was thoroughly reviewed [10]; it is generally attributed to carbonaceous species deposition, as well as sintering and metal particle oxidation. The following reactions are possible routes for hydrogen production from ethanol: steam reforming (1), partial oxidation (2), and decomposition (3 and 4): C2H5OH + H2O → 2CO + 4H2 2C2H5OH + 3O2 → 4CO2 + 6H2 C2H5OH → CH3CHO + H2 C2H5OH → CH4 + CO + H2
(1) (2) (3) (4)
Although steam reforming provides the highest hydrogen yield, it is highly endothermic; high conversions require high operation temperatures [8, 12–15]. The pathways for hydrogen production [11] are shown below: dehydrogenation: C2H5OH → C2H4O+H2 acetaldehyde decomposition: C2H4O → CH4 +CO acetaldehyde steam reforming: C2H4O+H2O → 3H2 + 2CO water gas shift reaction: CO + H2O → CO2 +H2 methanation: CO + 3H2 → CH4 +H2O
(5) (6) (7) (8) (9)
Hydrotalcites are a class of 2D inorganic layered matrices with the general formula [MII1−xMIIIx(OH)2]x+(An−)x/n·mH2O, where MII and MIII are bivalent and trivalent cations, x is proportional to the MIII/(MII+MIII) molar ratio and A is an interlayer anion with valency n. Such compounds, especially Ni-Al layered double hydroxides, are prospective catalysts for ethanol steam reforming. This work is licensed under the Creative Commons Attribution-
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E.V. Korneeva, et al.
Ni-containing catalysts possess redox properties. For NiO there is one peak at 360оС corresponding to NiO reduction by H2 to Ni [16, 18]. Tungsten incorporation into nickel oxide shifts reduction peaks to higher temperatures [17] apparently caused by interaction between small NiO particles and tungsten oxide nanoparticles (surface area significantly increases with W addition) [18]. NiO doping by Nb cations results in the opposite effect, shifting the reduction peak to lower temperatures [19]. NiO reduction in supported NiO-WO3/Al2O3 catalysts occurs in several steps [16]; bulk or weakly bound NiO is reduced below 400°С, Ni cations incorporated into the surface layers are reduced at 450–750°С, and NiAl2O4 spinel is reduced above 800°С. Reduction of Ni-Al-oxides obtained by 450оС calcination of Ni-Al layered double hydroxides (Ni/Al = 3) occurs in one step at 530оС [20]. Ethanol decomposition catalysts obtained by calcination of layered double МII-MIII-hydroxides have been investigated. For example, an Ni-Al precursor prepared by co-precipitation with a 3:1 Ni/Al mole ratio calcined at 500оС was tested at 250оС. It was more stable than Ni/Al2O3 prepared by Al2O3 impregnation with Ni(NO3)2 solution [21]. The Ni/Al2O3 catalyst provided full conversion and 70% H2 selectivity during ethanol steam reforming at 400оС [22]. Hydrogen selectivity reached 91% at >500оС and a 6:1 water-ethanol ratio but catalytic activity dropped rapidly due to carbon formation. Considering that the reduction and catalytic properties might be regulated by changing the interlayer anion in double Ni-Al-hydroxides, the aim of this work was to study the influence of [H2W12O40]6-, VO3-, and [NbO(C2O4)3]3- introduced into the interlayer space of layered double Ni-Al-hydroxides on structural, textural, redox and catalytic properties of calcined precipitates in ethanol steam reforming.
2 Experimental 2.1 Catalyst preparation Ni-Al hydroxide (NA) with a 2:1 Ni/Al mole ratio was prepared by co-precipitation. A mixture of aqueous Ni and Al nitrates was added dropwise to Na2CO3 solution at 70оС and constant pH 9 ± 0.2, maintained by addition of 2M aqueous NaOH. The suspension was aged 4 hours under the same conditions. The precipitate was filtered and washed until the filtrate was neutral. The precipitate was dried at room temperature, then at 110°С for 18 h. The dried products were calcined at 600оС for 4 h.
The Ni-Al hydroxide was modified by adding interlayer anions from aqueous (NH4)10[W12O41], NH4VO3, or (NH4)3[NbO(C2O4)3] and prepared by the above co-precipitation procedure and labeled NAW, NAV or NANb. All samples were characterized by the following physical and chemical methods.
2.2 Catalyst characterization Chemical analysis was carried out using a Perkin Elmer Optima 4300VD inductively coupled plasma emission spectrometer. X-ray diffraction measurements were performed on a Thermo ARL X’tra diffractometer using CuKα radiation and θ-θ geometry with Bragg-Brentano focusing. Data were recorded from 2Θ = 5–70o (step size 0.05o, time 3 s) with a single channel Si(Li) detector. Instrumental broadening was determined using an external standard (quartz NIST SRM 640b). Diffraction patterns were analyzed using the ICDD PDF-2 database [24]. Structural phase parameters were obtained from the ICSD database [25]. Theoretical X-ray patterns were built using PowderCell V.4.2 [26]. Rietveld refinement was performed using the TOPAS v.4.2 program package [27]. Ar adsorption-desorption isotherms were obtained with a SORP-4.1 instrument and the SBET method [28] used to calculate the total surface area within ± 10%. Temperature-programmed reduction by H2 was carried out in a flow installation with 10 vol. % H2 in Ar at 40 ml/min and a temperature ramp from 40oC to 900oC at 10oC/min. The catalyst was 0.2 g of the 0.25–0.5 mm fraction. Before reduction, samples were pretreated in oxygen at 500oC for 0.5 h. Product water was frozen out at – 80oС. The H2 concentration was determined by a thermal conductivity detector. The rate of hydrogen consumption (molecule H2/m2∙s) was calculated by
VT =
VT ,g ⋅ N A S BET
.
VT,g is the rate of hydrogen consumption at temperature T, (mol Н2/g∙s); NA, Avogadro’s number; SBET, specific surface area (m2/g). Catalytic ethanol steam reforming was carried out in a flow apparatus (contact time 70 ms) at atmospheric pressure with on-line chromatographic analysis of products. Pelletized catalyst (0.25–0.5 mm fraction, 0.18 g) mixed with 1.8 cm3 of quartz sand was used. The catalyst was initially heated to 500°C under N2, followed by feeding the reaction mixture (10% C2H5OH + 40% H2O in
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Catalytic steam reforming of ethanol over W-, V-, or Nb–modified Ni-Al-O hydrotalcite-type precursors
N2). The temperature was raised to 700°С and maintained for 1 h. The temperature was then decreased to 500°С in 50°C steps. Ethanol conversion (X) was calculated by C Nin in out − C EtOH ⋅ out2 C EtOH CN 2 X= in C EtOH
⋅100%
.
Сin and Сout are concentrations at the reactor inlet and outlet (%). The selectivity (S) was evaluated as the ratio of concentration of each product to the total concentration of products:
S=
ni ⋅ Ci ⋅ 100% ∑ ni ⋅ Ci , i
Ci is concentration and ni is the number of C atoms in product i. Hydrogen yield (Y) was calculated by
Y=
C Hout2
in 4 ⋅ C EtOH
⋅ 100%
. The 4 in the denominator is from ethanol steam reforming equation (1).
3 Results and discussion 3.1 Chemical analysis The experimental Ni to Al ratio (Table 1) for the NA sample is close to the nominal 2:1. Introduction of the interlayer anion does not change this ratio substantially. The ratio of the modifying component changes from 0.16 to 0.43 with respect to Al (Table 1). The Na content does not exceed 0.03 %.
3.2 XRD analysis and specific surface area Fig. 1 presents XRD patterns of Ni-Al samples modified by different interlayer anions and dried at 110оС.
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The unmodified sample pattern is typical of layered double hydrotalcite-like hydroxides Ni-Al-СО3 [29] with asymmetrical distortion of the 012, 015 and 018 reflections. Model calculations suggest that these changes result from stacking faults in the 3R1 polytype due to inclusion of a large number of 2H1 polytype fragments. For the NA sample d003 and d006 values are 0.771 and 0.383 nm, respectively. Samples modified by interlayer anions also have the hydrotalcite-like structure (Fig. 1а). However, even larger distortions are observed for them. Thus, 015, 018 and 113 reflections almost completely disappear while the 012 peak is asymmetrical. Further, the 003 and 006 reflections are greatly broadened (in all cases the 006 reflection is broader than the 003) and shifted. The d003 interlayer distance changes depending on the anion (Table 2). Changing from СО32- to [W12O41]10- increases d003 from 0.771 to 0.799 nm. In agreement with existing ideas about the layered double hydroxide structure, the effect can be due to both charge and anion size. The ionic radii of СО32- and [W12O41]10- equal 0.189 and 0.8 nm respectively [30, 31]. Increased anion charge decreases d003 due to increased electrostatic interaction, while larger anions increase d003 [32, 33]. Similar considerations hold for samples modified by VO3- and [NbO(C2O4)3]3-, anions with different charge and size. Diffraction patterns of samples calcined at 600оС contain peaks typical for the NiO structure (Fig. 1b). The unit cell parameter (a) of cubic NiO in the NA sample of 0.4143 nm (Table 2) is less than the 0.4177 nm for pure NiO [34]. The lower а suggests structural disordering and incorporation of Al3+ with its smaller ionic radius than Ni2+. For modified samples only reflections corresponding to a disordered cubic NiO phase are observed (Fig. 1b). There are broad halos in 2Q up to 30° corresponding to amorphous or strongly disordered phases containing doping anions. At the same time, the a parameter decreased for modified samples. The NiO crystallite size of 2.4 – 3.0 nm indicates high dispersion. The specific surface area (SBET) of non-modified sample dried at 110оС is 70 m2/g. Modifying with interlayer anions resulted in a negligible increase of the surface area (Table
Table 1. Sample composition. Sample
NA NAW NAV NANb
Composition, wt.% Ni
Al
M
Na
57.2 44.5 49.6 44.3
12.9 9.7 11.0 9.4
W 10.4 V 9.0 Nb 13.5
