Abstract-- In this paper, the catalytic behavior of nonstoichiometric nickel oxides was investigated in the ethane oxidative dehydrogenation reaction (ODH). The oxide was obtained by heat treatment in the range 250 - 400°C of β-Ni(OH)2 synthesized by precipitation-oxidation. The structural properties of the catalysts were characterized by XRD, FTIR, UV-vis, and SEM. The selective ODH reaction was quantified by gas chromatography. The samples showed pure NiO cubic structure with different degrees of crystallinity, crystallite size, and specific surface area. The increase of calcination temperature caused an increase in the average crystallite size from 28 Å at 250°C to 104 Å at 400 °C, whereas the shape of the particles showed no significant differences; pore diameter increased from 35 Å to 69 Å and specific surface area decreased from 245 to 82 m2/g. The test carried out at 240°C showed a decrease in conversion, from 8.75% to 4.25%; the more crystalline samples displayed the highest intrinsic activity with selectivity to ethene close to 60%. No influence of band-gap energy on catalytic properties was observed.
Keywords-- Oxidative Dehydrogenation, Ethane, Ethene, Nickel Oxide.
I. INTRODUCTION
One of the greatest difficulties is the development of suitable ODH catalysts, which minimize secondary oxidation reactions and the total oxidation of ethane (Arnold et al. (2010). The olefin product tends to be more reactive than ethane, undergoing secondary oxidation and generating undesired by-products such as CO and CO2 (Aimmermann and Walzl, 2009; Grabowski, 2006). In this context, several materials with redox properties have been developed and applied in order to assist the mechanism of reactions at the molecular level, and reactions in the gas phase (Gärtner et al., 2013). Among those materials, nickel oxide stands out for being a cation-deficient semiconductor as well as being an excellent oxidation catalyst able to activate C-H bond at low temperature (Verwey et al., 1950; Zboray et al., 2009). Many studies on catalytic performance and kinetic behavior suggested that ethane ODH on nickel oxide follows the Mars-van Krevelen redox mechanism (Mars and van Krevelen, 1954). The oxidation-reduction cycle begins with the activation of a hydrogen atom from an ethane molecule by a surface O2- species, generating the ethyl radical that is desorbed from the catalyst in the form of ethene (ethylene). The remaining hydroxyl species are eliminated as water, leaving a reduced metallic site available for subsequent oxidation by molecular dioxygen from the gas phase (Gärtner et al., 2013; Popescu et al., 2015). Activation of the C-H bond is the rate-limiting step of the overall reaction due to the high stability of the C-H bond (419.5 kJ/mol) (Zboray et al., 2009; Batiot and Hodnett, 1996).
The NiO surface reactions, as well as catalytic and chemisorptive properties, depend strongly on the preparation methods and surface defects created during the heat pretreatment of precursor (Roberts and Smart, 1984; Wepfer et al., 1980). The literature abounds with reported ways to prepare nickel oxide; the choice of the appropriate method to synthesize the precursor depends on the particular chemical or physical desired properties and the production scale. Among these methods, coprecipitation-calcination carried out at a moderate temperature is a simple, potentially scalable method, with advantages of large-scale production, flexible process, less time consumption, and low-cost production (Wu et al., 2007). In the case of β-Ni(OH)2, the oxide NiO probably comes from partial dehydration of nickel hydroxide. It has been claimed that dehydration promotes structural and surface defects leading to the formation of Ni3+ ions (Moroney et al., 1983). Cation defects (Ni2+ vacancy) at the surface lead to the formation of Ni3+ ions, associated with the existence of extra oxygen in the non-stoichiometric p-type oxide Ni1-xO (Gray and Darby, 1956; Heracleous and Lemonidou, 2006; Varghese and Mushrif, 2017). But O2- vacancies can also be formed. Both O and Ni vacancies tend to play important roles in the activity and selectivity of the catalyst in the oxidative dehydrogenation of ethane (Che and Tench, 1982; Savova et al., 2010). The participation of Ni-O pairs adjacent to Ni vacancies on NiO(100) and NiO(110) surfaces has been examined from a theoretical point of view (Varghese and Mushrif, 2017). The role of the oxygen excess has also been investigated experimentally (Zhao et al., 2021). An understanding of the chemistry of those surface processes relies on defining, in some detail, the electronic configurations of metal and oxygen species in the surface region (Dubey et al., 2018).
The thermal dehydration of Ni(OH)2 has some influence on the physicochemical properties of NiO such as crystallite size, surface area, pore size and volume, bandgap energy, among others. Abbas and Jung (2016) observed a decrease of surface area and pore volume of the mesoporous microspherical NiO samples with increasing calcination temperature of mesoporous β-Ni(OH)2. Surface areas of NiO samples prepared at 250, 300, 350, 400 and 500 ºC were 295, 197, 133, 92, and 10.5 m2/g, respectively. Similar behaviors of surface area and pore volume were observed by Lin et al. (2003) during the thermal decomposition of Ni(OH)2 synthesized by the sol-gel method. These authors also observed an increase in the average crystallite size with an increase in the calcination temperature. Other researchers (Akinkua-de et al., 2018) found that an increase in the average crystallite sizes of the NiO films with annealing temperature was accompanied by a decrease in the optical band-gap. Zhu et al. (2011) showed that the gap energy as well as the crystallite size of NiO increase with Ni(OH)2 calcination temperature.
Recently, Getsoian et al. (2014) proposed that the apparent activation energy for propene oxidation to acrolein over mixed metal oxides with the scheelite structure correlates with the bandgap of the catalyst, a result which was also supported by theoretical calculations. They noted that the model proposed to explain the correlation could also apply to any oxidation reaction in which activation of the substrate involves LMCT-like excited states of the catalyst, whatever its crystalline structure.
The effects of thermal treatments on the catalytic activity of metal/oxide-based materials are commonly studied (Ghadamgahi, 2018). In this work, we analyzed the influence of calcination temperature on the morphology, on the band-gap energy, and on the catalytic performance of nickel oxide in the ODH reaction of ethane into ethylene.
II. METHODS
A. Catalyst Preparation
The synthesis of nickel oxide catalysts was conducted by precipitation-oxidation in an aqueous media following a methodology adapted from Tang et al. (2008). The precipitation process was carried out at 50 °C with 15 mL of 0.6 M Ni(NO3)2·6H2O solution added dropwise to 30 ml of 3.2M NaOH solution; 30 mL of H2O2 (30 wt%) was then introduced under constant stirring. The precipitate was then filtered, washed with distilled water, and dried in an oven at 80 °C for 24 h. The dried Ni(OH)2 was submitted to heat treatment under airflow at different temperatures (250 - 400 ◦C).
B. Physical and Chemical Characterization
X-Ray diffraction (XRD) patterns of the different samples were obtained using a diffractometer (Ultima IV Rigaku) operated with Cu-Kα radiation (λ = 1.5418 Å) at 40 kV and 20 mA with Kβ filter. The data were recorded by continuous scanning over a range of 2θ angles from 5° to 80° with steps of 0.05. Crystal structures were identified using the Inorganic Crystal Structure Database (ICSD) files. The application of the Scherrer equation allowed the calculation of the average crystallite size of the samples (Young, 1993). Lattice parameters were determined using the Maud program for Rietveld refinement (Kubelka, 1931).
Infrared analysis was performed by Fourier-transform infrared spectroscopy (FTIR) in the wavenumber range of 400 to 4000 cm−1 (Nicolet-6700 spectrometer). Precursors and calcined samples were dispersed in KBr and pressed into waffles at 7 tons/cm2. Diffuse reflectance UV-visible spectra of the samples have been collected in the range of 200-900 nm under ambient conditions (Shimadzu 2600 spectrophotometer) with an integration sphere. NiO samples were analyzed in powder form, after dilution in BaSO4 (5 wt % NiO), used as a reference. The bandgap energy (Eg) was calculated according to the Kubelka-Munk theory (Kubelka, 1931) represented in Eq. 1:
(1)
where 
is the
diffuse reflectance of the sample. Using the Tauc and Menth (1972) function, Eq.
1 can be written as Eq. 2:
(2)
where 
is a proportionality constant, and 
is photon
energy.
Nitrogen adsorption/desorption isotherms at 77 K were acquired
using an Autosorb-1 analyzer after degassing for about 12 h at 300 °C. The
specific surface area of the samples was analyzed using the BET multipoint
method, and the pore volumes were estimated from data at 
. Scanning
Electron Microscopy (SEM) was performed on a Jeol FEG JMS-2100F microscope
operated at 10 keV.
C. Catalytic Performance
The catalytic oxidative dehydrogenation of ethane was performed in a conventional fixed-bed reactor operating at atmospheric pressure. The experiment was conducted in the range 150 - 310°C with a catalyst amount of 0.5 g and fed with a mixture of 3% C2H6/ 1% O2 in N2 (mol/mol) with a contact time of W/F =1 g.s.mol-1. Ethane and the reaction products were analyzed by on-line gas chromatography (Agilent GC-7820) with an FID detector. Ethane and COx were separated (Porapak Q capillary column) before passing through a methanizer.
The intrinsic activity of the
catalysts was calculated as 
, where 
[m³/s] is the
flow of ethane at the inlet of the reactor, 
[%] is the total conversion, 
[g] is the
weight of sample, SBET [m2/g] is the specific surface area of the
catalyst and 
= 0.0224 [m3/mol]
is the volume occupied by one mole of gas.
III. RESULTS AND DISCUSSION
A. Characterization of Precursor and Catalysts
The precursor, as well as the catalyst oxides obtained after thermal
treatment at different temperatures, were identified through XRD patterns and
FTIR spectra. All the characteristic peaks could be indexed to β-Ni(OH)2
phase with the lattice parameters of a =3.08 Å and c = 4.62 Å (ICSD No.
169978), with no impurity detected. A brucite type structure Mg(OH)2
was formed with space group P-3m1 (164) (Hall, 2015). Major diffraction peaks
Figure 1: XRD spectra of non-stoichiometric NiO samples after heat treatment of β-Ni(OH)2 in the temperature range of 250 to 400 °C.
(2θ = 19.2°, 33.2°, 38.4°, 51.9°, 59.3°, 62.8°, 70.1°, and 72.8°) can be attributed to the diffractions of (001), (100), (101), (102), (110), (111), (103) and (201) crystal planes, respectively. The unusual broadening of the (001) and (100) lines in XRD patterns of Ni(OH)2 may be attributed to stacking faults within the structure along the direction of the crystallographic c-axis (Delmas and Tessier, 1997; Tessier et al., 1999).
Additionally, FTIR spectrum presented the band at ν = 3634 cm-1, which can be assigned to O-H stretching mode of the hydroxide. The broad absorption band centered at 3438 cm−1 is attributed to O-H stretching mode of water and the band at 1637 cm−1 is related to the bending mode of adsorbed water. The two other peaks that appear at 518 and 446 cm−1 are attributed to δ(O-H) and ν(Ni-O) vibrations in β-Ni(OH)2, respectively (Hall, 2015). The peaks at about 1455, 1382, 1053, and 640 cm-1 are attributed to carbonate groups, which originate from the chemisorption of CO2 from air. The bands at 1455 and 1382 cm-1 are related to asymmetric stretching (υ3 band), while that at 1053 cm-1 corresponds to symmetric stretching (υ1 band). The band at 640 cm-1 is attributed to in-plane bending vibration (υ4 band). A shoulder at about 880 cm-1 is assigned to out-of-plane bending mode (υ2 band). Broken degeneracy of the υ3 band indicates symmetry lowering of the adsorbed ion. Thus, the IR spectrum confirms the β-phase structure and basicity of the sample.
Figure 1 depicts the XRD patterns obtained after heat treatment of beta nickel hydroxide at different temperatures (250 to 400 °C). The observed peaks in all recorded XRD patterns correspond to the (111), (200), (220), (311), and (222) crystallographic planes, which can be ascribed to the cubic crystal structure of NiO (space group Fm-3m) in agreement with standard data (ICSD-9866). Overall, the XRD analysis showed a single phase with small variations in the cell parameter. The values of crystallite size, which were calculated by the Scherrer equation, are also shown in Table 1. The NiO crystal size increased with the calcination temperature of precursors.
BET surface areas present a similar behavior to that found by Abbas and Jung (2016) after calcination of mesoporous Ni(OH)2. Particles coalescence during the temperature rise, leading to an increase of particle sizes, and a decrease of specific surface areas. The increasing number of contacts between the particles triggers an increase in mesoporosity (Bakovets et al., 2009).
In the corresponding FTIR spectra, the strong band at 423 cm−1 corresponds to the bending vibration of NiO (Nakamoto, 1986). A broad band at about 3445 cm−1, and a less intense band at 1624 cm−1 are related to the stretching and bending vibrations of water molecules adsorbed by the sample. As above, the bands observed at 1450, 1382 and 1053 cm-1 are attributed to superficial carbonate groups; it is known that the calcined powder tends to chemisorb acidic compounds as CO2 at basic sites (Gravelle and Teichner, 1969; Pease et al., 1986).
Diffuse reflectance UV-vis spectra of NiO presented strong band at 14,500 cm−1 and the weak band at 26,500 cm−1, ascribed to the 3A2g → 3T1g(F) and 3A2g → 3T1g(P) electronic transition in octahedral Ni(II) ion, are fingerprints for NiO. The weak absorption band at 30,500 cm−1 and the broad band at 35,500 cm−1 are supposed to be absorption lines corresponding to 4T1g(F) → 4T1g(P) and 4T1g(F) → 4A2g(F), respectively, and are assigned to the transitions in octahedral Ni(III) (3d7) ion. Moreover, it can be seen that the peaks attributed to Ni(III) ions in relation to Ni(II) increase proportionally for the sample calcined at 250 to 300 °C and from 300 to 350 °C but decrease from 350 to 400 ºC. Samples calcined at 300 and 400 °C provide very similar Ni(III) concentrations.
The optical band gaps Eg have been estimated by extrapolating
the linear fitted region at 
in plots of 
versus 
. The highest
values of bandgap energy were observed for the beta nickel hydroxide precursor
calcined at 350°C (3.41 eV). This value is close to that obtained by Zhu et
al. (2012) for NiO with flower-like morphology, which presented gap energy
between 3.12-3.41 eV, increasing with calcination temperature (300 to 500°C).
However, NiO thin films developed by Shadrach et al. (2018) showed a
decrease in gap energy, 3.71, 3.64, and 3.55 eV with an increase in the
calcination temperature (400 to 600 °C). This shows that the bandgap energy is
strongly influenced by the morphology of the oxide.
Figure 2 shows SEM images of the β-Ni(OH)2 samples calcined between 250 and 400 °C. The samples presented heterogeneity of particle sizes, mostly over 1 μm. In general, they are sponge-like with laminar structures and high porosity. There are no important differences in shape regardless of the calcination temperatures.
B. Catalytic Performance
Nickel
oxide activity was tested in the ODH reaction of ethane. Ethane conversion and selectivity
to ethylene are shown in Fig. 3 as a function of ODH reaction temperature. All
catalysts in this series show dependence with the reaction temperature. Highest
conversions reached ~20% with selectivity up to ~60% ethylene. The samples
calcined at 250 and 400 °C showed a maximum selectivity of 55%. NiO-300
exhibited the highest conversion at 270 °C with an appreciable selectivity of
60%.
