# Structure, Surface Nature, Thermal Stability, and Biomass Gasification Process of NiO/SiO2 and NiO-Pr2O3/SiO2 Nanocomposites Obtained through Facile Deposition Precipitation Method

Authors: Kannaiyan Shanmuganandam; Sethuramachandran Thanikaikarasan; Tansir Ahamad; Sajid Ali; Venkatesa Prabhu Sundramurthy
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

Recently, composites of metal matrix attracted many researchers in mechanical, biomedical, chemical, and electrical fields due to its structure, thermal, and some interesting chemical properties. Nanocomposites of NiO/SiO2 and NiO-Pr2O3/SiO2 considered much interest due its wide range of applications in medical, engineering, physics, chemistry, and material science. In the present work, synthesizing of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites was carried out using simple and low-cost deposition precipitation technique. Structural features represent that the prepared samples found to be nanocrystalline nature. Surface morphology showed that there is formation of particle with spherical in shape. Transmission electron microscopy supported that the structure of composite with size is in the range of nanometer. The Brauner-Elmer-Teller method showed the surface area of the synthesized nanoparticles. The pore volume present in the nanoparticle was determined by Barrett–Joyner–Halenda method. Thermogravimetric analysis revealed that the thermal stability of the synthesized nanocomposite is up to 1000°C. The hydrogen enrichment in the producer gas was evident by the tar cracking activity of the synthesized NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.


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## Body

## 1. Introduction

Composite materials play a vital role in designing and fabrication of modern devices by the virtue of its enhanced properties than alloy materials. In addition to that, it provides the requirement of mechanical, electrical, chemical, environmental and wastewater remediation fields due to its superior material characteristics, durability, reliability, and better performance capability [1–5]. Recently, nanocomposites found enormous applications in various fields due to its enhanced properties than microcomposites. Nanocomposites have been extensively used in photothermal, photocatalysis, nanofiller and sensor applications [6–10]. Besides, metal matrix nanocomposites have prepared by several techniques, viz., liquid metal infiltration [11], electrodeposition [12], spray pyrolysis [13], colloidal technique [14], and sol-gel process [15]. When compared to the techniques mentioned, the deposition precipitation technique provides numerous advantages due to the reason of its relatively low cost, low temperature processing, homogeneity of component distribution, and the formation of fine agglomerated particles uniformly [16, 17]. Mohammed and Harraz have studied the influence of NiO/SiO2 nanocomposites with Pd and Yt for photocatalytic applications. They have reported that the reaction rate of photocatalysis was found to be increased by the addition dopants such as Pd and Yt [18]. Dutta et al. reported the complete blocking of UV radiation at 15 wt.% loading of the NiO@SiO2 nanoparticles on polyvinylidene fluoride [19]. The process of hydrogen enhancement in lignite chemical looping gasification by the usage of NiO–CaSO4 was reported by Yang et al. [20]. Zhao et al. reported the hydrogen enhancement by the process in lignin photo reforming process. They showed that the Ni loading of 3.25% on TiO2 yields the highest hydrogen generation of 23.5 mmol h−1 g−1 from water splitting [21]. The enrichment of hydrogen found to increase six times by the process of doping NiO with CuO/TiO2 during photocatalytic activity [22]. Saket et al. studied the influence of praseodymium (III) coated AZ31-magnesium alloy for the process of corrosion resistance and reported the improvement of corrosion resistance by 64% [23]. Jayachandran and Kennedy [24] investigated the influence of praseodymium for solid state light applications and found that a concentration of Pr3+ ions at 0.04 remarkably increased the photoluminescence activity. To the best of our knowledge, very few reports focused the hydrogen enhancement process in biomass gasification technique using NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.In the present investigation, we have reported that the growth of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites by facile deposition precipitation technique. Structural features of the nanocomposites have been determined by following the method of X-ray diffraction. The particle size of the nanocomposites was estimated using high-resolution transmission electron microscopy. Surface morphology of the synthesized nanoparticles has been analyzed using high-resolution scanning electron microscopy. Brauner-Elmer-Teller method has been carried out to determine the surface area of the nanocomposites. The volume of the pores present in the synthesized nanocomposites was determined by Barrett–Joyner–Halenda technique. The stability of the nanocomposites in the range of temperature in between 600 and 1000°C has been analyzed using thermogravimetric analysis. The enrichment of hydrogen in biomass gasification process by the usage of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites within the temperature range in between 725 and 850°C has been investigated.


## 2. Experimental

### 2.1. Materials

The chemicals used in the present work were of AR grade reagents (procured from Merck): Tetraorthosilicates (TEOs), Nickel Nitrate Hexahydrate Ni(NO3)2.6H2O, Praseodymium Nitrate Hexahydrate Pr(NO3)3.6H2O, Cetyl Trimethyl Ammonium Bromide (CTAB), Sodium Hydroxide (NaOH), Ethanol (CH3CH2OH), and Hydrochloric acid (HCl).


### 2.2. Synthesis of NiO/SiO2 and NiO-Pr2O3/SiO2 Nanocomposites

The synthesis of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites was carried out by following the facile deposition precipitation method. The formation of silica support was obtained by the condensation process of Tetraorthosilicates (TEOs) dissolved in CH3CH2OH in addition with 0.1 M of HCl. The ratio of TEOs, HCl, and CH3CH2OH was maintained in the ratio 1 : 0.25 : 6, respectively. The reaction mixture was stirred slowly for the time period of 60 minutes using magnetic stirrer with heater leads to produce a silica gel. The incorporation of metal ions Ni and Pr was carried out by following the process of dissolving Ni(NO3)2.6H2O and Pr(NO3)3.6H2O in double distilled water. To this reaction, mixture of CTAB with a concentration of 0.21×10−3 mole litre-1 was added as a capping agent. This solution was transferred into a glass vessel containing silica gel with 0.1 M NaOH as a precipitating agent. The resultant product was filtered and washed with CH3CH2OH followed by double distilled water to remove the adsorbed ions. The prepared samples was dried in hot air oven at 120°C for the duration of 120 minutes with heating rate at 10°C/min. The process of calcination was carried out in a muffle furnace maintained at 600°C with heat rate of 20°C/min for the duration of 360 minutes. The mentioned process yielded the formation of nanocomposites, viz., NiO/SiO2 and NiO-Pr2O3/SiO2, respectively. The schematic diagram represents the growth of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites is shown in Figure 1.Figure 1
Schematic diagram for the growth of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.


### 2.3. Generation of Producer Gas with Catalytic System

The biomass gasifier with catalytic system was utilized to generate the producer gas. The system comprises of forced air, downdraft, dry bottom, fixed bed gasifier of 24 kg full load capacity at the rated output of 15 kWth, centrifugal blower, cyclone separator, tar sampling setup, and its associated instrumentation. At the downstream of the gasifier, a catalytic tar cracking system encompassing a guard bed reactor and a fixed bed catalytic tar cracking reactor was employed to crack the tar present in the producer gas. For the Casuarina wood, ambient air was used as a source materials and gasification agent, respectively. The proximate and ultimate analyses were carried out for the source material, viz., (ASTM E 871-82) moisture content, (ASTM E 872-82) volatile matter, and (ASTM D 1102-84) as content, and the remaining content was presumed as fixed carbon.


### 2.4. Characterization

The phase with crystalline nature of the prepared samples was determined using X-ray diffractometer (XPERT PRO PANalytical, Netherland) withCuKα radiation (λ=1.5406 Å) within the range of diffraction angle in between 20 and 80. Morphological features along with heterostructure were determined using scanning electron microscope (JEOL JSM-5600). The particle size of the nanocomposites was analyzed using transmission electron microscope (F30, Philips, Techani, 300 kV). The surface area of the samples was determined using Brauner-Elmer-Teller method (QUANTACHROME AUTOSORB-6B analyzer). The pore size present in the prepared samples was determined by Barrett–Joyner–Halenda method. The determination of thermal properties was carried out by using Discovery TGA 550 analyzer.


## 2.1. Materials

The chemicals used in the present work were of AR grade reagents (procured from Merck): Tetraorthosilicates (TEOs), Nickel Nitrate Hexahydrate Ni(NO3)2.6H2O, Praseodymium Nitrate Hexahydrate Pr(NO3)3.6H2O, Cetyl Trimethyl Ammonium Bromide (CTAB), Sodium Hydroxide (NaOH), Ethanol (CH3CH2OH), and Hydrochloric acid (HCl).


## 2.2. Synthesis of NiO/SiO2 and NiO-Pr2O3/SiO2 Nanocomposites

The synthesis of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites was carried out by following the facile deposition precipitation method. The formation of silica support was obtained by the condensation process of Tetraorthosilicates (TEOs) dissolved in CH3CH2OH in addition with 0.1 M of HCl. The ratio of TEOs, HCl, and CH3CH2OH was maintained in the ratio 1 : 0.25 : 6, respectively. The reaction mixture was stirred slowly for the time period of 60 minutes using magnetic stirrer with heater leads to produce a silica gel. The incorporation of metal ions Ni and Pr was carried out by following the process of dissolving Ni(NO3)2.6H2O and Pr(NO3)3.6H2O in double distilled water. To this reaction, mixture of CTAB with a concentration of 0.21×10−3 mole litre-1 was added as a capping agent. This solution was transferred into a glass vessel containing silica gel with 0.1 M NaOH as a precipitating agent. The resultant product was filtered and washed with CH3CH2OH followed by double distilled water to remove the adsorbed ions. The prepared samples was dried in hot air oven at 120°C for the duration of 120 minutes with heating rate at 10°C/min. The process of calcination was carried out in a muffle furnace maintained at 600°C with heat rate of 20°C/min for the duration of 360 minutes. The mentioned process yielded the formation of nanocomposites, viz., NiO/SiO2 and NiO-Pr2O3/SiO2, respectively. The schematic diagram represents the growth of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites is shown in Figure 1.Figure 1
Schematic diagram for the growth of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.


## 2.3. Generation of Producer Gas with Catalytic System

The biomass gasifier with catalytic system was utilized to generate the producer gas. The system comprises of forced air, downdraft, dry bottom, fixed bed gasifier of 24 kg full load capacity at the rated output of 15 kWth, centrifugal blower, cyclone separator, tar sampling setup, and its associated instrumentation. At the downstream of the gasifier, a catalytic tar cracking system encompassing a guard bed reactor and a fixed bed catalytic tar cracking reactor was employed to crack the tar present in the producer gas. For the Casuarina wood, ambient air was used as a source materials and gasification agent, respectively. The proximate and ultimate analyses were carried out for the source material, viz., (ASTM E 871-82) moisture content, (ASTM E 872-82) volatile matter, and (ASTM D 1102-84) as content, and the remaining content was presumed as fixed carbon.


## 2.4. Characterization

The phase with crystalline nature of the prepared samples was determined using X-ray diffractometer (XPERT PRO PANalytical, Netherland) withCuKα radiation (λ=1.5406 Å) within the range of diffraction angle in between 20 and 80. Morphological features along with heterostructure were determined using scanning electron microscope (JEOL JSM-5600). The particle size of the nanocomposites was analyzed using transmission electron microscope (F30, Philips, Techani, 300 kV). The surface area of the samples was determined using Brauner-Elmer-Teller method (QUANTACHROME AUTOSORB-6B analyzer). The pore size present in the prepared samples was determined by Barrett–Joyner–Halenda method. The determination of thermal properties was carried out by using Discovery TGA 550 analyzer.


## 3. Results and Discussion

### 3.1. Structural Properties

X-ray diffraction pattern was carried out to determine crystalline nature and structural features of the synthesized NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. XRD pattern recorded for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites is shown in Figure 2. The diffraction characteristics of NiO, Pr2O3, and SiO2 are resolved as a result of the decomposition of hydroxide leaving behind the metal oxides. The individual binary oxides, viz., NiO and Pr2O3, coexisted with SiO2 to form an individual binary nanocomposite. It is found that both metal oxides combined with SiO2 exhibit nanocrystalline nature with cubic structure and also in addition with SiO2 having trigonal structure. The observed diffraction peaks at 2θ values 43.51, 45.73, 69.95, and 77.05 correspond to the reflection planes of SiO2 along with planes of (200), (111), (002), and (301), respectively. The diffraction peaks of NiO are found at 2θ values of angle 37.09, 43.14, and 62.59 corresponding to the planes of reflection from (111), (200), and (220), respectively. Thereafter, the diffraction peaks of Pr2O3 are observed at 2θ values 31.02, 56.44, and 57.38 corresponding to the planes of reflection from (101), (112), and (201), respectively. The intensity of the diffraction peaks is proportional to the component concentration yielding it. The characteristic peak ratio of NiO (RNiO), Pr2O3 (RPr2O3), and SiO2 (RSiO2) can be obtained by using the following equations.
(1)RNiO=INiOINiO+ISiO2×100%,RSiO2=ISiO2INiO+ISiO2×100%,RNiO=INiOINiO+IPr2O3+ISiO2×100%,RPr2O3=IPr2O3INiO+IPr2O3+ISiO2×100%,RSiO2=ISiO2INiO+IPr2O3+ISiO2×100%,where INiO, IPr2O3, and ISiO2 are the characteristic peaks of NiO, Pr2O3, and SiO2 corresponding to the lattice plane (111), (101), and (200), respectively. The percentage of reflectance in XRD pattern for NiO, Pr2O3, and SiO2 is denoted in Figure 3. The estimated values of relative concentration given by the RNiO, RPr2O3, and RSiO2 are 37.09, 31.02, and 28.72. The highest volume fraction is obtained for NiO and SiO2, whereas the least fraction is noted for Pr2O3. Crystallite size is defined as the number of crystallites formed along the surface of the layer. The sizes of the crystallites are calculated from observed XRD pattern using FWHM data with Debye Scherrer formula [25].
(2)D=0.9λβDcosθ,where D is the crystallite size in nm, λ is the wavelength of CuKα target used, θ is Bragg diffraction angle at peak position in degree, and βD is the peak width at full wave half maximum of the peak related to instrument broadening. The value of crystallite size is found to be 28.65 and 18.07 nm for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. Strain is defined as the force which can act on the surface of the layer to restrict the formation of crystallites on its surface [26]. A plot of βcosθ versus 4 sinθ was carried out to determine the value of strain for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites, respectively. Williamson Hall plot recorded for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposite is shown in Figures 4(a) and 4(b). The estimated value of strain was found to be 4.3×10−3 and 5.6×10−3 line-2 meter-4 for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. The variation of crystallite size with strain for NiO-Pr2O3/SiO2 nanocomposites is shown in Figure 5. The NiO-Pr2O3/SiO2 exhibited higher strain value than NiO/SiO2 resulted decrement in value of crystallite size.
(3)βhklcosθ=kλD+4εtanθ.Figure 2
X-ray diffraction pattern of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.Figure 3
Intensity reflectance % in XRD pattern for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.Figure 4
(a) Williamson Hall plot for NiO/SiO2 nanocomposites. (b) Variation of crystallite size and strain for NiO-Pr2O3/SiO2 nanocomposites.
(a)(b)Figure 5
(a, b) High-resolution scanning electron microscopic image of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.
(a)(b)


### 3.2. Scanning Electron Microscopy

Surface morphology of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites was analyzed using JEOL JSM-5600 high-resolution scanning electron microscope. Figures 5(a) and 5(b) show the HRSEM images of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. The NiO/SiO2 nanocomposites found to be slightly elongates, whereas the NiO-Pr2O3/SiO2 has the distribution of spherically shaped morphology. This may be due to the variation ratio of capping agent added to the precursor. The presence of small amount of Pr2O3 and Ni on SiO2 inhibits the growth rate and resulted in the formation of NiO-Pr2O3/SiO2 nanocomposites with distribution of small size grains.


### 3.3. Transmission Electron Microscopy

High-resolution transmission electron microscopy (HR-TEM) is a versatile technique adopted for analyzing the particle size and surface morphology of the composites. Figures6(a) and 6(b) show the typographical transmission electron microscopic image of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. The shape of the particles is found to be spherical for NiO/SiO2 and NiO-Pr2O3/SiO2. The sizes of the particle were found to be in the range between 15 and 30 nm. The observation of different sizes of the particle may be due to the incorporation of metallic Pr3+ to NiO/SiO2 nanocomposite. The formation of large size particles may be due to agglomeration of the nanoparticles with smaller size. The sizes of the particles were determined using TEM pattern thus results in close agreement with the results of XRD pattern.Figure 6
(a, b) High-resolution transmission electron microscopic image of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.
(a)(b)


### 3.4. Surface Area and Porosity

The surface area of the synthesized nanocomposites was analyzed using Brunauer-Emmett-Teller (BET) method by the physical adsorption of gas molecules on the surface of nanocomposite. The BET analysis was carried out using the ASAP 2020 V3.OOH instrument, and the sample was outgassed at 200°C for the time duration of 12 hours. In addition to that, the sizes of the pore distribution were estimated by Barrett-Joyner -Halenda (BJH) method. The surface area was found to be 96 and 140 m2/g for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites, respectively. Besides, the volume of the pores was found to be 0.0986 and 0.256 cm3/g for NiO/SiO2 and NiO-Pr2O3/SiO2, respectively. Generally, a high surface area has a beneficial effect on the activity of nanocomposites. The surface area of NiO-Pr2O3/SiO2 nanocomposite is found to be 31% higher than that of NiO/SiO2 nanocomposites. So it is presumed that the hydrogen enhancement capability of NiO-Pr2O3/SiO2 nanocomposites would be higher than that of NiO/SiO2.


### 3.5. Thermal Stability

The thermal stability of the synthesized NiO-Pr2O3/SiO2 nanocomposite was determined by the technique of thermogravimetric analysis (Discovery TGA 550). TGA is one of the method to measure the change in weight of a sample over a temperature range. The temperature of the nanocomposites placed in a furnace was gradually increased, and the corresponding weight loss was measured using an analytical balance [27]. The observed weight loss may be due to evaporation of volatile components present in the sample. TGA curves recorded for typical NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposite are shown in Figure 7. The observation of weight loss around 15% at 200°C may be due to the desorption of water. In addition to that, the loss of weight in between 200 and 580°C results in a Pr3+ decomposition present in the sample. Moreover, the observed loss of weight above 580°C must be due to the reason of Ni2+ decomposition in the NiO-Pr2O3/SiO2 nanocomposite. The resulting Pr2O3 and NiO was found to be stable above 550°C. The TGA curve recorded for NiO/SiO2 is also shown in Figure 8. It is noted that the initial weight of the sample at 100°C was approximately 80% which may be due to the desorption of water molecule present in the sample. Thereafter above 550°C, the loss of weight of the sample occurs due to the reason of decomposition of nitrates of Ni2+. The observation of no loss of weight in between 600°C and 1000°C indicated the stability of the composites of NiO/SiO2.Figure 7
(a, b) Thermogravimetric analysis pattern of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.
(a)(b)Figure 8
Variation of catalytic bed temperature with hydrogen content for NiO-Pr2O3/SiO2 nanocomposites.


### 3.6. Enrichment of Hydrogen Generation

The enrichment of hydrogen gas by the usage of the synthesized NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites was determined by optimizing three critical parameters, viz., catalytic bed temperature, catalyst amount, and the gas feed rate. The enrichment of hydrogen by the variation of catalytic bed temperature was determined within the range of temperature between 725°C and 850°C with step increment of 25°C. The gasifier was loaded with 24 kg Casuarina wood with the optimized operating parameters such as throat temperature, equivalence ratio, and producer gas flow rate ,and catalyst amount was maintained at 720°C, 0.3, 0.008 litre sec-1, and 2 g, respectively. The catalytic tar cracking system was loaded with the synthesized nanocomposites NiO/SiO2 and NiO-Pr2O3/SiO2 in order to determine the enrichment of hydrogen in the producer gas.The elements present in the producer gas were found out by Siemens make Online Gas Analyzers, viz., Oxymat 61 (estimates O2 using paramagnetic principle), Ultramat 23 (estimates CO, CO2, and CH4 using nondispersive infrared multilayer technology), and Calomat 61 (estimates H2 using thermal conductivity principle). The tar and particulates present in the producer gas were determined by the usage of CEN BT/TF 143, 2005 international protocol of measurement of organic compounds in the producer gas. The quantity of tar present in producer gas was analyzed using a rotary vacuum flash evaporator with soxhlet extractor. The orifice meter and venturimeter were used to find out the rate of air and the producer gas, respectively. The measurement of temperature at different locations was found out by using the Chromel–Alumel K-type thermocouples interfaced with Agilent make (34907 A) data acquisition system. The electric resistance type heaters were wounded on guard bed and catalytic reactor to operate the system at desired temperatures. The tar cracking efficiency value was found to be 68% and 73% for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites while the catalytic bed temperature was maintained at 825°C. Thereafter, a slight increment in the value of tar cracking efficiency was evident by the increase of catalytic bed temperature up to 850°C. The optimization of catalytic bed temperature was fixed at 825°C due to the reason of energy penalty at higher temperature. The content of hydrogen present in the producer gas reached the value of 14.8 and 17.2 vol % for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites while the catalytic bed was maintained at 825°C. The variation of hydrogen content with respect to catalytic bed temperature is depicted as shown in Figure 8. The process of tar cracking enriches the content of hydrogen and calorific value of the producer gas. The tar existing in producer gas is converted into the gases of H2 and CO. The NiO-Pr2O3/SiO2 was found to exhibit better hydrogen enrichment than NiO/SiO2. The reformation of hydrocarbon and water shift reactions during the tar cracking process is indicated by the following equations.
(4)CnHm+2nH2O↔nCO+n+m/2H2CnHm+nCO2↔2nCO+m/2H2CO+H2O↔CO2+m/2H2C+H2O↔CO+H2The hydrogen enrichment capability of NiO-Pr2O3/SiO2 nanocomposites is found to be better than NiO/SiO2 which may be due to the reason of the particle size, surface area, and porosity nature of the synthesized nanocomposites. It is concluded that the particle size of NiO/SiO2 and NiO-Pr2O3/SiO2 was found to be 29 and 15 nm, respectively. The decrease in particle size value of NiO-Pr2O3/SiO2 may be due to the addition of Pr2O3 to NiO/SiO2. The smaller value of the particle size thus enhances the availability of active sites leads to increase the tar cracking with enrichment of hydrogen [28]. In addition to that, the surface area plays a significant role in the enrichment of hydrogen. It is evidenced from BET analysis that the surface area of NiO/SiO2 and NiO-Pr2O3/SiO2 was found to be 96 and 140 m2/g, respectively. The NiO-Pr2O3/SiO2 possesses a higher surface area thus enhances the adsorption of tar compounds on the surface of the catalyst, resulting higher tar cracking efficiency leads to the enrichment of H2 [29]. Moreover, porosity of the nanocomposites plays a vital role in the enrichment of hydrogen gas. It is observed from BJH results that the surface area of NiO/SiO2 and NiO-Pr2O3/SiO2 was found to be 0.0986 and 0.256 cm3/g, respectively. The dispersion of Ni2+ active sites present in the pore cracks the molecules of tar and enhances the H2 content present in the producer gas [30].


## 3.1. Structural Properties

X-ray diffraction pattern was carried out to determine crystalline nature and structural features of the synthesized NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. XRD pattern recorded for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites is shown in Figure 2. The diffraction characteristics of NiO, Pr2O3, and SiO2 are resolved as a result of the decomposition of hydroxide leaving behind the metal oxides. The individual binary oxides, viz., NiO and Pr2O3, coexisted with SiO2 to form an individual binary nanocomposite. It is found that both metal oxides combined with SiO2 exhibit nanocrystalline nature with cubic structure and also in addition with SiO2 having trigonal structure. The observed diffraction peaks at 2θ values 43.51, 45.73, 69.95, and 77.05 correspond to the reflection planes of SiO2 along with planes of (200), (111), (002), and (301), respectively. The diffraction peaks of NiO are found at 2θ values of angle 37.09, 43.14, and 62.59 corresponding to the planes of reflection from (111), (200), and (220), respectively. Thereafter, the diffraction peaks of Pr2O3 are observed at 2θ values 31.02, 56.44, and 57.38 corresponding to the planes of reflection from (101), (112), and (201), respectively. The intensity of the diffraction peaks is proportional to the component concentration yielding it. The characteristic peak ratio of NiO (RNiO), Pr2O3 (RPr2O3), and SiO2 (RSiO2) can be obtained by using the following equations.
(1)RNiO=INiOINiO+ISiO2×100%,RSiO2=ISiO2INiO+ISiO2×100%,RNiO=INiOINiO+IPr2O3+ISiO2×100%,RPr2O3=IPr2O3INiO+IPr2O3+ISiO2×100%,RSiO2=ISiO2INiO+IPr2O3+ISiO2×100%,where INiO, IPr2O3, and ISiO2 are the characteristic peaks of NiO, Pr2O3, and SiO2 corresponding to the lattice plane (111), (101), and (200), respectively. The percentage of reflectance in XRD pattern for NiO, Pr2O3, and SiO2 is denoted in Figure 3. The estimated values of relative concentration given by the RNiO, RPr2O3, and RSiO2 are 37.09, 31.02, and 28.72. The highest volume fraction is obtained for NiO and SiO2, whereas the least fraction is noted for Pr2O3. Crystallite size is defined as the number of crystallites formed along the surface of the layer. The sizes of the crystallites are calculated from observed XRD pattern using FWHM data with Debye Scherrer formula [25].
(2)D=0.9λβDcosθ,where D is the crystallite size in nm, λ is the wavelength of CuKα target used, θ is Bragg diffraction angle at peak position in degree, and βD is the peak width at full wave half maximum of the peak related to instrument broadening. The value of crystallite size is found to be 28.65 and 18.07 nm for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. Strain is defined as the force which can act on the surface of the layer to restrict the formation of crystallites on its surface [26]. A plot of βcosθ versus 4 sinθ was carried out to determine the value of strain for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites, respectively. Williamson Hall plot recorded for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposite is shown in Figures 4(a) and 4(b). The estimated value of strain was found to be 4.3×10−3 and 5.6×10−3 line-2 meter-4 for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. The variation of crystallite size with strain for NiO-Pr2O3/SiO2 nanocomposites is shown in Figure 5. The NiO-Pr2O3/SiO2 exhibited higher strain value than NiO/SiO2 resulted decrement in value of crystallite size.
(3)βhklcosθ=kλD+4εtanθ.Figure 2
X-ray diffraction pattern of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.Figure 3
Intensity reflectance % in XRD pattern for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.Figure 4
(a) Williamson Hall plot for NiO/SiO2 nanocomposites. (b) Variation of crystallite size and strain for NiO-Pr2O3/SiO2 nanocomposites.
(a)(b)Figure 5
(a, b) High-resolution scanning electron microscopic image of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.
(a)(b)


## 3.2. Scanning Electron Microscopy

Surface morphology of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites was analyzed using JEOL JSM-5600 high-resolution scanning electron microscope. Figures 5(a) and 5(b) show the HRSEM images of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. The NiO/SiO2 nanocomposites found to be slightly elongates, whereas the NiO-Pr2O3/SiO2 has the distribution of spherically shaped morphology. This may be due to the variation ratio of capping agent added to the precursor. The presence of small amount of Pr2O3 and Ni on SiO2 inhibits the growth rate and resulted in the formation of NiO-Pr2O3/SiO2 nanocomposites with distribution of small size grains.


## 3.3. Transmission Electron Microscopy

High-resolution transmission electron microscopy (HR-TEM) is a versatile technique adopted for analyzing the particle size and surface morphology of the composites. Figures6(a) and 6(b) show the typographical transmission electron microscopic image of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. The shape of the particles is found to be spherical for NiO/SiO2 and NiO-Pr2O3/SiO2. The sizes of the particle were found to be in the range between 15 and 30 nm. The observation of different sizes of the particle may be due to the incorporation of metallic Pr3+ to NiO/SiO2 nanocomposite. The formation of large size particles may be due to agglomeration of the nanoparticles with smaller size. The sizes of the particles were determined using TEM pattern thus results in close agreement with the results of XRD pattern.Figure 6
(a, b) High-resolution transmission electron microscopic image of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.
(a)(b)


## 3.4. Surface Area and Porosity

The surface area of the synthesized nanocomposites was analyzed using Brunauer-Emmett-Teller (BET) method by the physical adsorption of gas molecules on the surface of nanocomposite. The BET analysis was carried out using the ASAP 2020 V3.OOH instrument, and the sample was outgassed at 200°C for the time duration of 12 hours. In addition to that, the sizes of the pore distribution were estimated by Barrett-Joyner -Halenda (BJH) method. The surface area was found to be 96 and 140 m2/g for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites, respectively. Besides, the volume of the pores was found to be 0.0986 and 0.256 cm3/g for NiO/SiO2 and NiO-Pr2O3/SiO2, respectively. Generally, a high surface area has a beneficial effect on the activity of nanocomposites. The surface area of NiO-Pr2O3/SiO2 nanocomposite is found to be 31% higher than that of NiO/SiO2 nanocomposites. So it is presumed that the hydrogen enhancement capability of NiO-Pr2O3/SiO2 nanocomposites would be higher than that of NiO/SiO2.


## 3.5. Thermal Stability

The thermal stability of the synthesized NiO-Pr2O3/SiO2 nanocomposite was determined by the technique of thermogravimetric analysis (Discovery TGA 550). TGA is one of the method to measure the change in weight of a sample over a temperature range. The temperature of the nanocomposites placed in a furnace was gradually increased, and the corresponding weight loss was measured using an analytical balance [27]. The observed weight loss may be due to evaporation of volatile components present in the sample. TGA curves recorded for typical NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposite are shown in Figure 7. The observation of weight loss around 15% at 200°C may be due to the desorption of water. In addition to that, the loss of weight in between 200 and 580°C results in a Pr3+ decomposition present in the sample. Moreover, the observed loss of weight above 580°C must be due to the reason of Ni2+ decomposition in the NiO-Pr2O3/SiO2 nanocomposite. The resulting Pr2O3 and NiO was found to be stable above 550°C. The TGA curve recorded for NiO/SiO2 is also shown in Figure 8. It is noted that the initial weight of the sample at 100°C was approximately 80% which may be due to the desorption of water molecule present in the sample. Thereafter above 550°C, the loss of weight of the sample occurs due to the reason of decomposition of nitrates of Ni2+. The observation of no loss of weight in between 600°C and 1000°C indicated the stability of the composites of NiO/SiO2.Figure 7
(a, b) Thermogravimetric analysis pattern of NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites.
(a)(b)Figure 8
Variation of catalytic bed temperature with hydrogen content for NiO-Pr2O3/SiO2 nanocomposites.


## 3.6. Enrichment of Hydrogen Generation

The enrichment of hydrogen gas by the usage of the synthesized NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites was determined by optimizing three critical parameters, viz., catalytic bed temperature, catalyst amount, and the gas feed rate. The enrichment of hydrogen by the variation of catalytic bed temperature was determined within the range of temperature between 725°C and 850°C with step increment of 25°C. The gasifier was loaded with 24 kg Casuarina wood with the optimized operating parameters such as throat temperature, equivalence ratio, and producer gas flow rate ,and catalyst amount was maintained at 720°C, 0.3, 0.008 litre sec-1, and 2 g, respectively. The catalytic tar cracking system was loaded with the synthesized nanocomposites NiO/SiO2 and NiO-Pr2O3/SiO2 in order to determine the enrichment of hydrogen in the producer gas.The elements present in the producer gas were found out by Siemens make Online Gas Analyzers, viz., Oxymat 61 (estimates O2 using paramagnetic principle), Ultramat 23 (estimates CO, CO2, and CH4 using nondispersive infrared multilayer technology), and Calomat 61 (estimates H2 using thermal conductivity principle). The tar and particulates present in the producer gas were determined by the usage of CEN BT/TF 143, 2005 international protocol of measurement of organic compounds in the producer gas. The quantity of tar present in producer gas was analyzed using a rotary vacuum flash evaporator with soxhlet extractor. The orifice meter and venturimeter were used to find out the rate of air and the producer gas, respectively. The measurement of temperature at different locations was found out by using the Chromel–Alumel K-type thermocouples interfaced with Agilent make (34907 A) data acquisition system. The electric resistance type heaters were wounded on guard bed and catalytic reactor to operate the system at desired temperatures. The tar cracking efficiency value was found to be 68% and 73% for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites while the catalytic bed temperature was maintained at 825°C. Thereafter, a slight increment in the value of tar cracking efficiency was evident by the increase of catalytic bed temperature up to 850°C. The optimization of catalytic bed temperature was fixed at 825°C due to the reason of energy penalty at higher temperature. The content of hydrogen present in the producer gas reached the value of 14.8 and 17.2 vol % for NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites while the catalytic bed was maintained at 825°C. The variation of hydrogen content with respect to catalytic bed temperature is depicted as shown in Figure 8. The process of tar cracking enriches the content of hydrogen and calorific value of the producer gas. The tar existing in producer gas is converted into the gases of H2 and CO. The NiO-Pr2O3/SiO2 was found to exhibit better hydrogen enrichment than NiO/SiO2. The reformation of hydrocarbon and water shift reactions during the tar cracking process is indicated by the following equations.
(4)CnHm+2nH2O↔nCO+n+m/2H2CnHm+nCO2↔2nCO+m/2H2CO+H2O↔CO2+m/2H2C+H2O↔CO+H2The hydrogen enrichment capability of NiO-Pr2O3/SiO2 nanocomposites is found to be better than NiO/SiO2 which may be due to the reason of the particle size, surface area, and porosity nature of the synthesized nanocomposites. It is concluded that the particle size of NiO/SiO2 and NiO-Pr2O3/SiO2 was found to be 29 and 15 nm, respectively. The decrease in particle size value of NiO-Pr2O3/SiO2 may be due to the addition of Pr2O3 to NiO/SiO2. The smaller value of the particle size thus enhances the availability of active sites leads to increase the tar cracking with enrichment of hydrogen [28]. In addition to that, the surface area plays a significant role in the enrichment of hydrogen. It is evidenced from BET analysis that the surface area of NiO/SiO2 and NiO-Pr2O3/SiO2 was found to be 96 and 140 m2/g, respectively. The NiO-Pr2O3/SiO2 possesses a higher surface area thus enhances the adsorption of tar compounds on the surface of the catalyst, resulting higher tar cracking efficiency leads to the enrichment of H2 [29]. Moreover, porosity of the nanocomposites plays a vital role in the enrichment of hydrogen gas. It is observed from BJH results that the surface area of NiO/SiO2 and NiO-Pr2O3/SiO2 was found to be 0.0986 and 0.256 cm3/g, respectively. The dispersion of Ni2+ active sites present in the pore cracks the molecules of tar and enhances the H2 content present in the producer gas [30].


## 4. Conclusions

The NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites were synthesized by facile deposition precipitation method. Structural analysis revealed the combination of NiO and Pr2O3 combined with SiO2. The crystallites were found to exhibit most prominent reflection along (111) plane. The formation of nonagglomerated spherical-shaped grains was evident from surface morphology analysis. The surface area of NiO-Pr2O3/SiO2 nanocomposites was found to be 31.4% higher than that of NiO/SiO2 nanocomposites. It is also concluded that the pore volume of NiO-Pr2O3/SiO2 nanocomposites was found to be 61.4% higher than that of NiO/SiO2. The observed baseline value of H2 in the producer gas was 13.5 volume percentage at ER value 0.3. The enrichment hydrogen in the producer gas may be due to the process of tar cracking by the NiO/SiO2 and NiO-Pr2O3/SiO2 nanocomposites. The catalytic bed temperature was found to be optimized at 825°C.



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*Source: 1479808-2022-05-16.xml*