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Article

Synthesis of a SiO2/Co(OH)2 Nanocomposite Catalyst for SOX/NOX Oxidation in Flue Gas

by
Alon Khabra
1,
Haim Cohen
1,
Gad A. Pinhasi
2,
Xavier Querol
3,
Patricia Córdoba Sola
3 and
Tomer Zidki
1,*
1
Department of Chemical Sciences and Material Research Center, Ariel University, Ariel 4077625, Israel
2
Department of Chemical Engineering, Ariel University, Ariel 4077625, Israel
3
Institute of Environmental Assessment and Water Research (IDAEA-CSIC), 08034 Barcelona, Spain
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(1), 29; https://doi.org/10.3390/catal13010029
Submission received: 6 October 2022 / Revised: 19 December 2022 / Accepted: 20 December 2022 / Published: 24 December 2022
(This article belongs to the Special Issue Experimental and Theoretical Studies on Non-noble Transition Metals in Catalysis)
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Abstract

:
Sulfur and nitrogen oxides (SOX/NOX) are the primary air toxic gas pollutants emitted during fuel combustion, causing health and environmental concerns. Therefore, their emission in flue gases is strictly regulated. The existing technologies used to decrease SOX/NOX content are flue gas desulfurization, which necessitates high capital and operating costs, and selective catalytic reduction, which, in addition to these costs, requires expensive catalysts and high operating temperatures (350–400 °C). New wet scrubbing processes use O3 or H2O2 oxidants to produce (NH4)2SO4 and NH4NO3 fertilizers upon ammonia injection. However, these oxidants are expensive, corrosive, and hazardous. SiO2/Co(OH)2 nanocomposites are presented here as potential catalysts for SOX/NOX oxidation in wet scrubber reactors to scrub these toxic gases using atmospheric oxygen as the oxidant at relatively low temperatures of 60–90 °C. Several silica-cobalt-oxide-based nanocomposites were synthesized as potential catalysts at different concentrations and temperatures. The nanocomposite catalysts were characterized and exhibited excellent catalytic properties for SOX/NOX oxidation using atmospheric oxygen as the oxidant, replacing the problematic H2O2/O3. We thus propose SiO2-supported Co(OH)2 nanoparticles (NPs) as excellent catalysts for the simultaneous scrubbing of polluting SOX/NOX gases in flue gases using atmospheric O2 as the oxidation reagent at a relatively low-temperature range.

Graphical Abstract

1. Introduction

In the coming decades, fossil fuels will likely remain as the main fuel for power production, despite the fact that their combustion results in the emission of carbon dioxide (the greenhouse effect) and pollutants. These include sulfur oxides (SOX=2,3) and nitrogen oxides (NOX=1,2), which are the primary toxic gas pollutants emitted into the atmosphere in flue gas upon fossil fuel combustion [1]. These pollutants are responsible for numerous environmental and health concerns, along with economic losses [2]. Thus, current regulations (e.g., The Clean Air Act [3]) only allow for the release of small concentrations of these compounds in flue gas emissions from the stacks of combustion sources [4]. SO2 and NO molecules undergo oxidation in the atmosphere to produce SO3 and NO2, with H2SO4 and HNO3, respectively, being the final products, leading to the appearance of acid rain [5].
At present, two main processes are used to reduce the SOX and NOX concentrations in flue gas emissions and thus comply with current regulations, namely, flue gas desulfurization (FGD) and selective catalytic reduction (SCR). FGD and SCR are usually used in utilities, as well as in large- and mid-sized combustion plants [6,7]. In FGD, SO2 is oxidized and converted into gypsum (CaSO4) via injection of ground calcium carbonate and compressed air [8]. The operating costs of FGD are high, and several attempts to reduce them were reported [9,10,11]. SCR uses injected ammonia (NH3) or urea (CO(NH2)2), which reacts with NOX on a V2O5/TiO2 catalyst, along with WO3 or MoO3, to reduce NOX to molecular nitrogen (N2) [12,13]. SCR, moreover, is performed under relatively high-temperature conditions (350–400 °C) and is significantly expensive, leading to increased operating expenses [7,14]. Thus, more efficient and cost-saving processes are needed.
Recently, the efficient removal of SOX/NOX from flue gas via wet scrubbers, often spray towers, involving simultaneous gas-phase oxidation of SO2, and water-insoluble NO gas has been described [15,16]. Briefly, SO2 is oxidized to SO3, forming H2SO4 upon reaction with water, whereas NO is oxidized to water-soluble NO2, which similarly produces HNO2 and HNO3. Usually, strong oxidation reagents (e.g., hydrogen peroxide (H2O2) ozone (O3) or even plasma) oxidize NO to NO2 and SO2 to SO3, which are scrubbed in water. These systems can work efficiently at low temperatures (<100 °C). The H2SO4 and HNO3 produced can then be converted into a mixture of ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4), which can be used as nitrogen fertilizer upon injection of ammonium hydroxide (NH4OH). However, in all existing methods, the strong oxidation reagents used require that expensive and corrosion-resistant systems and safety equipment be installed in the wet scrubber units, which significantly increases the production costs [17].
At the same time, it is essential to confirm that the precipitated NH4NO3/(NH4)2SO4 mixture does not contain trace element contamination in order to use it as an N-fertilizer. In addition, new international maritime organization (IMO) regulations [18,19] require the removal of excessive SOX/NOX for ships within the next decade, calling for the installation of efficient scrubbers in relatively small systems (e.g., large diesel engines), which use a few hundred tons/day of diesel fuel compared to the large utilities, which may use up to 10,000 tons/day of fossil fuel (e.g., coal). However, the FGD and SCR methods are very large and expensive units that are only suitable for large utilities and not for ships. Thus, a small spray tower unit that can simultaneously scrub and control SOX/NOX emissions represents an ideal solution for ships that need to comply with the new IMO regulations.
It was reported that cobalt hydroxide (Co(OH)2), precipitated as nanoparticle aggregates (NPs) on silica (SiO2) NP carriers, is an efficient catalyst for water oxidation [20]. Thus, this material could serve as a potential catalyst for SOX/NOX oxidation in flue gases, using atmospheric O2 as an oxidant. Cobalt species are excellent oxidation catalysts with a very high redox potential. Unfortunately, cobalt is a relatively expensive and toxic metal. However, if cobalt is only used at the silica surface, the amounts of precious cobalt metal can be appreciably reduced, resulting in a much less costly catalyst with low toxicity. In the current study, SiO2 particles were chosen as the Co(OH)2 carrier given how SiO2 particles are relatively inexpensive and are available in different sizes, surface areas, and shapes [21]. It was also reported that SiO2 NPs might not be inert as catalyst carriers, as is commonly assumed [22,23,24,25,26]. Furthermore, previous research found that SiO2 has catalytic properties suitable for NO2 hydrolysis [27]. Thus, nanocomposite particles (Co(OH)2 impregnated on SiO2 NPs) dispersed in water might serve as an efficient catalyst for SOX/NOX oxidation with atmospheric oxygen (O2) serving as the oxidation catalyst.
This research thus aimed to develop an efficient catalyst for the simultaneous oxidation of SOX and NOX, which can be operated in a wet scrubber system using atmospheric oxygen as the oxidation reagent at relatively low temperatures (<100 °C). Moreover, while the catalyst commonly used in SCR for NOX treatment costs between 4000 to 8000 USD/m3, the preparation catalyst described here is estimated to cost some 2400 USD/m3 [28,29]. Moreover, the catalyst is developed to treat SOX and NOX in a united system at low working temperatures, reducing installation and operation costs. Such a unit will, therefore, prove far less expensive when used in large-scale utilities or in smaller fossil fuel users, such as ships.

2. Results and Discussion

In all experiments, the removal efficiency of SO2 was >99%, with SO2 being oxidized to SO3 to form H2SO4. Hence, the main challenge for ensuring catalyst efficiency was the conversion of NO to NO2. Therefore, nanocomposite catalytic efficiencies are always displayed in the text below as NO conversion efficiency.

2.1. Comparison of SiO2 Carriers

The different SiO2 nanocarrier types of nanocomposite catalysts were tested for SOX/NOX oxidation in the wet scrubber pilot system to assess their activities. The suspension contained 1 wt% of the nanocomposite catalyst (prepared by deposition of cobalt from a 6 mM cobalt sulfate solution) and denoted below as ([SiO2] = 1 wt%, [Co] = 6 mM). Although these catalysts exhibited excellent conversion ratios for SO2 oxidation of >99%, their activities for NO oxidation by atmospheric O2 were appreciably lower (Figure 1). Only the catalyst synthesized with Cab-O-Sil SiO2 carrier was efficient enough to cross the 50% level, reaching ~60% NO oxidation. The four other SiO2-based carrier catalysts, namely, Stöber SiO2, 10 nm colloidal SiO2, and 1.0- and 0.5-micron SiO2, reached 21.4%, 42.1%, and 26.6% and 36.8%, respectively, as opposed to 60.2% conversion with the Cab-O-Sil-based catalyst. In comparison, two reference samples were tested for SOX/NOX oxidation, one comprising pH 10 water and one comprising 1 wt% Cab-O-Sil silica suspended in water at pH 10. As expected, the oxidation levels were low, namely, 9.8% and 19.2%, respectively.

2.1.1. Catalyst Synthesis Using Cab-O-Sil M5

The SiO2 carrier is synthetic fumed SiO2 Cab-O-Sil consisting of an amorphous structure of 10 nm spheres connected to form 200–300 nm chains with a relatively high surface area of 200 m2/g [30,31]. The heterogenous nanocomposite catalysts were prepared using solutions of different [Co]/[SiO2] ratios, specifically, 3, 6, and 12 mM Co and 0.5, 1, and 2 wt% SiO2. The best-performing catalyst was obtained using deposition of cobalt from a 6 mM solution of CoSO4 in a 1 wt% SiO2 suspension. The effect of the synthesis temperature (25–95 °C) was studied for this catalyst composition (Figure 2). Figures S9 and S10 compare the efficiencies using different catalyst compositions.
Figure 2 demonstrates the effect of synthesis temperature on the Cab-O-Sil-based catalyst. The catalyst synthesized at 95℃ possessed the highest activity toward NO oxidation, i.e., 61.3% conversion. As already stated, SO2 conversion was excellent for all catalysts (>99%). The structure and Co distribution of the best Cab-O-Sil-based catalyst were determined using FESEM-EDX (Figure 3).
As shown in Figure 3, the surface of the larger SiO2 particles (the gray structures in Figure 3b) was covered by small islands of Co (hydroxide) (the green dots in Figure 3b) fixed on the SiO2 surface, as revealed by FESEM-EDX. Thus, the amount of expensive cobalt metal needed for the catalyst was relatively small, such that the price of the catalyst is not affected by the cobalt content. The cobalt concentration was ~0.07% in the catalyst suspension, which pricewise, is negligible. It was clearly demonstrated that the Co active sites were homogeneously distributed on the SiO2 carrier surface.

2.1.2. X-ray Photoelectron Spectroscopy (XPS)

XPS analysis was used to determine Co oxidation states and the chemical environment of the active sites in the nanocomposite catalysts by analyzing binding energies (BE). Three powder samples of the Cab-O-Sil-based nanocomposites prepared at 25, 70, and 95 °C were analyzed using XPS (Figure 4, Figures S2 and S3, respectively). The results of the XPS analysis are summarized in Table 1.
The evaluated curves show four prominent intensive peaks, two around 781 and 786 eV, associated with Co 2p3/2, and two around 797 and 803 eV, associated with Co 2p1/2. These peaks verified the existence of Co(OH)2 [32] and possibly Co(O)OH or CoO [33], which have a similar BE as Co(OH)2 [34]. While it is challenging to determine whether Co(OH)2, Co(O)OH, or CoO exist, it is most probable that Co(OH)2 dominates, mainly due to the Co 2p3/2 satellite peak averaging at 786.0 eV, which is closer to the 787.50 eV of Co(OH)2 than to the 789.10 eV of CoO [32,34]. Note that the main Co 2p3/2 peak values of these species were all very close and lay between 780 and 782 eV [34]. There is no trend in the values with respect to the synthesis temperature. As such, it would appear that synthesis temperature does not affect Co species on the surface of the nanocomposite catalyst.

2.1.3. X-ray Diffraction

The XRD patterns of the three Cab-O-Sil-based nanocomposite powder samples prepared at different temperatures are shown in Figure 5, Figures S7 and S8. The samples were prepared by filtering the suspension using a 0.22 μm nylon filter, followed by drying under ambient conditions. The filtered samples before and after drying are shown in Figures S4 and S5, respectively. Two prominent peaks were observed. The first corresponded to low crystallinity SiO2 at 2Ɵ = 21.5° [35], while the second was α-Co(OH)2 at 2Ɵ = 11.5° [36,37]. Typically, the α-Co(OH)2 XRD pattern presented additional, less pronounced peaks; however, these peaks were hindered by the low crystallinity SiO2 XRD hump, or they are too weak to be detected. Moreover, this analysis could not differentiate between synthesized catalysts at diverse temperatures. Nevertheless, it is clear that Co(OH)2 is a prominent feature of catalyst composition. It should be noted that α-Co(OH)2 is unstable and is usually transformed into the thermodynamically stable β-Co(OH)2 phase, with a prominent peak at ~18° [38]. Owing to the broad peak around 20°, the β-Co(OH)2 signal might have been hindered. In such a case, a mixture of α–β phases exists. The XRD samples were dried prior to analysis. Figures S4 and S5 show that wet samples were pink and that the dry samples were blue, demonstrating color shifts associated with the α–β phase transformation. The α–Co(OH)2 phase was detected in XRD. Note that the catalyst used in aqueous suspension was pink, i.e., β-Co(OH)2.

2.1.4. Cobalt Leaching—The Effect of pH

The acidity of the nanocomposite catalyst suspension in the wet scrubber system was kept at a pH range of 4–10 to maintain efficient scrubbing. However, SOX/NOX oxidation produces acids (H2SO4 or HNO3), resulting in acidification of the catalytic suspension, which might dissolve Co from the nanocomposite active sites and reduce the activity. To prevent this undesired process, ammonium hydroxide was injected into the reactor to keep the pH level above 4. The pH effect on Co leaching from the nanocomposite catalyst was studied to follow this potential adverse process. Figure 6 summarizes the ICP-OES results of the Co leaching experiment. The results indicated that Co leaching was highly pH-dependent. At pH < 6, cobalt leached from the silica into the solution. Moreover, in the pH range of 8–10, there was no appreciable dissolution of Co from the active site of the catalyst. This observation indicated that Co(OH)2 was efficiently fixed to the SiO2 surface at pH 8–10. Therefore, the catalyst suspension in the reactor should be maintained at a pH range of 8–10 to achieve effective catalytic SOX/NOX oxidation. The pH level can be maintained at the optimal range by injecting an NH4OH solution. This addition of NH4OH maintains the pH at the required level and produces fertilizers by precipitating as (NH4)2SO4 and NH4NO3. These precipitates did not affect catalyst performance during the experiment. Moreover, Co leaching tested after seven days of air bubbling at pH 9.5 showed a Co concentration in the leachant of only 0.082 ppm, far below the acute (maximum) guideline of 110 ppm [39].

2.2. Activation of Atmospheric Oxygen by the Nanocomposite Catalyst

Interestingly, the nanocomposite catalyst was highly efficient in activating atmospheric O2 as the SOX/NOX oxidation reagent in flue gases. The novelty of the catalyst is based on the fact that the process for eliminating SOX/NOX is performed in one stage using O2; namely, both pollutants are oxidized in one process by atmospheric oxygen. This is in contrast to current industrial processes that eliminate SOX/NOX via two separate methods, specifically, FGD for SOX oxidation and SCR for NOX reduction. Additionally, although SCR also uses heterogeneous catalysts, it requires relatively high temperatures (350–400 °C) as compared with the process developed here, which operates at 60–90 °C.
The Co(OH)2-based catalyst developed in this study can adsorb and activate atmospheric O2 and enhance its role as an efficient oxidation reagent at relatively low temperatures. The surface species are negatively charged at a pH higher than the isoelectric point of fumed SiO2 (~pH 4) [40]. Therefore, SiO2 is relatively hydrophilic, yet presents hydrophobic properties [41].
All nanocomposite catalysts studied were very efficient for SO2 oxidation (>99% conversion), whereas much lower oxidation efficiencies were found for NO oxidation. Note that SO2 can be adsorbed by an alkaline solution and oxidized by O2 even without catalysts. However, all catalysts activated the O2 molecule, denoted as O2ǂ, and increased its reactivity at 60–90 °C as an oxidation catalyst. Hence, the O2 molecule is adsorbed to the Co(OH)2 catalytic site at the nanocomposite surface in the first step via:
Cat + O2 ⇌ O2(ads)
The activation forms a chemisorbed O2 moiety with a weaker O=O bond via Scheme 1:
O2(ads) → ≡ O2ǂ (Activated)
In other words, partial cleavage of the double bond in the adsorbed O2 molecule occurs, likely yielding the activated species in the catalyzed oxidation process. The activated O2 molecule, O2ǂ, oxidizes NO or SO2 to produce NO2 or SO3. O2ǂ oxidizes water-soluble SO2 in the SIV oxidation state (HSO3−, SO32−, or dissolved SO2—SO2·H2O) to create SVI, generating H2SO4 as the final product. The fact that NO oxidation efficiency was much lower than that of SO2 oxidation probably stems from the fact that SO2 is hydrophilic and dissolves well in water, whereas NO is hydrophobic and thus is only slightly soluble in water.
As shown by control experiments (Section 3.1), the active species in the catalytic process was cobalt. The fact that the different nanocomposite SiO2-based catalysts yielded different catalytic activities probably stems from the various adsorption qualities at the Co centers deposited on the SiO2 carriers. Apparently, the activity of the catalysts increases with their surface area. However, among the silica carriers used, Cab-O-Sil silica possessed a high surface area of ~200 m2/g [42], but not the highest such value. Importantly, isolated silanol groups on the Cab-O-Sil-fumed silica surface are not bound via hydrogen bonds and are, therefore, active [42]. Additionally, the synergetic cobalt-hydroxide redox interaction increased appreciably due to the polarity effect of the adjacent silica carrier. This effect varied between the different silica carriers and resulted in different activities.
The efficiencies of the three nanocomposite catalysts for NOX oxidation were not the same, and they increased with preparation temperature (Figure 2). This observation can be explained by the different structures or the chemical natures of the active Co sites in the nanocomposite catalyst. Temperature affects the degree of Co hydrolysis and its rate, possibly by influencing several factors: (i) the different ratios of oxo-hydroxo species of cobalt; (ii) the different sizes and shapes of the Co island produced on the SiO2 carrier; or (iii) the different Co-Si or Co-O-Si bond strengths, which affect the activation efficiency of the adsorbed molecular O2 at the chemisorption sites.

3. Materials and Methods

3.1. Reagents

All chemicals used were of analytical grade and were used without further purification. CoSO4·6H2O was purchased from J.T Baker (Sigma-Aldrich, St. Louis, MO, USA). NaOH was supplied by Alfa Aesar, Haverhill, MA, USA. The 25% NH4OH solution was purchased from Frutarom, Hifa, Israel. Ultra-pure deionized (UPDI) water with a resistivity of >15 MΩ cm was used in this study. Four types of SiO2 were used: untreated fumed SiO2 powder, Cab-O-Sil M-5, which comprises synthetic, amorphous 300 nm colloidal chains of 10 nm silicon dioxide spheres with a high surface area of 200 m2/g and was supplied by Sigma Aldrich, Rehovot, Israel, denoted as Cab-O-Sil; 1.0- and 0.5-micron spherical silicon oxide powders (99.9%) with narrow particle size distributions provided by Alfa Aesar with a surface area range of 2–7 m2/g; a self-synthesized powder of 500 nm silica NPs synthesized by the Stöber process (see the SI for the synthesis protocol and Figure S1 for SEM and elemental mapping images), denoted as Stöber SiO2; and 30% colloidal 10 nm SiO2 NPs suspended in pH 10.5 water with a surface area of 305 m2/g from Alfa Aesar.

3.2. Instrumentation

The size, morphology, and elemental distribution of the different catalysts were determined using a Field-Emission Scanning Electron Microscope (FESEM) equipped with an Energy Dispersive X-Ray Spectrometer (EDX) (Ultra-High-Resolution FESEM-EDX MAIA3 by Tescan (JEOL, Tokyo, Japan), Brno–Kohoutovice, Czech Republic). An inductively coupled plasma–optical emission spectrometer (ICP-OES, Ametek, SPECTRO ARCOS, Kleve, Germany) was used to determine metal concentrations. X-ray photoelectron spectroscopy (XPS) was used to analyze the oxidation states of Co and their shifts due to environmental changes by a PHI 5500 Multitechnique System (from Physical Electronics, Chanhassen, MN, USA) with a monochromatic X-ray source (Aluminium Kα line, E = 1486.6 eV, and 350 W), placed perpendicular to the analyzer axis and calibrated using the 3d5/2 line of Ag with a full width at half maximum (FWHM) of 0.8 eV. X-ray diffraction (XRD) patterns of the Co hydroxides were acquired by a D8 A25 Advance powder diffractometer with monochromatic Cu Kα radiation (l =1.5405 Å) operated at 40 kV and 40 mA, a PSD-XE detector, and an energy discriminator (DAVINCI) from Bruker, Billerica, MA, USA.

3.3. Catalyst Synthesis

Four nanocomposite catalysts were prepared using different SiO2 particles as the carrier for Co(OH)2. The islands of the small Co NPs were precipitated on the surface of the larger SiO2 particles to produce an effective catalyst for the SOX/NOX oxidation process. This was achieved by using two different methods, one for silica powders and one for silica suspensions.

3.3.1. Catalyst Synthesis Using Nanopowders

The SiO2 used to produce the nanocomposite catalysts were Cab-O-Sil, 1.0- and 0.5-micron SiO2 powders, and Stöber SiO2. The nanocomposites were synthesized at various temperatures (25–90 °C) in the sequence presented in Scheme 2:
(i)
Dissolution of CoSO4·6H2O in water at different concentrations.
(ii)
Addition of the SiO2 NPs powder to the Co solution with constant stirring to form a homogeneous suspension.
(iii)
Addition of 1.0 M NaOH, along with rapid stirring, until reaching pH 10.
The SiO2/Co(OH)2 nanocomposites were synthesized via hydrolysis and deposition of CoII by sodium hydroxide on the silica carrier as Co(OH)2 (Scheme 2). In the first step, CoSO4·6H2O was dissolved in water at the desired temperature, and SiO2 particles were added to the solution and suspended by stirring for 10 min, creating a slightly acidic suspension. Consequently, the pH was adjusted to 10 by the dropwise addition of 1.0 M NaOH to start the Co hydrolysis. For the 1.0- and 0.5-micron SiO2 powders, some particles were precipitated after 30 min. The nanocomposite suspensions were stirred for homogenization prior to using them as catalysts in the wet scrubber pilot. Gas bubbling maintained the homogeneity of the suspension during the catalysis process.
After the hydrolysis, the suspension color changed from reddish pink to blue. After ~1 h of stirring and heating, a further color change was observed due to the phase transition from the metastable tetrahedral α phase (blue) to the stable octahedral β phase (pink), which is typical of Co(OH)2.

3.3.2. Catalyst Synthesis Using the Colloidal Suspension

Equal volumes of the two precursor solutions (2 wt% SiO2 at pH 9.0 and 12 mM Co) were flow-mixed using a Y-junction tube. The flow-mixing procedure was carried out to homogeneously bind Co(OH)2 NPs to 10 nm SiO2 and prevent local hydrolysis of Co, which may form large Co(OH)2 aggregates. The final solution was stirred and heated to 70 °C, affording a clear pink suspension. The concentrations of Co and SiO2 in the final suspension were 6 mM and 1 wt%, respectively. Next, 1.0 M NaOH was added dropwise until reaching pH 10 and heating at 70 °C continued for 1 h. A phase transition was observed when the blue (α-Co(OH)2) color appeared after NaOH addition had changed it to pink (β-Co(OH)2). The content of cobalt in the wet and dry catalysts was 0.035% and 3.42 wt%, respectively.
The synthesized heterogeneous particles were characterized and examined as catalysts for SOX/NOX oxidation using atmospheric O2 as the oxidation reagent in the pilot system. The best catalyst type (Cab-O-Sil-based catalyst, see below) was further evaluated. The nanocomposite catalyst was synthesized using different [SiO2]/[Co] molar ratios to achieve the optimal SOX and NOX oxidation catalyst in the wet scrubber pilot system. The reactions were carried out at a concentration range of 3–12 mM CoSO4 and 0.5–2 wt% SiO2 and a temperature range between 25 and 90 °C.

3.4. Cobalt Leaching Tests

To study Co leaching from the Cab-O-Sil-based SiO2/Co(OH)2 nanocomposite catalyst into the aqueous phase as a function of pH, the catalyst suspension samples were first acidified with nitric acid to reach a pH range of 4–10. A 50 mL suspension sample of the nanocomposite catalyst SiO2/Co(OH)2 at the desired pH was shaken in an orbital shaker (Analog Orbital Shaker, TS-400, MRC) at 100 rpm for 24 h. After 24 h, the nanocomposite catalyst was filtered. In another experiment, 500 mL fresh catalyst at pH 9.5 was kept in the reactor pilot system at room temperature and fed 0.5 litem per minute (LPM) bubbled air for seven days. After seven days, a 50 mL sample aliquot of the suspension was filtered. All samples were filtered using a 0.22 µm PES membrane filter to remove the SiO2/Co(OH)2, and the Co content in the filtrate was determined using ICP-OES.

3.5. Bubble Column Tests

To evaluate nanocomposite catalyst activities, a laboratory pilot system unit was designed based on the wet scrubber technique, using a bubble column as a substitute for the spray tower due to its simplicity and flexibility of operation in a laboratory environment. SOX/NOX oxidation using atmospheric O2 as oxidation reagent were performed in the bubble column shown in Scheme 3 and Figure S6. The system is described in detail by Khabra et al. [43]. The main reactor was a vertical cylindrical tube reactor with a diameter of 5 cm and a gas inlet at the bottom. Briefly, the synthetic contaminated flue gas was injected into the bottom of the reactor and bubbled through the liquid phase using a sparger to form fine bubbles, with an optimal flow rate of 0.5 LPM. The contaminated flue gas simulation was carried out using a synthetic mixture of air contaminated by SO2 and NO and bubbled through the catalyst suspension. All catalytic tests were performed at various operational parameters, specifically, temperature, pH, gas flow rate, and different contamination levels of SOX/NOX in the air. The concentrations of the various gases in the gas mixture were monitored pre- and post-wet scrubber. The reactor was filled with 500 mL of catalyst suspension. The synthetic flue gas contained 200–500 ppm SO2 and/or NO. The sources of the SOX/NOX used to prepare the synthetic contaminated feed gas were 5 L lecture bottles of 1 wt% (SO2 or NO balanced with nitrogen; Airgas). The gas mixture flow rate was measured using digital mass flow meters (indicated as Qgas in Scheme 3) and a rotameter (Qair). The mixture flow rate and concentration were controlled using needle valves (V1 and V2 in Scheme 3). The SO2, NO, and NO2 concentrations were measured pre- and post-bubble column application using an Optima 7 portable flue gas analyzer based on electrochemical sensors (Cin, Cout in Scheme 3). The SOX/NOX conversion for each case was calculated from the inlet to the outlet concentration ratio using Equation (3).
R i [ % ] = 100 ( 1 - C i , out C i , in ) ,   ( i =   NO ,   SO 2 )
The reactor temperature was controlled by a digital PID controller temperature control system using textile glass heating tape (500 W, 250 °C) and a K thermocouple for temperature sensing (TC in Scheme 3). The working temperature was kept up to 60 °C, the expected equilibrium temperature of the flue gases. The acidity values of the suspension were monitored using a pH electrode and controlled manually by adding an alkaline solution to the reactor (pHC in Scheme 3). Initially, the reactor was charged with the catalyst suspension at pH 10.
The flow characteristics in the bubble column, such as pressure drop, void fraction, residence time, and average bubble size, were studied in a preliminary study to set the operating parameters, i.e., the liquid volume and gas flow rate [43]. The operational requirements were set for a residence time of 3 s and a bubble flow regime (void fraction—gas/liquid volumetric ratio) of ε < 0.3 to maximize the available interfacial area. The operational flow rate ranges were between 0.5 and 2.5 LPM, and the net liquid height was 300 mm (500 mL). The maximum bubble diameter was 5 mm.
All efficient nanocomposite catalysts demonstrating high conversion rates for SOX/NOX oxidation were further analyzed using FESEM-EDS, XPS, ICP-OES, and XRD.

4. Conclusions

This study proposed a more unified solution to NOX/SOX elimination in flue gases than do the FGD and SCR processes regularly used by utilities. SiO2-supported Co NPs can serve as excellent catalytic scrubbing reagents for the oxidation of polluting SOX/NOX gases in flue gases using atmospheric O2 as the oxidation reagent at a relatively low-temperature range (70–100 °C). The SOX scrubbing process is much more efficient than that of NOX, probably due to the hydrophobic nature of NO, which possesses lower solubility in aqueous solutions than does SO2. SOX/NOX emissions in flue gases can be efficiently and simultaneously reduced upon their oxidation using atmospheric oxygen as the oxidation reagent in cost-effective wet scrubbers. To obtain better understanding of the catalytic oxidation mechanism, additional studies should be performed. Still, this new type of catalysts, as described here, can be used in spray tower units of large utilities and for smaller fossil fuel users, like ships.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal13010029/s1, Figure S1: Electron microscopy images of Stöber silica-based nanocomposite catalyst morphology and elemental mapping. (a) SEM image of SiO2/Co(OH)2. The surface roughness stems from the gold coating procedure for SEM imaging. (b) SEM-EDX mapping of the nanocomposite particles. Cobalt atoms are marked by purple dots; Figure S2: High-resolution XPS spectrum of SiO2/Co(OH)2 synthesized at 75 °C; Figure S3: High-resolution XPS spectrum of SiO2/Co(OH)2 synthesized at 95 °C; Figure S4: Filtered catalysts, 25–95 °C; Figure S5: Ground powder of the catalysts, 25–95 °C; Figure S6: The laboratory wet scrubbing pilot unit; Figure S7: Powder XRD pattern of SiO2/Co(OH)2 synthesized at 25 °C; Figure S8: Powder XRD pattern of SiO2/Co(OH)2 synthesized at 95 °C; Figure S9: Effect of silica concentration on catalyst activity (SiO2/Co(OH)2; [Co] = 6 mM); Figure S10: Effect of cobalt concentration on catalyst activity (SiO2/Co(OH)2; [SiO2] = 1 wt%).

Author Contributions

Conceptualization, T.Z., H.C., G.A.P. and A.K.; Methodology, T.Z., H.C., G.A.P., X.Q. and P.C.S.; Validation, T.Z. and H.C.; Investigation, A.K.; Resources, T.Z., H.C., G.A.P. and X.Q.; Writing—Original Draft, A.K.; Writing—Review and Editing, T.Z. and H.C.; Visualization T.Z., H.C. and G.A.P.; Supervision, T.Z. and H.C.; Funding acquisition, T.Z., H.C. and G.A.P. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Israel Innovation Authority “Kamin” program [grant number #55299].

Data Availability Statement

Not applicated.

Acknowledgments

A.K. thanks Gifty, Sara Rolly, and Mor Oved for silica particle synthesis and Ariel University for a Ph.D. fellowship and the Institute of Environmental Assessment and Water Research (IDAEA-CSIC), Barcelona, Spain for a visiting student fellowship.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Percent NO and SO2 oxidation on different SiO2 nanocarriers (all the catalysts are Co oxide impregnated on silica (SiO2/Co(OH)2) homogeneously dispersed in aqueous solution); [SiO2] = 1 wt%, [Co] = 6 mM).
Figure 1. Percent NO and SO2 oxidation on different SiO2 nanocarriers (all the catalysts are Co oxide impregnated on silica (SiO2/Co(OH)2) homogeneously dispersed in aqueous solution); [SiO2] = 1 wt%, [Co] = 6 mM).
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Figure 2. Effect of synthesis temperature on catalyst activity (SiO2/Co(OH)2; [SiO2] = 1 wt%, [Co] = 6 mM).
Figure 2. Effect of synthesis temperature on catalyst activity (SiO2/Co(OH)2; [SiO2] = 1 wt%, [Co] = 6 mM).
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Figure 3. FESEM-EDX image of the Cab-O-Sil-based SiO2/Co(OH)2 nanocomposite catalyst prepared at 70 °C ([SiO2] = 1 wt%, [Co] = 6 mM) (a) with elemental mapping of Co on the catalyst. (b) Co atoms are denoted by green dots. (c) The EDX-selected area for elemental analysis is highlighted.
Figure 3. FESEM-EDX image of the Cab-O-Sil-based SiO2/Co(OH)2 nanocomposite catalyst prepared at 70 °C ([SiO2] = 1 wt%, [Co] = 6 mM) (a) with elemental mapping of Co on the catalyst. (b) Co atoms are denoted by green dots. (c) The EDX-selected area for elemental analysis is highlighted.
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Figure 4. High-resolution XPS spectrum for SiO2/Co(OH)2 synthesized at room temperature (25 °C).
Figure 4. High-resolution XPS spectrum for SiO2/Co(OH)2 synthesized at room temperature (25 °C).
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Figure 5. Powder XRD pattern for SiO2/Co(OH)2 synthesized at 75 °C.
Figure 5. Powder XRD pattern for SiO2/Co(OH)2 synthesized at 75 °C.
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Figure 6. Effect of solution pH on Co leaching from a Cab-O-Sil-based SiO2/Co(OH)2 nanocomposite into the aqueous solution, as revealed by ICP-OES analysis of the leachates.
Figure 6. Effect of solution pH on Co leaching from a Cab-O-Sil-based SiO2/Co(OH)2 nanocomposite into the aqueous solution, as revealed by ICP-OES analysis of the leachates.
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Scheme 1. Chemisorption of oxygen molecule on the catalyst’s surface.
Scheme 1. Chemisorption of oxygen molecule on the catalyst’s surface.
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Scheme 2. Schematic diagram of SiO2/Co(OH)2 nanocomposite synthesis.
Scheme 2. Schematic diagram of SiO2/Co(OH)2 nanocomposite synthesis.
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Scheme 3. The bubble column reactor with temperature (TC) and pH (pHC) controllers and flow rate (Qgas, Qair) and gas concentration (Cin, Cout) sensors.
Scheme 3. The bubble column reactor with temperature (TC) and pH (pHC) controllers and flow rate (Qgas, Qair) and gas concentration (Cin, Cout) sensors.
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Table
Table 1. XPS binding energies (eV) for Co at different synthesis temperatures for Cab-O-Sil-based SiO2/Co(OH)2 nanocomposite catalysts *.
Table 1. XPS binding energies (eV) for Co at different synthesis temperatures for Cab-O-Sil-based SiO2/Co(OH)2 nanocomposite catalysts *.
2p3/22p1/2
TemperatureMPSSDSMPSSDS
25781.53786.384.85797.42803.265.84
70780.60785.605.00796.22802.015.79
95781.23785.884.65797.22803.766.54
(SiO2/Co(OH)2; [SiO2] = 1 wt%, [Co] = 6 mM). * MP = Binding energy of the main peak; SS = binding energy of the satellite peak; DS = energy separation between the main and satellite peaks. Error limits are ± 0.2 eV.
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Khabra, A.; Cohen, H.; Pinhasi, G.A.; Querol, X.; Córdoba Sola, P.; Zidki, T. Synthesis of a SiO2/Co(OH)2 Nanocomposite Catalyst for SOX/NOX Oxidation in Flue Gas. Catalysts 2023, 13, 29. https://doi.org/10.3390/catal13010029

AMA Style

Khabra A, Cohen H, Pinhasi GA, Querol X, Córdoba Sola P, Zidki T. Synthesis of a SiO2/Co(OH)2 Nanocomposite Catalyst for SOX/NOX Oxidation in Flue Gas. Catalysts. 2023; 13(1):29. https://doi.org/10.3390/catal13010029

Chicago/Turabian Style

Khabra, Alon, Haim Cohen, Gad A. Pinhasi, Xavier Querol, Patricia Córdoba Sola, and Tomer Zidki. 2023. "Synthesis of a SiO2/Co(OH)2 Nanocomposite Catalyst for SOX/NOX Oxidation in Flue Gas" Catalysts 13, no. 1: 29. https://doi.org/10.3390/catal13010029

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