Catalytic Hydrogen Evolution from H2S Cracking over CrxZnS Catalyst in a Cylindrical Single-Layered Dielectric Barrier Discharge Plasma Reactor

The use of non-thermal plasma technology in producing green fuels is a much-appreciated environmentally friendly approach. In this study, an Al2O3-supported CrxZnS semiconductor catalyst was tested for hydrogen evolution from hydrogen sulfide (H2S) gas by using a single-layered dielectric barrier discharge (DBD) system. The Al2O3-supported CrxZnS catalyst (x = 0.20, 0.25, and 0.30) was produced by using a co-impregnation method and characterized for its structural and photocatalytic characteristics. The discharge column of the DBD system was filled with this catalyst and fed with hydrogen sulfide and argon gas. The DBD plasma was sustained with a fixed AC source of 10 kV where plasma produced species and UV radiations activated the catalyst to break H2S molecules under ambient conditions. The catalyst (hexagonal-cubic-sphalerite structure) showed an inverse relationship between the band gap and the dopant concentration. The hydrogen evolution decreased with an increase in dopant concentration in the nanocomposite. The Cr0.20ZnS catalyst showed excellent photocatalytic activity under the DBD exposure by delivering 100% conversion efficiency of H2S into hydrogen. The conversion decreased to 96% and 90% in case of Cr0.25ZnS and Cr0.30ZnS, respectively.


Introduction
Hydrogen sulfide (H 2 S) is a poisonous gas and its production is harmful to both human health and equipment [1]. Hydrogen can be produced from various raw materials like coal, water, natural gas, hydrogen sulphide, biomass and boron hydrides using various methods (electrolytic, thermal and photolytic) [2,3]. Currently, the yearly global production of hydrogen is 50 million tons and more than 95% of it is obtained from fossil fuels. The CO 2 released by fossil fuels contributes to environmental pollution [3]. Hydrogen can also be produced by cracking hydrogen sulfide (H 2 S) over a suitable catalyst. Hydrogen gas is produced through different methods [4,5]. A large amount of H 2 gas is used in industrial applications, such as the production of chemicals, oils, fats, fuels, and metal reforming [6]. Currently, the Claus method is considered to be an important hydrogen-sulfide-removal technology. This technique is generally not preferred owing to its high working cost and related environmental issues. In the Claus method, hydrogen accumulating in hydrogen sulfide cannot be regained [7]. Various approaches have been proposed for the decomposition of H 2 S to produce hydrogen (H 2 ). These methods include the thermo-chemical method, catalytic decomposition, thermal-diffusion photochemical, electrochemical, and different ratios of chromium (Cr). A wet-impregnation method was adopted to prepare the catalyst samples. In this method, the ZnS amount was taken as 15 g, which is 10 wt% of γ-Al 2 O 3 . An aqueous solution was prepared by adding 5 g of a Zn-nitrate solution to 15 mL of distilled water. The Cr-nitrate and Zn-nitrate were mixed with different molar ratios (0.20, 0.25, and 0.30) by comparing the previous research. The prepared solution and γ-Al 2 O 3 were mixed with a gentle shake. The mixture was filtered by a filtration process and then dried at 120 • C for 12 h in the oven. The calcination of the material was performed in the furnace for 5 h. A fine powder was formed after crushing the calcinated material. The sulfide catalysts were formed when oxide precursors were sulfidated in the presence of sulfiding gas. Eventually, Cr x ZnS catalysts (x= 0.20, 0.25, and 0.30) was prepared.

Preparation of Photocatalyst
The procedure of synthesis of photocatalyst is illustrated in Figure 1. Using the illustrated procedure, a series of Cr-doped ZnS with Al2O3 support was prepared with different ratios of chromium (Cr). A wet-impregnation method was adopted to prepare the catalyst samples. In this method, the ZnS amount was taken as 15 g, which is 10 wt% of γ-Al2O3. An aqueous solution was prepared by adding 5 g of a Zn-nitrate solution to 15 mL of distilled water. The Cr-nitrate and Zn-nitrate were mixed with different molar ratios (0.20, 0.25, and 0.30) by comparing the previous research. The prepared solution and γ-Al2O3 were mixed with a gentle shake. The mixture was filtered by a filtration process and then dried at 120 °C for 12 h in the oven. The calcination of the material was performed in the furnace for 5 h. A fine powder was formed after crushing the calcinated material. The sulfide catalysts were formed when oxide precursors were sulfidated in the presence of sulfiding gas. Eventually, CrxZnS catalysts (x= 0.20, 0.25, and 0.30) was prepared.

DBD Plasma-Assisted Hydrogen Evolution
The schematic and photographic views of the DBD setup, used for the production of hydrogen by cracking H2S molecules over the composite catalyst, are given in Figure 2. This laboratory-built system consists of a 30 cm DBD vertical column with an active plasma column length of 23 cm. A quartz tube with a 4 mm wall thickness and a 12 mm internal diameter was used as a DBD column. A copper rod of 8 mm diameter was passed through the tube to work as one of the two electrodes. The tube was wrapped with a copper wire to work as an electrode for uniform radial and spatial distribution of the applied power and plasma. The upper end of the tube was used as a gas inlet and the lower end was connected with the gas analyzer. The discharge column of the DBD system was filled with this catalyst and fed with hydrogen-sulfide and argon gas.

DBD Plasma-Assisted Hydrogen Evolution
The schematic and photographic views of the DBD setup, used for the production of hydrogen by cracking H 2 S molecules over the composite catalyst, are given in Figure 2. This laboratory-built system consists of a 30 cm DBD vertical column with an active plasma column length of 23 cm. A quartz tube with a 4 mm wall thickness and a 12 mm internal diameter was used as a DBD column. A copper rod of 8 mm diameter was passed through the tube to work as one of the two electrodes. The tube was wrapped with a copper wire to work as an electrode for uniform radial and spatial distribution of the applied power and plasma. The upper end of the tube was used as a gas inlet and the lower end was connected with the gas analyzer. The discharge column of the DBD system was filled with this catalyst and fed with hydrogen-sulfide and argon gas.
The DBD plasma was sustained with a fixed AC source of 10 kV where plasmaproduced species and UV radiations were used to activate the catalyst to break the H 2 S molecules under ambient conditions. The Al 2 O 3 -supported Cr x ZnS semiconductor catalyst was tested for hydrogen evolution from H 2 S gas using this single-layered DBD system [15]. The discharge volume of the dielectric-barrier-discharge reactor was 22 mL [16]. One end of the battery was attached to the wire and the other to the rod. About 10 g of the Cr x ZnS catalyst (x = 0.20, 0.25, and 0.30) was loaded in the discharge column. At the same time, the gas (Ar + H 2 S) was passed through the loaded column. The gas product of the reaction in the discharge column was analyzed. The relationship between the H 2 S (X Hydrogen sulfide ) and H 2 yield (X Hydrogen ) is shown as follows: where A is the value of the H 2 peak area of effluence. A o has represented the hydrogen peak area at 100% hydrogen sulfide conversion. The area of the represents the energy lost during a single voltage cycle in the discharge. The total input energy used in the plasma during the process was calculated by the specific input energy (SIE) as: where V is the flowrate of gas (L/s) and P is the discharge power (W). The energy utilization for the H 2 generation (E, eV) was calculated from the specific input energy as: FOR PEER REVIEW 4 of 14 The DBD plasma was sustained with a fixed AC source of 10 kV where plasma-produced species and UV radiations were used to activate the catalyst to break the H2S molecules under ambient conditions. The Al2O3-supported CrxZnS semiconductor catalyst was tested for hydrogen evolution from H2S gas using this single-layered DBD system [15]. The discharge volume of the dielectric-barrier-discharge reactor was 22 mL [16]. One end of the battery was attached to the wire and the other to the rod. About 10 g of the CrxZnS catalyst (x = 0.20, 0.25, and 0.30) was loaded in the discharge column. At the same time, the gas (Ar + H2S) was passed through the loaded column. The gas product of the

FTIR Analysis of Catalyst
With the Fourier transform infrared (FTIR) analysis, the absorbance of the species in the crystal surface and the nanoparticle formation of ZnS were checked. It is reported that this analysis also gives information about the chemical bonding of the chemical [17]. The FTIR absorbance spectra of Cr x ZnS with different molar ratios are shown in Figure 3. As shown in Table 1, FTIR analysis showed the same peaks for Cr x ZnS samples with different ratios (x = 0.20, 0.25, and 0.30) within the range of 500-4000 cm −1 . The FTIR peaks were located around 3700 cm −1 , 1588 cm −1 , 1531 cm −1 , and 1020 cm −1 . All the peaks exist in the group frequency region (GFR) except 1020 cm −1 because its range was lower than the other three peaks, so it was observed in the fingerprint region (FPR) [18]. The peak at 3700 cm −1 was due to O-H stretching vibration. This peak shows an alcohol group of compounds with intermolecular forces based on their structure [19]. The peaks at 1588 cm −1 and 1020 cm −1 exhibited the same amines groups with no intermolecular force at medium peaks. Both peaks have different vibrations, i.e., 1588 cm −1 represents the N-H bending due to GFR and 1020 cm −1 represents the C-N stretching vibration in FPR. There is a strong peak appearance at 1531 cm −1 caused by N-O stretching. It exists in a nitro-compound group with no bonding forces.

FTIR Analysis of Catalyst
With the Fourier transform infrared (FTIR) analysis, the absorbance of the species in the crystal surface and the nanoparticle formation of ZnS were checked. It is reported that this analysis also gives information about the chemical bonding of the chemical [17]. The FTIR absorbance spectra of CrxZnS with different molar ratios are shown in Figure 3. As shown in Table 1, FTIR analysis showed the same peaks for CrxZnS samples with different ratios (x = 0.20, 0.25, and 0.30) within the range of 500-4000 cm −1 . The FTIR peaks were located around 3700 cm −1 , 1588 cm −1 , 1531 cm −1 , and 1020 cm −1 . All the peaks exist in the group frequency region (GFR) except 1020 cm −1 because its range was lower than the other three peaks, so it was observed in the fingerprint region (FPR) [18]. The peak at 3700 cm −1 was due to O-H stretching vibration. This peak shows an alcohol group of compounds with intermolecular forces based on their structure [19]. The peaks at 1588 cm −1 and 1020 cm −1 exhibited the same amines groups with no intermolecular force at medium peaks. Both peaks have different vibrations, i.e., 1588 cm −1 represents the N-H bending due to GFR and 1020 cm −1 represents the C-N stretching vibration in FPR. There is a strong peak appearance at 1531 cm −1 caused by N-O stretching. It exists in a nitro-compound group with no bonding forces.

UV-Visible Analysis
The absorption spectra of catalysts Cr x ZnS (x = 0.20, 0.25, and 0.30) were examined by UV-Vis analysis within the wavelength range of 200 nm to 800 nm and obtained results are shown in Figure 4. The absorption edges at 367 nm, 376 nm, and 379 nm correspond to Cr 0.20 ZnS, Cr 0.25 ZnS, and Cr 0.30 ZnS respectively observed along the x-axis [20]. In Figure 5, the Cr 0.30 ZnS catalyst showed a superior shift in absorption edge (red-shift) towards the visible light region in contrast to other samples, showing a maximum absorption upto 379 nm [21,22]. Cr 0.20 ZnS, Cr 0.25 ZnS and Cr 0.30 ZnS catalysts represented the absorbance values of 0.162 nm, 0.324 nm and 0.563 nm, respectively. Bodke et al. [14] reported that the concentration of doped Cr 3+ had a pronounced effect on the optical properties of the ZnS catalyst and witnessed a significant red-shift in the absorption of Cr-doped ZnS.
The absorption spectra of catalysts CrxZnS (x = 0.20, 0.25, and 0.30) were examined by UV-Vis analysis within the wavelength range of 200 nm to 800 nm and obtained results are shown in Figure 4. The absorption edges at 367 nm, 376 nm, and 379 nm correspond to Cr0.20ZnS, Cr0.25ZnS, and Cr0.30ZnS respectively observed along the x-axis [20]. In Figure  5, the Cr0.30 ZnS catalyst showed a superior shift in absorption edge (red-shift) towards the visible light region in contrast to other samples, showing a maximum absorption upto 379 nm [21,22]. Cr0.20ZnS, Cr0.25ZnS and Cr0.30ZnS catalysts represented the absorbance values of 0.162 nm, 0.324 nm and 0.563 nm, respectively. Bodke et al. [14] reported that the concentration of doped Cr 3+ had a pronounced effect on the optical properties of the ZnS catalyst and witnessed a significant red-shift in the absorption of Cr-doped ZnS.  of 0.162 nm, 0.324 nm and 0.563 nm, respectively. Bodke et al. [14] reported that the concentration of doped Cr 3+ had a pronounced effect on the optical properties of the ZnS catalyst and witnessed a significant red-shift in the absorption of Cr-doped ZnS.

X-ray Diffraction Analysis
The XRD analysis of as-synthesized catalysts CrxZnS (x = 0.20, 0.25, and 0.30) are shown in Figure 6. All prepared samples showed similar diffraction peaks, identifying no The band gap values of the catalysts with different Cr compositions are reported in Figure 5. The band gap of Cr x ZnS with x = 0.20, 0.25, and 0.30 was found to be 2.68, 2.48, and 1.69 eV, respectively. These band gap values are lower than the standard value of bulk ZnS (3.6 eV) [23].

X-ray Diffraction Analysis
The XRD analysis of as-synthesized catalysts Cr x ZnS (x = 0.20, 0.25, and 0.30) are shown in Figure 6. All prepared samples showed similar diffraction peaks, identifying no variation in the host crystal structure after introducing Cr 3+ ions into its lattice. The different diffraction peaks were found at 2θ values of 31 • , 36 • , 47 • , and 56 • , which correspond to (002), (001), (110), and (112) planes of ZnS, respectively. There was no other obvious indication of any other diffraction peak found except for the alumina peak. Among all samples, only the Cr 0.20 ZnS catalyst showed the origination of diffraction peak related to Cr impurity [24]. The information about the existence of the characteristic peak of (110) plane was confirmed from JCPDS#65-0309. The crystal structure of the Cr x ZnS catalyst is cubic sphalerite.

X-ray Diffraction Analysis
The XRD analysis of as-synthesized catalysts CrxZnS (x = 0.20, 0.25, and 0.30) are shown in Figure 6. All prepared samples showed similar diffraction peaks, identifying no variation in the host crystal structure after introducing Cr 3+ ions into its lattice. The different diffraction peaks were found at 2θ values of 31°, 36°, 47°, and 56°, which correspond to (002), (001), (110), and (112) planes of ZnS, respectively. There was no other obvious indication of any other diffraction peak found except for the alumina peak. Among all samples, only the Cr0.20ZnS catalyst showed the origination of diffraction peak related to Cr impurity [24]. The information about the existence of the characteristic peak of (110) plane was confirmed from JCPDS#65-0309. The crystal structure of the CrxZnS catalyst is cubic sphalerite. The surface area decreased with an increase in a molar ratio of Cr/Zn. Ramasamy [25] reported that the lattice constants were reduced with Cr doping because of the ionic radius The surface area decreased with an increase in a molar ratio of Cr/Zn. Ramasamy [25] reported that the lattice constants were reduced with Cr doping because of the ionic radius (0.63 Å and 0.74 Å) of Cr 3+ and Zn 2+ ions. In our study, as the Cr 3+ content increased, the lattice parameters were decreased in the case of all as-prepared Cr x ZnS catalysts. The Scherrer equation was used to calculate the average crystallite size of the catalyst. The grain sizes of Cr x ZnS were estimated to be 18.30, 17.89, and 17.49 nm, corresponding to the Cr 0.20 ZnS, Cr 0.25 ZnS, and Cr 0.30 ZnS, respectively. Since the band gap and the grain size are inversely related to each other; therefore, our measured band gap and crystallite size are in good agreement, as illustrated in Table 2 [26].

STEM Morphology Analysis
The morphology of the as-prepared samples was analyzed using the STEM technique and the results are displayed in Figure 7. The STEM analysis confirmed the successful formation of nanoparticles. The fine doping of the catalyst at a ratio of x = 0.30 appeared as a dark area in the images. A rough spherical morphology of the particles was observed in STEM images [27]. grain sizes of CrxZnS were estimated to be 18.30, 17.89, and 17.49 nm, corresponding the Cr0.20ZnS, Cr0.25ZnS, and Cr0.30ZnS, respectively. Since the band gap and the grain s are inversely related to each other; therefore, our measured band gap and crystallite s are in good agreement, as illustrated in Table 2 [26].

STEM Morphology Analysis
The morphology of the as-prepared samples was analyzed using the STEM techniq and the results are displayed in Figure 7. The STEM analysis confirmed the success formation of nanoparticles. The fine doping of the catalyst at a ratio of x = 0.30 appear as a dark area in the images. A rough spherical morphology of the particles was observ in STEM images [27]. The statistical distribution of CrxZnS (x = 0.20, 0.25, and 0.30) is expressed within range of 1-10 nm [28]. Figure 8 shows the distribution of particle sizes measured from STEM images. The average particle size of Cr0.20ZnS, Cr0.25ZnS, and Cr0.30ZnS was me ured at about 82 nm, 79 nm, and 76 nm, respectively. The statistical distribution of Cr x ZnS (x = 0.20, 0.25, and 0.30) is expressed within the range of 1-10 nm [28]. Figure 8 shows the distribution of particle sizes measured from the STEM images. The average particle size of Cr 0.20 ZnS, Cr 0.25 ZnS, and Cr 0.30 ZnS was measured at about 82 nm, 79 nm, and 76 nm, respectively.

Photoluminescence Analysis
The catalysts were further characterized with PL technique to determine the ext of the photoinduced electron-hole recombination rate. Principally, high-PL-emission tensity represents the rapid recombination of charge carriers and vice versa [29]. Figu shows the PL emission spectra of CrxZnS catalyst samples measured at room temperat and an exciton wavelength of 325 nm. The Cr doping has successfully altered the surf of the ZnS and promoted the migration of surface carriers, causing an increment in lig harvesting, which is consistent with the UV-Vis results [30]. The CrxZnS (x = 0.20) cata demonstrated the lowest emission intensity compared to the other two catalysts, ident ing its effective suppression of charge carriers. It is worth mentioning that the PL inten was reduced with Cr doping in the UV and visible zone because of the effective role Cr 3+ ions in trapping the electrons to prolong their recombination with holes [31]. Ad tionally, Cr 3+ dopants provide electrons reaching the surface of the ZnS to effectively tiate the reaction to accelerate the photocatalytic process [32]. Hence, it is concluded t the PL intensity is reduced owing to a strongly inhibited recombination of photoindu charge carriers because Cr 3+ captured the electrons. The CrxZnS (x=0.20) catalyst dem

Photoluminescence Analysis
The catalysts were further characterized with PL technique to determine the extent of the photoinduced electron-hole recombination rate. Principally, high-PL-emission intensity represents the rapid recombination of charge carriers and vice versa [29]. Figure 9 shows the PL emission spectra of Cr x ZnS catalyst samples measured at room temperature and an exciton wavelength of 325 nm. The Cr doping has successfully altered the surface of the ZnS and promoted the migration of surface carriers, causing an increment in lightharvesting, which is consistent with the UV-Vis results [30]. The Cr x ZnS (x = 0.20) catalyst demonstrated the lowest emission intensity compared to the other two catalysts, identifying its effective suppression of charge carriers. It is worth mentioning that the PL intensity was reduced with Cr doping in the UV and visible zone because of the effective role of Cr 3+ ions in trapping the electrons to prolong their recombination with holes [31]. Additionally, Cr 3+ dopants provide electrons reaching the surface of the ZnS to effectively initiate the reaction to accelerate the photocatalytic process [32]. Hence, it is concluded that the PL intensity is reduced owing to a strongly inhibited recombination of photoinduced charge carriers because Cr 3+ captured the electrons. The Cr x ZnS (x = 0.20) catalyst demonstrated the least intensity; therefore, it is more appropriate for hydrogen production.

Hydrogen Evolution Activity
The catalytic performance of the CrxZnS catalyst samples was evaluated for hyd gen production under non-thermal plasma treatment. The catalytic performance of doped ZnS and Al2O3 as a support material was also presented. The decomposition of over the tested catalyst compositions in a single-layered DBD plasma environment is ported in Figure 10. In the case of Al2O3 support, both discharge diffusion and plas produced reactive species may be influenced. The residence time of these species may extended by the adsorption capacity of the Al2O3 support [33]; however, in the literat the electric field was enhanced by using porous materials. Both the discharge and p longed residence time are useful for H2S decomposition [34]. More micro-discharges curred in the Al2O3-filled gap, which led to the beginning of chemical processes involv H2S molecules, radicals, and electrons. All prepared CrxZnS catalysts showed better p formance of H2S conversion than that of pure ZnS and Al2O3 support. The CrxZnS cata with a molar ratio of x = 0.20 showed the highest decomposition of H2S.

Hydrogen Evolution Activity
The catalytic performance of the Cr x ZnS catalyst samples was evaluated for hydrogen production under non-thermal plasma treatment. The catalytic performance of un-doped ZnS and Al 2 O 3 as a support material was also presented. The decomposition of H 2 S over the tested catalyst compositions in a single-layered DBD plasma environment is reported in Figure 10. In the case of Al 2 O 3 support, both discharge diffusion and plasma-produced reactive species may be influenced. The residence time of these species may be extended by the adsorption capacity of the Al 2 O 3 support [33]; however, in the literature, the electric field was enhanced by using porous materials. Both the discharge and prolonged residence time are useful for H 2 S decomposition [34]. More micro-discharges occurred in the Al 2 O 3filled gap, which led to the beginning of chemical processes involving H 2 S molecules, radicals, and electrons. All prepared Cr x ZnS catalysts showed better performance of H 2 S conversion than that of pure ZnS and Al 2 O 3 support. The Cr x ZnS catalyst with a molar ratio of x = 0.20 showed the highest decomposition of H 2 S.
The results after comparison revealed that H 2 S conversion varied for different Cr/Zn molar ratios. The catalytic activity greatly depends upon the dopant concentration. The H 2 S conversion levels significantly impact the energy needed to break down its molecules [35]. The Cr 0.20 ZnS catalyst outperformed the other tested catalysts in terms of catalytic performance and fully converted H 2 S at significantly lower energies. The H 2 S decomposition was 100%, 96%, and 90% when the gap was filled with Cr 0.20 ZnS, Cr 0.25 ZnS, and Cr 0.30 ZnS, respectively. The characterization of the catalyst showed that physical and chemical properties changed with Cr/Zn molar ratio. The cubic sphalerite structure of the catalyst was shown by XRD analysis [36,37]. Cr 3+ ions of chromium revealed uniformly scattering over the ZnS without introducing separated impurity phases. The results after comparison revealed that H2S conversion varied for different Cr molar ratios. The catalytic activity greatly depends upon the dopant concentration. H2S conversion levels significantly impact the energy needed to break down its molecu [35]. The Cr0.20ZnS catalyst outperformed the other tested catalysts in terms of catal performance and fully converted H2S at significantly lower energies. The H2S decomp tion was 100%, 96%, and 90% when the gap was filled with Cr0.20ZnS, Cr0.25ZnS, Cr0.30ZnS, respectively. The characterization of the catalyst showed that physical chemical properties changed with Cr/Zn molar ratio. The cubic sphalerite structure of catalyst was shown by XRD analysis [36,37]. Cr 3+ ions of chromium revealed uniform scattering over the ZnS without introducing separated impurity phases.
The H2S conversion with specific input energy varies for different H2S concentrati over the Cr0.20ZnS catalyst. The conversion rate was higher at the lower H2S concen tions. H2S decomposition increased with increasing the specific input energy. Chivers Lau [36] showed similar results for the H2S conversion under non-thermal plasma con tions. When a large number of electrons collide with Ar balance gas at lower H2S conc trations, air balance gas is also crucial to the breakdown. Cr0.20ZnS was selected to evalu the stability of the catalytic after 100% decomposition of H2S [38]. The long-term H2S c version reaction of the Cr0.20ZnS catalyst is shown in Figure 11. The H2 evaluation sho a maximum value up to 15 h and thereafter starts to decrease over time. Three differ readings were noted at different time periods. The H2 evolution decreased from 100% 94% over the Cr0.20ZnS after 22 h of reaction time. A decrease in H2 production over t might be due to the deactivation of the catalyst. The H 2 S conversion with specific input energy varies for different H 2 S concentrations over the Cr 0.20 ZnS catalyst. The conversion rate was higher at the lower H 2 S concentrations. H 2 S decomposition increased with increasing the specific input energy. Chivers and Lau [36] showed similar results for the H 2 S conversion under non-thermal plasma conditions. When a large number of electrons collide with Ar balance gas at lower H 2 S concentrations, air balance gas is also crucial to the breakdown. Cr 0.20 ZnS was selected to evaluate the stability of the catalytic after 100% decomposition of H 2 S [38]. The long-term H 2 S conversion reaction of the Cr 0.20 ZnS catalyst is shown in Figure 11. The H 2 evaluation shows a maximum value up to 15 h and thereafter starts to decrease over time. Three different readings were noted at different time periods. The H 2 evolution decreased from 100% to 94% over the Cr 0.20 ZnS after 22 h of reaction time. A decrease in H 2 production over time might be due to the deactivation of the catalyst. Table 3 summarizes the findings of hydrogen production efficiency over the Cr x ZnS catalyst samples. The H 2 production during the conversion of H 2 S was 100%, 96%, and 90% for x = 0.20, 0.25, and 0.30, respectively. Different energy conversion was observed with the same SIE (specific input energy) values for all catalysts.  Figure 11. H2 production of the Cr0.20ZnS catalyst with time. Table 3 summarizes the findings of hydrogen production efficiency over the CrxZnS catalyst samples. The H2 production during the conversion of H2S was 100%, 96%, and 90% for x = 0.20, 0.25, and 0.30, respectively. Different energy conversion was observed with the same SIE (specific input energy) values for all catalysts.

Conclusions
This laboratory-built non-thermal plasma system with a vertical DBD column was used to decompose H2S over the CrxZnS catalyst for the production of hydrogen gas. The catalyst was prepared using the co-impregnation method. A FTIR spectrum showed the materials' absorbance in different regions (fingerprint and group frequency region) and functional groups. X-ray diffraction displayed the surface morphology of the catalyst. The values of intensity, millar indices, grain size, and dspacing were decreased with increasing the Cr concentration. Hydrogen evolution was maximized (100%) after 15 h of reaction over the Cr0.20ZnS. Hydrogen evolution then decreased to 94% after 22 h of reaction time, showing a decrease in catalytic activity over time. The Cr0.20ZnS, Cr0.25ZnS, and Cr0.30ZnS catalysts showed 100%, 96%, and 90% conversion, respectively, after 15 h of processing time. The earlier reported works are time-consuming and energy-intensive compared to our work. This study produced reasonably good results in relatively shorter periods. The Cr0.20ZnS showed 100% conversion of H2S within 15 h of the process.