Effects of CaO addition into CuO/ZnO/Al 2 O 3 catalyst on hydrogen production through water gas shift reaction

. Hydrogen is a promising renewable energy carrier and eco-friendly alternative to fossil fuels. Water-gas-shift reaction (WGSR) is commonly used to generate hydrogen from renewable biomass feedstocks. Enriching hydrogen content in synthesis gas (syngas) production can be made possible by applying the WGSR after gasification. WGSR can achieve a maximal carbon monoxide (CO) conversion using a commercially patented CZA (Cu/ZnO/Al 2 O 3 ) catalyst. This study proposed three in-lab self-synthesized CZA catalysts to be evaluated and compared with the patented catalyst performance-wise. The three catalysts were prepared with co-precipitation of different Cu:Zn:Al molar ratios: CZA-431 (4:3:1), CZA-531 (5:3:1) and CZA-631 (6:3:1). The catalysts characteristics were determined through X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis and Scanning Electron Microscopy (SEM) techniques. CO gas was mixed with steam in a catalytic reactor with a 3:1 molar ratio, running continuously through the catalyst at 250 °C for 30 mins. All three catalysts, however, performed below expectations, where CZA-431 had a CO conversion of 77.44%, CZA-531 48.75%, and CZA-631 71.67%. CaO, as a co-catalyst, improved the performance by stabilizing the gas composition faster. The CO conversion of each catalyst also improved: CZA-431 improved its CO conversion to 97.39%, CZA-531 to 96.71%, and CZA-631 to 95.41%. The downward trend of the CO conversion was deemed to be caused by copper content found in CZA-531 and CZA-631 but not in CZA-431, which tended to form a Cu-Zn metal complex, weakening the catalyst's activity.


Introduction
The Indonesia government is promoting using biomass for energy to support clean energy transition program, particularly achieving Indonesia's renewable energy targets of 23% by 2025 (RI 2014).In the quest for achieving such targets and seeking sustainable energy solutions to address the issues of greenhouse gas emissions and energy security, hydrogen has emerged as a suitable secondary energy source and promising clean energy carrier due to its high energy efficiency, high calorific value, and environmentally friendly nature, with the potential to revolutionize a wide range of sectors, including transportation, industry, and power generation (Aziz, Darmawan, & Juangsa 2021;Veziroğlu & Şahi˙n 2008).Emissions emitted by hydrogen are lower compared to other commercial or drop-in fuels (Speight 2019;Watanabe et al. 2022).H2 can be utilized for various purposes, from energy storage utilities to prospective transport fuel (Nagar et al. 2023;Saeidi et al. 2017).It has also been commonly used in refineries as essential feedstock (Al-Baghdadi, Ahmed, & Ghyadh 2023; Damayanti, Sarto, & Sediawan 2020;Ratnawati et al. 2022).
Unfortunately, the availability of H2 is not particularly natural for energy use.Most of the H2 in the world can only be created or produced synthetically (Shuai et al. 2015;Stiegel & Ramezan 2006).Nowadays, steam reforming is capitalizing 95% of the captive market of synthetic H2 production, despite the nonsustainable characteristic of the process (Pal et al. 2018;Sahrin et al. 2022), followed by water splitting via electrolysis (4%) (Arregi et al. 2018;Pal et al. 2018).Nevertheless, steam reforming utilizes fossil and carbonaceous materials up to 96% as feedstock, predominantly natural gas (48%) and oil (30%), which renders the process unsustainable (Arregi et al. 2018).Despite the clean characteristics of hydrogen, the unsustainable characteristics of its production process will impair the initial idea of hydrogen application to reduce carbon emissions.As the world transitions towards a low-carbon future and the need to develop clean and sustainable energy sources increases significantly, hydrogen production from renewable sources has gained increasing attention worldwide due to its ability to decarbonize energy systems, reduce greenhouse gas emissions, and minimize dependence on fossil fuels (Yana et al. 2022).Consequently, this condition urges the utilization of renewable and more environmentally friendly feedstock in hydrogen production.
Among the various pathways for hydrogen production, biomass stands out as a promising, abundant, versatile, and widely available resource.Biomass is promising as a renewable energy source that may be easily obtained in Indonesia from agricultural, plantation, forest areas, and industrial waste such as palm oil, sugarcane, tapioca, pulp and paper, wood, and rice (Pambudi et al. 2023;Yana et al. 2022).Biomass is considered a neutral carbon source since photosynthesis captures carbon from CO2 in the air (He et al. 2023).It also can be renewed in the short term, giving it considerable potential to be utilized as feedstock in hydrogen production, which can later be notated as biohydrogen.The International Energy Agency (IEA) also underlines the considerable potential of biohydrogen in decreasing greenhouse gas emissions and encouraging growth in hydrogen production from renewable sources.
Biohydrogen production based on biomass feedstock is mainly conducted via thermochemical and biological routes (Kalinci, Hepbasli, & Dincer 2009), which later is considered a time-consuming and inefficient process (Chen et al. 2023;Ghodke et al. 2023;Peng et al. 2017).The main thermochemical methods for the conversion of biomass into hydrogen-rich gas are gasification, pyrolysis, and aqueous phase reforming (Duman & Yanik 2017;Heidenreich & Foscolo 2015;Huber, Iborra, & Corma 2006;Lepage et al. 2021;Muharto et al. 2023;Tian et al. 2024).Biohydrogen-based microalgae, involving biophotolysis, is also starting to develop (Sahrin et al. 2022;Wang et al. 2021).The cheapest method for producing H2 (2 US$/kg) is the thermochemical method of gasification, followed by dark fermentation in the biological method (2.3 US$/kg) (Ghodke et al. 2023).Compared to chemical and biochemical methods, the thermochemical method offers a more straightforward approach, high efficiency, and low-cost production to hydrogen generation (Lanjekar, Panwar, & Agrawal 2023).It typically does not necessitate the addition of chemical agents, and it can also convert a diverse range of wet biomass and use the feedstock obtained from biomass (Dou et al. 2019).Thus, the thermochemical process has been considered the most promising technology for hydrogen production from biomass (Agaton, Batac, & Reyes Jr 2022;Dou et al. 2019;Tleubergenova, Han, & Meng 2024;Younas et al. 2022).
The biomass gasification process, however, is still grappling with its technical challenges.Syngas products generally have a low hydrogen-to-carbon monoxide ratio (H2/CO< 1), indicating low H2 concentration due to changing chemical reactions based on operating conditions.Higher CO in the gas product will be the consequence of the biomass's abundance of carbonyl groups, which are simpler to bond-scission.(He et al. 2023).This situation seriously limits the large-scale applications of biomassto-hydrogen technology.Employing steam instead of air as a gasifying agent can bolster the syngas production, enriched with H2 (Acharya, Dutta, & Basu 2010), enhancing the syngas calorific value.Steam will accelerate reforming reactions and generate gasification products with the highest H2/CO ratio and the fewest impurities like CH4, H2S, and NH3 (Havilah et al. 2022;Sansaniwal et al. 2017).Without steam, the gasification product will be dominated by CH4 and CO2 (Watson et al. 2018).Steam gasification provides hydrogen's most significant stoichiometric yield, nearly three times that of air gasification.The generation of H2 and CO2 is greatly enhanced when the steam-to-biomass ratio (S/B) rises from 0 to 10.The yields of CO and CH4 are almost constant at the same period.At S/B 7.5, the H2/CO ratio reaches its maximum of 0.96, which is three times higher than the ratio in anhydrous conditions of 0.34.As S/B increases further to 10 levels, the H2/CO ratio decreases.The increased steam levels enhanced the decomposition of tar, while simultaneously reducing the concentration of hydrocarbons in the syngas, particularly C3-C4 and unsaturated C2 (Martínez et al. 2022;Martínez et al. 2020).Steam also promotes water gas shift reaction (WGSR) to occur more intensively in the biomass gasification process, shifting water vapour (H2O) to H2 and carbon dioxide (CO2), harnessing the existence of CO (Younas et al. 2022).
Even though steam gasification gives better performance in producing a higher H2/CO ratio in gas products, further hydrogen concentration is needed.Adding steam to syngas in a downstream catalytic reactor after gasification can also be a method to convert CO gas to H2 and CO2, hence increasing the H2/CO ratio (Ostadi, Rytter, & Hillestad 2019).Comparatively, the gasification process combined with a following WGSR reaction, as shown in Figure 1, is more adaptable for large-scale applications due to its capability to continually convert feedstocks into high-value hydrogen without limiting the composition of biomass (He et al. 2023).
Extensive research has been conducted on WGSR to increase biohydrogen from renewable energy sources like solid waste and biomass via gasification (Ahn et al. 2020;Shen et al. 2023;Zhou et al. 2023).WGSR has been an essential intermediate reaction in industrial applications, especially in H2/CO ratio adjustment of the gaseous product from methane steam reforming by removing CO and producing hydrogen (Shen et al. 2023).Simultaneously completing the removal of CO and H2 production is a significant benefit of the WGSR process (Zhou et al. 2023).The exothermic equilibrium in WGSR depends highly on factors like operating temperature, feed composition, and the presence of a catalyst, as described by Equation (1) (Poggio-Fraccari et al. 2022).Thermodynamically, WGSR is more efficient at lower temperatures, but kinetically, it tends to prefer higher temperatures with different catalyst types (Dasireddy et al. 2020;Shen et al. 2023).
Catalysts play a vital role in achieving maximum CO conversion and efficiency of the process (Dawood, Anda, & Shafiullah 2020).Significant progress has been made in improving the WGSR process, specifically on the catalyst type.Novel catalysts utilizing nanoparticles on a supporting medium have been extensively explored.Various catalyst candidates, such as iron, copper, cobalt, gold, platinum, and rare earth metals (e.g., cerium, samarium, gadolinium, and lanthanum), were investigated and tested extensively for their potential as WGSR catalysts (Gradisher, Dutcher, & Fan 2015).Current research is focussing more on the catalyst's support and solid additives.Catalysts must be mixed or supported with other substances to prevent or slow down sulfur poisoning.WGSR is sensitive to sulfur poisoning that might cause catalyst deactivation (Baraj, Ciahotný, & Hlinčík 2021).WGSR is typically classified as (1) high-temperature shift (HTS) at 300-350 °C using an iron-based catalyst which focuses on accelerating the reaction, and (2) low-temperature shift (LTS) at 200-250°C using a copper-based catalyst which focuses to remove CO gas to less than one ppm (Gogate 2020;Poggio-Fraccari et al. 2022;Shen et al. 2023).The LTS-WGSR is more challenging to perform without a suitable catalyst.The LTS-WGSR catalyst is typically a combination of CuO, ZnO, and Al2O3.As an active site, copper has high selectivity and activity performance, which makes it most appropriate for WGSR at low CO concentrations and temperatures (Baraj, Ciahotný, & Hlinčík 2021).ZnO can adequately absorb sulfur from poisoning the Cu crystallites, gives solid structural support for the catalyst (Fajín & Cordeiro 2021;Pal et al. 2018), and increases Cu dispersion.The close interactivity of Cu and ZnO will strain the copper lattice with higher catalytic activity than bulk-structured copper (Günter et al. 2001).Meanwhile, Al2O3 makes dispersion easier and minimizes pellet shrinkage (Loganathan, & Shantha 2010).Zinc aluminate, formed when zinc and aluminium ions (Al 3+ ) react, stabilizes the highly dispersed Cu/ZnO crystallites and prevents Cu agglomeration.During the reduction and reaction stages, Cu's surface enrichment and stabilization are accelerated (Baltes, Vukojević, & Schüth 2008;Chen et al. 1999).The high activity of the CZA catalyst is highly attributed to metallic Cu and ZnO's synergistic interaction (Na et al. 2019).The catalyst activity is mainly affected by the Cu/Zn/Al ratio (Lucarelli et al. 2018).High Cu concentrations can further intensify the occurrence of WGSR (Gunawardana, Lee, & Kim 2009).
The relatively affordable copper-based catalysts can maximize CO gas conversion even in low temperatures (Dasireddy et al. 2020;Fajín & Cordeiro 2021).Although metalbased catalysts have performed excellently in LTS-WGSR (Jang et al. 2019;Zhu et al. 2019;Zhu & Wachs 2016), the high price and the tendency of active components to aggregate hinder the application in the industry (Qin et al. 2018;Shen et al. 2023;Yao et al. 2017).Hence, a growing interest is in developing low-cost catalysts or other catalytic techniques for enabling lowtemperature WGSR (Shen et al. 2023;Xiang et al. 2020).
Calcium oxide (CaO) is a known alkaline earth metal catalyst for gasification.CaO acts as a CO2 absorbent and tar reducer via cracking/reforming (Fan et al., 2015;Zhou et al., 2019).CaO breaks the p-electron cloud's stability of condensed aromatic compounds in tar (Yongbin et al., 2004).It promotes the gasification reaction of char and generates more condensable products and less residual char.CaO also acts as a catalyst for WGSR and steam reforming of methane (Guoxin et al., 2009).Its catalyst effect on the WGS reaction was more potent than that on the methane steam reforming (Hu and Hao, 2009).CaO will facilitate the H2O dissociation that restricts the rate of the WGSR process (Figueiredo et al. 1998;Yan et al. 2021).CaO can also capture CO2 before creating the CaCO3 product layer (Živković et al. 2016).By capturing the CO2, the WGSR might be shifted to the product side, increasing the production of H2, especially in syngas produced from the biomass gasification process.
The commercially available CZA catalyst is renowned for performing well in WGSR to maximize the CO conversion to H2.Nonetheless, the role of CaO as support of the catalyst and a cocatalyst has scarcely been explored.Therefore, this research investigated the impact of CaO addition on catalytic WGSR, using CZA as a catalyst in three different molar ratios of Cu:Zn:Al (4:3:1; 5:3:1; and 6:3:1) through the co-precipitation process.The main parameters of interest were the total CO conversion, the average H2 concentration in the gas product, and the time required to reach exothermic equilibrium.A commercial CZA catalyst from Sud-Chemie (CZA-SC) was used for comparison.

Materials
Reagents for synthesized CZA catalysts were zinc nitrate tetrahydrate (Zn(NO3)2.4H2O),copper (II) nitrate trihydrate (Cu(NO3)2.3H2O),aluminium nitrate nonahydrate (Al(NO3)3.9H2O),and sodium carbonate (Na2CO3) anhydrous.All the chemicals above were provided by Merck KGaA Germany and designated for analysis grade under the EMSURE ® label.Merck KGaA Germany also provided calcium oxide (CaO) from marble in small lumps with assay (complexometric) ≥ 97.0%.Commercial CZA catalyst MDC-03 from Sud-Chemie was utilized as the benchmark.In this work, the WGSR utilized CO derived from the syngas produced during biomass gasification to produce hydrogen.Therefore, CO was employed as the gas model for this investigation.A mixture of steam and carbon monoxide gas was supplied to the WGSR with a ratio of 3:1.

Catalyst Preparation
The CZA catalysts were synthesized via co-precipitation method with different compositions.Precise ratios of the Cu(NO3)2, Zn(NO3)3, and Al(NO3)3 solutions were mixed to create the precursor solution and then placed in the burette.100 mL of purified water was filled in a beaker glass and put in a water bath.The precursor solution and 1 M Na2CO3 solution were dripped concurrently into 100 mL of distilled water while maintaining a neutral pH and a temperature of approximately 60°C.The precipitate in the form of carbonate metal salt was left to stand for 120 minutes at 70 °C.It was filtered and rinsed with water until pH six was reached, then dried at 80 °C for 18 hours.Hydroxy-carbonate precursors (hydrotalcite-like crystals) were calcined at 330 °C for 3 hours in 100 mL/min air to produce a CZA catalyst.

Catalyst Characterization
Both commercial and synthesized CZA catalysts involved in the experiments were characterized.The crystal-oxide formation in the catalyst was examined through Energy Dispersive X-ray diffraction Rigaku-Int 2000 X-ray generator with CuKα radiation source (Rigaku Corp., Japan), while the catalyst composition was characterized using the Shimadzu EDX 7000 Energy Dispersive X-ray Fluorescence Spectrometer (Shimadzu Scientific Instruments, Japan) with the high-performance silicon drive detector and a high level of sensitivity.The highperformance BET analyzer model of NOVA touch 2LX (Quantachrome Instruments, USA) was employed to characterize the surface area and pore volume.The catalyst structure was analyzed using SEM Jeol JSM-IT200 (JEOL GmbH, Germany) with high-resolution imaging in HV/LV/SE/BSE and 5x-300,000x magnifications.

WGSR Experiment
The catalytic activity of the WGSR process was performed using a micro-activity flow reactor system.It has a 9 mm internal diameter, a 14.5 mm external diameter, and a 305 mm length and was made of SUS-316L stainless steel.Furthermore, it is equipped with measured and controlled parameters, including steam and gas flow rate, reactor temperature, and pressure.The system ensured the absence of gas leaks.A schematic representation of the experimental apparatus is illustrated in Figure 2.
The WGSR process in a micro-activity flow reactor system consists of several key steps: preheating, reduction, preheating 2, reaction, nitrogen flush, and cooling.Each run required 2.0 g of catalyst inserted into the tubular reactor.The reactor was progressively preheated to 200 °C with a heating rate of 10 °C per minute.Traces of trapped air were removed by flowing nitrogen at 150 mL/min for 10 minutes.The catalyst was reduced for 60 minutes at 200 °C with hydrogen gas at 30 mL/min.Afterward, the reactor was purged with 150 mL/min of nitrogen to remove the residual hydrogen.The reactor was further heated to 250 °C before the start of the reaction to remove the residual hydrogen.CO gas with a 200 mL/min flow rate and steam (H2O) was then introduced into the reactor with a steam-to-CO ratio of 3:1.
The reaction was carried out at 250 °C for 30 minutes.The temperature was maintained, and the flow rate ratio was continually and steadily controlled.The gas product composition was measured using a Gasboard 3100P gas analyzer.The CO conversion was determined using the formula illustrated in Equation (2) (Li et al. 2014). (2) XCO represented the conversion of carbon monoxide.The H2 and CO concentrations in the product gas were represented by CH2 and CCO, respectively.

Catalyst Characteristics
presents the metal composition of the commercial and synthesized catalysts based on the EDX result.CZA-431, CZA-531, and CZA-631 had a Cu percentage of 49.44%, 55.84%, and 62.18%, respectively.The Cu content was proportional to the molarity of the Cu solution in the catalyst precursors preparation step, which were 4 M, 5 M, and 6 M for CZA-431, CZA-531, and CZA-631, respectively.CZA-431 had the closest Cu:Zn:Al composition (49.44%: 41.74%: 8.82%) to the CZA-SC metal composition (41.61%: 49.01%: 9.38%).The impact of the metal composition on catalyst performance will be discussed in the discussion on catalyst performance results.
CuO, ZnO, and Al2O3 metal oxides are vital in CZA catalyst activity.Since the EDX result alone could not present the existence of the metal oxides mentioned above, XRD analysis was conducted to find the oxide phase of each metal observed in the EDX result.The XRD patterns of the CZA catalysts are displayed in Fig. .The CuO and ZnO oxide phases exhibited characteristic peak patterns in the 30° to 70° range.The intensity of the reflections associated with Cu becomes more prominent as the Cu content increases.However, the intensity of CuO and ZnO peaks in synthesized catalysts was still below the CZA-SC catalyst, suggesting that the amount of CuO and ZnO crystals in synthesized catalysts was fewer than in commercial ones.The typical CuO reflections were rather broad in all catalysts, indicating an unclear crystalline phase and likely overlapping with broad ZnO reflections.These broad reflections also suggest that the crystal size of both ZnO and CuO was relatively small (Ereña et al. 2003).Notably, no peaks associated with alumina were observed, indicating the amorphous nature of this low-concentration promoter.Along with CuO and ZnO phases, other peaks were observed in the 10-11° and 23-24° range.Those peaks were very intense in the CZA-SC, followed by CZA-431, which showed the highest peaks among the synthesized CZA.Those peaks corresponded to the hydrotalcite analog compound (CuxZn(6-x)Al2(OH)16CO3•4H2O) (Kowalik et al. 2019), which were identified as residual hydroxy carbonates existed in the CZA catalyst.This residual hydroxycarbonate was believed to enhance the catalyst activity due to its ability to form highly active sites (Baltes, Vukojević, & Schüth 2008) and ensuing production of copper suboxide species (Plyasova et al. 1995).The existence of residual carbonates likely delayed the catalyst structure's fragmentation before reducing the copper species and forming more petite, more interactive particles (Baltes, Vukojević, & Schüth 2008).
The spent catalysts of the synthesized CZA catalyst were also analysed, and the results were displayed in the diffractogram pattern as CZA-431-s, CZA-531-s, and CZA-631s.Several new peaks appeared in the 40-45° and 50° range for both CZA-531-s and CZA-631-s catalysts.These peaks were related to Cu-Zn complex metal (Cu0.951Zn0.049).Since this complex metal is only found in the catalyst with a higher content of Cu, it might correlate with the closer distance between Cu particles that promoted the metal complex formation with the Zn nearby during the reaction.The narrow reflections of the peaks indicated the big crystal size of this metal complex.Along with the formation of the Cu-Zn metal complex, it can be observed that ZnO peaks get narrower reflections compared to the fresh catalyst, which could indicate the growth of ZnO crystal, which further will affect the catalyst activity.
Surface characteristics, including surface area and pore volume, were crucial for the catalyst since the active site needed for the reaction was distributed on the surface area.The higher surface area will theoretically result in higher catalytic activity (Bernard et al. 2021).The surface characteristics of the CZA catalysts are presented in Table 2.The CZA-431 catalyst had the highest pore volume and surface area, followed by CZA-531 and CZA-631 catalysts.
Both pore volume and surface area decreased along with increased Cu content, which may correlate with decreased Al content.The specific surface area in the catalyst was known to be strongly affected by Al2O3 contents, which act as textural promoters or spacers between Cu active sites (Ahn et al. 2020).The higher the Al content in the catalyst, the wider the distance between Cu particles.Therefore, the Cu/Al ratio will inversely relate to the specific surface area.In this study, CZA-431, CZA-531, and CZA-631 have a Cu/Al ratio of 5.61, 7.91, and 11.71, respectively.CZA-431 had the lowest Cu/Al ratio of 5.61, giving the highest specific surface area of 76.88 m 2 /g.However, CZA-SC had a lower specific surface area than the synthesized catalysts, even though it had a lower Cu/Al ratio than the CZA-431 catalyst.It may correlate with the hydroxycarbonate content in the CZA-SC catalyst.The narrow reflections of the hydroxycarbonate peaks indicate this compound's big crystal size, which may reduce the specific surface area of the CZA-SC catalyst.Cu concentration affects the catalyst's surface area as well.A high Cu content prevents the opening of pores, resulting in a decrease in the catalyst's surface area.This finding is consistent with the report by Lucarelli et al. (2018).
The structure of the catalyst is also essential to be discovered as it displays the catalyst particle's form, condition, and dispersion.The structure of the catalyst can be characterized using SEM. Figure 4    Furthermore, the presence of Zn content acted as an inhibitor of Cu aggregation, reducing the size of Cu particles (Ye et al. 2023).The CZA-631 catalyst has the most excellent Cu/Al ratio of 5.61, indicating that the particle spacing in the catalyst is relatively close.The particles even stick together, forming some aggregates, as seen in Figure 4(d).This is due to the low Al and Zn concentration in the catalyst.The variance in catalyst particle size and dispersion may be attributed to differences in the catalyst's production and preparation processes.The preparation methods impact the physicochemical characteristics of catalysts, including active metal dispersion, specific surface area, reducibility, and crystallite size, which ultimately affect the catalytic performance despite identical loading (Shim et al. 2016).

WGSR on CZA Catalysts
The WGSR catalyst's performance can be determined by testing it in the reaction and comparing the influence of the catalyst in the reaction by plotting the gas concentration during the reaction.WGSR can be represented as Equation ( 1), where H2 production is expected, and more H2 production is preferred.
Figure 5 shows the CO, CO2, and H2 gas concentrations during the WGSR reaction.The hydrogen, CO2, and CO gas concentrations produced in the reaction utilizing the CZA-SC catalyst were relatively stable, and the stable condition was obtained 3 minutes after the reaction began, see Figure 5(a).This condition is also proper for reactions using CZA-431 and CZA-531 catalysts, as illustrated in Figure 5 (b) and 5(c).However, the gas concentration in the reaction using the CZA-631 catalyst still fluctuated until 10 minutes after the reaction, as shown in Figure 5(d).After 10 minutes of reaction, the concentration of gas produced remained stable until 27 minutes, then decreased until the end of the reaction.This fluctuation of the gas produced can be related to the Cu content in the catalyst.The Cu content of the CZA-631 catalyst was 62.18%, the highest among the other synthesized catalysts.Hypothetically, the increase in Cu content should increase the catalyst activity as it functions as the active site in the catalysts.A higher Cu content of up to 8% can increase the catalyst surface area, resulting in higher activity (Gradisher, Dutcher, & Fan 2015).The quantity of Cu° active sites is related to H2 generation.The formation of 1 mol H2 indicated that the 2 Cu° active site is oxidized to be Cu + (Taniya et al. 2023).However, in this research, the results suggest that the difference in Cu concentration in CZA-431 and CZA-631 catalysts resulted in almost the same concentration of H2 production, and on CZA-531, the H2 production was even lower.
The average H2 gas concentration obtained from the reaction using the CZA-SC catalyst was 53%, as shown in Table 3.While on the reactions using CZA-431, 531, and 631 catalysts, the average H2 gas production was lower, i.e., 44.06%, 31.59%, and 43.86%, respectively.Considering CO conversion, the highest hydrogen content was obtained from the CZA-SC catalyst reaction, reaching 99.28%.In contrast, for CZA-431, CZA-531, and CZA-631, the CO conversion reached 77.44%, 48.75 %, and 71.67% respectively.In this study, the CO conversion decreases when the Cu content increases.The decrease in CO conversion on CZA-531 and CZA-631 catalysts is due to more significant amounts of Cu that tend to form Cu-Zn complex metal (Cu0.951Zn0.049)as found in the XRD photo shown in Fig. .This complex metal affects reaction stability, as seen in CZA-531 and CZA-631.The conversion is also affected by the Zn content, which in this case is determined by the ratio of Cu/Zn (Reddy & Smirniotis 2015).The Cu/Zn ratios for 1.18,1.51,and 1.91, respectively.CZA-431, which has a Cu/Zn ratio of 1.18 and is similar in Cu/Zn ratio to CZA-SC at 0.85, resulted in higher conversion rates than CZA-531 and CZA-631. Meza Fuentes et al. (2021) reported that a Cu/Zn ratio of 1M was more efficient than materials with higher copper content.
Meanwhile, the hydrogen yield obtained from CZA-631 is similar to CZA-431 with unstable product distribution.Conversely, the hydrogen content yield and conversion of CZA-531 was low due to hydroxycarbonate in the CZA-431 and CZA-631 synthesized catalysts.However, this hydroxycarbonate was not found in the CZA-531, as discussed in the catalyst characterization chapter.Meanwhile, Cu-Zn complex metal (Cu0.951Zn0.049) was not found in CZA-431 and CZA-SC catalysts.This complex compound can contribute to reducing catalyst activity.Moreover, the Al content as a structural promoter cannot compensate for the high Cu content, resulting in the low dispersion of the catalyst particles (Zhang et al. 2018).

WGSR on CZA-CaO catalysts
Based on the catalytic testing data for the WGSR, the synthesized catalysts needed improvements to match the performance of commercial catalysts.The catalyst could be refined by changing the support or structure of the catalyst and adding some promoters to the catalyst (Desgagnés, Alizadeh Sahraei, & Iliuta 2023;García-Moncada et al. 2022).Adding promoters improves reaction activity while increasing the catalyst-specific surface area inhibits precipitation on the modified catalyst's surface and improves performance (Song et al. 2016).The improvement done in this research was the use of CaO to increase the hydrogen gas content.
Figure 6 shows the gas product concentration of the WGSR process using the CZA catalysts with the addition of CaO.With the addition of CaO, each synthesized catalyst improved its hydrogen production performance to 52.03%, 51.15%, and 46.46%, respectively, as illustrated in Figure 6 6(c), which can be attributed to the role of CaO.Adding CaO reduces metal particle size, enhances the anti-sintering properties of the active metal, and confirms the stability of the catalyst (Mo et al. 2019).Moreover, the CaO confinement effect would give additional active sites and improve CO2 adsorption (Sengupta & Deo 2015).It explains why the CZA catalyst performs better when CaO is added to the WGSR process.
CaO addition in CZA-531 exhibits tremendous effects regarding hydrogen gas concentration and CO conversion.The hydrogen gas concentration improved from 31.59% to 51.15% (61.93%), and CO conversion jumped from 48.75% to 93.78% (92.37%), as depicted in Table 4. CO2, H2, and CO gas conversions surged due to CaO addition.Overall, the increase in gas product quality and CO conversion indicates that CaO positively affects the WGSR process, as seen in Table 4.Even though the CaO can be a CO2 absorber to produce CaCO3 according to the chemical reaction in Equation ( 3), as shown in Figure 7, CaCO3 appears with very little intensity.However, this reaction can only occur at temperatures higher than WGSR (250 o C).CaO can also adsorb the CO2 produced in the reaction, allowing the reaction equilibrium to shift toward the products (Günter et al. 2001).Besides, CaO can also react with H2O in the feed to form Ca(OH)2, as shown in the chemical reaction in Equation ( 4) (El Bazi et al. 2022).Li, Liu and Cai (2012) reported that CaO was converted to Ca(OH)2 in 0% of the steam volume at a temperature of 250 °C (Li, Liu, & Cai 2012).CaCO3 emerged in CZA-431, while Ca(OH)2 appeared in all CZA-431, CZA-531 and CZA-631.On the other hand, CaCO3 is also seen in the diffractogram in Figure 7 but with a small intensity.

𝐶𝑎𝑂 + 𝐶𝑂 2 ↔ 𝐶𝑎𝐶𝑂 3
(3) The role of CaO in increasing H2 production can be attributed to dissociating H2O since this is the step that limits the rate of the WGSR process (Figueiredo et al. 1998;Yan et al. 2021).CaO can also adsorb the CO2 produced in the reaction, allowing the reaction equilibrium to shift toward the products (Günter et al. 2001).The existence of CaO can also reduce the metal's particle size by improving the active metal's antisintering properties (Mo et al. 2019;Hadiyanto et al 2016).Furthermore, the CaO confinement effect would give additional active sites and improve CO2 adsorption (Sengupta & Deo 2015).In this study, CaO addition to the CZA catalyst has significantly improved H2 gas production.
Moreover, adding CaO resulted in a more stable reaction process that practically corresponded to the commercial catalyst.The WGSR progresses on the surface of CaO through the redox-a route, wherein H2O decomposes into hydroxyl and atomic hydrogen, followed by hydroxyl dissociation and CO oxidation by atomic oxygen.The CaO surface allows H2O to spontaneously dissociate, which acts as the rate-determining step for the WGSR.The presence of CaO diminishes the energy barrier for the production of CO2 during the WGSR.Essentially, our research involved introducing CaO to the CuZnAl synthesis catalyst.It led to a change favoring higher hydrogen production in the WGSR reaction, improving conversion performance.
Based on the results from this study, the addition of CaO on CZA catalyst in WGSR can also be employed to convert biomass feedstock to produce renewable energy, such as biohydrogen or hydrogen-rich syngas, primarily through the gasification process-WGSR.However, some constraints and challenges must be faced, such as the gas separation process and the high energy required to produce steam.In addition, the CO2 composition of the gas product is still relatively high.Thus, in future studies, a suitable catalyst design, kinetics of the WGSR, optimization of the process parameters, and economical feasibility analysis are needed to overcome these challenges.Tleubergenova, Han and Meng (2024) stated that gasification technology is under TRL (Technology Readiness Level) 7 and will be TRL 9 in the next 20-30 years.Thus, hydrogen production using green technology and renewable energy sources for large-scale industrial production has recently gained worldwide attention.

Conclusion
This research synthesized CZA catalysts using the coprecipitation method at various Cu:Zn:Al molar ratios.Applied to WGSR, the CZA-431 catalyst (4:3:1) generated the highest H2 content in the gas product with a CO conversion of 77.44%.CZA-531 and CZA-631 catalysts had higher Cu concentrations, of which the excess tended to promote the formation of the Cu-Zn metal complex, affecting the catalyst activity.This performance of CZA-431 was still not comparable to that of the commercial catalyst CZA-SC, with a CO conversion of 99.28%.To boost the H2 production, CaO was added to the synthesized CZA catalyst.CaO was known to have surfaces that enabled instantaneous H2O dissociation, which was the controlling step in WGSR.Adding CaO to the CZA-431 catalyst improved the CO conversion to 97.39%.CaO addition to the CZA catalyst effectively increased the H2 gas production.It accelerated the exothermic equilibrium of WGSR, leading to the stability of gas product composition.The ability of the CaO-CZA-431 catalyst  to convert CO to produce H2 is encouraging, and it can be the answer to the challenge in the biohydrogen production process via the thermal route.
This work can be utilized in future research to explore the application of thermochemical conversion processes in biomass gasification technology.The focus would be on using renewable resources to produce hydrogen, which can be used as an energy source.
-4940/© 2024.The Author(s).Published by CBIORE The catalyst particles were found to be in an almost good spherical form for all Cu, Zn, and Al particles, as seen in the SEM images.The wider distance between Cu particles, decreased Cu/Al ratio, or increased Al content was also confirmed.The investigation reveals that CZA-431, CZA-531, and CZA-631 exhibit Cu/Al ratios of 5.61, 7.91, and 11.71, respectively.It indicates a higher concentration and denser distribution of Cu, as depicted in Figure 4(a), 4(b), 4(c), and 4(d).The CZA-431 sample had the lowest Cu/Al ratio of 5.61, indicating a slightly lower Cu density, as shown in Figure 4(b).

Figure
Figure6(c), which can be attributed to the role of CaO.Adding CaO reduces metal particle size, enhances the anti-sintering properties of the active metal, and confirms the stability of the catalyst(Mo et al. 2019).Moreover, the CaO confinement effect would give additional active sites and improve CO2 adsorption(Sengupta & Deo 2015).It explains why the CZA catalyst performs better when CaO is added to the WGSR process.CaO addition in CZA-531 exhibits tremendous effects regarding hydrogen gas concentration and CO conversion.The hydrogen gas concentration improved from 31.59% to 51.15% (61.93%), and CO conversion jumped from 48.75% to 93.78% (92.37%), as depicted in Table4.CO2, H2, and CO gas conversions surged due to CaO addition.Overall, the increase in gas product quality and CO conversion indicates that CaO positively affects the WGSR process, as seen in Table4.Even though the CaO can be a CO2 absorber to produce CaCO3 according to the chemical reaction in Equation (3), as shown in Figure7, CaCO3 appears with very little intensity.However, this reaction can only occur at temperatures higher than WGSR (250 o C).CaO can also adsorb the CO2 produced in the reaction, allowing the reaction equilibrium to shift toward the products(Günter et al. 2001).Besides, CaO can also react with H2O in the feed to form Ca(OH)2, as shown in the chemical reaction in Equation (4)(El Bazi et al. 2022).Li, Liu and Cai (2012) reported that CaO was converted to Ca(OH)2 in 0% of the steam volume at a temperature of 250 °C(Li, Liu, & Cai 2012).CaCO3 emerged in CZA-431, while Ca(OH)2 appeared in all CZA-431, CZA-531 and CZA-631.On the other hand, CaCO3 is also seen in the diffractogram in Figure7but with a small intensity.

Table 4
The gas production improvement under the effect of CaO addition to the CZA catalyst The diffractogram pattern of CaO-CZA Catalyst fresh and spent catalyst (s)