High-Pressure Catalytic Ethanol Reforming for Enhanced Hydrogen Production Using Efficient and Stable Nickel-Based Catalysts

Abstract

The urgent need to address the climate crisis demands a swift transition from fossil fuels to renewable energy. Clean hydrogen, produced through ethanol steam reforming (ESR), offers a viable solution. Traditional ESR operates at atmospheric pressure, requiring costly separation and compression of hydrogen. High-pressure ESR, however, improves hydrogen purification, streamlines processes like pressure swing adsorption, and reduces operational costs while enhancing energy efficiency. High-pressure ESR also enables compact reactor designs, minimizing equipment size and land use by compressing reactants into smaller volumes. This study evaluates two nickel-based commercial catalysts, AR-401 and NGPR-2, under high-pressure ESR conditions. Key parameters, including reaction temperature, steam-to-ethanol ratio, and weight hourly space velocity, were optimized. At 30 bars, 700 °C, and a steam-to-ethanol ratio of 9, both catalysts demonstrated complete ethanol conversion, with hydrogen selectivity of 65–70% and yields of 4–4.5 moles of H₂ per mole of ethanol. Raising the temperature to 850 °C improved hydrogen selectivity to 74% and yielded 5.2 moles of H₂ per mole. High-pressure ESR using renewable ethanol provides a scalable, efficient pathway for hydrogen production, supporting sustainable energy solutions.

1. Introduction

Today, fossil fuels such as coal, natural gas, and petroleum remain the dominant sources of global energy [1]. However, their extensive use comes with serious environmental consequences [2,3]. The extraction and combustion of these fuels release large amounts of greenhouse gases, especially carbon dioxide, contributing significantly to climate change and ecological degradation [4]. In light of these negative impacts, there is an increasing demand for alternative, sustainable energy sources such as solar energy [5,6,7,8,9]. Among these options, hydrogen emerges as a standout renewable energy prospect [10]. It serves as a remarkably efficient fuel and energy carrier, poised to meet future energy demands effectively [11]. The remarkable energy density of hydrogen facilitates its efficient storage and transportation [12]. Hydrogen can be generated through various processes, including different catalytic reforming processes such as partial oxidation reforming, dry reforming and steam reforming [13,14,15,16,17]. Steam reforming, in particular, is appealing as it allows for the extraction of hydrogen from both steam and the hydrocarbon being reformed. Notably, hydrogen sourced from renewable oxygenates like ethanol, methanol, and dimethyl ether holds particular promise for steam reforming [18,19]. Ethanol, which can be produced from biomass, is particularly advantageous as a hydrogen source. Its non-toxic nature and wide availability make it an environmentally friendly and sustainable option [20]. Furthermore, steam reforming of ethanol has been shown to yield more hydrogen compared to other hydrocarbon fuels like such as traditional steam methane reforming [10].

The primary reaction governing ethanol steam reforming (ESR) is represented by Equation (1):

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}+3{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{C}\mathrm{O}}_2+6{\mathrm{H}}_2\left(\mathsf{\Delta }\mathrm{H}°298\right)=+174{\displaystyle \frac{\mathrm{k}\mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}}}$$

(1)

However, ESR entails a complex series of reactions, including ethanol decomposition, ethanol dehydrogenation, ethanol dehydration, methanation, and water-gas shift reaction [21,22,23,24,25]. These processes contribute to a highly intricate side reaction network, leading to the formation of various by-products such as carbon monoxide, methane, ethylene, acetylene, and solid carbon [16,26]. The occurrence of these side reactions depends on the catalysts and reforming conditions employed, thereby influencing hydrogen yields, hydrogen selectivities, and ethanol conversions [26]. Consequently, the choice of catalysts and reaction conditions significantly affects the overall reaction pathway [27,28].

Studies investigating the steam reforming of ethanol have examined various supported metal and metal oxide catalysts [18,19,21,25,27]. Transition metals like nickel, copper, and cobalt have emerged as promising candidates for their favorable activity and selectivity in ESR reactions [26,27,29,30,31]. Nickel, in particular, is often preferred due to its efficacy in breaking carbon-carbon bonds and its relatively low cost [32,33,34]. However, challenges remain with nickel catalysts due to their propensity for coke formation and metal sintering, both of which can diminish catalytic efficiency over prolonged use [10,35,36,37]. To address these challenges, significant endeavor has been invested in combining nickel catalysts with suitable support materials. This endeavor greatly enhances the stability, activity and selectivity of supported nickel catalysts utilized in the steam reforming of ethanol [38]. γ-Al₂O₃ is commonly used as a support material in steam reforming due to its naturally high surface area and excellent thermal stability [39,40,41]. Nonetheless, γ-Al₂O₃ exhibits acidic properties, encompassing both Lewis and Brønsted acidity [42,43]. This acidity has been found to promote ethanol dehydration, leading to the formation of ethylene—an intermediate that facilitates carbon deposition and ultimately contributes to catalyst deactivation [21,43,44,45]. To combat catalyst deactivation from carbon buildup, alternative supports with basic or neutral characteristics, like MgO, ZrO₂, La₂O₃, and CeO₂, have been utilized [44,46,47]. Each of these supports influences product distribution, hydrogen selectivity, and ethanol conversion, and some can notably diminish the activity of ESR. To mitigate these issues, mixed acidic and basic supports have been employed [48]. An example is magnesium-aluminum mixed oxide supports, which yield a highly dispersed and nearly uniform MgAl₂O₄ spinel with minimal phase separation, forming regions of pure Al₂O₃ or MgO as well [49]. Nickel catalysts supported on MgAl₂O₄ have shown enhanced performance relative to those supported on pure oxides, highlighting the exceptional stability of nickel crystallites [32,48,50,51].

While traditional ESR typically operates at atmospheric pressure, the adoption of high-pressure conditions has emerged as a transformative strategy offering numerous benefits [52,53]. Running ESR at elevated pressures enhances heat recovery, which lowers energy consumption and operating expenses, thereby improving both economic feasibility and environmental sustainability. These benefits are especially pronounced in industrial-scale plants equipped with downstream hydrogen separation and purification processes. High-pressure hydrogen is easier to separate and purify, simplifying downstream processes like pressure swing adsorption and membrane separation, lowering both cost and complexity. Additionally, high-pressure ESR offers flexibility in selecting purification techniques, allowing for tailored systems [54]. The process also enables more compact reactor designs, reducing equipment size and space requirements, particularly beneficial in areas with limited space or high property costs. Additionally, pre-pressurizing liquid ethanol before reforming is more energy-efficient than compressing hydrogen afterward, helping to reduce overall energy use. Generating hydrogen directly at high pressure lessens the need for expensive compression during storage or transportation. High-pressure hydrogen can also be seamlessly integrated into industrial processes such as ammonia synthesis, methanol production, and hydrocracking, further reducing costs and improving operational efficiency. Research on high-pressure ethanol steam reforming, particularly using fixed packed-bed tubular reactors, is scarce. To our knowledge, no previous studies have explored this area with the level of detail presented here. This study pioneers the use of two commercially available nickel-based catalysts, AR-401 and NGPR-2, in high-pressure ESR to enhance hydrogen production efficiency. Performed at pressures up to 30 bar and temperatures as high as 850 °C, our study achieved full ethanol conversion, with hydrogen selectivity reaching 74% and yields of 5.2 moles of H₂ per mole of ethanol. By optimizing critical parameters, this study demonstrates the potential of high-pressure ESR as a scalable and sustainable method for hydrogen generation, addressing the urgent need for cleaner energy in the fight against climate change.

2. Results and Discussion

2.1. Catalysts Testing

Catalyst testing was conducted in a fixed-bed reactor, as illustrated in Figure 1.

The reactor is constructed from high-temperature-resistant Inconel alloy and is designed with an outer diameter of 1.050 inches and an inner diameter of 0.514 inches. Engineered with sufficient wall thickness to withstand pressures over 500 bar, the reactor is housed within an electric furnace. A Cisco pump is used to precisely control the flow rate of the ethanol–water mixture, while a thermocouple inserted into the reactor continuously monitors the catalyst temperature. A 0.5 g sample is positioned at the center of the packed-bed reactor and is subjected to a reduction temperature of 400 °C for the AR-401 catalyst and 875 °C for the NGPR-2 catalyst in 10% H₂ for one hour before evaluating the catalysts’ performances. These reduction temperatures were obtained from the TPR experiments conducted using the Micromeritics 3-Flex Surface Characterization system (Norcross, GA, USA). Throughout the reaction, the ethanol–water feed is vaporized in a preheater and introduced into the reactor with nitrogen as an inert carrier. Downstream, products pass through a condenser and enter a gas–liquid separator, where the liquid phase is collected for analysis and the vapor phase is directed to online gas chromatography.

2.2. Characterizations

Although the catalyst vendors provided the fundamental compositions of the two commercial catalysts, the proprietary nature of their formulations prompted us to conduct further characterizations. Our approach encompassed a range of analytical techniques, including scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller (BET) measurements, and powder X-ray diffraction. These methods allowed us to gather comprehensive data on various aspects such as morphology, particle sizes, surface area, and chemical composition of the commercial catalysts. Furthermore, we utilized TPR and other advanced characterization techniques to gain deeper insights into the physicochemical properties of the catalysts. This multifaceted analysis provided valuable insights into the catalysts’ behavior and performance, enabling a more thorough understanding of their suitability for ESR applications.

2.2.1. Microscopy

SEM and TEM were employed to examine the morphology and structure of the catalysts under investigation. Figure 2a,b display SEM images depicting the surface morphology of the AR-401 catalyst, while Figure 2c,d illustrate that of the NGPR-2 catalyst.

Despite attempts to discern particle sizes from Figure 2, the presence of aggregated nanoparticles in both catalysts rendered precise particle size analysis challenging. To determine the elemental composition of the catalysts, EDX elemental mapping was performed. As depicted in Figure S1, both catalysts contain magnesium, nickel, aluminum, oxygen and carbon. Notably, while the percentage composition of Ni, Mg, and O is comparable in both catalysts, the NGPR-2 catalyst exhibits higher aluminum content and lower carbon content compared to the AR-401 catalyst, as evident from Figure S1. Furthermore, TEM was utilized to assess the particle sizes of the catalysts. Figure 3a,b showcase TEM images of the AR-401 catalyst, while Figure 3c,d show the images of the NGPR-2 catalyst. The TEM analysis revealed that both catalysts consist of fine nanoparticles with particle sizes ranging from 10 to 30 nm. Also, the nanoparticles exhibit predominantly round or spherical shapes, as observed in Figure 3.

2.2.2. Powder X-Ray Diffraction

We utilized PXRD to examine the chemical composition and crystal structure of the two catalysts. Figure 4 displays the PXRD pattern obtained for the AR-401 catalyst, while Figure 5 shows the corresponding pattern for the NGPR-2 catalyst. Analysis of the PXRD pattern for the AR-401 catalyst (Figure 4) reveals the presence of metallic nickel, nickel oxide, and magnesium aluminate phases. Specifically, three distinct diffraction peaks corresponding to metallic nickel are observed at 2θ values of approximately 45°, 52°, and 76°, corresponding to the Ni(111), Ni(200), and Ni(220) crystallographic planes, respectively. These peaks signify the presence of metallic face-centered cubic (FCC) nickel in the structure of the AR-401 catalyst [55].

Also, the presence of nickel oxide is evident from the peak observed at an approximate 2θ value of 37°, consistent with the (111) crystallographic plane of NiO [56]. Moreover, in Figure 4, the presence of magnesium aluminate is discernible through five characteristic peaks, indexed to Miller indices (111), (220), (422), (511), and (440), at approximately 2θ values of 19°, 31°, 56°, 59°, and 65°, respectively. These peaks confirm the existence of MgAl₂O₄ in the structure of the AR-401 catalyst [57]. The well-defined diffraction peaks indicate a high degree of crystallinity, with no detectable amorphous phase. The metallic Ni is the active catalytic sites for ESR, while NiO contributes to catalyst stability by preventing excessive sintering [58]. Magnesium aluminate acts as a structural support, enhancing thermal stability and dispersion of nickel species. These findings indicate that the AR-401 catalyst has a well-structured composition, which is crucial for maintaining catalytic performance over extended operation. On the other hand, as illustrated in Figure 5, the NGPR-2 catalyst primarily consists of Ni and NiO. The NiO peaks in Figure 5 correspond to the (111), (200), (220), and (311) crystallographic planes of NiO [56,59,60]. The presence of these diffraction peaks is further supported by XPS measurements [61]. The XPS analysis reveals characteristic binding energy signals corresponding to metallic nickel (Ni⁰), NiO, and the spinel phase MgAl₂O₄. These findings confirm the coexistence of both metallic and oxidized nickel species, along with the formation of the MgAl₂O₄ support, in agreement with the crystalline phases identified in the PXRD diffraction patterns. Together, the diffraction and XPS results provide strong complementary evidence for the structural and chemical composition of the catalyst.

Furthermore, two small peaks attributed to Ni metal are observed at around 2θ values of 45° and 52°, corresponding to the (111) and (200) planes of Ni, respectively [55]. In addition to nickel and the nickel oxide, the NGPR-2 catalyst contains magnesium aluminate, indicated by the (400) plane at roughly 2θ values of 66° [57] and α-Al₂O₃, as evidenced by the peak around 2θ values of 26°, assigned to the Miller indices of (012) [62].

2.2.3. BET Analysis

BET surface area measurements were conducted using the Micromeritics 3-Flex Surface Characterization equipment to determine specific surface area, pore size, particle size, and pore volume of the catalysts. The adsorption/desorption isotherms of both catalysts were obtained at liquid nitrogen temperature (−196 °C). Before the measurements, the samples underwent a 24-h degassing process at 300 °C. The specific surface areas of both catalysts were calculated using the Brunauer–Emmett–Teller (BET) method, while the Barrett–Joyner–Halenda (BJH) method was employed to determine pore size and pore volume. For the AR-401 catalyst, the specific surface area, average pore width, and cumulative pore volume were measured at 66.47 m²/g, 12.06 nm, and 0.20 cm³/g, respectively. In contrast, the NGPR-2 catalyst exhibited a specific surface area, average pore width, and cumulative pore volume of 89.88 m²/g, 10.47 nm, and 0.24 cm³/g, respectively. The NGPR-2 catalyst exhibits a greater surface area, larger pore volume, and narrower pore width compared to the AR-401 catalyst. Figure S2 illustrates the absorption/desorption isotherms and pore width distribution of both catalysts. Both catalysts exhibit a type 1V isotherm pattern. Analysis of the pore width distribution reveals the existence of mesoporous pores stretching from 2 to 50 nm in both catalysts. The pore size distribution plots display a bimodal profile, with smaller pores primarily concentrated near 3 nm and larger pores centered around 10 nm. The presence of both small and large pores can contribute to an optimized balance between surface reactivity and structural stability, which is critical for sustained catalytic performance.

2.2.4. Temperature Programmed Reduction

Temperature programmed reduction (TPR) experiments were carried out using a Micromeritics 3-Flex Surface Characterization system. The gas mixture used comprised 10% hydrogen in argon, with a total flow rate of 30 mL per minute. The TPR analysis spanned a temperature range from ambient to 1000 °C, with a fixed heating rate of 10 °C min⁻¹. These experiments were conducted to assess the reducibility and identify the optimal reduction temperatures for the AR-401 and NGPR-2 catalysts used in ESR. Figure S3 illustrates the TPR profiles of these catalysts. In both catalysts, a major peak around 300 °C indicates the reduction of NiO [63,64]. Also, peaks observed for the NGPR-2 catalyst at approximately 800 °C signify the reduction of NiAl₂O₄ species [64,65]. The reduction temperature and peak width provide insights into the ease of reduction and the level of interaction between catalyst species and the supporting material. Higher reduction temperatures imply more challenging reduction processes, while wider peaks suggest stronger interactions between the catalyst and the support material [65,66,67]. For instance, NiO exhibits easy reduction with minimal support interaction, whereas NiAl₂O₄ is challenging to reduce and displays substantial support interaction. The absence of a reduction peak around 800 °C in the AR-401 catalyst, in contrast to the NGPR-2 catalyst, suggests a weaker interaction between NiO and the magnesium aluminate support. This could be attributed to the aggregation of NiO on the support surface, preventing fine dispersion and reducing metal-support interactions [68]. While variations in NiO particle size could also contribute to these differences, TEM analysis confirmed similar particle sizes for both catalysts, effectively ruling out this factor as a primary cause. Based on this data, catalyst reduction was conducted at optimal temperatures of 400 °C for the AR-401 catalyst and 875 °C for the NGPR-2 catalyst.

2.3. Catalyst Performance

The catalysts’ performance is assessed based on three key parameters: ethanol conversion, hydrogen yield, and hydrogen selectivity. These parameters are defined using Equations (2)–(4), where the F terms represent the molar flow rates of the corresponding components.

$$\mathrm{E}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{\%}\right)={\displaystyle \frac{\left[{\mathrm{F}(\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}{)}_{\mathrm{i}\mathrm{n}}-{\mathrm{F}(\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}{)}_{\mathrm{o}\mathrm{u}\mathrm{t}}\right]}{\left[{\mathrm{F}(\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}{)}_{\mathrm{i}\mathrm{n}}\right]}}\times 100$$

(2)

$$\mathrm{H}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{m}\mathrm{o}\mathrm{l}\right)={\displaystyle \frac{\left[{\mathrm{F}(\mathrm{H}}_2{)}_{\mathrm{o}\mathrm{u}\mathrm{t}}\right]}{{6\left[\mathrm{F}\right(\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}{)}_{\mathrm{i}\mathrm{n}}]}}$$

(3)

$$\mathrm{H}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{S}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\left(\mathrm{\%}\right)={\displaystyle \frac{\left[{\mathrm{F}(\mathrm{H}}_2{)}_{\mathrm{o}\mathrm{u}\mathrm{t}}\right]}{{6\left[\mathrm{F}\right(\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}{)}_{\mathrm{i}\mathrm{n}}]\times \mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}}\times 100$$

(4)

2.4. Effects of Operating Conditions on Ethanol Steam Reforming

We first investigated the effect of three different operating conditions on the performance of our catalysts in ESR. These variables include temperature, pressure and steam-to-ethanol ratio.

2.4.1. Effects of Steam-to-Ethanol Ratio on Catalysts Performances

The study investigated the influence of varying the steam-to-ethanol ratio on hydrogen selectivity and yield for the AR-401 and NGPR-2 catalysts. Notably, all tests were conducted at atmospheric pressure and a constant temperature of 700 °C to assess the effects of these ratio changes. The evaluations were carried out at steam-to-ethanol ratios of 6, 9, and 12. For AR-401, the hydrogen selectivity was recorded at 70.3%, 74.8%, and 71.1% for steam-to-ethanol ratios of 6, 9, and 12, respectively. In comparison, NGPR-2 showed hydrogen selectivities of 71.8%, 73.6%, and 72.3% for the corresponding steam-to-ethanol ratios, as depicted in Figure 6a. Moreover, the hydrogen yield of AR-401 was measured at 4.51, 4.72, and 4.68 moles of hydrogen per mole of ethanol that reacted for steam-to-ethanol ratios of 6, 9, and 12, respectively, corresponding to hydrogen yield of 75.2%, 78.7%, and 78.0%, respectively. On the other hand, the hydrogen yield of the NGPR-2 catalyst was recorded as 5.07, 5.12, and 5.09 moles of hydrogen per mole of ethanol reacted for steam-to-ethanol ratios of 6, 9, and 12, respectively, corresponding to hydrogen yield of 84.5%, 85.3%, and 84.8%, respectively, as illustrated in Figure 6b.

This consistent trend indicates that both catalysts exhibit similar hydrogen selectivities, while NGPR-2 demonstrates a higher hydrogen yield. This result is consistent with the higher nickel content in NGPR-2, which offers a greater number of active sites for cleaving ethanol C–C bonds and facilitating the reforming reaction to generate hydrogen [69]. Furthermore, the selectivity of each catalyst rises with an increase in the steam-to-ethanol ratio from 6 to 9 but slightly decreases when further increased from 9 to 12. This suggests that at higher steam concentrations, competitive adsorption occurs, with steam occupying more active sites and limiting the availability of sites for ethanol adsorption [70]. As a result, increasing the steam-to-ethanol ratio beyond 9 offers no additional benefit for the ESR reaction. Moreover, from an energy efficiency standpoint, higher steam-to-ethanol ratios are actually detrimental to the overall ESR process [71]. Notably, both AR-401 and NGPR-2 catalysts achieved complete ethanol conversion across all three tested steam-to-ethanol ratios.

2.4.2. Effects of Pressure on Catalysts Performances

The effect of pressure on the performance of AR-401 and NGPR-2 catalysts was evaluated at a constant reaction temperature of 700 °C and a steam-to-ethanol ratio of 9, resulting in complete ethanol conversion (100%). The catalysts were tested at pressures of 1 bar, 3 bars, and 6 bars. As shown in Figure 7a, the hydrogen selectivity of AR-401 was 74.8%, 70.1%, and 66.7% for pressures of 1 bar, 3 bars, and 6 bars, respectively. Similarly, for NGPR-2, the hydrogen selectivity was 73.6%, 71.2%, and 68.4% at pressures of 1 bar, 3 bars, and 6 bars, respectively. Also, the hydrogen yield of both catalysts was measured at these three pressures. For the AR-401 catalyst, the hydrogen yield decreased from 4.72 to 4.57 to 4.23 moles of hydrogen per mole of ethanol reacted as the pressure increased from 1 bar to 3 bars to 6 bars. Likewise, the hydrogen yield of NGPR-2 decreased from 5.12 to 4.94 to 4.68 moles of hydrogen per mole of ethanol as the reaction pressure increased from 1 bar to 3 bars to 6 bars, as illustrated in Figure 7b.

These results demonstrate a clear trend where both hydrogen selectivity and yield decrease as pressure increases. This outcome is expected, as higher pressures are detrimental to ESR. According to Le Chatelier’s principle, the equilibrium shifts toward the reactants, which have fewer moles [72].

2.4.3. Effects of Temperature on Catalyst Performance

The impact of temperature on hydrogen selectivity, hydrogen yield, and ethanol conversion of AR-401 and NGPR-2 catalysts was investigated using an ethanol-to-steam molar ratio of 9 and at atmospheric pressure. It was observed that for the AR-401 catalyst, hydrogen selectivity increased from 52.5% to 74.8% as the temperature was increased from 400 °C to 700 °C. Similarly, the hydrogen selectivity of the NGPR-2 catalyst increased from 57.8% to 73.6% with a temperature increase from 400 °C to 700 °C, as illustrated in Figure 8a.

Furthermore, both AR-401 and NGPR-2 catalysts exhibited an increase in hydrogen yield with rising temperatures. Specifically, the hydrogen yield of the AR-401 catalyst increased from 0.99 mol/mol of ethanol reacted at 400 °C to 4.72 mol/mol of ethanol reacted at 700 °C. Similarly, the hydrogen yield of NGPR-2 increased from 1.4 mol/mol of ethanol reacted at 400 °C to 5.12 mol/mol of ethanol reacted, as shown in Figure 8b. Also, ethanol conversion also increased with increasing temperatures. For instance, the ethanol conversion of the AR-401 catalyst increased from 73.5% at 400 °C to 100% at 700 °C, while for NGPR-2, it increased from 80.2% at 400 °C to 100% at 700 °C, as depicted in Figure 8c. This trend is expected because, at high temperatures, methane reforming contributes additional hydrogen to the product stream, while the water gas shift reaction also enhances hydrogen production [73].

2.4.4. Product Selectivities at Different Temperatures

The selectivities of the products from ESR were investigated at various temperatures for both AR-401 and NGPR-2 catalysts. These selectivities were analyzed as a function of time on stream (TOS). The results obtained for the AR-401 catalyst are illustrated in Figure 9. In Figure 9a, at 400 °C, the selectivities of H₂, CO₂, CH₄, and CO were 52.8%, 14.1%, 20.6%, and 11.8%, respectively. Upon increasing the reaction temperature to 500 °C, the H₂ selectivity increased to 68.9%, while that of CO₂ increased to 16.9%. However, the selectivity of CH₄ decreased to 9.2%, and that of CO decreased as well to 4.3%, as shown in Figure 9b. With a further increase in the reaction temperature to 600 °C, the selectivity of H₂ increased to 70.7%, and that of CO₂ increased to 20.3%. Conversely, the selectivities of CH₄ decreased to 2.1%. However, the selectivity of CO remained unchanged at 4.3%, as illustrated in Figure 9c. At 700 °C, the H2 selectivity further increased to 75.6%, while the selectivity of CO increased to 9.7%. Conversely, the selectivity of CO₂ decreased to 12.3%, and that of CH₄ decreased to 1.2%, as depicted in Figure 9d.

Similar results were observed for the NGPR-2 catalyst, as summarized in Figure 10. As the temperature increased, the selectivity of H₂ showed a consistent upward trend, rising from 47.5% at 400 °C to 63.6% at 500 °C, further to 70.7% at 600 °C, and finally reaching 73.6% at 700 °C. Conversely, the selectivity of CO exhibited a gradual increase from 2.7% at 400 °C to 8.5% at 700 °C. However, the selectivity of CH₄ decreased notably from 29.1% at 400 °C to merely 0.6% at 700 °C. In contrast, the selectivity of CO₂ remained relatively unchanged throughout the temperature range, hovering around 20%. These findings underscore the significant influence of temperature on the product selectivities in ESR.

3. High Pressure Ethanol Steam Reforming

After optimizing the reaction parameters of the AR-401 and NGPR-2 catalysts in Section 3.3, our focus shifted to testing these catalysts under higher pressures, mirroring conditions used in industrial-scale ethanol reforming operations.

3.1. The Case for High-Pressure Ethanol Steam Reforming

High-pressure ESR presents significant advantages over atmospheric pressure processes, particularly for industrial-scale hydrogen production. It offers benefits that span economic efficiency, operational performance, and industrial integration, making it a compelling choice for sustainable hydrogen generation.

(a)

Economic Viability and Energy Efficiency: Operating ESR at high pressures dramatically enhances the cost-to-output ratio, yielding a better return on investment for each kilogram of hydrogen produced. Additionally, the integration of heat recovery systems becomes more effective at elevated pressures, enabling efficient recycling of process heat and reducing overall energy consumption. This improved energy efficiency makes the process both economically and environmentally sustainable.

(b)

Simplified Hydrogen Purification: Hydrogen produced at high pressures is inherently easier to separate and purify. This simplifies downstream processes such as pressure swing adsorption and membrane separation, reducing both their cost and complexity. Additionally, high pressure ESR offers flexibility in selecting suitable hydrogen purification and separation techniques, allowing systems to be tailored to specific industrial needs [53]. Additionally, pressurized systems minimize contamination risks, ensuring the production of higher-purity hydrogen essential for various applications.

(c)

Reduced Equipment Size and Land Requirements: High-pressure ESR enables compact reactor designs by compressing reactants into smaller volumes. This reduces the physical footprint and equipment costs, which is particularly advantageous for industrial facilities in regions with limited space or high real estate costs.

(d)

Lower Hydrogen Compression Costs: The compression of hydrogen is widely acknowledged to demand significant energy input [74]. Pre-compressing liquid ethanol before reforming is more energy-efficient than compressing hydrogen gas post-production. By generating hydrogen directly at elevated pressures, the process minimizes the energy and economic costs associated with downstream compression for storage or transportation.

(e)

Seamless Integration with Industrial Applications: Hydrogen generated at high pressures can more easily integrate into high-pressure industrial processes, such as ammonia synthesis, methanol production, or hydrocracking, without requiring additional compression. This streamlines operations and aligns with existing industrial standards, enhancing efficiency and reducing overall costs.

(f)

Scalability and Future Readiness: High-pressure ESR’s inherent scalability makes it ideal for large industrial setups and future expansions. Furthermore, it aligns well with emerging technologies in hydrogen storage, fuel cells, and decarbonization strategies, ensuring its relevance as the hydrogen economy continues to grow.

3.2. Effects of Weight Hourly Space Velocity on AR-401 Catalyst Performance at High Pressure

Our initial investigation focused on assessing the impact of weight hourly space velocity (WHSV) on product selectivity and hydrogen yield using the AR-401 catalyst under a pressure of 27 bar. We then applied the WHSV value that yielded the highest hydrogen selectivity to other pressure conditions, including 18, 24, and 30 bar.

To optimize hydrogen selectivity and yield, we conducted tests on the AR-401 catalyst using varying weight hourly space velocities (WHSVs), specifically at rates of 28.3 h⁻¹, 56.7 h⁻¹, 85.0 h⁻¹ and 113.3 h⁻¹. These WHSV values corresponded to residence times of 2.1, 1.06, 0.71, and 0.53 min, respectively. For these experiments, we maintained a pressure of 27 bar and an ethanol-to-steam ratio of 9. The results, depicted in Figure 11, illustrate the product selectivities and hydrogen yields as a function of time on stream at different WHSVs. The product selectivities remain consistent across all WHSVs, with only a slight increase in hydrogen selectivity observed from 69.3% at WHSV of 56.7 h⁻¹ to 72.4% at WHSV of 85.0 h⁻¹.

In contrast, the hydrogen yield exhibits a decreasing trend, declining from 4.52 moles of H₂ per mole of ethanol reacted at a WHSV of 56.7 h⁻¹, to 3.92 moles of H₂ per mole of ethanol reacted at a WHSV of 85.0 h⁻¹, and further decrease to 3.84 moles of H₂ per mole of ethanol reacted at a WHSV of 113.3 h⁻¹. This trend is anticipated due to the shorter residence time of reactants within the reactor as the WHSV increases from 28.3 h⁻¹ to 113.3 h⁻¹ [75]. Consequently, to optimize hydrogen selectivity and hydrogen yield, a WHSV of 85.0 was selected for further application to various pressures, including 18, 24, and 30 bar, as detailed in the subsequent section.

3.3. Evaluating Catalyst Performance Under High-Pressure Conditions

The impact of pressure on hydrogen selectivity and yield was assessed for both the AR-401 and NGPR-2 catalysts, across a range of pressures: 18, 24, 27, and 30 bar. The results, illustrated in Figure 12, provide insights into how changes in pressure affect these important performance indicators. In Figure 12a, we observe a subtle yet discernible decrease in the hydrogen selectivity of the AR-401 catalyst, dropping from 67.7% at 18 bar to 64.7% at 30 bar. Similarly, Figure 12b demonstrates a decline in hydrogen yield as pressure increases. For the NGPR-2 catalyst, as illustrated in Figure 12c, hydrogen selectivity remains relatively similar across pressure levels of 18, 24, and 27 bar, maintaining a range of 67–69%. However, a marked decrease in selectivity is noted at 30 bar. This pattern suggests a potential threshold effect, where pressures higher than 27 bar may negatively impact selectivity. Further analysis could explore the specific causes of this decline at elevated pressures, which could relate to changes in reaction dynamics or catalyst surface behavior at these conditions. In contrast, Figure 12d illustrates a clear downward trend in hydrogen yields for the NGPR-2 catalyst as pressure rises from 18 bar to 30 bar, mirroring the trend observed for the AR-401 catalyst.

These findings highlight the influence of reaction pressure on hydrogen selectivity and yield in both catalysts. They offer valuable insights for optimizing ESR processes under varying pressure conditions, thus advancing hydrogen production technologies. Also, we delved into the effects of elevated temperatures surpassing 700 °C at elevated pressures on product selectivity and hydrogen yield for NGPR-2 catalyst. Specifically, we assessed the NGPR-2 catalyst at a temperature of 850 °C and pressures of 24, 27, and 30 bar. The outcomes of this investigation are depicted in Figure 13. Figure 13a,c,e show the observed product selectivities under these experimental conditions.

Interestingly, the selectivities for hydrogen, carbon dioxide, carbon monoxide, and methane remarkably remain consistent across all three pressure levels examined. Particularly noteworthy is the stability of hydrogen selectivity at approximately 74%, accompanied by carbon dioxide and carbon monoxide selectivities averaging around 15% and 10%, respectively. Methane selectivity remains consistently low at approximately 1%. Furthermore, the hydrogen yields at these pressures demonstrate remarkable consistency, ranging from 5.1 to 5.2 moles of hydrogen per mole of ethanol reacted as shown in Figure 13b,d,f. This translates to a hydrogen yield of approximately 85–87%, highlighting the reliability of the NGPR-2 catalyst across different pressure settings at elevated temperatures. These results underscore the resilience and stability of the NGPR-2 catalyst, indicating its potential for high-efficiency hydrogen production under elevated temperatures and pressures.

3.4. Catalyts Stability

The stability of the AR-401 and NGPR-2 catalysts was evaluated by conducting ESR at 27 bar for over 30 h. Both catalysts exhibited exceptional stability, with hydrogen yield and selectivity remaining nearly constant throughout the reaction duration. Figure S4 illustrates the hydrogen selectivity of the AR-401 catalyst over a 30-h run at 27 bar, demonstrating consistent performance. Similarly, the NGPR-2 catalyst displayed comparable stability in terms of hydrogen selectivity and yield, reaffirming its reliability under prolonged high-pressure ESR conditions. These results highlight the robustness of both catalysts, making them promising candidates for scalable hydrogen production. Furthermore, the spent catalyst after the 30-h run was analyzed using SEM-EDX. The elemental composition of the spent catalyst closely matched that of the fresh catalyst shown in Figure S1 with slightly higher carbon content observed in the spent catalyst revealing that higher pressures had very little effect of the carbon deposition on the catalyst.

3.5. Outlook

Our study offers significant insights into high-pressure ESR. To further improve the efficiency and economic viability of this process for industrial applications, we propose future research in the following key areas:

(a)

Optimization of Reaction Pressure and Temperature: While our findings demonstrate the benefits of high-pressure ESR, including enhanced hydrogen flow and recovery rates, further research is needed to determine the ideal pressure conditions. Refining pressure ranges will maximize hydrogen yield and recovery while minimizing the energy costs associated with high-pressure operation. Also, optimizing temperature ranges is vital for ensuring the economic feasibility of the process. A balance between pressure and temperature will be crucial to making large-scale ESR more cost-effective.

(b)

Catalyst Development: Our study shows that commercially available nickel-based catalysts (AR-401 and NGPR-2) perform well under high-pressure conditions. However, further research into advanced or doped catalyst formulations could improve resistance to coke formation and deactivation, extending the catalyst’s life and reducing operational costs. Investigating alternative support materials with higher thermal stability and lower acidity could also enhance ethanol conversion and hydrogen selectivity. Hybrid catalyst systems, combining nickel with other transition metals, could optimize multiple reaction pathways, minimizing undesirable by-products like methane and carbon monoxide.

(c)

Energy Efficiency in Hydrogen Production: High-pressure reforming can reduce energy demands, especially in hydrogen compression, which is a critical step in hydrogen storage and distribution. Future work should explore more energy-efficient separation techniques integrated into high-pressure reforming, which could improve the economic competitiveness of ethanol-based hydrogen production. Incorporating renewable energy sources, such as coupling ESR with solar or wind energy, may reduce costs and enhance environmental sustainability.

(d)

Understanding the Reaction Kinetics of ESR: A comprehensive understanding of the reaction kinetics in ESR is crucial for optimizing hydrogen production and improving process efficiency. Investigations into the reaction mechanisms and the formation of intermediate species can lead to more precise catalyst designs and better process control. This knowledge is key to enhancing large-scale applications, improving energy efficiency, and reducing costs in ethanol-based hydrogen production.

(e)

Scale-Up and Industrial Application: Scaling up ESR from lab settings to industrial applications will require a deeper understanding of reactor design under high-pressure conditions. Future studies should focus on reactor size, residence time, and flow dynamics to ensure consistent and reliable performance at a large scale. Collaboration with industry stakeholders will also be essential to address logistical and regulatory challenges associated with integrating ethanol-derived hydrogen into existing energy infrastructures.

(f)

Technoeconomic Analysis of ESR: Technoeconomic analysis (TEA) is essential for assessing the feasibility of ESR at an industrial scale. TEA integrates technical performance metrics, such as hydrogen yield and energy efficiency, with economic considerations like capital and operational costs. This analysis helps identify cost drivers, potential bottlenecks, and areas for optimization, ensuring that the process is both technically feasible and economically viable. Moreover, TEA evaluates financial risks, market conditions, and regulatory factors, providing a comprehensive framework for scaling up ethanol-based hydrogen production while addressing sustainability and environmental impacts.

(g)

Sustainability and Feedstock Flexibility: Ethanol from biomass has proven to be a promising sustainable feedstock for hydrogen production. Future research could explore the use of diverse ethanol feedstocks, such as lignocellulosic biomass and agricultural waste, to increase the sustainability of hydrogen production. Comparative lifecycle studies of hydrogen production from various ethanol sources will help establish ethanol-derived hydrogen as a viable renewable energy option.

To summarize, high-pressure ESR using nickel-based catalysts presents a promising pathway for producing clean hydrogen. Advancing catalyst technologies, optimizing the process, and integrating renewable energy systems will be crucial to making this technology both commercially viable and environmentally sustainable.

4. Experimental Section

4.1. Materials

This study employed two commercial catalysts, AR-401 and NGPR-2, both supplied by our industry collaborator, Proteum Energy LLC (Phoenix, AZ, USA). AR-401 is a nickel-based catalyst featuring activated magnesium aluminate spinel supported on alumina and manufactured by Topsoe (Lyngby, Denmark). while NGPR-2, a pre-reformer catalyst which has high nickel content, is produced by Unicat (Dettenheim, Germany). While the NGPR-2 catalyst contains proprietary components, it primarily comprises nickel and nickel oxide (NiO) phases.

4.2. Characterizations of Catalysts

Scanning electron microscopy equipped with EDX elemental analysis (SEM-EDX) was used to examine the morphology of the catalysts. Phase identification of the catalysts was carried out using powder X-ray diffraction (PXRD) on a Bruker (Billerica, MA, USA) D2-Phaser diffractometer, employing Cu Kα radiation at 30 kV. To investigate the internal structure, high-resolution environmental transmission electron microscopy (TEM) was performed using a Hitachi HF3300 (Tokyo, Japan) operating at an acceleration voltage of 300 kV. Additionally, Brunauer–Emmett–Teller (BET) surface area measurements and temperature-programmed reduction (TPR) analyses were conducted using a Micromeritics 3-Flex surface characterization system.

5. Conclusions

In this study, we investigated high-pressure ethanol steam reforming using two commercially available nickel-based catalysts: AR-401 and NGPR-2. Following characterization of the catalysts, we assessed the influence of various operating conditions—temperature, pressure, steam-to-ethanol-ratio and weight hourly space velocity—on the performance of ESR. This evaluation was based on key metrics such as hydrogen selectivity, hydrogen yield, and ethanol conversion. We conducted high-pressure ethanol reforming under conditions mimicking those found in industrial applications, with pressure ranging from 18 to 30 bar. Our results demonstrated that at pressures up to 30 bar, both AR-401 and NGPR-2 achieved complete ethanol conversion. Also, both catalysts exhibited remarkable hydrogen selectivity (65–70%) and hydrogen yields ranging from 4 to 4.5 moles of hydrogen per mole of ethanol. Furthermore, we outlined potential directions for future research aimed at enhancing the efficiency of the ESR process. These include optimizing reaction conditions, developing advanced catalysts that minimize deactivation and coke formation, and conducting in-depth reaction kinetics studies and techno-economic analyses. The findings from this work underscore the importance of performing ESR at high pressures. High-pressure conditions promote a more efficient separation of hydrogen from the product stream, a crucial factor for improving hydrogen recovery rates in large-scale industrial reforming processes.