*3.1. Catalyst Characterization*

As a preliminary study for applying a pelletized catalyst to a commercial process, reactions were performed using a commercial Ni-based catalyst suitable for mass production of hydrogen. The textural properties and composition of the catalyst are listed in Table 4.


**Table 4.** Textural properties of the catalyst used herein.

For structural characterization, the commercial catalyst was ground and meshed to a size of ≤250 μm. The Brunauer–Emmett–Teller (BET) specific surface area, micropore area, pore volume, and pore size distribution of the powdered catalyst were estimated from the N2 adsorption and desorption isotherm obtained at −195.7 ◦C using a constant-volume adsorption apparatus (Micromeritics, ASAP-2020, Norcross, GA, USA). The pore volumes were determined at a relative pressure (P/Po) of 0.99. The catalyst was degassed at 300 ◦C for 4 h before the measurements. The pore size distributions of the samples were calculated using the Barrett–Joyner–Halenda (BJH) model.

#### *3.2. Steam Reforming Reaction*

#### 3.2.1. Methane Steam Reforming Reaction in the Lab-Scale Reactor

The catalytic activity of the powdered catalyst for the methane steam reforming reaction was tested in a fixed-bed tubular Inconel reactor (ID = 10 mm). Prior to feeding the reactants, the catalyst was activated by flowing H2 at a rate of 50 mL/min at 800 ◦C for 120 min. A TC was placed at the center of the catalyst bed to monitor the reaction temperature, and the feed flow was controlled using a mass flow controller (Brooks, 5850E, Hatfield, PA, USA). The gas products were analyzed using an online gas chromatograph (GC) (Young Lin Acme 6000, Gyeonggi-do, Korea) with a 40/60 carboxen-1000 column (2.0 ft × 1/8 in. × 2.1 mm) and a thermal conductivity detector (TCD). Nitrogen was used as an internal standard gas to verify the composition of the analytical gas (methane) as a volume or half volume.

The activity data shown in Figure 1 were collected by varying the reaction temperature (500 to 850 ◦C), steam/methane ratio (2 to 3.3), and reaction pressure (0.2 to 1 MPa). The methane conversions shown in Figure 5 (lab-scale) were evaluated under the following reaction conditions: heater temperature = 800 ◦C; pressure = 0.6 MPa; steam/methane ratio = 3; feed composition of CH4/H2O/N2 = 1/3/1; and gas hourly space velocity (GHSV) = 2.0–40.0 L CH4/(h·gcat).

The equilibrium conversion was calculated using "HSC chemistry" software (Outotec, Espoo, Finland).

#### 3.2.2. Methane Steam Reforming Reaction in the Bench-Scale Reactor

The bench-scale reactor consisted of three heaters, a stainless-steel reactor with an inner diameter of 32.52 mm and length of 110 cm, and five TCs. Figure 9 shows the bench reactor in detail. The TCs of the three heaters were located 22, 55, and 88 cm from the reactor inlet. Five TCs were located inside the reactor to monitor the temperature of the catalyst bed, at positions of 10, 21, 54, 87, and 103 cm. To prevent localization of heat and mass, the reactor was filled with a mixture of a pellet-type catalyst and spherical diluent (alpha-alumina). Prior to the SRM reaction, the catalyst mixture was activated by flowing H2 at a rate of 1 L/min at 800 ◦C for 120 min. The experiment was conducted under the conditions mentioned above, and the analysis method was the same as that of the lab-scale reaction.
