*3.2. Catalyst Characterization*

Scanning transmission electron microscopy (STEM) measurements over reduced samples (850 ◦C, 16 h, 10% H2/N2) were conducted with a FEI Titan 80–300 microscope (Hillsboro, OR, USA) operated at 300 kV. The FEI Titan is equipped with a CEOS GmbH double-hexapole aberration corrector (Heidelberg, Germany) for the probe-forming lens, which allows imaging at ∼0.1 nm resolution in STEM mode. The STEM images were acquired on high angle annular dark field with an inner collection angle of 52 mrad. In general, sample preparation involved mounting powder pre-reduced samples on copper grids covered with lacey carbon support films and then loading them immediately into the instrument airlock to minimize an exposure to atmospheric O2.

Nitrogen adsorption was measured at 77 K with an automatic adsorptiometer (Micromeritics ASAP 2000, Norcross, GA, USA). The samples were pre-treated at 383 K for 12 h under vacuum. The surface areas were determined from adsorption values for five relative pressures (P/P0) ranging from 0.05 to 0.2 using the Brunauer–Emmett–Teller method.

Temperature programmed reduction (TPR) and H2 chemisorption experiments were performed using an AutoChem II 2920 automated chemisorption analyzer (Micromeritics, Norcross, GA, USA). Metal reducibility was determined by TPR using changes in a thermal conductivity signal of the effluent gas. Samples (100 mg) were heated to 800 ◦C at 5 ◦C/min in a gas stream of 5% H2 in Ar (50 sccm). Volumetric pulse H2 adsorption measurements were carried out at 600 ◦C and 800 ◦C. First, 50 mg of the sample was reduced at 850 ◦C for 16 h using H2 (10% in N2, 100 mL/min) and purged for 4 h in pure N2. After ramping to the adsorption temperature, 5% H2/Ar was pulsed using a 100 μL loop with 1 min intervals between injections.

The amount of solid carbon deposited on the spent catalysts was measured by a Shimadzu Total Carbon Analyzer (TOC-5000A with a SSM-5000A Solid Sample Module, Shimadzu, Kyoto, Japan). A 1% O2/Ar mixture was blown through the sample starting from 40 ◦C until the temperature reached 900 ◦C. The temperature ramping rate was 10 ◦C/min.

## *3.3. Activity Measurements*

Catalytic activity tests were conducted in an 8 mm inner diameter Inconel fixed-bed reactor. For each test, the catalyst (9 mg, 60–100 mesh), diluted with α-Al2O3 (90 mg, 60–100 mesh), was loaded between two layers of quartz wool inside the reactor. Temperature was monitored with a thermocouple placed in the middle of the catalyst bed. Before reaction, the catalyst was reduced at 850 ◦C for 16 h using H2 (10% in N2, 100 mL/min).

Methane, ethane and natural gas simulant (94.5% methane, 4% ethane, 1% propane, 0.5% butane) gases were supplied by Matheson (Longview, WA, USA). Deionized water (18.6 mΩ) was introduced using a high-performance liquid chromatography pump (ChromTech series 1500, Bad Camberg, Germany) through <sup>1</sup>/16 inch stainless steel tubing into a vaporizer where the temperature was set at 250 ◦C. The catalysts were tested at atmospheric pressure at temperatures ranging from 550–900 ◦C, over a range of gas-hour-space-velocities (22,000–356,000 <sup>h</sup>−1). Note that we commonly refer to throughput in terms of contact time (τ) which ranges from 10–170 ms. Flow rates of dry gas products in the effluent gases were monitored by a digital flow meter (DryCal) (Mesa Labs, Butler, NJ, USA). Gas products were analyzed online using a two-channel Agilent Micro GC (3000A series) (Santa Clara, CA, USA) equipped with thermal conductivity detector.

In a typical experiment, a mass of 9 mg of catalyst was loaded to the reactor, 220 sccm of gas was fed down flow to the catalytic bed (80 sccm of N2, 35 sccm of methane, 105.8 sccm of H2O [0.085 mL/min]). Under those conditions, the calculated space velocity (GHSV) was 119,223 <sup>h</sup>−1, which corresponds to a τ of 30.2 ms and an S/C molar ratio of 3.0. For ethane reforming experiments, changes in space velocities were achieved with changes in gas flow (3.5–35 sccm of ethane for the Ir catalyst case, 35–100 sccm of ethane for Rh catalyst case) while concentrations (14.8 mol% ethane) and S/C ratios were maintained constant (S/C = 2.75).
