*3.2. Catalyst Characterization*

Fresh, reduced and spent samples were characterized by several techniques to identify and infer the effects of combustible materials such as C2H5NO2 and NH4NO3 on the catalyst morphology and the resulting catalytic performance. N2 adsorption-desorption isotherms for each sample were collected on a Micromeritics ASAP 2020 system (Micromeritics, Norcross, GA, USA). The surface area, pore size and pore volume were calculated with the N2 adsorption-desorption isotherms via the conventional Barrett–Joyner–Halenda (BJH) and Brunauer–Emmett–Teller (BET) methods. Prior to the measurements, the samples were outgassed under vacuum for 5 hours at 200 ◦C. The X-ray diffraction (XRD) patterns of each reduced sample were obtained with a Bruker AXS D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) with Cu K α radiation (λ=1.5406 Å) at a scanning rate of 6◦/min with the 2θ range of 10–90◦. The reducibility of the catalyst was studied by the H2 temperature-programmed reduction (H2-TPR) in an auto-controlled flow reactor system of TP-5076, which is equipped with a thermal conductivity detector (TCD, Tianjin Xianquan Co., China). The sample of 50 mg was pretreated in N2 stream at 200 ◦C for 1 hour. Additionally, when the temperature cooled down to 30 ◦C, the sample was heated to 950 ◦C at a heating rate of 10 ◦C/min in the H2/N2 flow (5 vol.% H2 in N2) of 30 mL/min. The H2-TPR spectra were obtained at the temperature range of 50–950 ◦C. The carbon accumulation in spent samples after reaction for 50 hours was determined by thermogravimetric (TG) analysis on a Mettler–Toledo TGA-1100SF thermogravimetric analyzer (Mettler-Toledo, Greifensee, Switzerland).

## *3.3. Catalytic Test*

The dry reforming of CH4 with CO2 was performed at atmospheric pressure in a continuous-flow fixed bed quartz tube reactor with an inner diameter of 9 mm. For the typical experiment, 200 mg of shaped catalyst was filled into the center of the reactor. Before starting the reforming reaction, the catalyst was pre-reduced to 750 ◦C and atmospheric pressure for 2 hours in an H2 flow of 60 mL/min with a heating rate of 10 ◦C/min. After that, the reactor temperature was elevated to 800 ◦C, and then a flow of gas mixture with a molar ratio of CH4/CO2/N2 = 9/9/2 was fed with a flow rate of 160 mL/min. The products were analyzed by online gas chromatography (Agilent GC 7820A, Agilent, USA). CH4, CO2, H2, N2 and CO were measured by a TCD detector with a 5A molecular sieve column and a Porapak Q column. Additionally, 10% of N2 was employed as an internal standard. The conversions of CH4 and CO2 were calculated with the following formulas:

$$X\_{CM4} = (F\_{CM4\cdot in} - F\_{CM4\cdot out}) / F\_{CM4\cdot in} \times 100\% \tag{5}$$

$$X\_{\rm CO2} = (F\_{\rm CO2\cdot in} - F\_{\rm CO2\cdot out}) / F\_{\rm CO2\cdot in} \times 100\% \tag{6}$$

where *X* and *F* indicate the conversion and flow rate of *i* gas in the feed or the effluent, respectively.
