*2.4. Catalyst Characterization*

Powder X-ray diffraction (XRD) analysis of fresh catalyst was conducted by employing a Rigaku (Miniflex) diffractometer with a Cu Kα1 radiation (λ = 0.15406 nm) operated at 40 mA and 40 kV. The 2θ range and scanning step for analysis were 10–80◦ and 0.02◦ , respectively.

The N<sup>2</sup> adsorption and desorption data at −196 ◦C was analyzed for determining the specific surface area (BET) of the fresh catalysts by using Micromeritics Tristar II 3020 surface area analyzer. In order to get rid of other adsorbed gases and moisture, all samples were degassed before analysis. For each analysis, a load of 0.2–0.3 g of catalyst was used. The pore size distribution of catalysts was calculated from the adsorption branch of N<sup>2</sup> isotherm by using the Barrett, Joyner & Halenda (BJH) method.

Temperature-programmed hydrogen reduction (H2-TPR) and temperature-programmed carbon dioxide desorption (CO2-TPD) measurements were performed on a chemisorption device (Micromeritics AutoChem II).

A known amount of catalyst was pre-treated with high purity argon (Ar) at 150 ◦C for about half an hour for TPR analysis. Then, the samples were heated in an automatic furnace to 1000 ◦C at a steady heating rate of 10 K/min under 40 mL/min of H2/Ar mixture (volume ratio = 10/90) at atmospheric pressure. The H<sup>2</sup> signal was monitored by a thermal conduction detector (TCD).

For TPD experiments, first the adsorption of carbon dioxide onto the samples was carried out for half an hour at 50 ◦C under 10%CO2/He gas at 30 mL/min. Then, the CO<sup>2</sup> desorption was done by increasing the temperature at a rate of 10 K/min to 800 ◦C.

The scanning electron microscopy (SEM) was employed in order to investigate the surface morphology of the catalysts. The SEM images of the spent catalyst samples were taken by using under 10% O2/He at 30 mL/min.

JSM-7500F (JEOL Ltd., Tokyo, Japan) scanning electron microscope. The TEM study was carried out at 200 kV with an aberration-corrected JEM-ARM200F (JEOL, Corrector: CEOS). The microscope is fitted with a JED-2300 (JEOL) energy-dispersive X-ray-spectrometer for chemical analysis. from ambient temperature to 1000 °C at a heating rate of 20 K/min, and the weight loss was recorded. For this purpose, catalyst samples recovered after 5 h on stream at 700 and 800 °C as well as Co-800 after long term test (24 h) at 800 °C were used. All analyses were carried out under air atmosphere.

*Processes* **2019**, *7*, 141 5 of 15

to ambient temperature. Afterwards, the temperature was raised with a ramp of 10 K/min to 800 °C

gravimetric analyzer (Shimadzu, Kyoto, Japan). The spent catalysts weighing 10–15 mg were heated

Temperature-programmed oxidation (TPO) experiments were conducted to determine the carbon accumulation on the spent catalyst after prolonged activity tests. The samples recovered from partial oxidation were dried at 150 ◦C for half an hour under helium at 30 mL/min and then cooled to ambient temperature. Afterwards, the temperature was raised with a ramp of 10 K/min to 800 ◦C under 10% O2/He at 30 mL/min. **3. Results and Discussion**  *3.1. X-ray Diffraction (XRD)*  Typical XRD patterns in the range 2θ = 10–80° of fresh cobalt and/or nickel catalysts supported

The quantitative analysis of coke deposition on the spent catalysts was carried out using thermo-gravimetric analyzer (Shimadzu, Kyoto, Japan). The spent catalysts weighing 10–15 mg were heated from ambient temperature to 1000 ◦C at a heating rate of 20 K/min, and the weight loss was recorded. For this purpose, catalyst samples recovered after 5 h on stream at 700 and 800 ◦C as well as Co-800 after long term test (24 h) at 800 ◦C were used. All analyses were carried out under air atmosphere. on the composite support (Al2O3 + ZrO2) calcined at 550 and 800 °C are presented in Figure 1. In the case of samples calcined at 550 °C, broad reflections are observed. It is not possible to distinguish the species due to broadening and superimposing of reflections. Therefore, it implies that metal species are made of smaller crystallites and are well dispersed on the supports, which makes them amorphous and insensitive to X-ray radiations. This finding is consistent with the results obtained by BET and TPR which will be discussed later. Also, it is well known that the addition of zirconia to alumina leads to signal enlargement as a result of the formation of smaller particles [18,21]. Moreover,

#### **3. Results and Discussion** the decline in the intensity of the diffraction signals of catalysts Ni-550, Co-550 and Ni–Co-550 may

#### *3.1. X-ray Diffraction (XRD)* also be caused by the distortion or defects in the Al-O bonds due to Zr presence in the support [22]. With regard to the catalysts calcined at 800 °C, diffraction signals of sharp intensity observed;

Typical XRD patterns in the range 2θ = 10–80◦ of fresh cobalt and/or nickel catalysts supported on the composite support (Al2O<sup>3</sup> + ZrO2) calcined at 550 and 800 ◦C are presented in Figure 1. In the case of samples calcined at 550 ◦C, broad reflections are observed. It is not possible to distinguish the species due to broadening and superimposing of reflections. Therefore, it implies that metal species are made of smaller crystallites and are well dispersed on the supports, which makes them amorphous and insensitive to X-ray radiations. This finding is consistent with the results obtained by BET and TPR which will be discussed later. Also, it is well known that the addition of zirconia to alumina leads to signal enlargement as a result of the formation of smaller particles [18,21]. Moreover, the decline in the intensity of the diffraction signals of catalysts Ni-550, Co-550 and Ni–Co-550 may also be caused by the distortion or defects in the Al-O bonds due to Zr presence in the support [22]. those represent more crystalline phases. Furthermore, the reflex intensity of the bimetallic catalyst is higher than for the monometallic catalysts. For the Ni-800 catalyst, the reflections obtained at 2θ = 63°, 75.3° and 79.4° are attributed to cubic NiO phase (JCPDS 01-73-1519). Actually, it is hard to identify the nickel oxide in the catalysts because its reflexes coincide with those of the tetragonal phase of zirconia [10]. The reflections observed at 2θ = 50.2°, 59.9°, 62.8° and 75.2° are ascribed to monoclinic ZrO2 (JCPDS: 00-007-0343). The signals found at 2θ = 60.5° may be assigned to γ-Al2O3 (JCPDS: 00-029-0063). Only in case of Ni–Co-800, extra peaks detected at 2θ *=* 65.53° and 66.4° correspond to the formation of NiAl2O4 spinel phase. It is noteworthy that for both mono- and bimetallic catalysts the increase of the calcination temperature increases the reflex intensity which may be attributed to the formation of larger crystal size.

**Figure 1. Figure 1.**  XRD patterns for fresh Ni and/or Co-based catalysts calcined at 550 and 800 XRD patterns for fresh Ni and/or Co-based catalysts calcined at 550 and 800 °C. ◦C.

With regard to the catalysts calcined at 800 ◦C, diffraction signals of sharp intensity observed; those represent more crystalline phases. Furthermore, the reflex intensity of the bimetallic catalyst is

higher than for the monometallic catalysts. For the Ni-800 catalyst, the reflections obtained at 2θ = 63◦ , 75.3◦ and 79.4◦ are attributed to cubic NiO phase (JCPDS 01-73-1519). Actually, it is hard to identify the nickel oxide in the catalysts because its reflexes coincide with those of the tetragonal phase of zirconia [10]. The reflections observed at 2θ = 50.2◦ , 59.9◦ , 62.8◦ and 75.2◦ are ascribed to monoclinic ZrO<sup>2</sup> (JCPDS: 00-007-0343). The signals found at 2θ = 60.5◦ may be assigned to γ-Al2O<sup>3</sup> (JCPDS: 00-029-0063). Only in case of Ni–Co-800, extra peaks detected at 2θ *=* 65.53◦ and 66.4◦ correspond to the formation of NiAl2O<sup>4</sup> spinel phase. It is noteworthy that for both mono- and bimetallic catalysts the increase of the calcination temperature increases the reflex intensity which may be attributed to the formation of larger crystal size. *Processes* **2019**, *7*, 141 6 of 15

#### *3.2. Textural Properties 3.2. Textural Properties*

The surface texture was assessed by using the nitrogen adsorption–desorption isotherms. Figure 2 illustrates the adsorption isotherms of the fresh catalysts calcined at 550 ◦C and 800 ◦C, while BET surface area, average pore diameter and pore volume are tabulated in Table 1. As per the IUPAC classification, catalysts demonstrate Type II isotherms. In Figure 2a it can be found that the BET surface area of Co-550 is highest and that of Ni-550 is lowest, while the surface area of Ni–Co-550 takes an intermediate value. On the other hand, the catalysts calcined at 800 ◦C (Figure 2b) exhibited a similar trend (Co > Ni–Co > Ni), however, the surface area of these catalysts was lower compared to those calcined at 550 ◦C, which may be due to the sintering. In a previous study, we showed that the addition of ZrO<sup>2</sup> to Al2O<sup>3</sup> increased the surface area. For instance, the surface area of pure supported Ni/ZrO<sup>2</sup> and Ni/Al2O<sup>3</sup> catalysts were 3.1 m2/g and 122.0 m2/g, respectively [23]. However, the surface area of binary supported Ni/Al2O<sup>3</sup> + ZrO<sup>2</sup> catalyst had risen to 212 m2/g. The surface texture was assessed by using the nitrogen adsorption–desorption isotherms. Figure 2 illustrates the adsorption isotherms of the fresh catalysts calcined at 550 °C and 800 °C, while BET surface area, average pore diameter and pore volume are tabulated in Table 1. As per the IUPAC classification, catalysts demonstrate Type II isotherms. In Figure 2a it can be found that the BET surface area of Co-550 is highest and that of Ni-550 is lowest, while the surface area of Ni–Co-550 takes an intermediate value. On the other hand, the catalysts calcined at 800 °C (Figure 2b) exhibited a similar trend (Co > Ni–Co > Ni), however, the surface area of these catalysts was lower compared to those calcined at 550 °C, which may be due to the sintering. In a previous study, we showed that the addition of ZrO2 to Al2O3 increased the surface area. For instance, the surface area of pure supported Ni/ZrO2 and Ni/Al2O3 catalysts were 3.1 m2/g and 122.0 m2/g, respectively [23]. However, the surface area of binary supported Ni/Al2O3 + ZrO2 catalyst had risen to 212 m2/g.

**Figure 2.** N2 adsorption-desorption isotherms for fresh Ni and/or Co-based catalysts (**a**) calcined at 550 °C and (**b**) calcined at 800 °C. **Figure 2.** N<sup>2</sup> adsorption-desorption isotherms for fresh Ni and/or Co-based catalysts (**a**) calcined at 550 ◦C and (**b**) calcined at 800 ◦C.

**Table 1.** BET surface area, pore volume (P.V.) and pore diameter (P.D.) of fresh Ni and/or Co-based catalysts calcined at 550 °C and 800 °C. **Table 1.** BET surface area, pore volume (P.V.) and pore diameter (P.D.) of fresh Ni and/or Co-based catalysts calcined at 550 ◦C and 800 ◦C.

