The solids NiO/ZnO (ZnO (Kadox) were prepared by impregnation of ZnO support with x% Ni(NO₃)₂, 6H₂O (1 M) (Merck) (x = 4 and 10%) [14] . The solution was stirred during 2 h, and evaporated at 80˚C under agitation. The solid was dried overnight, calcined at 300˚C (2 h) and 500˚C (6 h) at a heating rate of 4˚C/mn in air flow (1.2 L∙h⁻¹). The catalysts were sieved to have a particle diameter less than 0.16 mm.

The solids were analyzed by atomic absorption with a Perkin-Elmer 1100 B spectrometer. The specific surface areas were determined by the BET method using an analyzer of surface type (Coultronics 2100E). The crystalline structures were identified by X-ray diffraction (XRD) using a diffractometer (q/2q CGF) provided with a proportional detector and using Kα molybdenum anticathode (λ = 0.70930 Å). The patterns were obtained for 2θ values ranging from 20˚ to 70˚ with a step size of 0.02˚. The average crystallite size of the particles was calculated from the formula (L = 0.9 λ/βcosθ), where β is the width of the full width at half-height. The morphology of composite particles was observed By SEM (JEOL JSM-118 6360-LV). TPR was conducted on a U-shaped quartz tube embedded in a programmable furnace. 100 mg of the catalyst was pre-treated under pure He flow at 500˚C for 1 h. The reducibility of the catalyst 4% Ni/ZnO at (TPR) was realized under hydrogen diluted in argon (5%) with a heating rate of 5˚C/min in the range (25˚C - 600˚C), the TPR patterns were obtained by using a recorder connected to a GC equipped with TCD, the H₂ consumption is recorded during the test and quantified after calibration. and then reduced under a gas mixture flow (5% H₂, 95% Ar, 50 mL/min)

The catalyst was introduced in a fixed-bed quartz reactor placed within a tubular furnace, equipped with a programmed controller under atmospheric pressure. The bed temperature was monitored by a chromel-alumel thermocouple. 100 mg of catalyst were activated in situ under H₂ atmosphere at 500˚C/12h and tested between 450˚C and 600˚C. The reactant gas mixture (H₂O/CH₄ = 3.3) was achieved by flowing CH₄ (10%)/Ar through a water saturator working at 65˚C and then introduced in the reactor at a flow rate of 1.2 L/h. Before each run, the effluent passed through a water-trap at 0˚C to remove water. The gas composition was analyzed by TCD chromatograph (Hewlett-Packard 5730) containing two 2 m carbosieve B columns (1/8 inch, 100 - 200 mesh). The conversion rates and the selectivity of CO₂ and CO were calculated from the following formulas:

Total rate of transformation:

Rate of transformation in compound “i”:

Selectivity in compound “i”:

with:

i: CO, CO₂.

n_i: number of moles of compound “i”.

: number of moles of methane.

The studied catalytic systems are summarized in Table 1 whereas the XRD patterns after calcination are illustrated in Figure 1.

The results show that the nominal composition of the metal is very close to the theoretical one for the catalyst 4%. On the contrary, the nickel percentage remains weak compared to the nominal values (7% instead 10%).

Before reaction, the presence of NiO and the support are observed for all the catalysts. The results of XRD (Table 1 and Figure 1), before reaction, show ZnO zincite in all catalysts as well as NiO. This result means that for the three catalysts, NiO is dispersed uniformly on the surface.

Even though the processes are related to extrinsic factors, a detailed understanding of the physical properties is needed for a high-quality catalyst. As expected, the XRD patterns of the preparation is mixed phase (Figure 1), the powder exhibits a good crystallinity with strong peaks corresponding to cubic NiO (JCPDS card N˚ 47 - 1049) and hexagonal Wurtzite ZnO (JCPDS card N˚ 36 - 1451) structures. NiO crystallizes in the rocksalt structure and the crystallite size of NiO, estimated from the full width at half maximum, average 30 nm. Nanoplates with smooth surface and high crystallinity can be observed from Figure 2.

^a: Evaluated from the relation (L = 0.9 λ/βcosθ) of the most intense XRD peak.

Figure 2 shows the NiO/ZnO hetero-system prepared by impregnation method; one observes that NiO nanoparticles are well-mixed with ZnO distributed uniformly.

The TPR profiles of the bulk nickel and NiO/ZnO (Figure 3) are totally different. The reduction of NiO starts below 300˚C and is completed rapidly while temperature above 460˚C is required to reduce NiO/ZnO. The hetero-system presents one reduction peak at high temperature (460˚C), probably due to the reduction of NiO with more or less strong interaction with the support ZnO. The reduction of NiO interacts weakly with the support occurs at 450˚C [14] [15] and the calculated reduction rate reach 99.7%. Ni/ZnO is well dispersed onto the support ZnO with no chemical bond. In addition, NiO exists with small crystallites uniformly spread onto the ZnO surface, thus leading to their facile reduction under H₂/Ar flow.

The TPR profiles for the catalyst Ni/ZnO, exhibit one narrow peak with strong intensity at 460˚C (Figure 3), in agreement with the literature. According to Jasik et al. [16] , the narrow peak indicates that that the solid has a better dispersion with a good homogeneity of the active phase NiO over the ZnO support. However, the high reduction temperature implies a difficult reducibility. The temperature peak in the range (300˚C - 500˚C) has been assigned to NiO reduction [17] ; the NiO reduction peak around 350˚C agrees with the literature [18] .