**4. Discussion**

Prior to DRM reaction study, as-synthesized Ni based catalysts were characterized to assess their potential performance in the reforming reaction. High specific surface areas in zeolites are known to influence the catalytic performance during DRM [39]. It is noteworthy that despite the higher specific surface area of the study Y-zeolite, as compared with H-ZSM-5 and alumina, the Y-zeolite supported Ni catalyst (NY) exhibited less conversions at lower reaction temperatures (Figure 1). This could be attributed to the zeolite having pores filled with nickel particles and/or the presence of nickel particles entrapped in the pores of zeolite and hence these were not available for DRM reaction [40]. Furthermore, the loss of specific surface area (Table 1) in the spent catalysts also demonstrates the drastic role of carbon deposition during DRM. The XRD diffraction profiles from the as-synthesized catalysts (Figure 3) have helped to identify crystalline phases (nickel oxide, alumina, and zeolites) in the composites. The presence of spinel species such as nickel aluminate was also obvious. Moreover, the deposition of crystalline graphitic carbon was further confirmed by XRD data of spent NA catalyst. The reduction profiles using TPR (Figure 5) demonstrate easier reduction of Ni oxides or higher reducibility of the Y-zeolite supported catalysts (NY) as compared to the alumina (NA) and H-ZSM-5 supported catalysts (NH). The basicity of the composites, assessed in terms of temperature programmed desorption data using carbon dioxide as probe gas, showed that the H-ZSM-5 supported catalyst (NH) exhibited larger CO2 adsorption capacity than the rest of the study catalysts. The pre-reforming characterization results indicated larger specific surface area, higher number of reducible species and relatively lower basicity in the Y-zeolite supported catalyst (NY). These characteristics lead to expect better performance by NY during the reforming reaction however the activity results (Figures 1 and 2) did not follow the prediction. The DRM mechanism requires adsorption of reactants over the active sites of the catalyst in its initial step. Subsequently, they will react leading to products that ultimately will leave the catalyst surface following desorption [30]. CH4 adsorption requires Ni in its metallic form as the active site [40].

The catalytic stability (Figure 2) in terms of both CH4 and CO2 conversions showed loss of activity over time for all catalysts with varying deactivation degrees. The decrease in initial conversions for zeolite supported catalysts can be assigned to loss of Ni active sites residing inside the pores in the zeolite. The CO2 adsorption capacity is also found to influence the catalytic performance and hence more CO2 conversion is evidenced from H-ZSM-5 supported catalyst (NH) as compared to other catalysts in agreemen<sup>t</sup> with CO2-TPD results (Figure 6). Higher CO2 conversion helps to provide an oxidative environment to gasify carbon deposited over the surface of the catalyst and hence this can lead to less deactivation. Thus, the catalyst with the highest CO2 adsorption capacity and CO2 conversion i.e., H-ZSM-5 supported catalyst (NH), has shown to present the lowest deactivation factor: just 2%. The main cause of deactivation was found to be carbon deposition according to TPO (Figure 4) and TGA (Table 1) results. The catalyst system studied in this work demonstrates the role of catalyst's properties such as reduction behavior, CO2 adsorption capacity and basicity of commercial zeolites in comparison with conventional alumina supported catalysts during DRM. A comprehensive study of the surface chemistry of the hierarchical zeolites using spectroscopic techniques and their performance evaluation in relation to commercial zeolites is planned.
