*3.2. Catalytic Stability*

The deposition of carbon over the surface of a catalyst is crucial to investigate in examining DRM reactions. The stability of a catalyst is evaluated by testing its activity for a prolonged time to estimate the loss in activity. Figure 2 presents CH4 and CO2 conversions as a function of time for 9 h time on stream at fixed reaction temperature of 700 ◦C. The decrease in both CH4 and CO2 conversions was obvious for all of the tested catalysts (NA, NY and NH) and it was associated with the deactivation of all the catalysts. Interestingly despite that all catalysts showed deactivation, the extent of deactivation was not the same for all the catalysts. From quantitative results in Table 1, the NH catalyst showed negligible loss in CH4 and CO2 conversions as compared with NA (2.3 and 5.2% respectively) and NY (5 and 1.1% respectively) catalysts. The deactivation factor as function of extent of deactivation or catalyst stability was found to be the lowest (2%) in the NH catalyst.

**Figure 2.** (**a**) CH4 and (**b**) CO2 conversion versus time on stream for Ni supported on alumina (NA), Ni supported on Y-zeolite (NY), and Ni supported on H-ZSM-5 (NH) catalysts.

**Table 1.** Performance of NA, NY, and NH catalysts in CO2 reforming of methane after 9 h.


a Deactivation Factor (%DF) = [([CH4]initial − [CH4]final)/[CH4]initial] × 100; b Determined by TGA; c Before reaction; d After reaction.

Thermodynamically two of the side reactions including methane decomposition and Boudouard reaction (Equation (2)) are mainly forming carbon over the catalyst surface under reaction conditions [30].

$$\text{2CO} \rightleftharpoons \text{CO}\_2 + \text{C} \tag{2}$$

In order to investigate whether carbon formation was the main source of deactivation, thermogravimetric analysis (TGA) was used to quantify the carbon deposition. It can be clearly seen from TGA results in Table 1 that the NH catalyst exhibited weight loss of 3.7 wt% much lesser than NY (14.7 wt%) and NA (8.5 wt%). Hence, it can be concluded that the NH catalyst proves to show long term stability and lowest amount of carbon deposition. These findings are further discussed based on the characterizations of the catalysts before and after reaction as described in the following paragraphs.

The stability of the best catalyst (NH) in the current study is compared with the catalysts supported on zeolite or oxide supports (Table 2) and deactivation factor was used as a measure of stability. The deactivation factor over 5%Ni-ZSM [32] catalyst for similar reaction duration of 9 h was much higher (24.2%) than the NH catalyst (2%). Even Ni and Co based bimetallic catalyst supported on ZSM5 (1Co2Ni-ZSM5) [28] showed deactivation factor over 6 times higher (13.6%) than the NH catalyst. These catalysts were even tested at higher temperature (800 ◦C) as compared with the current NH catalyst (tested at 700 ◦C). Comparing the NH catalyst with alumina (2.25%Sr-10%Co/Al2O3) [33], ceria (Ni-Ce Imp) [34], and MCM41 [35] supported catalysts, the later catalysts showed more deactivation even with shorter reaction duration. Hence it can be concluded that the NH catalyst presents potentially more stable performance than other zeolite supported catalysts as well as catalysts with higher metal contents or employed promoters.


**Table 2.** Comparison of current work with the previously reported work.
