*4.4.3 Carbon deposition*

Fouling is a physical (mechanical) deactivation that causes the loss of catalyst activity owing to coke deposition that blocks the reactive sites. Steam reforming utilizes catalysts primarily based on Ni. Coke deposition is a prevalent cause of deactivation of Ni-based catalysts. Temperature-programmed hydrogenation (TPH) and Temperature-programmed oxidation (TPO) methods are used to analyze carbon deposition on the used catalyst. The methods of TPH and TPO are used to define the features of the kinds of carbon species created during reaction on the catalysts [126]. According to Bartholomew and Farrauto [122], the types of carbon that may be formed during reforming are *Cα*, *Cβ*, *CV*, *C<sup>γ</sup>* and *CC* (see **Table 5**).

CH4 cracking (Eq. (1)) and CO disproportionation are the two primary reasons for coke deposition during DRM (Eq. (6)). There are three possible carbon fouling mechanisms for the metal catalyst. The first mechanism is carbon, which deposits reactive sites on the catalyst and impedes binding of the reactants to the active locations. The carbon would otherwise encapsulate the catalyst's reactive site and deactivate the catalysts. Another deactivation option resides in the coke being deposited in the catalyst pores, thereby stopping the reactants from crystallizing on it. The third mechanism involves carbon-forming needle-like filaments in the active site of the nickel catalyst, to some extent breaking the catalysts. **Figure 8** shows the conceptual model of the mechanisms of carbon fouling of a catalyst.

Quincoces et al. [135] used DRM catalyst Ni/γ-Al2O3. They found that there were no rises in carbon deposition while the molar ratio of the reactants, CH4/CO2, was maintained in unity. This finding shows that by changing the response circumstances, such as the molar ratio of reactant feed, carbon deposition can be

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#### **Table 5.**

*Forms and reactivity of carbon formed by decomposition of CO on Ni.*

#### **Figure 8.**

*Conceptual models of fouling, crystalline encapsulation and pore plugging of a supported metal catalyst (adapted from Ref [124]).*

minimized. In their research, they discovered that a filamentous or whisker-like morphology was shown by the carbon deposit on Ni/γ-Al2O3. This finding is comparable to Kępiński et al. [136] reporting. Meanwhile, on a backed metal catalyst, Toebes et al. [137] recorded carbon formation with metal crystallites in addition to carbon filaments. The growth of carbon filaments has pushed the metal crystallites from the surface of the catalyst support.

Ito et al. [138] also proposed that CO2 could reduce the impacts of the fouling system. While the increasing carbon filaments remove the Ni metal, the introduced CO2 responds to CO through a reverse-Boudouard response with the carbon

*Catalysts for the Simultaneous Production of Syngas and Carbon Nanofilaments… DOI: http://dx.doi.org/10.5772/intechopen.101320*

whiskers. One of the findings of their study was that after the removal of the carbon whisker, there is a decrease in bulk Ni. This renders the catalyst to be inactive for carbon deposition. However, there is an increase in the reforming activity of CH4, which is due to the newly exposed Ni active sites from the bulk Ni.

Cheng et al. [139] report a reduction in the Brunauer-Emmett-Teller (BET) surface area and the amount of pore used carbon catalyst. As a result of this phenomenon, catalyst activity is lost. Wagner et al. [140] noted that a vapor reforming catalyst's acidity is proportional to its coke formation tendency. They also asserted that using basic support or basic mixed oxide support named K, the coking strength of the reforming catalysts could be improved. Li et al. [141] and Zanganeh et al. [142] also endorsed this argument, whereby nickel catalyst deactivation can be weakened if the nickel is backed by a strong Lewis base oxide like MgO, CaO, SrO or BaO.

Subsequently, the present research project introduces DRM to investigate the level of resistance of the catalyst towards carbon formation. Zanganeh et al. [142] suggested that an increase in the CO2/CH4 ratio during DRM and increasing the temperature to a high level may minimize carbon formation thermodynamically.

Ito et al. [138] also agreed that the increased CO2-to-CH4 feed ratio would eliminate the CH4 decomposition reaction. Koo et al. [143] found that introducing less than 1wt percent of Mg into the Ni catalyst would enhance their coking strength. Adding promoter like Mo could therefore allay the coke formation phenomenon on the Ni catalyst. Another proposal to reduce the carbon deposition of a catalyst with a small surface area is to reduce the Ni load of the assistance. A CO2/CH4 molar ratio of more than 3.0 should be used to prevent the boudouard reaction.
