*4.3.2 Catalysts developed for CO2 reforming*

There are focuses on the development of DRM catalysts for catalysts with the following features: greater activity and greater stability towards coke formation, sintering, the formation of inactive chemical species and metal oxidation [115]. The catalytic efficiency could be improved by changing the catalyst's active sites by adding supports and promoters during catalyst preparing to increase conversion and selectivity [116]. **Table 2** shows several catalysts that have been developed recently, including Ni-based catalysts applied to the DRM reaction.

#### **4.4 Catalyst deactivation**

Deactivation of catalyst relates to loss of activity of catalyst during the response. It is the significant drawback of metal-based catalysts, as it not only creates product reductions that affect the response rate, but also costs industry millions of cash to replace the catalyst. Catalyst deactivation basically relates to three elements, according to Bartholomew and Farrauto [122] chemical, mechanical and thermal. Catalysts for metal reforming are frequently deactivated by coking, poisoning, fouling and sintering. **Table 3** describes the mechanisms of catalyst deactivation.

#### *4.4.1 Poisoning*

Poisoning relates to the powerful adsorption in the feed of chemical substances such as impurities. Poisoning of catalysts may be reversible (temporary) or irreversible (permanent) [122]. The catalyst may be retrieved by air oxidation or


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

#### **Table 2.**

*Catalysts developed for the DRM reaction.*

steaming to wash its surface for reversible toxicity. For irreversible poisoning, however, the toxins cannot be removed, so replacing current catalysts with a fresh batch is essential. Sulfur species such as hydrogen sulfide are common poisons in all catalytic processes with reduced metals as the active site. S-poisoning, as in procedures of F-T synthesis and steam reform, is always a disaster.

In 2011, Bartholomew and Farrauto illustrated the mechanism of sulfur poisoning [122]. Firstly, the S atom adsorbs or blocks the reaction or active sites of the catalyst physically (geometric effect). Then, the S atom alters the metal atoms electronically. The metal ions subsequently alter their adsorbability or their capacity to dissociate with reactant molecules like H2 and CO. The S atom also alters the surface area and creates major catalytic characteristics alterations. This hinders the accessibility of adsorbed reactants to each other and thus slows down the adsorbed reactants'surface propagation. **Table 4** describes the typical poisons of industrial catalysts for different types of reaction. The avoidance of sulfur toxicity and sulfur strength can be improved by modifying the structure of the catalysts by


#### **Table 3.**

*Mechanisms of catalyst deactivation.*


#### **Table 4.**

*Poisons of the industrial catalysts.*

incorporating certain additives, such as molybdenum and boron, which adsorb sulfur selectively or change the response circumstances. According to Bartholomew and Farrauto [122], reduction in the temperature of steam reforming over Ni/Al2O3 catalysts from 800 to 500°C will decrease the strength of S adsorption, hence reducing sulfur poisoning from 5 ppm to only 0.01 ppm.

### *4.4.2 Sintering effect (thermal degradation)*

Bartholomew and Farrauto [122], Christensen et al. [123], and Argyle and Bartholomew [124] describe the sintering of a heterogeneous catalyst as the loss of the catalytic layer, which is generally irreversible owing to the development of crystallite either on the supporting material or after thermal degradation in the active stage. Bartholomew and Farrauto [122] revealed two significant sintering parameters. The first is the sintering of temperature, including above the catalyst atmospheric temperature. The next is the sintering rate, which is impacted by the support structure and morphology, the metal particle size distribution, and the support's phase transition. These two catalyst sintering processes are crystallite migration (coalescence) and nuclear or vapor transport (ripening of Ostwald). Christensen et al. [123] outlined that crystallite migration involves entire crystallite migration followed by collision and coalescence. In the meantime, Argyle and Bartholomew [124] addressed that Ostwald ripening relates to the migration of

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

metal transport species emitted from one crystallite over the assistance or through the gas phase and caught by another crystallite. The author also stated that the sintering method is due to elevated temperatures and that owing to the presence of water vapor there is an increase in the sintering speed. Due to sintering impacts, **Figure 7** demonstrates the conceptual models of crystallite development.

Lif and Skoglundh [125] found that the co-impregnation of nickel catalysts with the oxides of alkali metals, alkaline earths or lanthanides suppresses the sintering effect. In addition, it was also shown that the catalyst preparation sequential impregnation technique improves the catalyst's stability towards sintering. To conclude, it is extremely desirable that it possesses the following characteristics for the growth of a fresh catalyst: heat resistance, coking resistance and stability in syngas manufacturing.

#### **Figure 7.**

*Conceptual models for crystallite growth due to sintering by (A) Ostwald ripening and (B) crystallite migration (adapted from Ref. [124]).*
