**2.2 Catalytic decomposition of biogas**

Due to overdependence on fossil-based fuels and increasing environmental concerns, the resources of renewable energy, in particular biogas, have gained massive attention around the world as a substitute for traditional fossil fuels. Biogas is obtained from the process of the anaerobic digestion of organic compounds. Methane (40–70%) and carbon dioxide (30–60%) are the primary compounds of biogas [46]. One of its most common applications is the direct combustion for energy recovery through co-generation plants that produce electricity and heat. Nevertheless, the use of renewable sources of methane like the one contained in biogas (biomethane) for different applications like the production of hydrogen is a more interesting option than the use of fossil methane [47].

In this context, the catalytic decomposition of methane (CDM) (Eq. (1)) is being studied as an alternative to steam reforming of methane (SRM) to produce CO2-free hydrogen. The CDM in a single step produces a mixture of hydrogen and unconverted methane, which can be directly used as fuel in internal combustion engines or, even directly used to power a fuel cell [47].

$$\text{CH}\_4 \rightarrow \text{C} + 2\text{H}\_2 \quad , \quad \Delta H\_{298K}^{\circ} = 75.0 \text{ kJ mol}^{-1} \tag{1}$$

The catalysts traditionally used in the CDM consist of transition metals belonging to group VIII (Ni, Fe, Co) supported over different metal oxides such as Al2O3, MgO, La2O3, and CeO2 [48, 49]. These catalysts are characterized by promoting the formation of carbon nanostructures (carbon nanofibers or carbon nanotubes) varying their textural and structural properties as a function of the catalyst composition and the operational conditions [50]. These carbon nanostructures have very

interesting properties for their use in applications where thermal and electrical conductivity of materials is a key factor. However, one of the problems of the CDM is the deactivation over time of the catalysts due to carbon deposition that encapsulates the metal particles disabling their active sites [51].

Co-feeding with CH4 different oxidizing agents such as H2, H2O or CO2, can increase the life of the catalyst. Co-feeding with H2, inhibits the deactivation of the catalyst at the expense of a desired product, which reduces the efficiency of the process while the use of CO2 as Co-feeding induces Boudouard reaction (Eq. (2)) thereby resulting in gasification of graphitic carbon produced during the CDM reaction.

$$\text{2CO} \rightarrow \text{C} + \text{CO}\_2 \quad , \quad \Delta H\_{298K}^{\circ} = \text{172.0 kJ mol}^{-1} \tag{2}$$

The use of CO2 in the CDM process has been studied by two approaches: some authors have suggested a cyclical process consisting of a methane decomposition step followed by another stage of gasification of the deposited carbon with CO2. Other authors have studied the decomposition of mixtures CH4:CO2 in conditions that favor the formation of nanostructured carbon. Nagayasu et al. [52] observed a slow deactivation of a Ni based catalyst to be used in the CDM in the presence of CO2. They also noted an increase in carbon accumulation capacity in the form of nanotubes by increasing the partial pressure of CO2 co-fed along with that of CH4.

Asai et al. [53] confirmed the inhibition of the deactivation of the catalyst studied in the decomposition of methane in the presence of CO2, suggesting a mechanism based on the gasification of graphitic carbon layers that encapsulate the catalyst particles, allowing the formation of carbon in the form of nanotubes. Indeed, co-feeding of CH4 and CO2, which are the main components of biogas as previously mentioned, modifies the reaction mechanism of methane decomposition into carbon and H2, to a process called dry reforming, which produces a mixture of H2 and CO. This is a highly endothermic reaction that takes place by way of a catalyst in the temperature range between 600 and 800°C, producing syngas with a molar ratio 1:1 [54].

This syngas can be used in multiple applications such as fuel for solid oxide fuel cells or Fischer-Tropsch synthesis to produce environmental friendly liquid fuels, when using a renewable source such as biogas [55]. If the aim is to produce H2, then a water gas shift reaction followed by CO2-H2 separation should be accomplished. The practical implementation of the dry reforming of methane (DRM) faces many key challenges, which also apply to the biogas decomposition, and one of the most important is the deactivation of the catalysts due to the formation of carbon during the reactions of CH4 decomposition and CO2 disproportionation [56]. Also, Edwards and Maitra [57] reported that it is convenient to work at high temperatures and low ratios of CH4:CO2 (<1), to minimize carbon formation from a thermodynamic point of view. However, from the industrial point of view it would be much more desirable to work at moderate temperatures and CH4:CO2 ratios close to one, despite these are conditions under which carbon formation is thermodynamically favored.

Another issue that should be addressed is the high sulfur content of the biogas. This can provoke severe metal catalysts deactivation, therefore an exhaustive desulphurization of the biogas fed to the catalytic decomposition of biogas (CDB) reactor would be required when using a real biogas. The most commonly used methods for hydrogen sulphide removal can be found in [58]. The more active catalysts that promote the lower carbon deposition are precious metals, but its high price provokes that the most widely used catalysts for dry reforming are based on Ni, Co and Fe [59], which are the same catalysts traditionally used in the CDM.

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

Since the typical CH4:CO2 ratio in biogas composition is higher than 1 (CH4 concentration in biogas can be as high as 70% depending on its origin), avoiding carbon deposition in the biogas decomposition reaction is not a task easy to accomplish. Thus, as previously mentioned, the presence of CO2 along with the selection of optimum operating conditions for the deposition of carbon could prevent the rapid deactivation of the catalyst, resulting in a new biogas recovery process in which a gas with a suitable composition for its use in an internal combustion engine and carbon nanofibers (CNF) with multiple applications in sectors such as energy and transport are obtained. Direct decomposition of a gas simulating a typical biogas composition by means of metal catalysts under conditions that are favorable for carbon deposition has been studied by Muradov and Smith [60]. The problem associated to carbon deposition through decomposition of CH4:CO2 mixtures with ratio >1 was solved by adding small amounts of steam, prolonging the catalyst life. Some previous works by De Llobet et al. [61] focused on a study of CDB, conducted at moderate temperatures and using typical catalysts previously used in the CDM, promoting the formation of nanostructured carbon and syngas. As per their report, the Ni/Al2O3 catalyst exhibited high activity as well as stability, allowing them to obtain high CH4 conversion together with the high-yield production of fishbonelike nanocarbon.

## **2.3 Chemistry of carbon dioxide**

**Figure 3** illustrates a key aspect of the thermodynamics of any possible CO2 conversion. The figure also demonstrates the free emission of CO2 from Gibbs and its associated substances. It is evident that CO2 is an extremely stable molecule; it therefore requires significant energy input, optimized reaction conditions and (almost invariably) active catalysts for any chemical conversion of CO2 into a carbonaceous fuel.

**Figure 3.**

*Gibbs free energies of formation of selected chemicals (adapted from Ref. [62]).*

However, it is important to note that chemical reactions (conversions) arise due to the difference in the Gibbs free energy between the reactants and products of a chemical reaction (under certain conditions). This is illustrated by the Gibbs-Helmholtz relationship (Eq. (3)):

$$
\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ \tag{3}
$$

Therefore, the comparative stability of the ultimate response products must be taken into consideration in the effort to use CO2 as a chemical feedstock compared to the use of reactants. Both terms (Δ*H*<sup>o</sup> and *T*Δ*S*<sup>o</sup> ) of the Gibbs free energy are not favorable for the conversion of CO2 to other molecules [63]. Since the carbonoxygen bonds are relatively strong, substantial energy input is necessary for their cleavage, in terms of carbon reduction. Similarly, the entropy term (*T*Δ*S*<sup>o</sup> ) makes little to no contribution to the thermodynamic driving force for any reaction involving CO2. Most importantly, the enthalpy term, Δ*H*<sup>o</sup> , can be taken as a good initial guide for the assessment of thermodynamic stability and feasibility of any CO2 conversions.

Freund and Roberts [63] highlighted the significant contribution of CO2 surface chemistry. They claimed that any progress in the use of CO2 as a useful reactant can be achieved in relation to fuel synthesis by using novel catalytic chemistry wisely. They attempted to illustrate that the greatest potential impact lies in this area of material chemistry, physics and engineering. These researchers also pointed out that a positive change in free energy should not be considered as a reason enough not to pursue potentially useful CO2 reactions. This is because, Δ*G*<sup>o</sup> only provides information as to the yield of products at equilibrium through the relationship (Eq. (4)), and the kinetics of such a process is indeed favorable.

$$
\Delta G^\circ = -\text{RT}\ln K\tag{4}
$$

Since the kinetics are favorable, CO2 decrease to CO (a key step in all conversion reactions), the primary step in all transformation responses, may also be feasible on metal surfaces or other catalytic materials, for instance on nano- and mesoporous metal particles [62]. Presently, a large number of industrial-scale chemical manufacturing processes worldwide operate on the basis of strong endothermic chemicals. The SRM to yield syngas and hydrogen is a classic example (Eq. (5)):

$$\text{CH}\_4 + \text{H}\_2\text{O} \rightarrow \text{CO} + \text{\textdegree H}\_2 \quad , \quad \Delta H\_{298\text{K}}^\circ = +206 \text{ kJ mol}^{-1} \tag{5}$$

It is important to emphasize that the above-mentioned, highly endothermic reaction is used to produce large quantities of 'merchant hydrogen' in the gas, food and fertilizer industries worldwide. The corresponding DRM reflects the important reaction of CO2 with hydrocarbons, which will be central to our idea of converting CO2 into flue gases to produce chemical fuels (Eq. (6)):

$$2\text{CH}\_4 + \text{CO}\_2 \rightarrow 2\text{CO} + 2\text{H}\_2 \quad , \quad \Delta H\_{298K}^\circ = +247.3 \text{ kJ mol}^{-1} \tag{6}$$

The energy input for DRM requires about 20% more energy input than the SRM, but there is definitely no restricted additional energy cost for this chemical reaction. It is important that these two reactions lead to syngas with different H2:CO molar ratios. For the final production of liquid fuels, both are useful for the formation of horns.

**Figure 4** shows the enthalpy of the chemical reactions of the CO2 conversion. This means that CO2 is thermodynamically much easier to use as a co-reactant, usually with a higher (i.e. less negative) Gibbs free energy, such as H2 or CH4. These hydrogen-containing energy carriers give their internal chemical energy to promote the conversion of CO2. Therefore, the heat of reaction (enthalpy of reaction) from CO2 to CO production is important and obvious as the individual reactive and CO2 energy as a key factor. Compare the thermal decomposition energies of CO2 (Eqs. (7) and (8)).

$$\text{CO}\_2 \rightarrow \text{CO} + \text{\textquotedblleft}\_2\text{O}\_2 \quad , \quad \Delta H^\circ\_{298\text{K}} = +293 \text{ kJ mol}^{-1} \tag{7}$$

With that of the reaction of CO2 with H2 (Eq. (8))

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

$$\text{CO}\_2 + \text{H}\_2 \to \text{CO} + \text{H}\_2\text{O} \quad , \quad \Delta H\_{298\text{K}}^{"\text{-}} = 41.2 \text{ kJ mol}^{-1} \tag{8}$$

This aspect may be further illustrated by the process of 'oxyforming', whereby the amount of oxygen in the dry reforming reaction is increased deliberately. In doing so, the reaction enthalpy of reaction is significantly reduced (Eqs. (9) and (10)):

$$\text{\textbullet CH}\_4 + \text{O}\_2 + \text{CO}\_2 \rightarrow 4\text{CO} + 6\text{H}\_2 \quad , \quad \Delta H\_{298K}^{\circ} = +165.8 \text{ kJ mol}^{-1} \tag{9}$$

$$\text{SCH}\_4 + 2\text{O}\_2 + \text{CO}\_2 \rightarrow 6\text{CO} + 10\text{H}\_2 \quad , \quad \Delta H\_{298\text{K}}^\circ = +104.6 \text{ kJ mol}^{-1} \tag{10}$$

The fundamental material challenge in this area lies in the fact that, generally, the reaction between CO2 and H2 occurs at high temperatures on multi-component heterogeneous catalysts [64].

#### **Figure 4.**

*The enthalpy of reaction for syngas production and Fischer-Tropsch (FT) synthesis of methanol and dimethyl ether (adapted from Ref. [62]).*

#### **2.4 Syngas**

Syngas is a blend of carbon monoxide and coal with a tiny quantity of methane and carbon dioxide. In the ever changing energy landscape, it is not only versatile, but also an increasingly important commodity. There are various carbon sources that happen through gasification or catalytic reformation for the manufacturing of syngas. Coal, natural gas (mainly methane), petroleum, and biomass could be the sources of carbon. The primary technical problem with fossil fuel syngas manufacturing is the complicated purification and conditioning procedures of syngas. The main reasons why the world has become more interested in the producing of biomass-derived syngas are therefore to decrease over-dependence on fossil fuels, to impose stricter CO2 emission standards and to verify the accessibility of resources. Roddy [65] claimed that biomass could originate from industrial, domestic, agricultural and urban waste sources as a feedstock for syngas production. The use of biomass or waste as the raw material for syngas manufacturing is theoretically two-pronged: the generation of clean energy and an effective way to reduce waste as reported by Markets and Markets [66], a compound annual growth rate (CAGR) of 8.7% is anticipated to achieve 117,400 MW (Megawatts) heat in 2018. Boerrigter and Rauch [67] estimated the future market for syngas to increase to 50,000 petajoules (PJ) per annum, equivalent to 13.9 � <sup>10</sup><sup>9</sup> MWh per annum in, 2040. This amounts to replacing an average 30% fossil fuel usage is 10% of the complete world power consumption. They also projected that syngas will be used

primarily in gas-to-liquid (GTL) procedures, with 49% for gas-to-product (GTP) procedures and 39% for renewable gas and hydrocarbon manufacturing. In, 1993, Shell Malaysia built the world's first commercial GTL plant in Bintulu, Sarawak. Since, 2003, as many as for 14,700 barrels of high-quality GTL products have been produced per day. This is clearly an upgrade in the production from its original capacity of 12,500 barrels per day. As reported by the Borneo Post, Shell's GTL plant plans to invest RM (Malaysian ringgit) 48.36 million to rejuvenate its plant in Bintulu in, 2015. The world's largest GTL plant is located in Qatar, with a production capacity of 140,000 barrels of product per day.

In short, the development of the market for syngas is accelerating, the important increase in syngas consumption is due to its use as an energy precursor. The presence of CO, H2 and CH4 gases, which possess certain heating value, makes it highly in demand. Syngas also includes approximately 50% of natural gas's power density. Subramani et al. [68] reported that 1 kg of H2 contains the same amount of energy as 2.6 kg of CH4, which is equivalent to 3.1 kg of gasoline. H2 is used at low temperatures because of its elevated energy content; fuel cells are used to produce electricity, power cars or even in the synthesis of Fischer – Tropsch. In addition to serving as an energy carrier, it has traditionally been used as a feedstock for the mass production of significant chemicals, such as methanol, ammonia or fertilizers.
