**4.1 Kinetics and mechanistics of dry reforming approach**

Studies of DRM's kinetics and mechanisms were conducted to determine an appropriate reaction rate model, either empirically or on the basis of a theoretical response mechanism to best suit the relevant experimental information and possibly describe the response rate and the chemical process. This understanding can further optimize the design and layout of the chemical system catalysts (the reactor), which can further improve DRM's overall development with more cost-effective technology [89]. Although, from a mechanistic point of perspective, steam reforming has received much attention, there has been a resurgence of interest in dry reforming over the previous centuries. A series of catalysts for DRM were researched as a consequence. This has resulted in a number of mechanistic measures for DRM being published in the literature. The DRM reaction mechanism was explored by Aldana et al. [90] over a Ni-based catalyst.

Aldana et al. reported that H2 dissociates on Ni0 locations while carbon dioxide is activated on ceria-zirconia assistance to generate carbonates that can be hydrogenated into formats and then into methoxy species. This mechanism includes weak fundamental support sites for carbon dioxide adsorption and includes a stable interface between metal and support. Compared to Ni-silica, which activates both carbon dioxide and hydrogen on Ni<sup>0</sup> particles, these characteristics lead in much better operations of these catalysts [90]. This mechanism is also supported by Pan et al. [91]. Meanwhile, Ayodele et al. [92] conducted a DFT analysis of the DRM over Ru nanoparticles supported on TiO2 (101).

#### **4.2 Influence of process variables on reaction rates**

Extensive research was carried out to study the impacts of altering process variables on catalyst performance for the DRM reaction. This inquiry is essential as various process factors may result in variable catalyst performance [93]. The notion of activation energy should be considered as it will determine the response rate.

**Table 1** tabulates the activation energy (*Ea*) values of CH4 and CO2 obtained from different types of Ni-based catalysts in DRM. For most catalysts, the activation energy of CH4 is higher than that of CO2 since the molecules of CH4 are more stable than those of CO2. Therefore, more energy is required to activate the more stable molecules. Moreover, the basicity of the assistance for the catalyst has resulted to variations in the activation barrier. Kathiraser et al. [93] think the activation energy in DRM is fully dependent on the catalyst's type of catalyst support, promoter and bimetallic interactions.


#### **Table 1.**

*Ea values over several Ni-based catalysts for DRM reaction.*

In the meantime, Cui et al. [100] conducted a thorough study of the DRM mechanism over Ni/α-Al2O3 using steady-state and transient kinetic methods at 550– 750°C temperatures. Their results show that the CH4 dissociation and CO2 conversion *Ea* values could be classified as follows: low (550–575°C), middle (575–650°C) and high (650–750°C). In low and high temperature areas, the response was constant but fluctuated in the region of medium temperature. It is suggested that the dissociation of CH4 into CHx and hydrogen species in the Ni active sites at temperatures above 650°C has attained a level of balance. In addition to the activation energy, it is essential to correctly formulate the suitable catalyst's inherent kinetic models based on basic measures in order to reach a compromise between economic feasibility and process effectiveness. However, this kinetics of reaction is affected by the reactants 'mass transport. When eliminating the impact of mass transport, the conversions observed can be directly ascribed to the catalyst's inherent kinetics.

According to Kathiraser et al. [93], distinct gas hourly space velocities (GHSVs) need to be tested to eliminate internal mass transport resistance. The aim of this experiment is to verify that the conversions have reached a stable value and that a further shift in GHSV does not influence the conversion of reactants. The contact time, which plays a significant part in CO2 and CH4 conversions, is another consideration. When the contact time value is high, CO2 or CH4 conversions stay unaffected. The particle size of the catalyst should be held as small as possible to eliminate inner mass transport resistance, so that a further reduction in size does not impact conversions.

Kim et al. [101], explored the use of a CO2-photoacoustic signal (PAS) to analyze kinetically the DRM reaction on a Ni catalyst supported on Al2O3 and TiO2. They

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

discovered that the reason why mass flow rates low are used is because this method generates heat periodically because when a material absorbs a modulated laser beam, the photoacoustic signal is produced. It is essential to remember the characteristics of kinetic curves that act as the reaction mechanism's blueprints. These include the point of inflection, a brief period of induction or breakpoints. No particular GHSV can be found from all the results to eliminate the impacts of constraints on mass transfer. This indicates that the development of inherent kinetic models is critical in preliminary research.
