The search for clean and low-cost fuels as alternative for petroleum is a popular research focus in the energy field. The demand of natural gas as an energy source has increased steadily. The high H:C ratio and the absence of heteroatoms make natural gas an attractive feedstock for synthetic fuels and chemicals that can replace those that are typically petroleum-derived. The search for efficient routes to convert methane to other higher added-value products is a challenge for the scientific community. In addition, new fields of oil and gas contain associated CO2 (8–18%), and, in some specific fields, the associated gas encloses a higher CO2 content (79%). In this context, the tri-reforming process combines two of the most problematic greenhouse gases (CH4 and CO2) to generate syngas for the synthesis of clean liquid fuels and valuable chemicals. Developments in tri-reforming processes, which include the new catalysts, are presented in this chapter.
- carbon dioxide
- hydrogen production
Significant efforts are being directed nowadays towards finding alternatives that could restrain the climate change. The consistent rise of CO2 concentration in the atmosphere is known to be significantly detrimental to the environment. Thus, mitigating CO2 is becoming an urgent need.
Current methods involving CO2 mitigation can be broadly divided into two major categories, which involve (1) CO2 capture and sequestration (CCS) and (2) CO2 capture and utilization (CCU). Since the production of fuels/chemicals is an added feature along with mitigation in CO2 valorization-based methods, they could be economically favorable. An energy-intensive CO2 capture step is a common drawback of most CO2 valorization methods that aim to mitigate CO2 from major CO2 emission sources (such as industrial flue gases).
Different methane-rich gas streams can be found, both of natural and of anthropogenic origin. A decrease in fossil fuels and environmental concerns across the globe enforced researchers to work on energy resources like methane, which is the most abundant natural gas on earth .
Therefore, it is of utmost importance to seek for technologies that could convert two of the main product gases responsible for the greenhouse effect, methane and carbon dioxide, avoiding their massive release into the atmosphere.
Reforming of methane is one of the most important industrial processes, which convert natural gas into synthesis gas. Syngas is an intermediate feedstock for the production of hydrocarbons and hydrogen for fuel cells. Synthesis gas is produced from natural gas via catalytic processes based on dry reforming of methane (DRM), steam reforming of methane (SRM) and partial oxidation of methane (POM) . In fact, the available natural gas can be exploited for the production of chemicals and fuels.
The reforming processes are classified based on the energetic demand of the process and the type of reforming agent. Steam reforming of methane (SRM) produces a high ratio of syngas (H2/CO = 3), suitable for the production of ammonia. This process is endothermic and requires high investments. The partial oxidation of methane, an exothermic reaction, is an alternative process with reduced capital and operation costs. However, the partial oxidation of methane (POM) needs oxygen, and the cost of its production is about 50% of the investment of the whole process. There is a high risk of explosion at an elevated temperature . On the other hand, the dry reforming of methane (DRM) is a valuable reaction for biogas utilization and transformation of greenhouse gases (CH4 and CO2) in high-valued products. DRM produces a low syngas ratio (H2/CO = 1), which is suitable for the syntheses of oxygenates [4, 5, 6].
Tri-reforming of methane (TRM) is nowadays of great interest, because it combines the steam and dry reforming and partial oxidation of methane (CH4 + O2 + CO2 + H2O) processes; however, it holds the main advantages and disadvantages of all processes, to some extent .
It is well known that the major limitation of methane-reforming processes is the rapid deactivation of the catalyst, which has been commonly attributed to coke deposition and catalyst sintering.
The tri-reforming of methane may drastically reduce the carbon deposition. Furthermore, the presence of O2 in the feed allows the generation of energy in situ, due to the exothermal oxidation of methane, which increases the energy efficiency of the process. Besides, the possibility of changing the reactants’ compositions, allows for a versatile synthesis of gas composition, which can be suitable for different applications of synthesis gas [8, 9].
2. Tri-reforming process
Energy is the most important issue to modern economies, and it is predicted that a fast-rising energy demand will require US $45 trillion for new infrastructure investment by 2030. In particular, natural gas processes increase the options for the production of high added-value chemicals and energy demand. The Fischer-Tropsch (FT) technology is the main technology for the production of liquid fuels, named GTL process, but this technology is yet very expensive, due to the high costs of syngas production using steam reforming of methane (SRM) . The tri-reforming process (TRM), introduced by Song et al. , allows to use flue gas and methane to produce syngas, which can be converted to methanol and higher hydrocarbons. This new process is a synergic combination of the endothermic CO2 and steam-reforming reactions with the exothermic oxidation of methane, as shown in Eqs. (1)–(4) , which are carried out in a single reactor.
In addition, during the tri-reforming process, methane cracking (Eq. (5)), CO disproportionation or Boudouard (Eq. (6)), water-gas shift (Eq. (7)) and complete oxidation of carbon reactions (Eq. (8)) occur simultaneously .
The heat released during the POM reaction is used to supply the heat needed for the SRM and DRM reaction, and therefore the TRM reaction is energetically more efficient . In addition, TRM offers several advantages for syngas production compared to the single reactions . TRM does not require pure CO2 supply in the reaction. This implies that the flue gas from the combustion processes of power plants or the coke oven gas (COG) from iron-making industries can be used directly as a CO2 source for TRM process [15, 16, 17]. TRM can also be used to upgrade the syngas quality produced from biomass or coal gasification [18, 19]. The H2/CO ratio in syngas produced from tri-reforming can be adjusted varying the amounts of reactants to satisfy the requirement for further processes, such as methanol and Fischer-Tropsch synthesis [20, 21]. In addition, integrating steam reforming and partial oxidation with CO2 reforming could dramatically reduce or eliminate carbon formation on a reforming catalyst, thus increasing the catalyst life and process efficiency  due to the addition of O2 in the feed, which also generates heat that increases the energy efficiency. Therefore, the tri-reforming has the advantage of using natural gas and flue gases from power plants. The syngas from tri-reforming is used for the production of chemicals (such as MeOH and dimethyl ether by oxo-synthesis), fuels (for the Fischer-Tropsch synthesis) and electricity in fuel cells, as shown in Figure 1 .
|Direct use of flue gases||Usually requires oxygen plant|
|High methane conversion||No existing industrial process|
|Elimination of CO2 separation||No existing commercial catalysts|
|Different H2/CO ratios||Would require high GHSV|
|Minimization of coke formation||Heat management|
|Use of waste H2O/O2||Mass management|
|Simplifying the processing system|
However, due to the inherent problems of the reforming processes, there is a need to improve catalysts for optimizing the TRM process, improving the oxygen tolerance, resistance to coke formation and sintering of the metal-active sites at a high temperature.
3. Catalysts for methane-reforming reactions
The drawback of methane-reforming processes is mainly the severe tendency to carbon formation that deactivates the catalysts [23, 24, 25]. Noble metal-based catalysts (Rh, Ru, Pt, Pd and Ir) presented a high activity and stability against coke formation [26, 27]. However, their costs are still highly prohibitive for feasible application in this process. In fact, nickel-based catalysts are more preferable in the CH4 reforming, due to their availability and lower costs [28, 29, 30]. However, the stability of the nickel catalysts at elevated temperatures and the coke formation are the main obstacles for industrial applications [31, 32].
The addition of promoters to Ni-containing catalysts led to the reduction of coke deposition, better metal dispersion or smaller particle size, and the synergic effect between Ni and the promoter [33, 34, 35, 36]. In fact, bimetallic catalyst exhibits a higher activity compared to noble metals but not totally eliminate the carbon deposition.
The metal dispersion influences the coke deposition, since this process is structure-sensitive. The build-up of carbon involves quite large active metal particles, which are usually formed at high reforming temperatures.
Alumina-based supports have been investigated mainly due to the high specific surface area, increasing the metal dispersion . Nevertheless, the alumina supports easily deactivate due to the coke deposition and sintering. The formation of coke has been associated with the dehydration, cracking and polymerization reactions, occurring on the acid sites, while sintering is due to the transition of crystalline phase during reaction .
Additional improvement can be achieved using well-developed supports. An effort to overcome these problems is to search for basic additives or promoters, such as CeO2, SiO2, La2O3, BaO, CaO, SrO, MgO and ZrO2 [37, 38].
Sintering of metal clusters can be prevented with supports having a strong interaction with the active component. In fact, ceria-based catalysts can minimize sintering and coke formation  compared to MgO, TiO2, Al2O3, SiO2 and ZrO2 supports [40, 41, 42, 43, 44, 45, 46]. On the contrary, these supports facilitate sintering when submitted to higher temperatures. Moreover, ceria-based catalysts present good redox properties and high oxygen mobility, and as reported in the study, without noticeable oxygen mobility, the deactivation of the catalyst occurs very fast . On the other hand, the thermal stability of pure ceria under the typical reforming conditions is quite poor.
3.1. Promising catalysts
Although tri-reforming has not yet been implemented commercially, similar to steam or to dry reforming, Ni catalysts supported on a wide range of different supported materials, such as Al2O3, ZrO2, MgO, TiO2, CeO2, TiO2, CeZrO and SiO2, are the most popular catalysts for tri-reforming of methane .
Song et al.  suggested that the supports should have a high oxygen storage capacity that promotes CO2 adsorption. They proposed a simplified mechanism for the CO2 reforming reaction. The first step occurs with the activation of methane, followed by the surface reaction and the adsorbed surface CO2 species or adsorbed oxygen atoms (Eq. (9)); CO2 is more acidic, and basic supports may preferentially interact with CO2. Therefore, the CO2 adsorption at the surface facilitates the reaction with CH4 producing CO and H2. Moreover, supports with a high oxygen storage capacity may also facilitate the dissociative adsorption of CO2 into CO and adsorbed oxygen, according to Eq. (9) 
where □ denotes an active site.
Perovskite-type oxides have attracted significant interest as promising catalytic materials with applications in a wide range of reactions, including total oxidation and partial oxidation of hydrocarbons, carbon monoxide oxidation, alkenes hydrogenation, alkanes hydrogenolysis, alcohol synthesis, dry reforming and water-gas shift reaction [49, 50, 51]. The perovskites contain metallic and non-metallic elements, with a well-defined crystal structure. In general, the molecular formula is represented by ABO3, where A refers to an alkali metal, an alkaline earth metal or a lanthanide and B to a transition metal. These solids exhibit interesting properties such as superconductivity, ferromagnetism, appreciable thermal stability and conductivity and finally a high catalytic activity. The intrinsic properties of each perovskite are dependent on the type of inserted element and principally on the preparation method. In fact, perovskites as catalysts showed a reductive capacity under appropriate conditions. The metal particles are highly dispersed in the oxide matrix (AOX), inhibiting sintering of metal particles and carbon deposition. In fact, the high thermal stability makes the perovskites promising catalysts for the reforming of methane. Therefore, they are attractive alternatives to classic catalysts traditionally used in these reactions such as supported nickel and noble metals.
Various perovskites, including LaFeO3, LaNixFe1-xO3, LaNiO3 or La1-xCexFe0.7Ni0.3O3, have been found to exhibit a high activity in the steam reforming of methane with a minimal coke deposition under low steam-to-carbon ratios [52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67]. However, the need for high-operating temperatures (e.g. T ≥ 600°C) of methane-reforming reactions provokes irreversible structural changes, including structural collapse and dissolution of (reactive or inactive) metal particles from the perovskite lattice [57, 58, 59, 60, 63, 64].
Choudhary et al.  verified that the oxygen from the La1-xSrxFeO3 perovskite-type oxides surface was responsible for the complete oxidation of CH4–CO2 and H2O, while the bulk lattice oxygen was responsible for the deep reduction of Fe3+–Fe2+, and this was suitable for the partial oxidation of CH4–H2 and CO. The La1-xSrxFeO3 has had good repeatability in the catalytic performance, and no significant deactivation was observed over five redox cycles.
The LaCrO3 and LaFeO3 oxides doped with alkaline earth (AE = Ba, Ca, Mg and Sr) metals were prepared and studied on how the atomic oxygen influences the partial oxidation of methane to syngas . A-site doping with AE metals generally increases the mobility of lattice oxygen ions and thus decreases the temperature for the hydrogen and CO production, when compared with the non-doped LaCrO3 and LaFeO3 oxides. There are minor structural changes during the partial oxidation of methane of LaCrO3, which can be regenerated by oxidation at 950°C. However, the LaFeO3 presented negligible structural modifications. The stability of the perovskites occurs during repeated reaction cycles of generation-regeneration.
The LaFeO3, La0.8Sr0.2FeO3 and La0.8Sr0.2Fe0.9Co0.1O3 perovskite-type oxides were investigated in a continuous flow and sequential redox reaction  for the partial oxidation of methane in the absence of gaseous oxygen. The authors observed that methane reacted with sub-surface oxygen species of perovskite oxides in the absence of gaseous oxygen. The sequential redox reaction revealed that the structural stability is attributed to the continuous oxygen supply in the redox reaction, which evidences an excellent structural stability of the perovskite materials.
Other perovskites were employed for the DRM, SRM and POM reactions [68, 69, 70, 71, 72]. The effect of replacing cobalt by iron in LaCo1−xFexO3 (x = 0.0, 0.5 and 1.0) perovskite-type oxides over its physical properties and catalytic performance in the partial oxidation of methane (POM) was investigated. The product distribution varying with space time and with perovskite-type catalyst employed is found to be remarkable. For lower W/F values, the major product was H2 for the LaCoO3 (55.8%) and LaCo0.5Fe0.5O3 (59.2%), with similar ratios H2/CO (1.8–1.9) and a low CO2 formation .
We studied the combined dry and partial oxidation reaction on LaCrO3 and perovskites, fed with CH4:CO2:O2 = 1:1:0.5 and using a GSVH 60,000 h−1 at 700°C for 4 h. The conversions were 17% CH4 and 94% O2, respectively, and no conversion of CO2. Results showed an increasing formation of CO2 and a H2/CO ratio equal to 2.7, which suggests that the partial and total oxidation of methane initially takes place, producing CO, CO2 and H2O, and subsequently the steam and dry reforming occur to produce syngas. In fact, the water-gas shift reaction also takes place due to the high H2:CO ratio.
3.2. Effect of O2 and H2O concentration
Steam reforming of methane is the only large-scale industrial process currently available for the production of synthesis gas, producing high-purity hydrogen with a H2/CO ratio equal to 3. The partial oxidation of methane produces synthesis gas with a H2/CO ratio of 2, as required for methanol synthesis. However, the POM reaction is exothermic, and the control of the temperature of this process is difficult. Tri-reforming of methane is energetically favorable compared to the steam reforming of methane and partial oxidation of methane. The process is energetically thermal neutral. Compared to the SRM and POM reactions, the tri-reforming process has the advantage to produce different H2/CO ratios.
Singha et al.  found the optimum feed ratio and the effect of O2 and H2O concentration (mole ratio) conditions for the reaction, by monitoring the feed mixture and keeping the methane to CO2 mole ratio constant. The addition of oxygen in the feed helps to attain a thermal-neutral balance and compensate the heat necessary for the endothermic reactions occurring during the whole process . A high oxygen concentration in the reaction feed inhibits the CO2 reforming and lowers the CO2 conversion  because the reaction between oxygen and methane is thermodynamically favored over the reaction between methane and CO2. The higher concentration of oxygen in the feed allows a maximum methane consumption, and the available methane for the dry and steam reforming is very low . Table 2 shows the effect of O2 concentration over methane, CO2 and H2O conversions and H2/CO ratios . The effect of concentration of O2 over the reactant conversion was mainly due to the heat generated by the partial oxidation and complete oxidation of methane and the enhanced coke removal process [76, 77]. Increasing O2 concentration, the total oxidation of methane also increases, due to the exothermic reaction, and the amount of energy is released. The heat generated is useful for the steam- and dry-reforming reactions, which are endothermic, minimizing the required temperature to obtain a higher CH4 conversion  and external energy. On the opposite, lower O2 concentrations led to lower conversion; however, increasing the temperature, H2O and CO2 react with methane to produce synthesis gas . The higher H2/CO ratio was attributed to the steam reforming of methane, producing a H2/CO ratio of 3, attributed to the water-gas shift reaction, which produces only hydrogen, without the production of CO . On the other hand, with increasing temperature, one observes that the RWGS (reverse water-gas shift) reaction outweighs other reactions .
|Catalyst||GHSV (ml.g−1.h−1)||Feed ratio, O2:CO2:H2O:CH4:He||CH4 conv. (%)||CO2 conv. (%)||H2O conv. (%)||H2/CO ratio|
3.3. Effect of space velocity and methane/oxygen ratio
The effect of replacing cobalt by iron in LaCo1-xFexO3 (x = 0.0, 0.5 and 1.0) perovskite-type oxides on its catalytic performance in the partial oxidation of methane (POM) process was investigated, varying the space velocity and methane/oxygen ratio. The inlet methane to oxygen proportion was fixed at 2:1. The methane conversion increased with the space time and the maximum conversion was 31% at 0.67 kg.s.mol−1 for the LF perovskite. In terms of product selectivity, the catalysts produced mainly H2 and CO, CO2, C2H4 and/or C2H6, as shown in Table 3. The product distribution varying with space time and perovskite type catalyst is found to be remarkable. The H2 production decreased by about a half and the CO decreased four times for both LC and LCF catalysts. However, the CO2 formation increased by a factor of about 10, and the H2/CO ratio also increased by a factor of 2. Different was the product distribution of the LF perovskite presenting low H2 and CO formations and a high production of CO2, but a significant higher formation of C2 hydrocarbons compared to the other samples as W/F increases .
|W/F (kg.s.mol−1)||Catalysts||X CH4 (%)||H2/CO||Selectivity (%)|
Catalytic tests with the LF and LCF perovskites were also performed with a methane/oxygen ratio of 4 (W/F = 0.67 kg.s.mol−1). Table 4 shows that increasing the CH4/O2 ratio to 4, the methane conversion was halved, compared to the previous condition at a CH4/O2 ratio of 2 .
|CH4:O2||Catalysts||X CH4 (%)||H2/CO||Selectivity (%)|
The formations of ethane and ethylene are attributed to secondary reactions. In particular, the oxidative coupling of methane reaction takes place, which increased the C2H4 and C2H6 to C2H4 at high temperatures (2CH4 + 1/2O2 → H2O + 1/2C2H6). Parallel reactions of oxidative or non-oxidative dehydrogenation of ethane would occur, converting also C2H6 to C2H4 and then ethane could be oxidized to CO2. This last hypothesis is reinforced due to the increasing CO2 concentration at higher temperatures, most likely due to the oxidation of part of C2H6 (which leads to H2O and CO2), according to the following reactions, suggesting different reaction paths: C2H6 + 1/2O2 → C2H4 + H2O and C2H6 → C2H4+ H2.
Different reactions may occur in the whole process; the formation of the desired product with maximum selectivity depends on the input feed mixture. Steam increases the methane reforming and the water-gas shift (Eq. (7)) (WGS) reaction. It also helps to reduce the carbon deposition, which occurs during the dry reforming of methane . Therefore, the addition of H2O is thermodynamically more favorable for the methane reforming than for the dry reforming . For a lower H2O concentration, the methane conversion was lower than the CO2 conversion, which is assigned to the competition between H2O and CO2 molecules with methane. Increasing the H2O concentration input, the CO2 conversion decreases. Both WGS and steam reforming are equally important at a temperature below 650°C; however, with increasing temperature, the H2O conversion increases. Above 650°C, the RWGS reaction prevailed, producing less H2 and decreasing the H2/CO ratio [77, 80].
The reaction mechanisms are yet unknown for oxide catalysts and in particular for perovskite structures, which apparently are the most promising catalysts for the tri-reforming, based on the combined SRM, DRM and POM reactions. One explanation is that these materials present defects which promote the modification of electronic effects. Indeed, electronic effects may arise in the presence of ions with different charges of those belonging to the ions of the network, or as a consequence of the transition energy levels of electrons normally filled (usually the valence band) to empty levels (the conduction band). In all cases, when an electron is missing, that is, when there is an electron deficiency, this is usually called electronic holes. In the absence of an electric field, the ionic networks of the oxide structures tend to be electronically neutral, which requires that charge defects are compensated by the presence of other filler defects in order to obtain the condition of electro-neutrality, making the structure more stable. This means that charge defects are always present as a combination of two or more types of failures .
A reaction mechanism on mixed oxides can be suggested, assuming that CH4 is activated by the metal at the surface, forming carbon and H2. The carbon atoms adsorbed at the surface can react directly with oxygen, forming CO and H2. These intermediate species may react with the adsorbed CO2 species or dissociated steam. Song et al.  claim that the different extent of interaction between CO2 and catalysts could be responsible for this mechanism. They assumed that the interaction between CO2 and the catalyst could change the CH4 conversion rate, based on a simplified Langmuir-Hinshelwood (L-H) mechanism (Eqs. (10)–(12)).
where □ are the metallic surface sites.
They observed that the reaction order of CH4 on Ni/MgO is strongly compared to the adsorption of CO2 over Ni/MgO/CeZrO which is close to zero. This suggests that the CH4 conversion rate almost does not change with the partial pressure of CO2. However, it was found that Ni/MgO/CeZrO has even more stronger interaction with CO2 than Ni/MgO. In fact, the sites for a strong CO2 adsorption over Ni/MgO/CeZrO are probably not the same as for CH4 adsorption. It is important to note that the metal is itself believed to be able to activate CH4, as suggested by Rostrup-Nielsen , while the types of supports, like MgO, facilitate the adsorption of CO2. Hence, the locations of the interfaces between Ni and supports are fundamental, where the adsorption and reaction take place.
The energy crisis is a problem which will get exacerbated with depleting crude oil reserves around the world. There is an urgent need for alternative fuels around the world. The conversion of CO2 to a high-valued product could provide the necessary economic incentive towards both CO2 mitigation and fuel generation. The study reported new strategies of CO2 valorization. The tri-reforming produces directly synthesis gas from flue gases using methane as a co-feed. The utilization of CO2 without pre-separation from its sources saves energy, since a substantial energy input is required for CO2 separation from its concentrated sources . Tri-reforming of methane can be carried out by using CO2, H2O and O2 as a co-feed with natural gas or methane, and flue gas can be a very good source of highly concentrated feed for the tri-reforming process. New catalysts have been suggested with suitable promoters, mixed oxides and different supports, resistant to coke formation and sintering of the metal-active sites and stable at an elevated temperature. Stable and active catalysts for industrial application are under development, and researches are expected to bridge the gaps in science and technology for the tri-reforming process, providing further improvements and economically feasible.
The authors would like to thank FAPESP and Shell for supporting the “Research Centre for Gas Innovation—RCGI” (FAPESP Proc. 2014/50279-4), hosted by the University of São Paulo.