Effects of operating parameters for dry reforming of methane: A short review

. Dry reforming of methane (DRM) which also known as CO 2 reforming of methane is a well-investigated reaction to serve as an alternative technique to attenuate the abundance of greenhouse gases (CO 2 and CH 4 ). The syngas yielded is the main component for the liquid fuels and chemicals production in parallel with the fluctuating price of oil. Major researches were executed to seek for the well-suited catalysts before the commercialization of DRM can be realized. However, severe deactivation due to the carbon formation restricted the usage of promising Ni-based catalysts for DRM. Meanwhile, the deactivation on these catalysts can be associated with the operating conditions of DRM, which subsequently promoted the secondary reactions at different operating conditions. In fact, the parametric study could provide a benchmark for better understanding of the fundamental steps embodied in the CH 4 and CO 2 activation as well as their conversions. This review explores on the inﬂuences of the reaction operating parameters in term of the reaction temperatures, reactant partial pressures, feed ratios, and weight hourly space velocity (WHSV) on catalytic performance and carbon accumulation for


Introduction
The past few decades have witnessed tremendous progress in the research on greenhouse gases (CH 4 and CO 2 ) utilization. This utilization can ensure the continuity of the energy supply for the future generation, which can be applied in internal combustion engines or in the fuel cells to generate electricity while mitigating pollutant emissions [1]. On the other hand, synthesis gas (syngas, a mixture of H 2 and CO) is of paramount importance as a building block towards the generation of the praiseworthy chemicals and synthetic fuels by the means of Fischer-Tropsch Synthesis (FTS) [2,3]. To date, even though methane steam reforming has achieved the commercial grade, the contribution of the primary component of greenhouse gases, CO 2 prompted an urgent substitution over the existing reforming technology [4,5]. Owing to the capability to alleviate the CO 2 gas release and transform into useful products, thereupon, dry reforming of methane (DRM) has been instigated as a prestigious replacement for syngas generations [3,5,6]. However, challenges related to the carbon accumulation which can be related to the natures of the catalysts and also the operating conditions of DRM have to be addressed before meeting the commercialization level of DRM.
Previously, researchers paid immense focus on the catalysts investigations on catalyst selection. Intriguingly, Ni-based catalysts have been acknowledged their vast prospects over DRM reaction due to its low cost, satisfactory catalytic performance as well as readily available [3,7]. In fact, the optimal catalytic activity for DRM reaction not only affected by the catalysts adopted, but also the parametric factors such as reaction temperature, reactant partial pressure, feed composition, weight hourly space velocity (WHSV) and reactor type. As far we are concerned, the operating conditions of DRM are indispensable for the kinetic studies as well as efficient reactor design [8].
Therefore, in-depth understanding of the parametric study over DRM is crucial. Thermodynamics studies indicated the DRM process requires a larger amount of energy to operate due to its endothermic nature. Previous review by Usman et al. [9] has been reported on the influence different types of catalysts, active metals, promoters, particle size and reactor selection on catalytic performance and carbon deposition on DRM. Meanwhile, the operating conditions of DRM reaction are also crucial in affecting the catalytic performance for various catalysts. In addition, there are many other review papers [10][11][12][13][14] focused on discussing strategies to remove carbonaceous deactivation of catalysts by relating to the role of catalytic properties affected by the types of supports, active metal, promoters used, metal loading effect, catalysts preparation methods as well as thermodynamics study. However, there is almost no literature focus on the comparison of the DRM operating conditions, which are also key factors affecting the reaction performance. Thus, the parametric effects due to variation in reaction temperature, reactant composition, WHSV and the reactor design, which have been reported in the previous literature are summarized in this review to impart better interpretation over the challenges for DRM technology.

Reaction Temperature
The promotion of the side decomposition of the reactants induced by the nature of the reactions and the catalysts used led a positive temperature influence on the reactants' conversions and product yields. DRM is inevitably accompanied with carbon formation and supported catalysts are prone to catalyst deactivation due to the carbon formed and metal sintering effect [7,15]. Thus, effect of operating temperature of DRM have been studied in numerous previous literatures to provide clear justification on the deactivation happened on DRM. Herein, Table 1 provides a list of the influence of reaction temperature on DRM.
In a study carried out by Omoregbe et al. [8], the catalytic activity of the 10%Ni/SBA-15 over DRM were evaluated under the operating parameters of reaction temperatures (650-750ºC) at ambient pressure with varying CH4/CO2 feed ratios and partial pressure. Results ( Fig. 1) indicated both CO 2 and CH 4 conversions increased and less fluctuated with increasing reaction temperature. This can be explained by the carbonaceous deposit removal by CO 2 from the catalyst surface via reverse Boudouard reaction (C + CO 2 →2CO) which thermodynamically preferred at high reaction temperature of 700-750ºC [16,17]. A decline in the activity at 650ºC, was due to the CH 4 decomposition reaction that produce carbon deposition resulted from the thermodynamically favoured at 650ºC.
In the meantime, Cao and co-researchers [18] optimized the operating conditions of DRM for carbon deposition elimination by using thermodynamic calculations to investigate the influence of various operating temperatures (550-1200°C), on the H2/CO ratio and the carbon deposition. At constant pressure (P=0.1MPa) and CH 4 /CO 2 ratio (1.0), a reverse trend can be noticed with an increase in temperature ranged from 550-700ºC. The results also inferred that significant and severe carbon depositions were observed between 546ºC and 703ºC, which can be ascribed to the secondary reactions of CH 2 cracking and CO dissociation that are referred as the primary reactions devoting to coke deposition. The former was stimulated at 550ºC≤ T ≤ 1000ºC and P ≤ 0.1MPa, whereas the moderately exothermic latter reaction was enhanced at T ≤ 700ºC and P ≥ 0.1MPa. Meanwhile, the carbon deposits consumer, moderately endothermic CO 2 gasification happened at T ≥ 703 ºC. They also claimed that reducing the reaction pressure or reducing the CH 4 /CO 2 mole ratio could possibly have switched the carbon free regime at a lower reaction temperature. conversions over 10%Ni/SBA-15 catalyst. Adapted from [8].
Same goes to the study carried out by Sidik et al. [19] in which the reaction rate increases with the increased temperature due to the endothermic nature of DRM process [CH 4 +CO 2 → 2CO + 2H 2 (∆H 298K = +247 kJ mol -1 )] when using Ni-Co/MSN as catalyst [20]. Poor activity of the catalyst (low CH 4 and CO 2 conversions) can be observed at T < 500ºC (Fig. 2), yet approximately CH 4 and CO 2 conversions of 80% were reached at T > 700ºC. Analysis of variance (ANOVA) confirmed that most significant variable that affected the CH 4 conversions was operating temperature. Besides, Zhang et al. (2003) [21] claimed that the composition of the reactant gases (CO 2 and CH 4 ) decreased whilst the conversions of reactants were increased (38-92%; 28-94%) with the concurrent increased of water production when the temperature was raised (450-800ºC). The incline trend tended to reached plateau and then flattened out at T >700ºC. These results can be attributed to the hot spots formation at a high operating temperature in comparison with the bulk catalysts bed's average temperature.
Furthermore, Ayodele and co-workers [22] varied the reaction temperatures at the range of 650-750ºC to investigate the feasibility of the 20 wt%Co/80 wt%Nd2O3 catalyst over DRM. Under the feed ratio of CH 4 /CO 2 = 1 and T = 750ºC, 62.7% and 82% were the maximum CH 4 and CO 2 conversions achieved, while the maximum H 2 and CO products yield were 59.9% and 62.02%, respectively. At 750 ºC, the CH 4 conversion increased from 12.8% to 62.7% at CH 4 : CO 2 of 0.1 to 1.0, while the conversion of CO 2 inclined from 50% to roughly 80% in the same CH 4 : CO 2 range (CH 4 : CO 2 =0.1). Since the conversions recorded by both CH 4 and CO 2 were not similar, as opposed to their proposed methane dry reforming reaction (refers to Equation (1)), we posit that the CH 4 may exhibit poorer affinity to the catalyst, most likely due to the presence of stronger basic sites that favoured CO 2 adsorption as indicated by the TPD results.

Fig. 2.
Catalytic activity and stability of the Co/MSN, Ni/MSN and Ni-Co/MSN. Adapted from [19].
Meanwhile, Khavarian and Mohamed (2013) [23] suggested the CH 4 and CO 2 conversions over the synthesized MWCNTs were greatly influenced by the reaction temperatures within the range of 750-1000ºC which also related to the endothermic nature of DRM. The catalyst exhibited high activity and stability with 82.68% conversion of CH 4 at 950ºC, accompanied with insignificant activity loss. As such, the reaction rate of CH 4 and CO 2 over carbon nanotubes was affected significantly by the reaction temperature. Within the range of the reaction temperature studied, almost no coke formation over the catalyst surface and the syngas ratio was close to unity. The CH 4 conversion marked a drastic rise from 41.24% at 750ºC to 98.86% at 1000ºC. The CO 2 conversion was slightly surpassed the CH 4 conversion at the temperatures lower than 825ºC but then excelled the CO 2 conversion at higher temperatures. This can be correlated with the CH 4 and CO 2 adsorption and reaction rate competition over the MWCNTs with the temperature.
Furthermore, a thermodynamic equilibrium analysis for DRM was done by Nikoo and Amin [24]. Reaction temperatures were set at 200-1200ºC to investigate the equilibrium conversions, product compositions and solid carbon formation at different CO2/CH4 ratios (0.5-3) as well as reaction pressure (101.3-2533.1 kPa). For all CO 2 /CH 4 ratios, CH 4 conversion almost drastically increased with increasing temperature up to 727ºC, meanwhile, for CO 2 conversion, a gradual decline can be observed with temperature start from 300ºC to about 550-600ºC. The decreasing trend for CO 2 conversion can be mainly described by CO 2 + 2H 2 ↔C+ 2H 2 O. This exothermic reaction spontaneously occurs at low temperature but diminishes as the equilibrium constant decreases and reduces CO 2 conversion. In addition, side reaction namely carbon dioxide methanation (CO 2 + 4H 2 ↔CH 4 + 2H 2 O) which is exothermic and promoted at a lower temperature (300ºC-650ºC) also devoted to the same declining trend. Additionally, Schwengber at al. [25] performed DRM catalytic reaction tests by using 15% Ni/Al 2 O 3 and 30% Ni/Al 2 O 3 catalysts under the reaction temperature (600-700°C range). In general, increasing the reaction temperature in the DRM led to higher H 2 yields and higher conversions of CH 4 and CO 2 . Higher CH 4 conversion (average of 59%) was found to be at the 650ºC by 30% Ni/Al 2 O 3 catalyst, while other conversions fall in the range of 30 to 40%. The H 2 yield was obtained at 700ºC for both 15% Ni/Al 2 O 3 and 30% Ni/Al 2 O 3 catalysts, but the formation rate dropped at the 4th hour time-on-stream. These results showed that not only complete DRM occurred, but also other undesired reactions. The results proposed the co-existence of secondary reactions, which included reverse water-gas shift reaction (CO + H 2 O ↔ CO 2 + H 2 ), Boudouard reaction (2CO ↔ C + CO 2 ) and CO reduction (CO + H 2 ↔ C + H 2 O) that led to coke formation, low product yield and low reactants' conversions [34,35].
After all, coke accumulations are the main contributor over the catalysts deactivation which leading to severe reactor tubing blockage and physical disintegration of the catalyst framework. This circumstance was evidenced to have close connection on the reaction temperature conducted as DRM reaction was inevitably accompanied by numerous side reactions. Due to the endothermic nature of the reaction, high temperature (>750ºC) is the main accomplishment to attenuate coke deposition.

CO2 and CH4 partial pressure
Owing to the vital role of reactants partial pressures on DRM on providing the quantities of reactants, a number of the reported papers are dedicated to elucidate the relationship between the reactants (CO 2 and CH 4 ) partial pressure on the product yield and carbon deposition over DRM. Appropriate CO 2 and CH 4 partial pressures can minimize the happening of secondary reactions that were known to be main culprits for coke formation and catalyst deactivation. Table 2 summarizes the comparison study on the effect of reactants partial pressure on the catalytic performance over DRM.
Omoregbe et al. [8] reported on the DRM activity over 10%Ni/SBA-15 catalyst at feed partial pressure of 20-60kPa (Fig. 3). When CO 2 partial pressure (P CO₂ ) was increased, an increasing trend was observed for CH 4 conversion, whilst CO 2 conversion showed a substantial decline and exhibited an optimum performance at P CO₂ = 30-50kPa. The incline trend for CH 4 conversion with increasing of P CO2 (20-60kPa) was credited to the escalating intermediate by-product, H 2 O formation by RWGS reaction (CO 2 + H 2 → CO+ H 2 O) which promoted CH 4 steam reforming (CH 4 + H 2 O ⇌ CO + 3 H 2 ) as well. On the contrary, the considerable decline of CO 2 conversion with rising P CO2 (20-60kPa) was owing to the superabundance of CO 2 and inadequate quantity of CH 4 to act as limiting reactant for transforming CO 2 -rich feed composition [24,36]. Furthermore, the drop in CO 2 conversion with rising P CO2 also can be linked to the active Ni 0 metallic site oxidation at the catalyst surface to NiO (Ni + CO 2 → NiO + CO) in excess CO 2 circumstance. However, the rising in the P CH4 (20-60kPa) remarkably decreased both the reactants conversions, which was due to the increased carbon formation rate along with the occurrence via CH 4 cracking (CH 4 → C+ 2H 2 ) in CH 4rich feed [32]. Furthermore, CH 4 decomposition was easily facilitated in the CH 4 -excess environment further enhance the decomposition rate and promoting carbon deposition [4]. Likewise, Cao et al. [18] found that the reactants partial pressure reaction also affected the carbon formation rate during DRM. Results indicated that carbon deposition decreased when the partial pressures varied from 0.05 to 5MPa at 1200ºC. This finding was due to the CO dissociation (2CO ⇌ C + CO 2 ) to form carbon was inhibited as the partial pressure increased, thus the carbon formation tendency shifted to higher temperature region [18,37]. Therefore, it would be preferable to yield syngas suitable for the long-chain hydrocarbons synthesis at high pressure, as post syngas compression by using high CO content is not a technically liable mission. Another previous literature reported by Ayodele and co-researchers (2017) [22] also found that reactants consumption rates increased proportionally with the increase in pressure (5-50 kPa) when Co/Nd2O3 was employed as catalyst according to the proposed Langmuir-Hinshelwood kinetic mechanism.
Furthermore, Nikoo and Amin (2011) [24] investigated on the effect of system pressure on CH 4 and CO 2 conversions, main products distribution and H 2 /CO ratio at 900 ºC, CO 2 /CH 4 ratio of 1. CO 2 and CH 4 conversions were always excelled at lower pressures than those at higher pressures during reaction temperature at 900ºC. This proposes that at such a high temperature, greater pressures can impede the effect of temperature on increased reactant conversion. These decreased trends can be expressed by the endothermic trait of CRM. Besides, CH4 decomposition (CH 4 → C + 2H 2 ) and CO disproportionation (2CO → CO 2 + C) facilitate in lowering CH 4 and CO 2 conversions, as well as obstructing CO and H 2 formation at the higher pressures. Another research done by Ayodele et al. (2015) [38] investigated on the influences of reactants' (CH 4 and CO 2 ) partial pressures on the catalytic performance of the ceria-supported cobalt catalyst. The experiment was conducted by maintaining the partial pressure of one reactant constant at 50 kPa and varied the other reactant pressure between 5-50 kPa and vice versa at reaction temperatures of 650-750ºC. The highest conversion of CH 4 and CO 2 were acquired to be 78 and 80% at CH 4 and CO 2 partial pressure of 45 and 25 kPa, respectively. Syngas ratio of 1.0 was yielded at CH 4 partial pressure of 40 kPa. They also expressed that catalysts with basic support (electron-rich surface) such as ceria could improve the acidic gas adsorption such as CO 2 . In the region of low partial pressure of CO 2 , due to the prevalence of excess CH 4 and lack of CO 2 , most likely, CH 4 underwent catalytic decomposition into C and H 2 . Consequently, the reverse Boudouard reaction is favored leading to high conversion of CO 2 .
In a nutshell, neither CO 2 nor CH 4 surplus environment can escaped from the co-occurrence of the secondary reactions that arose depends on the supply of the reactant gases. Thereupon, the optimal partial pressures for reactant gases varied with distinct catalysts adopted as well as other operating conditions.

CH4/CO2 ratio
According to the stoichiometric equation of DRM (CO 2 + CH 4 → 2CO + 2H 2 ), the CO 2 /CH 4 = 1 case represents the stoichiometric oxidant supply of reactants. Since the role of CO 2 in DRM is similar to the oxidant in combustion, CO 2 /CH 4 with values less or greater than 1 represent the oxidant-lean and oxidant-rich cases, respectively. Thus, investigations on the CH 4 /CO 2 ratio using different catalysts for DRM have been reported and compared as well in Table 3. Intriguingly, the results obtained by Cao et al. (2017) [18] evidenced that the CH 4 /CO 2 mole ratio was the key factor to adjust H 2 /CO mole ratio, rather than adjusting the reaction pressure. Moreover, carbon formation decreased as CH 4 /CO 2 mole ratio decreased, which indicates that reaction CH 4 decomposition (CH 4 → C + 2H 2 ) was promoted to eliminate carbon formation with larger CH 4 /CO 2 mole ratio for the whole temperature range.  [39] implied that at the oxidant-rich condition in which the CO 2 supply was excessive, CH 4 conversion can be further improved as compared to the lower CO 2 conversion results for the oxidant-rich case.
Under the same CH 4 supply but less than the stoichiometric amount CO 2 supply, less CO can be produced with excessive CO 2 supply, since CO is one of the product elements [24]. Meanwhile, the CH 4 conversion increased gradually with the increase of CO 2 : CH 4 ratio from 1 to 5 as suggested by Sidik et al. (2016) [19] (Fig. 4). This observation implied that the CO 2 has a positive impact on the CH 4 conversion as it can act as an active oxidant [38]. This result could also be interpreted through the disproportionation reaction by the Le Chatelier's principal which explained that the surpass CO 2 could enhance the amount of CH 4 being converted to CO and H 2 . Besides, the stoichiometric effects of feed ratio on DRM to produce H 2 and CO also examined by Osazuwa and Cheng (2017) [40] using three different stoichiometric feed ratios (CO 2 /CH 4 = 0.5, 1, 2) at temperature of 750ºC. At CO 2 /CH 4 ratio of 0.5 where CO2 was the limiting reagent, 66 % CH 4 conversion was achieved. When DRM was carried out at equimolar feed ratio (CO 2 /CH 4 = 1), the highest CH 4 conversion was marked at 84% due to the exact matching with the stoichiometry ratio. Moreover, an increase in feed ratio from 0.5 to 1.0 witnessed a noticeable rise in the H 2 production from 45% to 60%. Moreover, the reverse gas shift (CO 2 + H 2 → CO + H 2 O) uses up the H 2 produced, thus leading to a drop in H 2 yield. Fig. 4. Response surface plot of the combined (a) CO 2 :CH 4 ratio and GHSV, (b) CO 2 :CH 4 ratio and reaction temperature. Adapted from [19].
In addition, the effect of varying the feed molar ratios of CO 2 : CH 4 on the conversions and product selectivity was investigated over the catalyst Pt (8%)/CeO 2 (20%)/α-Al 2 O 3 in a range of feed molar ratios (CO 2 : CH 4 ) from 1.0: 3.0 to 3.0: 1.0 at 650ºC were reported by Zhang et al. (2003) [21]. The CH 4 conversion was found to increase with the increment of CO 2 : CH 4 ratio, while the CO 2 conversion decreased. The secondary reaction between CO 2 and H 2 took place when CO 2 was in excess to produce the by-products, thus resulted in the decrease in H 2 :CO ratio in the product. This demonstrated that the reforming reaction was inactive while accompanied with the reverse watergas shift reaction (CO 2 + H 2 → CO + H 2 O). Gaur et al. (2012) [26] studied the effect of the CH 4 /CO 2 feed ratio (0.5, 1, 2) on La 1.97 Sr 0.03 Ru 0.05 Zr 1.95 O 7 (LSRuZ) and 0.5% Ru/Al 2 O 3 catalyst [26]. The superior performance of 0.5% Ru/Al 2 O 3 over LSRuZ suggested kinetically faster happening of RWGS reaction than on LSRuZ.
The effects of feed ratios (CH 4 : CO 2 ) ranged 0.1-1.0 also investigated by Ayodele et al. (2016) [22]. Maximum CH 4 and CO 2 conversions of 62.7% and 82%, respectively, were obtained at feed ratio of 1.0 (highest ratio employed) and reaction temperature of 750ºC (Fig.  5). Moreover, the production of syngas was observed to increase with feed ratio, reaching the maximum product yield of 59.9% and 62.02% for H 2 and CO. Additionally, the effects of CO 2 /CH 4 ratio (0.5-3) on equilibrium conversions, product compositions and solid carbon was studied by Nikko and Amin (2011) [24]. Meanwhile, CH 4 conversion increases with CO 2 /CH 4 ratio implying the CO 2 gas as a soft oxidant has a positive effect on CH 4 conversion. When CO 2 /CH 4 ratios <1, the amount of H 2 produced enhances within the whole investigated temperature, as CO 2 is the limiting reactant and the RWGS reaction cannot simultaneously improve along with partial oxidation of methane (CH 4 + ½ O 2 ↔ CO + 2H 2 ). Meanwhile, the number of H 2 moles produced decreases with increasing CO 2 /CH 4 ratio from 0.5 to 1 at a specified temperature. The declining trend of H 2 either for specified CO 2 /CH 4 ratios (>1) versus different temperatures or for specified temperature (> 973 K) versus different CO 2 /CH 4 ratios (>1) are presumably ascribed to RWGS reaction in which H 2 produced reacts with CO 2 to form water and CO. Generally, H 2 production becomes lower with increasing CO 2 /CH 4 ratio due to CH 4 being a more intensive limiting reactant restricted the source of hydrogen atoms.
Fakeeh et al. [41] also found that an increase in CH 4 /CO 2 ratio (0.25-2.33) over Ni/SiO 2 increased the CO 2 conversion, but decreased the CH 4 conversion. The higher CH 4 conversion than thermodynamic equilibrium suggested the happening of side reaction, CH 4 decomposition. When CH 4 /CO 2 ratio was > 1, CH 4 conversion increased and was quite similar with the thermodynamic equilibrium. Similar trends also reported by Serrano-Lotina et al. [31], Xu et al. [32], Meshkani et al. [42], and Meshkani and Rezaei [43]. Indeed, high CH 4 composition in the reactants facilitates CH 4 cracking and coke deposition, resulting in catalyst instability. When the feedstock of CH 4 = CO 2 , the coke removal rate by CO 2 is less than that of coke formation by CH 4 cracking. Adapted from [22].
In summary, a superabundance of CO 2 led to lower CO 2 conversion but higher CH 4 conversions. At this circumstance, higher quantity of H 2 was taken by RWGS reaction, so decreased H 2 selectivity. Thus, when CH 4 /CO 2 > 1, CH 4 conversion decreased but H 2 selectivity higher. From the previous studies, it can be concluded that CH 4 /CO 2 ratios between 1 and 1.43 seem to be the most optimal feedstock ratio to attain the best catalytic performance with low coke deposition and metal sintering.

WHSV
Weight hourly space velocity (WHSV) is related to the residence time for the interaction between catalyst particles and the reactants on the catalyst bed. Optimal WHSV can facilitated the reactants' conversions by providing satisfactory catalyst-reactant interaction.
Published journals on the effects of weight hourly gas velocity (WHSV) on DRM using distinct catalysts were tabulated in Table 4. Parametric study on the DRM performance over the reactant volumetric flow rate effect in terms of catalyst weight versus reactant flow rate (W/F0) were executed by Chein et al. [39]. In this study, W is fixed while F 0 was varied throughout the investigation. With the increased W/F 0 by decreasing the reactant flow rate, the higher reaction rate was achieved which led to higher reactants conversions. This enhancement was due to the increased residential contact time of the reactants with the catalyst. In addition, the high reaction rate resulted from higher W/F 0 , resulted in higher carbon yield as well. Meanwhile, CH 4 and CO 2 conversions became independent of W/F 0 when operating temperatures are > 1000ºC and 800ºC, respectively. Similar finding was also reported by Sidik et al. [19] , that CH 4 conversion also increased with the increased in the WHSV until reached the optimal point, the further increment in the WHSV value decreased the CH 4 conversion. This was also related to the effect of residence time brought by WHSV, that resulted in the shorter contact time for the interaction between reactants and the catalyst, thus lowering its catalytic activity. Moreover, similar observation was reported by Schwengber et al. (2016) [25] that demonstrated the catalytic reaction tests at different space velocities (WHSV of 15 and 45 L·h -1 ·g cat -1 ) by using 15%Ni/Al 2 O 3 and 30%Ni/Al 2 O 3 catalyst. From the results acquired, CH 4 conversion was also found to be decreased when WHSV increased. Apart from relating with the residence time, the finding can be understood in another way round that larger quantity of catalyst or longer beg length (lower WHSV) in the reaction bed favoured the reactants conversion and product formation [44].
In another studies by Xu et al. [32] and Meshkani et al. [42] whom adopted Ni-Co/La2O3-Al2O3 and Ni/MgO catalysts also witnessed the decline trends of the conversions with increasing GHSV. They proposed that high GHSV is beneficial in reducing metallic sintering and increasing the crystallites sizes during the reaction. They further declared that even though high GHSV offers higher contact frequency between the reactants and the catalyst but a shorter residence time, thus lower CH4 and CO 2 conversions were resulted. This phenomena were also in agreement with the findings obtained by Meshkani and Rezaei. [43], which also reported the negative effect on conversions upon increasing GHSV.
The effect of gas hour space velocity (GHSV) on the catalytic performance of Ni/La 2 Zr 2 O 7 and Ni/hydrotalcite-like precursor catalysts were also studied by le Saché et al. [33] and Serrano-Lotina et al. [36]. Similar findings also observed when a greater extent of activity drop was witnessed after doubling the GHSV; however, the overall conversions achieved were still comparably good especially for CO 2 . Furthermore, it was also noted that CH 4 conversion decreased by a greater extent than CO 2 , owing to the difficulty in overcoming the relatively stable C-H bonds present in CH 4 for its activation.
In summary, large WHSV was not favoured for the conversions of reactants and product yield, which can be claimed on the shorter residence time between both catalyst and reactants. Lower WHSV was somehow preferable in enhancing DRM activity which enable a longer contact time for catalyst to activate the reactants' behaviours. At these conditions, mass transfer dominates and kinetic control is the decisive factor when the reactants conversions achieved up to the thermodynamics equilibrium points.

Conclusion
The great potential of dry reforming of methane (DRM) to be served as energy transformation and storage system that provide alternatives energies is undeniably the most crucial technology for the sake of future chemical industry and environment. The greatest strength of this reaction is the consumption of the two main components of greenhouse gases (CO 2 and CH 4 ) to generate syngas (H 2 +CO). Meanwhile, this reaction was prone to catalyst deactivation due to the thermodynamic nature of the reaction impelled coke formation with the happening of several side reactions. Researches on seeking the excellent and efficient catalysts had achieved the desired accomplishment with the Ni-based catalysts are the most promising in term of its application and economic value. Although the nature and morphology of the supports, active metals used or even the promoters adopted affected the operation of DRM, still the operating conditions during DRM are the other issues that result in the carbon deposition. High temperature (>750ºC) is favourable for the endothermic DRM reaction, where this temperature will result in minimal of coke deposition. The optimal partial pressures for reactant gases could not be determined specifically as they may vary with different types of catalysts used. It can be concluded that CH 4 /CO 2 ratios between 1 and 1.43 seem to be the most optimal feedstock ratio to attain the best catalytic performance with low coke deposition and metal sintering. WHSV between 15-30 L·h -1 ·g cat -1 has been suggested to be able in enhancing DRM activity by enabling an appropriate contact time for catalyst to activate the reactants' behaviours. In short, this review summarized the various operating conditions studied in the previous literature to provide a clear benchmark for the future DRM studies for the sake to realize the commercialization of this technology in the foreseeable future.