catalysts

Article Low Temperature Activation of Dioxide by in Dry Reforming—A Thermodynamic Study

Anand Kumar

Department of Chemical Engineering, Qatar University, P. O. Box 2713, Doha, Qatar; [email protected]

 Received: 23 September 2018; Accepted: 18 October 2018; Published: 22 October 2018 

Abstract: Methane dry reforming (MDR) is an attractive alternative to methane reforming for production with low harmful environmental emissions on account of utilizing in the feed. However, carbon formation in the product stream has been the most challenging aspect of MDR, as it to catalyst deactivation by coking, prevalent in reforming reactions. Common strategies to limit coking have mainly targeted catalyst modifications, such as by doping with rare , supporting on refractory , adding /steam in the feed, or operating at reaction conditions (e.g., higher temperature), where carbon formation is thermodynamically restrained. These methods do help in suppressing carbon formation; nonetheless, to a large extent, catalyst activity and product selectivity are also adversely affected. In this study, the effect of ammonia addition in MDR feed on carbon suppression is presented. Based on a thermodynamic equilibrium analysis, the most significant observation of ammonia addition is towards low temperature carbon dioxide activation to methane, along with carbon removal. Results indicate that ammonia not only helps in removing carbon formation, but also greatly enriches .

Keywords: methane dry reforming; carbon dioxide conversion; ammonia-assisted reforming; thermodynamic equilibrium analysis

1. Introduction

The reaction CH4 + CO2 → CO + H2 or methane dry reforming (MDR) for producing hydrogen from methane provides an alternative way to the industrially used methane (MSR) [1–5]. The advantage of MDR reaction is that it uses CO2 as a feed, thus ameliorating the environmental impact by decreasing the of this ubiquitous , and generating a ratio of H2 and CO that can be adjusted to obtain the required synthesis gas () composition. Starting from syngas, using Fischer–Tropsch , long chain and can be produced on transition catalysts, as described in Equation (1):

nCO + (2n + 1)H2 → Cn H2(n+1) + nH2O (1)

The main disadvantage of the MDR reaction is that the catalysts used in this reaction undergo severe deactivation, primarily due to carbon formation, also known as coking, limiting the lifetime of the catalysts [3,6–11]. Supported noble metals suffer less deactivation, but are costly, and the use of promoters increases time on stream (TOS), but decreases activity and selectivity [3,9,12–15]. Mechanistic studies indicate the dissociation of methane on metal surfaces proceeds via two widely accepted reaction pathways: Firstly, by direct dissociation, and secondly, by indirect dissociation of intermediate species [7,10,16,17]. The direct dissociation is anticipated to take place at a high

Catalysts 2018, 8, 481; doi:10.3390/catal8100481 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 481 2 of 12 temperature, while the low temperature conditions are expected to favor indirect dissociation through reaction intermediates (e.g., CHx or formyl ) [18], where the oxygen is supplied from CO2 or via the support [19]. Some reports indicate the activation of methane to be favored on metal active sites, while that of carbon dioxide on the support during MDR [20]. Other reports indicate that supports do play a role in methane activation and the acidity or basicity of the support can influence the activation of methane [20,21] and, in some cases, the activation of CH4 is reported to require the availability of adsorbed oxygen, usually coming from the supports [22]. Methane reforming in the availability of CO2 or other oxidants (steam, oxygen, or air) indicates low carbon formation and high syngas yield [23–26] with a varying ratio of H2/CO based on the of support used. Recent literature [19,27–29] has summarized the status of formation in various hydrocarbon reforming catalysts susceptible to coke poisoning. Mechanistic studies conducted on different catalyst surfaces suggest the following intermediate reactions to be the main contributors in coke formation in MDR and other hydrocarbon reforming reactions [19]:

• The : 2CO → CO2 + C (2)

• Reverse carbon gasification: CO + H2 → H2O + C (3)

• Dehydrogenation of hydrocarbons:

Cm Hn → n/2H2 + mC (4)

• of to coke: C2 H4 → coke (5)

• Conversion of to mesityl oxide ((CH3)2C = CHCOCH3), which can further oligomerize to generate coke [30].

The Boudouard reaction is understood to be the main cause of carbon formation at a low temperature, whereas dehydrogenation of intermediate hydrocarbons at a high temperature favors coke formation [3,19]. These intermediate reactions are observed in both hydrocarbon and oxygenated hydrocarbon reforming, indicating similar catalysts could work in both cases for suppressing coke formation. Although various studies have been conducted on the design of effective catalysts for MDR reactions and a considerable progress has been made in the direction of understanding the mechanism of deactivation, nonetheless, carbon formation still remains a great challenge that limits the MDR catalysts’ cycle. Thermodynamic studies indicate a significant decrease in carbon formation in reforming oxygenated compounds when certain chemical additives are introduced in the feed [31,32], where carbon suppression is also accompanied by hydrogen enrichment [31,32]. In this work, the effect of ammonia addition in the MDR feed stream on coke formation is investigated. This study is based on the thermodynamic equilibrium analysis to obtain product distribution between 300–1000 K. My research group is primarily working on the catalysts’ synthesis using -based techniques for hydrocarbon reforming reactions [33–48]. While the experimental work on catalyst development for MDR is still underway, the results on thermodynamic equilibrium analysis are presented here to understand the favorable reaction conditions. Considering the ongoing work on ammonia to be used as a chemical [49,50], this study becomes more relevant and also investigates the total hydrogen enrichment and quality of syngas produced in ammonia-assisted MDR. Unlike methane partial oxidation, hydrogen amount is not expected to be compromised while removing the deposited carbon here. The thermodynamic calculation methodology applied in this study is briefly presented in the following section. Catalysts 2018, 8, 481 3 of 12

2. Thermodynamic Calculations The conditions of equilibrium product distribution at a given temperature were calculated using a software package “THERMO”. The program works on the basis of minimization of Gibbs free in a multiphase and multicomponent system, assuming the condensed phases to be immiscible and the to follow law. The details of the program can be found in the reference manual and other publications [31,32,51]. Table1 contains the enthalpy of formation of various compounds that are used in thermodynamic analysis. It should be noted that the “THERMO” program has a comprehensive database containing products with other possible atomic combinations of C, H, N, and O, and does not limit the optimization to only to the components listed in Table1.

Table 1. The standard enthalpy of formation for various compounds used in this study [52].

0 Compound Enthalpy of Formation (∆Hf ), KJ/mol

CH4 −74.6 CO2 −393.51 H2O (g) −241.826 NH3 −45.94 CO −110.53 H2 0 N2 0 C () 0

3. Results and Discussion Thermodynamic equilibrium product distribution for methane dry reforming using 1:1 molar ratio of CH4:CO2 (Equation (5)) at 1 atm. pressure and a temperature range of 300–1000 K is shown in Figure1a. 0 CH4 + CO2 ↔ 2CO + 2H2 : ∆Hf = +247.05KJ/mol (6) Looking at the distribution of various compounds (Figure1a), it is clear that elemental carbon and are the two most stable products at low temperature and both start to decrease as temperature increases. Methane concentration, however, increases to a maximum value of 0.39 moles at 600 K before starting to decrease and gradually reaching ~100% conversion at 1000 K. Carbon dioxide shows a similar trend, with an increasing value at lower temperature reaching to a maximum of ~0.55 moles at 800 K before starting to decrease. There is no CO formation until 600 K, whereas hydrogen starts to appear as early as 400 K itself and both CO and H2 show an exponential increase in concentration with temperature, indicating that higher temperature is more favorable for syngas formation and low temperatures are suitable for coking. Similar trends were obtained by Pakhare et al. [3,53] while investigating the effect of temperature on carbon formation in dry reforming of methane, indicating that only temperatures above 900 ◦C (1173 K) are suitable for carbon-free products. Catalysts 2018, 8, 481 4 of 12 Catalysts 2018, 8, x FOR PEER REVIEW 4 of 12

FigureFigure 1.1.Product Product distribution distribution in ( ina) methane (a) methane dry reforming, dry reforming, (b) methane (b) methane steam reforming, steam reforming, (c) methane (c) partialmethane oxidation, partial oxidation, (d) total ( carbond) total formation carbon formation in all the in threeall the reactions. three reactions. All the All calculations the calculations were performedwere performed at 1 atm at 1 pressure. atm pressure.

ExtendingExtending the the thermodynamic thermodynamic investigations investigations to other other commonly commonly used used methane-reforming methane-reforming + reactions,reactions, thatthat is, is, methanemethane steamsteam reforming reforming (MSR: (MSR: CH퐶퐻44 + H퐻22푂O, Figure 11bb)) and and methane methanepartial partial + 1 oxidationoxidation (M-POx:(M-POx: CH퐶퐻4 + 2 O푂2, Figure 1 1c),c), aa decreasedecrease inin carboncarbon formationformation isis veryvery clearclear inin thethepresence presence 4 2 2 of steam and oxygen, respectively. However, a careful observation indicates that methane conversion of steam and oxygen, respectively. However, a careful observation indicates that methane conversion is also inferior in these two reactions at low temperature values. Starting with the same initial methane is also inferior in these two reactions at low temperature values. Starting with the same initial amount of 1 , comparing the equilibrium amount of methane present in the three systems at methane amount of 1 mole, comparing the equilibrium amount of methane present in the three 800 K, the lowest conversion is observed in MSR with 0.64 moles of unreacted methane, followed systems at 800 K, the lowest conversion is observed in MSR with 0.64 moles of unreacted methane, by M-POx with 0.37 moles and MDR with only 0.27 moles. Nonetheless, conversion of methane followed by M-POx with 0.37 moles and MDR with only 0.27 moles. Nonetheless, conversion of directly relates to the amount of carbon formed at lower temperature values, as the other product methane directly relates to the amount of carbon formed at lower temperature values, as the other containing carbon (i.e., CO) is insignificant for temperatures below 800 K. Figure1d combines the product containing carbon (i.e., CO) is insignificant for temperatures below 800 K. Figure 1d trend of carbon formation in the three reactions, showing profiles with maximum carbon generation in combines the trend of carbon formation in the three reactions, showing profiles with maximum MDR, followed by MSR and M-POx respectively. The effect of temperature on carbon content is clearly carbon generation in MDR, followed by MSR and M-POx respectively. The effect of temperature on indicated by exposing two regions of distinct carbon growth patterns. For T < 700 K, carbon content carbon content is clearly indicated by exposing two regions of distinct carbon growth patterns. For T monotonically decreases in MDR and M-POx, whereas no carbon was observed in MSR. A deflection < 700 K, carbon content monotonically decreases in MDR and M-POx, whereas no carbon was in carbon profile is observed in MDR and M-POx between 700 K–1000 K, while in MSR, carbon starts observed in MSR. A deflection in carbon profile is observed in MDR and M-POx between 700 K–1000 to appear at 700 K, reaches a maximum value of 0.21 moles at 900 K before decreasing to 0.04 moles K, while in MSR, carbon starts to appear at 700 K, reaches a maximum value of 0.21 moles at 900 K at 1000 K. Throughout the investigated temperature range of 300–1000 K, MDR shows the highest before decreasing to 0.04 moles at 1000 K. Throughout the investigated temperature range of 300– carbon formation, though gradually decreasing with temperature, forming an ideal system for our 1000 K, MDR shows the highest carbon formation, though gradually decreasing with temperature, investigation of ammonia-assisted methane dry reforming and observing the effect of ammonia on the forming an ideal system for our investigation of ammonia-assisted methane dry reforming and reduction of carbon. observing the effect of ammonia on the reduction of carbon.

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Currently, a number of research activities are targeting ammonia synthesis as a means of hydrogenCurrently, storage a number [49,50,54] of. research Ammonia activities is a clean are sourcetargeting for ammonia on-demand synthesis hydrogen as a generation means of, hydrogenproviding storagehigh hydrogen [49,50,54 ]. Ammonia and no isharmful a clean environmental source for on-demand emissions, hydrogenas generation, is the only providingbyproduct high [49,50,54] hydrogen. The density addition and of no ammonia harmful alongside environmental carbon emissions, dioxide in as MDRnitrogen could is the help only in byproductalleviating [ coke49,50 , formation54]. The addition and increasing of ammonia the catalyst alongside life , along dioxide with in MDR increasing could the help total in alleviatinghydrogen production. coke formation The thermodynamic and increasing feasibility the catalyst tests life are cycle, being along performed with increasingat this stage the and total the hydrogenexperimental production. validations The are thermodynamic to follow up in feasibility future communications. tests are beingperformed at this stage and the experimentalFigure 2 validations shows the are effect to follow of ammonia up in future (1 mol) communications. addition on the product distribution in MDR systemFigures. Ammonia2 shows the is effectanticipated of ammonia to decompose (1 mol), additionproducing on thenitrogen product and distribution hydrogen in gases MDR, as systems.displayed Ammonia by a constant is anticipated amount to ofdecompose, 0.5 moles producing of nitrogen nitrogen throughout and hydrogen the temperature gases, as displayed range of byinvestigation a constant amountand an increase of 0.5 moles in hydrogen of nitrogen amount throughout as compared the temperature to MDR without range of ammonia. investigation It should and anbe increasenoted that in ammonia hydrogen is amount completely as compared utilized and to MDRpractically without no a ammonia.mount is present It should starting be noted from that 300 ammoniaK until 1000 iscompletely K (Figure 2a). utilized Other and products practically’ distribution no amount indicates is present a considerable starting from increase 300 K in until CH 10004 moles K (Figureand a slight2a). Other increase products’ in H2O distributionand CO moles indicates, whereas a considerable a decrease in increase carbon and in CH CO42 molesmoles andis observed. a slight increaseThis trend in His2 Oclea andrly CO illustrated moles, whereas by comparing a decrease the inproduct carbon distribution and CO2 moles at 800 is observed.K, as shown This in trend Figure is clearly2b. A significantillustrated by increase comparing in CH the4, product H2, and distribution H2O is seen at as 800 compared K, as shown to a in decrease Figure2b. in A C significant and CO2, increasewhereas inthe CH amount4,H2, and of CO H2 Ois isonly seen slightly as compared affected. to An a decrease increase in in C CH and4 COmoles2, whereas and a simultaneous the amount ofdecrease CO is only in CO slightly2 moles affected. indicates An an increase improved in CH CO4 2moles activation and aby simultaneous ammonia towards decrease CH in4, COC, 2andmoles CO indicatesformation. an improved CO2 activation by ammonia towards CH4, C, and CO formation.

FigureFigure 2.2. ((aa)) ProductProduct distributiondistribution in ammonia ammonia-assisted-assisted methane methane dry dry reforming reforming (CH (CH4 4+ +CO CO2 +2 NH+ NH3); (3b);) (comparisonb) comparison in product in product amount amount at 800 at 800 K with K with and and without without NH NH3 addition.3 addition.

TheThe additionaddition ofof oneone molemole ofof ammoniaammonia inin anan MDRMDR systemsystem hashas certainlycertainly helpedhelped inin reducingreducing thethe carboncarbon content,content, butbut onlyonly toto aa limitedlimited extent.extent. InIn order to eliminate carbon completely, anan extraextra amountamount ofof ammoniaammonia isis required,required, asas presentedpresented inin FigureFigure3 .3. For For a a low low temperature temperature range range of of 300–800 300–800 K, K, carbon carbon cancan bebe completelycompletely removedremoved byby increasingincreasing thethe ammoniaammonia contentcontent toto 44 molesmoles forfor eacheach molemole ofof methanemethane used.used. CarbonCarbon contentcontent atat 300300 KK graduallygradually decreasesdecreases fromfrom aa valuevalue ofof 22 molesmoles inin thethe absenceabsence ofof ammoniaammonia toto valuesvalues ofof 1.25,1.25, 0.5,0.5, andand 00 molesmoles asas thethe ammoniaammonia moles are increased to 1, 2,2, and 4,4, respectively. Thus,Thus, 44 molesmoles ofof ammoniaammonia isis sufficientsufficient forfor aa carbon-freecarbon-free productproduct inin methanemethane drydry reformingreforming atat lowlow temperaturetemperature values, values, but but it isit not is not enough enough for high for high temperature temperature operations operations with T with > 800 T K, > where800 K, carbonwhere formationcarbon formation peaks at peaks T ~900 at K T before~900 K decreasing before decreasing (as shown (as in shown Figure in3a). Figure Comparing 3a). Comparing the carbon the content carbon incontent MDR productsin MDR products at 900 K (Figureat 900 K3b), (Figure additional 3b), additional ammonia upammonia to 6 moles up to will 6 moles be required will be for required complete for removalcomplete of removal carbon. of carbon.

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FigureFigure 3. Effect3. Effect of of ammonia ammonia content content on on carbon carbon formationformation in methane dry dry reforming; reforming; (a ()a at) at temperature temperature 300–1000300–1000 K, K, (b )(b at) at temperature temperature = = 900 900 K. K. Figure 3. Effect of ammonia content on carbon formation in methane dry reforming; (a) at temperature 300–1000 K, (b) at temperature = 900 K. AsAs the the inclusion inclusion of ammonia of ammonia in MDR in MDR eliminates eliminates carbon carbon formation, formation, questions questions arise arise about about the nature the of productsnature Asof this productsthe carboninclusion this gets carbonof convertedammonia gets convertedin to. MDR Asshown eliminates to. As earlier shown carbon in earlier Figure formation, 2inb, Figure among questions 2b, the among carbon-containingarise aboutthe carbon the - compounds,containingnature of only compoundsproducts methane this, andcarbononly carbon methane gets converted and carbon to. show As monoxideshown an increase earlier show in in amountFigure an increase 2b, upon among ammoniain amountthe carbon addition. upon- Thisammonia observationcontaining addition. compounds indicates This that, only observation ammonia methane lowersindicates and carbon the that overall monoxide ammonia conversion show lowers an of increasemethane the overall in to amount carbon, conversion upon whereas of it facilitatesmethaneammonia carbon to carbon,addition. dioxide whereas This conversion observation it facilitates as indicates some carbon amount that dioxide ammonia of carbon conversion lowers dioxide asthe is some overall transformed amount conversion ofinto carbon carbon of monoxidedioxidemethane (Figureis transformed to carbon,2b). This whereas into effect carbon it was facilitates monoxide further carbon studied (Fig ure dioxide in 2b). detail, Thi conversions keeping effect was as in some mindfurther amount that studied approximately of carbonin detail, 6 moleskeepingdioxide of ammonia in is mindtransformed is that required approximately into carbon for carbon-free monoxide 6 moles product (Fig ofure ammonia distribution2b). Thi s iseffect required throughout was further for the carbon studied temperature-free in detail, product range distributionkeeping in throughout mind that theapproximately temperature 6 molesrange of 300ammonia–1000 K is thatrequired is expected for carbon to - createfree product a strong of 300–1000 K that is expected to create a strong reducing environment. Looking at the trends in reducingdistribution environment. throughout Looking the temperature at the trends range in Figure of 300 4a,–1000 addition K that of ishigh expected concentration to create of a ammonia strong Figure4a, addition of high concentration of ammonia not only decreases the reactivity of methane, but notreducing only decreases environment. the Looking reactivity at the of trends methane in Figure, but 4a, also addition promotes of high methane concentration formation of ammonia at low also promotes methane formation at low temperature ranges until 700 K (Figure4a). The methane temperaturenot only decreasesranges until the 700 reactivity K (Figure of 4a). methane The m, ethanebut also amount promotes increases methane from formationzero moles at at low300 K amount increases from zero moles at 300 K in MDR to 0.75, 1.5, and 2 moles upon addition of 1, 2, in MDRtemperature to 0.75, range 1.5, ands until 2 moles 700 K upon (Figure addition 4a). The of m 1, 2, and amount4 moles increases of ammonia from (Figure zero moles 4b); afterwardsat 300 K , and 4in moles MDR ofto 0.75, ammonia 1.5, and (Figure 2 moles4 b);upon afterwards, addition of the1, 2, CHand 4 molesvalue of remains ammonia constant (Figure 4b) due; afterwards to the lack, of the CH4 value remains constant due to the lack of a carbon source, as only 2 moles of CH4 can be a carbonthe source,CH4 value as remains only 2 moles constant of CHdue tocan the be lack produced of a carbon in the source MDR, as system only 2 (CHmoles+ of CO CH).4 can Therefore, be produced in the MDR system (CH44 + CO2). Therefore, thermodynamically, it 4 is very2 clear that produced in the MDR system (CH4 + CO2). Therefore, thermodynamically, it is very clear that thermodynamically,ammonia can completely it is very convert clear that carbon ammonia dioxide can to completely methane at convert lower temperature, carbon dioxide which to methane follows at ammonia can completely convert carbon dioxide to methane at lower temperature, which follows loweruntil temperature, 600 K as no decline which followsin the methane until 600 amount K as no is observed decline in for the T methane< 600 K. Increasing amount is temperature observed for until 600 K as no decline in the methane amount is observed for T < 600 K. Increasing temperature T

Figure 4. (a) Effect of ammonia content in MDR on methane; (b) comparison in methane amount in presence of NH at 300 K and 1000 K with regard to no NH in DMR. 3 3 Catalysts 2018, 8, x FOR PEER REVIEW 7 of 12

CatalystsFigure2018, 84., 481(a) Effect of ammonia content in MDR on methane; (b) comparison in methane amount in7 of 12 presence of NH3 at 300 K and 1000 K with regard to no NH3 in DMR.

ApplyingApplying thethe principleprinciple ofof conservationconservation of elements,elements, an increase in thethe methanemethane amountamount atat lowlow temperaturetemperature couldcould only only result result from from a a decrease decrease in in the the carbon carbon dioxide dioxide amount. amount. As As indicated indicated in Figurein Figur5a,e the5a, amountthe amount of CO of2 COconsistently2 consistently decreases decreases by increasing by increasing the ammonia the ammonia content, content, while while with CO with being CO absentbeing absentin the product in the product until 600 untilK and 600 the K amount and the of carbonamount consistently of carbon decreasing, consistently the decreasing, selectivity forthe directselectivity methane for direct formation methane from formation CO2 increases from CO rapidly2 increases in the rapidl presencey in ofthe NH presence3. No CO of 2NHis observed3. No CO2 ≥ foris observed NH3 4 for moles NH until3 ≥ 4 600moles K and until only 600 at K higher and only temperature, at higher whereas temperature, some COwhereas2 is observed some CO with2 is ≈ maximumobserved with value maximum at T 800–900 value at K. TKeeping ≈ 800–900 track K. Keeping of CO2 trackmoles of at CO 9002 moles K, only at 0.19900 K, moles only are0.19 left moles for NHare 3left= 6for moles NH3 that = 6 moles also decreases that also furtherdecreases at 1000 further K. at 1000 K.

FigureFigure 5.5. ((aa)) EffectEffect ofof ammoniaammonia contentcontent inin DMRDMR onon COCO22 inin product;product; ((bb)) aa decreasingdecreasing trendtrend inin COCO22 amountamount inin presencepresence ofof NHNH33 atat 900900 K.K.

LookingLooking atat thethe otherother carbon-containingcarbon-containing product—COproduct—CO inin FigureFigure1sb, 1b, Figure 2a, and2a, and6a— Figureit is clear6a—it that isCO clear is not that generated CO is not until generated 600 K even until if 600 large K even amounts if large of amountsNH3 is added of NH to3 theis added MDR tosystem. the MDR The system.generation The of generation CO happens of CO only happens at T > only 600 atK Tand > 600 grows K and exponentially grows exponentially thereafter. thereafter. Addition Addition of NH3 ofonly NH has3 only a slight has effect a slight, which effect, is not which as significant is not as compared significant to compared other carbon to other-containing carbon-containing products like productsCO2, CH4 like, and CO C.2 Nonetheless,, CH4, and C. th Nonetheless,e overall impact the overallof ammonia impact is ofa slight ammonia improvement is a slight in improvement CO content, inmost CO significantly content, most at significantly T = 1000 K at (Figure T = 1000 6b) K, where (Figure CO6b), moles where vary CO moles from 1.2vary moles from to1.2 1.4 moles moles. to 1.4Hydrogen moles. Hydrogen,, on the other on the hand other, is hand, expected is expected to experience to experience the biggest the biggest increase increase in quantity in quantity due due to toammonia ammonia being being an anadditional additional source source along along with with methane methane (Figure (Figure 7).7 ).For For MDR MDR and and low low amounts amounts of ofNH NH3 (≤23 ( ≤mol2 mol),), hydrogen hydrogen generation generation starts starts only only after after T T≥ ≥600600 K, K, a atre trendnd similar similar to to CO production,production, beforebefore growinggrowing rapidly (Figure(Figure7 7a).a). LowLow temperaturetemperature conditions favor water production ( (FigureFigures1 a,1a, Figure2a, and2a, 7b), and Figure which7 b), decreases which decreases gradually gradually with an with increase an increase in temperature in temperature while while increasing increasing the thehydrogen hydrogen content content in inthe the product. product. As As ammonia ammonia concentration concentration is is increased, increased, water water is preferentially producedproduced andand reachesreaches thethe maximummaximum valuevalue ofof 22 moles,moles, limitedlimited byby thethe numbernumber ofof oxygenoxygen atomsatoms inin thethe systemsystem comingcoming fromfrom COCO22, before thethe excessexcess hydrogenhydrogen atomsatoms areare distributeddistributed amongamong otherother products,products, suchsuch as CH CH44 andand H H2 (2Figure(Figuress 4a4 anda and 7).7 ).Looking Looking at the at thetrends trends of CH of4 CH, CO24, ,H CO2, and2,H H22,O and in the H2 Opresence in the presenceof excess ofamount excess of amount NH3 (≥4 ofmol), NH it3 (appears≥4 mol), as itif appearscarbon dioxide as if carbon is transformed dioxide isinto transformed methane at intolow methanetemperature at low values temperature (T < 600 valuesK), which (T < 600K),undergoes which steam undergoes reforming steam toreforming produce to syngas produce that syngas is a thatcombination is a combination of hydrogen of hydrogen and carbon and . monoxide. Therefore, Therefore, it can it can easily easily be be said said that that syngas syngas productionproduction in in thethe presencepresence of of ammonia ammonia is is a a result result of of MSR MSR at at high high temperature, temperature, where where ammonia ammonia facilitatesfacilitates excessexcess methanemethane andand waterwater generationgeneration at lowlow temperaturetemperature to bebe furtherfurther utilizedutilized inin MSRMSR atat highhigh temperature.temperature.

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(a) (b) (a) (b)

Figure 6. (a) Effect of ammonia content in DMR on CO in product; (b) CO amount in presence of Figure 6. 6.(a )(a Effect) Effect of ammoniaof ammonia content content in DMRin DMR on COon CO in product; in product (b); CO(b) amountCO amount in presence in presence of NH of3 NH3 at 1000 K. at 1000 K. NH3 at 1000 K.

(a) (b) (a) (b)

FigureFigure 7.7. ((aa)) EffectEffect ofof ammoniaammonia contentcontent inin DMRDMR onon HH22 inin product,product, andand ((bb)H) H22O in product.product. Figure 7. (a) Effect of ammonia content in DMR on H2 in product, and (b) H2O in product. Figure8 8 summarizes summarizes the the results results obtained obtained so so far far while while showing showing the the carboncarbon contentcontent inin thethe productproduct Figure 8 summarizes the results obtained so far while showing the carbon content in the product withwith syngas syngas quality quality (H 2 (H/CO2/CO ratio, ratio, Figure Figure8a) and 8a) simultaneously and simultaneously comparing comparing the influence the influenceof ammonia of withinammonia all syngascarbon-containing in all quality carbon (H-containing2 products/CO ratio, products at 1000Figure K at (Figure8a) 1000 and K8b). ( simultaneouslyFigure The addition8b). The a of comparingddition ammonia of ammonia theremoves influence removes carbon of ammoniafromcarbon product, from in all product while carbon it, - increaseswhilecontaining it increas hydrogen productses hydrogen production. at 1000 Kproduction. (Figure On the 8b). other On The the hand, addition other the hand, additionof ammonia the of addition ammonia removes of carbondoesammonia not from affect does product COnot affect production, while CO itproduction increas as much,es as andhydrogen much, favors and production. methane favors metha formation. On nethe formation. other As a hand, result, As thea carbon result, addition atomcarbon of is ammonia does not affect CO production as much, and favors methane formation. As a result, carbon distributedatom is distributed among CO among2, CH CO4, and2, CH C4 as, and the C ammonia as the ammonia amount isamount changed is changed while maintaining while maintaining a constant a is distributed among CO2, CH4, and C as the ammonia amount is changed while maintaining a levelconstant of CO level in theof CO product. in the Thus, product. in the Thus absence, in the of absence further COof further generation, CO generation, the H2/CO the ratio H appears2/CO ratio to constantincreaseappears linearlytolevel increase of withCO linearl in ammonia they product. with (Figure ammonia Thus8a)., in(Figure the absence 8a). of further CO generation, the H2/CO ratio appears to increase linearly with ammonia (Figure 8a).

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(a) (b)

FigureFigure 8. 8.( a(a)) Effect Effect of of ammonia ammonia content content on on syngas syngas quality quality (H (H2/CO2/CO ratio)ratio) andand carboncarbon contentcontent atat 10001000K; K; (b(b)) distribution distribution of of carbon-containing carbon-containing products products at at 1000 1000 K K as as a a function function of of ammonia ammonia content. content. 4. Conclusions 4. Conclusions Carbon formation is a severe challenge in methane-reforming reactions, particularly MDR Carbon formation is a severe challenge in methane-reforming reactions, particularly MDR (methane dry reforming). Based on the thermodynamic calculations for equilibrium product (methane dry reforming). Based on the thermodynamic calculations for equilibrium product distribution, it is clear that the carbon content in product can be minimized while simultaneously distribution, it is clear that the carbon content in product can be minimized while simultaneously enhancing hydrogen generation by utilizing ammonia as a in the MDR feed. Based on enhancing hydrogen generation by utilizing ammonia as a reducing agent in the MDR feed. Based detailed product analysis with temperature, it is concluded that ammonia favors water and methane on detailed product analysis with temperature, it is concluded that ammonia favors water and production at low temperature (T < 600 K), which, through the mechanism of methane steam reforming, methane production at low temperature (T < 600 K), which, through the mechanism of methane steam produces hydrogen and carbon monoxide at elevated temperature values (T > 600 K). The addition of reforming, produces hydrogen and carbon monoxide at elevated temperature values (T > 600 K). The ammonia somehow does not significantly affect CO, as most of the carbon gets distributed among addition of ammonia somehow does not significantly affect CO, as most of the carbon gets distributed CO2, C, and CH4. A complete carbon-free product distribution is possible by using NH3 ≥ 4 moles for among CO2, C, and CH4. A complete carbon-free product distribution is possible by using NH3 ≥ 4 each mole of methane, resulting in an H2/CO ratio above 5.5 mol/mol. The most significant role of moles for each mole of methane, resulting in an H2/CO ratio above 5.5 mol/mol. The most significant ammonia is towards CO2 conversion to methane at low temperature, in addition to producing high role of ammonia is towards CO2 conversion to methane at low temperature, in addition to producing quantity of hydrogen. high quantity of hydrogen. Funding: This research was funded by Qatar National Research Fund, grant number NPRP8-145-2-066 and NPRP8-509-2-209.Funding: This research The APC was was funded funded by by Qatar NationalNational Library. Research Fund, grant number NPRP8-145-2-066 Acknowledgments:and NPRP8-509-2-209This. The publication APC was was funded made possible by Qatar by National NPRP grant Library (NPRP8-145-2-066. and NPRP8-509-2-209) from the Qatar National Research Fund (a member of Qatar foundation). The statements made herein are solely Acknowledgments: This publication was made possible by NPRP grant (NPRP8-145-2-066 and NPRP8-509-2- the responsibility of the author(s). The publication of this article was funded by the Qatar National Library. 209) from the Qatar National Research Fund (a member of Qatar foundation). The statements made herein are Conflictssolely the of responsibility Interest: The of authors the author(s). declare The no conflict publication of interest. of this Thearticle founding was funded sponsors by the had Qatar no role Nat inional the Library. design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decisionConflicts to of publish Interest: the The results. authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the Referencesdecision to publish the results.

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