Luận văn thạc sĩ Kỹ thuật dầu khí: The effect of carbon monoxide on carbon dioxide methanation over nickel oxide catalysts

ABSTRACT This research was to investigate the effect of carbon monoxide CO on carbon dioxide CO2 methanation over a variety of NiO catalysts including different promoters MgO, CeO2, Pt,

INTRODUCTION

High consumption of fossil fuels worldwide resulted in rising carbon dioxide (CO2) concentration in the atmosphere, leading to global warming and climate shift

One of the most effective ways of dealing with the issue is hydrogenation of CO2 into synthetic fuel CH4 [1] This reaction was catalyzed by noble and transition metals [2- 5] such as rhodium (Rh), ruthenium (Ru), palladium (Pd), iridium (Ir), cobalt (Co), gold (Au), iron (Fe) and nickel (Ni) Among these metals, Ni was seen as the best one, thanks to high catalytic activity and reasonably manufacturing cost compared to noble metals [5-7] Metals and promoters were usually supported on γ-Al2O3, santa barbara amorphous (SBA-15) and mesostructured silica nanoparticles (MSN) While γ-Al2O3 is beneficial for CO2 hydrogenation due to its high selectivity toward methane and relatively low cost [8-10], SBA-15 and MSN are appealing to scientists because their pore structures as well as fundamental properties such as catalytic, conductive, adsorbed and magnetic can be adjustable [11-19] Overall, NiO/γ-Al2O3, NiO/SBA-15 and NiO/MSN were considered as potential catalysts for methanation To further increase the activity of catalysts in hydrogenation of CO2, a variety of promoters was used

Alkaline or alkaline earth metal oxides were used to weaken the catalyst acidity; precious metals were added to increase the reduction of NiO For instance, MgO was used to alkalize and increase the thermal stability of catalyst and the NiO dispersion [20], that led to improve theactivity and selectivity of CO2 hydrogenation into CH4 CaO was reported to be able to stabilize NiO catalyst, increase CO2 adsorption [21], preventing the decomposition of CH4 to produce coke [22], led to the increase in activity of Ni catalyst for methanation of CO [23] In addition to increasing the dispersion of NiO, CeO2 also showed the ability to improve the reduction of Ni 2+ into high active Ni o for hydrogenation of carbon oxides [24] Pt was also an additive of interest for NiO catalysts Pt had the ability to form Pt-Ni alloy to increase the dispersion of the active phase and reduce the size of metal particles [25], improving the catalyst stability [26], enhancing H2 adsorption, that made the activity of NiO catalyst in dry reforming of methane to be raised [27] Plus, Urea is a promising promoter because it was formed by a carbonyl ( C  O ) functional group and two

 groups which may have attraction to CO2, thereby increasing CO2 adsorption capacity on the supports

As for CO2 methanation over supported metal catalysts, experimental evidence shows that the first step of the mechanism of this reaction is the dissociation of CO2

(CO2(g)=CO(adsorbed)+O(adsorbed)) on the support It is also known as reversed water gas shift reaction (RWGS reaction, CO2 + H2  CO + H2O) endothermic, which enhances thermodynamically and kinetically at high temperature [28] Next, in the second step, these dissociated species reacts to H2 adsorbed on the metal to create CH4, known as exothermic CO methanation (CO + 3H2 → CH4 + H2O) which is favored for high CH4 selectivity at lower reaction temperature [29-33] As can be seen, CO methanation may supply heat from its reaction to the first step of CO2 methanation In the result, the methanation reaction can be carried out in a lower range of temperature to produce high selectivity of methane Addition of CO in feedstock probably accelerates the reaction rate of CO2 hydrogenation

Thus, this research aims to these targets:

- Understand the effect of CO on CO2 methanation

- Explain the relation between physico-chemical properties and catalytic activity of Nickel oxide catalysts on carbon dioxide methanation with the presence of carbon monoxide

This research was conducted at Department of Petro-chemistry & Catalysis, Institute of Chemical Technology, Vietnam Academy of Science and Technology.

LITERATURE REVIEW

Carbon dioxide in the Earth’s atmosphere

Figure 2.1 The Keeling curve of atmospheric CO2 concentrations measured at Mauna

Loa Observatory Carbon dioxide in Earth's atmosphere is a trace gas The current concentration of CO2 is about 0.04% (410 ppm) by volume (or 622 parts per million by mass) [34] having risen from pre-industrial levels of 280 ppm

Atmospheric concentrations of carbon dioxide fluctuate slightly with the seasons, falling during the Northern Hemisphere spring and summer as plants consume the gas and rising during northern autumn and winter as plants go dormant or die and decay Concentrations also vary on a regional basis, most strongly near the ground

4 with much smaller variations aloft In urban areas concentrations are generally higher and indoors they can reach 10 times background levels [35]

The concentration of carbon dioxide has risen due to human activities

Combustion of fossil fuels and deforestation has caused the atmospheric concentration of carbon dioxide to increase by about 43% since the beginning of the age of industrialization Most carbon dioxide from human activities is released from burning coal and other fossil fuels Other human activities, including deforestation, biomass burning, and cement production also produce carbon dioxide Human activities emit about 29 billion tons of carbon dioxide per year, while volcanoes emit between 0.2 and 0.3 billion tons [36, 37] Human activities have caused CO2 to increase above levels not seen in hundreds of thousands of years Currently, about half of the carbon dioxide released from the burning of fossil fuels remains in the atmosphere and is not absorbed by vegetation and the oceans

Figure 2.2 CO2 in Earth's atmosphere if half of global-warming emissions are not absorbed (NASA computer simulation) While transparent to visible light, carbon dioxide is a greenhouse gas, absorbing and emitting infrared radiation at its two infrared-active vibrational frequencies Light emission from the earth's surface is most intense in the infrared region between 200 and 2500 cm -1 , as opposed to light emission from the much hotter sun which is most

5 intense in the visible region Absorption of infrared light at the vibrational frequencies of atmospheric carbon dioxide traps energy near the surface, warming the surface and the lower atmosphere Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption Increases in atmospheric concentrations of CO2 and other long-lived greenhouse gases such as methane, nitrous oxide and ozone have correspondingly strengthened their absorption and emission of infrared radiation, causing the rise in average global temperature since the mid-20th century Carbon dioxide is of greatest concern because it exerts a larger overall warming influence than all of these other gases combined and because it has a long atmospheric lifetime (hundreds to thousands of years)

Not only do increasing carbon dioxide concentrations lead to increases in global surface temperature, but increasing global temperatures also cause increasing concentrations of carbon dioxide This produces a positive feedback for changes induced by other processes such as orbital cycles [38] Five hundred million years ago the carbon dioxide concentration was 20 times greater than today, decreasing to 4–5 times during the Jurassic period and then slowly declining with a particularly swift reduction occurring 49 million years ago [39]

Local concentrations of carbon dioxide can reach high values near strong sources, especially those that are isolated by surrounding terrain At the Bossoleto hot spring near Rapolano Terme in Tuscany, Italy, situated in a bowl-shaped depression about 100 m (330 ft) in diameter, concentrations of CO2 rise to above 75% overnight, sufficient to kill insects and small animals After sunrise the gas is dispersed by convection [40] High concentrations of CO2 produced by disturbance of deep lake water saturated with CO2 are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986 [41].

Carbon monoxide in the Earth’s atmosphere

Carbon monoxide (CO) is present in small amounts (about 80 ppb) in the Earth's atmosphere About half of the carbon monoxide in Earth's atmosphere is from the burning of fossil fuels and biomass [42] Most of the rest of carbon monoxide comes from chemical reactions with organic compounds emitted by human activities

6 and plants Small amounts are also emitted from the ocean, and from geological activity because carbon monoxide occurs dissolved in molten volcanic rock at high pressures in the Earth's mantle [43] Because natural sources of carbon monoxide are so variable from year to year, it is difficult to accurately measure natural emissions of the gas

Carbon monoxide has an indirect effect on radiative forcing by elevating concentrations of direct greenhouse gases, including methane and tropospheric ozone

CO can react chemically with other atmospheric constituents (primarily the hydroxyl radical, OH ) that would otherwise destroy methane Through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide and ozone Carbon monoxide is both short-lived in the atmosphere (with an average lifetime of about one to two months) and spatially variable in concentration.

Dual benefits from converting CO and CO 2 into fuel gas

Fossil fuels pose a fundamental dilemma for our human society On the one hand, their importance cannot be overstated - the combustion of coal, oil and natural gas supply close to 90 percent of our current energy needs and makes much of what we do possible On the other hand, their widespread use comes at a cost the gases emitted during the burning of fossil fuels are strongly implicated as the main drivers of climate change

There are several reasons why fossil fuels remain so popular Firstly, they are accessible in one form or another in almost all regions of the world Secondly, humankind has learned how to use them effectively to provide energy for a myriad of applications at every scale Thirdly, they are without equal as fuels for transportation, they are portable and contain a considerable amount of stored chemical energy The overwhelming energy source for transportation derives from oil stocks Considering all of this, it becomes obvious that our energy supply for the foreseeable future will surely be based on fossil-derived hydrocarbon fuels, with the unavoidable production of CO2 2.3.1 Environmental benefits

The only way to stabilize the Earth’s climate is to stabilize the concentration of greenhouse gases in the atmosphere A successful long-term local and global strategy

7 must surely be able to stabilize the atmospheric CO2 levels by substitution of fossil fuels by renewable energy sources It is often written that the potential of renewable energy sources is higher - by several orders of magnitude - than any estimated world energy demand Unfortunately, the vast majority of large renewable energy sources are almost always located far away from the main consumption areas One possibility is the production of electricity to join an electric grid If this is not a realistic possibility, the renewable energy must be harvested in the form of energy carriers As well as conventional energy storage for sustainable electricity, the generation of chemical energy carriers is another attractive alternative, with hydrogen, gas and liquid carbonaceous carriers as the primary candidates

Figure 2.3 A generic energy cycle using captured or sequestered CO2 and sustainable or renewable hydrogen to yield carbon-neutral or renewable carbonaceous fuels

The great value of gas and liquid carbonaceous fuels (e.g methane, petrol, diesel and others) lies both in their intrinsic (high) chemical energy content and in the ease with which they are stored and transported using existing infrastructure It is of course possible to reduce CO2 directly with hydrogen (hydrogenation), or potentially electricity, to synthesize carbonaceous fuels However, such an approach would not

8 impact positively on the global carbon balance since hydrogen and electricity as produced today are largely derived from fossil fuels, which, themselves, produce large amounts of CO2

If, however, renewable sources could be used as the energy vector to transform CO2 into fuels, one has a most attractive route to providing carbonaceous fuels that would not contribute to net CO2 emissions (figure 2.3)

Converting CO and CO2 into fuel gas especially methane is able to meet the demands of energy which has been rapidly increased

Table 2.1 Natural gas reserve and CO2 concentration in several gas fields in Vietnam

9 In Vietnam, CO2 has found in many natural gas fields with high concentration and large reserve as shown in table 2.1 [44] CO2 methanation is a promising solution to generate surplus energy Besides, not only does the hydrogenation of CO2 enhance heating value but it resolves several issues related to CO2 in gas processing such as corrosion and CO2 concretion as well.

Carbon oxides methanation

Methane is an energy carrier of significant importance to the industry, energy, and transportation sectors worldwide Its existing distribution infrastructure in many countries makes it a constitutive element of modern economies The major share of industrially used methane comes from fossil natural gas resources However, the debate of the finiteness of fossil resources and climate change caused the research expenditures relating to catalytic and biological methane production from carbon oxide-rich gases (methanation) to increase over the last years Biological methanation proceeds at low temperatures ( 99,9%) - 3-aminopropyltriethoxysilane: H2N(CH2)3Si(OC2H5)3 (Sigma-Aldrich, ≥ 99%) - N,N-dimethyl-acetamide: CH3CON(CH3)2 (Sigma-Aldrich, ≥ 99%)

- Ammonia: NH4OH (Xilong, 25-28 %) - Magnesium nitrate: Mg(NO3)2.6H2O (Xilong, ≥ 99%) - Chloroplatinic acid: H2PtCl6 (Merck)

- Ethanol: C2H5OH (Prolabo, ≥ 99%) - Water: H2O (Vietnam, 100%) 3.1.3 Procedure of SBA-15 catalytic support synthesis

SBA-15 was synthesized by dissolving 4 g Pluronic (P123) in 105 g water, stirring until gaining a homogeneous solution (solution 1) Next, 8.25g TEOS was slowly dropped to the solution 1, stirring 30 minutes to obtain solution 2 Then, the solution 2 was acidified with 28.6 g HCl 37% and stirred half an hour Thereafter, the acidified solution 2 was annealed at 60 o C for 24 hours to transform to suspensions

Afterwards, the suspensions were washed with water and then dried at 80 o C, 100 o C, 120 o C for 2 hours each Finally, the dried products were calcined at 550 o C for 10 hours to acquire SBA-15 The procedure diagram is presented in figure 3.1

Figure 3.1 SBA-15 synthesis procedure diagram

Preparation of MSN catalytic support

Hot plate magnetic stirrer, oven, furnace, fume hoods, centrifuge, vacuum filter kit, ultrasonic cleaner, cooling condenser, analytical balance, glass rod, pyrex tubes, becher, pipette, measuring cylinder, chromatography equipment

- N-cetyl-N,N,N-trimethyl-ammonium bromid: CTAB (Himedia, ≥99%) - Ethylene glycol: EG (Xilong, ≥99,9%)

- Ammonia: NH4OH (Xilong, 2528%) - Tetraethyl orthosilicate: TEOS [(C2H5O)4Si] (Merck, ≥ 99%) - Magnesium nitrate: Mg(NO3)2.6H2O (Xilong, ≥ 99%)

- Water: H2O (Vietnam, 100%) - 3-aminopropyltriethoxysilane: H2N(CH2)3Si(OC2H5)3 (Sigma-Aldrich, ≥ 99%) 3.2.3 Procedure of MSN catalytic support synthesis

MSN catalytic support was synthesized by using sol-gel method following the procedure described in figure 3.2 In the first step of the procedure, 0.032 mol CTAB, 2 mol EG, 2 mol NH4OH and 1 mol H2O were mixed together and stirred at 80 o C for 30 minutes Then, 12 mmol TEOS were slowly dropped to the blended solution while it continues stirring for 2 hours at 80 o C Next, it is annealed for 24 hours to form suspensions After that, the suspensions were filtered, washed and then dried at 60 o C

Finally, the obtained product was calcined at 550 o C for 3 hours with air (3 L/h) to acquire MSN

Figure 3.2 MSN synthesis procedure diagram

Preparation of promoted Nickel oxide catalysts

3.3.1 Preparation of promoted Nickel oxide catalysts using wet impregnation method

Wet impregnation is the simplest method to add NiO and promoters to the supports in heterogeneous catalyst synthesis The procedure diagram of Nickel oxide and promoters preparation is illustrated in figure 3.3

Figure 3.3 Nickel oxide and promoters impregnation procedure diagram

37 First of all, a certain amount of Nickel (II) nitrate (Ni(NO3)2.6H2O) and an identified amount of promoters were dissolved in water Then, the solution was impregnated with catalytic support by intended ratio to obtain catalytic components shown in table 3.1 Next, the impregnated solution was oscillated for 5 minutes in ultrasonic cleaner Later, it was dried at 80 o C, 100 o C, 120 o C for 2 hours each

Thereafter, the dried product was cooled down to room temperature before calcining at 600 o C for 4 hours to obtain planned catalyst

3.3.1.1 Preparation of non-promoted and promoted NiO catalyst supported on γ-Al2O3

The non-promoted NiO and promoted NiO catalysts were prepared by the impregnation method as below: dissolved Ni(NO3)2.6H2O and Mg(NO3)2.6H2O or Ce(NO3)3.6H2O or [Ca(NO3)2.6H2O and H2PtCl6] with the intended content to reach their components of the catalyst (table 3.1) in distilled water The solution was impregnated with γ-Al2O3, and after stirring in 1 hour the obtained suspension was overnight aged and dried at 80 o C, 100 o C and 120 o C for 2 hours for each temperature and calcined at 600 o C for 4 h as described in [166]

3.3.1.2 Preparation of non-promoted and promoted NiO catalyst supported on SBA-

15 Ni(NO3)2.6H2O and Ce(NO3)3.6H2O were dissolved with the identified ratio to reach their components (table 3.1) in distilled water Then, the solution was impregnated with SBA-15 The next steps are followed as figure 3.5 to acquire the catalysts

3.3.1.3 Preparation of non-promoted and promoted NiO catalyst supported on MSN

Ni(NO3)2.6H2O and Urea were also dissolved with the intended contents to fit their components of the catalyst (table 3.1) Next, the solution was impregnated with MSN After oscillating in 1 hour, the obtained suspension was dried at 80 o C, 100 o C and 120 o C for 2 hours for each temperature The product was cooled down to room temperature and then calcined at 600 o C for 4 hours to obtain the catalysts

38 In this thesis catalysts with optimal composition will be prepared and used The eight prepared catalysts are listed in Table 3.1 The explanation of using these catalysts will be mentioned in section 4.1

Order The catalytic components (%wt) Notation

4 0.1%Pt; 11%CaO; 37.7%NiO; 51.2% Al2O3 0.1Pt11CaNiAl

6 4%CeO2; 50%NiO; 46%SBA-15 4Ce50NiSBA15

8 Urea (1U : 3Nitrate); 50%NiO; 50%MSN 50NiMSN-Urea

X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase identification of a crystalline material and can provide information on unit cell dimensions The analyzed material is finely ground, homogenized, and average bulk composition is determined

X-ray diffractometers consist of three basic elements: an X-ray tube, a sample holder, and an X-ray detector X-rays are generated in a cathode ray tube by heating a filament to produce electrons, accelerating the electrons toward a target by applying a voltage, and bombarding the target material with electrons The specific wavelengths are characteristic of the target material (Cu, Fe, Mo, Cr) Copper is the most common target material for single crystal diffraction, with CuKα radiation = 1.5418Å These X-

39 rays are collimated and directed onto the sample As the sample and detector are rotated, the intensity of the reflected X-rays is recorded When the geometry of the incident X-rays impinging the sample satisfies the Bragg Equation, constructive interference occurs and a peak in intensity occurs

A detector records and processes this X-ray signal and converts the signal to a count rate which is then output to a device such as a printer or computer monitor The instrument used to maintain the angle and rotate the sample is termed a goniometer

For typical powder patterns, data is collected at 2θ from ~5° to 70°, angles that are preset in the X-ray scan

Debye-Scherrer in the form of Equation 3.1 shows the relationship between the crystalline (Db) and the broadening (β) of the diffraction line corresponding to Bragg angle (θ) using wavelength (λ)

K = Scherrer constant, 0.94 λ = X–ray wave length (A o )

Bd = angular width of peak in term of Δ (2θ) (radian) θ= Bragg angle of the reflection (degree)

It is important to note that the result from the Scherrer Equation is a crystallite thickness that is perpendicular to the diffraction planes rather than an actual particle size It is necessary to apply a correction factor that depends on the actual sharp of the crystallites and on the Millers indices of the diffracting planes in order to obtain the actual crystallite size from the thickness

XRD can also give some information on the dispersion of a supported catalyst only if it is in the form of a separate crystallite phase

In this thesis, the XRD patterns of the catalytic samples were determined by using XRD D2-PHASER (Brucker) The equipment changed θ angle to generate diffraction; the rotating speed of detector is as twice as that of the samples X-ray beam was formed from radiation of Cu (λ = 0,154056 nm) Reflections, derived from interaction between the samples and X-ray beam, were counted by Geiger counter to

40 produce XRD patterns The results were going to compare with data in standard XRD pattern bank to find out phase identification

3.4.2 Brunauer-Emmett-Teller (BET) method

One of the most important properties of a heterogeneous catalyst is its surface area since the reaction takes place on the catalyst surface The determination of surface area and pore size was done using the Autosorb-1 Gas Sorption System (Quantachrome Corporation) This equipment is based on Brunauer-Emmett-Teller (BET) method N2 gas with cross-sectional area of 16.2x10 -20 m 2 /molecule was used as the adsorbate at liquid temperature of 77 K Before measurement, sample was outgassed by heating under vacuum at 150 o C for 2h to eliminate adsorbed species at the surface Twenty-one point adsorption isotherms at P/P0 ratio less than 1 was utilized to obtain the surface area The result was analyzed by Autosorb Anygas Software Version 2.1, which were calculated using BET equation as show in Equation below:

P= pressure of gas Po= saturated vapor pressure of the liquid at the operating temperature W= weight of gas adsorbed at a relative pressure Po

Wm= weight of adsorbate constituting a monolayer of the surface coverage C= constant that is related to the energy of absorption in the first adsorbed layer and magnitude of the adsorbent/adsorbate integration

The surface area was calculated by following equation:

S= specific surface area (m 2 /g) Anitrogen = cross-sectional area of one molecule N2 = 0,162 nm 2 (at 77 K) Mw(nitrogen) = molecular weight of N2, 28

41 In this thesis, the BET surface areas of the catalytic samples were determined by using BET method on BET NOVA 2200 E with the properties as follows:

+ Analysis Bath Temperature: 77.300 K + Automatic Degas: Yes

At first, the samples were undergone heat treatment from 150 to 300 o C for 2 hours to remove adsorbed species Then, the isotherm adsorption process started at 0.05 of relative pressure (P/P0) and ended at 0.3 to build multi-point BET plot which was used to acquire Wm (equation 3.2) The specific surface area of the catalysts was calculated from equation 3.3

Catalyst activity testing system

Figure 3.5 CO2 methanation reaction system

- ON/OFF valve, check valve, pressured control valve, flow meter

- Reactor (1), temperature controller (3), steam condenser (4), gas chromatography connector (5), flow meter using soap (2)

45 - CO2 sensor (6), gas chromatography installed professional software

Procedure for conducting reaction

In storage, all the catalysts are of metal oxide, so they must be reduced to be active metal for conducting reaction Moreover, in reduction process, most contaminants such as water, adsorbents will be removed from catalytic surface

The reduction process includes three fundamental steps First of all, 0.2 g catalyst were put into reactor and fixed by 0.8 g quartz Next, H2 stream (2 L/h) was supplied and sent through the catalytic fixed-bed The reducing reaction was performed at 450 o C for 4 hours Finally, the catalyst was cooled down to 225 o C to prepare for CO2 methanation

After activating the catalyst, gaseous reactants, 20% mol (CO2 + CO)/H2 or 20% mol (CO2 + N2)/H2, were fed at the total flowrate of 3 L/h CO2 methanation was conducted at 225 o C, 250 o C, 275 o C, 300 o C, 325 o C, 350 o C, 375 o C and 400 o C The reaction mixture was analyzed by gas chromatography using flame ionization detector (FID) with capillary column DB624 and thermal conductivity detector (TCD) with capillary column HP-PLOT MoleSieve 5A)

46 Besides, CO2 concentration was recorded by CO2 sensor Vaisala GMP221KONO at each temperature level

3.6.3 Calculate CO, CO2 conversion and CH4 selectivity

Base on the results of CO2 sensor, the conversion of CO2 is determined as follow:

C C is the amount of CO2 reacted;

C CO is the initial CO2 concentration

Similarly, the conversion of CO is calculated as follow:

(3.6) Where: C CO o C CO is the amount of CO reacted;

C CO o is the initial COconcentration

C is the amount of CH4 calculated from the GC CH4’s peak;

C C is the amount of CO2 reacted; o

C C is the amount of CO reacted

RESULTS AND DISCUSSION

Explanation of catalyst’s selection

In previous study of group of Prof Luu Cam Loc on CO methanation [176], the optimal composition of NiO on -Al2O3 was determined to be 37.7 wt% and the optimal regime for catalyst calcination is 600 – 4h and reduction at 450 o C - 4h

Therefore, in this study, the catalyst 37.7 wt% NiO/-Al2O3 was prepared and modified with different additives In addition, the optimal contents of promoter were 3 wt% MgO [166] and 11 wt% CaO [23] To further increase the activity of the 11CaNiAl catalyst, Pt as reduction additive, was added To determine the optimal content of additives, NiAl and CaNiAl catalysts modified by CeO2 and Pt are prepared and investigated in the solo-CO2 hydrogenation reaction The activities of NiAl catalysts modified by CeO2 and Pt + CaO are showed in figure 4.1 and figure 4.2

Figure 4.1 The CO2 conversion in CO2 hydrogenation over CeNiAl catalysts It can be seen in figure 4.1, CO2 conversion in solo-methanation over all samples showed upward trends Although at a temperature lower 325 °C, catalysts

48 NiAl modified by 6% CeO2 don’t have the highest CO2 conversion, but in temperatures range from 325 o C to 400 o C, the main temperature regime of CO2 methanation, the highest CO2 conversion belonged to 6CeNiAl sample, its CO2 conversion at 325 o C was 74.0% and increased with increasing temperature and reached at 94.2% at 400 o C, compared to 82.3%, 78.0% and 61.9% of 4CeNiAl, 2CeNiAl and NiAl samples, respectively at the same condition

Figure 4.2 The CO2 conversion in CO2 hydrogenation over Pt11CaNiAl catalysts

The figure 4.2 shows that CO2 conversion in methanation over 0.1Pt11CaNiAl was always higher than that of others in temperature range of 225325 o C In detail,

11CaNiAl sample modified by 0.1 wt.% Pt reached 87.6% of CO2 conversion at 325 oC, compared to 79.8%, 77.5%, 79.7% and 53.9% of 11CaNiAl catalysts modified by 0.4, 0.3, 0.2 and 0 wt.% Pt respectively At a temperature of 350 °C or higher the activity of promoted catalysts is approximately the same (X ~ 88-90%) and higher than the non-promoted one From the results in figure 4.1 and figure 4.2, it followed

49 that both cerium oxide and platinum gave a promoting effect The optimal content of CeO2 and Pt was found to be 6 wt% and 0.1 wt% respectively

Similarly, the optimal content of NiO for SBA-15 was 50 wt% [177] and MSN was also 50 wt% shown in figure 4.3; the optimal contents of promoter were 4wt% of CeO2 on SBA-15 and 16.7wt% of Urea on MSN as shown in figure 4.4 and figure 4.5

Figure 4.3 The CO2 conversion in CO2 hydrogenation over NiMSN catalysts

It can be observed in figure 4.3 that all samples show upward trends; however, CO2 conversion line graph of sample 50NiMSN is always above that of others In detail, it started at 15.11% of CO2 conversion at 225 o C and finished at 96.66% at 400 oC compared to 87.98%, 95.72% and 94.81% of 30NiMSN, 40NiMSN and 60NiMSN at the same temperature, respectively

Figure 4.4 The CO2 conversion in CO2 hydrogenation over Ce50NiSBA15 catalysts In figure 4.4, samples 2Ce50NiSBA15 and 4Ce50NiSBA15 reached 100% of CO2 conversion at 400 o C In addition, in lower temperatures (< 400 o C) CO2 conversion of sample 4Ce50NiSBA15 is always higher than that of others For reasons above, we can conclude that 4Ce50NiSBA15 is the optimal sample in this case

Figure 4.5 The CO2 conversion in CO2 hydrogenation over Urea50NiMSN catalysts As can be seen in figure 4.5, in low temperature zone (< 325 o C), samples containing 0%, 10% and 16.7% of Urea show considerably sastified results However, in high temperature zone (> 325 o C), CO2 conversion of samples with 0% and 10% of Urea is much lower than that of samples with 12.5% and 16.7% of Urea Overall, 50NiMSN-Urea catalyst presented the best outcome.

Catalyst characterization

The X-ray diffraction spectra of nickel catalysts with different promoters supported on aluminium oxide (γ-Al2O3) are shown in figure 4.1:

Figure 4.6 X-ray diffraction spectra of (a) NiAl, (b) 3MgNiAl, (c) 6CeNiAl, (d)

XRD spectra of the catalysts (figure 4.6) showed the characteristic peaks of NiO at 2θ = 37.04, 43.03, 62.78, 75.4, and 79.4, CeO2 at 2θ = 28.5, CaCO3 at 2θ

= 29.4, the weak peak of γ-Al2O3 at 2θ = 67.37, and very weak diffraction peaks for MgO [166] In addition, diffraction peaks for NiO in XRD pattern of 0.1Pt11CaNiAl were considerably less intense than that of NiAl and 3MgNiAl This result indicated that CaO and Pt increased the dispersion of NiO particles on γ-Al2O3 The NiO particle size values of the catalysts, calculated from the Scherrer equation, varied from 16 nm to 28 nm (shown in figure 4.7)

Figure 4.7 NiO crystallite sizes of catalysts supported on γ-Al2O3

The sizes of crystallite NiO on the catalysts supported on γ-Al2O3 calculated by Scherrer equation at 2θ = 43.3 o were shown in figure 4.7 The crystallite sizes of NiO on NiAl, 3MgNiAl and 6CeNiAl were analogous at 23.4 nm, 22.7 nm and 27.5 nm respectively Thanks to the effect of Pt and CaO, the catalyst 0.1Pt11CaNiAl had the lowest size of NiO which was 15.9 nm

XRD spectra of catalysts are shown in figure 4.8 It is clear that the diagrams indicated typical peaks of NiO crystallite structure (2θ = 37.3 o ; 43.3 o ; 62.9 o ; 75.3 o and 79.4 o ) for 50NiSBA15, 4Ce50NiSBA15, 50NiMSN and 50NiMSN-Urea Plus, there was a small peak at 2θ = 28.6 o in the XRD pattern of 4Ce50NiSBA15, which approves the existence of CeO2 in fluorite-type cubic crystal structure [178] The peak presenting SiO2 was not observed because of its amorphous shape In figure 4.8a, b, there was no peak for Ni2SiO4, which confirms a weak interaction between NiO and SiO2 As for catalysts supported on MSN, the peaks were more significantly intense

54 when the support was modified by Urea due to its larger NiO partcle size (shown in figure 4.9) Ni2SiO4 phase was also not detected

Figure 4.8 X-ray diffraction spectra of (a) 50NiSBA15, (b) 4Ce50NiSBA15, (c)

Figure 4.9 NiO crystallite sizes of catalysts supported on SBA-15 and MSN

The results from Scherrer equation computed at 2θ = 43.3 o of catalysts 50NiSBA15, 4Ce50NiSBA15, 50NiMSN and 50NiMSN-Urea were illustrated in figure 4.9 As mentioned above, the existence of CeO2 phase may interact with NiO, generating bulk species Thus, the crystallite size of NiO on 4Ce50NiSBA15 was 2 nm larger than that on 50NiSBA15 which was 13.7 nm The greatest NiO crystallite size belonged to 50NiMSN-Urea because of its narrowest pore diameter, that led to generate bulk NiO on MSN’s surface

Table 4.1NiO crystallite sizes of all the catalysts at 2θ = 43.3 o

Order Catalyst NiO crystallite size (nm)

It can be observed in table 4.1 that catalysts supported on SBA-15 had the smallest NiO particle size (13-16 nm) because SBA-15 possessed wide pore diameter, high porosity, high specific surface area (table 4.3) and high thermal stability leading to prevent NiO particles from sintering NiO particles on MSN (17-19 nm) were slightly larger than those on SBA-15, attributed to narrow pore diameter of MSN (seen in table 4.3) Thanks to high specific surface area and high porosity, the NiO crystallite sizes of MSN were still significantly smaller than those of γ-Al2O3 The biggest NiO particles belonged to catalysts supported on γ-Al2O3 due to its low specific surface area However, Pt and CaO dramatically enhanced NiO dispersion The evidence in table 4.1 showed that 0.1Pt11CaNiAl produced tiny NiO particles (around 16 nm) which were even smaller than those on MSN From the results in table 4.1, the order in the NiO dispersion can be concluded as follow: SBA-15 > MSN >> γ-Al2O3

SEM images of the catalysts supported on Al2O3 are presented in figure 4.5

Figure 4.10 SEM images of catalysts supported on -Al2O3

SEM images of the NiAl, 3MgNiAl, and 6CeNiAl catalysts (figure 4.10 a,b,c) showed uniform particles and almost sphere shape (tens nm in size), ascribed to NiO and big blocks (hundreds of nm in size), which is attributed to carrier The big blocks on non-promoted sample have larger size than promoted one, indicating that the additives increase the thermal stability of the catalyst

There are significant differences in the shape of NiO particles on NiAl, 3MgNiAl, and 6CeNiAl samples compared to 0.1Pt11CaNiAl one The NiO particles on the first three samples have a smooth, rounded shape, while on the fourth one the fine, spongy and sharp edges particles were presented This shows that the first three catalysts were sintered to a certain degree when were heated at 600 o C, while the last one was not From this observation it can be concluded that CaO exhibited higher ability to increase the thermal stability of the NiAl catalyst than other additives

Figure 4.11 SEM images of catalysts supported on SBA-15 and MSN

SEM images of the catalysts supported on SBA-15 and MSN are given in figure 4.11 Overall, all the catalysts possessed large porosity It is clear that NiO particles of 50NiSBA15 and 4Ce50NiSBA15 were evenly allocated, which may result in the high specific surface area of these catalysts SEM image of 4Ce50NiSBA15 showed many particles in fluorite-type cubic shape, attributed to CeO2 phase as mentioned in XRD results According to SEM images, the smallest NiO particle size belonged to sample 50NiSBA15, compared to the biggest NiO particle size of 50NiMSN-Urea NiO conglomeration was observed in SEM image of 50NiMSN, which may explain to its lowest specific surface area From this observation it can be concluded that SBA-15 is better thermal stability than MSN

Figure 4.12 TEM images of catalysts supported on -Al2O3

It can be derived from the TEM analysis (figure 4.12) that nanometer-sized NiO particles have been successfully prepared although the NiO content in catalysts is as high as 37.7 wt.% thanks to the high porosity and large pore diameter (> 4 nm) of - Al2O3 The NiO particles of several nm inside the pore (bright colored spots) and particles of dimensions from several nm to several tens nm on the outer surface of - Al2O3 (dark spots) can be observed Two samples modified with MgO and CeO2 showed smaller and discrete particles than others On the 3MgNiAl sample besides very small dark-colored particles, 5080 nm blocks covering the surface of the carrier

60 were also observed Therefore this sample showed the smallest specific surface area (table 4.2) Compared to MgO promoted sample, 6CeNiAl and especially 0.1Pt11CaNiAl catalysts showed higher densities of dark spots on the surface This is explained by the fact that in these two samples CeO2 and CaCO3 crystals coexist with NiO crystalline particles while MgO exists in the amorphous state (from the XRD analysis) TEM image of sample 0.1Pt11CaNiAl (figure 4.12d) indicated that there is coexistence of NiO particles of a few nm and square blocks of tens nm, ascribed to CaCO3, as evidenced from the XRD spectra and CO2-TPD profiles (figure 4.16) The additional surface covering of big block of CaCO3 also contributed to a slight reduction in the surface area of the final catalyst compared to non-promoted one (table 4.2) The CeO2-modified catalyst is the most evenly distributed and uniform NiO particles compared to the other catalysts, resulting in the highest surface area of this sample In particular, specific surface area of 6CeNiAl catalyst (95.0 m 2 g -1 ) is greater than that of the non-promoted catalyst NiAl, which may be explained that CeO2 enhanced the dispersion of NiO on surface and into pore of the support, it also prevents NiO particles from sintering

Figure 4.13 TEM images of catalysts supported on SBA-15 and MSN

Overall, nanometer-sized NiO particles have been formed although the NiO loading content is extremely high, at 50 wt% In term of catalysts supported on SBA- 15, TEM images showed discrete and uniform particles The NiO particle sizes of several tens nm can be observed both inside the pore (bright color) and on the surface (dark color) of SBA-15 As can be seen, the NiO particle size of sample 50NiSBA15 was slightly smaller than that of 4Ce50NiSBA15, which may account for the higher specific surface area at 214.7 m 2 g -1 of the former compared to 176.2 m 2 g -1 of the latter due to pore filling and space occupation of CeO2 (detected in XRD analysis)

However, dark regions in TEM image of 50NiSBA15 was denser and higher densities than that of 4Ce50NiSBA15 This result indicates that CeO2 improved the NiO dispersion into the pore and on the surface of the support As for catalysts supported on MSN, there were several huge conglomerations in TEM image of 50NiMSN, attributed to NiO sintering This led to increase its crystallite size of NiO, thereby decreasing its specific surface area Compare to 50NiMSN, the particles in TEM image of 50NiMSN-Urea was fine, uniform and evenly distributed, which leads to its highest specific surface area although there were still several small agglomerations, attributed to its highest NiO particle size This result approves that Urea enhanced the thermal stability of catalysts supported on MSN and prevented NiO particles from sintering Thanks to wide pore diameter of SBA-15, its NiO particles were more discrete than those on MSN

The specific surface area, pore volume and pore diameter of nickel oxide catalysts with several promoters loading supported on Al2O3 are shown in table 4.2

It can be observed that the specific surface area was diminished when promoters were added into the catalysts, which attributes to pore filling of the support [179] However, with a right content, CeO2 enhanced the catalyst’s surface area, which increases from 89.4 m 2 g -1 of the pure to 95.0 m 2 g -1 of the promoted [129]

Analogously, MgO improved the pore volume and pore diameter compared to the opposite variation of the other promoters

Table 4.2 Specific surface area, pore volume and pore diameter of catalysts supported on -Al2O3

0.1Pt11CaNiAl 83.0 0.062 4.48 the specific surface area of Al 2 O 3 (S BET ): 225 m 2 g -1

Table 4.3 Specific surface area, pore volume and pore diameter of catalysts supported on SBA-15 and MSN

50NiMSN-Urea 271.6 0.187 1.96 the specific surface area of SBA-15 (S BET ): 639.1 m 2 g -1

63 the specific surface area of MSN (S BET ): 1352 m 2 g -1

There were tremendous drops in the specific surface area of all the catalysts shown in table 4.3, thereby the decline of the pore volume and pore diameter This may correspond to pore filling and space occupation of NiO and promoters on the carriers It is paradoxical that 50NiMSN-Urea had 271.6 m 2 g -1 of the specific surface area which was 113.7% greater than that of 50NiMSN This phenomenom can be attributed to the increase of NiO dispersion of Urea As can be seen in SEM and TEM images of 50NiMSN-Urea, NiO particles were evenly distributed and highly uniform

Figure 4.14 H2-TPR diagrams of catalysts supported on γ-Al2O3

H2-TPR diagram of the NiAl catalyst indicated a single reduction peak at Tmax 380 o C, attributed to the reduction of free NiO particles The strong interaction of NiO with the MgO additive is demonstrated by the shifting the reduction peaks into the higher temperature zone (Tmax = 415 o C) on H2-TPR diagram of the MgNiAl catalyst

In contrast, the 6CeNiAl and 0.1Pt11CaNiAl profiles displayed similar outcomes,

The effect of CO on CO 2 methanation

4.3.1 The effect of CO on CO2 methanation over catalysts supported on γ-Al2O3

Table 4.4 CO, CO2 conversion and CH4 selectivity in the hydrogenation of CO2 over catalysts supported on -Al2O3 with (I) and without (II) the addition of CO in reactants

Figure 4.18 CO2 conversion in hydrogenation of carbon oxides mixture (solid lines) and of single CO2 (dashed lines) over catalysts supported on γ-Al2O3 It is clear from figure 4.18 that all promoters greatly increased the activity of catalyst in hydrogenation of CO2 On the modified catalysts, the reactions began to take place at temperatures of 225250 °C, in contrast, at temperatures under 300 C, both reactions on non-promoted NiAl did not happen

It has been shown in figure 4.18, CO2 conversion in the reaction with the presence of 1 wt.% CO in feedstock (case I) was much better than those without the addition of CO (case II) In term of the NiAl catalyst, the gap between CO2 conversions of two cases of hydrogenation increased markedly corresponding to the rise of temperature in a range of 300400 o C The 3MgNiAl catalyst produced a substantial CO2 conversion As for CO2 methanation in the first case, the started reaction temperature is 250 o C which is lower than 300 o C of the NiAl catalyst This is because MgO prefers a high CO2 adsorption capacity Consequently, CO2 conversion at 400 o C for both cases of CO2 methanation on the 3MgNiAl catalyst was somewhat

72 satisfied, the former finished at 88.1% of CO2 conversion, the latter ended at nearly 4% lower Observably, CO2 conversion graph of the CO-added reaction was always above that of the non-added one

Figure 4.19 CO conversion in hydrogenation of CO + CO2 mixture on catalysts supported on γ-Al2O3

Paradoxically, at 250 o C, while the hydrogenation reaction on the 6CeNiAl catalyst in the second case occurred, the first case did not happen This phenomenon may be due to the adsorption competition between CO and CO2 on the catalyst In a higher temperature range of 275400 o C, CO2 conversion in the first case was always greater than that in the second case because at that temperature CO has completely converted, as seen in figure 4.19

The 0.1Pt11CaNiAl showed the best improvement of the addition of CO in reactants This is because Pt and CaO simultaneously promoted this catalyst, the former has a strong attraction to H2, and the latter raises the support’s basicity leading to high CO2 adsorption ability At 275 o C, CO2 conversion of both cases of hydrogenation reactions was analogous, at around 6.3% At the next temperature level

73 (300 o C), although CO2 conversion in the second case was three-fold, that in the first case was approximately 10 times as large as its CO2 conversion at 275 o C Finally, at 400 o C, the first case finished at 90.2% of CO2 conversion and the second case ended at 72.3% Apparently, CO2 conversion soars under the addition of CO into reactants

Comparing the results in figures 4.18 and 4.19 indicated that the activity of catalysts in hydrogenation of CO was higher than that of CO2 Even at 225 o C the CO conversion on catalysts has reached a value of several tens of percent while CO2 hydrogenation has begun at 275 o C Similar to hydrogenation of CO2, the improvement in activity of NiAl catalysts by adding of promoters was also clearly expressed in hydrogenation of CO In detail, at 250 o C CO conversions in hydrogenation of carbon oxides mixture over 3MgNiAl, 6CeNiAl and 0.1Pt11CaNiAl catalysts reached 100%, 94.0% and 57.1% respectively compared to 25.6% on non-promoted catalyst NiAl

The temperature of 100% CO conversion were 250 o C and 275 o C respectively for 3MgNiAl and 6CeNiAl, 0.1Pt11CaNiAl catalysts, compared to 325 o C for the non- modified catalyst

In general, the order in the activity of catalysts in single CO2 methanation (case II) and methanation of CO in the mixture was observed as follows: 3MgNiAl  6CeNiAl > 0.1Pt11CaNiAl >> NiAl It indicated that hydrogenation of carbon oxides take place on the one kind of active center The presence of CO in feedstock activated the hydrogenation of CO2 thanks to generated heat in the CO hydrogenation providing to the reaction of CO2 In this case the activity of the promoted catalysts was almost equal: 0.1Pt11CaNiAl  6CeNiAl > 3MgNiAl >> NiAl

Figure 4.20 CH4 selectivity in CO2 hydrogenation over catalysts supported on γ-Al2O3

Generally, CH4 selectivity in CO2 methanation in the first case was higher and more stable than that in the second case (figure 4.20) In detail, CH4 selectivity of the single CO2 hydrogenation on NiAl catalyst in the second case at 325 o C was approximately 31% and gradually rose to 73-77% when the reaction temperature was increased to 400 o C In the first case, on this catalyst there was a dramatic improvement in CH4 selectivity, it was ranged from 93 % to 95% in the reaction temperature range of 350400 o C and even reached 100% at 325 o C There were incredible tendencies for CH4 selectivity of CO2 methanation over 3MgNiAl depicted in figure 4.20 They almost remained stable at 100%, which can be ascribed to the enhancement of the support’s basicity, thermal stability and the diminution of NiO sintering at high temperatures when the catalyst was modified by MgO As seen, CH4 selectivity of the reactions over the 6CeNiAl and 0.1Pt11CaNiAl catalysts in the second case was analogous to that of the MgO-promoted catalyst They also surged to

75 approximately 97% and the levelled off, while in the first case, CH4 selectivity of these catalysts was always higher than 99%

4.3.2 The effect of CO on CO2 methanation over catalysts supported on SBA-15 and

Table 4.5 CO, CO2 conversion and CH4 selectivity in the hydrogenation of CO2 over catalysts supported on SBA-15 and MSN with (I) and without (II) the addition of CO in reactants

Figure 4.21 CO2 conversion in hydrogenation of carbon oxides mixture (solid lines) and of single CO2 (dashed lines) over catalysts supported on SBA-15 and MSN

77 It can be observed in figure 4.21, CO2 conversion in the reaction with the addition of CO in reactants (case I) was higher than those without the attandance of CO (case II) As for 50NiSBA15, CO2 conversion in case II (the second case) was nearly 4% at 275 o C At the same temperature, the methanation of CO2 in case I (the first case) did not happen because CO2 can not adsorb on the support due to the competition of CO CO2 conversion of the reaction in both cases went up slightly until 325 o C In temperature range of 325-400 o C, there was a jump in CO2 conversion in both cases This is because the first step of CO2 methanation received sufficient heat to accelerate the reaction rate In detail, CO2 conversion of the methanation in case I started at 17.4% and finished at 89.4% which was noticably better than 82.6% of the methanation in case II, thanks to excessive heat supply from the addition of CO

The 4Ce50NiSBA15 catalyst gave the best outcome in low temperatures (< 350 oC) compared to 50NiSBA15 catalyst as seen in figure 4.21 The started reaction temperature for both cases was 250 o C which was lower than 300 o C of 50NiSBA15

This can be explained in figure 4.22, CO conversion in hydrogenation of the mixture (CO + CO2) over 4Ce50NiSBA15 was 100% at 250 o C compared to 325 o C of 50NiSBA15 This result indicates that the number of active metal species of sample 4Ce50NiSBA15 was significantly better than that of 50NiSBA15, attributed to the improvement of NiO dispersion of CeO2 In contrast, between 350 o C and 400 o C, CO2 conversion in the methanation in case I and case II increased from 74.5% to 80.9% and 51.8% to 73.8% respectively, which was much lower than those of 50NiSBA15, ascribed to the lower specific surface area of 4Ce50NiSBA15

In term of catalysts supported on MSN, the started reaction of 50NiMSN for both cases was 275 o C which was also the temperature of 100% CO conversion (shown in figure 4.22) Between 275 o C and 325 o C, CO2 conversion in both cases was similar The gap between CO2 conversion in case I and case II widened at higher temperatures, at 400 o C, the former reached 85.3% of CO2 conversion which was approximately 5% higher than that of the latter

In comparison, despite of highest specific surface area, CO2 conversion in the methanation in the first case over 50NiMSN-Urea was similar with that over 50NiMSN in temperatres below 350 o C This may be explained in figure 4.22, the

78 temperature of 100% CO conversion of 50NiMSN was lower than that of 50NiMSN- Urea, that led to the higher RWGS reaction rate of 50NiMSN Additionally, according to Schaber et al [187], Urea will be vaporized and decomposed above 152 o C

CONCLUSION AND RECOMMENDATION

Conclusion

In the study, nano-structure promoted NiO catalyst on porous alumina was successfully prepared by the impregnation method Alkaline and precious metal additives dramatically change physico-chemical properties of NiO/γ-Al2O3 catalyst that increased the activity and CH4 selectivity of hydrogenation of CO2-rich gas

Owning high activity for methanation of carbonoxides promoted NiO/γ-Al2O3 catalysts completely converted CO at temperatures as low as 250 - 275 o C and converted 80-90% CO2 at 400 o C CO activated CO2 hydrogenation, its presence increased CO2 conversion by 3-25% and raised methane selectivity up to 95-100%

Additives increased the dispersion of NiO particles and protected the nanostructured NiO particles from sintering MgO and CaO enhanced the support’s basicity and CO2 adsorption, while CeO2 and Pt improved the reduction of active phase Ni 2+ MgO was the best promoter for hydrogenation of CO and single CO2 However, the catalyst simultaneously modified by additives of the two group (Pt and CaO) boosted CO2 conversion and CH4 selectivity in mixture methanation, which is attributed to the strong hydrogen attraction of Pt and the increase of the CO2 adsorption of CaO

In term catalysts supported on SBA-15 and MSN, nano-structured promoted NiO catalysts were also successfully synthesized by the impregnation method

Additives considerably changed the characteristics of catalysts, leading to increase the efficacy of the CO2 hydrogenation In the single CO2 methanation, the catalysts totally converted 73-83% CO2 at 400 o C In the hydrogenation of the mixture (CO + CO2), CO was completely converted at 300 o C and 80-90% CO2 were converted at 400 o C

CO has a promoted effect on CO2 methanation, its addition into feedstock boosted CO2 conversion by 4-8% and increased CH4 selectivity up to 90-100%

CeO2 increased the reducibility of NiO, resulting in high activity of the catalyst in both single CO2 methanation and hydrogenation of the mixture (CO + CO2) Urea protected NiO particles from sintering, leading to improve specific surface area

Besides the positive effect of Urea, it was decomposed and vaporized at above 152 o C,

82 causing preventing the adsorption of CO, CO2 and H2 on the support leading to decrease CO2 conversion and CH4 selectivity of the methanation at low temperatures

Adding CO to reactants and introducing promoters into NiO/γ-Al2O3, NiO/SBA-15 and NiO/MSN should be simultaneously conducted to surge the effectivity of CO2 methanation, contributing to the effort of reducing greenhouse gas in the atmosphere.

Pt11CaNiAl