WO2022108440A1

WO2022108440A1 - Method of regenerating carbon and reactivating a catalyst

Info

Publication number: WO2022108440A1
Application number: PCT/MY2021/000005
Authority: WO
Inventors: Mohd Ambar YARMO; Mohammad Kassim; Mohd Rahimi YUSOP; Wan Nor Roslam WAN ISAHAK; Masli Erwan ROSLI; Mohammad Wahab MOHAMMED HISAM; Maratun Najiha ABU TAHARI; Salma SAMIDIN
Original assignee: Universiti Kebangsaan Malaysia (Ukm)
Priority date: 2020-11-19
Filing date: 2021-11-18
Publication date: 2022-05-27
Also published as: AU2021381240A1; WO2022108440A8; CN116669847A

Abstract

The present invention relates to a method of regenerating carbon and reactivating a catalyst (10) in a catalysis reaction i.e. preparing the catalyst and distributing the prepared catalyst to at least one reactor (11); activating the catalyst with carbon monoxide; injecting additional carbon monoxide for a first carbon production (13) and forming a reduced catalyst simultaneously; injecting steam to split the reduced catalyst to regenerate carbon in a second carbon production (14); wherein, in the first carbon production, the catalyst converts carbon monoxide into carbon and carbon dioxide, and in the second carbon production, the reduced catalyst interacts with the steam to form a metal oxide, hydrogen and carbon and the hydrogen is used to regenerate carbon and reactivate the catalyst for a continuous cycle of carbon formation in the reactor (15).

Description

METHOD OF REGENERATING CARBON AND REACTIVATING A CATALYST

Technical field

The present invention relates to a method of regenerating carbon and reactivating a catalyst in a reactor.

Background of the Invention

There is a huge demand for quality carbon production both as research materials and for large scale industrial applications. Several production methods have been developed such as arc- discharged and oven laser evaporation. Both techniques have the advantages of producing high quality carbon tubes but at the same time, these methods produce relatively high amount of impurities. Besides, the temperature operation is very high i.e. around 3000'C; which is neither economical and/nor convenient. Both methods require extensive amount of energy to produce carbon tubes. Both methods require solid graphite/carbon as a target and hit by a pulsed laser or as an anode which has to be evaporated to get nanotubes. It is difficult to get large amount of graphite and this is one of the limits for large scale production.

Previous researches commonly used transition metals from sources like organometallocenes such as ferrocene, cobaltocene, nickelocene especially for carbon nanotube (CNT) growth. In most researches, they utilized iron pentacarbonyl (Fe(CO)₅) as a catalyst and carbon monoxide (CO) as source of carbon. Other sources of carbon include methane (CH₄), ethylene (C₂H₄), acethylene (C₂H₂), plastic, biomass, hydrocarbon. There had been some researches that reported the use of hydrogen gas (H₂) during carbon production process to enhance the yield of carbon.

Catalysts are substances that are added to a reaction to increase its rate of reaction by providing an alternate reaction pathway with a lower activation energy (Ea). However, the journey of finding the best catalyst remain unresolved, therefore it is a challenge to the chemists to find the best catalyst which can reduce the usage of energy, cost and time effectively.

One of the prior arts for the production of carbon i.e. US1974744 disclosed the use of a catalyst produced from the combination of a metal and a metal oxide and CO as a source of carbon to produce carbon black and EP0062912A1 disclosed the method of producing carbon by injecting steam into the reactor to convert carbon monoxide into carbon and hydrogen gases in a catalytic process. Although the presence of the catalysts in the above prior art manage to reduce the energy involved in the reaction, it is best to find other alternative option to reduce the energy required to produce carbon, whilst maintaining the effectiveness and quality of the carbon produced at a lower cost and time-saving. On the same note, finding an effective catalyst that is able to selectively promotes the production of carbon without damaging the end product is one of the problems to be solved. Therefore, improvement of catalysts is still in need in order to demonstrate much better method for carbon production with better quality and effectiveness.

Summary of the Invention

Accordingly, the present invention relates to a method of regenerating carbon and reactivating a catalyst in a catalysis reaction, the method comprising the steps of: preparing the catalyst and distributing the prepared catalyst to at least one reactor; activating the catalyst with carbon monoxide at 300°C - 900°C to produce an activated catalyst; injecting additional carbon monoxide for at least 1 hour for a first carbon production and forming a reduced catalyst simultaneously; injecting steam to split the reduced catalyst to regenerate carbon in a second carbon production; wherein, in the first carbon production, the catalyst converts carbon monoxide into carbon and carbon dioxide, and in the second carbon production, the reduced catalyst interacts with the steam to form a metal oxide, hydrogen and carbon and the hydrogen is used to regenerate carbon and reactivate the catalyst for a continuous cycle of carbon formation in the reactor.

The advantages and elements of the present invention will be described explicitly from the description of the preferred embodiments which illustrate the principles of the invention.

Brief Description of the Drawings

Fig. 1 illustrates the summary of the method of regenerating carbon and reactivating a catalyst in a reactor.

Fig. 2 illustrates the summary of the catalyst preparation.

Fig. 3. illustrates a schematic diagram of Micromeritic Autochem 2920 Chemisorption Analyzer.

Fig. 4 illustrates an embodiment of the present invention in which the gas will flow from top of the column of the reactor.

Fig. 5 illustrates a schematic diagram to explain the overall summary of the experiment design to produce carbon using carbon monoxide as the source of carbon.

Fig. 6 illustrates the Temperature Programme Reduction (TPR) profiles of iron oxide (Fe₂O₃) by (a) 10% CO/N₂, (b) 20% C0/N₂, CO 40% CO/N₂ and (d) 60% C0/N₂.

Fig. 7 shows the effect of CO stream flow towards formation of carbon by Fe₂O₃ catalyst.

Fig. 8 shows the effect of promoter loading on iron oxide towards production of carbon for Ihour exposure.

Fig. 9 shows the screening of various promoter oxide as catalyst for carbon production.

Fig. 10. shows the effect of reaction temperature towards formation of carbon bylO%Co/Fe₂03 and 10%Ni/Fe₂O₃.

Fig. 11 shows the effect of exposure time (1, 2 and 6 hour) on carbon production by Fe₂O₃, 10%Ni/Fe₂O₃ and 10%Co/Fe₂O₃ catalysts.

Fig. 12 illustrates the surface morphology of Fe₂O₃, 10%Co/Fe₂O₃ and 10%Ni/Fe₂O₃ before and after 1 hour of carbonization.

Figure 13 illustrates the influence of carbonization temperature towards carbon production. Figure 14 illustrates the XRD patterns of catalyst after carbon production reaction at four different temperature.

Fig. 15 shows the effect of amount water vapour dosing towards weight increment of catalyst.

Fig. 16 shows XRD patterns of catalyst after carbon production process at five different water vapour dosage.

Fig. 17 shows a surface morphology of Fe₂O₃ catalyst after carbonization with 5 and 40 water vapour dosage

Fig. 18 shows HRTEM images of Fe₂O₃ catalyst after carbonization together with 40 water vapour dosage.

Fig. 19 shows the reduction profile of flow gas occurring at TPR analysis process with different catalyst NiO, Cr₂O₃ and various Cr (5%-35%) doped to NiO catalyst.

Fig. 20 illustrates the percentage of carbon weight increment (w/w%) of different doped on NiO catalyst: (a) Low percent metal salt loading (b) high percent metal-salt loading.

Fig. 21 illustrates the effect of reaction temperature on carbon production.

Fig. 22 illustrates the effect of different meta] loading of Cr doped NiO on carbon production (a) without hold time (b) hold 1 hour.

Fig. 23 illustrates the effect of flow rates to the production of carbon.

Fig. 24 illustrates the reaction time dependence of carbon yield at constant reaction temperature 500^ºC and 40%CO:20ml/min.

Fig. 25 illustrates the XRD results of as-prepared products synthesized under constant precursor 40%CO, flowrate 20ml/min, reaction temperature of 400-900°C (a] without hold time (b) 60 min

Fig. 26 illustrates the FESEM image of the morphology of deposited carbon produced by decomposition of CO, at 500°C and with flow-rate at 20ml/min on the 25%Cr-NiO catalyst. Fig. 27 illustrates the TEM image of carbon prepared by decomposition of CO on 25%Cr-NiO catalyst, (a) unreduced catalyst (b) image of carbon growth on catalyst

Fig. 28 illustrates the carbon production based on NiO with 10%CO/N₂.

Fig. 29 illustrates the carbon production based on NiO with 40%CO/N₂.

Fig. 30 illustrates the carbon production based on NiO with 60%CO/N₂.

Fig. 31(A) illustrates the XRD pattern of NiO catalyst after reaction at 10%, 40% and 60% CO.

Fig. 32 illustrates the TEM image of NiO catalyst after treatment at T;700°C in 10%CO.

Fig. 32 illustrates the TEM image of NiO catalyst

Detailed Description of the Preferred Embodiments

The present invention will now be described in relation to the accompanying drawings Figs. 1 to 32, either individually or in any combination thereof.

Broadly, the present invention relates to a method of regenerating carbon and reactivating a catalyst in a catalysis reaction, the method comprising the steps of: preparing the catalyst and distributing the prepared catalyst to at least one reactor; activating the catalyst with carbon monoxide at 300°C - 900°C to produce an activated catalyst; injecting additional carbon monoxide for at least 1 hour for a first carbon production and forming a reduced catalyst simultaneously; injecting steam to split the reduced catalyst to regenerate carbon in a second carbon production; wherein, in the first carbon production, the catalyst converts carbon monoxide into carbon and carbon dioxide, and in the second carbon production, the reduced catalyst interacts with the steam to form a metal oxide, hydrogen and carbon and the hydrogen is used to regenerate carbon and reactivate the catalyst for a continuous cycle of carbon formation in the reactor. in details, the catalyst is a metal oxide-based catalyst or an impregnated catalyst wherein the impregnated catalyst is a combination of a metal oxide and a promoter.

Accordingly, the metal oxide-based catalyst is iron oxide or nickel oxide.

In addition to the above, the promoter in the impregnated catalyst is the form of nitrate salts selected from cobalt, chromium, zinc, molybdenum, zirconium, manganese, cerium, magnesium, nickel, iron, calcium and barium.

In a preferred embodiment, the carbon collected is in a form of carbon amorphous or carbon nanotubes (CNTs) or a combination of both.

In a preferred embodiment, an optimum temperature to produce the reduced catalyst is 500°C.

In view of the above preferred embodiment, the reduced catalyst is a metal carbide.

In another preferred embodiment, flow rate of the carbon monoxide in steps (12) and (13) is in a range of 5 ml/min - 50 ml/min. In one preferred embodiment, the flow rate of the carbon monoxide is optimized at 10 ml/min and 20ml/min for iron oxide-based catalyst and nickel oxide-based catalyst respectively.

In an embodiment, the steam is injected using a Pulse Chemisorption Water Vapour (PCWV) at a dose range of 5 to 40 and each dose consists of at least 0.23 cm3 of water vapour.

Advantageously, the carbon produced using the method above is able to produce selectively pure carbon. For example, the physical character of the CNT produced can be obtained in various forms, including powder, thin or thick films, aligned or entangled, straight or coiled, or even a desired architecture of nanotubes (Mukul & Yoshinori 2010).

Advantageously, the method above utilizes the carbonization process and the design of the reaction is simple and carbon growth parameters can be easily controlled compared with other synthesis method [Mukul & Yoshinori 2010 & Krzystof et al., 2010).

Advantageously, the carbon growth occurs at much lower temperature around 550-1000°C (Dresselhaus et al. 2001 & Ci et al. 2005) and therefore the process does not require a huge amount of energy to produce carbon and can be considered as a cheaper option.

Advantageously, CO can easily be produced through the conversion of CO₂ to CO via thermal catalytic reaction.

Instead of adding pure hydrogen gas (H₂) (prepared separately) to reactivate the impregnated catalyst, the present invention utilizes steam that contains water vapour. The purpose of water vapour addition into the system is mainly to regenerate the catalyst by breaking metal carbide into metal oxide and carbon. H₂ is produced in-situ after breaking up the metal carbide and it is used to reduce the catalyst during in-situ regeneration and reactivation. Therefore, active catalyst is formed and ready for next carbon formation cycles and continuously active to produce lot of carbon amount in a single reactor.

In this present invention, the carbon is produced through a chemical vapour deposition (CVD) and chemical reaction method in which thermal decomposition of carbon monoxide (CO) occurred in the presence of catalyst This process involves catalyst activation and followed by carbon production. H₂ gas is commonly used for catalysts activation. In this present invention, CO is selected as a reductant to activate the catalyst and also as a source of carbon. Generally, once the catalyst is activated, it is further exposed with CO gas in order to initiate the formation of carbon nanotubes (CNTs) and/or carbon amorphous, depending on the catalyst presence in the reaction system.

In the process of activating the impregnated catalyst, the chemical reaction that takes place is shown in Equation 1 below:

In the presence of the activated impregnated catalyst (M⁰/M_xO_y), the thermal decomposition of CO is shown in the Equation 2 for the first carbon production, followed by the reduction of the impregnated catalyst into a metal carbide in Equation 3. The reactions are shown below:

In the process of carbon regeneration and reactivation of the impregnated catalyst, the reaction that takes place is shown in Equation 4:

Hereinafter, examples of the present invention will be provided for more detailed explanation. The advantages of the present invention may be more readily understood and put into practical effect from these examples. However, it is to be understood that the following examples are not intended to limit the scope of the present invention in any way.

EXAMPLE 1

METHODOLOGY

Catalyst preparation

The composition of the catalyst used in this present invention for carbon production comprising of 100% metal oxide and 5wt% - 35wt% promoter-metal oxide. The metal oxide selected from d block elements which are iron oxide and nickel oxide. The selected promoter is either cobalt, chromium, copper, zinc, molybdenum, zirconium, manganese, cerium, magnesium, nickel, iron. calcium and barium. These promoters are in the form of nitrate salt. The selected metal oxide i.e. iron oxide or nickel oxide, went through a heat treatment at 400°C for 4 hours before being used as a catalyst in carbon production reaction. The promoter-metal oxide was synthesized through wet impregnation in which metal oxide is mix with nitrate salt of promoters. The chemicals, used in preparation of the metal catalyst, were supplied by FLUKA under analytical grade and used without further purification. Calculated amount of nitrate and metal oxide with corresponding to metal cation were dissolved in H₂O to form aqueous solution, stirring the mixture at 40°C - 50°C to form a homogenous impregnated catalyst. After 4 hours, the aqueous solution was dried in an oven at 110°C for 24 hours. The fully dried catalyst was grinded and sieved to obtain the particle size of 60 microns. After grinding, the catalyst was calcined at 400°C - 600°C for 4 hours and stored in a vacuum tight glass jar. Iron oxide and nickel oxide containing various percentage of promoter were labelled as x%M/Fe₂O₃ andx%M/NiO. Fig. 2 represented the summary of catalyst preparation.

Catalyst activation

The metal oxide and promoter-metal oxide were exposed with 10% - 60% carbon monoxide to activate the catalyst at temperature 300°C - 900°C. This activation step also known as reduction in which the metal oxide was reduced to a lower oxidation number. Temperature Programmed Reduction (TPR) has been used to activate the catalyst and measured by Micromeritic Autochem 2920 Chemisorption Analyzer under atmospheric pressure and non-isothermal conditions, as shown in Fig. 3. 50 mg of catalyst was distributed on quartz wool in the quartz u-tube and connect to the reactor. Firstly, the catalyst heated up to 150°C for 10 minutes under pure N₂ with 20 ml/min to remove any moisture before activate with 20 ml/min of carbon monoxide gas. A temperature ramp of 10’C/min from 150°C to 900°C to get complete TPR profiles. A thermal conductive detector (TCD) was used to record the TPR profiles.

Carbon Production

The synthesis of carbon was carried out at atmospheric pressure via catalytic decomposition of carbon monoxide in a small-scale reactor (micromeritics instrument). The thermal catalytic CVD method took place in a fix bed reactor type in which the gas will flow from top of the column as shown in Fig. 4. A 50 mg of sample was distributed on the quartz wool in the quartz u-tube connected to the reactor. After catalyst being activated, the active catalyst is continuously exposed with 10 ml/min - 20 ml/min of carbon monoxide for lhr - 6hr for carbon production process at temperature similar with the activation step. Adding water vapor into the reaction allows the regeneration of the catalyst for the next carbon formation. The water was added through pulse chemisorption water vapour (PCWV). During carbon production process, 10 ml/min of carbon monoxide is flow continuously and at the same time 10 ml/min of nitrogen will carry the water vapour and injected into the system. Every injection of vapours varies from 5 - 40 doses for each test. Specifically, about 0.23 cm³ or 10.4 pmol of water vapour using 0.98 cm³ of sample loops in the N₂ stream dosed for each pulse to the reduced metal oxide to undergo water splitting reaction in Equation 4 to yield carbon, metal oxide and hydrogen as the by-products. The hydrogen is further utilized to regenerate and reactivate the catalyst for the next carbon formation cycle.

Fig. 5 shows a schematic diagram to explain the overall summary of the experiment design to produce carbon using carbon monoxide as the source of carbon.

Analysis of carbon yield

The total amount of carbon deposited during the time on stream was determined by percentage weight increment Weight increment is calculated gravimetrically after cooling the product to ambient temperature (about 30°C). The carbon produced is defined by percent ratios of the weight of carbon formed per the weight of active catalyst using the following formula:

EXAMPLE 2

Iron oxide catalyst

A number of applications of iron-based elements have been developed which include catalysis, pigments, coagulants, gas sensors, ion exchange and lubricants (Mohapatra & Anand 2010). The extensive use of iron oxide as a catalyst is due to its stability during catalysis in high temperature reactions. Iron oxide is also widespread and viable in nature which can be considered as an inexpensive catalyst.

Reduction properties of catalysts by using CO-TPR technique

Based on thermodynamic studies, the process of carbon production requires Fe₂O₃ to be reduced to FeO and/or Fe,C active phase. Therefore, TPR analysis is important to study the reduction potential to activate the catalyst. The analysis of Fe₂O₃ reduction in non-isotherma] is shown in Fig. 6. According to the profile in Fig, 6, there are 4 sharp peaks which clearly indicates the reduction process followed by a four-step reduction. In this case, the reductant, CO with concentrations of 10%, 20%, 40% and 60 % were utilized and each gave a significant difference in their TPR profiles.

The first reduction peak showed Fe₂O₃ reduced to Fe₃O₄ phase which also agreed by (Kuo et. all 2013). A very clear separated peak around 415 to 570°C could attribute to the reducing step ofFe₃O₄ to a mixture of FeO and Fe_xC. A mixture of that FeO and Fe_xC could be formed especially in high concentration of CO (40% and 60%). By further reducing the Fe₂O₃ at higher temperature, this encouraged the transformation of FeO to Fe_xC and lastly metallic iron (Fe) will be formed.

Effect of CO flow rate on carbon production

According to the thermodynamic consideration, the formation of carbon from CO is optimized at temperature 500°C. In this case, an optimized concentration of 40% of CO was selected to interact with the Fe₂O₃ catalyst to produce carbon black and/or carbon nanotubes (CNTs) because it was the lowest concentration that initiate the iron carbide phase.

As illustrated in Fig. 7, there were 3 different flow rates of CO studied i.e. 5, 10 and 20 ml/min in order to obtain the best amount of carbon source during carbonization process.

In the first 1 hour of carbonization, 20 ml/min CO gave the highest weight gain which is 12w/w% compare to 5 and 10 ml/min which indicates the highest percentage of carbon produced. Referring to Fig. 7, it was clear that after 2-4 hours, there was a gradual increase in the carbon production and it could be deduced that the carbon production was directly proportional with the exposure time and the weight increment.

However, when comparing to the flow rate of carbon source and time of reaction, 10 ml/min exhibited the highest carbon growth. Carbon formation resulted by 20 ml/min of CO also considered quite high, but the percentage of increment is not as high as 10 ml/min. Therefore, 10 ml/min of CO was chosen as the best flow rate for the carbonization and this value was used throughout the whole studies.

Effect of promoter-oxide loading on iron oxide towards carbon production

The impregnated catalyst comprising of iron oxide and promoter-oxide managed to yield carbon and operated at reduction and carbonization temperature 500°C. Iron oxide without any promoter manages to produce 6w/w% of carbon. For impregnated catalyst, the presence of nickel oxide improved the carbon production in percentage range of 39-55w/w%. For example, cobalt oxide on iron also gave a good result with percentage range of 35-49w/w% of carbon production.

Referring to Fig. 8, other promoters, in combination with iron oxides such as Cr/Fe₂O₃, Cu/Fe₂O₃, Zn/Fe₂O₃, Zr/Fe₂O₃, Mn/Fe₂O₃ and Ce/Fe₂O₃ also showed further improvement in the carbon growth compared to single iron oxide catalyst. However, only Mo/Fe₂O₃ catalyst gave negative weight increment. This could be because of the 1-hour exposure to CO was just only used to reduce the catalyst and no carbon was produced hence, the weight of the catalyst decreased.

Meanwhile, nickel, cobalt, zirconium and cerium with iron oxide exhibited similar trend in their carbon growth in which the weight increment is directly proportional to the amount of promoter loading. However, in this case, the 10% promoter was considered the best metal loading since their difference with 20% promoter was insignificant.

The oxides of promoter also went through carbonization screening as shown in Fig. 9. All promoter oxides were firstly reduced and then exposed to CO for 1 hour at temperature 500°C. Results showed that only molybdenum able to increase their weight by 4w/w% whereas the other promoter oxide gave negative values. These proved that single metal oxide powder is not active to catalyse the CO to C.

Effect of reaction temperature on carbon production

Both impregnated catalyst of 10%Ni/Fe₂O₃ and 10%Co/ Fe₂O₃ worked the best for production of carbon. The reactions were done at temperature 500°C for both reductions and carbonization processes with carbon percentage of 50w/w% and 45w/w% respectively.

Further studies on these two catalysts had been done to acquire their best reaction temperature as shown in Fig. 10. Reduction and carbonization were run at similar temperature at 450, 500, 550 and 600°C. The weight of the catalyst shown significant increment at all temperature range. However, carbonization at 500’C showed the highest carbon growth compared to others. This is because at 500°C, the catalyst comprises of iron carbide which encourage the formation of carbon when exposed too long with CO. The carbon weight increment is disproportionate with temperature since entropy of reaction became more negative with increases reaction temperature. Therefore, the temperature of 500°C was optimized and the same parameter was used throughout the forthcoming experiments. Effect of time exposure on carbon production byFe₂O₃ , Ni/Fe₂O₃ and Co/Fe₂O₃ catalysts

The best promoter-iron oxide catalysts were compared with single metal Fe₂O₃ catalyst by further increasing the exposure of time with CO. The exposure time varied from 1, 2 to 6 hours for each catalyst and carbon growth were measured gravi metrically.

As depicted Fig. 11, carbon growth drastically improved by the presence of promoter during the first hour of carbonization. Carbon production further increased by 40-45w/w% using 10%Co/Fe₂O₃ and 10%Ni/Fe₂O₃, respectively. Carbon continued to grow and consistent with the elongated exposure time.

In this case, the promoter enhanced the active sites of the catalyst, therefore, allowing more catalyst-CO interactions to occur; at the same time, weaking the C-O bond and producing more carbon during 1-2 hour of carbonization. After 6-hour of reaction with CO, the conversion of CO became slower than the single metal catalyst and this could be due to the slower diffusion rate of the CO stream to the promoted catalysts.

On the other hand, Fe₂O₃ catalyst managed to produce the highest percent of carbon i.e. 134w/w% after 6 hours of exposure to CO compared to others i.e. 127 w/w% and 117 w/w% for Ni/Fe₂O₃ and Co/Fe₂O₃ respectively. Even though carbon formation at the initial reaction was lower, however, the CO conversion increased tremendously along with the exposure time.

Catalyst and Carbon Characterization

FESEM image for production of carbon

Morphologies of the catalyst before and after carbonization were analysed using FESEM with magnification 50K. The presence of promoter changed the morphology of the iron catalyst by forming small holes on the catalyst particles. The iron particles in the promoted catalyst also became smaller with the presence of small and irregular shape of Co and Ni particles. After 1- hour of carbonization as shown in Fig. 12, 10%Co/Fe₂O₃ produced a mixture of short carbon tubes and coated carbon whereas 10%Ni/Fe₂O₃ developed initial growth of big and short carbon tubes as shown in Fig. 13. However, no formation of carbon tubes on the Fe₂O₃ catalyst since the carbon formation was very low i.e. 6w/w% compared to the other catalysts. Therefore, carbon produced on the Fe₂O₃ catalyst was formed in the form of amorphous carbon instead of carbon nanotubes. EXAMPLE 3

Production of carbon using CO as a carbon source and addition water vapour

In view of the above Fe₂O₃ catalyst was selected for further studies using another carbonization and water splitting method for carbon regeneration and reactivating the catalyst for the next carbon formation. This allows continuous production of carbon at the same time, saving costs and time.

Generally, in this study, the catalyst was reduced for catalyst activation, followed by carbonization which involved CO exposure together with addition of water vapour.

Effect of different reduction temperature on carbon production

In this study, the reduction temperature of Fe₂O₃ has been varied to identify their activation phase towards carbon production. Various concentrations of CO were used which are 10, 20, 30, 40, 50 and 60% and parameter of the reduction and carbonization flow rates are as follows:

1. CO flow rate of reduction: 20 ml/min

2. CO flow rate of carbon production: lOml/min

3. Water vapour dose: 10 folds in 1 hour

Below are the results obtained based on the above parameters:

Tablel 10% CO/N₂

Table 2. 20% CO/N₂

Table 3. 30% CO/N₂

Table 4. 40% CO/N₂

Table 5. 50% CO/N₂

Table 6. 60% CO/N₂

Based on the tables 1 to 6, 40% CO/N₂ recorded the lowest concentration that gave positive weight increment at various reduction temperatures and followed by 50% and 60% CO/N₂. The highest weight increment recorded was 72.98% (g/g) at reduction temperature 415 °C for 60% CO/N₂. The catalyst was in form of FeO and Fe₃C phases after being reduced at this temperature.

The other concentrations i.e. 10%, 20% and 30% did not contribute to any weight increment because the amount of CO gas was insufficient to initiate the formation of iron carbide. In another word, excess CO was required to form iron carbide and encourage the formation of carbon Whilst 50% and 60% CO/N₂ showed the best concentrations in production of carbon with the most positive weight increment, however due to the limitation of the present instrument, CO at the concentrations of 50 to 60% would be too concentrated, toxic and would damage the filament of the detector in the instrument. Besides, further reactions involving addition of water vapour dosing caused the pressure to increase in the reactor. Therefore, 40% CO is used in order to study the effect of other parameters on carbon production.

Effect of different carbon production temperature on weight increment

Since the best reduction temperature recorded was at 500°C, carbonization was tested at various temperatures i.e. 400, 500, 600 and 700°C under 40% CO as tabulated in Table 7.

The reaction parameters are as follows:

1. 40% CO flow rate of reduction: 20 ml/min

2. 40% CO flow rate of carbon production: lOml/min

3. Water vapour dose: 10 folds in 1 hour

In view of the above and as shown in Fig. 13, 500°C was the best carbonization temperature with carbon formation 26.40w/w%. By increasing the temperature up to 600-700°C, the value of the weight increment became negative because the FeO and Fe₃C phases were reduced to metallic Fe as showed in Fig. 14. Metallic Fe was in an inactive phase for the carbonization. Therefore, reduction and carbonization temperatures of 500 °C was recorded as the optimum temperature since it gave the best weight increment compared to other temperature.

Table 7. Effect of carbonization temperature on carbon production

Effect of different flow rate of 40% CO and water vapour carrier during carbonization in carbon production

As seen in Table 8, 5 ml/min of 40% CO/N₂ and water vapour carrier (N₂) gave a higher weight increment compared to 10 ml/min. Slower flow rate would increase contact time between catalyst and CO gas, therefore effective collision between catalyst and CO molecules occurred and encouraged the formation of carbon.

Table 8. Influence of CO flow rate during carbonization towards carbon production

Effect of different number of water dosing on carbon production

The purpose of water vapour addition during the carbonization was to enhance the carbon production and at the same time, regenerating and reactivating the catalyst for the next carbon formation.

In this study, the reaction parameters were set as follows:

1. Reduction temperature: 500°C

2. 40% CO flow rate of reduction: 20 ml/min

3. 40% CO flow rate of carbon production: 10ml/min

4. Water vapour dose: 10-40 folds

5. Carbonization temperature: 500°C

Referring to Fig. 15, 1 hour of carbonization by Fe₂O₃ without water addition produced only 6w/w% of carbon. However, 30 minutes of carbonization together with 5 dose of water vapour showed further increment in the carbon formation i.e. 1 lw/w%. Therefore, by increasing dosage of water vapour, the carbon yield increases as well. Catalyst and Carbon Characterization

XRD Results

Referring to Fig. 16, the XRD patterns of catalyst after carbonization with five different water vapour dosages showed that carbon signal became intense and increased proportionally with the water vapour dosage. Excess CO exposure resulted in a decrease in Fe₂O₃ phase signal due to the transformation of Fe₂O₃ to Fe₃C. The presence of water vapour also enhanced the formation of carbon by oxidising the Fe₃C to form Fe₃O₄ and carbon. The Fe₃O₄ is then reduced again to form Fe₃C again. These processes repeated simultaneously during carbonization and the catalyst will be activated continuously by the water vapour.

FESEM image for production of carbon

Figure 17 shows a morphology comparison of Fe₂O₃ catalyst after carbonization at 5 and 40 dose of water vapour. During 5 water dosage, smooth surface of catalyst could be seen, where it indicated that carbon had coated the catalyst particles [Fig. 17 (a-b)). Meanwhile, formation of carbon tube clearly detected when the Fe₂O₃ was exposed to an extended period of CO and water vapour (Fig. 17 (c-d)).

HRTEM image for production of carbon

On the other hand. Fig. 18 shows HRTEM images of Fe₂O₃ catalyst after carbonization with 40 water vapour dosage. The catalyst comprises of short carbon tubes and carbon coated around the catalyst particles.

EXAMPLE 4

Nickel Oxide catalyst

Nickel oxide is commonly used as a catalyst due to its high surface oxidation properties in catalysis process, which makes it a suitable candidate for the production of carbon in this study. Doping is one of the extensive used methods to alter the electron structures of nanoparticles to achieve new or improved catalytic, electro-optical, magnetic, chemical, and physical properties (Liao et al. 2008). Similar to other transition metal catalysts, the NiO catalyst requires reduction to provide an active phase [i.e. metallic Ni) prior to their use. For industrial applications, the catalyst reduction is usually conducted with either hydrogen-containing gases or natural gassteam mixtures. Reduction conditions are important as they have influences on subsequent catalytic activity. For instance, high temperatures and rapid reduction may result in lower Ni dispersions and less 10 activity, the introduction of carbon or sulphur may accelerate catalyst deactivation (Sehested 2006 & Valle et al. 2014), Therefore, in this study, Ni was chosen as a catalyst for carbon production and studies of its chemical properties after regeneration.

Activation of catalyst by CO-TPR technique

Figure 19 shows a reduction profile of flow gas occurring at TPR analysis process with different catalyst NiO, Cr₂O₃ and various Cr (5%-35%) doped to NiO catalyst. Peak i(a) showed the 1^st reduction stage of NiO and the high intensity at Peak i(a) of Fig. 19(a) indicated the strong interaction between nickel oxide and carbon monoxide. This interaction also known as the Boudouard reaction. Then, NiO converted CO to C and CO₂.

Besides that, the peak in i (a) shows a very high intense peak due to the gas reduction process that occurred on the pores of the catalyst's surfaces. In this case, the carbon monoxide adsorbs and dissociates on the oxide surface of the catalyst. The carbon atoms then reacted with oxygen part of the nickel oxide and formed carbon dioxide and the metal nickel.

The peak i i (a) of the TPR profile shows a low intense peak due to reaction occurred at the inner surface of the catalyst. Peak ii(a) also due to a partial reduction phase of Ni^2t and Ni°. While peak iii(a) shows a very low intense peak, it was probably due to the reaction that occurred on the surface of the catalyst. The high temperature was needed for the chemical reaction to occur at the bulk surface of the catalyst due to the diffusion of particles that might trigger the rate of reaction.

The peak as shown in Fig. 19 also indicated there was a chemical reduction and transformation of CO to CO₂ occurred during the chemical process. The chemical reaction that took place is illustrated in the Equation below:

Therefore, it can be concluded, a complete reduction of Ni²⁺->Ni⁰ was at > 700°C with CO.

As illustrated in Fig. 19(b), the reduction profile for Cr₂O₃ could be insignificant as the reduction peak has very low intensity and there was no change of colour observed after the sample was treated at 900°C.

Figs. 19(c) to 19(i) show doping catalyst of chromium to nickel oxide, 5% to 35% of Cr doped to NiO. Figs. 19(c) and 19(d), the TPR curve shows only single peak of CO consumption, with a maximum temperature of 526°C and 429°C respectively. For Figs. 19(e) to 19 (i), the TPR curve shows two peaks at range 271°C to 279°C for 1^st peak, and the highest broad peak at range of 427°C to 455°C.

Addition ofCr to NiO catalyst as a promoter enhanced the reducibility of NiO as it was supported by the data obtained from XRD. NiO completed the reduction to Ni° at temperature 500°C when doped to Cr.

Effect of promoter loading on nickel oxide towards carbon production

The effect of promoter with NiO catalyst was investigated using various metal salt loading with low and high percentage of metal salt i.e. 5% and 25% to NiO for the production of carbon. Addition of promoter to the NiO catalyst was denoted as Cr-NiO, Co-NiO, Cu-NiO, Ce-NiO, Mo-NiO, Ba-NiO, Zr-NiO, Fe-NiO, Ca-NiO and Mg-NiO with percentage of 5% and 25%.

The addition of promoter to the catalyst NiO triggered the carbon growth and increased the catalyst activity. All the catalysts went through reduction process using a TPR chemisorption at 500°C with flow rate of CO at 20ml/min and continued exposure for 1 hour at 10°C/min before cooling to room temperature.

Fig. 20 shows a weight increment (w/w%) of carbon production. The chromium doped to NiO had the highest value compare to other metal salts either in 5% or 25% of catalyst loading. Meanwhile for Fe-NiO, by increasing the percentage of promoter, it would trigger the formation of carbon and the results obtained confirmed that the carbon formation proportionally increased with the increment in the percentage of promoter loading.

Effect of reaction temperature on carbon production

Effect of reduction temperature on the production of carbon was investigated as different temperature during process of carbon production would form different phase in which would affect the value of yield product.

In this case, 25%Cr-NiO was selected as the best catalyst in this process as it is showed highest yield compare to other metals salt doping. The determination of highest potential to the carbon production activity was selected from five different temperatures 400°C, 500°C, 600°C, 700°C and 800ºC. Then, the reaction was experimented in 2 ways i.e. reaction in which until end of the selected temperature (without hold time) and added of another hour to the selected temperature (hold time of 1 hour). Referring to Fig. 21, the yield of carbon at low temperature i.e. 400°C, the carbon yield was negative value (-4%). This means that only the catalyst was reduced but no conversion of CO took place yet. After raising the temperature to 500°C and 600°C the carbon yield was 18% and 35%, which correspond to 22% and 17% of carbon gained respectively. However, the increment of carbon weight at high temperatures i.e. 700°C and 800°C was only 6% and 5%, which indicated that these values were not suitable for carbon production. This might due to a deep reduction occurred at temperature above 700-C to a metal, agglomerated and formed metallic metal, inhibited the formation of CNT or carbon. In our case NiO reduced completely to metal at temperature 700^ºC.

In view of the above, the reduction of catalyst occurred at 400°C when temperature was hold to 1 hour. Based on TPR profile Fig. 19, at 400°C the reduction of NiO already started and when exposure time increased to 1 hour, more reduction happened, which could be due to the increasing number of active sites (Ni⁰) of the catalyst. This encouraged the growth of carbon due the presence of active phase in the Bourdouard reaction.

Interestingly, at temperature 500°C, the carbon yield without prolong was +18% and increased to +173% after an hour of exposure to CO. This was due to increase in the number of active sites (Ni⁰) formed at 500°C and triggered more carbon growth during prolong time compared to 400°C.

At T <500°C, carbon yield increases with increased reaction temperature and achieved maximum yield at T: 500°C. However, at T>500°C, the yield of carbon started to decrease due to an exothermic reaction which caused the negative effect towards the reaction. This was the consequence of more metallic nickel particles being generated at higher temperature such as at 700-C wherein they tend to agglomerate, forming larger metallic clusters. Most probably, these clusters led to the formation of encapsulated surface carbon from CO decomposition, which in turn de-activated the catalyst.

Effect of different loading Cr doped NiO (CrNiO) on carbon production

In order to investigate the influence of the catalyst concentration on the production of carbon production, the synthesis was fulfilled at 500°C, prolonged reaction time of 60 min and 0 min in a flow rate CO/N₂ 20ml/min using Cr/NiO catalyst at various concentrations, Cr- 5%, 10%, 15%, 20%, 25%, 30%, and 35%. The effect of different metal loading of Cr doped NiO on carbon production (a) without hold time (b) hold 1 hour was illustrated in Fig. 22. Effect of CO flowrates on production of carbon

The contact time has been reported to have important role in catalytic activity, therefore, the influence of residence time was investigated by varying the volumetric feed flow rate in term of ml/min.

As notice from the bar graph of Fig. 23, the carbon yield increased as increasing the precursor flow rates. At low flow rate of 5 ml/min, only small value of carbon yield was formed (37%). It reflects that the CO flow rate was not enough to initiate the fast growth of CNT on thermally activated catalyst. The catalyst reacted inadequately with the carbon precursor due to least availability of the carbon atoms to diffuse into the metal catalyst.

Furthermore, part of the carbon was unable to be transferred away in time due to the relatively low rate of the carbon diffusion in the bulk of the metal particle at this temperature and, thus, blocked the active catalyst surface, leading to a decrease in the reactivity.

At flow rate lOml/min, although the carbon production exhibited slight increased, the increment was only +48%. This might due to the precursor flowrate still was not enough to fully fluidize the catalytic particles and to produce sufficient carbon atoms to diffuse and grow into continuous CNT structures.

However, as the flow rate of CO was raised further to 20ml/min, the carbon production increased to 173% (about +88% gained of carbon weight). This flow rate provided a sufficient push to the catalyst particles to suspend in the bed column and to make good contact with the gas molecules. With further increase the CO flowrate from 20ml/min, the weight increment only gained +5%, +40% and +36% for flow rate 30ml/min, 40ml/min and 50ml/min. With further increase in CO flowrate >20ml/min, it shows that only smaller numbers of reactive carbon atoms resulted in growth of fewer nanotubes with higher structural defects.

Besides of that, when the temperature was kept constant (i.e 500-C) and the flow rate of CO increased to 30 ml to 50 ml, the conversion of CO dropped might due to enhancement in the rate of formation of carbon.

With the data obtained, it shows that the rate of formation and removal ofthe carbon atoms from the surface of the catalyst exactly matches at this flowrate (20ml/min) and yields equilibrium among the supply of the reactive carbon and nucleation of CNT structures. At optimized process, an equilibrium condition appears between the rate of decomposition of CO and the rate of diffusion of carbon into the catalyst particles.

Effect of time exposure on production of carbon

Fig. 24 shows the effect of reaction time on carbon yield and the different on increasing reaction time under the constant reaction temperature of 500°C and CO flow rate ratio of 20 mL/min, indicating that carbon yield increases with extended reaction time.

The trend of the average growth rate indicates that productivity declines after 60 min, evidently because of loss of catalyst activity with reaction.

At reaction time of 60 minutes, growth rate showed the highest rate compare to others with 1.5 mg/min.

Catalyst and Carbon Characterization

CHNS Results

Table 14 confirms the formation of carbon by using NiO and 20%Cr-NiO catalysts under holding time and non-holding time (or untreated]. About 63.14% (w/w) carbon detected through CHNS instrument analysis in which is equivalent to 118% (w/w) of weight increment.

Table 14 tabulates the carbon content for different type of catalysts

XRD Results

Fig. 25 showed XRD pattern of carbon production by using catalyst 25%Cr-NiO, flow under 40% of carbon monoxide at various temperature (400°C-700°C).

The experiment was run at two conditions;

(a) without hold time, and

(b) with hold time of 1 hour. As shown in Fig. 25, the formation of carbon can be approved through the appearance of peak 2o: 22.23. Additional of Cr as promoter to NiO catalyst trigger the carbon growth and by increasing the time exposure of precursor increased the yield of carbon.

FESEM image for production of carbon

From morphology analysis using FESEM (Fig. 26), it was clearly showed the production of carbon nanotubes type of carbon (CNTs) after different reaction time.

The CNTs grew on the surface of active metal catalysts when reaction temperature was hold under the flow of CO gas.

TEM image for production of carbon

Fig. 27. Illustrates the TEM image of carbon produced after decomposition of CO on 25%Cr-NiO catalyst, wherein (a) shows the image of the carbon using an unreduced catalyst and (b) image of carbon growth on activated catalyst. This image confirms the growth of carbon nanotubes at constant reaction temperature of 500°C and reaction time of 60 mins, with the CO flow rate of 20 ml/min.

Production of carbon using CO as a carbon source and addition of water vapour

In this method, catalyst went through reduction step to activate the catalyst and followed by carbonization which involve CO exposure together with addition of water vapour. Here, 10%CO, 40%CO and 60% were choosen as an precursor as it is can conclude the effect of low, medium and high concentration of CO to the production of carbon on NiO catalyst.

Screening of NiO catalyst under 10%CO/N₂ with water vapour

Reduction temperature of NiO had been varied to identify their activation phase towards carbon production. Various concentration of CO was used which are 10%, 40% and 60% with constant flow rate 20ml/min, water vapour dose:10 dose. As shown in Fig. 28, the weight increments towards carbon production gave negative value which indicates no formation of carbon under 10%CO. At temperature 500°C to 800°C, the catalyst was not active to generate carbon formation even with addition of H₂O. Therefore, the study confirms that the low concentration of CO is insufficient to catalyse the carbon formation.

Screening of NiO catalyst under 40%CO/N₂ with water vapour

The study of screening of carbon through NiO further continued by increasing the concentration of the precursor to 40%(v/v). Based on Fig. 29, it shows a negative value of yield production. Even though, the temperature of carbon production with water vapour was increased to 500°C, the value of carbon yield was still negative. Therefore, it could be concluded that, at this rate, the rate of carbon feedstock was not fully efficient yet to form carbon nanotubes. Besides, a deep reduction of the Ni²⁺ to Ni° and aggregation of the Ni° occurred and formed metallic nickel at the surface and inhibited the formation of CNT growth on this catalyst.

Screening of NiO catalyst under 60%CO/N₂ with water vapour

Fig. 30 shows the data of carbon production processed under 60%CO/N₂ and the temperature was increase to 600°C to 700°C with additional 10 dose of water vapour. There was no carbon formation when T<600°C, at reduction temperature 410°C, 450°C, 500°C, 55O°C and 600°C respectively. However, when temperature of water vapour increased to 700°C, there were increments to the weight of the catalyst, hence, confirming the formation of carbon on the catalyst.

Catalyst and Carbon Characterization

XRD Results

Fig. 31(A) shows XRD diffraction pattern of NiO catalyst after reaction process consist of diffraction pattern of NiO reacted under atmosphere 10%CO/N₂, 40%CO/N₂, 60%CO/N₂ reduction temperature 700°C, carbon production with water vapour at temperature 700°C.

At 10%CO and 40%CO, the catalyst completely reduced to Ni° meanwhile at 60%CO there is a formation of peak (002) which indicated a graphite peak.

FESEM image for production of carbon

The figure in Fig. 31(B), shows an image of raw NiO at reduction temperature of 700°C. The carbonization on NiO with additional of water proved the presence of amorphous carbon deposition on catalyst and proved further with XRD diffraction through the formation of carbon graphite at peak (002).

TEM image for production of carbon

Fig. 32 shows the TEM images of NiO at temperature 700°C, 40% CO concentration, CO flow rate of 20ml/min and exposure time of 1 hour. Transmission electron microscopy (TEM) was performed to confirm the formation of carbon nanotubes. As seen in Fig. 32, there was no carbon formation on the catalyst and the time exposure didn’t trigger the growth of carbon on nickel oxide catalyst. Summary

Iron oxide-based catalyst manages to produce highest carbon yield (134w/w%) among Ni/Fe and Co/Fe catalyst with elongation CO exposure at temperature 500°C. The presence of 10% promoter significantly improved the carbon yield at initial reaction time especially during the first hour (45 and 50w/w%). Addition of water vapour was further enhanced the carbon production by 20w/w% for 1-hour reaction with 10 water vapour doses under 40% CO. Carbon yield was proportional with the carbon concentration, time exposure and amount of water vapour dosage (5 doses: 11 w/w%, 10 doses: 26 w/w%, 15 doses: 55 w/w%, 20 doses: 79 w/w% and 40 doses: 107 w/w%). The existence of water vapour in the system was initiated the in-situ production of H₂ in which the H₂ is essential for in-situ carbon regeneration and catalyst reactivation. Therefore, active catalyst is formed and ready for next carbon formation cycles and continuously active to produce lot of carbon. Cr-NiO was selected and showed the best catalyst in carbonization reaction to produced highest yield of CNT with catalyst loading of 25% of Cr on nickel oxide. NiO catalyst showed positive value of carbon weight percentage when use high concentration of CO (60%) which is quite high to run in a daily experiment. However, the weight gained obtained is lower than 25%CrNiO (173%, w/w%). Proposed optimal conditions are as follow: gas reaction 40%CO, reaction time 60 min, reaction temperature 500^ºC, and flow rate 20ml/min. The yield of the product is over 173%., with growth rate 1.5 mg/min.

Claims

Claims:

1. A method of regenerating carbon and reactivating a catalyst [10] in a catalysis reaction, the method comprising the steps of: preparing the catalyst and distributing the prepared catalyst to at least one reactor [11); activating the catalyst with carbon monoxide at 300°C - 900°C to produce an activated catalyst (12); injecting additional carbon monoxide for at least 1 hour for a first carbon production (13) and forming a reduced catalyst simultaneously; injecting steam to split the reduced catalyst to regenerate carbon in a second carbon production (14); wherein, in the first carbon production, the catalyst converts carbon monoxide into carbon and carbon dioxide, and in the second carbon production, the reduced catalyst interacts with the steam to form a metal oxide, hydrogen and carbon and the hydrogen is used to regenerate carbon and reactivate the catalyst for a continuous cycle of carbon formation in the reactor (15).

2. The method as claimed in claim 1, wherein the catalyst is a metal oxide-based catalyst or an impregnated catalyst wherein the impregnated catalyst is a combination of a metal oxide and a promoter.

3. The method as claimed in claim 2, wherein the metal oxide-based catalyst is iron oxide or nickel oxide.

4. The method as claimed in claim 2, wherein the promoter is the form of nitrate salts selected from cobalt, chromium, zinc, molybdenum, zirconium, manganese, cerium, magnesium, nickel, iron, calcium and barium.

5. The method as claimed in claim 1, wherein carbon collected is in a form of carbon amorphous or carbon nanotubes (CNTs) or a combination of both.

6. The method as claimed in claim 1, wherein an optimum temperature to produce the reduced catalyst is 500°C.

7. The method as claimed in any of the claims 1 to 3, wherein the reduced catalyst is a metal carbide.

8. The method as claimed in claim 1, wherein flow rate of the carbon monoxide in steps (12) and (13) is in a range of 5 ml/min - 50 ml/min.

9. The method as claimed in 8, wherein the flow rate of the carbon monoxide is optimized at 10 ml/min and 20ml/min for iron oxide-based catalyst and nickel oxide-based catalyst respectively.

10. The method as claimed in claim 1, wherein the steam is injected using a Pulse Chemisorption

Water Vapour (PCWV) at a dose range of 5 to 40 and each dose consists of at least 0.23 cm³ of water vapour.