De-oxygenation of CO 2 by using Hydrogen , Carbon and Methane over Alumina-Supported Catalysts

The de-oxygenation of CO2 was explored by using hydrogen, methane, carbon etc., over alumina supported catalysts. The alumina-supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts were first reduced in hydrogen atmosphere and then used for the de-oxygenation of CO2. Furthermore, experimental variables for the de-oxygenation of CO2 were temperature (range 50 to 650 oC), H2/CO2 mole ratios (1.0 to 5), and catalyst loading (0.5 to 10 wt %). During the de-oxygenation of CO2 with H2 or CH4 or carbon, conversion of CO2, selectivity to CO and CH4 were estimated. Moreover, 25.4 % conversion of CO2 by hydrogen was observed over 1 wt% Pt/Al2O3 catalyst at 650 oC with 33.8 % selectivity to CH4. However, 8.1 to 13.9 % conversion of CO2 was observed over 1 wt% Pt/Al2O3 catalyst at 550 oC in the presence of both H2 and CH4. Moreover, 42.8 to 79.4 % CH4 was converted with 9 to 23.1 % selectivity to CO. It was observed that the de-oxygenation of CO2 by hydrogen, carbon and methane produced carbon, CO and CH4. © 2012 BCREC UNDIP. All rights reserved


Introduction
In the present environment, high thermal energy required for the reaction of abundant and thermodynamically stable methane and carbon dioxide molecules which are main constituents of green house gases.These green house gases are produced from combustion of bio-fuel and petroleum products.In order to maintain the CH4 and CO2 concentration within the threshold value limit in the atmosphere, there is a need to reduce their emission .
Therefore, in this paper, de-oxygenation of CO2 by either individual or/and combination of hydrogen, carbon and methane over reduced alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts were reported .

Chemicals and apparatus
In the experiments, alumina-supported ruthenium, rhodium, platinum, molybdenum, vanadium, and magnesium catalysts were prepared by using the chemicals ruthenium chloride (ruthenium content 45-55%), rhodium chloride (98%), platinum chloride (99.9+%), ammonium molybdate (99%), vanadate and magnesium nitrate (99%) (Aldrich Chemicals, CO.,) over alumina.In addition, the alumina support was prepared by the calcination of (Böhmite) aluminium oxide hydroxide (γ-AlO(OH)) at 500 o C for 1.5 h.Moreover, the high purity hydrogen, carbon dioxide, helium gases (Deluxe India Ltd.) and methane (Alphagaz) were used to explore the activation of carbon dioxide over alumina supported catalysts.The supported catalysts were calcined at 700 o C for 4 h in a furnace (Thermax Co. Ltd.).The reduction of alumina-supported ruthenium, rhodium, platinum, molybdenum, vanadium, and magnesium catalysts were carried out in hydrogen atmosphere.The deoxygenation of CO2 was carried out in a split furnace (Carbolite USA).Carbon dioxide, hydrogen, carbon monoxide, and methane were analyzed by using Porapak-Q as column, TCD and FID as detectors equipped with GC (Nucon India Ltd).The reactants and products of the deoxygenation reactions of CO2 were presented in terms of % conversion and % selectivity, respectively.

Characterization of catalysts
The alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts were characterized by X-Ray diffraction (Philips Power XRD) for XRD patterns.Particle sizes of catalysts were examined by SEM (QUANTA 200 3D).

Set up and procedure for the deoxygenation of CO2
The de-oxygenation of CO2 was carried out over the reduced supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts in a split furnace as illustrated in Figure 1.A variable ratio was used of a mixture of carbon dioxide, H2 or/and CH4 in helium gas.A quartz fixed bed reactor was used with dimension 6 mm OD.The quartz reactor was modified at the center with dimension 10 mm OD and 10 cm length.In order to make complete set up, 4 mm OD stainless steel tubes, four three ways valves, a gas sampling valve, a carbolite split furnace with temperature controller, a Nucon GC and flow control valves were used.With four three way valves, flow of hydrogen, helium and carbon dioxide gases were controlled (Figure 1).For flushing a catalyst bed, helium gas was used.For reduction of alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalyst a gas mixture of hydrogen and helium in 1:1 mol ratio was used at 500 o C for 1 h.De-oxygenation of carbon dioxide was carried out by using H2, carbon, or/and CH4 over reduced alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts at a temperature by keeping a molar ratio of CO2 and helium 1:1.The analysis of carbon dioxide, hydrogen, methane and carbon monoxide was carried out by pulse technique by using Porapak-Q column with a Nucon GC equipped with TCD and FID (methane) detectors.

Bulletin of Chemical Reaction
The alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts 0.1 to 0.5 gm with particle size -22 to -30 mesh was placed in the quartz reactor at centre with support of quartz wool.A certain temperature of catalyst bed was fixed with a temperature controller.Catalyst bed was flushed with helium gas in order to remove the stresses of other gases.After that, catalyst bed was reduced by using a mixture of hydrogen and helium gases in mol ratio 1:1 at 500 o C for 1 h.Then, catalyst bed was cooled down.The reactions of carbon dioxide were carried out by using hydrogen or/and CH4 at certain temperature by passing a mixture of carbon dioxide and hydrogen or/and CH4 in presence of helium.Further, the product was analyzed by online GC using a Porapak-Q column and also a thermal conductivity and flame ionization detector (methane).However, the results were confirmed by means of three runs.Furthermore, the results of the de-oxygenation reaction of CO2 were presented as % conversion for reactants and % selectivity for products.

Results and discussion
The results of de-oxygenation reaction of carbon dioxide by using hydrogen or/and methane over alumina-supported ruthenium or rhodium or platinum or molybdenum or vanadium or magnesium catalysts are described in the following sections.

Characterization of catalysts
The catalyst particle size and the crystalline phases are important for the de-oxygenation of CO2 over supported catalyst.Therefore, the alumina supported ruthenium, rhodium, platinum, molybdenum and vanadium catalysts were characterized by SEM images and XRD patterns.Characterized the 1 wt% Ru/Al2O3, 5 wt% Ru/Al2O3, 1 wt% Rh/Al2O3, 4 wt% Rh/Al2O3, 2 wt% Pt/Al2O3, 4 wt% Pt/Al2O3, 4 wt% Mo/Al2O3, 4 wt% V/Al2O3 and 7.5 wt% Mg/Al2O3 catalysts by SEM for particle size (Figure 2 (a-i)).The SEM images clearly show the particle sizes of alumina supported noble metals catalysts.The particle size of catalyst is an important factor the deoxygenation reaction of CO2 because the reaction of CO2 with hydrogen, methane, carbon and combining the mixture of hydrogen, methane and carbon could occur on the supported catalysts.Therefore, the large number of fine particles in the same amount of catalyst with their uniformity and uniform distribution enhances the de-oxygenation reaction of CO2 with reactants hydrogen, methane and carbon.Here, the alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts were observed with fine and uniform particles as shown in the Figure 2(a-i).The observed de-oxygenation reactions of CO2 with reactants hydrogen and methane are efficient (activities of catalysts are good) over alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts in the studied temperature range 500 to 650 o C.
The status of supported catalyst is observed by the XRD pattern of metal catalyst component over the alumina support.In order to identify the crystalline phases of catalysts, the XRD patterns for alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium were recorded for different loadings of these metals over alumina (Figures 3-5).In the Figure 3, the XRD patterns for the alumina supported molybdenum, vanadium and magnesium catalysts were recorded.However, the results show that as the loading of ruthenium increases over alumina from the 0.5 to 5 wt.%, the intensities of 2θ values of ruthenium were increased.The XRD patterns of 4wt.%Pt/Al2O3 and 2 wt.%Rh/Al2O3 were shown in the Figure 4.In the Figures 3-5, it was observed that as the loading of metal catalyst component ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium over the alumina support increases, the crystalline phases of these metal catalyst component increases indicating that the de-oxygenation reaction of CO2 with hydrogen and methane increases.The results (Figures 3-5) of intensity in the XRD pattern of metal catalyst component increases with the loading of metal catalysts component over the alumina support.However, the uniform distribution of metal catalyst component over the alumina support is also an important aspect.The uniform and equal distribution metal catalyst over alumina support was observed in a certain range of metal catalyst loading otherwise metal catalyst component either get coagulated or observed in bulky form.However, the crystalline phase of metal catalyst component is also an important aspect to allow the deoxygenation reaction of CO2 with hydrogen and methane.The conversion of de-oxygenation reaction of CO2 was increased over the alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and (basicity) magnesium catalysts in the studied temperature range.
The acidities of alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and (basicity) magnesium oxide catalysts were given in Table 1.Although, CO2 is an acidic gas, in a certain proportion it is adsorbed over the surface of the metal catalyst ruthenium, rhodium, platinum, molybdenum, vanadium and (basicity) magnesium components.The alumina supported molybdenum, vanadium and (basicity) magnesium catalysts deoxygenate the CO2 to carbon along with other products.Therefore, alumina supported molybdenum, vanadium and (basicity) magnesium catalysts are highly active for the de-oxygenation reaction of CO2.The conversion of de-oxygenation reaction of CO2 was increased with the increased in

The de-oxygenation of CO2 by using
hydrogen.
However, the de-oxygenation reaction of CO2 to methane is an eight-electron exchange process with the significant kinetic limitations.The deoxygenation of carbon dioxide with hydrogen over catalyst would be given as.
The de-oxygenation of carbon dioxide by using hydrogen were carried out over 1 wt% 1 wt% Rh/Al2O3, 1 wt.%Pt/Al2O3, 2 wt% Mo, 7.5 wt% V and 5 wt% Mg catalysts in a temperature range 50 to 650 o C (Figures 6-7).The conversion of CO2 and selectivity for CO and CH4 were estimated.The conversion of carbon dioxide was observed 0.1 to 2.6 % in a temperature range from 50 to 300 o C over 1 wt% Ru/Al2O3, 1 wt% Rh/Al2O3 and 1 wt.%Pt/Al2O3 catalysts.The observed selectivity for methane was from 1.2 to 1.4 % over 1 wt% Rh/Al2O3.In the temperature range from 50 to 300 o C, the activation energy required to activate the catalyst for the de-oxygenation reaction of CO2 with hydrogen is not enough.Therefore, the conversion of de-oxygenation reaction of CO2 was observed low.
The conversion of carbon dioxide was increased after the increasing the temperature 400 o C over 1 wt% Ru/Al2O3, 1 wt% Rh/Al2O3 and 1 wt.%Pt/Al2O3 catalysts.The selectivity to methane was also observed with the increased above 400 o C. The conversion of carbon dioxide and selectivity of carbon monoxide increases from the temperature 550 to 650 o C over 2 wt% Mo, 7.5 wt% V and 5 wt% Mg catalysts.25.4% CO2 conversion was observed over 1 wt% Pt/Al2O3 catalyst while the selectivity to methane was 33.8%.However, above the temperature 400 o C, the catalyst get higher energy of activation, therefore, the conversion of deoxygenation reaction of CO2 by hydrogen was observed higher.

De-oxygenation of CO2 with variation of H2/CO2 mol ratio
From the above reactions, it can be seen that the H2/CO2 mol ratio is an important aspect to explore for the de-oxygenation reaction of CO2 by hydrogen as the de-oxygenation of CO2 and the product formation depend on the reaction mixture composition of CO2 and hydrogen.The described in the de-oxygenation reactions of CO2 by variation of mol ratio of CO2 to H2, the different products such as C, CO and CH4 were formed.An effect of H2/CO2 mol ratio in a range from 1.0 to 5 for the deoxygenation of CO2 by H2 was studied over 1 wt.%Ru/Al2O3, 1 wt.%Rh/Al2O3, 1 wt.%Pt/Al2O3, 2 wt% Mo, 7.5 wt% V and 5 wt% Mg at the 550 o C. Deoxygenation of CO2 by H2 showed the converted 12.1 % carbon dioxide with selectivity 12.5 % to methane over 1 wt.%Ru/Al2O3 catalyst at (H2/CO2) = 2 mol ratio (Figures 8-9).However, over 1 wt.%Pt/Al2O3 ) catalyst at the 550 o C, 9 % conversion of CO2 with 14.6 % selectivity to methane was observed.The results showed that the conversion of carbon dioxide at 2 mol ratios was observed higher along with selectivity to carbon monoxide over 2 wt% Mo/Al2O3 catalyst.Over 7.5 wt%

Bulletin of Chemical Reaction Engineering & Catalysis, 7 (1), 2012, 65
Copyright © 2012, BCREC, ISSN 1978-2993 V/Al2O3 and 5 wt% Mg/Al2O3 catalysts, conversion of carbon dioxide and selectivity to carbon monoxide was higher at 1 mol ratio.However, conversion of carbon dioxide and selectivity to carbon monoxide decreases with the increased in the H2/CO2 mol ratio above 3.

De-oxygenation of CO2 with variation of catalyst loading
The uniform distribution of total amount of metal catalyst component over the alumina support is an important aspect to explore.The uniform and equal distribution metal catalyst amount over alumina support was observed in a certain range of metal catalyst loading otherwise metal catalyst component either get coagulated or observed in bulky form.De-oxygenation reaction of CO2 by using H2 was explored over alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts.Ruthenium, rhodium and platinum loadings over alumina were varied from 0.5 to 5 wt%.The molybdenum, vanadium and magnesium loadings over alumina were varied from 1.0 to 10 wt.%.In the Figures 10-11, the results were given of conversion of carbon dioxide and selectivity to methane or carbon monoxide.The conversion of carbon dioxide and selectivity to methane was observed in the range 0.4 to 17 % and 0.2 to 18 %, respectively over ruthenium, rhodium and platinum catalysts.The conversions of carbon dioxide and selectivity to carbon monoxide were increased with the increased in the molybdenum, vanadium and magnesium loading from 1.0 to 10.0 wt%.For 10 wt% Mg/Al2O3, conversion of carbon dioxide and selectivity to carbon monoxide were 12.23 % and 20.73 % respectively.

De-oxygenation of CO2 by methane
A reforming reaction of CO2 with methane could be given as:

Bulletin of Chemical Reaction Engineering & Catalysis, 7 (1), 2012, 66
Copyright © 2012, BCREC, ISSN 1978-2993  The de-oxygenation reactions of CO2 by methane could be given as follows: The product formation of de-oxygenation reaction of CO2 by methane depends on the composition of CO2 and methane over the catalyst bed.The results of reforming of CO2 by methane were given in Tables 2-3 over 2.5 wt% Ru/Al2O3, 1.0 wt% Rh/Al2O3, 1.0 wt% Pt/Al2O3, 2.0 wt% Mo/Al2O3, 7.5 wt% V/Al2O3 and 5.0 wt% Mg/Al2O3 catalysts at different mol ratios of CH4/CO2.However, 5.3 to 8.6 % CO2 conversion, 5.3 to 8.6 % CH4 conversion and 4.7 to 5.3 % selectivity to CO were observed over 2.5 wt% Ru/Al2O3 catalyst.Moreover, 0.2 to 2.5 % CO2 conversion, 0.2 to 2.4 % CH4 conversion and 0.1 to 1.3 % selectivity to CO were observed over 1.0 wt% Rh/Al2O3 catalyst.In addition to this, 5.8 to 8.8 % CO2 conversion, CH4 5.9 to 8.9 % conversion and 4.6 to 6.3 % selectivity to CO were observed over 1.0 wt% Pt/Al2O3 catalyst.Moreover, over 2.0 wt% Mo/Al2O3 catalyst, 0.83 to 2.66 % CO2 conversion, 0.04 to 3.0 % CH4 conversion and 0.08 to 0.63 % selectivity to CO were observed.Furthermore, 0.42 to 2.21 % CO2 conversion, 0.2 to 3.0 % CH4 conversion and 0.03 to 0.48 % selectivity to CO were observed over 7.5 wt% V/Al2O3 catalyst.In addition to this, the results of 0.23 to 1.12 % CO2 conversion, 0.12 to 1.24 % CH4 conversion and 0.005 to 0.20 % selectivity to CO were observed over 5.0 wt% Mg/Al2O3 catalyst.There is no carbon formation over the alumina supported Ru, Rh, Pt and Mg catalysts, however, there was carbon formation over the alumina supported Mo and V catalysts.However, alumina supported platinum catalyst shows good performance for reforming of CO2 by CH4.

De-oxygenation of CO2 by using H2 and CH4
The composition of reaction mixture during the de-oxygenation reaction of CO2 over catalyst bed is an important aspect to investigate.De-oxygenation reactions of carbon dioxide by using a mixture of hydrogen and CH4 over alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts were explored.Hydrogen and methane both de-oxygenate the CO2 over the alumina supported catalyst.Therefore, the combined effect hydrogen and methane for deoxygenation of CO2 was explored.In Tables 4-5

De-oxygenation reactions of CO2 in presence of carbon
The effect of carbon in presence of methane for the de-oxygenation reaction of CO2 is also important aspect to explore.The de-oxygenation of CO2 by using carbon could be given as follows: CO2 + C→ 2CO ΔΗ = 172.5 kJ/mol In the Figure 12, de-oxygenation reactions of CO2 by using carbon and methane were studied over the 1 wt% Pt/Al2O3 catalyst.The results show that the methane conversion was higher in absence of carbon.The conversions of CO2 and selectivity to CO trends were observed similar as earlier reported in the reforming of CO2 by methane.

Conclusions
The de-oxygenation reactions have been explored of carbon dioxide by using hydrogen, methane and carbon over alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts.De-oxygenation reactions of CO2 were efficient by using hydrogen or methane and hydrogen and methane over alumina supported ruthenium, rhodium, platinum, molybdenum, vanadium and magnesium catalysts.There was carbon formation during the deoxygenation reactions of CO2 over Mo and V supported catalysts.However, there is no carbon formation of over alumina supported ruthenium,

Figure 1 .
Figure 1.The schematic presentation of set up of deoxygenation reaction of CO2

Figure 4 .
Figure 4.The XRD patterns of the alumina supported the (a) platinum and (b) rhodium catalysts

Figure 10 .Figure 11 .
Figure 10.De-oxygenation of carbon dioxide by using hydrogen at 550 o C with variation of loading of Ru, Rh and Pt catalyst over alumina

Figure 12 .
Figure 12.De-oxygenation of carbon dioxide by using methane at 550 o C (a) in absence of carbon and (b) in presence of carbon (25 wt% ) over 1 wt% Pt/Al2O3 catalysts