Advanced
Synthesis Gas Production via Partial Oxidation, CO<sub>2</sub> Reforming, and Oxidative CO<sub>2</sub> Reforming of CH<sub>4</sub> over a Ni/Mg-Al Hydrotalcite-type Catalyst
Synthesis Gas Production via Partial Oxidation, CO2 Reforming, and Oxidative CO2 Reforming of CH4 over a Ni/Mg-Al Hydrotalcite-type Catalyst
Clean Technology. 2014. Jun, 20(2): 189-201
Copyright © 2014, The Korean Society of Clean Technology
  • Received : March 26, 2014
  • Accepted : May 16, 2014
  • Published : June 30, 2014
Download
PDF
e-PUB
PubReader
PPT
Export by style
Article
Author
Metrics
Cited by
TagCloud
About the Authors
Hoon Sub Song
Department of Chemical Engineering Education, Chungnam National University 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Korea
Soon Jin Kwon
Graduate School of Energy Science and Technology, Chungnam National University 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Korea
William S. Epling
Department of Chemical and Biomolecular Engineering, University of Houston 4800 Calhoun Road Houston, TX, 77004, United States
Eric Croiset
ecroiset@uwaterloo.ca
Sung Chan Nam
Greenhouse Gas Department, Korea Institute of Energy Research 140, Yuseong-daero 1312beon-gil, Yuseong-gu, Daejeon 305-343, Korea
Kwang Bok Yi
Department of Chemical Engineering Education, Chungnam National University 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Korea
ecroiset@uwaterloo.ca

Abstract
Partial oxidation, CO 2 reforming and the oxidative CO 2 reforming of CH 4 to produce synthesis gas over supported Ni hydrotalcite-type (Ni 0.5 Ca 2.5 Al catalyst) catalysts were carried out and the effects of metal supports (i.e.; Mg and Ca) on the formation of a stable double-layer structure on the catalysts were evaluated. The CH 4 reforming stability was determined to be affected by the differences in the interaction strength between the active Ni ions and support metal ions. Only a Ni-Mg-Al composition produced a highly stable hydrotalcite-type double-layered structure; while the Ni-Ca-Al-type composition did not. Such structure provides excellent stability for the catalyst (-80% efficiency) as confirmed by the long-term CO 2 reforming test (-100 h), while the Ni-Ca-Al catalyst exhibited deactivation phases starting at the beginning of the reaction. The interaction strength between the active metal (Ni) and the supporting components (Mg and Al) was determined by temperature-programed reduction (TPR) analyses. The affinity was also confirmed by the TPR temperature because the Ni-Mg-Al catalyst required a higher temperature to reduce the Ni relative to the Ni-Ca-Al catalyst. The highest initial activity for synthesis gas production was observed for the Ni 0.5 Ca 2.5 Al catalyst; however, this activity decreased quickly due to coke formation. The Ni 0.5 Ca 2.5 Al catalyst exhibited a high reactivity and was more stable than the other catalysts because it had a higher resistance to coke formation.
Keywords
1. Introduction
In recent years, the replacement of fossil fuels has received much attention as a result of various environmental issues as well as the limited amount of resources [1] . Therefore, the utilization of natural gas (mainly CH 4 ) for useful energy applications in transportation, such as methanol, hydrogen, or synthesis gas, is an attractive technology. There are many approaches to develop of highly resistant catalysts for the CH 4 reforming process and improve simultaneously the lifetime of the catalysts. One of the most common active metal components is Ni supported by stable materials (i.e.; Al 2 O 3 ). Supported Ni catalysts are the typical choice for methane reforming, with Ni metal being an active component in CH 4 dissociation [2- 4] . Typically, supported Ni catalysts are prepared by the wet-impregnation method. However, the catalysts prepared by wet-impregnation are often not fully capable of reduction by H 2 and the active Ni metal is often not homogenously distributed on/in the catalyst [5] . Additionally, Ni particles typically sinter during reformation reactions at high temperatures [6] , and the larger Ni particle sizes promote carbon formation. Under reforming conditions, catalyst deactivation also leads to a reduction in pressure due to the build-up of carbonaceous species [4 , 7 , 8] . A catalyst with a homogeneously distributed active sites would therefore be more resistant to sintering and coke formation under extreme conditions [9 - 12] . Hydrotalcite (HT)-like compounds, a class of layered double hydroxides (LDHs), contain 2+ and 3+ metal ions randomly distributed throughout the layered structure. The Mg-Al HT precursors, based on [Mg 1-x 2+ Al x 3+ (OH) 2 ] x+ (CO 3 - x )⋅ m H 2 O, in which a portion of the Mg 2+ ions can be replaced by Ni 2+ , contain exchangeable anions located within the layered structure [6 , 9 , 13 , 14] . Upon high temperature calcination, this type of catalyst maintains a high surface area. Additionally, the HT-type compounds contain very small crystallites comprising oxide mixtures, which are stable during high temperature reactions and can also be reduced to thermally stable metal particles [15 , 16] . These attributes render the Mg-Al HT, with Ni substitution, an optimal catalyst candidate for reforming reactions. In recent years, many investigations have been conducted to increase the stability of the HT-type catalysts by adding promoters. For example, Yu inserted La into the Ni-Mg-Al HT catalyst to inhibit carbon formation and enhance CO 2 adsorption.
Eq. (1) represents the general expression of the combined partial oxidation reaction where CO 2 reforming is converted to CH 4 , where x represents the amount of CO 2 added. The stoichiometric amount of oxygen can then be determined by the amount of CO 2 present. If x is zero, then Eq. (1) is equivalent to the partial oxidation reaction, Eq. (2). If x is 6, then Eq. (1) is equivalent to the CO 2 reforming reaction, Eq. (3). Changing x between 0 and 6 changes the H 2 /CO ratio between 2 and 1. Therefore, by varying x, the resulting synthesis gas ratio (i.e. H 2 /CO ratio) can be tailored to be between 2 and 1. For methanol production, the conversion of CO 2 reforming of methane would require an additional process to adjust the H 2 /CO ratio because it results in a synthesis gas ratio less than 2 [17 , 18] . The partial oxidation of CH 4 , which meets the H 2 /CO ratio of 2 for downstream methanol production, is difficult to control due to the formation of hot-spots and the subsequent risk of explosion [2] . In the oxidative CO 2 reforming of CH 4 (where x ranges from 0 to 6 in Eq. (1)), the H 2 /CO ratio can be controlled by adjusting the feed ratio of O 2 and CO 2 . Therefore, the oxidative CO 2 reforming of CH 4 is an interesting approach to synthesis gas production, especially for downstream applications with specific H 2 / CO ratio needs, such as methanol synthesis or the Fischer-Tropsch process [19] . Additionally, the oxidative CO 2 reforming of CH 4 overcomes some of the limitations of independent partial oxidation and CO 2 reforming reactions [20] . For example, CO 2 reforming of CH 4 is endothermic and typically requires a substantial heat input to maintain the reaction temperatures. However, combining it with the partial oxidation reaction results in a lower energy input being required to maintain the catalyst bed at high temperatures, because the exothermic partial oxidation reaction can provide the heat for the endothermic reforming component [13] .
PPT Slide
Lager Image
PPT Slide
Lager Image
PPT Slide
Lager Image
As described above, a variety of catalysts have been previously employed in steam reforming, partial oxidation, and dry reforming to produce synthesis gas. However, most of these studies did not include long-term experiments to determine the stability of the HT-type catalyst. In this study, a typical Ni-Mg/Al HT-type catalyst was used for synthesis gas production. Additionally, the effects of various cations on the formation of the HT-type catalyst were evaluated with respect to stability.
2. Experimental
- 2.1. Catalyst preparation
Two co-precipitated catalysts (Ni 0.5 Mg 2.5 Al and Ni 0.5 Ca 2.5 Al) were prepared. The preparation procedure for both catalysts was identical, with either Mg or Ca nitrates being used as the precursors. For the preparation of the Ni 0.5 Mg 2.5 Al catalyst, 30 mL of a 1 M Ni nitrate (Ni(NO 3 ) 2 ⋅6H2O, Puratronic ® , 99.9985%) aqueous solution, 150 mL of a 1 M Mg nitrate (magnesium nitrate hexahydrate, Alpha-Aesar, ACS grade 98.0-102.0%) aqueous solution and 60 mL of a 1 M Al nitrate (Al(NO 3 ) 3 ⋅9H 2 O, Alpha-Aesar, ACS grade 98.0-102.0%) aqueous solution were prepared and mixed. The mixed nitrate solution was then added drop-wise into 240 mL of a 0.50 M Na 2 CO 3 aqueous solution at room temperature under vigorous stirring (400 rpm). A 3.0 M NaOH aqueous solution was simultaneously added drop-wise to maintain the pH at 10.0±0.1. The resulting precipitate was aged in the mother liquor at 120 ℃ for 12 h. The aged precipitate was then cooled to room temperature and held for 1 h. The precipitate was then washed and filtered with distilled and de-ionized water until the residual Na + in the aged precipitate was removed (pH -7). The washed precipitate was then dried at 120 ℃ for 12 h. The Ni 0.5 Ca 2.5 Al catalyst was prepared in the same manner using a mixture of 30 mL of 1 M Ni nitrate, 150 mL of 1 M Ca nitrate (Ca(NO 3 ) 2 ⋅4H 2 O, Alpha-Aesar, ACS grade 98.0-102.0%), and 60 mL of 1 M Al nitrate. In preparing the 10 wt% Ni/Al2O3 catalyst, -75 mL of distilled water was heated to 80 ℃ while stirring, and γ-Al 2 O 3 powder (Alpha-Aesar, 99.97% (metal basis)) was then added to the distilled water. 1 M Ni nitrate was added drop-wise, until the target 10 wt% of Ni relative to the Al 2 O 3 was obtained. For all catalysts, water was evaporated under continuous stirring at 80 ℃. The resulting residue was then heated in an oven at 120 ℃ for 12 h, and the dried precipitate was crushed to powder form. The powder precipitates were calcined at 850 ℃ in air for 5 h to produce an oxide-phase catalyst. The calcined catalysts were pelletized to attain a particle size of 354-500 µm.
- 2.2. Catalyst characterization
The catalyst crystal structures of the fresh and spent catalysts were characterized using X-ray diffraction (XRD). Powder XRD patterns were measured on a Rigaku D/Max-III C using standard Bragg-Brentano geometry with Ni-filtered Cu Kα radiation (λ 1 =1.5406 Å, λ 2 =1.5444 Å) and 40 kV/100 mA X-ray radiation. The XRD data were collected in the range of 5° to 80° 2θ range using a step size of 0.01 and a count time of 1s. The diffraction patterns were identified upon comparison with spectra in the JCPDS data base (International Centre for Diffraction Data, USA). Catalyst surface areas were measured using a Micromeritics Gemini 3 2375. Approximately 100 mg of the pelletized catalysts (particle size in the range 354-500 µm) was loaded and the surface area was measured at -196 ℃, using liquid nitrogen as the adsorbate. Eleven points within the P/P 0 range of 0.05 and 0.3 were collected and used to produce the BET plot and subsequently calculate the surface area. The BET surface areas of the Ni 0.5 Mg 2.5 Al, Ni 0.5 Ca 2.5 Al, and Ni/Al 2 O 3 catalysts after calcination and reduction are listed in Table 1 and compared to some reference values. After reduction in 10% H 2 /N 2 at 720 ℃ for 1 h, the surface areas increased, which implied re-construction of the catalyst surfaces. The Ni0.5Ca2.5Al catalyst had the highest increase in surface area after reduction.
Catalyst surface areas after calcimation and reduction
PPT Slide
Lager Image
a)Calcined at 1,123 K for 5 h in airb)Reduced at 993 K for 1 h in 10% H2/N2
The catalyst morphology was examined using scanning electron microscopy (SEM). A Leo FESEM 1530 SEM was utilized in this investigation, and the vacuum pressure was less than 1.5 ×10 -5 mbar. TPR and TPO were also used to characterize the catalysts. For the TPR test, 100 mg of the calcined catalyst was placed in a quartz tube and the temperature was ramped at 10 ℃/min from 25 to 800 ℃ in the presence of a 10% H 2 /N 2 reducing gas mixture flowing at 30 mL/min. A thermal conductivity detector (TCD) was then used to measure the amount of H 2 consumption. The amount of coke produced during the CO 2 reforming of CH 4 was determined by TPO in a Hiden Catlab micro-reactor system. 25 mg of the spent catalyst was placed in the quartz tube and a 30 mL/min 10% O 2 /He gas mixture was utilized. The sample temperature was increased from 25 ℃ to 850 at 10 ℃/min.
- 2.3. Catalytic reaction tests
A fixed-bed reactor was designed to examine the catalyst performances. The catalyst bed resided in a 10 mm I.D. quartz tube with a highly porous quartz frit to support the catalyst. A thermocouple located at the top of the catalyst bed was used to control the furnace temperature. Before the reaction tests, the effects of the external and internal mass transfer limitation at a constant GHSV of 240,000 cm 3 /g-h were examined. For the external mass transfer, a fixed residence time (W/F=0.015) through a variety of feed velocities (50, 100, 150, 200 and 250 mL/min of total feed) at 700 ℃ was chosen. The internal mass transfer was evaluated using a feed mixture composed of CH 4 / CO 2 /N 2 =1/1/3 at 700 ℃ for various sizes of catalysts (average 303, 427, and 605 μm). When the CH 4 consumption rate was unchanged for the external and internal mass transfer limitations, the reaction can then be controlled by the reaction limitation instead of the diffusion limitation. The activation energy of CH 4 and CO 2 consumption was determined by the Arrhenius plot. For each reaction test, 50 mg of the calcined catalyst with a particle size between 354-500 µm was diluted with 400 mg of SiC (422-599 µm). The SiC was used to prevent an increase in pressure due to coke formation and subsequently improve the thermal dispersion throughout the catalyst bed. The catalysts were reduced in situ at 720 ℃ in 200 mL/min of 10% H 2 /N 2 for 1 h. After the reduction, the bed temperature was reduced to the desired reaction temperature under 100% N 2 . During the catalyst activity tests, the total gas flow rate was set to 200 mL/min (80 mL/min of reactant gases and 120 mL/min of N 2 ). The feed ratio for the partial oxidation of CH 4 was CH 4 /O 2 /N 2 =26/14/60 and for CO 2 reforming of CH 4 was CH 4 /CO 2 /N 2 = 20/20/60. For the oxidative CO 2 reforming of CH 4 , the x represented the amount of CO 2 added in Eq. (1) was (4) in this study. The ratio was therefore CH 4 /CO 2 /O 2 /N 2 =21.8/14.5/3.6/60, and the stoichiometry corresponding to x =4 is shown in Eq. (4)
PPT Slide
Lager Image
The resulting gases were collected and sampled using a gas chromatography (GC) column (Varian CP-3800 with a Carboxen-1000) to quantify the components of the product gases. The CH 4 conversion rate (Eq. (5)) and the synthesis gas ratio, H 2 / CO, (Eq. (6)) were calculated from the concentrations measured by GC, which were then converted into molar flow rates. The activation energy of CH 4 and CO 2 were calculated from the Arrhenius plot (Eq. (7)).
PPT Slide
Lager Image
PPT Slide
Lager Image
For the pre-treatment, the catalysts were reduced at 720 ℃ in 10% H 2 /N 2 for 1 h. The three reactions-partial oxidation, CO 2 reforming, and oxidative CO 2 reforming of CH 4 -were carried out with a GHSV=240,000 cm 3 /g-h at 700 ℃ for 20 h during the first set of experiments.
3. Results and discussions
- 3.1. Catalyst activities
- 3.1.1. External and internal mass transfer limitation
Reliable kinetic data could be obtained when the mass transfer limitation (external and internal) was negligible. The effects of the feed flow rate and catalyst particle size on the reaction rate were experimentally determined using a feed mixture composed of CH 4 /CO 2 /N 2 (1/1/3) at 700 ℃. For the external diffusion, a constant rate (W/F=0.015 g*s/mL) was applied in order to maintain the amount of contact time. The effects of the flow rate with a consistent amount of contact time are shown in Figure 1 . It is clear that the rate of CH 4 consumption increased until the flow rate was 200 mL/min. This result indicated that up to the flow rate of 200 mL/min, the external mass transfer had some influence on the overall reaction rate. However, when the flow rate exceeded 200 mL/min, the rate of CH 4 consumption remained constant. Therefore, experiments with a flow rate of 200 mL/min are adequate to obtain reaction data that are not masked by the external transport limitation.
PPT Slide
Lager Image
External mass transfer limitation (Catalyst: Ni0.5Mg2.5Al-HT; W/F = 0.015 at 700 ℃).
In order to investigate the possible effects of internal diffusion limitation, three different particle sizes were tested: (i) 251-354 μm (average 303 μm), (ii) 354-500 μm (average 427 μm), and (iii) 500-710 μm (average 605 μm). The flow rate was chosen to be 200 mL/min based on the external limitation. The effects of the particle size on the rate of CH 4 consumption is shown in Figure 2 . It was clear that the rate of CH 4 consumption was independent of the average diameter of the particles. This result indicated that particle sizes between 250 and 700 μm are small enough for the reaction to not be affected by internal mass limitation at 700 ℃.
PPT Slide
Lager Image
Internal mass transfer limitation (Catalyst: Ni0.5Mg2.5Al-HT; Feed flow rate = 200 mL/min at 700 ℃).
- 3.1.2. Partial oxidation of CH4
The conversions attained during the CH 4 partial oxidation and the resulting synthesis gas ratios (H 2 /CO) are shown in Figure 3 . All catalysts exhibited -85% CH 4 conversion and the Mg- and Ca-containing samples exhibited no significant deactivation over the 20 h test. Additionally, the levels of reactivity were all determined to be near the equilibrium levels. The Ni/Al 2 O 3 catalyst exhibited some deactivation beginning after 13 h of reaction, which confirmed that the Ni/Al 2 O 3 catalyst deactivates over time, as confirmed by many other previous studies [21- 23] . The synthesis gas ratios (H 2 /CO ratio) obtained with each catalyst exhibited similar patterns. The Ni 0.5 Mg 2.5 Al and Ni 0.5 Ca 2.5 Al catalysts resulted in a syngas ratio of around 2, close to the theoretical level. This result indicated that since H 2 and CO were produced in proportion, the stoichiometry in Eq. (4) was applied and neither product was preferentially consumed via side reactions afterwards. The syngas ratio decreased with time over the Ni/Al 2 O 3 catalyst, which corresponded to the reduced conversion. In conclusion, for the partial oxidation of CH 4 , the two co-precipitated catalysts (Ni 0.5 Mg 2.5 Al and Ni 0.5 Ca 2.5 Al) exhibited stable reactivity for 20 h of reaction with conversions close to the equilibrium level and syngas ratios near the predicted values based on the reaction stoichiometry.
PPT Slide
Lager Image
CH4 conversion profiles and gas ratios (H2/CO) for the CH4 partial oxidation as a function of time for the three catalysts. Equilibrium values, calculated using ASPEN PlusTM, are also plotted (total gas flow rate: 200 mL/min; CH4/O2/N2 = 26/14/60 vol%; GHSV = 240,000 cm3/ gcat-h).
- 3.1.3. CO2reforming of CH4
Coke covering the active Ni sites will ultimately lower the activity of the catalyst. It is therefore assumed that the dry reforming is a good indicator of the carbon deposition resistance of a given catalyst. Under CO 2 reforming of CH 4 , two reactions typically occur CO 2 reforming of CH 4 (Eq. (3)) and the reversible water-gas shift (RWGS) reaction (Eq. (8)). Therefore, in addition to the main reaction, the RWGS also affects the overall pathway. For example, due to the RWGS reaction, the syngas ratio (H 2 /CO) for CO 2 reforming of CH 4 was always less than 1.
PPT Slide
Lager Image
The CH 4 conversions at 700 ℃ for the three catalysts, along with the equilibrium conversion, are shown in Figure 4 . At 700 ℃, the theoretical CH 4 conversion level was -82%. The CH 4 conversion using the Ni 0.5 Ca 2.5 Al catalyst increased slightly at the beginning and reached the equilibrium CH 4 conversion (82%) within 4 h time-on-stream. However, after 4 h it slowly deactivated over time, decreasing from 82% to 75% after 20 h. This reduction was likely caused by carbon build-up on the surface, which was confirmed by TPO and will be discussed later. The Ni/Al 2 O 3 catalyst exhibited continuous deactivation from an initial CH 4 conversion of 60%, decreasing to 48% conversion after 20 h. It is important to note that Ni/Al 2 O 3 also had the lowest initial CH 4 conversion. Alternatively, the Ni 0.5 Mg 2.5 Al catalyst exhibited very stable conversion (-68%) compared to the other catalysts. This result indicates that the Ni 0.5 Mg 2.5 Al catalyst exhibited the highest resistance to deactivation. However, the conversion was lower than the equilibrium conversion, and lower than the conversion obtained with the Ca-containing catalyst. The syngas ratios (H 2 /CO ratios) were also less than unity due to the reverse watergas shift (RWGS) reaction (Eq. (8)). Additionally, even though the CH 4 conversion over the three catalysts was distinguishable, the syngas ratios were very similar and all catalysts started with a ratio of -0.9, which was close to the equilibrium value. These results confirmed that the CH 4 conversions were dependent on the catalysts, but the synthesis gas ratios were not, which suggested that the same reaction pathway was occurring over each catalyst. These results also indicated that the RWGS reaction rapidly reaches equilibrium for CO 2 reforming of CH 4 at 700 ℃.
PPT Slide
Lager Image
CH4 conversion profiles and synthesis gas ratios (H2/CO) as a function of time for CO2 reforming of CH4 using the three different catalysts (total gas flow rate: 200 mL/min; CH4/CO2/N2 = 20/20/60 vol%; GHSV = 240,000 cm3/ gcat-h).
During 20 h of time-on-stream, the Ni 0.5 Mg 2.5 Al catalyst exhibited high stability, and the Ni 0.5 Ca 2.5 Al catalyst exhibited deactivation. However, even after 20 h, the activity of the Cabased catalyst was still higher than the Mg-based catalyst. It is therefore difficult to conclude after 20 h time-on-stream which of these two catalysts was better. Therefore, more prolonged experiments were performed at 100 h time-on-stream for these two catalysts. The pre-treatment processes were identical to those described previously. The experimental results for the Ni 0.5 Mg 2.5 Al catalyst and the Ni 0.5 Ca 2.5 Al catalyst are shown in Figure 5 for over 100 h of the dry reforming. The Ni 0.5 Mg 2.5 Al catalyst exhibited a relatively stable reactivity based on the CH 4 convertsion and the H 2 /CO ratio. The CH 4 conversion decreased by about 4% over 100 h, from 67 to 63%. However, for the Ni 0.5 Ca 2.5 Al catalyst, the deactivation trend observed during the first 20 h continued throughout 100 h. The CH 4 conversion decreased from 78 to 62% but the syngas ratio only slightly decreased. Based on these longer-term experiments, it was clear that the Ni 0.5 Mg 2.5 Al catalyst suffered considerably less deactivation than the Ni 0.5 Ca 2.5 Al catalyst and exhibited a much higher resistance to coke formation, as will be discussed below. In conclusion, the Ni 0.5 Mg 2.5 Al catalyst exhibited the highest stability, even though the activity remained lower than the equilibrium-based expectation. The Ni 0.5 Ca 2.5 Al catalyst initially exhibited the highest activity, reaching equilibrium after about 4 h time-onstream, but past 4 h its activity continuously decreased at a rate of -0.2%/h. Finally, the Ni/Al 2 O 3 catalyst exhibited the worst reactivity and stability for the dry reforming.
PPT Slide
Lager Image
Long-term Ni0.5Mg2.5Al and Ni0.5Ca2.5Al catalytic performance profiles for CO2 reforming of CH4 (total gas flow rate: 200 mL/min; CH4/CO2/N2 = 20/20/60 vol%; GHSV = 240,000 cm3/ gcat-h).
- 3.1.4. Oxidative CO2reforming of CH4
The basic concept of the oxidative CO 2 reforming of CH 4 is to combine the exothermic partial oxidation with the endothermic CO 2 reforming. The change in reaction enthalpy (ΔH) at 700 ℃ for the range of x [Eq. (1)] between 0 and 6 was simulated using ASPEN PlusTM, the results of which are shown in Figure 6 . When x was 0, Eq. (1) represented the partial oxidation of the CH 4 reaction and when x was 6, it represented the CO 2 reforming of CH 4 . Figure 6 confirms that oxidative CO 2 reforming of CH 4 at 700 ℃ was an endothermic reaction when x was greater than 1. In order to confirm the ASPEN simulation ( Figure 6 ) with actual experimental data, temperature data obtained during the experiments are shown in Figure 7 . With the two reforming reactions, there was an immediate decrease in temperature, which then increased as the heat of the furnace compensated for the drop in temperature. For the partial oxidation reaction, the opposite process occurred. The oxidative reforming led to a smaller decrease in temperature relative to the dry reforming alone, confirming that the former requires less heat to maintain the reaction temperature while reducing the chance of hot spots formation as a result of partial oxidation.
PPT Slide
Lager Image
Change in enthalpy (ΔH) at 700 ℃ as a function of the x value described in Eq. (1) (calculated using ASPEN PlusTM).
PPT Slide
Lager Image
Temperature profiles during partial oxidation, CO2 reforming, and the oxidative CO2 reforming of CH4 upon reducing the reaction temperature from 720 ℃ to 700 ℃.
The catalytic activities for the oxidative CO 2 reforming of CH 4 over the three catalysts are shown in Figure 8 . Co-feeding O 2 with CH 4 and CO 2 was expected to reduce carbon deposition as a result of the increased oxidation of surface carbon species and also increase the conversion of CH 4 at high temperatures (-700 ℃). The reactivities of the Ni 0.5 Ca 2.5 Al and Ni 0.5 Mg 2.5 Al catalysts during the initial period (< 5 h) were similar (-75%). The reactivity of the Ni 0.5 Ca 2.5 Al catalyst for oxidative CO 2 reforming of CH 4 was less than that of CO 2 reforming of CH4; however, the reactivity of the Ni 0.5 Mg 2.5 Al catalyst was quite similar. Since the feed for the oxidative CO 2 reforming of CH 4 contains oxygen that might oxidize surface carbon, the Ni 0.5 Ca 2.5 Al catalyst did not exhibit deactivation, and conversion actually slightly increased after 20 h of reaction. The syngas ratios for the catalysts were similar, even though the syngas ratio for the Ni/Al 2 O 3 was slightly lower than that of the other catalysts. These trends (different conversions but same syngas ratios) were also observed in the partial oxidation and CO 2 reforming reactions, which suggested that the same reaction mechanisms existed over the three sample types. Additionally, except for the Ni/Al 2 O 3 catalyst, the H 2 /CO ratio was very close to its equilibrium value, which implied that the RWGS reaction reached equilibrium in the presence of the Ni 0.5 Mg 2.5 Al and Ni 0.5 Ca 2.5 Al catalysts at 700 ℃.
PPT Slide
Lager Image
CH4 conversion profiles and synthesis gas ratios (H2/CO) for the oxidative CO2 reforming of CH4 on the three catalysts (total gas flow rate: 200 mL/min; CH4/CO2/O2/N2 = 21.8/14.5/3.6/60 vol%; GHSV = 240,000 cm3/ gcat-h).
Overall, based on the reactivity tests for all three sets of reactions, the Ni 0.5 Mg 2.5 Al catalyst exhibited the most stable reactivity among other catalysts under the most severe conditions (e.g. CO 2 reforming of CH 4 ). Additionally, the reactivity of the Ni 0.5 Mg 2.5 Al catalyst, although lower than the equilibrium conversion, still resulted in greater than 70% CH 4 conversion. The Ni 0.5 Ca 2.5 Al catalyst exhibited the highest initial CH 4 conversion for all reactions; however, its reactivity began to degrade shortly after the onset of the dry reforming test. Finally, as expected, the tests on oxidative CO 2 reforming of CH 4 exhibited convertsions between those of the partial oxidation and the dry reforming. An important difference between the oxidative CO 2 reforming and dry reforming in the presence of the Ni 0.5 Ca 2.5 Al catalyst was that the reactivity was more active when oxygen was added and its activity decreased initially but eventually stabilized at a level similar to that of Ni 0.5 Mg 2.5 Al. Nonetheless, the Ni 0.5 Mg 2.5 Al catalyst exhibited superior stability under dry reforming and oxidative reforming.
- 3.1.5. Activation Energy and Effects of the Partial Pressures of CH4, CO2, and H2
Typically, the kinetic studies were conducted at the temperature in which the conversion was far from the equilibrium. It was confirmed that the CH 4 consumption rate from the reactivity test ( Figure 4 ) at 700 ℃ was lower than the equilibrium level. The temperature sensitivity of the CH 4 consumption rate for CO 2 reforming of CH 4 over the Ni 0.5 Mg 2.5 Al-HT catalyst was determined by the Arrhenius plot ( Figure 9 ) in the temperature range of 650-750 ℃ at a constant GHSV of 240,000 cm 3 /g-h. It was clear that the support of Ni crystallites significantly influenced the activation energy by affecting the rate-controlling step in the reaction sequence. The activation energy for the consumption of CH 4 and CO 2 and the formation of H 2 and CO on the Ni 0.5 Mg 2.5 Al-HT catalyst were calculated to be 18.9, 18.0, 25.3, and 18.7 kJ/mol, respectively. Based on the present investigation, it was determined that the rate determining step of CO formation closely corresponds to the CH 4 and CO 2 consumption step. One possible reason for obtaining a high energy barrier for H 2 formation was the RWGS process. Since the activation energy barrier of the CH 4 consumption step was slightly greater than that of the CO 2 consumption step, it was assumed that the CH 4 dissociation step could be the rate determining step. The lower activation energy barrier for CO 2 consumption might be caused by the presence of the strong Lewis base MgO, which can facilitate the activation of CO 2 .
PPT Slide
Lager Image
Arrhenius plot for CH4 and CO2 consumption at GHSV = 240,000 cm3/g*h; Feed composition: CH4/CO2/N2 = 1/1/3 vol%).
The influence of the partial pressures of CH 4 , CO 2 , and H 2 on the Ni 0.5 Mg 2.5 Al-HT catalyst at atmospheric pressure for the CH 4 consumption rate (-r CH4 ) was evaluated in the temperature range of 650-750 ℃. A constant CO 2 (or CH 4 ) partial pressure of 20.26 kPa was used as the CH 4 (or CO 2 ) partial pressure was varied. N 2 gas was used to balance the total GHSV to 240,000 cm 3 /g-h for all conditions. As shown in Figure 10 , the CH 4 consumption rate was strongly affected by the partial pressure of CH 4 at a CO 2 partial pressure of 20.26 kPa, since the CH 4 consumption rate increased as the CH 4 partial pressure increased. As shown in Figure 11 , the CH 4 consumption rate was strongly influenced when the partial pressure of CO 2 was lower (in the range of 10.13 to 20.26 kPa) than the stoichiometric ratio. The CH 4 consumption rate was then unchanged at higher CO 2 partial pressures (≫20.26 kPa). Therefore, it can be concluded that the reaction rate was more sensitive to the CO 2 partial pressure than to the CH 4 partial pressure when compared in the low partial pressure ranges (10.13 to 20.267 kPa). However, at high CO 2 and CH 4 partial pressures (≫20.26 kPa), the CH 4 partial pressure exhibited a stronger influence on the CH 4 consumption rates than
PPT Slide
Lager Image
CH4 consumption rate as a function of PCH4 at a constant PCO2 of 20.26 kPa.
PPT Slide
Lager Image
CH4 consumption rate as a function of PCO2 at a constant PCH4 of 20.26 kPa.
the CO 2 partial pressure. These results can be attributed to the stronger adsorption of CH 4 to the surface of the catalyst compared to that of CO 2 at higher partial pressures. Figure 12 displays the rates of CH 4 consumption at various partial pressures of H 2 at constant CH 4 and CO 2 partial pressures of 10.13 kPa. As the H 2 partial pressure increased, the rate of CH 4 consumption decreased because the reverse reaction and RWGS reaction then become dominant.
PPT Slide
Lager Image
CH4 consumption rate as a function of PH2 at a constant PCH4 and PCO2 of 20.26 kPa.
- 3.2. Catalyst characterization
- 3.2.1. Structure of fresh and spent catalysts
The fresh Ni 0.5 Mg 2.5 Al and Ni 0.5 Ca 2.5 Al catalyst crystal structures were investigated using XRD, and all diffraction patterns were matched with JPCDS references. The XRD patterns obtained from the Ni 0.5 Mg 2.5 Al sample ( Figure 13 ) confirmed that the as-prepared samples contained a well-crystallized HT-type (or LDH) phase. The LDH structure for the Ni 0.5 Mg 2.5 Al catalyst implied that Ni 2+ + was incorporated in the Mg 2+ site positions and dispersed uniformly throughout the brucite layer of the HT structure. In particular, a set of three reflection peaks at 2 θ values of 11.3, 22.7, and 34.5° indicated that the HT-type structure possessed a layered structure in which both Ni 2+ and Al 3+ substituted Mg 2+ sites in the brucite-like sheet [15 , 24] . After calcnation at 820 ℃ in air for 5 h, the HT peaks disappeared and MgO (at 37.0, 43.1, 62.6, and 79.0°) and NiO (at 37.0, 43.1, 62.6, 74.9, and 79.0°) peaks appeared and overlapped. After reduction at 720 ℃ in 10% H2/N2 for 1 h, the NiO peaks were weak and the Ni metal peaks at 44.1, 51.7, and 76.1° were clearly visible. This result indicated that during reduction, some of the NiO crystals transformed into active Ni metal phases, as expected. Therefore, the Ni 0.5 Mg 2.5 Al catalyst prepared by the co-precipitation method clearly contained active Ni phases substituted into a portion of the Mg 2+ phases, resulting in the Ni/MgAl HT-type catalyst [25] . After thermal treatment at 850 ℃ in air for 5 h, and the reduction by 10% diluted H 2 /N 2 at 720 ℃ for 1 h, the active Ni phases within the LDH become the active catalytic metal.
PPT Slide
Lager Image
XRD patterns obtained from the Ni0.5Mg2.5Al catalyst: (a) fresh; (b) calcined at 820 ℃ in air for 5 h; (c) reduced at 720 ℃ in 10% H2/N2 for 1 h.
For the Ni 0.5 Ca 2.5 Al catalyst, the XRD patterns of the fresh, calcined, and reduced catalysts are shown in Figure 14 . The fresh Ni 0.5 Ca 2.5 Al catalyst did not exhibit the HT-like structure. However, after being calcined at 820 ℃ in air for 5 h, NiO, CaO, and Ca 12 Al 14 O 33 peaks appeared. Interestingly, Ni metal peaks were observed with the calcined Ni 0.5 Ca 2.5 Al catalyst but not with the Ni 0.5 Mg 2.5 Al catalyst, which implied that the Ni-Ca interaction was weaker than Ni-Mg interaction.
PPT Slide
Lager Image
XRD patterns obtained from the Ni0.5Ca2.5Al catalyst: (a) fresh; (b) calcined at 820 ℃ in air for 5 h; (c) reduced at 720 ℃ in 10% H2/N2 for 1 h.
The XRD patterns of the spent Ni 0.5 Mg 2.6 Al catalysts after 20 h of partial oxidation, CO 2 reforming and the oxidative CO 2 reforming of CH 4 at 700 ℃ are shown in Figure 15 . Both Ni metal and NiO phases were observed after CH 4 partial oxidation and the oxidative CO 2 reforming of CH 4 . The oxygen species supplied for the partial oxidation of CH 4 and the oxidative CO 2 reforming of CH 4 likely oxidized some of the Ni. However, the H 2 or CO generated from the reactions can simultaneously reduce the formed NiO, subsequently regenerating the active Ni phases for further reactions, resulting in evidence of both chemical states being present. Additionally, a weak graphite peak was observed at 26.4° after CO 2 reforming of CH 4 and the oxidative CO 2 reforming of CH 4 tests, but not after the CH 4 partial oxidation. The likely reason for its absence during partial oxidation is that the C:O ratio was the lowest (26:28) compared to the other two processes.
PPT Slide
Lager Image
XRD patterns obtained from the Ni0.5Mg2.5Al catalyst (a) after reduction; (b) after partial oxidation of CH4; (c) after CO2 reforming of CH4; (d) after the oxidative CO2 reforming of CH4
The XRD patterns of the different catalysts after 20 h of CO 2 reforming of CH 4 at 700 ℃ are shown in Figure 16 . The diffraction patterns of Ni 0.5 Mg 2.5 Al indicated the presence of both metal and oxide Ni, as well as MgO. However, the XRD data obtained from the Ni 0.5 Ca 2.5 Al exhibited weaker Ni peaks, with significantly less Ni metal being observed. Most importantly, much stronger graphite peaks were observed on the Ni 0.5 Ca 2.5 Al and Ni/Al 2 O 3 catalysts than on the Ni 0.5 Mg 2.5 Al catalyst. These results implied that the Ni 0.5 Mg 2.5 Al catalyst had a stronger resistance to coke formation.
PPT Slide
Lager Image
XRD patterns after CO2 reforming of CH4 at 700 ℃ for 20 h from: (a) Ni0.5Mg2.5Al; (b) Ni0.5Ca2.5Al; (c) Ni/ Al2O3 catalysts.
- 3.2.2. TPR of catalysts
The H 2 -temperature-programmed reduction (H 2 -TPR) data obtained from the calcined Ni 0.5 Mg 2.5 Al and Ni 0.5 Ca 2.5 Al catalysts are shown in Figure 17 . The reducibility of the Ni-based catalysts during the reduction process was an important factor in determining the level of reactivity since metallic Ni produced by the reduction process was in the active phase to initiate the reaction (e.g. CH 4 dissociation). The reducibility of the catalyst was affected by, and can be estimated by measuring, the strength of the interaction between the active phase and the support. The Ni 0.5 Mg 2.5 Al catalyst exhibited a small peak at low temperatures (-600 ℃) and slightly increased at high temperatures (<700 ℃) while the Ni 0.5 Ca 2.5 Al catalyst exhibited two reduction peaks at lower temperatures, 580 and 770 ℃. Those two peaks can be assigned to complex NiO x species corresponding to Ni/support interactions[39]. Since the Ni 0.5 Mg 2.5 Al catalyst did not exhibit any H 2 consumption until very high temperatures were reached (>750 ℃), it was determined that the Ni 0.5 Ca 2.5 Al catalyst exhibited relatively free NiO species. Previous investigations confirmed that the reduction process for the NiO-MgO would be initiated at very high temperatures (-900 ℃) since it produced a solid solution [26] . Additionally, the area under the TPR curve was clearly greater for Ni 0.5 Ca 2.5 Al than for Ni 0.5 Mg 2.5 Al and it can therefore be concluded that more Ni was reduced over the Ni 0.5 Ca 2.5 Al catalyst compared to the Ni 0.5 Mg 2.5 Al catalyst. This result also implied that the Ni 0.5 Mg 2.5 Al catalyst exhibited stronger interactions between the Ni and support since the reduction peaked at a higher temperature over the Ni 0.5 Mg 2.5 Al catalyst. These phenomena could be explained by comparing of the MgO, CaO, and NiO lattice sizes. Both MgO and CaO are face-centered cubic type oxides. However, only MgO has lattice parameters (a=4.2112 Å) and bond distances (A-B = 2.11 Å) close to those of NiO (a = 4.1946 Å, A-B = 2.10 Å), with the lattice parameters and bond distances of CaO being a=4.8105 Å and A-B=2.40 Å [27] . This result suggested that NiO can be more easily substituted into the MgO lattice [24 , 28] . A catalyst that is more susceptible to reduction can ultimately provide more active sites on the catalytic surface, leading to higher reactivity. Based on the TPR results, the Ni 0.5 Ca 2.5 Al was the more reducible catalyst and had higher initial reactivity in the partial oxidation, CO 2 reforming and the oxidative CO 2 reforming of CH 4 processes relative to the Ni 0.5 Mg 2.5 Al catalyst, as shown in the reactivity data ( Figure 3 , Figure 4 , and Figure 8 ). Additionally, since 720 ℃ was the reduction temperature used prior to the catalytic testing, the Ni 0.5 Mg 2.5 Al catalyst was only partially reduced.
PPT Slide
Lager Image
TPR results for the calcined Ni0.5Ca2.5Al and Ni0.5Mg2.5Al catalysts.
SEM images of the fresh Ni 0.5 Mg 2.5 Al catalyst are shown in Figure 18 , which display a layered structure (also referred to as a “card house” shape [15] ) at the surface of the catalyst. Alternatively, the Ni 0.5 Ca 2.5 Al catalyst did not exhibit a layered structure [27] . This result corresponds with the XRD results ( Figure 13 ), where the data indicated that the Ni 0.5 Mg 2.5 Al catalyst possessed a layered structure. Additionally, this layered structure provided stronger interactions between the active site (Ni) and the support (Mg-Al), resulting is a lower reducibility, as confirmed above.
PPT Slide
Lager Image
SEM of the fresh Ni0.5Mg2.5Al catalyst (100,000× magnification).
- 3.2.3. Carbon deposition analysis
TPO experiments were performed to estimate the amount of carbon deposited on the catalytic surface after the reaction. The temperature of the catalyst bed was increased from 25 to 850 ℃ at a rate of 10 ℃/min. Mass spectrometry (MS) was used to detect the carbon oxides formed, which was then used to determine the amount of carbon present. The amount of carbon deposited on the catalysts can be estimated by calculating the area under the TPO curves plotted in Figure 19 . The amounts of carbon deposited are listed in Table 2 and confirm that the Ni 0.5 Mg 2.5 Al catalyst exhibited a significantly stronger resistance to coke formation relative to the Ni 0.5 Ca 2.5 Al catalyst. The resistance to coke formation might be related to the stronger interaction between Mg and Ni ions, which subsequently inhibited coke formation. Alternatively, the Ni 0.5 Ca 2.5 Al catalyst, which was more easily reduced and resulted in Ni/Ca interactions being weaker, was more susceptible to deactivation via coke build-up.
PPT Slide
Lager Image
TPO results for Ni0.5Ca2.5Al and Ni0.5Mg2.5Al catalysts after CO2 reforming of CH4 at 700 ℃ for 20 h.
Amount of carbon deposition on the Ni0.5Mg2.5Al and Ni0.5Ca2.5Al catalysts after 20 h, CO2reforming at 700 ℃.
PPT Slide
Lager Image
Amount of carbon deposition on the Ni0.5Mg2.5Al and Ni0.5Ca2.5Al catalysts after 20 h, CO2 reforming at 700 ℃.
SEM images of spent Ni 0.5 Mg 2.5 Al ( Figure 20 ) and spent Ni 0.5 Ca 2.5 Al ( Figure 21 ) catalysts after 20 h of CO 2 reforming of CH 4 at 700 ℃ exhibited significant differences in the carbon deposition traits. Using the Ni 0.5 Mg 2.5 Al catalyst resulted in significantly less carbon fiber evidence relative to the Ni 0.5 Ca 2.5 Al catalyst.
PPT Slide
Lager Image
Spent Ni0.5Mg2.5Al catalyst after 20 h of CO2 reforming of CH4 (5,000×magnification).
PPT Slide
Lager Image
Spent Ni0.5Ca2.5Al catalyst after 20 h of CO2 reforming of CH4 (5,000×magnification).
4. Conclusion
Different approaches for the synthesis gas production using Ni-supported catalysts have been investigated. The typical methods to produce synthesis gas (partial oxidation and CO 2 reforming of CH 4 ) by HT-like catalyst were performed for short and long reaction periods in order to the investigate the effects of the structure of the catalyst on the stability and reactivity. Partial oxidation, CO 2 reforming and the oxidative CO 2 reforming of CH 4 processes to produce synthesis gas were evaluated at 700 ℃ for 20 h in the presence of three different catalysts: Ni 0.5 Mg 2.5 Al, Ni 0.5 Ca 2.5 Al, and Ni/Al 2 O 3 . For the partial oxidation of CH 4 during 20 h of reaction, the Ni 0.5 Mg 2.5 Al, Ni 0.5 Ca 2.5 Al, and Ni/Al 2 O 3 catalysts exhibited similar levels of activity, which were close to the equilibrium levels, with the Ca- and Mg-containing samples exhibited no deactivation and the Ni/Al 2 O 3 catalyst being deactivated. During CO 2 reforming of CH 4 , the Ni/Al 2 O 3 and Ni 0.5 Ca 2.5 Al catalyst conversions decreased as a result of coke formation, leading to deactivation of the catalyst. The Ni 0.5 Mg 2.5 Al catalyst exhibited high and stable reactivities for over 100 h. The Oxidative CO 2 reforming of CH 4 , which combined the exothermic partial oxidation with the endothermic CO 2 reforming of CH 4 , can facilitate heat transfer between the reactions. The addition of O 2 reduced coke deposition on the catalysts since the oxygen can combust the surface carbon. Therefore, the catalytic activity levels for the partial oxidation and the oxidative CO 2 reforming of CH 4 processes could be maintained close to the equilibrium levels with less deactivation compared to the CO 2 reforming of CH 4 . The Ni 0.5 Ca 2.5 Al catalyst exhibited the highest initial activity, but was deactivated quickly due to coke deposition. The Ni 0.5 Mg 2.5 Al catalyst exhibited the most stable reactivity over 20 h of reaction. Additionally, the Ni 0.5 Mg 2.5 Al catalyst exhibited excellent stability for the CO 2 reforming of CH 4 , even in the absence of oxygen in the feed gas. TPR data indicate stronger interactions between the Ni and Mg compared to that with Ca, which seemingly correlates to catalyst stability in terms of decreased coke formation and subsequently allowing conversions and product yields to remain constant.
Acknowledgements
The authors gratefully acknowledge Technology Convergence Inc. (TCI) and the Ontario Centres of Excellence (OCE) for financial support, and a grant from Korea Institute of Energy Research (KIER) under Korea Research Council for Industrial Science and Technology, the Ministry of Knowledge and Economy, South Korea.
References
González A. R. , Asencios Y. J. O. , Assaf E. M. , Assaf J. M. 2013 “Dry Reforming of Methane on Ni-Mg-Al Nano-Spheroid Oxide Catalysts Prepared By the Sol-gel Method from Hydrotalcite-like Precursors,” Appl. Surf. Sci. http://dx.doi.org/10.1016/j.apsusc.2013.05.082 280 876 - 887
Tang S. , Lin J. , Tan K. L. 1998 “Partial Oxidation of Methane to Syngas over Ni/MgO, Ni/CaO and Ni/CeO2,” Catal. Lett. http://dx.doi.org/10.1023/A:1019034412036 51 169 - 175
Di M. , Dajiang M. , Xuan L. , Maochu G. , Yaoqiang C. 2006 “Partial Oxidation of Methane to Syngas over Monolithic Ni/gama-A12O3Catalyst-Effects of Rare Earths And other Basic Promoters,” J. Rare Earths http://dx.doi.org/10.1016/S1002-0721(06)60142-7 24 451 - 455
Choudhary V. R. , Rane V. H. , Rajput A. M. 1997 “Beneficial Effects of Cobalt Addition To Ni-catalysts for Oxidative Conversion of Methane to Syngas,” Appl. Catal. A Gen. http://dx.doi.org/10.1016/S0926-860X(97)00101-4 162 235 - 238
Shinozuka Y. , Ohishi Y. , Shishido T. , Takaki K. , Takehira K. 2005 “Nickel Containing Mg-Al Hydrotalcite-type Anionic Clay Catalyst for the Oxidation of Alcohols with Molecular Oxygen,” J. Mol. Catal. A Chem. http://dx.doi.org/10.1016/j.molcata.2005.04.035 236 206 - 215
Bhattacharyya A. , Chang V. W. , Schumacher D. J. 1998 “CO2Reforming of Methane to Syngas I: Evaluation of Hydrotalcite Clay-derived Catalysts,” Appl. Clay Sci. http://dx.doi.org/10.1016/S0169-1317(98)00030-1 13 317 - 328
Bartholomew C. H. 2001 “Mechanisms of Catalyst Deactivation,” Appl. Catal. A Gen. http://dx.doi.org/10.1016/S0926-860X(00)00843-7 212 17 - 60
Ito M. , Tagawa T. , Goto S 1999 “Suppression of Carbonaceous Depositions on Nickel Catalyst for the Carbon Dioxide Reforming of Methane,” Appl. Catal. A Gen. http://dx.doi.org/10.1016/S0926-860X(98)00251-8 177 15 - 23
Takehira K. , Shishido T. , Wang P. , Kosaka T. , Takaki K. 2004 “Autothermal Reforming of CH4over Supported Ni Catalysts Prepared from Mg-Al Hydrotalcite-like Anionic Clay,” J. Catal. http://dx.doi.org/10.1016/j.jcat.2003.07.001 221 43 - 54
Sukenobu M. , Morioka H. , Kondo M. , Wang Y. , Takaki K. , Takehira K. 2002 “Partial Oxidation of Methane over Ni/ Mg-Al Oxide Catalysts Prepared by Solid Phase Crystallization Method from Mg-Al Hydrotalcite-like Precursors,” Appl. Catal. A Gen. http://dx.doi.org/10.1016/S0926-860X(01)00732-3 223 35 - 42
Tomishige K. , Nurunnabi M. , Maruyama K. , Kunimori K. 2004 “Effect of Oxygen Addition to Steam and Dry Reforming of Methane on Bed Temperature Profile over Pt and Ni Catalysts,” Fuel Process. Technol. http://dx.doi.org/10.1016/j.fuproc.2003.10.014 85 1103 - 1120
Basini L. , Amore M. D , Fornasari G. , Guarinoni A. , Matteuzzi D. , Del Piero G. , Trifiro F. , Vaccari A. 1998 “Ni/ Mg/Al Anionic Clay Derived Catalysts for the Catalytic Partial Oxidation of Methane Residence Time Dependence of the Reactivity Features,” J. Catal. http://dx.doi.org/10.1006/jcat.1997.1942 173 247 - 256
Inaba M. , Tsunoda T. , Suzuki K. , Takehira K. , Hayakawa T. 2004 “Combined Partial Oxidation and Dry Reforming of Methane to Synthesis Gas over Noble Metals Supported on Mg-Al Mixed Oxide,” Appl. Catal. A Gen. http://dx.doi.org/10.1016/j.apcata.2004.07.030 275 149 - 155
Sukenobu M. , Morioka H. , Furukawa R. , Shirahase H. , Takehira K. 2001 “CO2Reforming of CH4over Ni/Mg-Al Oxide Catalysts Prepared by Solid Phase Crystallization Method from Mg-Al Hydrotalcite-like Precursors,” Catal. Lett. 73 21 - 26
Olsbye U. , Akporiaye D. , Rytter E. , Rønnekleiv M. , Tangstad E. 2002 “On the Stability of Mixed M2+/M3+Oxides,” Appl. Catal. A Gen. http://dx.doi.org/10.1016/S0926-860X(01)00740-2 224 39 - 49
Shiraga M. , Atake I. , Shishido T. , Oumi Y. , Sano T. , Takehira K. 2007 “Partial Oxidation of Propane over Ru Promoted Ni/Mg(Al)O Catalysts: Self-activation and Prominent Effect of Reduction-oxidation Treatment of the Catalyst,” Appl. Catal. A Gen. http://dx.doi.org/10.1016/j.apcata.2007.01.043 321 155 - 164
Oyama S. T. 2003 “Novel Catalysts for Advanced Hydroprocessing: Transition Metal Phosphides,” J. Catal. http://dx.doi.org/10.1016/S0021-9517(02)00069-6 216 343 - 352
Bradford M. C. J. , Vannice M. A. 1998 “CO2Reforming of CH4over Supported Pt Catalysts,” J. Catal. 171 157 - 171
Ruckenstein E. , Wang H. Y. 2001 “Combined Catalytic Partial Oxidation and CO2Reforming of Methane over Supported Cobalt Catalysts,” Catal. Lett. 73 99 - 105
Amin N. A. S. , Yaw T. C. 2007 “Thermodynamic Equilibrium Analysis of Combined Carbon Dioxide Reforming with Partial Oxidation of Methane to Syngas,” Int. J. Hydro. Energy 32 1789 - 1798
Zhu Y.-A. , Chen D. , Zhou X.-G. , Yuan W.-K. 2009 “DFT Studies of Dry Reforming of Methane on Ni Catalyst,” Catal. Today http://dx.doi.org/10.1016/j.cattod.2009.08.022 148 260 - 267
Zhang Y. , Xiong G. , Sheng S. , Yang W. 2000 “Deactivation Studies over NiO/γ-Al2O3Catalysts for Partial Oxidation of Methane to Syngas,” Catal. Today http://dx.doi.org/10.1016/S0920-5861(00)00498-3 63 517 - 522
Ginsburg J. M. , Pin J. , Solh T. El , Lasa H. I. De. 2005 “Coke Formation over A Nickel Catalyst Under Methane Dry Reforming Conditions: Thermodynamic and Kinetic Models,” Ind. Eng. Chem. Res. http://dx.doi.org/10.1021/ie0496333 44 4846 - 4854
Hu Y. H. , Ruckenstein E. 1996 “Temperature-programmed Desorption of CO Adsorbed on NiO/MgO,” J. Catal. 311 306 - 311
Arena F. , Frusteri F. , Parmaliana A. , Plyasova L. , Shmakov N. 1996 “Effect of Calcination on the Structure of Ni/MgO Catalyst: an X-ray Diffraction Study,” J. Chem. Soc., Faraday Trans. http://dx.doi.org/10.1039/ft9969200469 92 469 - 471
Świerczyński D. , Libs S. , Courson C. , Kiennemann A. 2007 “Steam Reforming of Tar from a Biomass Gasification Process over Ni/olivine Catalyst Using Toluene as a Model Compound,” Appl. Catal. B Environ. http://dx.doi.org/10.1016/j.apcatb.2007.01.017 74 211 - 222
Ruckenstein E. , Hu Y. H. 1995 “Carbon Dioxide Reforming of Methane over Nickel/Alkaline Earth Metal Oxide Catalysts,” Appl. Catal. A Gen. http://dx.doi.org/10.1016/0926-860X(95)00201-4 133 149 - 161
Shimizu Y. , Sukenobu M. , Ito K. , Tanabe E. , Shishido T. , Takehira K. 2001 “Partial Oxidation of Methane to Synthesis Gas over Supported Ni Catalysts Prepared from Ni-Ca/ Al-layered Double Hydroxide,” Appl. Catal. A Gen. http://dx.doi.org/10.1016/S0926-860X(01)00525-7 215 11 - 19