Next Article in Journal
Organocatalytic Enantioselective Epoxidation of Some Aryl-Substituted Vinylidenebisphosphonate Esters: On the Way to Chiral Anti-Osteoporosis Drugs
Next Article in Special Issue
Catalytic Transfer Hydrogenation of Biobased HMF to 2,5-Bis-(Hydroxymethyl)Furan over Ru/Co3O4
Previous Article in Journal / Special Issue
Recent Advancements in Stereoselective Olefin Metathesis Using Ruthenium Catalysts
Article Menu
Issue 3 (March) cover image

Export Article

Catalysts 2017, 7(3), 88; https://doi.org/10.3390/catal7030088

Article
The Role of Ruthenium in CO2 Capture and Catalytic Conversion to Fuel by Dual Function Materials (DFM)
1
Earth and Environmental Engineering, Columbia University in the City of New York, New York 10027, NY, USA
2
Chemical Engineering, Columbia University in the City of New York, New York 10027, NY, USA
*
Author to whom correspondence should be addressed.
Academic Editors: Albert Demonceau, Ileana Dragutan and Valerian Dragutan
Received: 7 February 2017 / Accepted: 14 March 2017 / Published: 17 March 2017

Abstract

:
Development of sustainable energy technologies and reduction of carbon dioxide in the atmosphere are the two effective strategies in dealing with current environmental issues. Herein we report a Dual Function Material (DFM) consisting of supported sodium carbonate in intimate contact with dispersed Ru as a promising catalytic solution for combining both approaches. The Ru-Na2CO3 DFM deposited on Al2O3 captures CO2 from a flue gas and catalytically converts it to synthetic natural gas (i.e., methane) using H2 generated from renewable sources. The Ru in the DFM, in combination with H2, catalytically hydrogenates both adsorbed CO2 and the bulk Na2CO3, forming methane. The depleted sites adsorb CO2 through a carbonate reformation process and in addition adsorb CO2 on its surface. This material functions well in O2- and H2O-containing flue gas where the favorable Ru redox property allows RuOx, formed during flue gas exposure, to be reduced during the hydrogenation cycle. As a combined CO2 capture and utilization scheme, this technology overcomes many of the limitations of the conventional liquid amine-based CO2 sorbent technology.
Keywords:
supported Ru and sodium carbonate; dual function material; CO2 capture in flue gas; conversion to synthetic natural gas; redox properties of Ru

1. Introduction

As predicted by the Intergovernmental Panel on Climate Change (IPCC), carbon dioxide in the atmosphere may reach 570 ppmv by the year of 2100, accompanied by a rise in the mean global temperature of 1.9 °C and mean sea level by 38 cm [1,2,3]. Excess carbon dioxide emission results primarily from anthropogenic activities by fossil fuel combustion for power generation, industrial manufacturing, and transportation [4]. Due to the CO2 accumulation and the rising demand of energy, sustainable energy development [5,6], and CO2 capture, storage and utilization (CCSU) [7] are two efficient methods to combat green-house gas emissions and associated climate change.
For a decade, various processes and materials have been proposed for CO2 capture, with extensive reviews available [8,9]. The aqueous amine–based sorbent technology is still the most popular and developed approach toward capturing CO2 from flue gas. However, the sorbent regeneration for separation of CO2 and H2O from the amine solution by distillation is highly energy intensive. The separated CO2 must then be transported to a recovery site for processing, which adds to the expense. Furthermore, the O2 containing flue gas deteriorates the amine, limiting its ability to be recycled [10].
Recently, several unique material combinations and processes for upgrading CO2 have been reported. A super-dry reforming process involving chemical looping in which CO2 is utilized for enhancing CO production at 750 °C has been reported. Nickel catalyzes the dry reforming of CH4 leading to a redox reaction of iron oxides. CO2 is adsorbed on CaO/Al2O3, providing a series of equilibrium shifts enhancing CO production for ultimate use in synthesis gas reactions [11]. Another interesting approach for enhanced CO production combines a reduced Fe, Cr, Cu reverse water gas shift (RWGS) catalyst with K2CO3 as a CO2 adsorbent. Approximately 6% CO2 in N2 is adsorbed on K2CO3 at 550 °C. At the same temperature, injected H2 reacts with adsorbed CO2 generating CO and H2O via the RWGS reaction. A weakness in the process is the deterioration of performance in O2, limiting this to O2-free CO2 streams [12].
Ruthenium is a well-known noble metal catalyst often used commercially for homogeneous or heterogeneous catalytic reactions requiring a high degree of product selectivity, including aromatic ring saturation to cyclo-hexyl derivatives, saturation of allyl compounds, organic acid hydrogenation to alcohols, hydrogenation of sugars [13], steam reforming of hydrocarbons for H2 production [14], selective homogeneous catalysis for the oxidation of primary alcohols to aldehydes [15], and homogenous chiral synthesis [16]. It is also used for ammonia synthesis [17], preferential oxidation of CO in H2 [18], Fischer-Tropsch processes [19], electrochemical chlorine evolution [20], and methanation of CO and CO2 [21,22,23]. In representative reactions, such as those mentioned above, all require either the metal, its oxide, or an organo-metal complex form of Ru as the catalyst. A unique feature of Ru/RuOx is its redox chemistry of rapid and reversible transformation between its oxide and metallic states.
We are now reporting a work-in-progress combining a series of adsorption and catalytic reactions for converting adsorbed CO2 emitted from power plant oxygen-containing flue gas where the redox property of Ru/RuOx is critical for successful use. The material used for this process is composed of a nano-dispersed [24] alkaline component in intimate contact with Ru on a high surface area carrier (Al2O3), referred to as a Dual Function Material (DFM). It adsorbs CO2 on its alkaline component and catalytically converts it to synthetic natural gas (i.e., CH4) in the subsequent step over the Ru catalyst approaching a closed loop for carbon utilization. The sequential steps will be discussed below.
Alkaline carbonates have been reported to be efficient for CO2 adsorbents [25]. What is the most relevant to our new materials is the ability to adsorb CO2 and then hydrogenate catalytically to methane. The catalytic hydrogenation of a series of carbonates was first reported by Tsuneto et al. in 1992 [26] and Yoshida et al. in 1999 [27]. A variety of transition metal catalysts (but not Ru) were physically mixed with the carbonates and hydrogenation was conducted. The carbonate was completely hydrogenated but there was no attempt to reform the carbonate by CO2 capture.
We have demonstrated that a DFM, containing 5% Ru, 10% Na2CO3/Al2O3, captures CO2 both on the carbonate surface and depleted bulk carbonate sites, which are subsequently hydrogenated to CH4. In a typical implementation of this material, the methane produced is dried and compressed and recycled to the inlet of the power plant or integrated into the natural gas grid, ideally closing the carbon cycle. Thus, this material provides an additional source of sites for CO2 capture by carbonate reformation, yielding an increase in subsequent methane production. For this scheme to be used, H2 used for methanation needs to be generated from water electrolysis, the energy of which is provided by excess solar or wind energy [22]. Installations for water electrolysis powered by solar energy for H2 vehicle fueling stations has been announced between Guangdong Synergy Hydrogen Power Technology (Guangdong, China) and Proton On Site (Wallingford, CT, USA). Installations in Foshan and Yunfu, cities in China, will be initial locations [28]. Thus it is expected that renewable H2 will be available in selected locations.
The two-step sequential reactions involve first the selective adsorption of CO2 from O2-containing flue gas on the DFM. During this step, the Ru in the DFM is oxidized to RuOx. Upon CO2 saturation, the flue gas is diverted to an identical parallel reactor where CO2 capture continues. Renewable H2 is added to the reactor containing the saturated DFM where it rapidly reduces the RuOx to its metallic and active state, and methanation proceeds via CO2 spill over to the Ru sites. All steps are conducted at the same temperature (~320 °C) [29]. Therefore, the reversibility of the redox chemistry of Ru/RuOx makes it unique for this process relative to commonly used Ni, which when oxidized will not reduce under the conditions of this process [30,31,32].
This paper reports cyclic aging studies from a simulated flue gas demonstrating stable repeatable cycles of CO2 adsorption and methanation. The importance of H2 exposure intensity will also be discussed. The price of Ru should not be confused the more expensive among precious metal catalysts. Its price per gram as of 21 December 2016, was Ru $1.25, Rh $24.53, Pt $28.40, and Pd $20.60 with all recycled for continued use.

2. Results and Discussion

2.1. Na2CO3 Loading Study

Various carbonate-containing materials were evaluated on a thermal gravimetric analysis (TGA) for reversible CO2 adsorption. The CO2 adsorbed and methanation in mmol per gram on Al2O3 supported K2CO3, Na2CO3, and MgO powder adsorbents with weight loadings from 5% to 20% were previously reported [30]. In this study K2CO3 and Na2CO3 showed the largest CO2 adsorption relative to Al2O3 supported CaO and MgO. 10% Na2CO3 on Al2O3 achieved the highest adsorption capacity (0.11 mmol/g), and was therefore selected for additional adsorption studies. Figure 1 shows TGA results for CO2 adsorption and reversible desorption for dispersed Na2CO3 on Al2O3 at different loadings.
Figure 1 indicates 10 wt % Na2CO3/Al2O3 obtained the highest capacity of CO2 adsorption (0.11 mmol/g) among other Na2CO3 loadings. The pure N2 purge verifies that Na2CO3 dispersed on γ-Al2O3 powder has the ability of desorb the CO2 reversibly in situ, consistent with dispersed CaO/Al2O3 substantiating that the binding interaction between CO2 and adsorbents is weak [33]. This property insures the desorption and spill-over of CO2 to Ru catalytic sites for methanation.
Based on the methanation capacities, DFM composed of 5% Ru, 10% Na2CO3/Al2O3 generated the most methane (1.05 mol CH4/kg of DFM) compared to other adsorbents at the same loading [30]. Herein, further studies were conducted on Al2O3 supported Ru-Na2CO3 DFM.

2.2. Carbonate Hydrogenation Study

A H2 activation (reduction) step was conducted using unsupported Ru-Na2CO3 to understand the decomposition profile of Ru nitrosyl nitrate used in the powder preparation procedure. However, a greater weight loss than expected was observed indicating hydrogenation of the carbonate in the sample was also occurring. Results all conducted at 320 °C using the TGA is shown in Figure 2 in which the final step shows CO2 adsorption.
During the sample preparation, the 50 mg of 1.44% Ru/Na2CO3 was dried at 120 °C for 2 h and calcined at 250 °C for 2 h in air. When the precursor Ru(NO)(NO3)3 is heated, it has been reported that a mixture of oxides such as RuO, RuO2, RuO3, and RuO4 [33,34] form depending on the temperature.
After a one-hour N2 purge at 320 °C, the sample achieved stable weight as water and/or other volatile components evaporated. At this point, 2% H2/N2 was introduced. During the two-hour H2 exposure, Ru is expected to reduce, nitrate decompose, and some carbonate hydrogenate. The stable weight decreased by 1.763 mg. Assuming the Ru(NO)(NO3)3 remained in its original form as it is in the precursor, the expected weight loss would be 1.332 mg or 65.37% of the total mass loss. Therefore, the residual mass loss (an additional 34.63%) can be explained by carbonate hydrogenation catalyzed by the newly formed Ru metal. It is clear from Figure 2 that a continuing weight loss likely occurs due to the slow hydrogenation of the carbonate. After about 120 min of pretreatment, the sample showed an ability to re-adsorb CO2 in depleted sites, replacing some of that which was hydrogenated.

2.3. Hydrogen Exposure Time Study

50 mg of 5% Ru, 10% Na2CO3/Al2O3 powder sample was subjected to a one-hour pretreatment in 2% H2/N2 at 320 °C. In Figure 3, a rapid adsorption of CO2 up to 1.2 mg from a feed gas of 5% CO2 in N2 was observed. Upon the addition of 2% H2/N2 three distinct slopes of weight loss vs time are observed. The largest slope, between 30 and 35 min, was 0.086 mg/min. The second between roughly 35 and 70 min was 0.00952 mg/min and the third about 0.00195 mg/min from 70 to 660 min. The first loss, associated with the largest slope, occurs from 1.2 mg to 0.66 mg is likely due to methanation of the CO2 adsorbed on the Ru metal. This weight loss is approximately 50% of the Ru atoms present assuming 1 CO2 adsorbed per Ru site. It is also possible 1 CO2 adsorbs on two Ru sites for dissociative adsorption to CO and atomic O [23]. The second slope has a weight loss of about 0.45 mg which is twice that observed for CO2 adsorbed on 10% Na2CO3/Al2O3 (see Figure 1) suggesting that additional sites for CO2 adsorption have become available due to the depleted sites generated by the hydrogenation of the bulk carbonate itself upon the addition of H2. The second slope is speculated to be the hydrogenation weakly adsorbed CO2 on the carbonate surface most of which has spilled over to the Ru sites where the heat liberated (not shown) is indicative of methanation. The weight loss associated with the lowest slope is less than 0.2 mg as is believed to be hydrogenation of the bulk carbonate. This is speculative, since Figure 1 was generated with a feed gas of only 0.5% CO2/N2 while Figure 3 was conducted with 5% CO2/N2. However, CO2 chemisorption reaches a plateau where it becomes independent of partial pressure.
For the 2nd cycle, the mass gain for CO2 adsorption was similar to the first cycle. However, with only 30 min of hydrogen exposure, the weight loss was significantly less than after the longer time exposure in cycle 1. In the third cycle it is likely only the Ru metal and the surface of Na2CO3 adsorb CO2 since insufficient hydrogenation of the bulk carbonate had occurred with only a 30-min exposure. It is clear that bulk carbonate hydrogenation is dependent on exposure time and hydrogen partial pressure. This will be demonstrated in the following studies.

2.4. 10-Cycle Test in CO2 Feed

Although the primary commercial target of our research is focused on power plant exhaust gases containing O2 and steam, there are applications where they are not present, such as fermentation, land fill, and cement manufacturing effluents. Therefore, an aging test consisting of 10 cycles using 7.5% CO2/N2 for the adsorption was conducted in our scaled up reactor with DFM pellets. This is more consistent with industrial processes since fine powdered materials create operational, technical and health problems for workers. For example, in a fixed bed reactor, powders have high pressure drop [35] and expose operators to possible risk of chronic human health issues such as respiratory problems and lung cancer [36]. In order to avoid such issues and facilitate commercialization, a process was conducted with 10 grams of freshly prepared pelleted DFM (5 mm × 5 mm tablets of γ-Al2O3) and cycle tested in a larger scale fixed bed reactor.
The results for CO2 adsorption and methanation under O2-free conditions are shown in Figure 4. Given the slow kinetics for bulk carbonate hydrogenation experienced in Figure 3, the H2 was increased to 5% H2/N2. The Ru-Na2CO3 DFM achieved stable adsorption and methanation performance for 10 successive cycles and is expected to maintain this advantage over many more cycles. However, the average conversion efficiency of the adsorbed CO2 converted to methane was only 60.5%. A large amount of desorbed CO2 was observed at the reactor outlet. The 39.5% unreacted CO2 released is likely due to its premature desorption caused by the large exotherm of methanation. We intend to address this by performing parametric studies for feed gas components (i.e., H2 partial pressure) and process variations. It is also logical to locate a small Ru catalyst downstream to complete methanation of the unconverted CO2. Conditions of our experiments were controlled to avoid CO formation. CO is only produced at higher temperatures and space velocities over Ru as shown in reference [23].

2.5. 12-Cycle Test in Simulated Flue Gas

Performance of the pelleted Ru-Na2CO3 DFM in simulated O2-and-steam-containing flue gas is another key factor for its commercialization and application. A 12-cycle test was conducted in the fixed bed reactor with the performance shown in Figure 5. The results of BET specific surface area of the fresh and after-12-cycle Ru-Na2CO3 pelleted DFM sample is shown in Table 1, in which the small change is consistent with the stable performance of the sample. Table 1 reports the BET and Ru crystallite sizes for fresh and cyclic aged DFM. Some sintering is observed. However, given the low temperatures of the process, this is not expected to be a major source of deactivation. Extended cyclic aging will be necessary to more definitively determine the major sources of deactivation. These studies will be soon initiated. Crystallite sizes (d) were calculated from the equation d = (103/%D) where %D = % dispersion and 103 is the constant related to the number of Ru atoms present in the samples determined from room temperature H2 chemisorption [37].
The CO2 adsorption capacity was reduced compared to the O2 free process of Figure 4, because Ru was inactivated—for CO2 adsorption—to RuOx by O2 in the simulated flue gas. Both adsorption and methanation capacity remained relatively stable until the 9th cycle, when reduction in both CO2 adsorption and CH4 produced was noted. Since both CO2 adsorption and methanation are linked to Ru activity, it was reasonable to assume that the RuOx was not sufficiently reduced, causing a loss in both performances.
Before the 11th cycle, the material was given an extra 150-min of hydrogen exposure, resulting in a recovery of both CO2 adsorption and methanation. When the extra hydrogen exposure was extended to 48 h before the 12th cycle, both capacities reached even higher levels. This apparent deactivation was not observed in the O2 free cycle tests of Figure 4. This again demonstrated the importance of H2 partial pressure in maintaining Ru reduced while enhancing the rate of bulk carbonate hydrogenation.
The average conversion of adsorbed CO2 converted to methane during the 12-cycle test was 89.6%, which is a great improvement compare to the 10-cycle test in the dry CO2 feed (60.5%). Though the overall CO2 adsorption of the 12-cycle test in the simulated flue gas was lower than in the 10-cycle test, the conversion efficiency of 12-cycle test is preferred. Water generated at the surface during methanation may oxidize the surface of Ru. Therefore, a minimum H2 partial pressure is required to maintained Ru in the reduced and active state. We have found that H2 partial pressure is a critical parameter necessary for reducing the RuOx formed during CO2 adsorption (Figure 5) and methanation (Figure 6). This will be confirmed in the future.

2.6. Effect of H2 Partial Pressure on CO2 Hydrogenation and RuOx Reduction

As observed for Figure 5, the loss in performance was reversed by increasing the exposure time of the DFM to H2. It is reasonable to assume that increasing the partial pressure of H2 would have the same effect by maintaining the Ru in the metallic and active state while increasing the extent of carbonate hydrogenation. This was demonstrated in Figure 6 where 10% H2/N2 dramatically increased the amount of methane produced (131 mL) compared to 5% H2/N2 (58.7 mL). This also points to the importance of the redox chemistry of Ru/RuOx. It is logical to assume that during the 12-cycle simulated flue gas test (Figure 5), Ru was oxidized and there was insufficient H2 to re-reduce it.
The rate and extent of RuOx reduction is critical for the flue gas operation. In TGA experiments the rate and extent of RuOx reduction was to understand the minimum H2 content needed for rapid reduction of the oxides of Ru. Results are shown in Figure 7. It is clear the reduction rate (slope) increases with H2 content (0.069 mg/min at 1.30% H2 to 0.370 mg/min at 2.86% H2). The most dramatic increase occurs for H2 > 1.3%. Equally as significant is the decrease in time for initiation of reduction as the H2 content increases. This points to the reducibility of RuOx to Ru, making it more suitable for the DFM flue gas process relative to NiOx to Ni metal [32] that requires temperatures in excess of 500 °C for reduction.

2.7. Speculated Functional Pathway of the Ru-Na2CO3 DFM

The speculated pathways for each step and component in DFM are summarized in Table 2. The RuOx, as prepared, must be activated to its active metallic state for methanation. For feeds containing O2, it will be oxidized and lose its ability to adsorb CO2. However, during methanation it will be reduced and re-activated for methane production. For O2-free feed gas, Ru is both active for CO2 adsorption and methanation.
For Ru-Na2CO3 DFM, bulk carbonate hydrogenation occurs during pretreatment, producing NaOH and CH4 with CO2 spill-over to Ru sites. Upon exposure to CO2, it is reformed to a bulk carbonate species where CO2 adsorbs on the surface Na2CO3⋯CO2 and reforms bulk carbonate. The slowest step is the hydrogenation of the reformed bulk carbonate. Table 2 is still speculative and is the focus of on-going kinetic and characterization studies.

3. Experimental

3.1. Material Synthesis

The powder materials tested were prepared via incipient wetness method on a high surface area γ-Al2O3 (SBA-150, BASF, Iselin, NJ, USA) support using ruthenium(III) nitrosylnitrate (Ru(NO)(NO3)3, Ru 31.3% min, Alfa Aesar, Tewksbury, MA, USA) and Na2CO3 (Sigma-Aldrich, St. Louis, MO, USA) as the precursors for Ru and Na2CO3, respectively. Repeated impregnations were conducted to reach the designed loadings if necessary. The Al2O3 supported alkaline materials (Na2CO3) were dried at 120 °C for 2 h and then calcined at 400 °C for 4 h in air. For Ru deposition, the material was dried at 120 °C for 2 h followed by calcination at 250 °C for 2 h in air after metal catalyst (Ru) impregnations.
The pellet materials (5 mm × 5 mm tablets) used for the cyclic tests were prepared via a precipitation method with the target composition of 5% Ru, 10% Na2CO3/Al2O3 (TH100, SASOL, Hamburg, Germany). The principle procedure of the precipitation method was derived from the work done by Zheng et al. [31], but the impregnation order, selected precursors, and calcination programs were modified (see Figure 8 for the preparation process flow). Ruthenium(III) chloride hydrate (RuCl3·xH2O, 99.9% PGM basis, Ru 38% min, Alfa Aesar, Tewksbury, MA, USA) and sodium carbonate (Sigma-Aldrich) were the chosen precursors for Ru and Na2CO3, respectively. Using RuCl3 as the Ru precursor for pellet sample instead of Ru(NO)(NO3)3 achieved more uniform penetration within the pellets, consequently improved dispersion of Ru, and avoided residual of precursor in the sample. The highest calcination temperature was 250 °C to avoid generation of high temperature volatile ruthenium oxides (>300 °C in air). The water wash was introduced, after precipitation of water-insoluble Ru(OH)x to remove unwanted chlorides. The carbonate salts were then impregnated and processed as shown in Figure 8.

3.2. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis were conducted using either a TG 209 F1 Libra NETZSCH or SAT 449 F3 Jupiter NETZSCH unit for the Na2CO3 loading, carbonate hydrogenation, and hydrogen exposure time study. For each test, 50 mg of the prepared powder sample was placed in an alumina crucible and located in the instrument. The temperatures for carbonate hydrogenation and nitrate decomposition, CO2 adsorption/desorption, and CO2 methanation, were all conducted at 320 °C, while the feed gases and corresponding time periods were varied. Table 3 shows the programs in detail for each test. The total flow rates were always 40 mL/min for each feed gas. Before each test, the sample was heated to 320 °C in pure nitrogen until a stable final weight was observed. Hydrogen was introduced in pre-treatments to decompose the ruthenium nitrate to active metallic ruthenium while decomposing the remaining nitrate compounds until stable mass was observed. Pure nitrogen was used only to de gas each sample.

3.3. Scaled-Up Fixed Bed Reactor

Cyclic tests of CO2 adsorption and catalytic conversion to methane in a fixed bed reactor are shown in Figure 9. The reactor consisted of gas feed system, furnace, fixed bed reactor, and product analyzer. The gas feed system includes cylinders of compressed designed gases (TechAir, White Plains, NY, USA), rotameters (Key Instruments, Hatfield, PA, USA) calibrated with a precise bubble meter for flow rate control, and a syringe pump (Cole-Parmer, Vernon Hills, IL, USA) along with heating tape (maintains temperature at 125 °C) for steam introduction. The fixed bed reactor is housed in a microthermal furnace (MTSC12.5R-0.75x18-1Z-IOT, Mellen, Concord, NH, USA) and is composed of a standard quartz tube (O.D. = 25 mm, I.D. = 20 mm, length = 570 mm), a thermocouple (K-type, Omega, Norwalk, CT, USA) was placed at gas inlet. An uncoated ceramic monolith was used to secure the pellets and glass beads (DI of 6 mm, Fisher Scientific, Hampton, NH, USA), the latter used to fill the downstream dead volume. Water (steam) in the gas outlet, from the feed gas and water produced from methanation, was condensed by an ice cold trap before flowing into the analytical unit. The exhaust gas analyzer was an Enerac Model 700 equipped with electrochemical sensors for CO, CO2, and hydrocarbons, and nondispersive-infrared sensor for O2. It operates with a frequency of 1 Hz, allowing gas measurements every one second. No pressure drop was observed during the reactor tests from the inlet to the outlet.
The two cyclic tests each contained 10 grams of 5% Ru, 10% Na2CO3/Al2O3 pellets at 320 °C and 300 mL/min (GHSV = 1432 h−1) for total flow rates. The process flow for each cycle in these two tests is shown in Figure 10. The pre-reduction step for both was designed to reduce the RuOx to Ru metal during preparation. For the 10-cycle test, only 7.5% CO2 in N2 was used for the CO2 adsorption step. No N2 purge between adsorption and methanation was needed since no O2 was present in this series of tests.
The 12-cycle test was performed in simulated natural gas fired power plant flue gas, containing 7.5% CO2, 4.5% O2, 15% H2O and balance N2. A 3-min N2 purge between CO2 adsorption and methanation and the reverse was added to avoid contact of H2 and O2. The first ten cycles were operated continuously and identically, while the last two cycles required a prolonged H2 purge before CO2 feed was introduced. The 11th cycle underwent a 150 min 5% H2/N2 exposure and the 12th for 48 h. The prolonged H2 exposure was needed to regenerate the re-oxidized ruthenium.

4. Conclusions

The bulk carbonate present in Ru-Na2CO3 DFM can adsorb and convert CO2 to CH4. Additionally, the bulk carbonate can also be catalytically hydrogenated to CH4 providing “depleted” sites for additional CO2 adsorption from flue gases. Ru plays a key role as a methanation catalyst for O2-containing flue gas due to its ease of reduction once oxidized. Once in its reduced state it provides an additional site for CO2 capture as well as effective methanation. This technology overcomes some of the limitations of aqueous amine CO2 scrubbing by having the dual capability of both capturing CO2 in O2-flue gas and subsequently methanating the CO2 at one temperature using renewable H2. The methane produced is dried, compressed, and recycled to the front of the plant ideally closing the carbon balance, with conversion of 89.6% of the captured CO2 in simulated flue gas.
The Ru-Na2CO3 DFM demonstrated stable performance for at least 10 cycles of CO2 adsorption and methanation in an O2-free CO2 feed gas. In simulated flue gas, due to the presence of oxygen, part of active metallic Ru is oxidized and loses its activity for CO2 adsorption and methanation. Stable performance was observed in the flue gas up to the point where Ru becomes less effective due to oxidation. This apparent deactivation can be reversed by prolonged exposure to H2 or preferably by a high H2 content. The reversibility is due to the ease of reduction of RuOx to Ru.

Acknowledgments

Financial support from AngloAmerican is greatly appreciated. The authors would like to thank SASOL and BASF for their material support.

Author Contributions

The present work was conducted under the supervision of Robert J. Farrauto, with Shuoxun Wang, Erik T. Schrunk, and Harshit Mahajan as authors at Columbia University. Robert J. Farrauto did the main research consulting and paper editing, and is named the correspondent author of the submitted work. Shuoxun Wang (Ph.D. candidate) designed and conduct most of the experiments and paper writing, and is named the primary author. Erik T. Schrunk helped with the experimental designs and operations on the scaled-up fixed bed reactor. Harshit Mahajan completed the experiment of RuOx reduction.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Stewart, C.; Hessami, M.-A. A study of methods of carbon dioxide capture and sequestration––The sustainability of a photosynthetic bioreactor approach. Energy Conver. Manag. 2005, 46, 403–420. [Google Scholar] [CrossRef]
  2. Hasib-ur-Rahman, M.; Siaj, M.; Larachi, F. Chemical Engineering and Processing: Process Intensification. Chem. Eng. Process. Process Intensif. 2010, 49, 313–322. [Google Scholar] [CrossRef]
  3. Audus, H.; Freund, P.; Smith, A. Global Warming Damage and the Benefits of Mitigation; IEA Greenhouse Gas R&D Programme: Paris, France, December 1995. [Google Scholar]
  4. Heintz, Y.J.; Sehabiague, L.; Morsi, B.I.; Jones, K.L.; Pennline, H.W. Novel Physical Solvents for Selective CO2 Capture from Fuel Gas Streams at Elevated Pressures and Temperatures. Energy Fuels 2008, 22, 3824–3837. [Google Scholar] [CrossRef]
  5. Carmo, M.; Fritz, D.L.; Mergel, J.; Stolten, D. A comprehensive review on PEM water electrolysis. Int. J. Hydrog. Energy 2013, 38, 4901–4934. [Google Scholar] [CrossRef]
  6. Ursua, A.; Gandia, L.M.; Sanchis, P. Hydrogen Production from Water Electrolysis: Current Status and Future Trends. IEEE Proc. 2012, 100, 410–426. [Google Scholar] [CrossRef]
  7. Lackner, K.S. A guide to CO2 Sequestration. Science 2003, 300, 1677–1678. [Google Scholar] [CrossRef] [PubMed]
  8. Saeidi, S.; Amin, N.A.S.; Rahimpour, M.R. Hydrogenation of CO2 to value-added products—A review and potential future developments. J. CO2 Util. 2014, 5, 66–81. [Google Scholar] [CrossRef]
  9. Spigarelli, B.P.; Kawatra, S.K. Opportunities and challenges in carbon dioxide capture. J. CO2 Util. 2013, 1, 69–87. [Google Scholar] [CrossRef]
  10. Rochelle, G.T. Amine Scrubbing for CO2 Capture. Science 2009, 325, 1652–1654. [Google Scholar] [CrossRef] [PubMed]
  11. Buelens, L.C.; Buelens, L.C.; Galvita, V.V.; Poelman, H.; Detavernier, C.; Marin, G.B. Super-dry reforming of methane intensifies CO2 utilization via Le Chatelier’s principle. Science 2016, 354, 449–452. [Google Scholar] [CrossRef] [PubMed]
  12. Bobadilla, L.F.; Riesco-García, J.M.; Penelás-Pérez, G.; Urakawa, A. Enabling continuous capture and catalytic conversion of flue gas CO2 to syngas in one process. J. CO2 Util. 2016, 14, 106–111. [Google Scholar] [CrossRef]
  13. Bartholomew, C.; Farrauto, R.J. Fundamentals of Industrial Catalytic Processes; Wiley and Sons: New York, NY, USA, 2006; Chapter 7; pp. 490–491. [Google Scholar]
  14. Bartholomew, C.; Farrauto, R.J. Fundamentals of Industrial Catalytic Processes; Wiley and Sons: New York, NY, USA, 2006; Chapter 6; p. 355. [Google Scholar]
  15. Bilgrien, C.; Davis, S.; Drago, R. The selective oxidation of primary alcohols to aldehydes by oxygen employing a trinuclear ruthenium carboxylate catalyst. J. Am. Chem. Soc. 1987, 1987, 3786–3787. [Google Scholar] [CrossRef]
  16. Huang, Y.Y.; Yang, X.; Feng, Y.; Verpoort, F.; Fan, Q.H. Chiral Ru/Ir bimetallic dendronized polymer catalysts constructed through sequential metal coordination and applied in asymmetric hydrogenation of quinaldine. J. Mol. Catal. A Chem. 2014, 393, 150–155. [Google Scholar] [CrossRef]
  17. Saadatjou, N.; Jafari, A.; Sahebdelfar, S. Ruthenium Nanocatalysts for Ammonia Synthesis: A Review. Chem. Eng. Commun. 2014, 202, 420–448. [Google Scholar] [CrossRef]
  18. Shen, L.; Zhang, C.; Liu, Y. Meso-macroporous Al2O3 supported Ru catalysts for CO preferential oxidation in hydrogen-rich gases. J. Nat. Gas Chem. 2012, 21, 653–660. [Google Scholar] [CrossRef]
  19. Steynberg, A.P.; Dry, M.E. Fischer-Tropsch Technology, Studies in Surface Science and Catalysis; Elsevier: Amsterdam, The Netherlands, 2004; Chapter 7; pp. 560 and 566. [Google Scholar]
  20. Menzel, N.; Ortel, E.; Mette, K.; Kraehnert, R.; Strasser, P. Dimensionally stable Ru/Ir/TiO2-anodes with tailored mesoporosity for efficient electrochemical chlorine evolution. ACS Catal. 2013, 3, 1324–1333. [Google Scholar] [CrossRef]
  21. Garbarino, G.; Bellotti, D.; Riani, P.; Magistri, L.; Busca, G. Methanation of carbon dioxide on Ru/Al2O3 and Ni/Al2O3 catalysts at atmospheric pressure: Catalysts activation, behaviour and stability. Int. J. Hydrog. Energy 2015, 40, 9171–9182. [Google Scholar] [CrossRef]
  22. Götz, M.; Lefebvre, J.; Mörs, F.; McDaniel Koch, A.; Graf, F.; Bajohr, S.; Reimert, R.; Kolb, T. Renewable Power-to-Gas: A technological and economic review. Renew. Energy 2016, 85, 1371–1390. [Google Scholar] [CrossRef]
  23. Janke, C.; Duyar, M.S.; Hoskins, M.; Farrauto, R. Catalytic and adsorption studies for the hydrogenation of CO2 to methane. Appl. Catal. B Environ. 2014, 152–153, 184–191. [Google Scholar] [CrossRef]
  24. Gruene, P.; Belova, A.G.; Yegulalp, T.M.; Farrauto, R.J.; Castaldi, M.J. Dispersed Calcium Oxide as a Reversible and Efficient CO2—Sorbent at Intermediate Temperatures. Ind. Eng. Chem. Res. 2011, 50, 4042–4049. [Google Scholar] [CrossRef]
  25. Wang, S.; Yan, S.; Ma, X.; Gong, J. Recent advances in capture of carbon dioxide using alkali-metal-based oxides. Energy Environ. Sci. 2011, 4, 3805–3819. [Google Scholar] [CrossRef]
  26. Tsuneto, A.; Kudo, A.; Saito, N.; Sakata, T. Hydrogenation of solid state carbonates. Chem. Lett. 1992, 21, 831–834. [Google Scholar] [CrossRef]
  27. Yoshida, N.; Hattori, T.; Komai, E.; Wada, T. Methane formation by metal-catalyzed hydrogenation of solid calcium carbonate. Catal. Lett. 1999, 58, 119–122. [Google Scholar] [CrossRef]
  28. Proton OnSite Awarded 13 Megawatt Electrolyzers. Available online: https://www.linkedin.com/pulse/proton-onsite-awarded-13-megawatt-electrolyzers-kathleen-mullins (accessed on 20 December 2016).
  29. Duyar, M.S.; Treviño, M.A.A.; Farrauto, R.J. Dual function materials for CO2 capture and conversion using renewable H2. Appl. Catal. B Environ. 2015, 168, 370–376. [Google Scholar] [CrossRef]
  30. Duyar, M.S.; Wang, S.; Arellano-Treviño, M.A.; Farrauto, R.J. CO2 utilization with a novel dual function material (DFM) for capture and catalytic conversion to synthetic natural gas: An update. J. CO2 Util. 2016, 15, 65–71. [Google Scholar] [CrossRef]
  31. Zheng, Q.; Farrauto, R.; Chau Nguyen, A. Adsorption and Methanation of Flue Gas CO2 with Dual Functional Catalytic Materials: A Parametric Study. Ind. Eng. Chem. Res. 2016, 55, 6768–6776. [Google Scholar] [CrossRef]
  32. Bartholomew, C.H.; Farrauto, R.J. Chemistry of nickel-alumina catalysts. J. Catal. 1976, 45, 41–53. [Google Scholar] [CrossRef]
  33. Kato, T.; Usami, T.; Tsukada, T.; Shibata, Y.; Kodama, T. Study on volatilization mechanism of ruthenium tetroxide from nitrosyl ruthenium nitrate by using mass spectrometer. J. Nucl. Mater. 2016, 479, 123–129. [Google Scholar] [CrossRef]
  34. Sawada, K.; Ueda, Y.; Enokida, Y. Ruthenium Release from Thermally Overheated Nitric Acid Solution Containing Ruthenium Nitrosyl Nitrate and Sodium Nitrate to Solidify. Procedia Chem. 2016, 21, 82–86. [Google Scholar] [CrossRef]
  35. Islam, A.; Ravindra, P. Biodiesel Production with Green Technologies; Springer International Publishing Switzerland: Cham, Switzerland, 2017; Chapter 1. [Google Scholar]
  36. Batholomew, C.; Farrauto, R.J. Fundamentals of Industrial Catalytic Processes; Wiley and Sons: New York, NY, USA, 2006; Chapter 10. [Google Scholar]
  37. Batholomew, C.; Farrauto, R.J. Fundamentals of Industrial Catalytic Processes; Wiley and Sons: New York, NY, USA, 2006; Chapter 2; p. 81. [Google Scholar]
Figure 1. Thermal Gravimetric Analysis (TGA) adsorption studies conducted at 320 °C for 50 mg of powder. Before the adsorption the samples were degassed in 2% H2/N2 for 2 h, then exposed to 0.5% CO2 in N2 and subsequently pure N2.
Figure 1. Thermal Gravimetric Analysis (TGA) adsorption studies conducted at 320 °C for 50 mg of powder. Before the adsorption the samples were degassed in 2% H2/N2 for 2 h, then exposed to 0.5% CO2 in N2 and subsequently pure N2.
Catalysts 07 00088 g001
Figure 2. TGA result of Ru salts and carbonate hydrogenation study operated at 320 °C for 50 mg of 1.44% Ru/Na2CO3 powder sample.
Figure 2. TGA result of Ru salts and carbonate hydrogenation study operated at 320 °C for 50 mg of 1.44% Ru/Na2CO3 powder sample.
Catalysts 07 00088 g002
Figure 3. TGA result of the 3-cycle test for 50 mg of 5% Ru, 10% Na2CO3/Al2O3 at 320 °C. The plot excluded the over-night activation step at 320 °C in 2% H2/N2. The long tail of low weight loss in the first cycle was compressed indicating an overnight (10.5 h) hydrogen exposure at 320 °C.
Figure 3. TGA result of the 3-cycle test for 50 mg of 5% Ru, 10% Na2CO3/Al2O3 at 320 °C. The plot excluded the over-night activation step at 320 °C in 2% H2/N2. The long tail of low weight loss in the first cycle was compressed indicating an overnight (10.5 h) hydrogen exposure at 320 °C.
Catalysts 07 00088 g003
Figure 4. CO2 adsorption and methanation cycles (10) using 10 grams of the Ru-Na2CO3 DFM (5% Ru, 10% Na2CO3/Al2O3) pellets in a fixed packed bed reactor with 7.5% CO2/N2 at 320 °C. A 150-min of 5% H2/N2 exposure was used during pretreatment followed by 10 identical CO2 adsorption-methanation cycles. 5% H2/N2 for 30 min was used for the methanation cycles. The volumes of CO2 adsorbed and CH4 produced are shown in the figure.
Figure 4. CO2 adsorption and methanation cycles (10) using 10 grams of the Ru-Na2CO3 DFM (5% Ru, 10% Na2CO3/Al2O3) pellets in a fixed packed bed reactor with 7.5% CO2/N2 at 320 °C. A 150-min of 5% H2/N2 exposure was used during pretreatment followed by 10 identical CO2 adsorption-methanation cycles. 5% H2/N2 for 30 min was used for the methanation cycles. The volumes of CO2 adsorbed and CH4 produced are shown in the figure.
Catalysts 07 00088 g004
Figure 5. CO2 adsorption and methanation using 10 grams of the Ru-Na2CO3 DFM (5% Ru, 10% Na2CO3/Al2O3) pellet for the 12-cycle test conducted in the fixed bed reactor in simulated flue gas at 320 °C. Pretreatment of the DFM was conducted with 5% H2/N2 for 150 min before the first cycle. For cycles 2–10, hydrogen exposure was 30 min. Before cycle 11 another 150-min 5% H2/N2 exposure was added, and for cycle 12, 48 h H2 exposure. The volumes shown in the figure are for CO2 adsorbed and CH4 produced.
Figure 5. CO2 adsorption and methanation using 10 grams of the Ru-Na2CO3 DFM (5% Ru, 10% Na2CO3/Al2O3) pellet for the 12-cycle test conducted in the fixed bed reactor in simulated flue gas at 320 °C. Pretreatment of the DFM was conducted with 5% H2/N2 for 150 min before the first cycle. For cycles 2–10, hydrogen exposure was 30 min. Before cycle 11 another 150-min 5% H2/N2 exposure was added, and for cycle 12, 48 h H2 exposure. The volumes shown in the figure are for CO2 adsorbed and CH4 produced.
Catalysts 07 00088 g005
Figure 6. Methane production comparison of 5% H2/N2 and 10% H2/N2. The test was operated using 10 grams of 5% Ru, 10% Na2CO3/Al2O3 pellets in the large fixed bed reactor. The operational conditions were at 320 °C, 300 mL/min of 7.5% CO2/N2 for CO2 adsorption. The volume of methane produced are shown.
Figure 6. Methane production comparison of 5% H2/N2 and 10% H2/N2. The test was operated using 10 grams of 5% Ru, 10% Na2CO3/Al2O3 pellets in the large fixed bed reactor. The operational conditions were at 320 °C, 300 mL/min of 7.5% CO2/N2 for CO2 adsorption. The volume of methane produced are shown.
Catalysts 07 00088 g006
Figure 7. TGA results of RuOx reduction on 50 mg 5% Ru/Al2O3 powder samples H2 partial pressures from 1.3% to 2.86% at 300 °C. Only reduction sections are shown in the figure.
Figure 7. TGA results of RuOx reduction on 50 mg 5% Ru/Al2O3 powder samples H2 partial pressures from 1.3% to 2.86% at 300 °C. Only reduction sections are shown in the figure.
Catalysts 07 00088 g007
Figure 8. Ru-Na2CO3 DFM pellet preparation process flow using the precipitation method. A slight excess of NaOH was used with a ratio of Ru:OH = 1:4.5 to precipitate Ru(OH)x. A water wash and filtration (40 times of water immersion and separation) was used to remove any NaCl. Drying and calcination steps were conducted in air.
Figure 8. Ru-Na2CO3 DFM pellet preparation process flow using the precipitation method. A slight excess of NaOH was used with a ratio of Ru:OH = 1:4.5 to precipitate Ru(OH)x. A water wash and filtration (40 times of water immersion and separation) was used to remove any NaCl. Drying and calcination steps were conducted in air.
Catalysts 07 00088 g008
Figure 9. Diagram of scaled-up fixed bed reactor. The N2 cylinder was used either exclusively or simultaneously with the 50% CO2/N2 cylinder to dilute the CO2 concentration to 7.5% for feeding. The computer connected to the Enerac unit equipped with infrared/electrochemical sensors recorded data every second. Total flow rate of each gas feeding was 300 mL/min.
Figure 9. Diagram of scaled-up fixed bed reactor. The N2 cylinder was used either exclusively or simultaneously with the 50% CO2/N2 cylinder to dilute the CO2 concentration to 7.5% for feeding. The computer connected to the Enerac unit equipped with infrared/electrochemical sensors recorded data every second. Total flow rate of each gas feeding was 300 mL/min.
Catalysts 07 00088 g009
Figure 10. Process flow for the two different cyclic tests operated on the scaled-up fixed bed reactor. (a) O2- and steam-free CO2 adsorption and hydrogenation for the 10-cycle test. Pre-reduction was utilized only once before the first cycle. The subsequent ten cycles were conducted repeatedly under identical experimental conditions; (b) Simulated flue gas 12-cycle test with the pretreatment used only before the first cycle followed by 10 identical cycles. A N2 purge was needed between each cycle to avoid H2-O2 contact. Before the 11th and the 12th cycle, a prolonged H2 exposure was utilized while subsequent steps remained the same as the first ten cycles.
Figure 10. Process flow for the two different cyclic tests operated on the scaled-up fixed bed reactor. (a) O2- and steam-free CO2 adsorption and hydrogenation for the 10-cycle test. Pre-reduction was utilized only once before the first cycle. The subsequent ten cycles were conducted repeatedly under identical experimental conditions; (b) Simulated flue gas 12-cycle test with the pretreatment used only before the first cycle followed by 10 identical cycles. A N2 purge was needed between each cycle to avoid H2-O2 contact. Before the 11th and the 12th cycle, a prolonged H2 exposure was utilized while subsequent steps remained the same as the first ten cycles.
Catalysts 07 00088 g010
Table 1. Brunauer-Emmett-Teller (BET) specific surface area of the fresh and flue gas cyclic aged Ru-Na2CO3 DFM pellets. Ru crystallite sized for fresh and cyclic aged samples based on H2 chemisorption assuming H/Ru = 1/1.
Table 1. Brunauer-Emmett-Teller (BET) specific surface area of the fresh and flue gas cyclic aged Ru-Na2CO3 DFM pellets. Ru crystallite sized for fresh and cyclic aged samples based on H2 chemisorption assuming H/Ru = 1/1.
SampleBET Specific Surface Area (m2/g)Ru Crystallite Sizes (nm)
Fresh Ru-Na2CO3 DFM pellet1033.98
Flue gas cyclic aged Ru-Na2CO3 DFM pellet895.15
Table 2. Speculated function pathway of Ru and Na2CO3 in the Ru-Na2CO3 DFM.
Table 2. Speculated function pathway of Ru and Na2CO3 in the Ru-Na2CO3 DFM.
SectionRu PathwayNa2CO3 Pathway
Pretreatment RuO x + x H 2 Ru + x H 2 O Na 2 CO 3 + 4   H 2   Ru   2   NaOH + CH 4 + H 2 O
O2-free
CO2 feed 1
Ru + CO 2 Ru CO 2 4   NaOH + 4   CO 2 Na 2 CO 3   bulk + Na 2 CO 3   surface CO 2 + 2   H 2 O
O2-containing CO2 feed Ru + x 2 O 2 RuO x 4   NaOH + 4   CO 2 Na 2 CO 3   bulk + Na 2 CO 3   surface CO 2 + 2   H 2 O
Methanation & hydrogenation RuO x + x H 2 Ru + x H 2 O Na 2 CO 3   surface CO 2 + 8   H 2   Ru   2   CH 4 + H 2 O + Na 2 CO 3   surface
Na 2 CO 3   bulk + 4   H 2   Ru   2   NaOH + CH 4 + H 2 O
1 Na2CO3 bulk and Na2CO3 surface are the same component but bulk and surface are distinguished.
Table 3. Programs of TGA tests at 320 °C related to the results discussed in this paper.
Table 3. Programs of TGA tests at 320 °C related to the results discussed in this paper.
TestPre-TreatmentStep 1Step 2Step 3
Na2CO3 Loading Study2% H2/N2, 2 h0.5% CO2/N2, 1 hPure N2, 1 h-
Carbonate Hydrogenation StudyPure N2, 1 h2% H2/N2, 2 h5% CO2, 2 h-
Hydrogen Exposure Time Study 12% H2/N2, 1 h5% CO2/N2, 30 m2% H2/N2, 10.5 hPure N2, 30 m
1 The step 2 shows in the table was for the first cycle of the test only. For the 2nd and the 3rd cycle, step 2 lasted only for 30 min, while the other steps were the same as it was in cycle 1.
Catalysts EISSN 2073-4344 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
Back to Top