SiO2 Catalyst

Mar 26, 2010 - A CO2temperature-programmed desorption study gives credence to our findings on the role of MgO. Our results confirm the bifunctional ...
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7128

J. Phys. Chem. C 2010, 114, 7128–7131

Bifunctional Mechanism of CO2 Methanation on Pd-MgO/SiO2 Catalyst: Independent Roles of MgO and Pd on CO2 Methanation Hyun You Kim,† Hyuck Mo Lee,† and Jung-Nam Park*,‡ Department of Materials Science and Engineering, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701, Korea and Department of Chemistry, Sungkyunkwan UniVersity, Suwon 440-746, Korea ReceiVed: February 1, 2010

We scrutinized the reaction mechanism of CO2 methanation catalyzed by a Pd-MgO/SiO2 catalyst. Density functional theory studies showed that MgO and Pd nanoparticles play completely different roles. We found that MgO initiates the reaction by binding a CO2 molecule, forming a magnesium carbonate species on the surface, and that a supply of atomic H is essential for further hydrogenation of magnesium carbonate to methane. A CO2temperature-programmed desorption study gives credence to our findings on the role of MgO. Our results confirm the bifunctional mechanism of CO2 methanation by a Pd-MgO/SiO2 catalyst. 1. Introduction CO2 emission caused by fossil-fuel burning is a primary reason for rapid global warming.1–5 It is obvious that increasing atmospheric CO2 concentration is creating a critical risk for earth’s climate system.3–5 However, unfortunately, it is an undeniable fact that our civilization depends mostly on the energy gained by burning fossil-fuels. Therefore, the potential utilization of CO2 as an abundant and inexpensive chemical feedstock is of widespread interest.6 The utilization technologies for CO2, such as CO2 capture and storage,4,5,7–13 separation,14,15 and sequestration5,16 and CO2 chemical conversion,17,18 are becoming a key issue in our efforts to reduce the risk of future devastating effects. In this context, utilizing CO2 as a feedstock in chemical processes for synthesis of fuels or valuable products is one of the most practical methods.19–21 As part of a more general investigation of CO2 activation, CO2 methanation (CO2 + 4H2 f CH4 + 2H2O) has been studied over Ni,22,23 Ru,24,25 and Rh26 catalysts. The methanation reaction is exothermic; however, an 8-electron process is required to reduce the fully oxidized carbon to methane, and there are significant kinetic limitations that require a catalyst to achieve acceptable rates and selectivities. An Ni(II) ferrite catalyst with spinel structure27 has been proposed for CO2 methanation, as is active for CO2 methanation, showing high selectivity to methane; however stability issues remain, and the reaction mechanism has not been closely investigated yet. Previously, we reported that the Pd-MgO/SiO2 catalyst, synthesized from a reverse microemulsion, is active and selective for CO2 methanation.28 It was found that the Pd-MgO/SiO2 catalyst had a greater than 95% selectivity to CH4 at a carbon dioxide conversion of 59%, wherease Pd/SiO2 has activity only for CO2 reduction to CO, and Mg/SiO2 alone is relatively inactive.28 In this study, in aiming to provide clearer insights into the role of Pd and MgO we systematically use computational and experimental methods to investigate the reaction mechanism of CO2 methanation catalyzed by the previously designed28 PdMgO/SiO2 catalyst. As demonstrated by density functional * To whom correspondence should be addressed. E-mail: pjungnam@ gmail.com. Tel: +82-31-299-4882. Fax: +82-31-290-7075. † KAIST. ‡ Sungkyunkwan University.

theory (DFT) calculations, MgO initiates the reaction by binding a CO2 molecule, forming a magnesium carbonate species on the surface, and Pd allows the reaction to proceed by supplying atomic hydrogen, which is essential for further dehydrogenation of magnesium carbonates to methane. The finding of such a bifunctional reaction mechanism agrees with the previously proposed reaction scheme in our previous study.28 Upon desorption of the methane the carbonate is reformed by gas phase CO2. 2. Experimental Procedure 2.1. Catalyst Preparation. The Pd-MgO/SiO2 catalyst was synthesized following a reverse microemulsion (ME) method28 by adding a mixture of Pd(NO3)2 · H2O (Aldrich, 7 mL, 0.028M) and Mg(NO3)2 · H2O (Aldrich, 3 mL, 0.169 M) to a vigorously stirred solution containing a nonionic surfactant (40 mL, Igepal CO-520, Aldrich) and cyclohexane (100 mL, Fisher Scientific). During the stirring, Hydrazine (64 µL, Aldrich 98%) was added and the mixture stirred for another hour. A NH4OH solution (EMD 28%) was then added to adjust the pH to 11. After another hour of stirring a mixture of TEOS (tetraethyl orthosilicate, 0.995 mL, Acros organic) and C18TMS (n-octadecyl trimethoxysilane, 0.357 mL, Gelest Inc.) was added dropwise. Hydrolysis and condensation of the silica precursors were allowed to proceed in the stirred mixture for 3 days at 20 °C. After condensation, the Pd-MgO/SiO2 was precipitated and washed with ethanol. The ethanol wash followed by centrifugation was repeated three times. The Pd-MgO/SiO2 was dried at 100 °C and calcined at 550 °C in air for 6 h; the heating rate was 1 °C min-1. For the synthesis of Pd/SiO2,29 a microemulsion process was used identical to that described above for the PdMgO/SiO2, but the Pd/SiO2 catalyst was synthesized using the nitrate precursor; Pd(NO3)2 · H2O (Aldrich, 10 mL, 0.02 M). 2.2. Characterization. X-ray photoelectron spectroscopy (XPS) was performed with a Kratos “Axis Ultra” spectrometer using a monochromated Al-KR source (1486.6 eV) for excitation and an eight-channel detector. The base pressure was 5 × 10-10 Torr and the spectra were calibrated to the C 1s electron emission at 285.0 eV and processed with Casa XPS. CO2 temperature-programmed desorption (TPD) experiments were performed in a fixed-bed reactor system connected with TCD (thermal conductivity detector). The sample of 0.06 g was

10.1021/jp100938v  2010 American Chemical Society Published on Web 03/26/2010

CO2 Methanation on Pd-MgO/SiO2 Catalyst

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Figure 1. XPS spectra of Mg 2p over (a) as-prepared Pd-MgO/SiO2 and (b) Pd-MgO/SiO2 after reaction. (Reaction conditions: catalyst weight ) 0.1 g, space time ) 1.1 s, mole ratio of reactants H2/CO2 ) 4, Flow rate ) 10.2 cm3 min-1, reaction temperature ) 450 °C, reaction time ) 9 h.)

first pretreated under 10 wt % H2 in He flow of 30 mL min-1 at 450 °C for 1 h, followed by cooling down to room temperature with a He flow of 30 mL min-1. In addition, 10 wt % CO2 in He flow of 30 mL min-1 was switched at room temperature and maintained for 1 h for CO2 adsorption to the sample. Subsequently, the He flow of 30 mL min-1 was switched at room temperature. Finally, the sample was ramped up to a rate of 10 °C min-1. The CO2-TPD patterns during the temperature-programmed operation were measured by TCD with plus polarity. 3. Computational Method Given the complexity of the Pd-MgO/SiO2 catalyst, we excluded the role of interfaces between consisting elements. Because we found that Mg plays a critical role in CO2 methanation and suggested that a carbonate that formed on the MgO surface is an important reaction intermediate, we initially studied the molecular adsorption of CO2 to MgO surfaces and whether it forms a carbonate. Figure 1 shows the XPS data from the Pd-MgO/SiO2 catalysts. Compared with the binding energy of metallic Mg-2p and Mg2+-2p core level spectra,30–32 Figure 1 confirms the presence of MgO phase. Because of the well-known fact that Pd nanoparticles and surfaces can dissociate molecular hydrogen,33–40 we did not directly implement Pd nanoparticles into our system. We revalidated the role of Pd by comparing the energetics of CO2 methanation with different hydrogen sources, H2 and H atoms. We performed spin-polarized Kohn-Sham DFT calculations with plane-wave VASP code41–44 and the RPBE45 functional. The plane wave energy cutoff was 500 eV. The ionic cores were described by the PAW method implemented in VASP.41–44 To describe the low-index surfaces of MgO, we prepared and cut [2 × 2] rock salt structure MgO supercell. Slabs of four types of MgO surfaces were prepared, MgO(100), MgO(110), Mgterminated MgO(111) (MgO(111)/Mg), and oxygen-terminated MgO(111) (MgO(111)/O) with a 20 Å of vacuum thickness (see Figure 2). To remove the large dipole, half of the surface atoms of the top layer of (111) surfaces were moved to the opposite side of the slab.46,47 The atomic positions in the top half layers were allowed to relax during geometry optimization. Applied to all calculations was 2 × 2 × 1 k-points mesh. The convergence criteria for the electronic wave function and for the geometry were 10-4 and 10-3 eV, respectively. We used the Gaussian smearing method with an initial window size of 0.05 eV, which was gradually decreased to 0 during geometry optimization, to prevent partial occupancy.

Figure 2. Morphology of four low-indexed MgO surfaces studied. These surfaces were sliced from the MgO[2 × 2] super cell. Green and red spheres denote Mg and O atoms.

TABLE 1: Energy of CO2 Adsorption of Four Tested MgO Surfaces CO2 (eV) Ead

MgO(001)

MgO(110)

MgO(111)/Mg

MgO(111)/O

0.07

-2.16

-2.95

0.28

4. Results and Discussion Table 1 shows CO2 adsorption energy on four tested MgO surfaces. Among these, MgO(100), the most stable surface of MgO, and MgO(111)/O does not bind a CO2 molecule, whereas MgO(110) and MgO(111)/Mg strongly bind a CO2 molecule. Figure 3 shows the structures and binding energies of the intermediates in CO2 methanation on an MgO (110) surface. Although the energy of CO2 adsorption (Step 1 -2.16 eV), predicts a strong chemical interaction between a CO2 and MgO(110), a Bader charge analysis shows that the CO2 did not acquire or lose electrons (see Figure 4). Rather, the C atom of the CO2 molecule directly binds to a protruded oxygen anion of MgO(110) and forms a monodentate carbonate. This carbonate perfectly replaces one oxygen anion of MgO(110). It is likely that most of the energy gained in step 1 of Figure 3 (CO2 adsorption on MgO(110)) originates from the formation energy of a carbonate. This Bader charge analysis and electron redistribution diagram after carbonate formation (Figure 4) show that the carbonate draws electrons from four nearby Mg atoms leading to electron polarization on these Mg atoms and thereby the subsequent local anisotropic surface disordering of MgO(110). Figure 5 shows the CO2-TPD profiles, demonstrating the base strength distribution of Pd-MgO/SiO2 catalysts with different loading amounts of Mg (0-3.6 wt %). A CO2 desorption peak below 450 °C was observed on Pd-MgO/SiO2 catalysts. By increasing Mg loading amount (0-3.6 wt %), the intensity of the CO2 desorption peak is increased over the Pd-MgO/SiO2 catalyst, indicating that the MgO of the Pd-MgO/SiO2 exhibits

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Figure 5. CO2-TPD patterns of a series of Pd-MgO/SiO2 with different loading amounts of Mg (0-3.6 wt %) (a) 0, (b) 0.3, (c) 0.9, (d) 1.8, and (e) 3.6 wt %.

Figure 3. The structures and the binding energies of the intermediates in CO2 methanation on MgO(110). The energy of the initial state (MgO(110) plus eight H atoms in the gas) is taken to be zero. Ex is the energy released during the xth reaction step. The total energy, Etotal, is the energy of the reaction of one CO2 molecule with eight H atoms to make two H2O molecules and one CH4 molecule. This has been calculated from the energies of the steps in the cycle, and it is equal to the energy calculated for the gas phase reaction, as it should be.

Figure 4. Electron distribution after CO2 adsorption on MgO(110). (a) Carbonate on MgO(110). (b) Differences in Bader charge of atoms caused by carbonate formation. The negative sign means that the atom gained electrons and was negatively charged. (c,d) The top and side views of the electron redistribution diagram after carbonate formation. Protruded deep blue and gold spheres show the regions where electrons are accumulated or depleted during carbonate formation.

a high CO2 adsorption-desorption capacity. This finding confirms our DFT result that MgO initiates the reaction by binding CO2 molecules.

It has generally been believed that the exterior of a nanosized catalyst is covered with the most stable surface of a material and that this surface is where the catalyzes take place. However, recent experimental and theoretical findings confirm that the less stable species of materials, such as a low-coordinated surface atom of Au nanoparticles48 or a side edge of a planar MoS2 catalyst,49–52 are responsible for their catalytic activity. In our case, we found that a carbonate, the reaction initiator of CO2 methanation, is not observed in the most stable MgO(100). On the other hand, the less stable MgO(110) and MgO(111)/ Mg strongly bind a CO2 molecule and form a carbonate. We postulate, therefore, that not the most stable Mg(001), but rather the less-stable MgO surfaces or defective surfaces with vertices, steps, or kinks where there are low-coordinated or protruded oxygen atoms, can bind a very stable CO2 molecule, being an active species for CO2 methanation. And this is presumably why a very small amount of Mg content dramatically improves the activity of CO2 methanation by a Pd-MgO/SiO2 catalyst. When we exposed the carbonate on MgO(110) to H atoms, the oxygen species of the carbonate was spontaneously hydrogenized and two H2O molecules were produced (see step 2 to 5 of Figure 3). Table 2 compares the energy from the reaction of the hydrogenation of oxygen species of the carbonate with two hydrogen sources, H atom or H2 molecule. Interestingly, we found that an H2 molecule cannot facilitate hydrogenation of the oxygen species of the carbonate on MgO(110). This fact indirectly confirms that the Pd of our Pd-MgO/SiO2 catalyst indeed dissociates H2 molecules and supplies H atoms to carbonates on MgO. Steps 6 to 9 of Figure 3 show that a residual C on MgO(110) was sequentially hydrogenized to CH4 in the presence of atomic H. We postulate that a supplement of H atoms by Pd also accelerates the reaction speed of the CH4 formation (see Table 2). In summary, we scrutinized the role of the MgO and Pd nanoparticles on CO2 methanation by the previously reported Pd-MgO/SiO2. We showed that MgO and Pd plays completely different roles; MgO initiates the reaction by binding a CO2, and the Pd core dissociates the H2 molecule and supplies H atoms, actions essential for further hydrogenation of the oxygen species of the carbonate and residual C atom. Our findings confirm the bifunctional mechanism of the Pd-MgO/SiO2 catalyst. From the standpoint of CO2 utilization, the present work is expected to be very useful for applications requiring the CO2

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TABLE 2: Energy of Hydrogenation of Oxygen Species of the Carbonate on MgO(110) and the Residual C Atoma ∆EH (eV) ∆EH2 (eV)

step 2

step 3

step 4

step 5

step 6

step 7

step 8

step9

-0.91 1.38

-1.58 0.71

-0.96 1.33

-0.72 1.57

-4.28 -1.99

-1.41 0.88

-4.60 -2.31

-3.81 -1.52

a ∆EH and ∆EH2 represent a reaction energy relative to atomic hydrogen and molecular hydrogen. Refer to Figure 3 for the geometries of each step.

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