Selective Conversion of Methane by Rh1-Doped Aluminum Oxide

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A: Molecular Structure, Quantum Chemistry, and General Theory 1

Selective Conversion of Methane by Rh-Doped Aluminum Oxide Cluster Anions RhAlO : A Comparison with the Reactivity of PtAlO 2

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Ya-Ke Li, Yan-Xia Zhao, and Sheng-Gui He J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02483 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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The Journal of Physical Chemistry

Selective Conversion of Methane by Rh1-Doped Aluminum Oxide Cluster Anions RhAl2O4‒: A Comparison with the Reactivity of PtAl2O4‒ Ya-Ke Li,a,b,c Yan-Xia Zhao,*a,b and Sheng-Gui He*a,b,c

a

State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of

Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China b

Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center of

Excellence in Molecular Sciences, Beijing, 100190, P. R. China c

University of Chinese Academy of Sciences, Beijing, 100049, China

*E-mail: [email protected], [email protected]; phone: +86-10-62568330; fax: +8610-62559373.

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ABSTRACT Studying the elementary reactions of single noble-metal-atom doped species can give theoretical guidance for the design of related single atom catalysis. Using a combination of mass spectrometry and density functional theory calculations, the reaction of RhAl2O4– with the most stable alkane molecule CH4 under thermal conditions has been studied. The methane tends to be converted into syngas (free H2 and adsorbed CO) with four C‒H bonds activation. In sharp contrast, formaldehyde was generated in the previously reported reaction of PtAl2O4– with CH4. Density functional theory calculations show that the difference in reactivity between RhAl2O4– and PtAl2O4– is found to be due to a higher energy barrier of the third C‒H bond activation for the Pt analogue. This work provides the first comparative study on the reactivity of single noble metal atoms (Rh, Pt) on the same cluster support (Al2O4–) and can be helpful for rational design of single-atom catalysts for selective methane conversion.


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1．．INTRODUCTION Single-atom catalysis with isolated noble-metal atoms dispersed on solid surfaces has attracted an increasing attention because it not only maximizes the effective use of the precious metals but also provide an alternative strategy to tune the activity and selectivity of a catalytic reaction.1-3 Oxide supported single noble-metal atoms, including Au, Pt, and Rh, have been demonstrated to show excellent performance in important reactions such as the oxidation of carbon monoxide1, 4-6 and water-gas-shift.1, 9-11 Single-atom catalysis also has a great potential for transformation of methane, the most stable alkane molecule.1, 12-18 Understanding the molecular and electronic level mechanisms governing single-atom catalysis can be very important to design better performing catalysts. Gas-phase studies of well-defined atomic clusters under isolated, controlled, and reproducible conditions provide one useful method to uncover the mechanistic details of elementary reactions in related single-atom catalysis.19-35 The reactivity of various bare single noble-metal species such as Rh+/o/,36-39 Pt+/-,40-43 and so on,34, 44-49 and liganded single noble-metal ions34,

50-59

toward methane has been widely studied. To have a better understanding of the

molecular level mechanisms and the role of single-noble metal atoms in methane conversion over realistic catalysts, it is quite necessary to study the reactions between methane and the composite systems with both the single-noble atoms and the oxide clusters support. Since the first study on the reaction between methane and single noble-metal-atom doped cluster AuNbO3+,60-61 the reactions of methane with other related clusters

PtAl2O4–,62 AuV2O6+,63

AuTi3O7,8–,64 and RhAl3O4+ 65 have also been investigated. All of these studies provide detailed information about the reactivity of the supported single noble-metal atoms, Au, Pt, and Rh, in methane activation and conversion. However, a comparison of the reactivity and product 3 ACS Paragon Plus Environment


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selectivity of these supported single noble-metal-atoms on a same cluster support toward methane has never been conducted before. Due to the high activity of supported Rh catalysts toward methane and the isotopically pure nature of Rh element, the reactions of Rh-doped metal oxide cluster anions with methane were explored. This study reports that the reaction of Rh1-doped aluminum oxide cluster anions RhAl2O4– with CH4 generates syngas (free H2 and adsorbed CO), which is in sharp contrast with the previously reported reaction of PtAl2O4– with CH4 that produces CH2O.62 The different product selectivity of supported single Rh and Pt atoms in methane conversion can thus be interpreted. This gas phase study parallels well with the condensed phase study66-67 that dispersed Rh on Al2O3 give a higher dihydrogen generation than the corresponding Pt system in methane oxidation. 2. METHODS 2.1 Experimental Methods A home-made reflection time-of-flight mass spectrometer (TOF-MS)68 coupled with a laser ablation cluster source, a quadrupole mass filter (QMF),69 and a linear ion trap (LIT)70 reactor was used in the experiments. The RhAl2O4‒ cluster anions were prepared by laser ablation of an Rh/Al mixed disk (molar ratio of Rh:Al = 1:1) in the presence of 0.01% 18O2 seeded in 6 atm He carrier gas. The cluster anions of interest were mass-selected by the QMF and entered the LIF reactor, where they were confined and thermalized by collisions with a pulse of about 3 Pa He gas for almost 0.8 ms (~550 collisions) and then reacted with a pulse of CH4, or CD4 for 2.2 ms. 2.3 Theoretical Methods The density functional theory (DFT) calculations using Gaussian 09 program71 were carried out to investigate the structures of RhAl2O4‒ and the mechanisms of reaction with CH4. The 4 ACS Paragon Plus Environment

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TPSS functional72 were tested (Table S1) and found to perform very well for the bond energies of Rh−O, Rh−C, Al−O, C−H, H−O, H−H, and C−O in diatomic molecules. Thus, the TPSS functional results were given throughout the work. The reaction mechanism calculations involved geometry optimization of reaction intermediates and transition states (TSs). The initial guess structures of the TS species were obtained through relaxed potential energy surface scans using single or multiple internal coordinates.73 Vibrational frequency calculations were performed to check that the IMs or TSs have zero and only one imaginary frequency, respectively. The intrinsic reaction coordinate calculations 74 were carried out to make sure that a TS connects two appropriate minima. The reported energies (∆H0) were corrected with zeropoint vibrations. The NBO analysis was performed with NBO 3.1. 75 3. RESULTS 3.1 Experimental Results As shown in Figure 1, upon the interaction of RhAl2O4– with 0.03 Pa CH4, the product peak assigned as RhAl2O4CH2– was observed (Figure 1b). The magnitudes of the RhAl2O4CH2– product peaks increase as the CH4 pressure increases (Figure 1c), which suggests the reaction (1) below: RhAl2O4‒ + CH4 → RhAl2O4CH2‒ + H2

(1)

In contrast, the reaction (2) was observed for the previously studied reaction between PtAl2O4– and CH4.62 PtAl2O4‒ + CH4 → PtAl2O3H2‒ + CH2O

(2)

Reaction (1) was confirmed by using the isotopic labelling experiment with CD4 (Figure 1d). In addition, very weak product peaks assigned as RhAl2O3H2– and RhAl2O3H4– were also observed (Figure S1). The pseudo first-order rate constant (k1) for reaction between RhAl2O4– 5 ACS Paragon Plus Environment


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and CH4 is estimated to be (1.4 ± 0.3) ×10‒11 cm‒3 molecule‒1 s‒1, corresponding to a reaction efficiency 76-77 (k1 / kcoll) of about 1.2%. The kinetic isotope effect [KIE=k1 (RhAl2O4– + CH4) / k1 (RhAl2O4– + CD4)] is 7±2.

Figure 1. Time-of-flight mass spectra for the thermal reactions of mass selected RhAl2O4− with (a) He, (b) 0.03 Pa CH4, (c) 0.09 Pa CH4, and (d) 0.30 Pa CD4. The reaction time is 2.2 ms. 3.2 Theoretical Results The TPSS functional was used to study the geometric structures of reactant RhAl2O4– (Figure S2) and product RhAl2O4CH2– (Figure S3) as well as the mechanisms of the reaction between RhAl2O4– and CH4 (Figures S4-S8). In the lowest-energy structure of RhAl2O4‒ cluster, the Rh atom is bonded with two O atoms and one Al atom. In contrast, the Pt atom is bonded with one O atom and one Al atom in the PtAl2O4‒ cluster (Figure 2a). RhAl2O4– is a triplet and almost all of the unpaired spin densities are distributed around the Rh atom, while the lowestenergy structure of product cluster RhAl2O4CH2– is a singlet state. Therefore, the spin conversion may take place during the reaction.78-79 6 ACS Paragon Plus Environment

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Figure 2. DFT calculated (a) structures of RhAl2O4‒ and PtAl2O4‒. Bond lengths (in pm) and spin densities (in µB and listed in the parentheses) are shown; (b) potential energy profile for the reaction of RhAl2O4– with CH4. A triplet-to-singlet crossing point is marked with CP and the 7 ACS Paragon Plus Environment


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details are shown in Figure S5. The zero-point vibration corrected energies (∆H0) with respect to the separated reactants are given in eV. The structures of reactants, products, and intermediates are plotted and those of transition states can be found in the Supporting Information. The lowest-energy reaction pathway found for Reaction (1) is shown in Figure 2. The Rh atom functions as the preferred trapping site to anchor the CH4 molecule. The natural charge on Rh atom is +0.45 e and the electrostatic potential map shown in Figure S2b also indicates that the Rh site of RhAl2O4– is less negatively charged than the other side of the cluster, suggesting that the Rh atom can be the preferred trapping site when the CH4 molecule approaches the RhAl2O4– cluster. Therefore, a larger binding energy of 0.44 eV is achieved when the methane molecule is trapped by the Rh site with the formation of encounter complex I1. The reaction proceeds through oxidative addition mechanism resulting in the cleavage of the first C−H bond of methane and the formation of Rh−H and Rh−CH3 bonds (3I1 → 3TS1 → 3I2). A spin conversion is involved in the second C−H bond activation step (3I2 → CP → 1I3). This can be understood by the fact that with the coordination number of Rh atom increasing, the C–H bond activation following a mechanism of oxidative addition that is much more favorable in the lowspin state than in the high-spin state.38, 80-82 Through a structure relaxation (1I3 → 1TS3 → 1I4), the Rh atom delivers one H atom to the bridging O atom through a small barrier (0.05 eV) to form a stable intermediate 1I5 (1I4 → 1TS4 → 1I5). A more stable intermediate 1I6 can be formed if the H atom is further transferred to the terminally bonded O atom (1I5 → 1TS5 → 1I6). After the formation of 1I6, the system has enough energy (1.98 eV) to move the second H atom and the CH2 moiety is ready to bond with the O atom to form a CH2O moiety (1I8 → 1TS8 → 1I9 → 1TS9 → 1I10). The CH2O moiety can be dissociated easily with the cleavage of two C−H bonds by Rh atom and formation of two 8 ACS Paragon Plus Environment

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Rh−H bonds and Rh−CO bond (1I10 → 1TS10 → 1I11 → 1TS11 → 1I12). It is noted that evaporation of CH2O from the reaction complex is kinetically unfavorable (Figure S4). The two H atoms on Rh make a H2 unit (1I12 → 1TS12 → 1I13). Finally, with enough energy released, the H2 molecule can be evaporated easily from Rh atom in 1I13 to form product ion RhAl2O4CH2−. Compared with the evaporation of H2 molecules, the evaporation of CO is much less favorable (Figure S4). This is consistent with the experimental results that (H2 + RhAl2O4CH2−) rather than (CO + RhAl2O3H4−) is the major product in the reaction between RhAl2O4− and CH4 (Fiugre S1). Note that CO is attached on the Rh atom in 1P1 with a binding energy of 2.29 eV, so heating the product may generate CO. One can consider that CH4 is converted to syngas (free H2 and adsorbed CO) by RhAl2O4−. The eletron detachment channel has also been considerred in this study. the vertical dissociation energies (VDEs) and adiabatic dissociation energies (ADEs) for intermediates 1I6, 1

I8, 1I9, 1I10, 1I11, 1I12, 1I13 and products 1P1 and 2P2 have been studied by the DFT

calculations (Table S2). The values of the VDEs and ADEs are very high, which means that the electron detachment channel is thermodynamically unfavorable. Thus, the electron detachment from the product ions or the reaction intermediate can be negligible in this study. 4 DISCUSSIONS Our previous work62 studied the reaction between PtAl2O4‒ and CH4, in which formaldehyde rather than syngas was produced. To further understand the difference of the function between Rh and Pt, the lowest-energy reaction pathway to produce H2 molecule in the previously studied reaction between PtAl2O4‒ and CH4 has also been considered (Figures 3, S9 and Table S3). The Pt atom is the active site in the reaction. A much higher energy barrier needs to be overcome when the third C‒H bond is cleaved by the Pt atom than that by Rh atom (Figure 3), leading to 9 ACS Paragon Plus Environment


the generations of CH2O and H2 in Pt and Rh reaction systems, respectively. It is noteworthy that there is still some problem with the DFT calculation method used for the reaction of PtAl2O4‒ with CH4. As shown in Figure S9, the free energy of Pt-TS10 is identical with that of Pt-P2. Compared with the experimental results, the energy of Pt-TS10 may be under-estimated or the energy of Pt-P2 may be over-estimated by the BLYP functional. Considering the error of the calculation method, the barrier for the third C‒H bond in reaction of PtAl2O4‒ with CH4 is still higher than that in the reaction of RhAl2O4‒ with CH4.

Figure 3. Simplified potential energy profiles for methane conversion to dihydrogen by (a) RhAl2O4‒ and (b) PtAl2O4‒. The zero-point vibration corrected energies (∆H0) with respect to the separated reactants are given in eV. Additional insights can be gained from a molecular orbital (MO) analysis, considering that the Rh system has one less valence electron than the corresponding Pt system. Figure 4 shows that the lowest unoccupied molecular orbitals (LUMOs) of 1I10 and 1TS10 correspond to the singly occupied molecular orbitals (SOMOs) of Pt-I10 and Pt-TS10, respectively. All of the 10 ACS Paragon Plus Environment

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occupied MOs of 1I10 and 1TS10 also have their counterparts in the occupied MOs of Pt-I10 and Pt-TS10, respectively (Figure S10). The bonding nature of the SOMOs of Pt system can be well related with the high reaction barrier for the activation of the third C‒H bond. As shown in Figures 4 and 3b, a bonding-type orbital between Pt and C atoms is consistent with the low energy of Pt-I10, while an antibonding-type orbital between the two atoms leads to higher energy of Pt-TS10. The reactivity of neutral PtAl2O4 toward CH4 conversion was also studied by DFT calculations (Figures S11-14). Without the extral valence eletron in the neutral PtAl2O4 system, the barrier for the third C‒H bond is still high (1.03 eV). The natural bond orbital analysis indicates that in neutral 1Pt-I10, the charge for Pt atom is -0.479 e which is much less negative than the charge of Rh atom (-0.852 e) in negative 1I10. This may lead to that the electrostatic attraction between Pt and the transferring H atoms in 1Pt-I10 is weaker than that between Rh and the transferring H atoms in 1I10, which leads to the higher barrier for the third C‒H bond activation during 1Pt-I10 → 1Pt-TS10 than that during 1I10 → 1TS10. In 1I10 and Pt-I10, the charge distribution is similar. Thus, in addition to the extra valence electron, the net charge (the charge state) also matters in the reactivity and selectivity of the cluster reactions.83-84



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Figure 4. DFT calculated lowest unoccupied molecular orbitals (LUMOs) of 1I10 and 1TS10 and corresponding singly occupied molecular orbitals (SOMOs) of Pt-I10 and Pt-TS10. This gas phase study parallels well with the condensed-phase study66-67 that the nanosized Rh supported on Al2O3 gives a higher dihydrogen generation than corresponding Pt in methane conversion to syngas. In addition, condensed-phase study85 indicates that formaldehyde can be decomposed to syngas by the supported single Rh atom, which parallels the gas phase result that the formed CH2O moiety (1I10) can be activated easily by the cluster supported Rh atom. One less valence electron of Rh atom leads to much easier activation of the third C‒H bond of methane and favorable generation of syngas (free H2 and adsorbed CO) rather than formaldehyde in the comparison with the Pt system. 5. CONCLUSIONS In conclusion, this work provides the first comparative study on the reactivity of single noble metal atoms (Rh, Pt) on the same cluster support (Al2O4‒) toward methane activation and conversion. The single Rh and Pt atoms are active sites in methane activation and conversion to syngas (free H2 and adsorbed CO) and formaldehyde, respectively. The much lower reaction barrier for the activation of the third C‒H bond of methane in Rh system than that in Pt system leads to the syngas rather than the formaldehyde generation. This study can be helpful for rational design of single-atom catalysts for selective methane conversion. ASSOCIATED CONTENT Supporting Information. Detailed description of theoretical methods, additional experimental and theoretical results are shown in the supporting information.

AUTHOR INFORMATION Corresponding Author 12 ACS Paragon Plus Environment

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*

Email: [email protected]; [email protected]

ORCID Ya-Ke Li: 0000-0003-1877-1922 Yan-Xia Zhao: 0000-0002-4425-5211 Sheng-Gui He: 0000-0002-9919-6909

Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was financially supported by the Chinese Academy of Sciences (No. XDA09030101) and the National Natural Science Foundation of China (Nos. 21773253, 91645203, and 21573247).



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TOC Graphic


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Figure 1. Time-of-flight mass spectra for the thermal reactions of mass selected RhAl2O4− with (a) He, (b) 0.03 Pa CH4, (c) 0.09 Pa CH4, and (d) 0.30 Pa CD4. The reaction time is 2.2 ms. 176x144mm (200 x 200 DPI)

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Figure 2. DFT calculated (a) structures of RhAl2O4‒ and PtAl2O4‒. Bond lengths (in pm) and spin densities (in µB and listed in the parentheses) are shown; (b) potential energy profile for the reaction of RhAl2O4– with CH4. A triplet-to-singlet crossing point is marked with CP and the details are shown in Figure S5. The zero-point vibration corrected energies (∆H0) with respect to the separated reactants are given in eV. The structures of reactants, products, and intermediates are plotted and those of transition states can be found in the Supporting Information. 156x294mm (300 x 300 DPI)


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Figure 3. Simplified potential energy profiles for methane conversion to dihydrogen by (a) RhAl2O4‒ and (b) PtAl2O4‒. The zero-point vibration corrected energies (∆H0) with respect to the separated reactants are given in eV. 209x156mm (232 x 232 DPI)



Figure 4. DFT calculated lowest unoccupied molecular orbitals (LUMOs) of 1I10 and 1TS10 and corresponding singly occupied molecular orbitals (SOMOs) of Pt-I10 and Pt-TS10. 144x108mm (156 x 156 DPI)


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Selective Conversion of Methane by Rh1-Doped Aluminum Oxide

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