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Theoretical insights and the corresponding construction of supported metal catalysts for highly selective CO2-to-CO conversion Xiaodong Chen, Xiong Su, Hai-Yan Su, Xiaoyan Liu, Shu Miao, Yonghui Zhao, Keju Sun, Yanqiang Huang, and Tao Zhang ACS Catal., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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Theoretical insights and the corresponding construction of supported metal catalysts for highly selective CO2-to-CO conversion Xiaodong Chen,†,‡,# Xiong Su,†,# Hai-Yan Su,*,§Xiaoyan Liu,† Shu Miao,† Yonghui Zhao,§ Keju Sun,∥ Yanqiang Huang,*,†,δ and Tao Zhang†,δ †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, No. 457 Zhongshan Road, Dalian 116023, P. R. China. ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

§

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics,

Chinese Academy of Sciences, No. 457 Zhongshan Road, Dalian 116023, China. ∥

Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering,

Yanshan University, Hebei 066004, China. δ

iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute

of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.

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ABSTRACT

Exploration of highly selective catalysts for the reverse water-gas shift reaction, which can be served as a pivotal stage of transitioning the abundant CO2 resource into chemicals or fuels, still remains a big challenge. Here, we firstly offered a guideline about this matter based on exploring the evolution of main intermediates during CO2 hydrogenation over a library of supported metal catalysts with density functional calculations. We identified that a high selectivity toward this reaction was correlated with the energy difference between the dissociation barrier and the desorption energy of metal carbonyls over supported metal catalysts. Generally, decreasing the coordination number of metal sites to single-atom or moving supported metals to the lower right corner in the transition-metal series hindered the carbonyl dissociation but improved CO desorption, giving rise to the increased CO selectivity. Furthermore, the above strategies were truly confirmed by measuring the catalytic performance that occurred on the real synthetic catalysts.

KEYWORDS: CO2 conversion; RWGS reaction; supported metal catalysts; theoretical insights; selectivity


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1. INTRODUCTION Highly efficient conversion of CO2 to energy-rich commodities is a promising way of addressing the “3Rs” (reduce, reuse, and recycle) associated with the ever-increasing CO2 levels, and is necessary for the sustainable development of long-term fossil-carbon industries.1–3 Catalytic reduction of CO2 to CO, also known as the reverse water-gas shift (RWGS) reaction, has recently received considerable attention because this process is pivotal in the transformation of CO2 to chemicals or liquid fuels through the industrially developed methanol and Fischer– Tropsch syntheses.4-7 Much efforts have been devoted to the design of reducible oxide supported metal catalysts, which often show higher catalytic activities at low temperatures than mixed oxide or irreducible oxide supported metal catalysts; this is because of the dual functionalities provided by the supported metal and adjacent vacancy sites.4, 8, 9 However, the RWGS reaction is always accompanied by undesired methanation over such supported metal catalysts. Therefore, an additional separation step is required and this decreases the efficiency of the subsequent syngas use.10 Various studies have been performed to improve the RWGS selectivity by elucidating the reaction mechanisms11-13 and the effects of metal particle size,14-19 support type,20-24 and promoters.9, 25-29 Two primary mechanisms, i.e., redox and associative mechanisms, have been proposed for the RWGS reaction. They differ in terms of whether CO2 activation occurs via direct or hydrogen-assisted pathways, and the intermediates involved, such as formate (HCOO) or carboxyl (COOH).30 According to the two mechanisms, the side product of methane can be produced by C-O bond scission in CO, from HCOO or COOH etc., followed by sequential hydrogen addition. Although considerable research has been performed in this area, mechanistic


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insights into the factors that govern the relative activities of the two competitive reaction pathways at the atomic level are still needed. Herein, density functional theory (DFT) studies were performed to identify the competitive reaction pathways of RWGS and methanation and engineer product selectivity by tuning the atomic coordination number (Nc) from nanoparticles to single-atoms and the nature of metal sites. Four model catalysts, namely, single-atom Ir embedded in TiO2 matrix (denoted by Ir1/TiO2) and TiO2 supported Ir, Pt, and Au nanoparticles (denoted by Ir5, Pt5, and Au5 sites), were used to illustrate the effects of Nc and the metal nature. Among these models, single-atom Ir1/TiO2 and TiO2 supported Au nanoparticle were found to show excellent selectivity toward RWGS reaction. Based on these findings, we demonstrated that the difference between desorption energy and dissociation barrier of metal carbonyls can be regarded as a critical factor for tuning CO/CH4 selectivity. More specifically, decreasing the coordination number of metal sites to single-atom or moving supported metals to the lower-right corner in the transition-metal series will hinder the carbonyl dissociation but improve CO desorption. The above strategies were also intentionally verified by the catalytic performance that occur in these real synthetic catalysts.

2. EXPERIMENTAL PROCEDURES DFT calculations. Spin-polarized DFT calculations were performed using the Vienna ab initio simulation package.31 The interactions between ionic cores and electrons were described using the projector-augmented wave method, and the Kohn–Sham valence electronic wave function was expanded in a plane-wave basis set with a kinetic energy cutoff at 400 eV. The exchange-correlation effects were represented within the generalized gradient approximation


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using the PW91 functional.32 The energies were converged to within 10−4 eV/atom, and the forces were converged to within 0.03 eV Å−1. The stepped Ir (111), Pt (111), and Au (111) surfaces were modeled using a four-layer slab with (7 × 3) surface unit cells, in which three neighbouring rows of metal atoms on the top layer were removed. The rutile TiO2 (110) surface was modeled using a four-layer slab in a (2 × 1) supercell. A vacuum region of 15 Å between any two repeated slabs was found to be sufficient to avoid interactions between repeated slabs in the z-direction. The surface Brillouin zone was sampled with (2 × 4 × 1) and (4 × 4 × 1) Monkhorst–Pack k-point grid meshes for the stepped metal and TiO2 surfaces, respectively.33 The top three (metals) or two (TiO2) layers and the adsorbates were fully relaxed, and the remaining layers were fixed in their bulk truncated positions. The lattice constants for bulk Ir, Pt, Au, and TiO2 were calculated to be 3.88, 3.99, and 4.18, and a = 4.66/c = 2.97 Å, respectively, in good agreement with the experimental values (3.84, 3.92, 4.08, and a = 4.59/c = 2.96 Å).34, 35 All the transition states (TSs) were located using the force reversed method36 and climbingimage nudged elastic band method.37 The relaxation was stopped when the residual forces in each atom were smaller than 0.03 eV Å−1. The elementary activation barrier was calculated with respect to the most stable state for the adsorption of adsorbates on the surfaces. The free energy G for gas-phase species is obtained by G = ETOTAL – TS + RTln(P/P0), where ETOTAL is the energy of gas-phase species, S is the entropy at temperature T, P and P0 is the partial pressure of gas-phase species and standard pressure. In this work, the reaction conditions (T = 600 K, P) on different models are employed separately based on experimental testings: stepped Ir (PCO = 0.006 atm, PH2 = 0.436 atm,


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PH2O = 0.011 atm), stepped Pt (PCO = 0.043 atm), stepped Au (PCO = 0.029 atm) and Ir1/TiO2 (PCO = 0.002 atm, PH2 = 0.448 atm, PH2O = 0.002 atm). Catalyst preparation. A pure rutile-phase TiO2 support (Brunauer–Emmett–Teller surface area 27 m2 g−1) was prepared using a previously reported method.38 The supported Ir catalysts were prepared via a wetness-impregnation method using the prepared TiO2 as the support. The Ir contents in the Ir/TiO2 samples were calculated to vary from 0.1% to 5%. In a typical synthesis of the 0.1% Ir catalyst, the required amount of 6.1 mg H2IrCl6 solution (16.28 wt% H2IrCl4 in ultra-pure water, Sinopharm Chemical Reagent Co., Ltd.) was diluted to 50 mL. Rutile TiO2 support (1.0 g) was then added and the mixture was stirred at 50 °C until the water was evaporated. The precipitate was dried at 120 °C for 12 h, and calcined in air at 500 °C for 4 h. The obtained samples were repeatedly washed with dilute ammonia solution (pH = 14) and deionized water to remove the residual chlorides, dried at 60 °C for 12 h, and pre-treated in a feed gas stream at specified temperatures for catalytic evaluation. The Pt/TiO2-1% catalyst was similarly prepared using H2PtCl6 solution (Sinopharm Chemical Reagent Co., Ltd.) as the Pt source. The Au/TiO2-1% catalyst (standard Au catalyst) was purchased from the Haruta-Gold Incorporated Co., Ltd. Characterizations. The sample structures were determined using X-ray diffraction [PANalytical X’Pert-Pro powder X-ray diffractometer, Cu Kα monochromatized radiation (λ = 0.1541 nm)]. Diffraction peaks were recorded in the 2θ range of 20° to 80°, at a scanning speed of 4° min−1. Sub-ångström-resolution HAADF-STEM was performed using a JEOL JEMARM200F STEM/TEM instrument with a guaranteed resolution of 0.08 nm. Prior to examination, the catalyst powder was ultrasonicated in ethanol and then dispersed on a TEM Cu grid covered with a lacy carbon film and dried at room temperature. A JEM-2100F field-


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emission electron microscope operated at an accelerating voltage of 200 kV was used. Samples for STEM measurements were prepared by suspending the ultrasonically dispersed catalyst powder in ethanol and placing a drop of the suspension on a Cu grid. X-ray absorption spectra, including XANES and extended XAFS spectra at the L3 edge of Ir, were recorded at the BL14W1 at the Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, China. A double Si (111) crystal monochromator was used for energy selection. The energy was calibrated using Ir foil. Before the experiments, the samples were all reduced at 500 °C and sealed in Kapton film in a glove box without exposure to air. The spectra were recorded at room temperature in fluorescence mode using a solid-state detector. The data were analyzed using Athena software packages. Catalytic activity measurements. The CO2 hydrogenation activities over Ir/TiO2, Pt/TiO2, and Au/TiO2 were investigated in a continuous-flow fixed-bed reactor under atmospheric pressure. The flow rate of the feed gas was controlled using a mass flow controller. The reaction temperature was measured and controlled using a K-type thermocouple located at the central axis of the reactor. Prior to the catalytic test, the catalyst (40–60 mesh, 0.2 g) was packed into a quartz reactor (inner diameter 11 mm) and purged with He at a flow rate of 30 mL min−1 at 350 °C for 60 min. The catalyst was reduced at 500 °C for 1 h in a feed gas containing 45 vol% CO2, 45% vol% H2, and balanced with 10 vol% He (also used as an internal standard gas) at a flow rate of 50 mL min−1. During the activity tests, the reaction temperature was set at 300–500 °C using the same feed gas flow. The effluent gas products were passed through an ice-bath unit to remove water vapor, and analyzed online using an Agilent 7890B gas chromatograph with a TDX-01 column connected to a thermal conductivity detector.


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The catalytic performance was evaluated by the conversion of CO2 based on the different concentrations between the inlet and outlet, which is defined as: ×

Conversion (CO2%) =

× 100%

The selectivities of CO and CH4 are defined as:

×

Selectivity (CO%) =

× ×

× 100%

Selectivity (CH4%) = 1 - Selectivity (CO%) Where CO (in), CH4 (in) and N2 (in) are the concentrations of CO, CH4 and N2 at inlet; CO2 (out), CO (out) and CH4 (out) are the concentrations of CO2, CO and CH4 at outlet, respectively. The reaction rate of CO2 was calculated using the following formula: r=

×

Where r is the reaction rate of CO2 (mol gcat−1 h−1), FCO2 is the feed gas flow rate (mol h−1), M is the mass of catalyst (g), and x is the conversion of CO2. Additionally, the Ir/Pt/Au particle size was calculated based on an assumption of a quasihemispherical model of metal particle.39 For Ir/TiO2, Pt/TiO2, Au/TiO2 catalysts, the metal dispersion is determined by assessing the portion of surface exposed atoms. DIr =

DPt =

DAu =

. "#$ . % "& . ' "(


8

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Where D is the metal dispersion; d is the diameter of metal particles (nm). The dispersion of Ir/TiO2-0.1% is defined as 100% since all Ir species are dispersed atomically on the catalyst surface. The turnover frequency (TOF) of CO2 is calculated depending on the metal dispersion, which is defined as the moles of CO2 converted per mole surface metal atom per second (s−1). TOF =

) × * × +

Where M is the atomic weight, and x is the metal content (gmet gcat-1).

3. RESULTS AND DISCUSSION The redox mechanism and associative mechanism for RWGS reaction and CO2 methanation are listed in Table S1. We first analyze the factors that affect product selectivity by the redox mechanism. We will also discuss the associative mechanism which involves the HCOO or COOH intermediates. According to the redox mechanism, CO2 decomposes directly into CO and O. The formed CO can either desorb or react further, leading to the liberation of CO product or the unwanted formation of methane. A selective RWGS catalyst should have a higher activation barrier for CO methanation than for CO desorption. Compared to CO desorption which can be generally characterized by the adsorption energy of CO, the methanation process is more complex. It has been suggested that CO dissociation, with (hydrogen-assisted dissociation) or without (direct dissociation) the presence of hydrogen, is the rate-limiting step (RLS) in CO methanation on a number of metal surfaces.40-45 Our calculations for the free-energy diagram of CO methanation on stepped Ir sites show that direct CO dissociation is the slowest among all the elementary steps (Figure 1a and Supplementary Table S3).


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Figure 1. (a) Free-energy diagram for CO methanation on B5 sites of stepped Ir (black line) and PR1-Ir1/TiO2 (110) surface (red line). TS structures for direct CO dissociation are shown in inset. (b) Structures of B5 sites on stepped metals (left: Ir, Pt, and Au) and PR1-Ir1/TiO2 (110) surface (right). Blue, dark yellow, and red balls indicate Ir, Ti, and O atoms, respectively. Metal atoms at step edge are highlighted, and B5 sites are indicated by black pentagons.

We will exploit these findings by treating CO dissociation as RLS for CO methanation on stepped Ir, and use its activity to tune the methane process in the following discussion. The CO dissociation has been shown to depend strongly on the coordination number (Nc) of metal sites which typically exist on the step edge (see Figure 1b). The B5 sites (consisting of 5 metal atoms) or open surfaces often present superior activity than the B4 sites on the close packed surfaces.46-48 The reason is that the dissociated C and O fragments will share common metal atoms with decreasing Nc, which destabilizes the TSs.49 Furthermore, as the Nc decreases to less than B5 site, the four-fold TS site of C fragment adsorption becomes unavailable.46 Thus, if we decrease the Nc of metal sites, e.g., to a single-atom metal site embedded in an oxide support, CO dissociation


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can be expected to be suppressed. This is verified by DFT calculations on the Ir1/TiO2 system. Several possible stoichiometric and reduced Ir1/TiO2 (110) structures were studied (see Figure 1b and Supplementary Figures S2). The PR1-Ir1/TiO2 (110) surface, on which each Ir coordinates with one bridging oxygen (50% O vacancy concentration), was chosen to represent the Ir1/TiO2 system based on the stability and activity assessment (see the experimental section and Supplementary Table S4-S6). Calculations of CO dissociation on the Ir1/TiO2 (110) surface identified only a stable TS for the direct dissociation pathway. Furthermore, compared with stepped Ir, which has active B5 sites, the reaction is more endothermic (2.29 vs 0.64 eV, see Figure 1a, Table S3 and S6) and the activation energy barrier is increased by 2.45 eV on Ir1/TiO2 (110). Since the two surfaces have comparable CO adsorption energies (−2.46 vs −2.37 eV, see Table S2 and S5), the difference between the two TSs may be responsible for changes in the CO dissociation activity. Figure 1a shows that the dissociated C binds only with the single Ir atom at the bridging oxygen vacancy, with O on the inert coordinatively unsaturated Ti nearby at the TS on Ir1/TiO2 (110). The single Ir atom and coordinatively unsaturated Ti on Ir1/TiO2 (110) bind the C and O fragments at the corresponding TS configurations much more weakly than the B5 sites on the stepped Ir, by 2.87 and 2.31 eV (Supplementary Table S7), respectively, primarily because of the lower Nc of the metal sites and their electropositive nature that weakens charge transfer to the fragments. Once CO is dissociated on Ir1/TiO2 (110), subsequent hydrogen addition to the resulting C intermediate is easier, with a maximum activation barrier of 2.32 eV. These results demonstrate that the RLS of CO dissociation is not changed on Ir1/TiO2 (110), and the CO dissociation activity is a highly efficient descriptor to tune methane formation. An efficient RWGS catalyst should not only inhibit methane formation but remain facile CO desorption together with high CO2 hydrogenation activity. The similar adsorption energy of


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CO on Ir1/TiO2 (110) and stepped Ir indicate that the two surface models have similar CO desorption ability. Therefore, by inhibiting CO dissociation, the CO selectivity on Ir1/TiO2 (110) can be largely promoted than that on stepped Ir. In addition, we also examined the activity of CO2 hydrogenation to CO on Ir1/TiO2 (110) by DFT calculations. We only considered the redox mechanism for this process, which represents the lower bound for CO2 hydrogenation activity. The data in Supplementary Table S7 show that CO2 decomposes to CO and O on Ir1/TiO2 (110), accompanied by the recovery of O vacancies; the activation energy is 1.04 eV. Dissociative adsorption of H2 at the adjacent bridging O sites leads to OH formation. Then, two OH at bridge sites combine into H2O (Ea = 0.83 eV), accompanied by the creation of new O vacancies. Based on the above insights, an integrated reaction loop of RWGS was proposed and shown in Figure 2. The activation barriers for the elementary steps in the RWGS reaction are moderate and the barrier for CO desorption is much lower than for CO dissociation, suggesting that both high activity and selectivity in the RWGS reaction can be achieved on the Ir1/TiO2 (110) surface.

Figure 2. The proposed reaction pathway of RWGS process over Ir1/TiO2 catalyst.


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These formal approaches were applied to the fabrication of a series of rutile-TiO2-supported Ir catalysts because the crystalline lattice of IrO2 matches that of rutile (the structure of support is shown in Supplementary Figures S4-S5), with similar lattice distances of 4.59 and 4.50 Å (along the a and b axes), respectively.50 This strategy has been successfully used in the stabilization of nanosized precious-metal catalysts, even after severe aging or exposure to real reaction conditions.51-53 Building further on these results, the fabrication of single-atom or nanoparticle Ir was achieved by tuning the Ir contents, i.e., Ir/TiO2-0.1 (0.1 wt% Ir content) and Ir/TiO2-5, using an incipient-wetness impregnation method. The aberration-corrected high-angle annular darkfield (AC-HAADF) images in Figure 3a and 3b show the morphologies of the two samples. The

Figure 3. HAADF-STEM images of (a) Ir/TiO2-0.1 and (b) Ir/TiO2-5 catalysts. In Ir/TiO2-0.1, only Ir single atoms (white ellipses) are uniformly dispersed on rutile TiO2 support. In Ir/TiO2-5, both single atoms and three-dimensional Ir clusters with particle size of about 2 nm (white squares) or less are observed. (c) Normalized XANES spectra at the L3-edge of the Ir foil, IrO2, and Ir/TiO2 samples with different loadings of Ir. (d) The Fourier transform of k3-weighted EXAFS spectra of the Ir/TiO2 samples with different loadings of Ir: experimental vs. data fitting results.


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images show that Ir species in Ir/TiO2-0.1 are totally single atoms, whereas in Ir/TiO2-5, nanoparticles with mean particle size of ~2 nm are observed. It should be noted that there is still a significant quantity of Ir single atoms in Ir/TiO2-5, indicating the strong interaction between Ir and the rutile support. X-ray absorption fine structure (XAFS) techniques were used to further probe the coordination structures and chemical states of the loaded Ir species in the TiO2 lattice. The L3 edge of the X-ray absorption near-edge structure (XANES) in the Ir/TiO2-0.1 sample shows a structure similar to that of the IrO2 phase but different from that of Ir foil (Figure 3c-3d and Supplementary Figures S6–S7). This is reflected by the white line intensity of these samples, showing that the peak on Ir/TiO2-0.1 is much higher than that on Ir foil, possibly due to the higher oxidation state54 or the decreased particle size55-57 of the Ir species. The statistical fitting data for Ir coordination (Supplementary Table S8) show that there is no Ir–Ir signals in the region of 2–3 Å, confirming the sole presence of highly dispersed Ir atoms in Ir/TiO2-0.1. The high oxidation state of the Ir species (denoted by the coordination number of Ir–O) in the Ir/TiO2-0.1 sample indicates that the Ir single atoms are positively charged; this is consistent with the DFT calculation results. The catalytic CO2 hydrogenation performances of the Ir/TiO2 catalysts were evaluated using a packed-bed reactor with various Ir loadings from 0.1% to 5%. HAADF images and particle size distribution analyses show that the mean particle size of these tested catalysts are ranging from single atom to less than 2 nm (Supplementary Figure S8). The CO2 conversion was controlled at about 2.1% (see Figure 4a) by regulating the catalyst loading and feed gas flow to enable a more accurate assessment of the effect of Nc on product selectivity. The CO product selectivity on Ir/TiO2-0.1 reached almost 100% but decreased to 21.8% on Ir/TiO2-5. Figure 4b shows the calculated turnover frequencies (TOFs) and activation energies (Ea) of the


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hydrogenation products as a function of the increasement of Ir loading. The decline in the Ea values (from 89 to 60 kJ mol−1, Supplementary Figure S9) for methane verifies that methanation was easier on large Ir clusters. However, the Ea values for CO formation remained the same (at about 69 kJ mol−1) on all these catalysts. These results agree well with the DFT calculation data in Figure 1, showing that the dissociation of C–O bonds on single-atom Ir sites (either direct or H-assisted) is difficult, whereas the increased amount of nanosized Ir clusters in the other catalysts favors methane formation.

12

6

40

20

3

0

0

85

-1

)

80

TOF (S

9 60

90

0.04

CO2 conversion (%)

80

0.05

0.03 75

0.02

70

-1

(b)

100

Ea (kJ mol )

(a)

CO selectivity (%)

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65

0.01

60

0.1

0.5

1

2

5

0.00 0.1

0.5

1

2

5

Iridium loading (wt%)

Ir loading (wt%)

Figure 4. (a) The CO product selectivities (bar chart) obtained over a library of Ir/TiO2 catalysts (with Ir loadings from 0.1% to 5%) when catalytic evaluations were controlled under similar CO2 conversions (open pentagons). Catalyst loadings and feed gas flow rates were set at 0.5, 0.2, 0.1, 0.05, and 0.025 (g), with corresponding flow rates of 25, 35, 75, 50, and 100 (mL min−1). (b) CO (grey bar chart) and methane (red bar chart) TOFs, and CO (solid olive pentagons) and methane (open olive pentagons) activation energies on different Ir/TiO2 catalysts. Reaction conditions: 0.5 g catalyst; the feed gas consists of 45 vol% CO2, 45 vol% H2, and balanced with He; the feed gas flow is 50 mL min−1; reaction temperature is 350 °C.

We then turned to investigate the effect of the nature of supported metal on CO dissociation. It has been reported that the CO dissociation activity gradually decreases as moving the


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supported metal to the right and down in the transition-metal series.58,

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59

Consequently, the

metals in the lower right corner, such as Pt and Au, can be expected to be the least active for CO dissociation. We firstly considered direct CO dissociation on stepped Pt and Au models. The activities of stepped Pt and Au in CO dissociation are lower (Ea = 2.72 vs 5.27 eV) than that of stepped Ir (Ea = 2.21 eV, Supplementary Table S3). Stepped Au, in particular, shows a sharp increase of 3.06 eV in the dissociation barrier. The TS decomposition energies (Supplementary Table S7) reflect weaker binding of intermediates on Pt and Au, making bond-breaking steps more difficult. In the presence of hydrogen, CO hydrogenation leads to HCO intermediate formation on stepped Pt and Au with barriers of 1.41 and 0.58 eV, respectively. The adsorbed HCO intermediate decomposes to O and CH, with barriers of 1.69 and 3.45 eV, respectively (Supplementary Table S3). In the H-assisted CO dissociation mechanism, Pt and Au have overall barriers of 2.68 and 3.66 eV, lower than the direct dissociation barriers by 0.04 and 1.61 eV, respectively. Despite the lower barriers in the H-assisted pathway, the CO dissociation barriers on Pt and Au are still much higher than that on Ir. To access a more comprehensive evaluation on the catalytic performance of RWGS over these catalyst models, the difference between CO dissociation barriers (Ea) and desorption free energies (∆Gdes) of CO on stepped Ir, Pt, Au, and Ir1/TiO2 (110) are calculated and shown in Figure 5 (left column). Compared with that on stepped Ir, the difference on Pt and particularly on Au and Ir1/TiO2 (110) is greatly enhanced, which rationalizes well with the selectivity between RWGS and methanation. Combinations of CO dissociation and desorption in the four model catalysts suggest that the difference between the desorption free energies and dissociation barrier of CO on the metal surface is the crucial factor for CO selectivity in RWGS reactions. The CO


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selectivities for the RWGS reaction on Au and Ir1/TiO2 (110) are substantially higher than those

5

5

Ir1/TiO2 4

Ir5 Pt5

4

Au5 3

3

2

2

1

1

0

0

Ea(HCO→HC+O)-Ea(HCO→H+CO) / eV

on stepped Pt and Ir.

Ea(CO dissociation)-∆Gdes(CO) / eV

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Figure 5. left: difference between activation energies Ea for CO dissociation and desorption free energies of CO; right: difference of activation energies between HCO→HC+O and HCO→H+CO on PR1-Ir1/TiO2 (Ir1/TiO2), stepped Ir (Ir5), Pt (Pt5), and Au (Au5) surfaces.

As well as the redox mechanism, an associative mechanism, characterized by intermediates, i.e., HCOO or COOH, has been discussed extensively for the RWGS reaction. In the HCOOmediated associative mechanism, the HCOO intermediate may undergo C–O bond scission, leading to the formation of HCO. The HCO intermediate then either decomposes to CH and O (HCO→HC+O, representing the methanation process) or dehydrogenates to CO (HCO→H+CO, representing RWGS), as shown in Figure 5 (right column). Compared to stepped Ir sites, the difference in Ea between the two reactions greatly increases, suggesting that the conclusions by HCOO-mediated associative mechanism is not altered. In the COOH-mediated associative mechanism, the COOH intermediate can undergo C–O bond scission, leading to the formation of CO or COH. Note that COH has been reported to be less stable than its isomer, HCO, on a number of metal surfaces.60 Our calculations further confirm that COH is energetically less favorable than HCO by 1.13 and 0.91 eV, respectively, on the two most promising candidates,


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Ir1/TiO2 (110) and stepped Au. Moreover, HCO binds with the surface through both C and O, but COH binds only through C, which is less favorable for C–O bond scission. The possibility of methanation via a COH intermediated pathway is therefore low.

Figure 6. (a) The CO selectivities and CO2 conversions collected over the Ir/TiO2-1% (black solid squares and open squares), Pt/TiO2-1% (red solid cycles and open solid cycles), and Au/TiO2-1% (blue solid triangles and open solid triangles) catalysts. (b) Activation energies of CH4 over the Ir/TiO2-1% (blue squares) and Pt/TiO2-1% (red cycles) catalysts. No methanation process was observed on Au/TiO2-1% catalyst.

We experimentally investigated the effects of different metal compositions on the catalytic properties using the TiO2-supported 1 wt% Ir, Pt, Au (standard Au) and Ir1/TiO2 catalysts, as seen in Figure 6 and Figure 7. The order of the CO selectivities on the catalyst surfaces was the opposite of the C–O bond dissociation orders, as predicted by DFT. In particular, the CO selectivities on Ir1/TiO2 and the standard Au catalyst were 100% over a wide range of reaction temperature, whereas methanation occurred on the 1% Pt- and Ir-based catalysts. The highest TOF value for the RWGS reaction, 0.32 s−1, was obtained over the standard Au catalyst. On the basis of these results, the Ea values of methane formation on Pt/TiO2-1 and Ir/TiO2-1 were estimated to be 91.0 and 73.7 kJ mol−1, respectively, confirming that methanation was easier


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over the Ir-nanoparticle-based catalyst. These experimental results correspond well with the above DFT calculation predictions.

Figure 7. The calculated TOFs for RWGS and methanation reactions on Ir/TiO2-0.1%, Ir/TiO21%, Pt/TiO2-1%, and Au/TiO2-1% catalysts. Reaction conditions: 0.5 g catalyst; feed gas consists of 45 vol% CO2, 45 vol% H2, and balance N2 with a flow rate of 50 mL min−1; reaction temperature is 350 °C.

4. CONCLUSIONS In summary, we have clearly demonstrated the essential factor that determines CO/CH4 selectivity in the CO2 hydrogenation process. All these DFT calculation results reveal that the C– O bond scission of main intermediates (HCOO-, -COOH, M-CO, et al.) is the rate-limiting step in CO2 hydrogenation for methanation. This is primitively attributed to the difference between desorption energy and dissociation barrier of metal carbonyls as determined by the catalyst structure at atomic level. Accordingly, we developed two strategies for engineering the CO product selectivity by tuning the coordination environment and nature of the supported metals. Single-atom Ir and Au nanoparticles dispersed on a TiO2 support showed high activities and selectivities for RWGS reaction under real working conditions. The synthesis of desired


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inorganic nano-architectures, which have been identified as promising catalysts by first principles, is expected to play a key role in bridging the existing gap between surface science and catalysis. These results may open up new avenues for the rational design of supported catalysts for catalytic CO2 hydrogenation and energy-conversion processes.


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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected]. Author Contributions #

These authors contributed equally.

Notes The authors declare no competing ﬁnancial interest. Supporting Information Simulation details, details of free energy calculations, material characterizations, catalytic results and activation energy evaluations. These materials are available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT The authors acknowledge the National Natural Science Foundation of China (Nos. 21476226, 21506204, 21273224), China Ministry of Science and Technology under contact of 2016YFB0600902, the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDA09030101, XDB17020400), Dalian Science Foundation for Distinguished Young Scholars (2016RJ04) and the Youth Innovation Promotion Association CAS for financial support. The authors also thank the BL14W at the Shanghai Synchrotron Radiation Facility for the XAS experiment.


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Graphical Abstract


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Theoretical Insights and the Corresponding ... - ACS Publications

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