Highly Active and Selective Hydrogenation of CO2 to Ethanol by

May 9, 2017 - College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China. ‡ Research Institute of I...
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Highly Active and Selective Hydrogenation of CO2 to Ethanol by Ordered Pd−Cu Nanoparticles Shuxing Bai,† Qi Shao,† Pengtang Wang,† Qiguang Dai,‡ Xingyi Wang,‡ and Xiaoqing Huang*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China Research Institute of Industrial Catalysis, School of Chemistry & Molecular Engineering, East China University of Science & Technology, Shanghai 200237, China

‡

S Supporting Information *

conventional disordered structures.17 Unfortunately, conventional structures usually suffer from low selectivity and limited activity.18,19 From the structural perspective, the ordered phases with strong interaction, charge transfer and orbital rehybridization,20 can provide the predictable control over structural and electronic effects for catalysis optimization, which cannot be afforded by the conventional disordered alloys. Herein, we report highly ordered Pd-Cu nanoparticles (NPs) are highly active, highly selective and stable catalysts toward CO2 hydrogenation to C2H5OH. With optimizing the Pd/Cu ratio as well as supports, the Pd2Cu NPs/P25 delivered high selectivity to C2H5OH of 92.0% and exhibited the highest turnover frequency (TOF) of 359.0 h−1, outperforming all previous catalysts. These catalysts also exhibited excellent stability. Mechanism studies demonstrated the Pd2Cu NPs/P25 underwent CO2 adsorption/ activation, formation of formate intermediates, transformation or hydrogenation of intermediates (formate, *CO, *CH3, *CO insertion to form *CH3CO) and hydrogenation to C2H5OH finally. Among these steps, *CO hydrogenation to *HCO is likely the rate-determining step for the CO2 hydrogenation to C2H5OH. The monodisperse Pd-Cu NPs were synthesized by using a wet-chemical approach (Please see Supporting Information for details). Figure 1a and Figure S1 show transmission electron microscopy (TEM) images of the prepared Pd-Cu NPs, in which the product consists of uniform NPs with spherical shape and an average diameter of 6.5 ± 1.0 nm (inset of Figure 1a). The X-ray diffraction (XRD) patterns of these Pd-Cu NPs (Figure 1b) match well with those of ordered body-centered cubic (B2) PdCu (ICDD No. 01-078-4406).21,22 The high-resolution TEM (HRTEM) images of the Pd-Cu NPs are displayed in Figure 1c-e, where the 0.21 nm spacing corresponds to the (110) plane of the B2 phase of PdCu, consistent with the XRD result. Elemental distributions of Pd-Cu NPs were confirmed by line scans (Figure 1f), where Pd and Cu atoms are uniformly distributed across the Pd-Cu NPs. The overall atomic ratios of Pd to Cu in Pd2Cu NPs, Pd1.5Cu NPs and Pd1Cu NPs are 66.1/33.9, 59.9/40.1 and 51.4/ 48.6, determined by scanning electron microscopy energydispersive X-ray spectroscopy (SEM-EDS, Figure S2). The asprepared Pd-Cu NPs were then deposited on different support (SiO2, CeO2, Al2O3 and P25) for catalytic investigations (see experimental section for details, Figure 1g and Figures S3 and S4).23,24 As shown in Figure S5, whereas only TiO2 phases were

ABSTRACT: Carbon dioxide (CO2) hydrogenation to ethanol (C2H5OH) is considered a promising way for CO2 conversion and utilization, whereas desirable conversion efficiency remains a challenge. Herein, highly active, selective and stable CO2 hydrogenation to C2H5OH was enabled by highly ordered Pd-Cu nanoparticles (NPs). By tuning the composition of the Pd-Cu NPs and catalyst supports, the efficiency of CO2 hydrogenation to C2H5OH was well optimized with Pd2Cu NPs/P25 exhibiting high selectivity to C2H5OH of up to 92.0% and the highest turnover frequency of 359.0 h−1. Diffuse reflectance infrared Fourier transform spectroscopy results revealed the high C2H5OH production and selectivity of Pd2Cu NPs/P25 can be ascribed to boosting *CO (adsorption CO) hydrogenation to *HCO, the rate-determining step for the CO2 hydrogenation to C2H5OH.

T

he excess emission of carbon dioxide (CO2) leads to several environmental issues, such as greenhouse effect,1,2 whereas CO2 hydrogenation provides a way for CO2 elimination and utilization.3−5 Highly valuable products of CO2 hydrogenation include hydrocarbons, carbon monoxide (CO), alcohols, Cn (n > 2) oxygenates, etc.4,6 Among them, ethanol (C2H5OH), as a renewable fuel additive and eminent industrial intermediate, is one of the most desirable products.5 However, achieving high selectivity of C2H5OH production is a challenge because methanol (CH3OH) or mixed alcohols (CxH2x+1OH, x = 1-5) is the main product for CO2 hydrogenation,5,7 whereas direct synthesis of C2H5OH is rarely reported. The unsatisfied selectivity and yield for CO2 hydrogenation to C2H5OH hinder practical application. Therefore, development of efficient catalysts for C2H5OH synthesis is an urgent issue.7 Previous research on catalysts for CO2 hydrogenation to C2H5OH generally centered on platinum (Pt) and its alloy catalysts,7 due to their extraordinary ability for the CO2 activation, whereas its scarcities and high price hamper further development.8,9 To overcome this limitation, many researchers are focusing on exploitation of Pt-free catalyst for effective CO2 hydrogenation to C2H5OH. Among various candidates, palladium (Pd)-based catalysts, as the most robust C-C coupling catalysts, have attracted much attention10−13 because the C-C coupling is the critical step to control the yields of C2H5OH.6 Various Pd-based catalysts have been created to enhance catalytic activity,14−16 but the major catalysts are limited to the © 2017 American Chemical Society

Received: March 28, 2017 Published: May 9, 2017 6827

DOI: 10.1021/jacs.7b03101 J. Am. Chem. Soc. 2017, 139, 6827−6830

Communication

Journal of the American Chemical Society

wt % Pd2Cu NPs/P25, 2.45 wt % Pd2Cu NPs/P25 and 4.91 wt % Pd2Cu NPs/P25 were 90.0, 31.4, 15.3 and 9.7 h−1, respectively and the TOFPd values of C2H5OH were 332.9, 359.0, 376.5 and 224.4 h−1, correspondingly (Figure S8a), showing lower Pd mass loading was more beneficial for CO2 hydrogenation to C2H5OH. Besides the mass loading, the support also plays a role in catalytic performance25 because it can provide new heterogeneous sites,26 change electrical properties,27 etc. Therefore, performances of Pd2Cu NPs over different supports (SiO2, CeO2, Al2O3 and P25) for CO2 hydrogenation were further explored (Figure 2b). CH3OH and C2H5OH yields for Pd2Cu NPs/SiO2, Pd2Cu NPs/CeO2, Pd2Cu NPs/Al2O3 and Pd2Cu NPs/P25 are 5.3, 7.1, 4.0 and 3.6 mmol g−1 h−1, and 14.8, 16.2, 19.7 and 41.5 mmol g−1 h−1, respectively. The Pd2Cu NPs/CeO2 shows the highest CH3OH yield, which is probably because CeO2 can enhance activity of Cu in the conversion of CO2 to CH3OH.28 Among these four catalysts, Pd2Cu NPs/P25 exhibits the best catalytic performance with the highest TOFPd and selectivity of C2H5OH (Figure S8b and Table S1), in which the oxygen vacancies on P25 can facilitate CO2 hydrogenation (Figures S9 and S10).29 Catalytic behaviors of CO2 hydrogenation over different Pd-Cu NPs/P25 are shown in Figure 2c,d. As a reference, commercial 10 wt % Pd/C (10 wt % Pd on activated carbon, Aldrich Company) was tested, which yielded 4.1 and 37.4 mmol g−1 h−1 of CH3OH and C2H5OH (Table S1). As shown in Table S1 and Figure S11, the 1.19 wt % Pd NPs/P25 displayed high CH3OH (8.9 mmol g−1 h−1) and C2H5OH (9.1 mmol g−1 h−1) yields. As shown in Figure 2c, the CH3OH yields exhibit a typical volcano-shaped curve with the ratio of Pd to Cu, as 3.6, 4.7 and 4.0 mmol g−1 h−1 are observed for the 1.23 wt % Pd2Cu NPs/P25, 1.14 wt % Pd1.5Cu NPs/P25 and 1.12 wt % Pd1Cu NPs/P25, respectively. However, the C2H5OH yields decrease with the ratio of Pd to Cu increase, as 41.5, 19.2 and 13.7 mmol g−1 h−1 for the 1.23 wt % Pd2Cu NPs/ P25, 1.14 wt % Pd1.5Cu NPs/P25 and 1.12 wt % Pd1Cu NPs/P25, indicating C2H5OH yields are higher than that for 1.19 wt % Pd NPs/P25 (9.1 mmol g−1 h−1). Therefore, selectivities of C2H5OH on Pd-Cu NPs/P25 (77.5%-90.1%) are higher than on Pd NPs/ P25 and decreases with the diminution of the ratio of Pd to Cu (Figure 2c). The TOFPd values of CH3OH of the 1.19 wt % Pd NPs/P25, 1.23 wt % Pd2Cu NPs/P25, 1.14 wt % Pd1.5Cu NPs/ P25 and 1.12 wt % Pd1Cu NPs/P25 were 79.6, 31.4, 43.8 and 37.5 h−1, respectively and the TOFPd values of C2H5OH increase to 81.7, 359.0, 178.6 and 129.1 h−1, correspondingly. The 1.23 wt % Pd2Cu NPs/P25 displays the best activity for C2H5OH synthesis with the TOFPd is 4.4, 2.0 and 2.8 times higher than those of the Pd NPs/P25, Pd1.5Cu NPs/P25 and Pd1Cu NPs/P25, respectively (Figure 2d). To the best of our knowledge, the TOF of the CO2 hydrogenation to C2H5OH by Pd2Cu NPs/P25 exceeds the previous reported catalysts (Table S2), which is 1.9-fold higher than the reported Au/α-TiO2.29 In addition, we also investigated the effect of temperature on catalytic activity of Pd2Cu NPs/P25. As presented in Figure 3a and Figure S12, CH3OH yields almost remained the same whereas C2H5OH yields increased with the raise of temperature. The C2H5OH selectivities at 150, 175 and 200 °C were 78.1%, 85.9% and 92.0%, respectively, suggesting high reaction temperature was favorable for CO2 hydrogenation to C2H5OH. The Arrhenius plot for C2H5OH production on Pd2Cu NPs/P25 is displayed in Figure 3b and the associated apparent activation energy (Ea) value is ∼161.3 kJ mol−1. The high Ea further infers high temperature is favorable to formation of C2H5OH.29 Catalytic behaviors of the 1.23 wt % Pd2Cu NPs/P25 were

Figure 1. (a) TEM image of Pd2Cu NPs. (b) XRD patterns of different Pd-Cu NPs. HRTEM images of (c) Pd2Cu NPs, (d) Pd1.5Cu NPs and (e) Pd1Cu NPs. (f) STEM line scans of Pd2Cu NPs. (g) TEM image of Pd2Cu NPs/P25. Inset in panel a is size distribution of Pd2Cu NPs. Scale bars in panels a, c-e and g are 20, 2 and 20 nm, respectively.

presented, the Pd-Cu NPs phases could hardly be observed due to their ultralow loadings in the catalysts. We start looking for the optimal catalytic condition for CO2 hydrogenation. The mass loading of noble metal is generally regarded as an important parameter for the catalytic evaluation because it closely correlates with cost and catalytic activity.23 We first explored mass loading effect on catalytic performances and P25 was chosen as the support. Both gaseous and liquid products were analyzed after completion of reaction, (Figures S6 and S7), where only CH3OH and C2H5OH were detected. As shown in Figure 2a and Table S1, as Pd loading increased from 0.43 to 4.91

Figure 2. Achieved product yields of CH3OH and C2H5OH (ROH) and selectivity of C2H5OH of Pd2Cu NPs with different loadings on (a) P25 and (b) over different support in CO2 hydrogenation at 200 °C for 5 h. (c) Achieved product yields of ROHs and selectivities of C2H5OH and (d) comparisons of the TOFPd values of Pd-Cu NPs/P25. Error bars correspond to deviations from three independent experiments.

wt %, CH3OH yields almost remained the same whereas C2H5OH yields went up. The selectivity of C2H5OH increases from 78.7% of 0.43 wt % Pd2Cu NPs/P25 to 96.1% of 2.45 wt % Pd2Cu NPs/P25. To compare the influences from the mass loading, Pd-based TOFs (TOFsPd) were estimated, assuming Pd atoms were the active sites in CO2 hydrogenation to C2H5OH.6 The TOFPd values of CH3OH of 0.43 wt % Pd2Cu NPs/P25, 1.23 6828

DOI: 10.1021/jacs.7b03101 J. Am. Chem. Soc. 2017, 139, 6827−6830

Communication

Journal of the American Chemical Society

Pd atoms ratio (48%) on the surface, the better performance motivtaed by Pd2Cu NPs/P25. To determine how electronic properties and surface atomic ratio influence the catalytic behavior, we investigated the interaction between different catalysts and CO2 or CO2 + H2 mixtures at 150 and 200 °C by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).32 Figure S18a shows the infrared (IR) adsorption spectra after exposure Pd2Cu NPs/P25 to CO2 at two different temperatures, measured by DRIFTS. After exposure to CO2 at 150 °C (black curve), two peaks at 1224 and 1242 cm−1 and four trace peaks at 1396, 1446, 1545 and 1606 cm−1 appeared. Two peaks at 1242 and 1606 cm−1 are assigned to weak adsorption species, carboxylate (CO2δ−), which successfully formed on the surface of Pd2Cu NPs and intermediate for CH3OH or C2H5OH synthesis.28 Trace peaks of unidentate carbonate (υs(OCO), 1396 and υas(OCO), 1446 cm−1) and bidentate carbonate (CO32−, 1224 and 1545 cm−1) correspond to strong adsorption species.33 To investigate the effect of temperature on species adsorbed on catalyst surface, a similar experiment was carried out by increasing the temperature to 200 °C (Figure S18a, red curve). We found CO2δ− species disappeared and the intensities of unidentate carbonate and bidentate carbonate species enhanced, confirming the interaction between the surface and CO2 was enhanced at higher temperature. To investigate further the intermediate species formed in the reaction condition, the Pd2Cu NPs/P25 was exposed in CO2 + H2 at 200 °C, where new IR peaks at 1186, 1309-1396, 1885, 2875 and 2958 cm−1 were observed (Figure S18a, blue curve). The peak at 1186 cm−1 is assigned to alkoxy species, whereas a broad peak at 1309-1396 cm−1 is assigned to formate (HCOO−) species28 and the peaks at 2875 and 2958 cm−1 are assigned for CH stretch modes (ν(CH)). The broad band at 1885 cm−1 is characteristic of 3-fold bridge-bonded adsorption CO species (*CO) on Pd, suggesting C2H5OH is likely formed via *CO insertion onto *CH3 species.6 In addition, the carbonate peaks decreased with these new peaks, suggesting these adsorption species were reduced to HCOO− or CH species.28 Compared with the IR adsorption spectra of Pd2Cu NPs/P25 after exposure to CH3OH (black curve), C2H5OH (red curve), carbon monoxide (CO, blue curve) or methane (CH4, purple curve) (Figure S18b), the two peaks for ν(CH) at 2875 and 2958 cm−1 result with C2H5OH and no observable adsorption peaks of gas CO (2173, 2127 cm−1) and gas CH4 (3016 cm−1) determined to negligible CO and CH4 as products.34 After removal of reactive gases at 200 °C, the peaks of HCOO− and alkoxy species disappeared, indicating only these adsorbed species were generated under the reaction conditions (Figure S18a, purple curve). The above results show the HCOO− species can be a transient species for CO2 hydrogenation to C2H5OH on Pd sites (Scheme S1).6 Owing to the impact of Pd/Cu ratio on activity (Figure 2c), we also studied the interactions of CO2 or CO2 + H2 mixtures with various catalysts at 200 °C. Figure S19a displays adsorptions of CO2 on Pd-based NPs/P25. In all catalysts, bands at 1200-1700 cm−1 are parallel, which suggest similar adsorption ability of various catalysts for CO2 hydrogenation to C2H5OH. After the exposure of Pd-based NPs/P25 to CO2 + H2, the results of Pdbased NPs/P25 are alike except for that of Pd2Cu NPs/P25 (Figure S19b). Compared with that on Pd2Cu NPs/P25 (Figure S19b, red curve), three differences were observed as follows: First, the band of HCOO− species became sharp at 1365 cm−1, instead of a broad peak at 1309-1396 cm−1. Second, a new band at 1572

Figure 3. (a) Achieved product yields of ROHs and selectivity of C2H5OH and (b) Arrhenius plot for the C2H5OH production of 1.23 wt % Pd2Cu NPs/P25 at different temperatures for 5 h. (c) Time course of the CO2 hydrogenation catalyzed by 1.23 wt % Pd2Cu NPs/P25 at 200 °C. (d) Product yields achieved with 1.23 wt % Pd2Cu NPs/P25 and 10 wt % Pd/C over six rounds of successive reactions. Error bars correspond to the deviations from three independent experiments.

further studied by exploring product yield and selectivity of C2H5OH as a function of reaction time. The C2H5OH yield and selectivity of C2H5OH increased linearly to 1.16 mmol and 92.0% before 5 h, and then increased moderately to 1.28 mmol and 93.4% after 10 h (Figure 3c), indicating 5 h was the best catalytic time for CO2 hydrogenation to C2H5OH over 1.23 wt % Pd2Cu NPs/P25. Finally, stability of Pd2Cu NPs/P25 was investigated by recycling the catalytic processes. After six rounds, the C2H5OH yield of the 1.23 wt % Pd2Cu NPs/P25 remained as high as 38.3 mmol g−1 h−1 (92.3% of the initial value) (Figure 3d and Figure S13). The small activity decay after six catalytic rounds is mainly caused by the loss of catalyst during the catalyst collection because Pd2Cu NPs/P25 largely maintained their structures (Figure S14). For comparison, only 20.6% of initial activities could be retained, and aggregation was observed for commercial 10 wt % Pd/C after durability tests (Figure S15). To reveal the reasons for high activity and selectivity for C2H5OH on Pd2Cu NPs/P25, we investigated possible factors in CO2 hydrogenation. Considering the parallel diameters of Pd-Cu NPs, the electronic properties of different Pd-Cu NPs/P25 were investigated by X-ray photoelectron spectroscopy (XPS) (Figure S16). It was revealed that the Pd 3d spectra of Pd NPs/P25 can be divided into four components located at 341.8 eV (Pd2+ 3d3/2), 339.8 eV (Pd0 3d3/2), 336.8 eV (Pd2+ 3d5/2) and 334.8 eV (Pd0 3d5/2).30 The increased binding energy of Pd0 (340.5 and 335.5 eV) and Pd2+ (342.2 and 337.2 eV) in the ordered Pd-Cu NPs/ P25 (Figure S16 and Table S3) suggests charge transfer.6 Based on the calculation, the ratio of Pd0/Pd2+ in Pd-Cu NPs/P25 is higher than that of Pd NPs/P25, as well as ratios of Cu0/Cu2+ over Pd-Cu NPs/P25 (Figure S17 and Table S3), indicating electronic interactions between Pd and Cu can enhance reducibility of surface oxide,31 beneficial for CO2 hydrogenation reaction since the metallic state atoms are considered as the active sites of CO2 activation.32 Considering the C2H5OH selectivities of Pd-Cu NPs/P25 were higher than that of Pd NPs/P25 (Figure 2c), charge transfer in Pd-Cu NPs/P25 can be the reason for improved selectivity. Additionally, the surface Pd/Cu ratios of Pd2Cu NPs/ P25, Pd1.5Cu NPs/P25 and Pd1Cu NPs/P25 were 48/52, 37/63 and 28/72 (Table S3). Considering the TOFsPd of Pd-Cu NPs/ P25 diminish with Pd/Cu ratios reduced (Figure 2d), the higher 6829

DOI: 10.1021/jacs.7b03101 J. Am. Chem. Soc. 2017, 139, 6827−6830

Journal of the American Chemical Society

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cm−1 was observed. Finally, bands of *CO blue-shift from 1885 cm−1 on Pd2Cu NPs/P25 to 1920 cm−1 on other Pd-Cu NPs/ P25, which is assigned to 2-fold bridge-bonded *CO on Pd. The *CO stretching frequency shifts to higher frequencies with increasing *CO coverage,6 suggesting the *CO coverage on other Pd-Cu NPs/P25 is higher than that on Pd2Cu NPs/P25 (Figure S19b). Combined with the result that the surface Pd atomic ratios of Pd1.5Cu NPs/P25 and Pd1Cu NPs/P25 decreased to 37% and 28% (Table S3), the higher *CO coverage may be caused by the lower surface Pd atomic ratio. After aerofluxus the reactive gases (Figure S18c), the slight decrease of the intermediate species over other Pd-based NPs/P25 is different from the complete disappearance of these species over Pd2Cu NPs/P25 (Figure S18c, red curve). With CO as probes, the bands at 1572, 1498 and 1365 cm−1 were generated on all catalysts after CO aerofluxus (Figure S18d), indicating the *CO with high coverage may poison the active sites Pd by forming the stable adsorption species.35 Therefore, *CO hydrogenation to *HCO, which can bring down *CO coverage and thereby weaken *CO poisoning, is likely the rate-determining step for CO2 hydrogenation to C2H5OH.8 The best activity of Pd2Cu NPs/P25 toward CO2 hydrogenation to C2H5OH is related to the 3-fold bridge-bonded *CO species with lower *CO coverage. In conclusion, ordered Pd-Cu NPs were synthesized as highly efficient catalysts toward CO2 hydrogenation to C2H5OH. By tuning catalyst composition and support, the optimized Pd2Cu NPs/P25 exhibited the highest TOFPd, selectivity and yield of C2H5OH, and also excellent durability. The improved selectivity is likely due to the charge transfer between Pd and Cu in the ordered Pd-Cu NPs/P25, which can enhance the reducibility of surface oxide. DRIFTS results showed *CO hydrogenation to *HCO, which is beneficial to decrease *CO coverage and weaken *CO poisoning, is the rate-determining step for CO2 hydrogenation to C2H5OH. The high activity of Pd2Cu NPs/P25 toward CO2 hydrogenation to C2H5OH can be ascribed to low coverage of *CO over Pd atoms as 3-fold bridge-bonded *CO species, which is more easily converted to C2H5OH than 2-fold bridge-bonded *CO species on others Pd-Cu NPs/P25.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b03101.

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Experimental details and data (PDF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Xiaoqing Huang: 0000-0003-3219-4316 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology (2016YFA0204100), the National Natural Science Foundation of China (21571135), Young Thousand Talented Program, the start-up supports from Soochow University, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). 6830

DOI: 10.1021/jacs.7b03101 J. Am. Chem. Soc. 2017, 139, 6827−6830