Highly Active Carbon Supported Pd-Ag Nanofacets Catalysts. for Hydrogen Production from HCOOH

Highly Active Carbon Supported Pd-Ag Nanofacets Catalysts for Hydrogen Production from HCOOH Wenhui Wang,, Ting He,, Xuehua Liu, Weina He, Hengjiang Cong, Yangbin Shen, Liuming Yan, Xuetong Zhang, Jinping Zhang, and Xiaochun Zhou*,, Division of Advanced Nanomaterials, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125, China. Department of Chemistry, College of Sciences, Shanghai University, 99 Shangda Road, Shanghai 200444, China. College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China. Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125, China. Correspondence and requests for materials should be addressed to X. Z. (email: xczhou2013@sinano.ac.cn). S-1

Table of Contents SI.1. Figure S1... 3 SI.2. Figure S2... 3 SI.3. Figure S3... 4 SI.4. Figure S4... 4 SI.5. Figure S5... 5 SI.6. Figure S6... 5 SI.7. Figure S7... 6 SI.8. Figure S8... 6 SI.9. Figure S9... 7 SI.10. Figure S10... 7 SI.11. Figure S11... 8 SI.12. Figure S12... 8 SI.13. Figure S13... 9 SI.14. Figure S14... 9 SI.15. Figure S15... 10 SI.16. Figure S16... 10 SI.17. Experimental details... 11 SI.18. References:... 20 S-2

SI.1. Figure S1 Figure S1. TEM images of as-synthesized Ag nanoplates. 1 (A) Ag nanoplates are flat-lying. (B) Ag nanoplates are stacked together and are standing vertically on their edges. (C) Amplified image of (B). The yellow arrow points the interface of fcc and hcp crystal. (D) Statistics of the edge length. (E) Statistics of the total thickness of Ag nanoplate. (F) Statistics of the thickness of hcp layer. SI.2. Figure S2 Figure S2. TEM images of as-synthesized Pd-Ag nanoplate catalysts with different ratios of Pd to Ag (npd:nag). (A-D) TEM images of Pd-Ag nanoplates by depositing Pd on Ag nanoplates at different mole ratios of npd:nag. (A) npd:nag =8.33 10-4 (B) npd:nag =0.17; (C) npd:nag =0.33; (D) npd:nag =1.0. (E-H) Schematic of the images in (A-D). S-3

SI.3. Figure S3 Figure S3. HRTEM images of as-synthesized Ag nanoplate. (A) Ag nanoplates are flat-lying. The inset shows the HRTEM image of corresponding Ag nanoplate. (B) Ag nanoplates are stacked together and are standing vertically on their edges. (C) HRTEM image of the structure within the red area in (B). Refer Aherne et al. s work for more details of structure analysis. 2 SI.4. Figure S4 Figure S4. HRTEM images of Pd-Ag nanoplate at different ratios of npd:nag. (A) npd:nag=8.33 10-4. (B) Zoom in (A). (C) npd:nag=0.33. (D) Zoom in (C). The hcp layer is destroyed at high ratio of npd:nag. S-4

SI.5. Figure S5 {111} Figure S5. Ag nanoplate has a three-layer structure. These three layers include a defect-induced hcp Ag crystal layer, which is sandwiched between two face centered cubic (fcc) layers. The triangular planes have {111} facet, and the edges have three facets including {111}, {100} and the facet on hcp Ag crystal (abbreviated as {hcp}). SI.6. Figure S6 Figure S6. XRD measurements of Pd-Ag nanoplates with different npd:nag. 1) 1.0, 2) 0.5, 3) 0.33, 4) 0.17, 5) 0.083, 6) 0.042, 7) 8.3e-3, 8) 8.3e-4, 9) 4.2e-4. S-5

SI.7. Figure S7 Figure S7. XRD patterns of Ag nanoparticle, Pd nanoparticle and Pd-Ag nanowire with different npd:nag. 1) Ag nanoparticle, 2) 0.0083, 3) 0.083, 4) 0.17, 5) 0.33, 6) 0.5, 7) 1.0, 8) Pd nanoparticle. SI.8. Figure S8 Figure S8. (A)(B) TEM image of Pd-Ag nanowire catalyst with a large field of vision. (npd:nag = 0.33). (C) TEM image of Pd-Ag nanowire catalyst. (npd:nag = 0.33). (D) Pd and Ag contents across the Pd-Ag nanowire in (C) measured by EDX. We can see that the surface of nanowire is composed of Pd and Ag, while the inner part is mainly composed of Ag. S-6

SI.9. Figure S9 Figure S9. Dark field images of single Ag nanowires and the statistics of the nanowire length. (A)-(D) Examples of dark field images of single Ag nanowires. (E) Statistics on the summary of Ag nanowire length per image. We took more than 200 dark field images with single Ag nanowire randomly, and built a program to calculate the summary of Ag nanowire length per image. The volume of drop, the area of Ag nanowire and the size of image are all known. Then we can calculate the summary of Ag nanowire length per volume through the measurement in Figure S9. This data could be used to calculate the specific area activity (TOF) of side wall. Here, we didn t consider the activity on the two ends of the Ag nanowires, since the aspect ratio of the Ag nanowires in this paper is usually larger than 120 (~50 nm diameter and > 6000 nm length), and the area of the two ends is less than 1% of the area of the side wall. It is very hard and also unnecessary to get the activity of Pd decorated two ends with {111} facet. SI.10. Figure S10 Figure S10. Activity of as-synthesized Ag nanoplate for formic acid (FA) decomposition. The measurement was performed in 5 ml solution with 5 M FA and 2.5 M sodium formate at 90. Any calculation will substrate the volume in green curve. S-7

TOF (h -1 ) SI.11. Figure S11 VH2+CO2 (ml min -1 g -1 Pd) 80k 60k 40k 20k 0 a Nanoplate Nanowire 1E-3 0.01 0.1 1 Ratio (n Ag : n Pd ) b 10k Nanoplate {111}+{100}+{hcp} Nanowire 8k {111} 6k {100} 4k 2k 0 1E-3 0.01 0.1 1 Ratio (n Pd : n Ag ) nh2 (mmol min -1 g -1 Pd) 1600 1200 800 400 0 c Nanoplate Nanowire 1E-3 0.01 0.1 1 Ratio (n Ag : n Pd ) Figure S11. Activity of Pd-Ag nanoplate and nanowire catalysts for FA dehydrogenation at the steady state after reaction for 10 min at different ratios of npd:nag. (a) Volume rate per Pd weight. (b) TOF. (c) The mole rate of H2 per Pd weight per time. The measurement was performed in 5 ml solution with 5 M FA and 2.5 M sodium formate at 90. The catalysts at some ratios of npd:nag have 2-4 parallel samples, and the averge performance of them are presented. SI.12. Figure S12 Figure S12. XRD patterns of Pd-Ag nanoplate after certain storage time, i.e. 0 week (black curve), 3 weeks (blue curve), 4 weeks (green curve). The peak for hcp was still there after 4 weeks. S-8

SI.13. Figure S13 Figure S13. Lifetime measurement of three catalysts. The measurement was performed in 5 ml solution with 5 M FA and 2.5 M sodium formate at 90. Although the activity of Pd-Ag nanoplate catalyst at low ratio of npd:nag, i.e. 8.33 10-4 dramatically drops in the beginning 10 minutes, the remaining activity after 10 minutes is still as high as ~50 k ml min -1 g -1 Pd. SI.14. Figure S14 Figure S14. Kinetic analysis of the Pd-Ag catalysts with different active facets, including {111}, {100} and facet on hcp. The activity is expressed by the volume per minute and per gram Pd. The measurement was performed in 5 ml solution with 5 M FA and 2.5 M sodium formate at 90. S-9

SI.15. Figure S15 Figure S15. GC spectrum. (a) CO measurement for generated gas from FA/SFA solution over Pd-Ag/C nanoplate catalyst using FID-Methanator (detection limit for CO: 10 ppm). (b) H2 measurement using TCD. The concentration of CO is ~60 ppm. SI.16. Figure S16 Figure S16. Meaning of parameters d1, d2 and d3. S-10

SI.17. Experimental details 1) NaOH trap experiment In order to determine H2 to CO2 molar ratio in the gas mixture generated from FA decomposition catalyzed by Pd-Ag/C nanoplate, the generated gas was tested after passing the trap containing 5 M NaOH solution, and the CO2 was captured. Then, the volume of the gas generated from the decomposition of FA and SFA solution was monitored and compared with those without NaOH trap test. It was found that the generated gas was composed of H2 and CO2 with a ratio 0.51:0.49. 2) Reusability of Pd-Ag/C nanoplate catalyst in the dehydrogenation of formic acid Figure S17 shows that the Pd-Ag/C nanoplate catalyst has no significant decrease in catalytic activity after 60 h. Figure S17. Reusability test of Pd-Ag/C nanoplate catalyst (npd:nag = 0.33) toward the FA dehydrogenation in 5 ml solution with 5 M FA and 2.5 M SFA at 90. 3) More details for TOF calculation As we all know that the calculation of TOF value for heterogeneous catalysts is always much more difficult than that for homogenous ones. The difficulty is due to that it is hard to get the accurate value of active site for catalytic reaction on the surface of heterogeneous catalysts. In this paper, we need to consider the active site on different facets on different locations of nanoplate. We agree that the calculation will never be simple. So we add more related S-11

information into the supporting information of this paper, and try our best to express it clearly. We can divide the calculation into several steps: It is necessary to present the principle for the TOF calculation. In this paper, TOF was calculated according to the literature, 3 TOF = (number of produced hydrogen molecule)/(reaction time number of Pd atom on the surface) (S1) We can divide above equation into two parts: P1 = (number of produced hydrogen molecule)/(reaction time) (S2) NPd = number of Pd atom on the surface (S3) Hence, TOF = P1/ NPd (S4) In above equation, the number of produced hydrogen molecule and reaction time is easy to be obtained from experiment. In the experiment, a volume versus time plot in Figure S18 is usually obtained. From the plot, we can get P1 = NAk/2Vm = N A VH 2 CO2 (S5) 2V t m Where Vm is molar volume of gas, i.e. 22.4 L/mol, and NA is Avogadro constant. But the number of Pd atom on the surface (NPd) needs to be specifically calculated for different cases, which is shown as follows. Figure S18. Schematic of volume versus time (V~t) plot for the FA dehydrogenation. Calculate the TOF for the Pd-Ag nanoplate catalyst at low ratio npd:nag ( 0.0417). Based on the TEM research, Figure 2a and d show that Pd prefers depositing onto the edges of Ag nanoplate at low ratio npd:nag ( 0.0417). And in this case, we assumed that the total S-12

weight of Pd in the reactor are located on the Ag nanoplate surface. The total number of Pd atom in the sample is used as the number of active site directly. Actually, many Pd atoms are not on the surface due to the migration of Pd from the surface to the bulk of nanoplate. The purpose of this assumption is to calculate the lower limit of TOF. In real experiment, the Pd atoms also could deposit on two faces of Ag nanoplate. But these Pd atoms will not show high activity due to too much Ag around them. In addition, Pd atoms on the edge surface of nanoplate could diffuse from surface to the body of nanoplate. It is very difficult to calculate the number of Pd atom, which are not on the surface of the edges of nanoplate. But, there is no doubt that the total number of Pd atom in the sample is much larger than the real number of active site. Therefore, according to equation (S4), the real TOF for the Pd-Ag nanoplate catalyst at low ratio npd:nag ( 0.0417) is much larger than the number (9170 h -1 ) given in the paper. We only present the floor level of TOF for the Pd-Ag nanoplate catalyst at low ratio npd:nag ( 0.0417). Here, we will present an example for the TOF calculation of the typical Pd-Ag nanoplate catalyst at low ratio npd:nag=8.33 10-4. From volume versus time plot, P1 in equation (S5) is calculated to be 1.396 10-4 NA h -1. The weight of catalyst is 30 mg, and the Pd content is 0.00541%. Therefore, the number of Pd atom on the surface is NPd = NA 30 10-3 0.00541%/MPd = 1.522 10-8 NA (S6) Where MPd is the molar mass of palladium. Therefore, the TOF will be TOF = P1/ NPd = 1.396 10-4 NA / 1.522 10-8 NA 9170 h -1 (S7) Calculate TOF for the Pd-Ag nanoplate catalyst at high ratio npd:nag (>0.0417). When the ratio npd:nag is above 0.0417, the Pd homogeneously deposits all the zones of Ag nanoplate including the edges and faces. In this case, there forms Pd-Ag alloy nanoplate (TEM in Figure 2 and XRD in Figure 4). The number of Ag atom layers for Ag nanoplate is m Ag d (S8) 2rAg Where, d refers to the thickness of Ag nanoplate; r Ag is the atomic radius of Ag atom. With the deposition of Pd, the thickness of nanoplate increases. According to the amount of added PdCl2 and the size of Ag nanoplate (~88 nm), we could calculate the number of Pd-Ag atom layers. m Ag d h Pd (S9) 2rAg 2rPd S-13

Where, h refers to the increased thickness of nanoplate; r Pd is the atomic radius of Pd atom. Assuming the ratio npd:nag is even in the body of the nanoplate, we can calculate the ratio (θpd) of Pd atom on the surface of two nanoplate faces to total Pd atom 2 Pd (S10) h / 2rPd d / 2rAg Then, the number of Pd atom on the surface of two nanoplate faces N Pd NPdtotal Pd (S11) Here, we will present an example for the TOF calculation of the typical Pd-Ag nanoplate catalyst at high ratio npd:nag= 0.33. From volume versus time plot, P1 in equation (S5) is calculated to be 3.821 10-3 NA h -1. The weight of catalyst is 30 mg, and the Pd content is 2.148%. Therefore, the θpd and the number of Pd atom on the surface is 2 Pd = 2/(2.35+18.46) = 0.0961 (S12) h / 2rPd d / 2rAg NPd = NPdtotal θpd=na 30 10-3 2.148% θpd /MPd = 5.819 10-7 NA (S13) Where MPd is the molar mass of palladium. Therefore, the TOF will be TOF = P1/NPd = 3.821 10-3 NA / 5.819 10-7 NA 6570 h -1 (S14) Calculate TOF for the Pd-Ag nanowire Based on the TEM measurement, we built a structural model for the as-synthesized Ag nanowire. We assume the structural outline of nanowire is a cylinder shape, since the perimeter difference of circle and its inscribed regular pentagon is only about 6%. For the Pd-Ag nanowire, we suppose Pd homogeneously deposits on the side wall of Ag nanowire according to TEM research. Figure S8 shows that the surface of nanowire is composed of Pd and Ag, while the inner part is mainly composed of Ag (Figure S19). So we define the inner part radius is R1 and the Pd-Ag nanowire total radius is R2, including the inner Ag part and the external Pd-Ag ring part. In addition, Pd-Ag alloy is assumed to be on the nanowires according to XRD research. Then the number of Pd on surface is calculated accordingly. NPd 2 R2 L SPd n ' Pd ' ' n Pd n Ag S-14 (S15) Where, L refers to the summary of the length of all Pd-Ag nanowires in reaction, SPd is area of a Pd atom, ' npd and n 'Ag are the Pd and Ag molar number in the Pd-Ag nanoring. L was measured by dark-field microscopy (Figure S9).

' n ' n Pd ' n Pd Ag mr 2 0 2 mr0 6( R 2 0 R 2 1 ) Where nag:npd=6:m; R0 is the radius of as-synthesized Ag nanowire. (S16) Figure S19. Structural model for the Pd-Ag nanowire. Here, we will present an example for the TOF calculation of the typical Pd-Ag nanowire catalyst at low ratio npd:nag= 8.33 10-3. From volume versus time plot, P1 in equation (S5) is calculated to be 3.21 10-4 NA h -1. The weight of catalyst is 30 mg, and the Pd content is 0.0667%. Therefore, according to the structural model, the number of Pd atom on the surface is 2 R S L Pd n ' Pd ' ' n Pd n Ag 2 NPd =7.864 10-8 NA (S17) Where MPd is the molar mass of palladium. Therefore, the TOF will be TOF = P1/NPd = 3.21 10-4 NA / 7.864 10-8 NA 4080 h -1 (S18) Calculate TOF based on Pd and Ag Indeed, several reports have shown that Pd-based catalysts have high activities by synergetic effects between Pd and other metal. But it is obvious that Pd atoms are the crucial active sites in the Pd-based catalysts. As we know, only the Ag atoms located near the Pd atom could interact with Pd atom. In our paper, the content of Pd is usually much lower than that of Ag, and many Ag atoms are far away from Pd atoms. Therefore, not all Ag in the catalyst have synergetic effects with Pd. In order to re-calculate the TOF values, it s needed to find the number of Ag, which have synergetic effects with Pd. Since most of literatures are still using Pd as the reference to calculate the TOF, this paper will use this methodology too. S-15

In literatures, the content of Pd significantly higher than or equal to that of other metal, for example, Pd0.44Ag0.19-Mn0.37/N-SiO2, 4 Co0.30Au0.35Pd0.35/C, 5 and Ag24.2Pd16.6-Hs/G. 6 The mole ratio npd:nag is usually less than 1:2. That is to say, in literatures, usually only less than 2 Ag atoms have synergetic effects on one Pd atom. Therefore, in our work, we could suppose 2 Ag atoms have synergetic effects on one Pd atom, if the content of Pd is much less than that of Ag, i.e. npd:nag is less than 1:2. In addition, if npd:nag is larger than 1:2, the summary of Ag and Pd atoms are used. Based on above rule, we re-calculated the TOF of our catalysts shown in Figure S20. The figure shows that the two activity peaks of Pd-Ag/C nanoplate catalysts at npd:nag=8.33 10-4 and 0.33 is still up to 3060±210 h -1 (90 ) and 2190±20 h -1 (90 ), respectively. In addition, at 60, the new TOF of Pd-Ag nanoplate catalysts at npd:nag=0.33 and 8.33 10-4 is up to 300 h -1 (60 ) and 1170 h -1 (60 ), and the Pd-Ag nanowire catalysts at npd:nag=8.33 10-3 is up to 430 h -1 (60 ). The activity of Pd-Ag{hcp} can reach 6590 h -1 (60 ) based on Pd and Ag. The activity of our catalysts is higher than most of the activity reported recently (Table S1). Figure S20. The TOF of Pd-Ag nanoplate catalysts for FA dehydrogenation at the steady state at different ratios of npd:nag, on the basis of the total amount of Ag+Pd used in the catalyst. The amount of Ag atoms is for the Ag atoms, which have synergetic effects on Pd atom. XPS analysis We carried out the XPS analysis in Figure S21. Figure S21a shows that the Pd 3d spectra of Pd- Ag nanoplate catalyst (npd:nag = 0.33) could be fitted into four symmetric peaks, including the existence of two states of Pd species. The Pd 3d 5/2 peak at 335.1 ev and Pd 3d3/2 peak at 340.4 ev are attributed to the metallic Pd(0), while the binding energy peaks shown at 335.7 and 341.0 ev are attributed to S-16

3d5/2 and 3d3/2 of oxidic Pd(Ⅱ). Higher peak area of Pd(0) indicates that the metallic state Pd is the dominant species on the surface of Pd-Ag nanoplate catalyst (npd:nag = 0.33). 4 Figure S21b shows that the 3d5/2 and 3d3/2 peaks of Ag appear at 368.1 and 374.1 ev, indicating the most Ag will be in oxidic state Ag(I). Figure S21c-d show that the catalyst has no chlorine element, but has abundant oxygen element. Therefore, the oxidic state Ag(I) is possibly a kind of silver oxide. Figure S21e shows the Pd 3d spectra of Pd-Ag nanoplate catalyst (npd:nag = 8.33 10-4 ). Since the content of Pd is so low that the XPS signal is noisy. Nevertheless, we still can identify the typical peaks for Pd 3d. Then, we tried to fit the Pd 3d spectra with four symmetric peaks. Figure S21f shows that peak area of Pd(Ⅱ) is much smaller than that of Pd(0), indicating that metallic Pd dominates on the surface of Pd-Ag/C nanoplate catalyst (npd:nag = 8.33 10-4 ). Figure S21f shows that the 3d5/2 and 3d3/2 peaks of Ag appear at 368.3 and 374.3 ev, indicating the most Ag will be in metallic states. 7 From above results, we can see that the high activity of Pd-Ag nanoplate catalyst is highly possibly due to the metallic state of Pd and Ag. The XPS analysis here is also added into manuscript and SI. S-17

Figure S21. XPS spectra of Pd and Ag for Pd-Ag/C nanoplate catalysts at different ratios, npd:nag = 0.33 and 8.33 10-4. (a)-(d) Pd 3d, Ag 3d, Cl 2p and O 1s for Pd-Ag nanoplate catalyst (npd:nag = 0.33). (e)-(h) Pd 3d, Ag 3d, Cl 2p and O 1s for Pd-Ag nanoplate catalyst (npd:nag = 8.33 10-4 ). S-18

Table S1. Comparison of the catalytic activity data for the currently studied PdAg/C nanoplate catalyst with the prior Pd-based heterogeneous catalyst reported for the dehydrogenation FA. Catalyst T ( ) TOF (h -1 ) Reference Pd-Ag{hcp}/C nanoplate 90 19000 a This work Pd-Ag{hcp}/C nanoplate 60 9790 a This work 3270 b Pd-Ag/C nanoplate (npd:nag=0.33) Pd-Ag/C nanoplate (npd:nag=8.33 10-4 ) Pd-Ag/C nanowire (npd:nag=8.33 10-3 ) 60 300 b This work 60 1170 b This work 60 430 b This work AuPd-MnOx/ZIF-8-rGO 25 382.1 8 PdAg-MnO x/n-sio 2 25 524 4 AgPd-Hs/G 25 333 6 AuPd/C 25 1120 9 Pd-B/C 30 1184 10 (Co 6)Ag 0.1Pd 0.9/RGO 50 2739 11 Pd/MSC-30 50 2623 12 Ag 42Pd 58/ C 50 382 13 Pd/PDA-rGO 50 3810 14 Pd/C_m 60 7256 15 Pd 50Cu 50/resin 75 810 16 Ag@Pd 90 626 17 PdAu/C 92 27 18 PdAu@Au/C 92 300 19 a TOF values calculated based on the amount of surface Pd. (refer to more details for TOF calculation in SI) b TOF values calculated based on the amount of Ag+Pd. We supposed 2 Ag atoms have synergetic effects on one Pd atom, if the content of Pd is much less than that of Ag, i.e. npd:nag is less than 1:2. In addition, if npd:nag is larger than 1:2, the summary of Ag and Pd atoms are used. S-19

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