Pd-PdO Interface as Active Site for HCOOH Selective Dehydrogenation at Ambient Condition

Transcription

1 Pd-PdO Interface as Active Site for HCOOH Selective Dehydrogenation at Ambient Condition Qing Lv 1,#, Qinglei Meng 1,3,#, Weiwei Liu 2, #, Na Sun 1 Kun Jiang 5, Lipo Ma 1, Zhangquan Peng 1, Wenbin Cai 5, Changpeng Liu 4, Junjie Ge 4, *, Limin Liu 2, *, Wei Xing 1, * 1 State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences Changchun, China. 2 Beijing Computational Science Research Center, Beijing , China 3 University of Science and Technology of China (USTC), Hefei, Anhui , China 4 Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, Changchun, China. 5 Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Centre of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai. # These authors contributed equally to this work. S1

2 Experimental Section Chemicals and materials Vulcan carbon powder XC-72 (Cabot Co.), PdCl 2 (59% Pd, Shanghai Chemical Reagent Co., Ltd), formic acid (HCOOH, 97%, Alfa Aesar), sodium formate dehydrate (HCOONa H 2 O, sinopharm Chemical Reagent Co., Ltd), HCOOD (95%, 99% D, Norell), DCOOD (95%, 98% D, Norell), D 2 O (99.8% D, Cambridge Isotope Laboratories) were all used as received. Highly purified argon ( 99.99%) was supplied by Changchun Juyang Co Ltd. Ultrapure water (resistivity:ρ 18 MΩcm -1 ) was used to prepare the solutions. 1 g PdCl 2 was dissolved into 0.1 M HCl aqueous solution to obtain H 2 PdCl 4 solution. Synthesis of Pd/C catalysts. Pd/C catalysts were synthesized by a microwave-assisted polyol process in ethylene glycol solution. Briefly, mg Vulcan XC-72 was dispersed into 100 ml ethylene glycol in a beaker under ultrasonic treatment for 30 min to form a uniform ink µl H 2 PdCl 4 solution (5.9 mg Pd ml -1 ) was added and stirred for 3 hours. Then proper amount of 1 M NaOH aqueous solution was added to adjust the ph of the solution, with final value varied between 4 and 11. Subsequently, the breaker was placed in the center of a microwave oven (800 W) with microwave heating for 90 s and then 10 s on and 10 s off for twice. The resulting solution was stirred for 8 hours, filtered and washed with abundant water, and dried in an oven at 60 for 12 hours to obtain Pd/C catalysts. Synthesis of PdO/C catalyst. PdO/C was synthesized according to ref. 1. First, mg Vulcan XC-72 was dispersed in 300 ml distilled water under ultrasonic treatment for 40 min µL H 2 PdCl 4 solution (5.9 mg Pd ml -1 ) was added and stirred for 3 hours. Then proper amount of 1 M NaOH aqueous solution was added dropwise to adjust the ph to ca. 11. The obtained mixture was continuously stirred for 8 hours to allow the complete precipitation of Pd 2+. The suspension was subsequently filtered and washed with S2

3 plenty of water, and dried in an oven at 60 for 12 hours. Then, the obtained solid was transferred to a tubular oven and calcined for 2 hours at 260 in O 2 /N 2 to obtain PdO/C catalyst. Surface area calculation for catalysts with varied particle size. The surface areas of varied catalysts were calculated from equation = where SA is surface area of Pd(m 2 /g), ρ is the density of Pd (12.02 g cm -3 ), and d is the average diameter (nm). Electrochemical reduction/oxidation of Pd/C catalyst. To obtain Pd/C catalyst with complete Pd (0) or Pd (II), the catalyst was treated with electrochemical method. Firstly, Pd/C catalyst with mean Pd particle size of around 3 nm was sprayed on carbon paper as work electrodes. The electrochemical reduction/oxidation was performed with an EG&G mode 273 potentiostat/galvanostat and a three-electrode test cell. Nafion membrane was used to isolate the solution in anode and cathode cells. The test cell can be sealed and the pressure in it can be measured with a pressure meter. A Pt foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. To evaluate the potential at which Pd can be oxidized or reduced, cyclic voltammetry experiment was performed in 0.1 M HClO 4 solution at a scan rate of 50 mv s -1 at first. Then the oxidation and reduction reaction was performed at a fixed potential until the current was essentially constant. Subsequently, proper amount of HCOOH + HCOONa solution was added under magnetic stirring to obtain 1.1 M HCOOH M HCOONa. The test cell was sealed immediately. The pressure change in the test cell was measured with the pressure meter and recorded by a computer. The volume of the generated gas was calculated. Catalyst characterization The transmission electron microscope (TEM) images were obtained using a JEOL 2010 microscope operating at 200 kv with nominal resolution. Samples were sonicated and dispersed in EtOH and S3

4 placed dropwise onto a holey carbon support grid for TEM observation. The X-ray diffraction (XRD) patterns were obtained using a Rigaku-D/MAX-PC 2500 X-ray diffractometer with CuK (= Å ) as a radiation source, which operated at 40 kv and 200 ma. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos XSAM-800 spectrometer with an Al Kα radiation source. All elemental analyses of catalyst samples were analyzed by ICP-AES-MS (Inductively Coupled Plasma-Atomic Emission Spectroscopy-Mass Spectrometry) on a Thermo Elemental IRIS Intrepid. Gas generation from HCOOH + HCOONa aqueous solution Typically, the as-prepared catalyst (100 mg) was kept in a round-bottom flask. 10 ml aqueous solution of 1.1 M HCOOH M HCOONa was added to the flask with magnetic stirring (400 r / min) at 30. Before the addition of the solution, Ar was used to remove air in the system. Pressure change of the system was monitored by a pressure meter, which was recorded every half a minute by computer. An empty bottle was connected between the flask and the pressure meter (Scheme S1) to keep the pressure below 50 kpa. The volume of this system has been measured by filling water. The volume of the generated gas can be calculated as follows: V= PV 1 P 0 Where V is the actual volume of generated gas under ambient atmosphere; P 0 is the barometric pressure; V 1 is the void volume of the system; P is the pressure measured by the pressure meter, which is the differential between the pressure in the flask and the barometric pressure. The measurement method has been compared with gas burette collecting method. These two results are consistent, which means the reaction rate is almost not influenced at this pressure. This measurement method can also supply more information which is difficult to be observed with the traditional method, which is shown in the HCOONa decomposition section. TOF was used to determine the intrinsic activity of catalysts. S4

5 = / Where is initial turnover frequency; is the volume of generated hydrogen under ambient atmosphere; R is the universal gas content; T is room temperature; is the mole number of Pd on the surface of the catalysts; and t is the reaction time. To determine the mass activity of Pd in the catalysts, was used. = / Where is the mole of all the Pd in the catalysts, MW Pd is the molar weight of Pd, i.e., g/mol. Cycle test of the catalysts After the decomposition of HCOOH + HCOONa for 2 hours (short time recycle test) or 10 hours (long time recycle test), the catalysts were filtered and washed with copious water, dried at 30 under vacuum. Then the washed catalysts were used for the 2 nd run. In-situ ATR-IR Measurements The ATR-IR measurement was conducted according to ref 2. Pd/C or PdO/C catalyst layer was covered on Au film chemically deposited on the basal plane of a hemicylindrical Si prism using Varian 3100 FT-IR Excalibur spectrometer equipped with an MCT detector, at a spectral resolution of 8 cm -1 with unpolarized IR radiation at an incidence angle of ca. 65. Catalyst ink was prepared by mixing 2 mg catalysts with 1 ml ethanol and 120 µl Nafion (5 wt.%. Aldrich), sonicated for 1 h. Then 100µL of the catalyst ink was transferred onto an electrochemically polished Au film via a pipette. All the spectra are shown in the absorbance unit as log (I/I 0 ), where I and I 0 represent the intensities of the reflected radiation of the sample and reference spectra, respectively. Experimental S5

6 details including chemical deposition of the Au films, preparation of the catalyst ink and the catalyst overlayer on Au films, setup of the ATR cell etc., can be found elsewhere. 3, 4 DFT study of reaction processes of formic acid decomposition on Pd(111) surfaces The energy calculations presented in this paper are performed within the framework of the density functional theory (DFT), using the Vienna ab initio simulation package (VASP) 5-7. The PBE generalized gradient approximation (GGA) is applied for the exchange correlation functional 8 and the projector-augmented wave (PAW) method 9,10 for representing the interaction between valence electrons and core ions. For determining activation energies, reaction pathways are obtained by means of the nudged elastic band (NEB) method as implemented in VASP 11. A spring interaction between adjacent images is added to ensure continuity of the path, thus mimicking an elastic band. An optimization of the band, involving the minimization of the force acting on the images, brings the band to the transition state. It should be noted that the accuracy of the transition state is mainly affected by the number of images or replicas. In order to check whether the 5 images are enough, we also examined the reaction of step 3 (trans-cooh* cis-cooh* on the Pd surface) with 7 images, which shows that the changes of energy barrier is within 2 mev (Fig S19.). The periodically repeated simulation cell includes slabs of four substrate layers for PdO (100) surface and three substrate layers for Pd (111) surface and both them with a vacuum gap between the slabs larger than 16 Å. Simulations are performed within the (2 2) surface unit cell of PdO(100) and Pd(111) respectively. The wave functions are expanded in plane waves up to a kinetic energy cut-off of 520 ev for PdO(100) related calculations and 400 ev for Pd (111) calculations. Integration in the first Brillouin zone is performed using Monkhorst Pack grids including k-points 12. Molecules are adsorbed only on one side of the slab. The adsorbed molecules and the two outer atomic layers of the PdO(100) and the one outer atomic layers of the slab of Pd(111) are allowed to relax, whereas the atom positions in the remaining layers of the slabs are kept fixed. The isolated species in the vacuum have been calculated in the same cell as the slab, with the same computational parameters. Reaction pathways of all the elementary steps involved in formic acid decomposition (FAD) on Pd (111) surface have been investigated by the other theoretical studies To the best of our knowledge, there is no computational studies of HCOOH decomposition on Pd(111) surface. Scheme S6

7 S2 shows the mechanism of FAD considered in this work. FAD was initiated by activating C H bond of the HCOOH to form carboxyl (COOH*). According to the surface morphology of Pd(111) and the interface of Pd(111) and PdO(100), we investigated the most stable configurations of the related molecules adsorbed on the surfaces involved in the reactions, as shown in Fig 4.(c) and (e). The reaction pathway for FAD on Pd(111) was calculated in this work. The energy profile is shown in Figure 4(d). All the total energies are calculated with DFT-PBE functional through optimizing the corresponding configuration, which are further used to determine the adsorption energies and so on. S7

8 Supplementary Figures and Tables Scheme S1. Gas generation and monitoring system. Scheme S2. The Reaction Mechanism of FAD a a Reaction species shown in italic font are the transition states, and those marked with an asterisk (*) are the reaction intermediates adsorbed on the surface. S8

9 Figure S1. XPS results of the Pd polycrystalline surface. S9

10 Figure S2. XRD patterns for all the catalysts. The black and red vertical bars stand for PdO, JCPDS: and Pd, JCPDS: , respectively. S10

11 Figure S3. TEM images and particle size distribution histograms of Pd/C with different average particle sizes of (a, d) 2.5 nm, (b, e) 3.6 nm, (c, f) 4.8 nm, (g, j) 7.2 nm, (h, k) 9.6 nm and (i, l) 16.2 nm. The bar in image (i) indicates 100 nm and bars in all the other TEM images indicate 50 nm. S11

12 Figure S4. Size dependence of initial activity of Pd/C catalysts for gas evolution from 1.1 M HCOOH M HCOONa. Figure S5. TEM image and histogram for PdO/C. S12

13 Figure S6. TEM images and particle size distribution histograms of Pd/C with average particle sizes of (a, b) 3.6 nm and after treated at (c, d) 200, (e, f) 400, (g, h) 600, (i, j) 800 in N 2 for 2 hours. The bars in image (a, c) indicate 50 nm and bars in image (g, i) indicate 100 nm. S13

14 Figure S7. (a) Time-course of gas evolution from 10 ml 1.1 M HCOOH M HCOONa at 30 in the presence of 100 mg Pd/C (ca. 5 wt% Pd) with Pd particle size of 3.5 nm and after calcined in N 2 for 2 hours at temperature between 200 and 800. (b) XPS spectra of Pd 3d for Pd/C catalysts with Pd particle size of 3.5 nm and after calcined in N 2 for 2 hours at temperature between 200 and 800. S14

15 Figure S8. TEM images of (a) PdO/C, (b) PdO/C-H2/N2, (c) Pd/C-9.6 nm, (d) Pd/C-9.6 nm-o2, (e) Pd/C, (f) Pd/C-O2, and (g) Pd/C-H2/N2. S15

16 Figure S9. XPS spectra of Pd 3d for (a) Pd/C-9.6 nm, Pd/C-9.6 nm-o 2 and (b) Pd/C, Pd/C-H 2 /N 2, Pd/C-O 2. Figure S10. The activity change of Pd/C-3.6 nm after redox treatment. S16

17 Figure S11. (a) Optical photograph for carbon paper modified by Pd/C catalyst. (b) Optical photograph for the electrochemical treatment of Pd/C catalysts. Figure S12. (a) Cyclic voltammogram on the Pd/C catalyst in 0.1 M HClO4 solution with a scan rate of 50 mv s -1. (b) Electrochemical oxidation reaction at fixed potential of 0.55 V (vs. SCE). (c) Electrochemical reduction reaction at fixed potential of 0.15 V (vs. SCE). (d) Time-course of gas evolution from 1.1 M HCOOH M HCOONa with the raw, oxidized and reduced Pd/C catalysts at ambient temperature. S17

18 Figure S13. Time-course of gas evolution from 10 ml 1.1 M HCOOH M HCOONa in the presence of 100 mg (a) Pd/C and (b) PdO/C catalysts at different temperatures from 25 to 45. (c) Arrhenius plot for HCOOH decomposition on Pd/C and PdO/C. S18

19 Figure S14. Time-course of gas evolution from 10 ml 0.8 M HCOONa in the presence of (a) Pd/C and (b) PdO/C with and without air in the system. Time-course of gas evolution from 10 ml 1.1 M HCOOH M HCOONa and 1.1 M HCOOH in the presence of (c) Pd/C and (d) PdO/C. Figure S15. Time-course of gas evolution from 10 ml HCOOH + HCOONa + H 2 O, HCOOD + HCOONa + D 2 O and DCOOD + DCOONa + D 2 O in the presence of 100 mg (a) Pd/C and (b) PdO/C catalysts at 30. S19

20 Figure S16. The XPS analysis of the Pt/C catalysts. S20

21 Figure S17. Short time recycle test. TEM images of (a) the raw Pd/C for the 1 st run and (b) recycled Pd/C for the 2 nd run after used for 2 hours. (c) XPS spectrum of the raw Pd/C for the 1 st run and recycled Pd/C for the 2 nd run. (d) Time-course of gas evolution from 10 ml 1.1 M HCOOH M HCOONa in the presence of 100 mg raw Pd/C for the 1 st run, the recycled Pd/C for the 2 nd run, the recycled Pd/C calcined in N 2 at 200 for 2 hours, the recycled Pd/C calcined in O 2 /N 2 at 200 for 2 hours. S21

22 Figure S18. Long time recycle test. TEM images of (a) the raw Pd/C for the 1 st run and (b) Pd/C recycled for the 2 nd run after used for 10 hours. (c) XPS spectrum of the raw Pd/C for the 1 st run and recycled Pd/C for the 2 nd run. (d) Time-course of gas evolution from 30 ml 1.1 M HCOOH M HCOONa in the presence of 100 mg raw Pd/C for 10 hours. (e) Time-course of gas evolution from 10 ml 1.1 M HCOOH M HCOONa in the presence of the raw Pd/C, the recycled Pd/C for the 2 nd run after used for 10 hours, the recycled Pd/C calcined in N 2 at 200 for 2 hours, the recycled Pd/C calcined in O 2 /N 2 at 200 for 2 hours. S22

23 Figure S17d shows that the catalytic activity of Pd/C has a little decrease for the 2 nd run after used for 2 hours. The TEM images (Figure S17a, b) show that the dispersion of Pd nanoparticles has no obvious changes. The decrease of activity should result from adsorption of intermediate species or reduction of PdO in the presence of HCOO -. The percentage of Pd (II) decreases by 1.2% after decomposing FA for 2 hours, calculated from the deconvolution of XPS (Figure S17c). The recycled Pd/C after the 1 st run was calcined in N 2 or O 2 /N 2 at 200 ºC. N 2 was used to exclude the influence of removing the adsorbed intermediate species at high temperature; and O 2 /N 2 was used to reoxidize Pd (0). As shown in Figure S17d, there was no obvious change after calcined in N 2, which indicated the adsorbed intermediate species had been washed away by filtration. However, after calcined in O 2 /N 2, the catalytic activity is recovered and even increased a little, because of the increase of PdO. The phenomenon for the recycle test after 10 hours was similar (Figure S18), except that some aggregation of Pd nanoparticles existed, which decreased the active sites, leading to the activity cannot be recovered completely by oxidation treatment. It demonstrates that the deactivation and reactivity of Pd/C is closely related to the change of PdO content as well as the Pd particle size in Pd/C catalysts. S23

24 Figure S19. The energy profiles of the step 3: trans-cooh* cis-cooh*, calculated with the 5 (black points )and 7 (blue points) images using NEB. The unit is ev. S24

25 Table S1. The nominal and actual loading of Pd and the percentage of Pd (II) in Pd/C catalysts and their initial gas generation rate and TOF. Catalyst Nominal (wt%) ICP-OES (wt%) Percentages of Pd (II) (%) Mass activity (mol H2 g -1 Pd h -1 ) Specfic activity (mmol H2 cm -2 h -1 ) Pd/C-2.5 nm Pd/C-3.6 nm Pd/C-4.8 nm Pd/C-7.2 nm Pd/C-9.6 nm Pd/C-16.2 nm PdO/C NA S25

26 Table S2. TOF and mass activity of reported heterogeneous catalysts for the decomposition of formic acid. Catalyst TOF (h -1 ) [a] Mass activity (mol H2 g -1 Pd h -1 ) T (K) Reference Pd/C Pd/C Pd/C Pd/C Pd/C Pd/C-2.5 nm this work PdAu/C-CeO PdAg/C-CeO Pd-B/C Co 0.30Au 0.35Pd 0.35/C PdNi@Pd/GNs-CB Ag 42Pd 58/C Ag@Pd/C AuPd MnO x /ZIF-8-rGO PdAg-MnO x/n-sio Pd/MSC PdO/C this work [a] The method for statistical quantity of active sites are different in these literature, as the true active sites were not confirmed. Mostly, the quantity of active sites was determined according to the sizes of nano-particles in the references. In our work, CO stripping measurements were used to precisely evaluate the number of active sites as described in the main text. The TOF of Pd/C-2.5 nm and PdO/C exhibited in Table S2 was calculated according to active sites calculated from CO stripping measurements. However, we also calculated the TOFs according to the sizes of nano-particles, similar to method adopted in the references, and the results are 2302 and 3172 h -1 for Pd/C-2.5 nm and PdO/C, respectively, which are also higher than that of the previously reported catalysts. S26

27 Table S3. Adsorption energy of varied species on Pd(111)and PdO(100) through DFT calculation Ea(eV) Pd(111) PdO(100) HCOOH Mid-HCOOH CO CO OH H S27

28 Table S4. The percentages and ratios of the evolved gas generated from different decomposition gas with different catalysts. catalysts-decomposition liquors H 2 / % O 2 / % N 2 / % CO 2 / % H 2 : CO 2 N 2 : O 2 Pd/C-HCOOH+HCOONa : : 1 PdO/C-HCOOH+HCOONa : : 1 PdO/C- HCOONa (much air in the sysem) : : 1 The real role of HCOO - in HCOOH + HCOONa for FAD is an important question. Different from some reported literatures, 2 we demonstrates that HCOONa can be decomposed by both Pd/C and PdO/C catalysts through measuring the change of pressure in the system,as shown in Figure S14. Most of the evolved gas is H 2 (10.5 : 1 for H 2 : CO 2 ) for HCOONa; and the decomposition rate is slow. Without HCOONa, the decomposition rate of HCOOH aqueous solution is also much slower than HCOOH + HCOONa solution. However, the percentages of the evolution gas from HCOOH 7, 12 and HCOOH + HCOONa are similar, ca. 1:1 for H 2 : CO 2. Therefore, for HCOOH + HCOONa solution, HCOOH is decomposed to generate H 2 and CO 2 ; and HCOONa can promote the decomposition rate of HCOOH. In previous literature, 2 it was reported that single HCOONa cannot be decomposed by Pd/C, which may be misled by a large amount of air in the decomposition system. It can be seen that the pressure in the system is decreased within the initial 20 minutes due to the consumption of O 2 during this process (GC spectrum results shows that the ratio of O 2 : N 2 in the evolved gas from HCOONa is much smaller than that in the air), which cannot be observed using traditional burette collecting method. Table S5. KIE values for FAD at 30 with Pd/C and PdO/C. Decomposition liquor Pd/C PdO/C HCOOD + HCOONa + D 2O DCOOD + DCOONa + D 2O S28

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