Luming
Wu
a,
Baoxia
Ni
a,
Rui
Chen
a,
Chengxiang
Shi
a,
Pingchuan
Sun
bc and
Tiehong
Chen
*ac
aInstitute of New Catalytic Materials Science, School of Materials Science and Engineering, Key Laboratory of Advanced Energy Materials Chemistry (MOE), Nankai University, Tianjin 300350, PR China. E-mail: chenth@nankai.edu.cn
bKey Laboratory of Functional Polymer Materials of Ministry of Education, College of Chemistry, Nankai University, Tianjin 300071, PR China
cCollaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, PR China
First published on 23rd September 2019
Ultrafine and highly dispersed PdAu nanoparticles were immobilized on amine functionalized carbon black (VXC-72-NH2) for dehydrogenation of formic acid (FA). The introduction of amines is of vital importance for the formation of ultrafine PdAu nanoparticles (∼1.5 nm). Moreover, the presence of the amino groups also increased the electron density of PdAu nanoparticles, and this effect facilitated the formation of metal-formate, which further enhanced the rate of the catalytic dehydrogenation of FA. The as-prepared Pd0.6Au0.4/VXC-72-NH2 exhibited high catalytic activity and 100% H2 selectivity for dehydrogenation of formic acid without any additive, with turnover frequency (TOF) values of 7385 h−1 at 298 K and 17724 h−1 at 333 K, which are the highest TOF values ever reported among heterogeneous catalysts for FA dehydrogenation.
Among all the available support materials, carbon-based materials with unique properties of high surface area, enhanced mobility of charge carriers and good stability have been widely used.5–7 However, for carbon-based materials, the surface hydrophobicity would result in poor dispersion in aqueous solution, and the lack of surface functional groups is to the disadvantage of interactions with supported metal NPs. Recently, nitrogen (N)-doped carbon has been prepared and used as the support for catalysts, and the electron-rich N dopant could bring about basic coordination sites to facilitate the anchoring of well-dispersed and ultrafine NPs for high activity.8–10 However, in most cases, the procedures presently used for the synthesis of N-functionalized carbon involve high temperature or long pyrolysis time, which would greatly hinder their practical applications.11,12 Hence, controllable surface modification of carbon materials by facile and low temperature methods to support metal NPs with high dispersion remains a critical challenge.
Hydrogen is a sustainable and clean energy carrier for the establishment of a fuel-cell-based hydrogen economy.13 Formic acid (FA, HCOOH) has emerged as one of the most promising hydrogen storage compounds due to its high hydrogen density (4.4 wt%), nontoxicity and excellent stability.14–17 Hydrogen stored in FA can be released via the dehydrogenation pathway (HCOOH → CO2 + H2) and the dehydration pathway (HCOOH → CO + H2O).18,19 The dehydration pathway is an undesired side reaction and should be strictly controlled because CO is highly toxic and capable of poisoning Pt-based fuel cell catalysts. In recent years, many heterogeneous catalysts have been developed for FA dehydrogenation,20–37 and among them amine functionalized graphene, MOFs and mesoporous silica are generally used to support gold (Au), palladium (Pd) and Pd-based alloy NPs. Depending on the nature of support materials and preparation methods, some catalysts exhibited efficient dehydrogenation activities. For instance, Jiang and coworkers prepared AuPd NPs supported on amine functionalized graphene, and the catalyst gave a TOF value of 4445.6 h−1 at 298 K.27 Masuda et al. reported PdAg NPs supported on mesoporous carbon functionalized with p-phenylenediamine, which exhibited a high TOF of 5638 h−1 at 348 K.25 Xu and coworkers prepared highly dispersed palladium nanoclusters immobilized on a nitrogen (N)-functionalized porous carbon support with a TOF of 8414 h−1 at 333 K.10
Compared with graphene, MOFs, mesoporous silica, mesoporous carbon, active carbons and carbon black are industrial-scale produced and cost-effective supports, which have widely been used as the support of noble metals for many industrial catalysts. Herein, we report a facile and efficient approach to synthesize well-dispersed PdAu NPs immobilized on amine functionalized carbon black (VXC-72). The introduction of amines is of vital importance for the synthesis of ultrafine PdAu nanoparticles (∼1.5 nm), and the as-prepared Pd0.6Au0.4/VXC-72-NH2 exhibited high catalytic activity and 100% H2 selectivity for dehydrogenation of formic acid without any additive, with TOF values of 7385 h−1 at 298 K and 17724 h−1 at 333 K, which are the highest TOF values ever reported among heterogeneous catalysts for FA dehydrogenation. This work may provide a general and applicable approach to improve the properties of carbon supported noble metal catalysts, which are widely employed in industrial applications such as hydrogenation, oxidation and electrocatalysis.
(1) |
The transmission electron microscopy (TEM) images of Pd0.6Au0.4/VXC-72-NH2 showed that PdAu NPs with an average particle size of 1.5 nm were homogenously dispersed on VXC-72-NH2 (Fig. 1a–c). In contrast, larger PdAu NPs with a mean size of 3.0 nm were observed for Pd0.6Au0.4/VXC-72 (Fig. 1d–f). In addition, VXC-72 is hard to disperse in water uniformly due to its hydrophobicity (the water contact angle is shown in Fig. 2a), while VXC-72-NH2 functionalized with amine groups exhibited excellent hydrophilicity. As shown in Fig. 2b, VXC-72-NH2 could be uniformly dispersed in aqueous solution, and the catalyst suspension remained stable even after 24 h. The hydrophilicity differences highlight the importance of amine functionalized VXC-72 as a support for mediating the ultrafine size and excellent dispersion of PdAu NPs. The energy-dispersive X-ray spectroscopy (EDX) result confirmed the existence of N, Au and Pd in Pd0.6Au0.4/VXC-72-NH2 (Fig. S1†). The accurate content of Au:Pd in Pd0.6Au0.4/VXC-72-NH2 was determined to be 0.42:0.58 by ICP-AES.
Fig. 1 TEM images and size distributions of Pd0.6Au0.4/VXC-72-NH2 (a–c) and Pd0.6Au0.4/VXC-72 (d–f). |
Fig. 2 Water-contact angle of (a) VXC-72 and (b) VXC-72-NH2 and the corresponding photos of VXC-72 and VXC-72-NH2 suspension. |
Brunauer–Emmett–Teller (BET) surface areas were calculated from the N2 adsorption desorption isotherms of the samples (Fig. S2†). The detailed textural parameters are summarized in Table S1.† It can be seen that compared with VXC-72 (SBET = 120 m2 g−1), the surface area of amine functionalized carbon black was slightly reduced (SBET = 103 m2 g−1). No obvious change was observed after the deposition of Pd and Au onto VXC-72-NH2 (SBET = 108 m2 g−1).
The XRD patterns of the as-prepared catalysts are shown in Fig. 3a. Two broad peaks at around 24.9° and 43.3° were ascribed to the (002) and (200) diffractions of amorphous carbon, respectively.39,40 For the sample Pd0.6Au0.4/VXC-72, the diffraction peak at 38.7° was just between the standard (111) planes of cubic Au (JCPDS: 65-2870) and cubic Pd (JCPDS: 65-2867), indicating the formation of a PdAu alloy structure.41–43 For the sample Au/VXC-72-NH2 the diffraction peaks at 38.2° could be assigned to the (111) plane of metallic Au (JCPDS: 65-2870). However, Pd/VXC-72-NH2 and Pd0.6Au0.4/VXC-72-NH2 showed no obvious diffraction peaks, indicating that ultra-small Pd and PdAu particles were well dispersed on the support.24 Therefore, the presence of amines was the key factor for the formation of the ultrasmall PdAu NPs. Moreover, after annealing of Pd0.6Au0.4/VXC-72-NH2 in an Ar atmosphere (773 K, 3 h), there appeared a diffraction peak at about 38.7°, confirming that the PdAu NPs were in an alloy structure (Fig. S3†).42 In the Raman spectra (Fig. 3b) the wide bands at around 1344 and 1591 cm−1 were ascribed to the disordered graphite carbon (D-band) and sp2-carbon (G-band) within aromatic carbon rings, respectively.44 After acid treatment, the D/G ratio increased, indicating a decrease of the sp2-hybridized structure that generates defects and edge planes. The D/G ratio of VXC-72-NH2 remained almost the same as that of acid treated VXC-72, implying that amine functionalization induced no damage to the structure of the carbon matrix.
Fig. 3 (a) XRD patterns of the as-synthesized catalysts and (b) Raman spectra of the as synthesized VXC-72 supports. |
X-ray photoelectron spectroscopy (XPS) investigation confirmed the existence of abundant O and N species after amine functionalization of VXC-72 (Fig. 4 and Table S2†). The binding energy of Au and Pd in Pd0.6Au0.4/VXC-72-NH2 indicated the valence of metallic Au0 and Pd0 (Fig. 5).45 Additionally, the XPS results showed that the binding energy of Pd 3d in Pd0.6Au0.4/VXC-72-NH2 was shifted to a higher value relative to that in Pd/VXC-72-NH2, while binding energies of Au 4f in Pd0.6Au0.4/VXC-72-NH2 were shifted to the lower value compared with that in Au/VXC-72-NH2. These shifts could be attributed to a partial electron-transfer from Pd to Au, which could be explained by the lower electronegativity of Pd than Au (Pd 2.2, Au 2.4) due to the alloy effect between Pd and Au. Moreover, the Pd 3d and Au 4f peaks of Pd0.6Au0.4/VXC-72-NH2 were both shifted to lower binding energies in comparison to those of Pd and Au on Pd0.6Au0.4/VXC-72, and the shifts indicated that some electrons were transferred from the VXC-72-NH2 substrate to the PdAu NPs, confirming the strong interaction between the VXC-72-NH2 support and PdAu NPs.41
Fig. 4 XPS spectra of O 1s for (a) VXC-72, (b) acid treated VXC-72 and (c) VXC-72-NH2 and N 1s for (d) VXC-72 (e) acid treated VXC-72 and (f) VXC-72-NH2. |
Fig. 5 XPS spectra of (a) Pd 3d and (b) Au 4f in Pd0.6Au0.4/VXC-72-NH2, Pd0.6Au0.4/VXC-72, Pd/VXC-72-NH2 and Au/VXC-72-NH2. |
Fig. 6 shows the catalytic activity of the as-prepared catalysts for the dehydrogenation of FA at 298 K. It can be seen that without Pd addition, the Au/VXC-72-NH2 catalyst did not show any activity. The catalyst Pd0.6Au0.4/VXC-72-NH2 showed a high catalytic activity and 245 mL of gas was released within 8 min at 298 K, affording a turnover frequency (TOF) value of 7385 h−1, the highest value at room temperature reported so far. In contrast, the catalyst Pd0.6Au0.4/VXC-72 showed a much lower activity for the decomposition of FA under the same conditions, over which only 73 mL of gas was released in 20 min (Fig. 6c). The calculated TOF value of Pd0.6Au0.4/VXC-72-NH2 (7385 h−1) was greater than that on the Pd0.6Au0.4/VXC-72 catalyst (497 h−1) by a factor of 15.
Obviously, the significant enhancement of the catalytic performance of Pd0.6Au0.4/VXC-72-NH2 could be ascribed to the hydrophilic VXC-72-NH2 support for the synthesis of ultrafine PdAu NPs. The catalytic activity is much higher than that of Pd or Pd-based alloy supported by amine functionalized rGO,21,24,26,27,37 mesoporous carbon,25 MOFs,29,35 and mesoporous silica,32–34,36 as listed in Table 1. As reported in the literature,15 for supported noble metal catalysts, the interaction between the metal and support is closely related to the catalytic activity. The functionalization of carbon black by amine converted the hydrophobic surface to a hydrophilic surface, which was beneficial to the formation of highly dispersed ultrafine PdAu NPs. Meanwhile the presence of amine groups modulated the electronic structure of PdAu NPs, and all these factors lead to greatly improved activity.
Catalyst | Temp. (K) | Additive | TOF (h−1) | E a (kJ mol−1) | Ref. |
---|---|---|---|---|---|
a TOF values calculated for total metal atoms. b TOF values calculated on surface metal sites or active sites. c Initial TOF values calculated at the initial time or initial conversion of FA. d TOF values calculated at the complete time of gas release. | |||||
Pd0.6Au0.4/VXC-72-NH2 | 298 | None | 7385a,c | 20.6 | This work |
Pd0.6Au0.4/VXC-72-NH2 | 333 | None | 17724a,c | 20.6 | This work |
(Co6)Ag0.1Pd0.9/rGO | 333 | HCOONa | 2739a,d | 43.1 | 1 |
Pd/N-MSC-30 | 333 | HCOONa | 8414a,d | 43.7 | 10 |
Pd@CN900 K | 298 | HCOONa | 5530b,c | 46.9 | 11 |
Pd/PDA-rGO | 298 | HCOONa | 727a,d | 54.3 | 21 |
Pd/S-1-in-K | 298 | HCOONa | 856a,d | 39.2 | 23 |
PdAg/amine-mesoporous carbon | 348 | HCOONa | 5638a,c | — | 25 |
NiPd/NH2–N-rGO | 298 | None | 954.3a,c | — | 26 |
Au0.5Pd0.5/NH2–N-rGO | 298 | None | 4445.6a,c | — | 27 |
Pd/SBA-15-PA | 298 | None | 355a,c | — | 28 |
Pd–NH2-MIL-125 | 305 | None | 214a,c | — | 29 |
Au@SiO2 | 403 | None | 958a,c | — | 31 |
Pd–MnOx/SiO2–NH2 | 323 | None | 1300a,c | — | 32 |
Pd60Au40/ZrSBA-15-AP | 298 | None | 1185a,c | 47 | 33 |
SBA-15-amine/Pd | 298 | None | 293a,c | — | 34 |
AuPd/MIL-101-NH2 | 298 | None | 526a,c | 32.5 | 35 |
CrAuPdN–SiO2 | 298 | None | 730a,c | 49.8 | 36 |
PdAuNi/f-GNS | 298 | None | 1090a,c | 55 | 37 |
AuPd–MnOx/ZIF-8-rGO | 298 | None | 382.1a,c | — | 41 |
Pd0.44Ag0.19–Mn0.37/N-SiO2 | 298 | None | 330a,c | 72.4 | 51 |
Pd0.6Ag0.4@ZrO2/C/rGO | 333 | HCOONa | 4300a,d | 50.1 | 52 |
The gas mixture generated from FA catalyzed by Pd0.6Au0.4/VXC-72-NH2 was analyzed by gas chromatography (GC) to be CO2 and H2, and no CO could be detected from the evolved gas (Fig. 7), suggesting that the as-prepared Pd0.6Au0.4/VXC-72-NH2 has a 100% selectivity to H2 from FA. Gas generation over Pd0.6Au0.4/VXC-72-NH2 was completed in 9, 8, 7.5, 7 and 5.5 min at 298, 303, 313, 323 and 333 K, respectively (Fig. 8), corresponding to TOF values of 7385, 8491, 11037, 14179 and 17724 h−1. The apparent activation energy (Ea) was determined to be 20.6 kJ mol−1, which was lower than most of the reported values (Table 1).
The recycling stability of Pd0.6Au0.4/VXC-72-NH2 was also evaluated by further addition of an equivalent of FA to the reaction mixture after the completion of the previous cycle. As shown in Fig. 9, there was no significant decrease in catalytic activity after the fifth run. In addition, the results of XRD (Fig. S4†) and TEM images (Fig. S5†) of Pd0.6Au0.4/VXC-72-NH2 after the fifth run showed no obvious change in the PdAu alloy size. The XPS result showed that after recycled use, the elements N, Au and Pd were stable on carbon black (Fig. S6†), and the atomic percentage is listed in Table S2.† The contents of Au and Pd in the samples determined by ICP-AES are listed in Table S3,† and the results showed that there was no obvious loss of N, Au and Pd in recycled Pd0.6Au0.4/VXC-72-NH2, indicating good catalytic durability and stability of the catalyst.
The results above suggested that VXC-72-NH2 could efficiently anchor the PdAu NPs, preventing aggregation and overgrowth during the synthetic and catalytic processes. The amine groups on the support could serve the following roles. First, –NH2 would convert the surface of carbon black to be hydrophilic and give rise to ultrafine sizes and excellent dispersion of nanoparticles. Second, the alkaline –NH2 group could facilitate O–H bond dissociation of FA to produce –[H2NH]+ and metal-formate species. Moreover, the electron modification of VXC-72-NH2 toward PdAu NPs would increase the electron density of PdAu active centers to facilitate the formation of metal-formate, which enhanced the rate of the catalytic dehydrogenation of FA.46–48 As illustrated in Scheme 2, the alkaline –NH2 group could serve as a proton scavenger, which benefits the O–H bond dissociation in the FA molecule, resulting in the formation of a metal-formate intermediate along with the [H2NH]+ group during the initial step of the reaction. Subsequently, the PdAu–formate species undergo β-hydride elimination to produce CO2 and a palladium hydride species. Finally, H2 is produced from the palladium hydride species and [H]+.21,49,50
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9na00462a |
This journal is © The Royal Society of Chemistry 2019 |