Zhongjie
Yang
ab,
Caoyu
Yang
ab,
Jianyu
Han
a,
Wenshi
Zhao
ab,
Shengxian
Shao
ab,
Siyu
Li
a,
Huiwen
Gao
ab,
Haijiao
Xie
c and
Xiaofei
Zhang
*a
aCAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China. E-mail: stone623717@outlook.com
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
cHangzhou Yanqu Information Technology Co., Ltd., Zhejaing 310003, China
First published on 7th June 2021
Selective electrochemical CO2 reduction to formic acid is an appealing strategy to achieve efficient utilization of CO2. Modification of the catalytic interface with organic linkers has been proven to promote the conversion of CO2 to CO, but the highly selective generation of formic acid is still a challenge. Herein, modification of the interface of graphene oxide (GO) with amide linkages to anchor SnO2 nanoparticles is presented to catalyze the CO2 reduction reaction (CO2RR). As expected, the as-synthesized SnO2/tert-GO shows a competitive high cathodic faradaic efficiency (FE) of 84.4% for formate at −0.96 V and shows long-term stability for the CO2RR. Further, density functional theory (DFT) calculations indicated that amide linkages facilitate the conversion of CO2 to formate.
Among the various products, HCOOH occupies a pivotal position in industry. As an important chemical intermediate, formic acid is undoubtedly strongly needed and widely utilized in textile manufacturing, antibacterial agents, fuel cells and livestock feed.15–17 Recently, immense efforts have been made to produce formic acid with high selectivity.18–21 Homogeneous and heterogeneous catalysts have been widely used.22–26 However some obvious flaws still exist. The separation of homogeneous catalysts requires high energy consumption and a complicated process. Compared with their homogeneous counterparts, heterogeneous catalysts overcome the problem of separation. Commonly used materials including Pd, Pb, Hg, In, and Cd exhibit good catalytic activity for HCOOH, but sometimes their application is hindered by the shortcomings of scarcity and toxicity.27
Among reported emerging catalysts, Sn containing catalysts are of particular interest due to the advantages of cost-effectiveness and nontoxicity. As a result, different catalytic systems including Sn containing alloys and SnO2 with specific crystal planes and composites are developed to improve the yield of HCOOH.28–30 Although some progress has been made, the utilization efficiency of Sn is still very low. Furthermore, the stability of the catalysts and the synergistic effect of multiple components are often low. Therefore, it is necessary to construct a new catalytic system to meet the requirements of high efficiency and stability for the CO2RR.
In recent years, two dimensional (2D) materials have been widely used in catalysis and greatly promote the development of heterogeneous catalysts.31,32 Whether the 2D materials themselves have active sites, or have active components loaded on their surface, they can maximize the utilization of metal atoms and help to increase the material transfer rate. However, only a few cases of Sn based 2D materials have so far been reported for catalyzing the CO2RR, which are subject to limitations of synthetic methods and poor stability. For instance, Li et al. reported a 2D SnO2 nanosheet catalyst for the CO2RR with a faradaic efficiency of more than 80%.30 However low conductivity hindered the transfer of electrons to the carbon black support. More recently, Tsujiguchi et al. explored Sn nanoparticles (NPs) loaded on reduced graphene oxide (rGO) for the CO2RR.28 The synthesized Sn/rGO composites showed stronger CO2 adsorption ability than bare Sn catalysts. Thus, the synergy effect between the active centers and support promotes the catalytic conversion of carbon dioxide. Obviously, the chosen supports not only can stabilize the active sites but also provide additional opportunities to enhance the catalytic activity.
The surface molecular approach has emerged as an effective means of regulating catalytic activity due to straightforwardness and controllability.33 The interaction between the catalysts and reactants determines the overall catalytic activity and selectivity of products. In our previous work, we demonstrated that the introduced amide linkages on the surface of graphene oxide can effectively capture CO2 and further accelerate the reaction of terminal alkyne with CO2 to generate acetylenic acid with Ag active sites.34 Motivated by the above exploration, we constructed SnO2/amide graphene oxide named SnO2/tert-GO, in which the amide group enhances the enrichment of carbon dioxide around tin oxide in ambient atmosphere (Fig. 1). To the best of our knowledge, although the ligand effect is widely studied in the electroreduction of carbon dioxide to carbon monoxide,35 its role in promoting the production of formic acid with high activity has been rarely explored.
Herein, SnO2/tert-GO was synthesized by a simple amide linkage functionalization of graphite oxide powder and hydrothermal transformation of SnCl2 to form SnO2. The as-obtained SnO2/tert-GO showed superior CO2RR catalytic performance compared to SnO2/GO without amide linkages and bare SnO2 NPs, and the FEHCOO− reached 84.4% at – 0.96 V vs. RHE.
In this process, the amide bond is formed by the condensation reaction between the carboxyl group on the surface of the graphene oxide and 4-tert-butylaniline.
CO2 + H + e− + * → OCHO* |
OCHO* + H + e− + * → HCOOH |
The free energy change ΔG of the reaction was calculated as the difference between the free energies of the initial and final states as shown below:
ΔG = ΔE + ΔZPE − TΔS | (1) |
Furthermore, SnO2/graphene oxide composites denoted as SnO2/GO and bare SnO2 NPs are synthesized under similar conditions as a comparison. The difference is that only GO or no substrate is added. The morphology of SnO2/GO remained almost the same as that of SnO2/tert-GO, which can be confirmed in the SEM and TEM images (Fig. S5 and S6a†). Further, the XRD pattern of SnO2/GO and SnO2 reveals the successful synthesis of SnO2 nanoparticles (Fig. 2h and S7†). Elemental mapping analysis shows that all the C, O and Sn elements are evenly distributed on the GO (Fig. S6c–e†). Meanwhile, the structure and morphology of SnO2 NPs are characterized by XRD, SEM and TEM (Fig. S7, S8a, and S8b†). Next, the electronic structures of SnO2/GO and SnO2 are further explored by XPS. The XPS spectra of Sn 3d (Fig. 2i) show peaks at 487.44 eV and 486.82 eV, and what needs to be emphasized is that the XPS peak deviation of SnO2 and SnO2/tert-GO comes from coordination unsaturated Sn–O bonds caused by the graphene oxide substrate. The tin atoms in SnO2/tert-GO have a higher oxidation state compared with SnO2, which helps to increase reactivity.
The electrochemical CO2RR performances of SnO2, SnO2/GO and SnO2/tert-GO were evaluated in CO2-saturated 0.1 M KHCO3 (pH = 6.8) electrolyte in an H-type cell. All catalysts were uniformly drop cast onto carbon papers and subjected to electrolysis. All potentials reported here were measured versus the Ag/AgCl (3.5 M KCl) electrode and then converted to values versus the reversible hydrogen electrode (RHE) without iR compensation. The liquid and gaseous products were detected by 1H NMR spectroscopy and on-line GC, respectively. The electrochemical activities of SnO2, SnO2/GO and SnO2/tert-GO were initially investigated by linear sweep voltammetry (LSV) which was performed at a scan speed of 10 mV s−1 in a CO2-saturated solution of 0.1 M KHCO3 (Fig. 3a). All three catalysts showed obvious catalytic activity indicating that SnO2 was the original active site for catalytic electroreduction of carbon dioxide. Among all three samples, the current density of SnO2 is significantly lower than that of SnO2/GO and SnO2/tert-GO. This indicated that the 2D GO substrate can effectively increase the conductivity for SnO2, which is beneficial for the fast transfer of electrons during the CO2RR. Notably, SnO2/tert-GO displays the largest current density in the potential range −0.6 to −1.3 V vs. RHE. Compared with the carboxyl functional group in SnO2/GO, the amide linkages in tert-GO show stronger CO2 adsorption ability, thus increasing the catalytic activity.31
Product selectivity of HCOOH is a vital evaluation index in the CO2RR. The gaseous products and liquid products were quantified by online GC and 1H-NMR, respectively. In order to obtain suitable electrolytic potentials, SnO2, SnO2/GO and SnO2/tert-GO modified electrodes were electrolyzed at a gradually increasing applied voltage range of −0.66 to −1.26 V vs. RHE for 3600 s, respectively (Fig. S9†). As shown in Fig. 3b and S10,† the maximum FEHCOO− is 62.2%, 75.7% and 84.4% for SnO2, SnO2/GO and SnO2/tert-GO at −0.96 V vs. RHE, respectively. Obviously, SnO2/GO and SnO2 showed lower FEHCOO− than SnO2/tert-GO. The hydrophilicity test is carried out to explore the effect of different linkers on products. The contact angle of SnO2/GO and SnO2/tert-GO is 13.7° and 42.2°, respectively. The weakening of hydrophilicity in SnO2/tert-GO can inhibit the HER reaction to a certain extent, thus enhancing the selectivity of formate (Fig. S12†). To achieve high CO2RR production rates, the concentrations of CO2 at the surface of obtained catalysts should be promoted at the molecular level. However, CO2 molecules are originally dissolved in the liquid electrolyte and hardly supplied to the surface of catalysts due to the low CO2 solubility. The introduced amide linkages on graphene oxide build an effective fence around the active center to capture carbon dioxide.
We also summarized the FEH2 and FECO of the three catalysts, as shown in Fig. S9.† For SnO2/tert-GO, the FEHCOO− and FECO added up to stay above 80% at the applied voltage range of −0.76 to −1.26 V vs. RHE. For SnO2 and SnO2/GO, the FEH2 was obviously higher than that of SnO2/tert-GO. The partial current densities of formate, hydrogen and carbon monoxide were calculated according to the FEs of the products and the LSV curves for SnO2/GO and SnO2/tert-GO. As shown in Fig. 3c, for SnO2/tert-GO, the partial current density of formate is higher than that of hydrogen and carbon monoxide, indicating that the reactions of both carbon monoxide and hydrogen are inhibited, which is also attributed to the GO surface modified by the hydrophobic group. For SnO2/GO, as shown in Fig. 3d, the partial current density of hydrogen is higher than that of carbon monoxide. This result indicated that SnO2 loaded on GO had improved electrical conductivity, and some active sites of GO for the HER were activated.
Furthermore, the electrochemical surface areas (ECSAs) of the cathodes were examined to understand the reaction mechanism. The double layer capacitance values of SnO2, SnO2/GO and SnO2/tert-GO were calculated as 1.82, 6.29 and 10.43 mF cm−2, respectively. By estimating the double-layer capacitance measured via cyclic voltammetry (CV, Fig. S11† and Fig. 3e), it can be inferred that the ECSA of SnO2/tert-GO is larger than that of SnO2/GO and SnO2. It can be concluded that both GO and tert-GO substrates could effectively increase the ECSA and the number of CO2RR active sites. Moreover, the durability of SnO2/tert-GO is obtained at an applied potential of −0.96 V vs. RHE (Fig. 3f). The cathodic current density was maintained at −4.5 mA cm−2 for 12 h with negligible activity decay and the corresponding FEHCOO− was kept at around 84% for the whole electrolysis period, suggesting that SnO2/tert-GO exhibited prominent stability under the test conditions. In addition, Fourier Transform Infrared Spectroscopy (FTIR) is utilized to prove the presence of amide groups and stability of SnO2/tert-GO (Fig. S13†). The signal of bending vibration of N–H in the amide bond appears at 1567 cm−1, and a small amount of reduced signal of carbonyl can be distinguished at 1726 cm−1 in the FITR spectrum of SnO2/tert-GO. After the CO2RR process, the signal of the amide bond still exists, indicating the good stability of amide groups in catalysis. As a result, compared with the reported literature, SnO2/tert-GO shows a competitive performance for CO2-to-formate (Table S1†).
To further understand the origin of catalytic activity of the SnO2/tert-GO catalyst, the calculation models in which a small SnO2 cluster was loaded on GO and tert-GO are established. The SnO2 could be stabilized by the abundant oxygen-containing groups on the surface of graphene oxide. Thus, the interface of SnO2/tert-GO and SnO2/GO is composed of SnO2 and different organic linkers, which could reasonably illustrate the effect of ligand on catalytic activity. The Gibbs free energies of converting CO2 into formic acid were calculated with SnO2 loaded on tert-GO and GO (Fig. 4). Furthermore, the OCHO* intermediate has been regarded as the key intermediate in HCOOH production. We discovered that SnO2/tert-GO is beneficial to form the OCHO* intermediate (ΔG = 0.63 eV). In contrast, SnO2/GO shows a lower value (ΔG = 0.98 eV) for the OCHO* intermediate. These results indicated that the amide linkages in surfaces of tert-GO could accelerate the rate-determining step of *COOH formation, thus, HCOOH formation was enhanced. In order to understand the reason of high selectivity for formate, the theoretical calculations including the competitive pathway of the HER and CO2 to CO are carried out. The free energy (ΔG) of SnO2/tert-GO and SnO2/GO in the HER is 0.95 eV and 1.43 eV, respectively. Meanwhile, the ΔG of SnO2/tert-GO and SnO2/GO in production of CO is 1.53 eV and 1.85 eV, respectively (Fig. S14†). Obviously, both of them show a higher energy barrier compared with production of formate and prove that SnO2/tert-GO facilitates the conversion of CO2 to formate.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ta02780h |
This journal is © The Royal Society of Chemistry 2021 |