Theoretical and Experimental Evidence for the CarbonâOxygen Group

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Theoretical and Experimental Evidence for the CarbonOxygen Group Enhancement of NO Reduction Jinyang Li, Yirui Wang, Jia Song, Qi Gao, Jia Zhang, Jingyi Zhang, Dong Zhai, Jizhi Zhou, Qiang Liu, Zhi Ping Xu, Guangren Qian, and Yi Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04213 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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Environmental Science & Technology

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Theoretical and Experimental Evidence for the Carbon-Oxygen Group

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Enhancement of NO Reduction

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Jinyang Li †,‡, Yirui Wang ‡, Jia Song†, Qi Gao†, Jia Zhang *,†,‡, Jingyi Zhang †, Dong

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Zhai ‡, Jizhi Zhou †, Qiang Liu †, Zhi Ping Xu †,§, Guangren Qian **,†, Yi Liu ‡

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†

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and Chemical Engineering, Shanghai University, No. 333 Nanchen Rd., Shanghai

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200444, P. R. China;

SHU Center of Green Urban Mining & Industry Ecology, School of Environmental

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‡

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200444, P. R. China;

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§

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Bioengineering and Nanotechnology, the University of Queensland, Brisbane, QLD

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4072, Australia

Shanghai Institute of Materials Genome, Shanghai, No. 99 Shangda Rd., Shanghai

ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for

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Corresponding authors

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* Phone: 86-21-66137746; fax: 86-21-66137761; e-mail: [email protected].

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** Phone: 86-21-66137758; fax: 86-21-56333052; e-mail: [email protected].

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ACS Paragon Plus Environment


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ABSTRACT: The relation between a catalytic center and the surrounding

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carbon-oxygen groups influences the catalytic activity in various reactions. However,

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the impact of this relation on catalysis is usually discussed separately. For the first

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time, we proved that carbon-oxygen groups increased the reducibility of Fe-C bonds

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toward NO reduction. Experimentally, we compared the reductive activities of

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materials with either one or both factors, i.e., carbon-oxygen groups and Fe-C bonds.

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As a result, graphene oxide-supported Fe (with both factors) showed the best activity,

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duration of activity and selectivity. This material reduced 100% of NO to N2 at 300°C.

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Moreover, theoretical calculations revealed that the adsorption energy of graphene for

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NO increased from -13.51 (physical adsorption) to -327.88 kJ/mol (chemical

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adsorption) after modification with Fe-C. When the graphene-supported Fe was

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further modified with carboxylic acid groups, the ability to transfer charge increased

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dramatically from 0.109 to 0.180 |e-|. Therefore, the carbon-oxygen groups increased

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the reducibility of Fe-C. The main results will contribute to the understanding of NO

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reduction and the design of effective catalysts.

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1. INTRODUCTION

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Nitric oxide (NOx) is discharged from industrial combustion and automobiles. It

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has drawn substantial public attention due to its contribution to various environmental

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problems, resulting in danger to ecosystems and human health.1,

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several technologies have been developed to control the discharge of NOx, and

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selective catalytic reduction (SCR) is most commonly used because of its high

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efficiency and selectivity.3 One of the key problems of SCR is the catalyst. In recent

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years, carbon-based transition metal catalysts (CTCs) have been widely investigated

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due to their many desirable advantages: (1) carbon and transition metals are widely

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available;4 (2) carbon has a large specific surface area and a good pore structure for

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physical adsorption as well as for chemical reaction;4 (3) various CTCs, such as

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activated-carbon-supported Cu, V, Ni and Fe, have shown effective catalytic

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activities.5-8 Among these CTCs, Fe was both environmental friendly and

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cost-effective.

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Until recently,

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On one hand, many studies have reported that carbon-metal bonds (Me-C) in

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CTCs play a significant role in NO reduction.9-13 In brief, the metal site is activated by

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interaction with an unstable carbon site, evolving N2 and resulting in C-O complexes.

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Afterwards, decomposition of the C-O complexes regenerates the unstable carbon

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sites. As a result, NO reduction continues.9-11 The reduction of NO is also governed

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by redox reactions, in which Me-C complexes serve as the catalytic centers for the

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transference of oxygen from NO to an external reducing agent.12 In our previous work,

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a sludge-derived catalyst showed activity toward the reduction of NO. During the



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process, the transfer of oxygen and electrons between the metal and carbon

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occurred.13 However, there has been little work on the reductive properties of Me-C

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bonds in NO reduction to date.

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On the other hand, carbon-oxygen groups also influence the activity of NO

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reduction. In general, three viewpoints have been proposed to explain the effects of

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carbon-oxygen groups. First, the carbon-oxygen group itself participates in NO

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reduction.14 With an increase in the content of the groups, the performance also

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increased.7, 10 Second, carbon-oxygen groups facilitate the dispersion of the metal,

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thus increasing the activity. Carbon-oxygen groups have even shown anti-sintering

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properties toward metals in CTCs.15 Third, carbon-oxygen groups are beneficial for

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the adsorption of the reductant, NH3, which helps the following catalytic reduction of

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NO.16 However, until recently, there have been few reports that discuss the effects of

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carbon-oxygen groups on Me-C bonds. Furthermore, no theory has been established

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to elucidate the interactions between carbon-oxygen groups and Me-C bonds.

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Therefore, this work aimed to investigate the influence of carbon-oxygen groups

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on the reducibility of Me-C bonds during NO reduction. For this purpose, graphene

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oxide-supported metal was synthesized to investigate the effect of carbon-oxygen

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groups on reducibility. As is well-known, graphene oxide has a rich content of various

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carbon-oxygen groups. Moreover, a DFT calculation was applied to reveal the

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reducibility of Me-C bonds surrounded by different carbon-oxygen groups. Through

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these investigations, we obtained both theoretical and experimental evidence that

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carbon-oxygen groups enhanced the reducibility of Me-C bonds.


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2. EXPERIMENTAL METHODS

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2.1. Material Preparation. In general, graphene oxide (GO) was prepared by a

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modified Hummers method.17 Graphene oxide-supported Fe (FGx) was synthesized by

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co-precipitation (x stands for the molar ratio of GO:Fe, x = 10, 2, 1.5, 0.5 and 0.1).

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For example, in FG1.5, the GO and Fe(NO3)3·9H2O contents with a molar ratio of 1.5

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were individually dispersed/dissolved in 10 mL of deionized water. The GO solution

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was added dropwise into the Fe solution under vigorous stirring. Then, 10 mL of a

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NaOH solution (molar ratio of Fe:OH = 1:3) was quickly added into the GO and Fe

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solution. The obtained mixture was vigorously stirred for 2 h at room temperature.

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After this period, the precipitate was separated from the mixture and washed four

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times by centrifugation. Finally, the solid was dried at 105°C. A physical mixture of

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Fe3O4 and GO with a molar ratio of 1:1.5 (i.e., a weight ratio of 2:9) was also

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prepared and denoted Fe&GO1.5. In addition, reduced graphene oxide (rGO) was

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synthesized by heating GO at 900°C under N2 for 4 h. The product was used to

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synthesize FrG1.5 through the same process as that used for FG1.5. All materials were

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activated at 300°C, milled to 100 mesh and stored in a desiccator for further use.

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2.2. NO Reduction. In each test, 1.0 g of solid was placed in a quartz reactor with

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an inner diameter of 4 mm (packing density = 1.27 g/mL). First, flowing N2 was used

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to flush the reactor for 30 min to remove residual air. NO gas (990 ppm of NO,

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balance Ar) was then introduced into the reactor at 30.0 mL/min for 30 min to



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stabilize the NO concentration. The outlet stream from the reactor was diluted with Ar

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by mass flowmeters (D07-7B and D07-19B, Beijing Sevenstar Electronics Co., Ltd.)

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to obtain an NO concentration of 51.2 ppm. Here, the NO concentration was detected

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by a chemiluminescence NO-NO2-NOx analyzer (42i, Thermo Scientific). This

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equipment was also used to determine the NO concentration after the reduction

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reaction. In addition, at certain time points, the concentrations of CO, CO2, N2 and

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N2O in the outlet stream were detected by a standard calibration method using GC

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(GC 7900). The gas space velocity was 14400 h-1. After the NO concentration became

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steady, a tube furnace was used to increase the reaction temperature.

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To evaluate the performance of FG1.5 at different temperatures, the temperature of

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the tube furnace was increased from 50 to 500°C. The NO concentration was

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monitored continuously for 20 min at each testing temperature. In addition, the

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performance at 250 and 400°C was examined for 120 min. Moreover, SO2 (500 ppm,

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3.0 mL/min) and O2 (0.03 mL/min) was mixed with NO to evaluate their influences

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on NO removals by FG1.5 and FrG1.5.

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2.3. Computational Simulation. The calculation was carried out through the

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DMol3 package in Materials Studio (Accelrys). The following settings were adopted

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together with the standard Perdew-Burke-Ernzerhof (PBE) generalized-gradient

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approximation (GGA) functional. The DFT-D correction method was included to take

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into account the van der Waals interactions; the DFT semi-core pseudo-potential

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(DSPP) was used for the core treatment, and the double-ξ numerical polarization


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(DNP) basis set was utilized; the convergence thresholds were set to 1×10−5 Ha

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(energy), 0.002 Ha/Å (force) and 0.005 Å (displacement). In addition, the real-space

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global orbital cutoff radius was set to 4.6 Å, and the smearing of electronic

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occupations was 0.005 Ha.

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A graphene sheet model was used in the calculation. This model was composed of

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42 carbon and 18 hydrogen atoms, which was similar to the numbers used in previous

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reports.18, 19 When building the model, the hydrogen atoms were used to saturate the

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carbon atoms with dangling bonds at the edges of the graphene sheet. To reveal the

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reducibility of metal-carbon bonds, an Fe atom was directly linked to C. This setting

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simulated an Fe oxide, which was reduced by the surrounding reductant and activated

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for the adsorption and further reduction of NO. The adsorption energies (∆Eads) of NO

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were calculated by the following equation (1):

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∆Eads = ENO/CTC - ECTC - ENO

(1)

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where ENO/CTC is the total energy after adsorption of NO; ECTC is the total energy of

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the pristine material; and ENO is the total energy of an isolated NO molecule. A

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negative or positive value of ∆Eads indicates an exothermic (-) or endothermic (+)

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process, respectively. Several models were calculated in this work, including

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graphene-supported Fe (GP-Fe), Fe supported on hydroxyl-functionalized graphene

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(OH-GP-Fe), Fe supported on lactone-functionalized graphene (O-GP-Fe), Fe

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supported on carboxylic acid-functionalized graphene (COOH-GP-Fe), graphene (GP)

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and carboxylic acid-functionalized graphene (COOH-GP).

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2.4. Characterization. The carbon-oxygen groups were determined by the

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temperature-programmed decomposition (TPD) technique. In general, 100 mg of

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sample was placed in a tube furnace. The target sample was heated from room

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temperature to 800°C at a rate of 3°C/min under a flow of N2. At the same time, GC

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equipment (GC 7900, Techcomp. Ltd) continuously detected the concentration of

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evolved CO2 from the sample. The curve obtained for the CO2 concentration

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represented the overlap of carbon-oxygen groups.20

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The X-ray photoelectron spectroscopy technique (XPS, ESCALAB, Thermo

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Fisher Scientific) was used to characterize FG1.5 before and after NO reduction. The

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XPS was equipped with a monochromatic Al-Kα source and a charge neutralizer. The

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microstructure of FG1.5 was analyzed using a HITACHI SU-1510 scanning electron

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microscope (SEM). Transmission electron microscopy (TEM) specimens were

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prepared from ion milling after mechanical slicing, polishing and dimpling, and their

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TEM images were recorded at an operating voltage of 200 keV (JEOL JEM2010,

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Japan). A dedicated scanning transmission electron microscope operating at 100 keV

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(STEM; HB501, Thermo/Vacuum Generator, UK) was also used to perform electron

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energy loss spectroscopy (EELS, Model PEELS-666, Gatan, USA) to characterize

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FG1.5.

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3. RESULTS AND DISCUSSION

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3.1 Enhanced NO Reduction by FG1.5. Figure 1A shows the NO reductions by

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GO, Fe3O4, FrG1.5 and FG1.5 at temperatures from 50 to 500°C. Obviously, the FG1.5


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had a better activity than GO, Fe3O4 and FrG1.5 at the tested temperatures. For

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example, FG1.5 removed 72.3% of NO at 250°C. In comparison, the removal of NO

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by GO (34.6%), Fe3O4 (13.9%) and FrG1.5 (8.0%) were all less than 40% at the same

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temperature. Even at a lower temperature of 100°C, FG1.5 removed 21.4% of NO,

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which was larger than the values achieved by the other materials, i.e., 1.5% (GO),

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5.7% (Fe3O4) and 12.9% (FrG1.5). On the other hand, the NO removal by FG1.5

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increased with increasing temperature. Specifically, FG1.5 removed 26.2% of NO at

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150°C. When the temperature increased above 300°C, the removal increased above

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99.7%. However, with increasing temperature, GO achieved a maximum removal of

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37.2% at 200°C. Afterwards, the removal by GO decreased to 11.2% upon increase of

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the temperature to 500°C. For Fe3O4, almost constant activities were observed at the

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tested temperatures, and the highest removal was less than 15%. Although the NO

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removal by FrG1.5 also increased with increasing temperature, its removal was still

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lower than FG1.5. For example, FrG1.5 removed 92.1% of NO at 500oC, lower than

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that of FG1.5 (99.9%). Hence, compared with FrG1.5, FG1.5 had an advantage of

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low-temperature activity at 250°C (72.3%) and 300°C (99.7%). This result indicated

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that carbon-oxygen groups on graphene oxide in FG1.5 accounted for a better removal

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of NO.

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Figure 1B compares the duration of the activities of GO, Fe3O4 and FG1.5 at a

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constant temperature of 400°C. The NO reduction of FG1.5 was maintained at 100%.

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However, Fe3O4 and GO showed removals of only < 20% at the same conditions.

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Therefore, FG1.5 showed not only the highest activity at the same temperature (Figure



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1A and 1B) but also the steadiest activity for NO reduction. Even at 250oC, FG1.5

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showed a stable activity of 70% for NO reduction, which was consistent with the data

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in Figure 1A. Moreover, compared to NO removals at 400oC in Figure 1A, Figure

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S1 (in supporting information) showed that SO2 slightly decreased NO removals by

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FrG1.5 and FG1.5, since SO2 occupied active sites. In other reports, SO2 and H2O both

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decreased carbon-metal activities toward NO removals because of similar reason.21, 22

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Figure S2 showed that O2 almost had no influence on NO removal by these two

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catalysts. Note during NO removal, almost no NO2 was detected. Thus, O2 seemed to

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have little relation with the mechanism of NO removal in this work.

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Considering that FG1.5 was synthesized from the metal and GO, the excellent

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activity and stability seemed to result from the special GO-Fe structures, such as the

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presence of carbon-oxygen groups and Fe-C bonds. Figure 1C compares the

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physically and chemically mixed materials, i.e., FG1.5 and Fe&GO1.5. Although the

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reaction conditions were the same, FG1.5 showed better activity than Fe&GO1.5. As

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illustrated in Figure 1A, FG1.5 was able to achieve a removal of 99.6% at 300°C.

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However, Fe&GO1.5 only achieved a removal of 5.6% at 300°C. Even when the

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reaction temperature was increased to 700°C, the removal was only 32.5%, which was

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still lower than the value of 34.2% obtained by FG1.5 at 200°C. Hence, the mixing

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method of Fe and GO and the resulting GO-Fe bonds had an influence on the activity.

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The gaseous species produced by NO reduction by FG1.5 were also detected, and

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the results are listed in Table 1. At room temperature (R.T.), FG1.5 had no activity. In

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this case, the elements N and O exhibited mass balance before and after reaction.


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When the reaction temperature was 300°C, NO was obviously decomposed, resulting

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in contributions of 25.72 of N2, 24.74 of CO2 and 1.24 of CO. According to the mass

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balance and standard deviation of each gas species, the total N and O remained nearly

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the same before and after reaction. In other words, the nitrogen of NO transformed

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into N2; the oxygen of NO transformed into a big amount of CO2 and a small amount

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of CO. Thus, NO reacted with carbon species to form N2 and CO2. This result was

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consistent with other report and our previous work,11,13 in which NO reacted with

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carbon was reduced to N2. This removal mechanism was also the reason why O2 had

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little influence on NO removal in Figure S2.

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3.2 Experimental Evidence of Carbon-Oxygen Group Enhancement of NO

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Reduction. Figure 2 compares the TPD results for GO, FG1.5 and FrG1.5. The area

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below the curve represents the amount of carbon-oxygen groups. Obviously, GO had

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the largest amount of carbon-oxygen groups. The peak of TPD attained 6000 ppm in

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Figure 2A. When GO was used to synthesize FG1.5, the peak decreased to 3000 ppm.

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At the same time, the amount of carbon-oxygen groups also decreased (Figure 2B).

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When rGO was used to produce FrG1.5, the obtained sample showed a small amount

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of carbon-oxygen groups (Figure 2C). To obtain more detailed information, the

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temperature-dependent curves were fitted to include carboxyl (240-450°C), anhydride

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(350-500°C) and lactone groups (450-800°C). The fitting results are listed in Table 2.

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Generally, the total amount of carbon-oxygen groups decreased in the order of

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GO>FG1.5>FrG1.5. There was 11.81 mmol/g of groups in GO. This value decreased to



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2.23 after the synthesis of FG1.5, which was attributed to the precipitation of Fe and

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the resulting consumption of acidic groups. After activation at 900°C, most of the

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carbon-oxygen groups were removed from the GO. Thus, when the resulting rGO was

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used to synthesized FrG1.5, a minimal amount (0.63 mmol/g) of carbon-oxygen groups

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remained. Among the specific group species, the lactones groups occupied the largest

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fraction in the tested samples. For example, there was 8.10 mmol/g of lactones in GO,

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corresponding to 68.6% of the total groups.

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When we compared the TPD results (Figure 2) with the NO reductions (Figure 1),

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two important results were obtained. On one hand, GO had 11.81 mmol/g of

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carbon-oxygen groups, which was five times more than the amount in FG1.5 (2.23

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mmol/g). However, FG1.5 was far more active than GO in NO reduction at the tested

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temperatures (Figure 1A). On the other hand, FG1.5 and FrG1.5 both contained iron.

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Nevertheless, FrG1.5 showed only minimal activity for NO reduction. Hence, the

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activity difference was attributed to the carbon-oxygen group content in FG1.5.

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According to the above discussions, metal doping and carbon-oxygen groups both

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contributed to the enhanced NO reduction by FG1.5. If either metal doping or

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carbon-oxygen group was introduced on GO, the obtained material showed a lower

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reducibility toward NO.

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3.3 Microstructure and Binding Energy of Fe-C in FG1.5. Figure 3A shows a

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scanning electron micrograph (SEM) of FG1.5. The SEM showed small particles

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doped on sheet-like graphene oxide. This observation was similar to that of other


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reports of Fe-doped GO.23,

24

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transmission electron (HRTEM) and dedicated scanning transmission electron

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microscopy (STEM) images of FG1.5. These characterization methods also displayed

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the microstructure of the Fe-doped GO structure, which was similar to that observed

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by HRTEM in other reports.25, 26 Therefore, FG1.5 contained a typical doping structure.

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Moreover, Figure S3 shows the SEM and HRTEM of FG1.5 after reaction at 400oC.

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Obviously, FG1.5 still showed a doping microstructure, in which Fe distributed on the

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carbon cloth. However, compared to that before reaction, Fe grown bigger. This was

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due to the consumption of carbon species around Fe (Table 1), which resulted in the

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growth of Fe reaction center. However, during the reaction process, the catalyst was

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still stable in activity, according to the performances in Figure 1B.

Figure 3B and 3C show the high-resolution

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Figure 3D shows three parallel measurements (I, II and III) of the Fe L2,3-edge

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spectra. These spectra exhibited two strong L3 and L2 lines (also called white lines),

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which were generated by the transition of electrons from the 2p1/2 and 2p3/2 core levels

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to unoccupied 3d states.27, 28 The relative intensity of these two lines (L3/L2) generally

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corresponds to the valence state of Fe. In Figure 3D, the L3/L2 ratios were 3.4, 3.7

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and 4.1 in the three areas. These ratios were higher than that of Fe0 (L3/L2 = 3.0).29

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Meanwhile, these values were lower than those for most iron oxides, such as FeO

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(L3/L2 = 4.6) and Fe3O4 (L3/L2 = 5.2).30 Thus, part of the Fe was in the reduced state.

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Since FG1.5 was activated at 300°C, the valence of Fe seemed to be reduced by the

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surrounding GO together with evolved CO2/CO. A similar phenomenon was reported

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in which L3/L2 was reduced from 5.5 to 4.6 by CO.30 Several reports even proved that



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the L3/L2 ratio of iron carbide was 3.0.29 Therefore, the binding between GO and Fe

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was different from that of iron carbide in FG1.5, since 3.4, 3.7 and 4.1 are larger than

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3.0 (L3/L2). Moreover, the L3 peak positions of FG1.5 were found to be located at

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709.0 and 712.0 eV. The 709.0 eV position was 0.9 eV higher than that of iron carbide

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(708.1 eV) and 1.1-2.6 eV lower than those of FeIIO (710.1 eV) and FeIII2O3 (711.6).

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However, the 712 eV peak was close to that of FeIII2O3. These comparisons suggested

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that the oxidation state of Fe in FG1.5 was distributed between those of iron carbide

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and iron oxide (FeO). These low-valent Fe species accounted for the high activity of

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FGx.

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Figure 4 compares the XPS results of FG1.5 before and after NO reduction at

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300°C, including the binding energies of the Fe 2p3/2 and C 1s spectra. Before the

289

reaction, the binding energy of Fe was 710.8 eV (Figure 4A). This was consistent

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with the results of EELS (Figure 3D) and XPS of Fe3O4 reported in the literature.31

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Meanwhile, the spectra showed no obvious signal at the position of 708 eV, indicating

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that there was minimal zero-valent iron in the material prior to the reaction. This

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phenomenon also indicated that after being activated at 300°C, the Fe was only

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partially reduced to lower valence states by the surrounding GO. However, after the

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reaction, the binding energy of Fe increased to 711.2 eV, thereby confirming the

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existence of Fe2O3;31 i.e., the low-valent Fe in FG1.5 was oxidized. In addition to the

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peak at a binding energy of 711.2 eV, there was a small signal at 710 eV. This peak

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indicated that some of the Fe was still in a lower valence state. In other words, during

299

the reduction of NO, the Fe was oxidized and reduced in FG1.5.


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Figure 4B shows the binding energies of the C 1s spectra. These two spectra were

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fitted by C-C, C-O, C=O and -COO- groups, the areas of which were also calculated.

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According to the fitting results, the percentage of oxygen-containing carbon in FG1.5

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increased from 0.73 to 1.27 after the reaction, indicating that conversion and

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generation of new oxygen-carbon groups occurred during the reduction of NO. Since

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there were no oxygen sources besides the NO molecule in the whole reaction, the

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oxygen that remained in FG1.5 came from NO reduction, which was consistent with

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the results in Table 1.

308 309

3.4. Theoretical Evidence of the Carbon-Oxygen Group Enhancement of

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Reducibility. Figure 5 shows the calculation models of NO adsorbed on various

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materials. With these models, the adsorption energies were calculated, and the results

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are compared in Table 3. The comparisons indicated several important results.

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First, the adsorption energies of GP and COOH-GP were only -13.51 and -17.37

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kJ/mol. These low values indicated that NO was only physically adsorbed on GP and

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COOH-GP. In other words, graphene and graphene oxide potentially had minimal

316

activity toward NO reduction. This conclusion was consistent with the result shown in

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Figure 1, in which graphene oxide showed low activities at the tested temperatures.

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In addition, since Eads only increased from -13.51 to -17.37 kJ/mol, the carbon-oxygen

319

groups seemed to have little effect on enhancing the activity without Me-C bonds in

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the CTC.



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However, when the graphene was doped with Fe, GP-Fe showed an Eads of

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-327.88 kJ/mol toward the NO molecule. Moreover, when the Fe-C bond was

323

surrounded by carbon-oxygen groups, the adsorption energy further increased. For

324

example, lactone-modified graphene oxide showed an Eads of -343.33 kJ/mol after it

325

was doped with Fe-C bonds. Furthermore, the different carbon-oxygen groups had

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different enhancement effects. The adsorption energy decreased in the order of

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NO/COOH-GP-Fe

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NO/OH-GP-Fe (-339.47 kJ/mol) > NO/GP-Fe (-327.88 kJ/mol). Similarly, the bond

329

distance between the Fe and N atoms also decreased in the above order. Among the

330

materials, COOH-GP-Fe showed the shortest bond distance of 1.625 Å. These results

331

indicated that the functionality of carbon-oxygen groups on graphene definitely

332

increased the adsorption energy of the graphene-supported Fe.

(-366.47

kJ/mol)

>

NO/O-GP-Fe

(-343.33

kJ/mol)

>

333

The positive Hirshfeld atomic charge of Fe-C indicates the ability to donate

334

electrons,32, 33 i.e., reducibility. In NO/GP-Fe, after the Fe-C bonds adsorbed NO,

335

charge of 0.109 |e-| was transferred to the gaseous molecule. Moreover, with the help

336

of the carbon-oxygen groups, the Hirshfeld charges of the Fe-C bonds all increased.

337

For example, the charge increased from 0.109 to 0.134 |e-| after graphene-supported

338

Fe was modified with lactone groups. According to Table 3, the transferred charges

339

of the three configurations were 0.180 (carboxylic acid), 0.134 (lactone) and 0.116

340

(hydroxyl). From the perspective of the NO molecule, the negative Hirshfeld atomic

341

charge indicates the amounts of accept electrons. The tendencies were consistent with

342

those of Fe-C bonds. When graphene-supported Fe was modified with carboxylic acid


Page 17 of 33


343

groups, it showed the biggest transferred charges of -0.178 |e-|. These charges were

344

potentially transferred from Fe-C bonds. Note that the transferred charges from Fe-C

345

bonds (0.180) were quite close to the amounts (-0.178) that NO accepted. Therefore,

346

the carbon-oxygen groups increased the activity of Fe-C bonds toward NO

347

reduction.34, 35

348 349

3.5 Discussions. Based on the above observations and analyses, Fe-C bonds were

350

the basic structure for effective NO reduction in FG1.5. The formation of Fe-C is

351

shown in Figure S1. As is well-known, graphene oxide (GO) has a better electrical

352

conductivity than other carbon species. After the activation of GO-Fe at 300°C, Fe-C

353

bonds formed between Fe and C (in graphene oxide) atoms. Because of the thermal

354

activation procedure, the valence state of the bonded Fe was reduced. As a result, the

355

detected valence state of Fe was between Fe(0) and Fe(III) (XPS results in Figure 4A

356

and EELS results in Figure 3C). However, this bonding species was quite reductive

357

toward NO reduction, which was described in Figure S1. The Fe-C bond acted as the

358

reductive center between NO and the reductant. The bonding also seemed to account

359

for the increased number of Lewis acid sites in FG1.5 (Figure S2).

360

On the other hand, the molar ratio of GO:Fe determined the amount of Fe-C bonds,

361

thus controlling NO reduction (Figure S1). When the molar ratio was too low (< 1.5

362

in Figure S3), i.e., there was too much Fe, the Fe atoms aggregated with each other,

363

and even the reduced Fe (by GO) was re-oxidized in air. When the molar ratio was too

364

high (> 1.5 in Figure S3), i.e., there was too much GO, the GO covered the formed



365

Fe-C bonds, protecting them from contact with NO. However, at a molar ratio of 1.5:1,

366

there was enough GO to reduce the valence state of Fe and form Fe-C bonds. At the

367

same time, the amount of remaining GO exposed part of the bonding, making NO

368

reduction feasible.

369

More importantly, we proved that the presence of carbon-oxygen groups increased

370

the reducibility of Fe-C bonds through theoretical and experimental designs.

371

Experimentally, FG1.5 showed better activity than FrG1.5 and GO (Figure 1). At the

372

same time, FG1.5 also contained many more carbon-oxygen groups than FrG1.5 did

373

(Figure 2). Thus, compared with the isolated participation of Fe-C bonds in NO

374

reduction, the carbon-oxygen groups undoubtedly helped the Fe-C bonds to perform

375

NO reduction by increasing their reducibility. This enhancement was revealed by the

376

results of theoretical DFT calculations shown in Figure 5 and Table 3. Furthermore,

377

according to the calculations, different carbon-oxygen groups had different abilities to

378

increase the reducibility of Fe-C bonds. In this work, carboxylic acid groups were

379

determined to provide the best enhancement. This result was consistent with those of

380

many reports. However, our work was the first to reveal the relation between Fe-C

381

bonds and carbon-oxygen groups in depth.

382

From the viewpoint of application, although V/Ti catalyst is widely used in

383

industrial reduction of NO, vanadium is a well-known hazardous metal,36 which

384

creates excessive cost for the disposal of the spent catalyst. Considering the

385

components of carbon-iron catalyst, it was made up non-toxic elements. Compared

386

with other catalysts, which also showed low-temperature activity together with low


Page 18 of 33

Page 19 of 33


387

toxicity, such as Sm-MnOx,37 the carbon-iron catalyst had the advantage of abundant

388

raw resources. Therefore, carbon-iron catalyst is a potential candidate for green NO

389

reduction at low temperature. The main results of the present work will contribute to

390

the understanding of NO reduction process and the design of effective carbon-iron

391

catalysts. Even though graphene oxide was not used, the carbon-iron catalyst would

392

show enhanced activity in NO reduction, if the catalyst had enough carbon-metal

393

bonds and carbon-oxygen groups. Moreover, if carbon-iron catalyst combined with

394

membrane, it would be much easy to use in a real appellation.

395 396

ASSOCIATED CONTENT

397

Supporting Information

398

Supplementary data associated with this article can be found in the online version.

399

The influence of SO2, O2 and catalyst component on NO removal, the SEM, HRTME

400

and NH3-TPD of catalyst, NO reduction mechanism.

401 402

AUTHOR INFORMATION

403

Corresponding Author

404

*

405

**

406

Notes

407

The authors declare no competing financial interest.

Phone: 86-21-66137746; fax: 86-21-66137761; e-mail: [email protected]. Phone: 86-21-66137758; fax: 86-21-66137761; e-mail: [email protected].

408



409

ACKNOWLEDGMENTS

410

This project was financially supported by the National Nature Science Foundation

411

of China, No. 21707087, No. 21477071 and No. 91543123, the Program for

412

Innovative Research Team in University, No. IRT13078, the Sail plan of Shanghai for

413

Youth, No. 15YF1400100, and a research grant (No. 16DZX2260601) from the

414

Science and Technology Commission of Shanghai Municipality. We appreciate the

415

help of the Instrumental Analysis & Research Center of Shanghai University in

416

sample characterization.

417 418 419 420 421

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Reduced Graphene Oxide/Fe2O3 Composite as a High-Performance Anode Material

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533

Tables

534

Table 1 Gaseous species after NO reduction by FG1.5. R.T 300°C

535 536 537 538

NO 51.20 0.16

N2 N.D. 25.72

CO2 N.D. 24.74

CO N.D. 1.24

N2 O N.D. N.D.

Page 26 of 33

Nt 51.20 51.60

Ot 51.20 50.88

Note: “N.D. stands for “not detected”. The standard deviation of each species was 0.01 (NO), 0.83 (N2), 0.28 (CO2) and 0.27 (CO). Total N (Nt) = NO + 2N2 +2N2O; total O (Ot) = NO + 2CO2 + CO; theoretically, Nt = Ot = original concentration of NO (51.2 ppm).

539 540 541

Table 2. Deconvolution of the carbon-oxygen groups (mmol/g) on GO, FG1.5 and FrG1.5. Species Range (°C) GO FG1.5 FrG1.5 Carboxyl 240-450 0.49 N.D. 0.11 Anhydride 350-500 1.79 0.22 0.12 Lactone 450-800 8.10 2.01 0.40 Total 11.81 2.23 0.63

542

Note: “N.D. stands for “not detected”.

543 544

Table 3. Calculated results for the adsorption of NO on different surfaces. Configuration NO/GP-Fe NO/OH-GP-Fe NO/O-GP-Fe NO/COOH-GP-Fe NO/GP NO/COOH-GP

545

Eads (kJ/mol)

RFe-N (Å)

-327.88 -339.47 -343.33 -366.47 -13.51 -17.37

1.655 1.645 1.630 1.625 N.C. N.C.

Hirshfeld atomic charges (e-) Fe-C

NO

0.109 0.116 0.134 0.180 N.C. N.C.

-0.109 -0.115 -0.134 -0.178 N.C. N.C.

Note: N.C. stood for “not calculated”.

546


Page 27 of 33


547

Figure Captions

548

Figure 1. The NO removal by Fe3O4, GO, FrG1.5 and FG1.5 (A) from 50 to 500°C and

549

(B) at 400°C for 120 min (performance of FG1.5 at 250oC was also supplied); and

550

(C) the difference in NO removal between Fe&GO1.5 and FG1.5.

551

Figure 2. The TPDs of GO (A), FG1.5 (B) and FrG1.5 (C).

552

Figure 3 The SEM (A), HRTEM (B), STEM (C) and EELS (D) images of FG1.5

553 554 555 556 557

activated at 300°C. I, II and III are three parallel measurements of FG1.5. Figure 4. The Fe 2p3/2 (A) and C 1s (B) XPS results of FG1.5 before and after NO reduction at 300°C. Figure 5. Calculation models of NO adsorbed on: (A) GP-Fe; (B) COOH-GP-Fe; (C) O-GP-Fe; (D) OH-GP-Fe; (E) GP; and (F) COOH-GP.

558



100

Page 28 of 33

A FG1.5 Fe3O4

Removal (%)

80

GO FrG1.5

60

40

20

0

50

100

150

200

250

300

350

400

450

500

Temperature (oC)

559 100

B

Removal (%)

80

FG1.5 (400oC)

60

GO (400oC) Fe3O4 (400oC)

40

FG1.5 (250oC)

20 0

0

20

40

60

80

100

120

Time (min)

560 100

Fe&GO1.5

80

Removal (%)

C

FG1.5

60

40

20

0

561

100

200

300

400

500

600

700

Temperature (oC)

562

Figure 1. The NO removals by Fe3O4, GO, FrG1.5 and FG1.5 (A) from 50 to 500°C and

563

(B) at 400°C for 120 min (performance of FG1.5 at 250oC was also supplied); and (C)

564

the difference in NO removal between Fe&GO1.5 and FG1.5.


Page 29 of 33


A

6000 4500 3000

CO2 concentration (ppm)

1500 0

B

6000 4500 3000 1500 0

C

6000 4500 3000 1500 0

0

100

200

300

400

500

600

565

Temperature (oC)

566

Figure 2. The TPDs of GO (A), FG1.5 (B) and FrG1.5 (C).

567


700

800


568

569

570 D

712.0

Fe-L2-edge

Relative Intensity (a.u.)

Fe-L3-edge III 709.0

II 712.0

I 640

571

660

680

700

720

740

760

780

800

Energy loss (eV)

572

Figure 3. The SEM (A), HRTEM (B), STEM (C) and EELS (D) images of FG1.5

573

activated at 300°C. I, II and III are three parallel measurements of FG1.5.


Page 30 of 33

Page 31 of 33



Fe(2p3/2)

710.8

A

711.2

Before Reaction

After Reaction

700

705

710

715

720

725

730

735

740

Binding energy (eV)

574

C-C 284.5


C 1s Before reaction

B

C-O 285.5 C=O 286.5 C(O)O 288.6

C-O:C=0.73

After reaction

C-O:C=1.27

280

285

290

295

300

Binding energy (eV)

575 576

Figure 4. The Fe 2p3/2 (A) and C 1s (B) XPS results of FG1.5 before and after NO

577

reduction at 300°C.

578



579 580

Figure 5. Calculation models of NO adsorbed on: (A) GP-Fe; (B) COOH-GP-Fe; (C)

581

O-GP-Fe; (D) OH-GP-Fe; (E) GP; and (F) COOH-GP.

582


Page 32 of 33

Page 33 of 33


583

Table of Contents

584


Theoretical and Experimental Evidence for the CarbonâOxygen Group

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