Rational Synthesis of Porous Graphitic-like Carbon Nitride Nanotubes

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Rational Synthesis of Porous Graphitic-like Carbon Nitride Nanotubes Codoped with Au and Pd as Efficient Catalyst for Carbon Monoxide Oxidation Kamel Eid, Mostafa H. Sliem, Halema Ali Al-Kandari, Mohammed A. Sharaf, and Aboubakr Moustafa Abdullah Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03588 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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Rational Synthesis of Porous Graphitic-like Carbon Nitride Nanotubes Co-doped with Au and Pd as Efficient Catalyst for Carbon Monoxide Oxidation Kamel Eid,a‡ Mostafa H. Sliem,a‡ Halema Al-Kandari,b Mohammed A. Sharaf,c and Aboubakr M Abdullaha*

a.

Center for advanced materials, Qatar University, Doha 2713, Qatar.

b.

Department of Health Environment, College of Health Sciences, Public Authority for Applied Education and Training, P.O. Box 1428, Faiha 72853, Kuwait.

c.

Department of Maritime Transportation Management Engineering, İstanbul University-Cerrahpaşa, Avcilar, Istanbul 34320, Turkey

ABSTRACT: The precise fabrication of efficient catalysts for CO oxidation is of particular interest in a wide range of industrial and environmental applications. Herein, a scalable up method is presented for the controlled synthesis of graphitic-like porous carbon nitride nanotubes (gC3N4NTs) co-doped with Au and Pd (Au/Pd/gC3N4NTs) as efficient catalysts for carbon monoxide (CO) conversion. This includes the activation of melamine with nitric acid in the presence of ethylene glycol and metal precursors followed by consecutive polymerization and carbonization. This drives the formation of porous one-dimensional gC3N4NT with an outstanding surface area of (320.6 m2 g-1) and an atomic level distribution of Au and Pd. Intriguingly, the CO conversion efficiency of Au/Pd/gC3N4NTs was substantially greater than that for the gC3N4NTs. The approach thus presented may provide new avenues for the utilization of gC3N4 doped with multiple metal-based catalysts for CO conversion reactions which had been rarely reported before.

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INTRODUCTION

Noble metals such as Pt, Pd, Ir, and Ru are well-known their superb activities in various catalytic oxidation and reduction reactions.1-9 However, their low-abundance in the nature, high-cost, and instability of noble-metals are the stumbling blocks that preclude their practical catalytic applications. Among these applications, the CO oxidation reaction is of great potential in numerous environmental remediation and industrial applications.10,11 Various efforts were devoted for developing efficient catalysts for CO oxidation and came to fruition in tailoring the synthesis of noble metals, metal oxides, and transition metals for various end use applications.12-15 Noble metals are among the most effective catalysts for low-temperature CO oxidation.16, 17 Unlike other noble metals, Au-based and Pd-based catalysts are famed for their outstanding CO oxidation activities at lower temperatures.18-26 This is originated from the unique ability of Au and Pd to precisely tailoring the adsorption of CO molecules and O2 dissociation, which determine the reaction kinetics.27-29 Meanwhile, Pd possesses an outstanding affinity for decreasing the CO-Pd interaction which prompts the CO-oxidation at low temperature along with the high tolerance for the binding of CO2 product.30 Various Au- and Pd-based catalysts on various supports were used for efficient CO oxidation conversion. For example, the CO conversion activity of the Au-bilayer nanostructure on Titania was substantially greater than that of its counterpart Au-monolayer nanostructures on Titania, attributed to the bilayer effect and the direct electronic interaction with titania.31 Coupling between Au and Pd can greatly enhance the CO oxidation activity and durability. This is owing to both the synergetic and electronic effects between Au and Pd which were preferred for the enhancement the reactant NTs adsorption and the retardation in the adoption of reaction products.

21

Meanwhile, combined Au with Pd enriches the O2 adsorption/activation along with

weakening the CO-Pd interactions, resulting in the prompt oxidation of the CO at low temperature. 32-37

For instance, Pd/Au/TiO2 catalyst was significantly more active than Pd/TiO2 and Au/TiO2


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catalysts.38 Likewise, Pd/CeO2-nanorods exhibited a superior CO oxidation activity than CeO2 nanorods.39-40 In addition to the combination between Pd and Au, both the support type and the morphology were a decisive factor in the CO oxidation reaction. In particular, the electronic interaction of support with Pd and/or Au can improve the O2 rate, which determines the CO oxidation kinetics. 18, 41-43

Towards this end, dumbbell Au0.8Pd0.2-FeOx/Al2O3 NCs were prepared by the metal oxide

overgrowth resulted in an enormous enhancement of the CO oxidation activity in comparison to that of Au0.75Pd0.25/ Al2O3 and Au-FexOy/Al2O3.44 Unlike other metal oxide supports such as SiO2, Fe2O3, TiO2, and CeO2, porous gC3N4NTs possess more thermal/chemical stability, high surface area, low cost, and high tolerance to poisoning species during catalytic reactions.

45-48

Also,

gC3N4NTs with its outstanding electron density can interact strongly with the metal-based catalysts. One should note that the nanotube structure with its inherent porosity can absorb a greater amount of O2 to enhance the overall CO oxidation activity. It is noteworthy that the utilization of gC3N4 based nanostructures as support for CO oxidation reactions had been rarely reported and had not been emphasized enough relative to other catalytic applications.

37, 45-50

For

example, Cu2O on two-dimensional 2D continuous lamellar g-C3N4 achieved a complete CO conversion at 200 °C, while support-free Cu2O displayed only 28 % conversion at 220 °C.51 This inspired us to contemplate the rational synthesis of gC3N4NTs atomically co-doped with Au and Pd based on the activation of melamine by nitric acid in the presence of ethylene glycol and metal precursors to construct polymer tubular-like structures followed by subsequent thermal annealing. The as-formed gC3N4NTs are featured with porous one-dimensional nanotube morphology and were co-doped atomically with both Au and Pd. Our presented catalyst represents a combination between the unique physicochemical properties of gC3N4NTs (e.g., great thermal/chemical stabilities, porous nanotube morphology, high surface area, low cost, great electron density and conductivity) with the outstanding intrinsically catalytic merits of Au/Pd


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(e.g., great CO/O2 adsorption and their high activation rates for O2). In addition, the simplicity, versatility, template-free and the high mass production of our developed approach are prerequisite features in the practical applications. The CO conversion efficiency of the as-synthesized Au/Pd/gC3N4NTs was benchmarked in reference to the gC3N4NTs.

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EXPERIMENTAL SECTION Materials.

Gold

(III)

chloride

trihydrate

(HAuCL4.3H2O,

99.99

%),

potassium

tetrachloropalladate (II) (K2PdCl4, 99.99 %), melamine (99 %), ethylene glycol, and nitric acid (70 %) were obtained from Sigma-Aldrich Chemie GmbH (Munich, Germany). Synthesis of porous Au/Pd/gC3N4NTs. In the typical synthesis of Au/Pd/gC3N4NTs 1 g of melamine was slowly added to 30 mL of an ethylene glycol solution containing 1 mL of HAuCL4.3H2O (20 mM) and 1 mL of K2PdCl4 (20 mM) followed by drop-wise addition of 70 mL of HNO3 (0.1 M) while stirring at room temperature for 30 min. The as-formed yellowish precipitate was washed with ethanol and dried at 80 °C for 12 h prior to annealing at 450 °C (3° / min) for 2 h. After cooling to room temperature, the final product was saved for further characterization. Materials Characterization. The morphology and composition of the as-synthesized materials were carried out by a scanning electron microscope (SEM, Hitachi S-4800, Hitachi, Tokyo, Japan), a transmission electron microscope (TEM, TecnaiG220, FEI, Hillsboro, OR, USA), equipped with an energy dispersive spectrometer (EDS), High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), elemental mapping, and high-resolution TEM (HRTEM). The X-ray photoelectron spectroscopy (XPS) was recorded on a Kratos Axis (Ultra DLD XPS Kratos, Manchester, UK) equipped with a monochromatic Al Kα radiation source (1486.6 eV) under a UHV environment (ca. 5x10-9 Torr). The X-ray diffraction pattern (XRD) was recorded on an X-ray diffractometer (X`Pert-Pro MPD, PANalytical Co., Almelo, Netherlands) using Cu Kα X-ray source (λ = 1.540598 Å). The N2-physisorption isotherms were


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measured on a Quanta chrome Autosorb-1 analyzer (Quanta chrome Instrument Corporation, Boynton Beach, FL, USA). The Fourier transform infrared spectra were recorded on a Thermo Nicolet Nexus 670 FTIR spectrometer (Thermo Scientific, Madison, WI, USA). The Raman spectra were recorded on a PerkinElmer RamanStation 400 spectrometer with a 785 nm laser as an excitation source. CO Oxidation Reaction. The CO conversion reaction on Au/Pd/gC3N4NTs was benchmarked relative to the gC3N4NTs in a fixed bed quartz tubular reactor connected to an online gas analyzer (IR200, Yokogawa, Japan). In particular, 50 mg of each catalyst was initially treated at 250 °C under an O2 flow of 50 mL min-1 for 1 h, followed by a flow of H2 (30 mL min-1) for 1 h. Then, the catalysts were consequently exposed to the reactant gas mixture consisting of 4 % CO, 20 % O2, and balanced was Ar with a total flow of 50 mL min-1 while heating. The percentage of CO conversion (% CO) was calculated using the following equation: %CO = [(COin - COout)]/ COin × 100 Where COin is the input amount and COOut is the output amount of CO. The cyclic voltammogram measurements were carried out on a Gamry electrochemical analyzer (reference 3000, Gamry Co., USA), using a three-electrode cell including a platinum wire, Ag/AgCl, glassy carbon (GC, 5 mm) as counter, reference, and working electrodes, respectively. The GC electrodes were covered with 10 µg of each catalyst followed by the addition of 5 μL Nafion (0.05 %) and left to dry before the measurements.

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RESULTS AND DISCUSSIONS

Scheme1 displays the fabrication process of the Au/Pd/gC3N4NTs where it is the polymerization of melamine in an ethylene glycol solution in the presence of both Pd and Au precursors with the assistance of nitric acid followed by carbonization. In particular, nitric acid resulted in polymerized melamine with the formation of melon sheets as revealed by the formation of a yellowish precipitate (Figure S1a). The as-formed precipitate was filtered and washed with ethanol and then dried at 80 °C to remove any


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impurities (Figure S1b). Then, the obtained yellowish powder was carbonized by thermal annealing at an elevated temperature (Figure S1c). This clearly displays the inherent capacity towards high mass production of the Au/Pd/gC3N4NTs. The SEM images displayed the formation of monodispersed and uniform one-dimensional nanotube morphology (Figure 1a-b). The TEM image of Au/Pd/gC3N4NTs also revealed the production of porous nanotube structure with an average length of 1.3 µm and width of 95 nm (Figure 1c-d). The high magnification TEM image of a single nanotube showed its smooth surface and well-defined porous interior (Figure 2a). The HRTEM image of a randomly selected area from the nanotube wall showed that the wall was composed of a polycrystalline graphitic layer of nanostructures (Figure 2b). In the meanwhile, the HRTEM image of the core area displayed the polycrystalline phase of carbon nanostructure having various curvatures of the nanosheets (Figure 2c). The resolved Fourier ﬁltered of lattice fringes (FFT) in the core area showed their twisting with multiple crystalline defects and lattice distortion as shown as an insight in (Figure 2c). These defects could plausibly be attributed to the co-doping effect with both Au and Pd. The interplanar distance among the adjacent lattice fringes was determined to be around 0.33 nm, which is assigned to the {002} facet of graphitic carbon structure and had been also demonstrated by the corresponding Fourier-transform (FT) pattern (Figure 2c). Both of the TEM and SEM images showed only the presence of nanotube structure with the absence of any kind of undesired nanocrystals such as nanoparticles, reflects on the uniformity of the as-formed Au/Pd/gC3N4NTs. It is noteworthy that, Au and Pd could not be observed by the TEM because both of them are not nanoparticles but are doped structure at the atomic level inside the carbon skeletal structure. Therefore, we have used the EDX, element mapping analysis, and XPS analysis to confirm the presence of Au and Pd as coming in the following sections. The HAADF-STEM image analysis also revealed the fruitful production of porous nanotube morphology with a smooth surface and well-defined thick-walls (Figure 3a). The average inner diameter of the nanotubes is about 70 nm and a wall thickness of 8 nm (Figure 3a). The element mapping analysis was called to get more insight into the composition of the materials thus


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obtained, which clearly depicted the obvious presence of C, N, Au, and Pd through the nanotubes (Figure 3b-e). Interestingly enough, both Au and Pd are found to be coherently distributed through the interior pore and exterior surface of the nanotube. The atomic contents of C, N, Au, and Pd were determined to be 40, 59, 0.52, and 0.48, correspondingly. These estimated atomic contents were almost in line with the initial precursor’s concentrations, demonstrating the purity of the as-obtained materials. The EDX analysis was further carried out to confirm the composition of the materials thus prepared. As expected, gC3N4NTs showed the presence of C and N with an atomic ratio of 45 and 55, respectively without any kind metal impurities except for the Cu element from the TEM copper grid (Figure 2f). Meanwhile, Au/Pd/gC3N4NTs, revealed the existence of C, N, Au, and Pd with atomic ratios of 41, 58, 0.51, and 0.49, correspondingly. This implies that, the ratio of Au/Pd is almost one, demonstrates their equal distribution in the as-made Au/Pd/gC3N4NTs. Meanwhile, the detection of Au and Pd by the EDX analysis implies their presence in the bulk or inside the pore of the nanotubes. Therefore, both element mapping and EDX analysis of Au/Pd/gC3N4NTs displayed the homogenous distribution of Au and Pd inside and outside the nanotubes. Metal-free gC3N4NTs porous nanotubes were prepared as a control via activation of the polymerization and carbonization of melamine in an ethylene glycol solution free of Au and Pd precursors (Figure S2). Porous gC3N4NTs nanotubes with an average length of 1.21 µm and an average width of 93 nm were formed (Figure S2). The average width of the as-fabricated gC3N4NTs was slightly narrower than that of its counterpart Au/Pd/gC3N4NTs which may be attributed to the Au and Pd dopants resulting in an expansion of the lattice. The crystallinity of the as-synthesized Au/Pd/gC3N4NTs and gC3N4NTs was investigated by the XRD analysis which revealed that the diffraction peak at 2 of 27° corresponds to {002} facet of graphitic-like carbon (Figure 4a). It is worth noticing that the diffraction peak of Au/Pd/gC3N4NTs was positively shifted relative to gC3N4NTs which can serve as an indirect evidence for the Au and Pd dopants.


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Meanwhile, we could not resolve any diffraction peaks for Au, Pd and/or their oxides, owing to their low content and the inherent doping at the atomic level in Au/Pd/gC3N4NTs. The electronic structure and surface composition of the Au/Pd/gC3N4NTs and gC3N4NTs were investigated by the XPS analysis. The full scan XPS clearly warranted the presence of C, N, Au, and Pd peaks in Au/Pd/gC3N4NTs, meanwhile, gC3N4NTs showed only the C and N peaks (Figure 4b). There was a slight positive shifting in the binding energies of both C 1s and N1s peaks in Au/Pd/gC3N4NTs relative to their counterparts in gC3N4NTs, which could be originated from the co-doping effect with Au and Pd. The C 1s peak was fitted into two main peaks that were attributed to either graphitic carbon (C-C or C=C) at 284.7 eV and the sp2 carbon-nitrogen (N-C=N) at 286 eV, of the aromatic rings-like such as the s-triazine unit and pyridine-like structure (Figure 4c).52 The N 1s spectrum had been deconvoluted into three peaks that were attributed to the pyridinic-N at 398.3 eV, pyrrolic-N at 399.1 eV, and graphiticN at 401.1 eV, which are ubiquitous features for the carbon nitride-based materials (Figure 4d). The highresolution XPS of Pd 4f displayed the two major peaks for Pd 3d5/2 at 335.0 V and Pd 3d3/2 at 340.5 eV with inferior oxide phases (Figure 4e). Similarly, Au 4f showed two main peaks at 84.3 eV for Au 4f7/2 and 88.2 eV for Au 4f5/2 alongside with insignificant oxide phases (Figure 4f). The surface composition of the Au/Pd/gC3N4NTs showed the presence of C, N, Au, and Pd with atomic contents of 40, 59, 0.51, and 0.49, respectively, which confirms the presence of Au and Pd over the surface of the nanotubes. Intriguingly enough, both Au and Pd were also detect after etching of around 20 nm from the surface of the typically synthesized Au/Pd/gC3N4NTs, which indicates the presence of Au and Pd inside the nanotubes. This results is in line with the EDX and element mapping analysis, which all in all revealed the homogenous atomic distribution of Au and Pd inside and outside the nanotubes. Figure 5 shows the N2-physisorption isotherms measurements of Au/Pd/gC3N4NTs compared to gC3N4NTs.54,57 The surface area and porosity of the as-fabricated materials were calculated using the density functional theory (DFT). The surface area of Au/Pd/gC3N4NTs (320.6 m2 g-1) (Figure 5a) was slightly larger than that of metal-free gC3N4NTs (275.7 m2 g-1) (Figure 5c). This indicates that, Au/Pd/gC3N4NTs could provide more active catalytic sites for the adsorption of reactant molecules during


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the catalytic reactions. The average pore diameter of Au/Pd/gC3N4NTs (57.6 nm) (Figure 5b) was almost close to that of gC3N4NTs (54.8 nm) (Figure 5d). This is in addition to other pores with different diameters in both samples. The porosity of Au/Pd/gC3N4NTs nanotubes was greatly required for accelerating the mass transfer and electron mobility during the catalytic applications. The formation of the Au/Pd/gC3N4NT and gC3N4NT had been additionally confirmed by the FTIR analysis, where both samples depicted the main absorption peaks assigned to the breathing mode of triazine at 810 cm−1 and the stretching mode of C-N heterocycles at 1200-1650 cm-1 (Figure 6). In the meanwhile, the small broad bands between 2900 and 3300 cm-1 could be possibly attributed to N-H vibrations that originated from the uncondensed amine groups. Noticeably, the bands at 2900-3300 cm-1 were less intensive and broader in the Au/Pd/gC3N4NTs than those in its counterpart gC3N4NTs; most plausibly are owing to the doping effect, owing to the strong attraction ability of N towards Au and Pd atoms (Figure 6). This is indicated the acceleration and improvement of the condemnation of the gC3N4NTs through Au/Pd mediated synthesis. Additionally, there is a slight shift in the bands of Au/Pd/gC3N4NTs relative to those of the gC3N4NTs which is an apparent demonstration of the effect of co-doping with Au and Pd. The successful production of the as-synthesized materials was further proved by the Raman spectroscopy using 785 nm laser light as an excitation source, because it is one of the most accurate approaches for investigation the disorder in sp2 carbon materials (Figure S3). Both gC3N4NTs and Au/Pd/gC3N4NTs displayed one prominent wide peak at 1550 cm-1 attributed to the crystalline G band, in line with previous reports (Figure S3).53-54 Meanwhile, the absence of the disordered D-band, indicates the high degree of graphitization of the as-synthesized materials, that is ascribed to the amorphous/crystalline structure of the obtained materials without any resolved lattice fringes as investigated by the HRTEM. Intestinally, both materials displayed a noticed peak at 2680 cm-1, which could be attributed to the symmetrical 2D, indicating their full dispersion.54


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Various reaction experiments were carried out to optimize the fabrication process of the Au/Pd/gC3N4NTs and to understand the reason behind the formation of nanotubes instead of other morphologies such as rods or sheets. The quick addition of melamine to the reaction solution drove the formation of aggregated flakes-like nanostructures (Figure S4a) that implied the significant effect of the slow addition of melamine to produce uniform nanotube structure. The polymerization of melamine with nitric acid in the absence of ethylene glycol formed sheet-like aggregated structure (Figure S4b). This is an unambiguous indication of the role of ethylene glycol as a structural directing agent in the formation of tubular structures. It was further proved through the usage of isopropanol solution instead of ethylene glycol that resulted in the formation of fiber-like architecture (Figure S4c). The fast addition of nitric acid produced non-uniform and aggregated nanotubes (Figure S4d). Unfortunately, we could not get uniform nanotubes atomically doped with Au and Pd, when we change the initial Au and Pd precursors ratios. Taking these results into consideration, the fabrication mechanism of the Au/Pd/gC3N4NTs in the form of nanotube structure could be ascribed to the polymerization of melamine with the assistance of nitric acid as an activator followed by carbonization at elevated temperatures (Scheme 2). In particular, nitric acid converts melamine to cyamelurine, which subsequently condense to form melon sheets. Meanwhile, metal precursors were adsorbed onto melamine sheets through N-atoms during the polymerization step, owing to the great binding affinity of N-atom towards Au and Pd atoms as noticed beforehand in less intensive and broader of N-H vibration in FTIR spectrum of Au/Pd/gC3N4NTs (Scheme 2). Then, the asformed melon sheets were polymerized and assembled into a tubular-like structure. These polymeric units were carbonized at high temperatures. The slow mixing of melamine and/or nitric acid led to the presence of prevalent Van der Waals Forces that bind the melon sheets together during the polymerization and facilitate the formation of uniform nanotube morphologies. The creation of the gC3N4-based materials has attracted much great attention for a few decades. However, their fabrication in the form of one-dimensional porous nanostructures had been rarely reported. Meanwhile, the fabrication of porous gC3N4 nanotubes undoped and/or doped with a metal-based catalyst for CO oxidation had not been highlighted enough as compared to other catalytic applications. Herein, we


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have successfully developed a versatile approach for the controlled synthesize of porous gC3N4 nanotubes co-doped with both Au and Pd for CO oxidation reaction. In contrast to previously reported synthesis methods for gC3N4NTs, our method is simple and allowed preparation of porous gC3N4NTs having high surface areas and co-doped with Au and Pd which are required features for the CO oxidation reactions. The catalytic activity of the as-synthesized materials was investigated for the gas phase CO oxidation reactions under ambient atmospheric conditions; owing to the importance of this reaction in various fundamental, industrial, and environmental applications. The CO conversion to CO2 (CO + ½ O2  CO2) employing the Au/Pd/gC3N4NTs relative to that for metal-free gC3N4NTs was estimated at different reaction temperatures that ranged from room temperature up to 400 °C (Figure 7a). The results displayed that the metal-free gC3N4 NT did not exhibit any significant CO oxidation activity, even after heating till 400 °C. Intriguingly enough, after co-doping of gC3N4NTs with both Au and Pd, the CO conversion efficiency increased substantially with an increase in the reaction temperature until the complete conversion occurred (Figure 7a). The complete CO conversion (100 %) was achieved on the Au/Pd/gC3N4NTs at 165 °C which is indicative of the significant doping effect in the improvement of the CO conversion efficiency when compared with gC3N4NTs. This is owing to the electronic effect of Au and Pd which enhance the adsorption of CO and activation of O2 leading to oxygen accelerating the CO oxidation kinetics. This was noticed in the earlier conversion temperature of the Au/Pd/gC3N4NTs than that of the gC3N4NTs (Figure 7b). Indeed, the CO conversion on the Au/Pd/gC3N4NTs started at 74 °C that was significantly lower than that for the gC3N4NTs (130 °C). Following that, the CO conversion increased sharply on the Au/Pd/gC3N4NTs; whereas there was not any noticed increase in the CO conversion on the gC3N4NTs by an increase in the reaction temperature. This could be mainly attributed to the presence of oxygenated species formed by a combination between Au and Pd which eventually leads to a decrease the in the temperature required for the CO conversion. To this end, the half conversion temperature (T50) and the full conversion temperature (T100) on Au/Pd/gC3N4NTs were found to be 152 °C and 165 °C,


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respectively. The quick CO conversion kinetics on the Au/Pd/gC3N4NTs gained further support by the observation of its lower time necessary for the full CO conversion. The CO conversion % on Au/gC3N4NTs and Pd/gC3N4NTs was about 8 and 7 %, respectively, which were slightly higher than that of gC3N4NTs (Figure S5). This implies that, gC3N4NTs should be co-doped with both Au and Pd to achieve a full CO conversion (100 %), owing to the electronic and synergetic effects of Au/Pd. The complete CO conversion time was noticed for the Au/Pd/gC3N4NTs at 30 min while at this same time the conversion was only 2 % of CO for the gC3N4NTs (Figure 7c). Intriguingly, the T100 of our newly synthesized Au/Pd/gC3N4NTs of 165 °C was lower than that previously reported for various Au-based and Pd-based catalysts such as: Au0.75Cu0.25/SiO2 of 300 C, Pd/La-doped -alumina of 175 C, Pt/CNx/SBA-15 of 250 C, and Pd-impeded 3D porous graphene of 190 C.55-57 Figure 7d illustrates the CO oxidation durability test carried out on the Au/Pd/gC3N4NTs for 10 durability cycles. The results showed that the Au/Pd/gC3N4NTs maintained its initial CO oxidation activity after accelerated 10 stability cycles without any significant loss. Additionally, after the durability test, Au/Pd/gC3N4NTs reserved its nanotube morphology without any noticed cracking or agglomeration (Figure S6). For purposes of providing further elucidations for the enhanced CO oxidation activity of our newly designed Au/Pd/gC3N4NTs, its ability to adsorb CO was measured. This includes benchmarking COstripping CVs in CO-saturated in an aqueous solution of 0.1 M KOH at a scan rate of 50 mVs-1 (Figure 8). The results displayed the superior CO-adsorption ability of the Au/Pd/gC3N4 NTs as compared to the metal-free gC3N4NTs (Figure 8a). Additionally, the onset potential and oxidation potential of CO on Au/Pd/gC3N4NTs manifested a substantial negative shift relative to the gC3N4NTs. Meanwhile, the CO oxidation kinetics for the Au/Pd/gC3N4NTs was significantly faster than that for the gC3N4 NTs under any applied potential, as is indicated by the dashed lines in (Figure 8b). These results all in all unambiguously warranted the superior CO oxidation activity and durability of the Au/Pd/gC3N4NTs which could be attributed to a unique combination between the intrinsic physicochemical properties of the gC3N4NTs and the impressive inherent catalytic merits of doping with


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Au and Pd. Indeed, the electronic effect of Au/Pd led to an enhancement of both the CO- and O2adsorption along with the activation of O2 that consequently led to an acceleration of the CO oxidation kinetics.19-51,57 Meanwhile, gC3N4NTs with its great electron density and conductivity interacted well with Au and Pd and resulted in discerned improvement in the CO oxidation activity.51 Noteworthy here is that the porous nanotube architecture provides a highly accessible surface area and various active sites for the adsorption of the reactant molecules along with accelerating their transfer and molecular mobility.

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CONCLUSIONS

In brief, we presented herein a scalable-up approach for tailoring the fabrication of the Au/Pd/gC3N4NTs via polymerization of melamine in an ethylene glycol solution that contains Pdand Au-precursors with the addition of nitric acid followed by subsequent carbonization at high temperatures. In contrast to previous synthetic approaches for the gC3N4, our method is simple and allowed for the high mass production of the gC3N4 porous nanotubes co-doped with Au and Pd and having high surface area. These unique merits endow the Au/Pd/gC3N4NTs for the CO oxidation activity with a significantly low T100 of 165 °C. The approach thus presented may pave the way for the tailored synthetic routes of gC3N4NTs doped with various metal-based catalysts for a wide range of catalytic and environmental applications.

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ASSOCIATED CONTENT

Supporting Information contains additional characterization data.

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AUTHOR INFORMATION

Corresponding Author *E-mail : [email protected] Author Contributions


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‡

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K. Eid and M. Sliem contributed equally to this work, meanwhile the manuscript was written

through equal contributions from all authors. Notes The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS

We gratefully appreciate the Center for Advanced Materials at Qatar University for supporting of this work.


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Figures and Figures Captions Scheme 1. The formation process of Au/Pd/gC3N4NTs. Figure 1. (a-b) SEM image and (c-d) TEM images at different magnifications of Au/Pd/gC3N4NTs prepared under typical conditions. Figure 2. (a) High magnification TEM image of Au/Pd/gC3N4NTs, (b and C) HRTEM images of the numbered areas in (a), respectively. The insight in (c) shows the FFT and FT. Figure 3. (a) HAADF-STEM image of a single Au/Pd/gC3N4NTs nanotube and its elemental mapping analysis for (b) C, (c) N, (d) Au, and (e) Pd. (f) EDX analysis of the as-fabricated materials. The indicated scale bars in b-d are 20 nm. Figure 4. (a) The Wide-angle XRD patterns of and (b) XPS survey of Au/Pd/gC3N4NTs and gC3N4 NTs. (c) high-resolution XPS spectra of C 1s, (d) N 1s, (e) Au 4f, and (f) Pd 3d. Figure 5. N2 adsorption-desorption isotherms and pore size distributions of Au/Pd/gC3N4NTs (a and b) and gC3N4NTs (c and d), respectively. Figure 6. FTIR analysis of a typically synthesized Au/Pd/gC3N4NTs as compared to gC3N4NTs. Figure 7. (a) CO conversion efficiency on typically prepared catalysts as a function of temperature, (b) the CO conversion kinetics at temperatures that ranged between 25 and 150 °C, and (c) CO conversion as a function of time, and (d) the stability tests of Au/Pd/gC3N4NTs measured for 10 cycles. Scheme 2. The proposed formation mechanism of Au/Pd/gC3N4NTs. Figure 8. (a) The CO-peaks adsorbed on Au/Pd/gC3N4NTs and gC3N4NTs benchmarked in CO-saturated an aqueous solution of 0.5 M KaOH at a sweeping rate of 50 m V s-1 and (b) the CO-oxidation kinetics at a potential ranged between -0.8 and 0.5 V.


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Scheme 1

Figure 1


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Figure 2

Figure 3


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Figure 4


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Figure 5

Figure 6


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Scheme 2


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Figure 7

Figure 8


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Table of Contents Porous Au/Pd/gC3N4 NTs were obtained by polymerization of melamine in an ethylene glycol solution containing metal precursors followed by carbonization and exhibited a substantial CO oxidation activity under ambient conditions.


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Cover Art Rational Synthesis of Porous Graphitic-like Carbon Nitride Nanotubes Codoped with Au and Pd as Efficient Catalyst for Carbon Monoxide Oxidation


Rational Synthesis of Porous Graphitic-like Carbon Nitride Nanotubes

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