Data on the catalytic CO oxidation and CO2 reduction durability on gC3N4 nanotubes Co-doped atomically with Pd and Cu.

Understanding the fabrication mechanism of graphitic carbon nitride (gC3N4) nanostructures is critical for tailoring their physicochemical properties for various catalytic applications. In this article, we provide deep insights into the optimized parameters for the rational synthesis of one-dimensional gC3N4 atomically doped with Pd and Cu denoted as (Pd/Cu/gC3N4NTs) and its fabrication mechanism. This is in addition to the CO oxidation durability along with the electrochemical and photoelectrochemical CO2 reduction durability of Pd/Cu/gC3N4NTs. The presented herein results are correlated to the research article entitled "Precise Fabrication of Porous One-dimensional gC3N4 Nanotubes Doped with Pd and Cu Atoms for Efficient CO Oxidation and CO2 Reduction (Kamel Eid et al., 2019).

Data

The presented data article is associated with the research article (Kamel Eid et al., 2019 [1]). This includes (i) the SEM and TEM images of metal-free gC3N4, (ii) the TEM images of Pd/Cu/gC3N4 prepared in different morphologies, (iii) the CO oxidation durability of Pd/Cu/gC3N4NTs, Pd/gC3N4NTs, and Cu/gC3N4NTs, (iv) the electrocatalytic and photoelectrochemical CO2 reduction of Pd/Cu/gC3N4NTs, and (v) the XRD, EDX, and TEM image of Pd/Cu/gC3N4 after the CO durability testes.

Experimental design, materials, and methods

CO oxidation

We tested the CO oxidation reaction in a fixed bed quartz tubular reactor connected to an online gas analyzer (IR200, Yokogawa, Japan) in the presence of 50 mg of each catalyst. Initial pretreatment was carried out at 250 °C under an O2 flow of 50 mL min−1 for 1 h, and then H2 (30 mL min−1) for 1 h. Following that, the catalysts were exposed to the gas mixture involving of 4% CO, 20% O2, and 76% Ar with a total flow of 50 mL min−1 under continuous heating from 25 °C to 400 °C (5°min−1) [1], [2], [3], [4], [5]. The percentage of CO conversion (% CO) was calculated using the following equation:where COin is, the input quantity and COOut is the output quantity.

Electrochemical reduction of CO2

The cyclic voltammogram (CVs), linear sweep voltammogram (LSV), and electrochemical impedance spectroscopy (EIS) measurements were measured on Gamry electrochemical analyzer (reference 3000, Gamry Co., USA) using a three-electrode system composed of a Pt wire (counter electrode), Ag/AgCl (reference electrode), and glassy carbon ((5mm) working electrode). The CVs, LSVs, and EIS were measured in a CO2-saturated aqueous solution of 0.5 M NaHCO3 at a sweep rate of 50 mV s−1. In the photoelectrochemical measurements, the light source was ozone-free xenon lamp (100 W, Abet Technologies, USA) with fluorine-doped tin oxide as a working electrode in a Quartz photo-glass cell (50 mm × 50 mm). The catalyst loading amount of each catalyst on the working electrode was fixed to 10 μg cm−2 using. After deposition of each catalyst on the working electrodes, a 5 μL of Nafion solution (1 wt %) was added on each electrode and left to dry completely under vacuum at 80 °C before the measurements.

Scheme 1 shows the fabrication process of Pd/Cu/gC3N4NTs, including the initial slow mixing of melamine in an aqueous solution of ethylene glycol solution, contains Pd- and Cu precursors [3]. Then, nitric acid was added dropwise to slowly deprotonate melamine and facilities the polymerization step to polymeric gC3N4, followed by annealing at elevated temperature to allow the carbonization process and formation of gC3N4NTs doped with Pd and Cu.

Scheme 1

Schematic shows the synthesis process of Pd/Cu/gC3N4NTs.

Fig. 1 shows the histogram chart of Pd/Cu/gC3N4NTs. The widths of thus obtained Pd/Cu/gC3N4NTs ranged from 60 to 90 nm. The average width of thus formed nanotubes is nearly 80 nm.

Fig. 1

The size distribution histogram of Pd/Cu/gC3N4NTs.

Fig. 2 shows the SEM and TEM images of metal-free gC3N4NTs that were prepared by the same method of Pd/Cu/gC3N4NTs but in the absence of Pd and Cu precursors. Fig. 2a reveals the SEM image of gC3N4NTs formed in high yield (nearly 100%) of nanotubes shape. The nanotube shape was uniform and mono distributed with an average width of 78 nm and an average length of 1.4 μm. The TEM image shows the absence of any undesired nanostructures such as spherical nanoparticles or other shapes.

Fig. 2

(a) SEM and (b) TEM images of gC3N4NTs.

Fabrication parameters optimization

Fig. 3a shows the TEM image of Pd/Cu/C3N4NTs nanoflakes prepared by the quick mixing of melamine (1 g) in an aqueous solution of ethylene glycol solution (30 mL) involving K2PdCl4 (20 mM) and CuCl2 (20 mM) followed by the slow addition of HNO3 (60 mL of 0.1 M) under stirring. The obtained precipitate was washed with ethanol and dried at 80 °C for 12 h before annealing at 550 °C (5 °C/min) for 2 h under nitrogen. The TEM image reveals the formation of aggregated flakes-like Pd/Cu/C3N4NTs nanostructures obtained in a high yield with an average dimension of ∼ 250 nm.

Fig. 3

(a) TEM images of Pd/Cu/gC3N4NTs obtained by (a) quick addition of melamine, (b) quick addition of HNO3, (c) using 60 mL of HNO3 (0.03 M), (d) using ethanol instead of ethylene glycol. (e) Pd/Cu/gC3N4NTs formed using 60 mM of K2PdCl4 and CuCl2 and (f) using 40 mM of K2PdCl4 and CuCl2.

Under the same conditions and parameters of nanoflakes, the quick addition of nitric acid produced sheet-like nanostructures. This arose from the quick deprotonation and polymerization process via rapid addition of nitric acid (Fig. 3b). Reducing the concertation of nitric acid to 0.03 M with fixing all other conditions and parameters formed aggregated and non-uniform Pd/Cu/C3N4 nanotubes (Fig. 3c). Using isopropanol solution instated of ethylene glycol led to the production of Pd/Cu/C3N4 nanofibers in line with our previous reports (Fig. 3d) [2]. The as-formed nanofibers were highly uniform with average dimensions of 1.5 ± 0.2 μm in length and 80 ± 3 nm in width.

Fig. 3e shows the formation of gC3N4 nanosheets decorated with aggregated Pd/Cu nanoparticles formed through increasing the concertation of Pd/Cu to 60 mM instead of 20 mM with fixing all other conditions. Similarly, decreasing the concertation of Pd/Cu to 40 mM drove the formation of nanosheets decorated with uniform Pd/Cu nanoparticles (Fig. 3f). These results warranted that the formation of Pd/Cu/C3N4NTs is highly sensitive to the concentration of reactants and their mixing conditions. In particular, the addition of melamine and nitric acid should be sluggish to provide enough time for a consistent polymerization into uniform nanotubes. Nitric acid facilitates the deprotonation of active –NH2 groups of melamine and allowing the conversion of melamine into melem and then to polymeric gC3N4 composed of triazine-based units after carbonization at an elevated temperature [1], [2], [3], [4], [5]. Meanwhile, the concertation of Pd/Cu precursors should be lower to be anchored on the N-atoms of melamine and then facilitating the atomic doping of Pd/Cu instead of formation of nanoparticles [1], [2], [3], [4], [5]. On the other hand, glycol-mediated solution acting as a structure-directing agent for driving the formation of nanotube shape.

CO oxidation stability tests

The CO oxidation durability is an important factor in large-scale environmental and industrial applications [1], [2], [3], [4]. Fig. 4 shows the accelerated durability tests of Pd/Cu/gC3N4NTs, Pd/gC3N4NTs, and Cu/gC3N4NTs measured for ten cycles at their complete CO conversion temperature (T100). The results show that Pd/Cu/gC3N4NTs is more durable than both Pd/gC3N4NTs and Cu/gC3N4NTs. Particularly, the CO oxidation kinetics and T100 of Pd/Cu/gC3N4NTs were almost maintained without any significant changes (Fig. 4a). Meanwhile, the T100 of Pd/gC3N4NTs, and Cu/gC3N4NTs increased only by around 11 °C (Fig. 4b) and 25 °C (Fig. 4c), respectively. However, the CO oxidation kinetics did not decrease substantially on both Pd/gC3N4NTs and Cu/gC3N4NTs, as shown in their light-off curves (Fig. 4b and c).

Fig. 4

The CO oxidation light-off stability tests measured on (a) Pd/Cu/gC3N4NTs, (b) Pd/gC3N4NTs, and (c) Cu/gC3N4NTs for ten cycles at their T100.

The sample was dispersed in ethanol and sonicated for 3 min and then mounted on a carbon-coated TEM grid. Fig. 5 reveals the TEM images of Pd/Cu/gC3N4NTs before (Fig. 5a) and after the CO oxidation stability tets (Fig. 5b). Comparing the TEM image of Pd/Cu/gC3N4NTs before and after the CO oxidation durability testes, we found that the structural stability of nanotube shape is fully maintained without any changes. Therefore, the nanotube morphology did not change after ten durability cycles.

Fig. 5

The TEM image of Pd/Cu/gC3N4NTs before (a) and after (b) the stability tests.

Fig. 6a shows the XRD analysis of Pd/Cu/gC3N4NTs after the CO durability tests, which displayed the one diffraction peak at 27.01° assigned to {002} facet and one peak at 13.15° attributes to{100} facet of gC3N4 nanostructure similar to those obtained for Pd/Cu/gC3N4NTs before the CO durability tests. Thus, the XRD result indicates that Pd/Cu/gC3N4NTs reserved its crystallinity after the CO oxidation durability tests. The EDX analyses after CO stability testes is carried out to confirm the compositional durability of Pd/Cu/gC3N4NTs (Fig. 6b). The results showed the presence of C, N, Pd, and Cu with atomic contents of 58, 40.9, 0.5, and 0.6, respectively (Fig. 6b). Thus, the EDX result implies that Pd/Cu/gC3N4NTs kept its composition without any deterioration, owing to the homogenous distribution of Pd and Cu inside the carbon matrix.

Fig. 6

(a) XRD analysis and (b) EDX analysis of Pd/Cu/gC3N4NTs after the CO durability tests.

Table 1 shows the comparison between the catalytic CO oxidation activity of our newly designed Pd/Cu/gC3N4NTs and the previously reported catalysts such as Pd-based, Au-based Cu-based, Pt-based, and Mn-based. The complete conversion temperature of our obtained Pd/Cu/gC3N4NTs was significantly lower than that of all the catalysts reported in the literature as shwon in Table 1 in addition to the low cost of our catalyst that was made of nearly 99% gC3N4NTs and 1% Pd/Cu.

Table 1

Comparison between the CO oxidation activity of our newly designed Pd/Cu/gC3N4NTs and various catalysts reported elsewhere.

Catalyst	Complete CO conversion, T100	Reference
Pd/Cu/gC3N4NTs	154 °C	Our work
Au0.75Cu0.25/SiO2	300 °C	[6]Catal. Today, 2017, 282 105–110.
Pd/La-doped γ-alumina	175 °C	[7]Nat. Commun., 2014, 5, 4885.
Pd-impeded 3D porous graphene	190 °C	[8]ACS Nano 2015, 9, 7343-7351
Pt/CNx/SBA-15	250 °C	[9]Chem. A Eur. J., 2014, 20, 2872–2878.
Nanoarray-based CuMn2O4/Washed-coated CuMn2O4	320 °C/350 °C	[10]J. Mater. Chem. A, 2018, 6, 19047-19057
Cu1/Mn1	180 °C	[11]Catal. Lett., 2016, 146, 2364-2375
MnOx	310 °C	[12]Catal. Sci. Technol., 2016, 6, 8222-8233

The TEM, XRD, and EDX results confirmed the structural and compositional stability of the as-synthesized Pd/Cu/gC3N4NTs after the CO oxidation stability tests. This probably originates from coupling between the unique physicochemical properties of 1D gC3N4 nanotubes (e.g., stability, massive accessible active sites, thermal stability nearly up to 600 °C, and chemical stability in various solvents) and the inherent catalytic merits of Pd/Cu (eg., electronic effect, synergetic effect, strong adsorption/activation/dissociation for CO/O2, and high tolerance for CO2 product) [1], [2], [3], [4], [5], [13], [14], [15], [16]. Chemically speaking, the atomic doping of gC3N4NTs with Pd and Cu stabilizes them against aggregation as well as protecting their active catalytic sites from the blocking by the reaction intermediates or products.

Scheme 2 shows the formation process and mechanism of typically prepared Pd/Cu/gC3N4. The strong binding affinity between N-atoms of melamine and Pd/Cu facilitate adsorption and anchoring of both Pd and Cu on N-atoms during the polymerization step that led to the homogenous atomic distribution of Pd, and Cu on the N-atoms of gC3N4.

Scheme 2

The formation mechanism of Pd/Cu/gC3N4NTs and the distribution of Pd and Cu inside gC3N4NTs.

Fig. 7a shows the CVs for CO2 reduction measured under various sweeping rates ranged from 25 to 200 mV s−1, which showed the steady enhancement in the current density with increasing the scan rate. The relationship between the obtained current densities and the square root of scan rates is linear (Fig. 7b).

Fig. 7

(a) The CVs measured on the as-made catalysts in CO2-saturated 0.5 NaHCO3 at 50 mV s−1 under different scan rates and (b) Randles-Sevcik equation. (c) The electrochemical CVs durability on Pd/Cu/gC3N4NTs measured under dark. (d) The photoelectrochemical CVs stability tested under continuous light irradiation (100 W). We performed all the measurements at the room temperature.

The electrocatalytic and photo-electrochemical CO2 reduction durability tests were carried out on Pd/Cu/gC3N4NTs via measuring the chronoamperometric test (I-T) for 30 min in CO2-saturated an aqueous solution of 0.5 NaHCO3 at 50 mV s−1. Then the CVs curve were measured again in CO2-saturated an aqueous solution of 0.5 NaHCO3 at 50 mV s−1. The CVs curves showed that Pd/Cu/gC3N4NTs kept its initial electrocatalytic CO2 reduction activity (Fig. 7c) without any significant deterioration in the current density, reduction kinetics, and reduction potential (Fig. 7d), [17].

Fig. 8 depicts the gas chromatography result that was obtained after calibration relative to pure formic acid and methanol under the same conditions. The results demonstrated the presence of formic acid as the main product as well as methanol as an inferior product (Fig. 8). Therefore, the gas chromatography indicates the ability of Pd/Cu/gC3N4NTs to reduce CO2 electrochemically to formic acid at room temperature.

Fig. 8

CO2 reduction products obtained from the Gas chromatography (Agilent Technologies 7890A) with using a column PerkinElmer Elite-624 at 35 °C.

Specifications Table

Subject area	Chemistry
More specific subject area	Catalysis
Type of data	Scheme, Images, Table, and Figures
How data was acquired	Transmission electron microscope ((TEM), TecnaiG220, FEI, Hillsboro, OR, USA), scanning electron microscope ((SEM), Hitachi S-4800, Hitachi, Tokyo, Japan), X-ray diffraction patterns ((XRD), X'Pert-Pro MPD, PANalytical Co., Almelo, Netherlands), CO oxidation (online gas analyzer IR-200, Yokogawa, Japan), CO2 reduction Gamry electrochemical analyzer (reference 3000, Gamry Co., USA).
Data format	The obtained data are imaged and analyzed.
Experimental factors	The thermal CO oxidation stability tests were measured under continuous gas mixture flow while heating (25–300 °C). The electrocatalytic CO2 reduction durability tests were benchmarked at the room temperature in 0.5 M NaHCO3 solution.
Experimental features	Changing the reaction parameters and conditions to optimizing the fabrication process of Pd/Cu/gC3N4. Investigation the thermal CO oxidation durability as well as the electrochemical and photoelectrochemical CO2 reduction of Pd/Cu/gC3N4. These results are beside the structural and compositional analysis of Pd/Cu/gC3N4 after the catalytic durability reactions.
Data source location	Center for advanced materials, Qatar University, Doha 2713, Qatar.
Data accessibility	The data are obtained and provided in this article.
Related research article	Eid et al., Precise Fabrication of Porous One-dimensional gC3N4 Nanotubes Doped with Pd and Cu Atoms for Efficient CO Oxidation and CO2 Reduction, Inorganic Chemistry Communications.” [1]

Value of the data• Optimization of the fabrication process of gC3N4 nanostructures doped with binary metals is essential in various catalytic applications. • Understanding the fabrication mechanism of Pd/Cu/gC3N4NTs is essential for tailoring their physicochemical and catalytic properties for various applications. • The catalytic CO oxidation and CO2 reduction durability of Pd/Cu/gC3N4NTs are central factors in commercial applications. • These data may open new avenues on using gC3N4-based materials for gas conversion reactions.