Nickel phthalocyanine@graphene oxide/TiO2 as an efficient degradation catalyst of formic acid toward hydrogen production

A new photocatalytic system was introduced to degrade formic acid toward hydrogen production using nickel(II) phthalocyanine (NiPc)@graphene oxide (GO)/TiO2 as the catalyst. Synthesis of NiPc was performed in the presence of GO leading to a homogeneous distribution of NiPc on GO. While TiO2 promoted the reaction using each of NiPc and GO under visible light, the reaction was carried out with superior rate using NiPc@GO/TiO2. In this reaction, GO minimized the band gap of TiO2 through contributing its Fermi levels and NiPc escalated the photocatalytic reaction rate as a sensitizing agent. The reaction released hydrogen with the rate of 1.38 mmol h−1 and TOF = 77 h−1.


Scientific Reports
| (2021) 11:16148 | https://doi.org/10.1038/s41598-021-95382-z www.nature.com/scientificreports/ electron microscopy (TEM) micrographs were obtained with Philips CM100 BioTWIN transmission electron microscope and TEM Philips EM 208S. Produced gasses were analyzed using an online gas chromatograph (GC-9890A) with thermal conductivity detector (TCD) and flame ionization detector (FID). Gas products (e.g., CO 2 and H 2 ) were detected by TCD equipped TDX-01 column. The powder X-ray diffraction (XRD) pattern was obtained using Bruker, D8 ADVANCE X-ray diffractometer with a Cu-K α radiation source (λ = 1.5406 Å) operating at 40 kV, 40 mA, and a scanning range of 5-80° 2θ, with a 2θ scan step of 0.015° and a step time of 0.2 s. Fourier transform infrared spectroscopy (FTIR) was used to characterize different functional groups of the composite using Jasco 6300 FTIR instrument in the range of 600-4000 cm −1 . XPS spectra were recorded on a Thermo ESCALAB 250 Xi using monochromatic Al K α radiation (1486.6 eV) with a spot size of 850 µm. The spectra acquisition and processing were carried out using the software Thermo Avantage. The sample was stuck on the sample holder using a double-sided carbon tape and then introduced into the preparation chamber and was degassed until the proper vacuum was achieved. Then it was transferred into the analysis chamber where the vacuum was 9-10 mbar. The analysis was carried out using the following parameters: Pass energy of 20  Typical procedure for the degradation of FA by NiPc@GO/TiO 2 . A 300 W xenon lamp with a glass filter (420 nm) was used as a light source in the photocatalytic degradation of FA. Typically, 0.05 g (18 µmol) of NiPc@GO and 0.04 g TiO 2 were added into 30 mL of FA. Before irradiation, the mixture was stirred in a dark box for 30 min to disperse the catalyst in the mixture. Next, the mixture was irradiated in a photochemical reaction chamber equipped to a gas collector under continuous stirring condition.

Results and discussion
FT-IR spectra were prepared for NiPc and NiPc@GO to characterize and compare them (Fig. 1). NiPc's FT-IR spectrum indicated some absorption bands at 1641, 3103, and 3409 cm −1 related to stretching vibrations attributed to C=N/C=C, aromatic C-H, and O-H, respectively. In the meantime, the spectrum of NiPc@GO also showed those peaks just with a little difference for the peak at 3400 cm −1 which was appeared in a high intensity due to overlapping of NiPc's OH peak with that of GO. One of the valuable methods to characterize GO composites is TEM which can obviously shows its structure. So, TEM micrographs were prepared for NiPc@GO expecting to see GO layers containing nanoparticles of NiPc (Fig. 2). The micrographs showed wide sheets of GO with homogeneous distribution of NiPc on the GO with the average particle size between 4.1 and 4.9 nm. Since the molecular sizes of MPcs are about 1.5 nm, each of the particles was constructed from aggregation of about 3 molecules. This is very ideal system to obtain very fine particles of a MPc in a heterogeneous system, which finally can run the reaction more effectively.
The Raman spectroscopy of NiPc@GO was considered for more confirming the structure since GO's D-band and G-band absorption peaks can be observed there. As can be seen from Fig. 3, the Raman spectrum of NiPc@ GO showed D-and G-bands of GO at 1543 and 1332 cm −1 , respectively. It is crucial to consider that peaks attributed to NiPc appeared in the spectrum such as the peak at 1605 cm −1 for C=N band 31 .
XRD pattern of NiPc@GO was prepared to determine diffraction peaks related to GO and NiPc. The (001) peak for GO 32 and (100), (102), (213), and (214) peaks of NiPc 19 obviously indicated the synthesized nanocomposite structure (Fig. 4). NiPc content of NiPc@GO was determined to be 0.37 mmol per 1 g of the catalyst by FAAS.
XPS analysis was performed on NiPc@GO to determine various elements on the nanocomposite (Fig. 5). The analysis showed peaks related to C 1s, N 1s, O 1s, Ni 2p 3/2 , and 2p 1/2 at 284.9, 402.3, 533.0, 856.1, and 874.3 eV, respectively. The carbon peak at 285.1 eV is the result of overlapping of peaks at 284.4 eV for C=C, 285.7 eV for C-N, and 286.3 eV for C-O bands, which approves presence of two different kinds of structures including Pc and GO. The peaks related to N, and Ni also obviously approve the presence of NiPc in the prepared composite.
Before starting the photocatalytic reaction using various composites based on NiPc, UV-Vis spectra were prepared for NiPc and NiPc@GO to achieve knowledge about their desired operation wavelength (Fig. 6). Both NiPc and NiPc@GO indicated absorptions under 440 nm and above 460 nm with higher absorption intensity for NiPc@GO. As a result, both NiPc and NiPc@GO should induce high efficiency under visible irradiation for TiO 2 which is employed as an eminent photocatalyst in the UV region. The H 2 production reaction of a dye-sensitized semiconductor composite is usually affected by a series of circumstances such as electron donating system, irradiation wavelength, sensitizing capability and so on 33 . NiPc@ GO/TiO 2 was considered for the H 2 production since the composite would afford most of the desired conditions for the photocatalytic reaction. Pcs as the potent sensitizers have a conjugated Π system with absorption areas at 620-700 nm known as Q band and about 350 nm known as Soret B band 34 . These absorptions permit to Pcs to have a great ability as a sensitizer in a wide range of wavelengths leading to use of NiPc as a sensitizer in semiconductors 35 . In addition, GO by contribution of its Fermi levels facilitates the electron transportation on TiO 2 levels as the semiconductor 36 . In this work, the photocatalytic activity of NiPc@GO/TiO 2 was evaluated   production was decreased to 0.63 mmol h −1 in the presence of NiPc/TiO 2 under visible irradiation, which approves contribution of GO in the reaction progress. The phenomenon is intelligible considering that GO's Fermi levels are inserted in TiO 2 band gaps. This can also be observed from the result of degradation reaction by GO/TiO 2 with 0.49 mmol h −1 H 2 production rate, while as mentioned the reaction did not proceed using bare TiO 2 . In the meantime, the reaction was examined employing NiPc@GO as the catalyst with 0.92 mmol h −1 hydrogen production rate. In spite of the fact that lack of TiO 2 as a semiconductor should significantly decreases the yield; this negligible decline is not strange when we find out that Pcs are also semiconductor 37 . The lowest amount of TiO 2 required for obtaining high yield was achieved 0.04 g. The reaction yield was reduced dramatically in a dark room revealing the photo-induced pathway for the reaction. Turnover frequency (TOF) was calculated for the reaction by NiPc@GO/TiO 2 about 77 h −1 which is a high number regarding mild conditions of the reaction. Furthermore, impression of NiPc loading on GO was evaluated on the H 2 generation rate as well as TOF. 18 µmol of NiPc on GO was recognized as the best catalyst amount regarding both values of the H 2 production rate and TOF. In all tests, only signals assigned to H 2 and CO 2 were detected by gas chromatography without any CO signal.
The kinetic study was carried out on the reaction via investigation of temperature effect on the reaction assuming that the reaction is not diffusion-limited below 57 °C. Temperature had an important influence on the catalytic activity of NiPc@GO/TiO 2 and NiPc@GO, where a high temperature was beneficial for the dehydrogenation  (Fig. 7). Moreover, the activation energy (E a ) was calculated to be 9.1 and 19.0 kJ/mol for the reaction by NiPc@GO/TiO 2 and NiPc@GO, respectively. These values are considerably lower than most previously reported E a values 13 . Potential NiPc leaching into the mixture of FA degradation reaction was also analyzed with FAAS analysis. For this purpose, filtrate of the heterogeneous degradation reaction of FA after 1 h was passed from a syringe filter and then dissolved in HNO 3 . The FAAS analysis of sample evinced that the Ni concentration in the reaction mixture was less than the detection limit. This result indicates that virtually no NiPc leaches from NiPc@ GO into the mixture. A hot filtration test was also performed on NiPc@GO by removing the catalyst from the reaction mixture after 10 min and monitoring the reaction progress. Under this condition, gas releasing was completely quenched confirming heterogeneously done the reaction. Finally, the catalyst stability was examined in the reaction mixture. For that, the light source was removed after 1 h and irradiation was performed again after 24 h. This cycle was repeated for 5 cycles with 1.38 mmol h −1 H 2 was produced in each of them, which shows as expected TiO 2 , NiPc, and GO are stable in FA.

Conclusion
NiPc@GO was prepared through synthesis of NiPc in the presence of GO, which uniform distribution of NiPc on GO was obtained. Superior photocatalytic activity was observed for NiPc@GO in the degradation reaction of formic acid under visible light toward production of H 2 . In addition, TiO 2 improved NiPc@GO catalytic activity significantly. Results showed that TiO 2 as a semiconductor, and NiPc as a semiconductor as well as photosensitizer contribute in this reaction. Additionally, GO by improving electron injection efficiency and retarding the charge recombination in semiconductors accelerate the reaction.