Nickel Phthalocyanine@graphene Oxide/TiO2 As an Efficient Degradation Catalyst of Formic Acid Toward Hydrogen Production

doi:10.21203/rs.3.rs-506855/v1

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Nickel Phthalocyanine@graphene Oxide/TiO₂ As an Efficient Degradation Catalyst of Formic Acid Toward Hydrogen Production

https://doi.org/10.21203/rs.3.rs-506855/v1

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published 09 Aug, 2021

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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 minimizes the band gap of semiconductor, TiO₂, through contributing its Fermi levels and NiPc escalates the photocatalytic reaction under visible light as a sensitizing agent. The reaction released hydrogen with the rate of 1.38 mmol.h^− 1 and TOF = 77 h^− 1.

Physical Chemistry

Catalysis

Nanoscience

catalyst

formic acid

hydrogen production

photocatalytic

With increasing concerns about application of fossil fuels, production of hydrogen as a green combustion source has been attracted a great deal of attention.^[1] The significance of this subject has led to the introducing various sources such as especial chemicals for the hydrogen production.^[2] Although decomposition of most chemicals toward hydrogen production gives toxic wastes, formic acid (FA) degradation is a green approach for this agenda. A great number of catalysts for the FA degradation have been reported in recent years. Ru as one of the effective catalysts have promoted the reaction in various forms such as [Ru(H₂O)₆]²⁺, RuCl₃,^[3] [RuCl₂(p-cymene)]₂,^[4] and ruthenium carbonyl complexes.^[5] Meanwhile, Pt and Ru-loaded CdS/Al-HMS catalyzed FA decomposition under visible light with the reaction rate of 7.63 ml.h^− 1.^[6] Titanium dioxide modified with various metals and metal oxides has been employed in the FA degradation reaction as photo-oxidation catalysts.^[7–11] As an interesting instance, N-doped titanium oxide/Au photocatalytically promoted the reaction.^[12] Palladium nanoparticles immobilized on nitride carbon-coated mesoporous tungsten oxide was also used in this reaction with high conversions in low catalyst loading.^[13]

Phthalocyanines (Pc) are considered as one of the most important categories of colorant. Easy production approach and high efficiency lead to the development of new applications in photodynamic therapy, optical data storage, reverse saturable absorbers, and solar screens.^[14] In addition, Pcs especially metalo-Pc such as NiPc are known as the catalysts for a variety of reactions.^[15–19] The structure of NiPc, consisted from a long conjugated Π-system, provides opportunity to trap light in order to promote a photocatalytic reaction. NiPc composites photocatalytically catalyzed degradation reactions of Rhodamine B, and 2,4-dichlorophenol.^{[20, 21]} Rh-complexes/NiPc was employed as efficient water splitting catalyst under visible irradiation.^[22]

In continue of our efforts to extend the application of phthalocyanines as a heterogeneous catalyst,^[23–25] herein a three-component catalytic system, NiPc@GO/TiO₂ NPs, efficiently promoted hydrogen production from formic acid degradation under visible irradiation. In this reaction, GO improves the electron injection efficiency and retards the charge recombination in the semiconductor, TiO₂.^[26] While NiPc as a sensitizer enhances photon interaction with TiO₂, obtained results showed that it can also act as a semiconductor like TiO₂. Therefore, two possible pathways are conceivable for degradation of FA in the presence of NiPc@GO/TiO₂ NPs (Scheme 1).

2.1. Material and Methods

All reagents were purchased from Sigma-Aldrich and used without further purification. Anatase form of TiO₂ with the particle size under 25 nm was purchased from Sigma-Aldrich. Ni determination was carried out on a Flame Atomic Absorption Spectroscopy (FAAS) (Shimadzu 105 model AA-680 atomic absorption spectrometer) with a hollow cathode lamp. Transition 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., CO₂ and H₂) 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 second. 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 eV, dwell time 50 ms and the Step size 0.1 eV. UV-Vis spectra were collected by a UV–visible spectrophotometer (Biowave II, Biochrom WPA Ltd., UK), which the solution was prepared by dispersing the photocatalyst in EtOH:H₂O (70:30) under ultrasonication, and then separation of solids by centrifugation.

2.2. Preparation of NiPc@GO

Phthalic anhydride (8 mmol), urea (40 mmol), ammonium molybdate (0.005 g), NiCl₂ (2.1 mmol), and GO (1.0 g) was mixed in a round-bottom flask and then placed in a microwave oven. The reaction vessel was irritated by 900 W instrument for 3 min. After that, the vessel was cooled to room temperature and 50 ml H₂O was added to it. The mixture was sonicated for 15 min at room temperature, filtered off as purple solid, washed with water (5 × 20 ml), and dried in oven at 80 ⁰C to obtain NiPc@GO (21 w%/79 w%) as a purple solid.

2.3. Typical procedure for the degradation of formic acid by NiPc@GO/TiO₂

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 TiO₂ 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.

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 show 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 4.1–4.9 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.^[27]

XRD pattern of NiPc@GO was prepared to determine diffraction peaks related to GO and NiPc. The (001) peak for GO^[28] and (100), (102), (213), and (214) peaks of NiPc^[29] 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 a 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 TiO₂ which is employed as an eminent photocatalyst in the UV region.

The H₂ 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.^[30] NiPc@GO/TiO₂ was considered for the H₂ 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.^[31] 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.^[32] In addition, GO by contribution of its Fermi levels facilitates the electron transportation on TiO₂ levels as the semiconductor.^[33] In this work, the photocatalytic activity of NiPc@GO/TiO₂ was evaluated in the H₂ production from formic acid under Vis irradiation. In the meantime, the effects of various ingredients of the catalyst were investigated on the reaction yield (Table 1).

The catalytic performance of NiPc@GO was investigated in the degradation of FA in the presence and absence of TiO₂ under ambient conditions (Table 1). The results exhibited a high activity for NiPc@GO/TiO₂ in the degradation reaction toward H₂ generation under visible light providing 1.38 mmol.h^− 1 H₂ using 18 µmol of the catalyst. H₂ production was decreased to 0.63 mmol.h^− 1 in the presence of NiPc/TiO₂ 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 TiO₂ band gaps. This can also be observed from the result of degradation reaction by GO/TiO₂ with 0.49 mmol.h^− 1 H₂ production rate, while as mentioned the reaction did not proceed using bare TiO₂. 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 TiO₂ as a semiconductor should significantly decreases the yield; this negligible decline is not strange when we find out that phthalocyanines are also semiconductor.³⁴ The minimum TiO₂ 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/TiO₂ about 77 h^− 1 which is a high number regarding mild conditions of the reaction. In all tests, only signals assigned to H₂ and CO₂ were detected by gas chromatography without any CO signal.

Table 1

H₂ production by various catalysts from FA degradation reaction
Entry	catalyst	H₂ produced (mmol)
Entry	catalyst	Under Vis	Dark room
1	TiO₂	-	-
2	GO	-	-
3	NiPc	0.61	0.067
4	TiO₂/NiPc	0.63	0.067
5	TiO₂/GO	0.49	-
6	NiPc@GO	0.92	0.045
7	TiO₂ (0.03 g)/NiPc /GO (0.05 g)	1.34	0.054
8	TiO₂ (0.04 g)/NiPc (0.05 g)/GO (0.05 g)	1.38	0.054
9	TiO₂ (0.05 g)/NiPc (0.05 g)/GO (0.05 g)	1.38	0.054
Reaction conditions: FA (30 ml), catalyst, r.t., 1h.

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 ⁰C. Temperature had an important influence on the catalytic activity of NiPc@GO/TiO₂ and NiPc@GO, where a high temperature was beneficial for the dehydrogenation reaction (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/TiO₂ 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 HNO₃. 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 H₂ was produced in each of them, which shows as expected TiO₂, NiPc, and GO are stable in formic acid.

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 H₂. In addition, TiO₂ improved NiPc@GO catalytic activity significantly. Results showed that TiO₂ 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.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

The authors would like to acknowledge the support received from the Research Council of Urmia University.

AUTHORS CONTRIBUTION

Sajjad Keshipour: Conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, project administration, resources, supervision, validation, visualization, writing-original draft, and writing-review & editing. Shima Mohammad Alizadeh: Data curation, formal analysis, investigation, methodology, resources, validation, visualization, and writing-review & editing.

SUPPORTING INFORMATION

Data sharing not applicable – no new data generated.

ORCID

Sajjad Keshipour https://orcid.org/0000-0003-3545-2907

Shima Mohammad Alizadeh https://orcid.org/0000-0003-0795-546X

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No competing interests reported.

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You are reading this latest preprint version

Nickel Phthalocyanine@graphene Oxide/TiO₂ As an Efficient Degradation Catalyst of Formic Acid Toward Hydrogen Production

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Material and Methods

2.2. Preparation of NiPc@GO

2.3. Typical procedure for the degradation of formic acid by NiPc@GO/TiO₂

Results And Discussion

Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1

Nickel Phthalocyanine@graphene Oxide/TiO2 As an Efficient Degradation Catalyst of Formic Acid Toward Hydrogen Production

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Material and Methods

2.2. Preparation of NiPc@GO

2.3. Typical procedure for the degradation of formic acid by NiPc@GO/TiO2

Results And Discussion

Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1

Nickel Phthalocyanine@graphene Oxide/TiO₂ As an Efficient Degradation Catalyst of Formic Acid Toward Hydrogen Production

2.3. Typical procedure for the degradation of formic acid by NiPc@GO/TiO₂