Kinetics Study on Photocatalytic Degradation of Methyl Orange Catalyzed by Sea Urchin-Like Cu 2

Sea urchin-like cuprous oxide with hollow glass microsphere as core was prepared using sodium sulfite as the reducing agent and sodium acetate-acetic acid as buffer solution in copper sulfate solution. Methyl orange was selected as degradation target for photocatalytic experiments. The photocatalytic activities were investigated by visible spectrophotometer. Photocatalytic kinetics parameters were studied by the Langmuir-Hinshelwood model and Arrhenius formula. It was observed that the sea urchin-like morphology dramatically improved the photocatalytic activity of cuprous oxide. The photo-degradation belongs to the first-order reaction and the maximum degradation rate could reach 94.37%. The activation energy and pre-exponential factor are 41.18 KJ·mol-1 and 1.07 × 106, respectively. After seven times recycling, the sample still showed high photo-catalytic efficiency and stability.


Introduction
Dye wastewater with the characteristics of complex composition, high color and emissions, high toxicity and poor biodegradability has serious polluted the environment.The common treatment methods of the dye wastewater are flocculation precipitation [1], electrolysis [2], adsorption [3], and biological vectors [4] in industry.One common disadvantage of these methods is that they can transfer the contamination from one phase to another rather than being destructive.Therefore, the invention of a new treatment method without secondary pollution is deemed necessary.In recent years, it has become a focus topic that catalyst is utilized to photodegrade the organic pollutants in wastewater.Cuprous oxide (Cu 2 O) has a direct band gap of 2.0 eV which can be excited by visible light, and it has high stability in solar cells [5].In the past decade, Cu 2 O with various morphologies, such as nanosize spheres [6], sea urchin-like [7], porous octahedron [8], nanowires [9] [10], bi-pyramids [11] star-like and flower-like [12], has been synthesized by different techniques.Furthermore, it has showed an ideal effect on the photocatalytic degradation organic pollutants in water.However, there are rare reports on the photocatalytic kinetics of Cu 2 O in previous studies.
In this work, the influence of initial methyl orange (MO) concentration on photo-degradation efficiency was studied.Various kinetics parameters including the reaction rate constants, the activation energy, the pre-exponential factor and the reaction order were obtained and the stability property of the sea urchin-like Cu 2 O was also studied.

Experimental
Preparation and characterization of the sea urchin-like Cu 2 O in detail has been investigated as in [7]    Compared with diffrentCu 2 O list in Table 1, sea urchin-like Cu 2 O showed much higher photocatalytic activity due to the three reasons.(a) Needle-like whiskers increase the surface area that determined by BET method is 3.3961 m 2 /g.While the BET surface areas of the octahedral morphology and the truncated octahedral morphology are 0.0308 m 2 /g and 0.1819 m 2 /g, respectively [13].(b) The large area exposure of the Cu 2 O whiskers, in which visible light can occur numerous times of reflection and diffuse reflection, augments the adsorption ability and utilization rate of visible light.(c) The width size of crystal whiskers of the sea urchin-like Cu 2 O is only about 100 nm, so •OH species formed could easily reach the surface of the crystal whiskers to oxidize MO.

Reaction Order of the Degradation of MO
The degradation of Cu 2 O to MO be supposed to first-order kinetics.So, the previous data (before 20 min) taken from Figure 2 were drawn into plots of −ln(C/C 0 ) and irradiation time shown in Figure 3 The results reveal that the photo-degradation conform the pseudo first-order kinetics really.

The Activation Energy (Ea) and Pre-Exponential Factor (A) of the Sea Urchin-Like Cu 2 O
The temperature can strongly increase the dye degradation, so the activation energy of degredation be studied by the Langmuir-Hinshelwood model [18]., where, Ea is activation energy and A is pre-exponential factor.In Arrhe- , where, Ea is activation energy and A is pre-exponential factor.Ea and A of the degradation are 41.18KJ•mol −1 and 1.07 × 10 6 , respectively, with the correlation coefficient of 0.997.Though the raction Ea is little higher than other Cu 2 O samples [13]- [15] [19]- [21], the significant increase of A also accelerate the reaction, hence the photo-degradation efficiency of MO is much higher than other Cu 2 O samples [13]- [16].

Conclusion
The sea urchin-like Cu 2 O has highly active to degradation of MO under visible light irradiation.Degradation rate was achieved to 94.37% within 25 min.The photo degradation of MO follows the Langmuir-Hinshelwood model and belonging to the first-order reaction.The activation energy and pre-exponential factor are 41.18KJ•mol −1 and 1.07 × 10 6 , respectively.After seven times recycling, the Cu 2 O still showed higher photo-catalytic and the SEM image of the sea urchin-like Cu 2 O shows in Figure 1.The sea urchin-like Cu 2 O of 0.50 g•L −1 was added into MO solutions with different initial concentrations according to the corresponding proportion.Then the system was illuminated under a 24 W fluorescent lamp (FSL) after the suspension was stirred in darkness for 40 min to ensure adsorption equilibrium.During the reaction, the beaker filled with the suspension was put into a water-bath to maintain the solution at a constant temperature.The distance between the lamp and the solution surface is 15 cm.The suspension was strongly stirred in order to keep the Cu 2 O well suspended in MO solution.During the course of irradiation, 10 mL of the suspension was drawn once from the mixture solution every 5 min and filtrated the Cu 2 O.The absorbance of MO aqueous solutions was measured by 7230 G visible spectrophotometer at 464 nm.

Figure 2
the degradation rate of MO first increases and then decreases with the increase of MO concentration from 20 mg•L −1 to 60 mg•L −1 .And the degradation rate of MO after 25 min reaches 90.9%, 92.23%, 94.37%, 91.48% and 35.35%, respectively.

2 Cu 1 k
OH − + h + → • OH, •OH + MO  → products.The degradation rate of MO be expressed as: time and the amount of Cu 2 O are constant, the concentrations of •OH radicals are also constant,

Figure 4
Figure 4 shows the results of lnk at different temperature, lnk are given as: 41183.7 ln 13.88 k RT = − .

3. 4 .
Photo-Catalytic Efficiency of Seven Times Recycling Use of Catalyst Stability of Cu 2 O was investigated by reuse 7 times shows in Figure 5 It indicates that the efficiency change from 95.84% to 76.1%.The X-ray patterns of residual Cu 2 O after the seventh recycled, as shown in Figure 6(b), is same as before use.So, the sea urchin-like Cu 2 O prepared is much stable in the photo-catalysis process under the acidic conditions.

Figure 4 .
Figure 4. Dependence of the first-order kinetics constant and temperature.

Figure 5 .
Figure 5.The degradation to MO of sea urchin-like Cu 2 O photo-catalyst at different reuse times.

Figure 6 .
Figure 6.XRD patterns of sea urchin like (a) before use (b) after reuse for seven times.

Table 1 .
The degradation rate of MO in the presence of different Cu 2 O.
Figure 3. First-order decay curve at different initial MO concentrations.