English

• Being a green environmental remediation method, the photocatalysis technology has emerged among many advanced oxidation process [1-4]. Traditionally, photocatalytic oxidation has been widely used in pollutant degradation [5-9], air purification [10-13], sterilization [14-16], and water splitting [17-21]; recently, our group has introduced an innovative photocatalytic technology in the field of precious metal recovery [22]. With the development of waste resources, the recovery of precious metals in secondary resources has gradually attracted increasing attention [23-28]. In this work, we focus on the dissolution of precious metals, which is a challenging process due to the strong chemical inertness of precious metals.

  The technology that is currently being used to dissolve precious metals is mainly based on aqua regia and cyanidation, both of which require high-risk experimental conditions and cumbersome procedures [29-34]. Furthermore, traditional technologies are prone to generate a large amount of secondary waste, and they cannot be used in all situations [26]. There is an urgent need for an environmentally friendly and efficient precious metal dissolution technology [34-36]. In a previous work, we used the superoxide radical (^·O₂⁻) derived from photogenerated electrons and coordination components to promote the delocalized oxidation of the outer electrons in precious metals [22]. Seven types of precious metals can be dissolved using a photocatalyst without the need for any acid, alkali, nor cyanide.

  To render the photocatalytic dissolution technology more suitable for industrial application, it is necessary to increase the dissolution rate of precious metals. Only by identifying the key active species in the reaction process can the reaction rate of the rate-determining step be accurately improved [37-41]. In preliminary explorations of the mechanism, we believed that ^·O₂⁻ and photogenerated electron–hole pairs are indispensable active substances for the dissolution of precious metals. Therefore, inhibiting the photogenerated electron–hole recombination or increasing the production of ^·O₂⁻ may be an effective strategy to accelerate the dissolution of precious metals.

  Phosphate particles can interact with hydroxyl groups and be strongly adsorbed on the surface of TiO₂, which can significantly affect the interface and surface chemical properties of TiO₂. On the one hand, phosphate has a good adsorption effect on O₂. On the other hand, phosphate decorated made the surface of TiO₂ nanoparticles negatively charged and promoted the migration of photogenerated holes to the surface of the photocatalyst. In the dissolution of precious metals, we need more O₂ to participate in the reaction and reduce the recombination of photogenerated e⁻ and h⁺, so phosphate decorated TiO₂ was chosen as catalyst. Based on these considerations, we prepared different salt-decorated TiO₂ materials and used them in the photocatalytic dissolution of precious metals. It is worth noting that decorating with PO₄³⁻ can effectively improve the photocatalytic dissolution efficiency (by 1.2–2.8 times compared with that of undecorated TiO₂). This is attributed to the dual functionality of PO₄³⁻, which can not only adsorb more oxygen molecules on the catalyst surface to promote the generation of ^·O₂⁻ but also suppress the recombination of electrons and holes in TiO₂. However, optimizing the oxygen adsorption or carrier separation alone cannot accelerate the dissolution of precious metals. In order to increase the photocatalytic dissolution rate of precious metals, the two abovementioned approaches must be combined. Phosphate-loaded TiO₂ can be used for the recycling of precious metals from central processing unit (CPU) boards or various waste catalysts, and the proposed strategy is feasible and can be expanded in the future.

  Phosphate-loaded TiO₂ (P-TiO₂) was synthesized using the one-step impregnation method. Firstly, a certain amount of Na₃PO₄ was added to 40 mL of deionized water to prepare the PO₄³ precursor. The preparation of 4% P-TiO₂ requires 5.26 g of Na₃PO₄. Different P-TiO₂ ratios reduce the Na₃PO₄ content. Then, 400 mg of commercial TiO₂ (P25) was added to form a milky white solution, which was kept at 75 ℃ for 4 h. The white precipitate produced was collected through centrifugation and dried at 80 ℃ in an oven for 12 h. An undecorated P25 sample was utilized for comparison and labeled TiO₂. Different phosphate-loaded P25 materials were prepared following the abovementioned preparation steps, and only the phosphate source was replaced for the different samples. We analyzed different contents of P-TiO₂ by SEM EDS, where the P contents of 1%, 2%, 3% and 4% P-TiO₂ were as 1.10%, 2.08%, 2.70% and 4.54%, respectively (Table S1 in Supporting information). In addition, the P decorating results were quantificationally obtained in Table S2 (Supporting information) by Eq. 1, which was consistent with Table S1.

  (1)

  The Au NPs were prepared by the sodium citrate reduction method [42]. First, 1% sodium citrate aqueous solution was prepared by 1 g sodium citrate and 100 mL DI. Then, 1 mL HAuCl₄ (10 mg/L) and 99 mL DI were boiled in the oil bath pan. After the solution was boiled to 100 ℃, 5 mL of 1% sodium citrate solution was dropped expeditiously, and continued boiling for 10 min to obtain Au NPs.

  Take a certain amount of pre-prepared Au NPs and 1 g SiO₂ into a 250 mL round-bottomed flask. In a rotary steamer, place the mixture at 80 ℃ and spin-evaporate the solvent at a speed of 60 rpm to prepare Au/SiO₂. The Au content in Au/SiO₂ dissolved in aqua regia was tested by inductive coupled plasma emission spectrometer to detect the Au loading.

  Disperse 50 mg of photocatalyst and 50 mg of 1 wt% Au/SiO₂ in a 50 mL quartz bottle containing 20 mL of organic mixed solution (15 mL CH₃CN and 5 mL CH₂Cl₂). Under 500 rpm stirring, put it under UV LED lamp to react (365 nm, 800 mW/cm²). Take samples every 20 min to test the dissolved amount of Au. The photocatalytic dissolution efficiency is calculated according to the actual detected metal content by the inductively coupled plasma spectrometer, and the dissolution rate was calculated according to Eq. 2 below:

  (2)

  This photocatalytic process is fluid–solid heterogeneous reaction and is commonly described by the shrinking-nucleus model (Eq. 3) [43, 44]:

  (3)

  where k_c represents the apparent rate constants (min⁻¹) for diffusion through the product layer chemical reaction and t is the reaction time, the leaching fraction of precious metal x = M_t/M₀, where M₀ is the mass of precious metal and M_t is the mass of precious metal in the leached solution.

  The dissolution method of other precious metals is the same as above, just change the dissolving object. Since Rh cannot be dissolved in aqua regia, Rh is dissolved in the hydrofluoric acid. And phosphate and silver (Ag) easily form silver phosphide precipitates, so the dissolution of Ag is not considered here.

  PO₄³⁻ modified TiO₂ was obtained through a simple solution impregnation with P25. Firstly, we focused on P25 loaded with 4% Na₃PO₄, which is here referred to as 4% P-TiO₂. It can be seen from the SEM image that the size and morphology of the sample did not change after the incorporation of PO₄³⁻ (Figs. S1a and b in Supporting information). The P element is evenly distributed on the surface of the material loaded with PO₄³⁻ (Fig. 1a). No change occurs in the lattice fringes of the modified P25 (Figs. S1c and d in Supporting information). From the XRD patterns of the catalysts with different decorating contents, it can also be demonstrated that the incorporation of PO₄³⁻ has no effect on the crystal structure of TiO₂ (Fig. 1b). The catalyst peaks in the Raman spectra are found at 145.6, 196.8, 398.5, 517.8 and 638.1 cm⁻¹ and thus have the same location as the TiO₂ peaks [45], further confirming that the incorporation of PO₄³⁻ does not affect the crystal structure of TiO₂ (Fig. S2 in Supporting information). Furthermore, it is clear from the FTIR spectra that PO₄³⁻ was successfully loaded on the surface of TiO₂. Compared with pure TiO₂, 4% P-TiO₂ exhibits new peaks located at 1010–1250 cm⁻¹ and 1260–1320 cm⁻¹, which are attributed to the tensile vibrations of Ti–O–P–O⁻ and P=O, respectively (Fig. 1c) [46-48]. The XPS spectra confirm the combination of P (PO₄³⁻) and O (TiO₂) in the catalyst. In addition to the characteristic peak of PO₄³⁻ in the P 2p spectrum at 133.18 eV, a peak corresponding to the P–O bond between P and TiO₂ can also be found at 132.38 eV (Fig. 1d) [49]. The Ti 2p_3/2 and Ti 2p_1/2 peaks for 4% P-TiO₂ are located at 457.9 eV and 463.6 eV, signifying the dominant state of Ti⁴⁺. Moreover, the high-resolution XPS result definitely confirms existence of Ti–O–P based on the peaks at 459.7 eV and 464.8 eV (Fig. S3a in Supporting information). In the O 1s spectrum of 4% P-TiO₂, the peak for the Ti-O bond in TiO₂ has been observed at 529.1 eV. Note that the peaks at 530.1 eV corresponded to the oxygen species Ti-O-P and P=O, and the peak at 530.8 eV was assigned to P–O–H (Fig. S3b in Supporting information). The high-resolution P 2p spectrum also confirms the presence of P–O–Ti and PO₄³⁻ (Fig. S3c in Supporting information). The Na 1s of the 4% P-TiO₂ is located at 1071.4 eV (Fig. S3d in Supporting information) [50]. In addition, the zeta potential of 4% P-TiO₂ remains always negative in the pH range of 2–13, indicating that the catalyst surface has negatively charged ions (PO₄³⁻) (Fig. S4 in Supporting information).

  Figure 1

  

  Figure 1.  Catalyst structure and morphology characterization. (a) SEM-EDS mapping of 4% P-TiO₂. (b) XRD pattern and (c) FTIR spectra of the different decorated amounts of P-TiO₂ sample. (d) The P 2p XPS spectra of 4% P-TiO₂ and TiO₂.

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  We then focused on the as-prepared 1 wt% Au/SiO₂ sample to explore the catalytic activity of P-TiO₂. Au is uniformly loaded on the surface of SiO₂ (Fig. S5 in Supporting information). Through a series of optimization experiments, it was found that the catalyst activity of P-TiO₂ is the best when the sample is immersed in an Na₃PO₄ aqueous solution (pH 13) at 75 ℃ for 4 h (Fig. S6 in Supporting information). To explore the effect of Na⁺ on the dissolution of precious metals, we chose sodium nitrate (NaNO₃), which does not adsorb oxygen and does not react with precious metals, as the source of Na⁺. When different concentrations of NaNO₃ solutions (0.01–1 mol/L) were added during the photocatalytic dissolution of Au, the dissolution rate of Au did not change significantly (Fig. S7 in Supporting information). Therefore, the addition of Na⁺ will not affect the dissolution effect of precious metals. In fact, when the concentration of Na₃PO₄ solution reaches 0.8 mol/L, the solution is saturated. In saturated Na₃PO₄ solution, the single maximum loading of PO₄³⁻ is 4%. After modified P-TiO₂ several times, the results showed that the PO₄³⁻ content on TiO₂ was not enhanced even after multiple dipping, and thus the dissolution efficiency was not significantly improved (Fig. S8 and Table S3 in Supporting information). Therefore, the activity of precious metal dissolution does not continue to increase when PO₄³⁻ doping exceeds 4%. The higher the PO₄³⁻ content, the faster the dissolution of the precious metals; this may be due to the increase in the number of PO₄³⁻ active sites on the TiO₂ surface (Fig. 2a). From the reaction kinetic curve, it can be observed that the dissolution rate of 4% P-TiO₂ is 2.8 times higher than that of TiO₂ (Fig. 2b). Over a period of 240 min, the performance of 4% P-TiO₂ was found to be 1.8 times higher than that of pure TiO₂ for the dissolution of 5% Pt/C, 1.4 times higher for the dissolution of 5% Pd/C, 1.3 times higher for the dissolution of 5% Rh/C, 1.2 times higher for the dissolution of 5% Ru/C, and 1.2 times higher for the dissolution of 5% Ir/C (Figs. 2c–f).

  Figure 2

  

  Figure 2.  Photocatalytic dissolution test of precious metals. (a) Dissolution percentage and (b) kinetic linear fitting curves of Au with different photocatalysts. (c) Dissolution percentage and (d) kinetic linear fitting curves of Pd, Pt, Ru, Rh and Ir using TiO₂. (e) Dissolution percentage and (f) kinetic linear fitting curves of Pd, Pt, Ru, Rh and Ir using 4% P-TiO₂. Reaction conditions: solution (20 mL, CH₃CN: CH₂Cl₂ = 3:1), 50 mg catalyst, 50 mg Au/SiO₂, 500 rpm/min, 365 nm LED lamp (800 mW/cm²) in air condition.

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  The increase in the dissolution rate of precious metals originates from the increase in the number of reactive species. The photocatalytic dissolution of precious metals is a typical photochemical reaction; thus, photogenerated electron–hole pairs are essential active substances. In addition, the capture experiment proved that the ^·O₂⁻ produced by the reaction between O₂ and electrons is a reactive radical species (Figs. 3a and b). The free radicals in the dissolution process were detected via ESR, and the results show that 4% P-TiO₂ results in a higher ^·O₂⁻ content than TiO₂ (Fig. 3c). Since the PO₄³⁻ on the catalyst surface is beneficial for the adsorption of O₂, the higher the PO₄³⁻ content, the larger the amount of O₂ chemisorbed on the catalyst surface, and the higher the number of ^·O₂⁻ generated (Fig. 3d). The mechanism through which P-TiO₂ increases the photocatalytic dissolution rate of precious metals is as follows (Fig. 3e). Firstly, the light excites the catalyst, which leads to the generation of electrons and holes, while the PO₄³⁻ on the catalyst adsorbs a large number of oxygen molecules. Then, the electrons migrate toward the surface of the catalyst and combine with the oxygen molecules, which results in the generation of ^·O₂⁻. The holes react with the organic molecules in the solvent to generate organic free radicals. Finally, the free radicals interact with the precious metals and dissolve them.

  Figure 3

  

  Figure 3.  (a) Dissolution profile and (b) kinetic linear fitting curves of Au under the capture of different living species (DDQ capture electrons (e⁻), EDTA-2Na capture holes (h⁺), p-benzoquinone capture superoxide radical (^·O₂⁻)). (c) ESR test for ^·O₂⁻ species. (d) O₂-TPD of TiO₂ and x% P-TiO₂. (e) Mechanism of P-TiO₂ activated oxygen enhancing photocatalytic precious metal dissolution. Reaction conditions: solution (20 mL, CH₃CN: CH₂Cl₂ = 3:1), 50 mg catalyst, 50 mg Au/SiO₂, 500 rpm/min, 365 nm LED lamp (800 mW/cm²) in air condition.

  DownLoad: Full-Size Img PowerPoint

  Theoretical calculations were conducted to simulate the movement of O₂ on the surfaces of TiO₂ and P-TiO₂ (Fig. S9 in Supporting information). In the simulations involving the O₂ adsorption on the TiO₂ and P-TiO₂ surfaces, the adsorption energy was defined as ΔE_ad [ΔE_ad = E_TiO − E_TiO − E_sub)]. During the TiO₂ catalysis, the O atoms in O₂ are adsorbed on TiO₂ and connect with the Ti atoms, forming a bond with a length of 1.976 Å. However, PO₄³⁻ has a stronger adsorption capacity for oxygen (Table S4 in Supporting information). The bond lengths between the Ti atoms in TiO₂ and the O atoms in O₂ and the P atoms in PO₄³⁻ and the O atoms in O₂ are 1.979 and 3.116 Å, respectively.

  On the other hand, the PO₄³⁻ incorporation on TiO₂ causes the TiO₂ surface to be negatively charged (Fig. S4), which favors the migration of photogenerated holes toward the catalyst surface, thereby promoting the separation of photogenerated electrons and holes. In Fig. S10 (Supporting information), transient photocurrent responses and electrochemical impedance spectroscopy (EIS) Nyquist plots of different decorated amounts of (0%−4%) P-TiO₂ show that the photocurrent is higher and the impedance is lower with the increasing of PO₄³⁻ decorated amounts, which further confirms that the charge carrier separation is enhanced after the PO₄³⁻ decoration. The time-resolved spectroscopic data confirm that 4% P-TiO₂ has a longer carrier life (τ_PL = 0.7556 ns) than TiO₂ (τ_PL = 0.7122 ns) (Fig. S11 in Supporting information). The dissolution reaction rate cannot be increased significantly when other salts (such as NaBr and NaCl) replace PO₄³⁻. It can be found that, compared with TiO₂, decorating with NaBr can increase the separation between photogenerated charges; however, NaBr has a weaker O₂ adsorption. On the other hand, NaCl can absorb more O₂, but decorating with NaCl does not result in a significant separation of the photogenerated carriers (Fig. S12 in Supporting information). Therefore, in order to achieve a high-efficiency dissolution of precious metals, it is necessary to increase the yield of ^·O₂⁻ and the separation of the photogenerated carriers at the same time.

  To verify the applicability of P-TiO₂, we selected representative CPU boards (containing Au), Pt/SiO₂, and Pd/molecular sieves for investigation. The dissolution rates of 4% P-TiO₂ for the CPU board, Pt/SiO₂, and Pd/molecular sieve are 1.8, 1.5 and 1.7 times higher than those of TiO₂, respectively (Figs. 4a–c). Before the reaction, the CPU board, Pt/SiO₂, and Pd/molecular sieve were characterized by golden yellow, gray–black, and yellow–brown colors, respectively; after dissolution, the precious metals detached from the carrier, causing the colors to fade (Figs. 4d–f). Not only 4% P-TiO₂ exhibits a reasonably good universality, but it also has a good stability. Since PO₄³⁻ only adheres to the surface of TiO₂ via electrostatic adsorption, agitation during the reaction may cause the active sites to detach. In the cyclic experiment, the activity of 4% P-TiO₂ decreased only by around 20% after 10 continuous cycles (Fig. 4g). From the SEM mapping, it can be found that the content of the P element on the surface of the catalyst is slightly reduced after the reaction (Fig. S13 and Table S1 in Supporting information). As we placed the deactivated catalyst again into the Na₃PO₄ solution for regeneration, the catalyst resumed its original activity. This confirms that the investigated catalyst can be used in practical applications.

  Figure 4

  

  Figure 4.  Photocatalytic dissolution diagram of precious metals from (a) CPU board, (b) Pt/SiO₂ and (c) Pd/molecular sieve. Photographs of retrieving precious metals from (d) CPU board, (e) Pt/SiO₂ and (f) Pd/molecular sieve before and after reaction. (g) Cycle stability test of 4% P-TiO₂. Reaction conditions: Solution (60 mL, CH₃CN: CH₂Cl₂ = 3:1), 1.0 g catalyst, 1.0 g precious metal material, 500 rpm/min, 365 nm LED lamp (800 mW/cm²) in air condition.

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  In this work, we used a simple one-step method to prepare P-TiO₂. Through the design and control of the catalyst surface interface, PO₄³⁻ was selected among various salts. This catalyst can not only promote the separation of photogenerated electrons and holes but also adsorb more oxygen molecules. Due to this dual functionality, the generation of ^·O₂⁻ is enhanced, which improves the efficiency of the photocatalytic dissolution of precious metals. This simple catalyst can be prepared at low cost, and offers the possibility to enhance the photocatalytic recovery of precious metals.

  Declaration of competing interest

  The authors declare no competing interests.

  Acknowledgments

  This work was supported by the National Key Research and Development Program of China (No. 2020YFA0211004), the National Natural Science Foundation of China (Nos. 22176128, 21876114), Sponsored by Program of Shanghai Government (Nos. 21XD1422800, 19DZ1205102, 19160712900), Chinese Education Ministry Key Laboratory and International Joint Laboratory on Resource Chemistry, and Shanghai Eastern Scholar Program. "111 Innovation and Talent Recruitment Base on Photochemical and Energy Materials" (No. D18020), Shanghai Engineering Research Center of Green Energy Chemical Engineering (No. 18DZ2254200). Shanghai Frontiers Science Center of Biomimetic Catalysis.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.03.117.

  
  
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  Figure 1  Catalyst structure and morphology characterization. (a) SEM-EDS mapping of 4% P-TiO₂. (b) XRD pattern and (c) FTIR spectra of the different decorated amounts of P-TiO₂ sample. (d) The P 2p XPS spectra of 4% P-TiO₂ and TiO₂.

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  Figure 2  Photocatalytic dissolution test of precious metals. (a) Dissolution percentage and (b) kinetic linear fitting curves of Au with different photocatalysts. (c) Dissolution percentage and (d) kinetic linear fitting curves of Pd, Pt, Ru, Rh and Ir using TiO₂. (e) Dissolution percentage and (f) kinetic linear fitting curves of Pd, Pt, Ru, Rh and Ir using 4% P-TiO₂. Reaction conditions: solution (20 mL, CH₃CN: CH₂Cl₂ = 3:1), 50 mg catalyst, 50 mg Au/SiO₂, 500 rpm/min, 365 nm LED lamp (800 mW/cm²) in air condition.

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  Figure 3  (a) Dissolution profile and (b) kinetic linear fitting curves of Au under the capture of different living species (DDQ capture electrons (e⁻), EDTA-2Na capture holes (h⁺), p-benzoquinone capture superoxide radical (^·O₂⁻)). (c) ESR test for ^·O₂⁻ species. (d) O₂-TPD of TiO₂ and x% P-TiO₂. (e) Mechanism of P-TiO₂ activated oxygen enhancing photocatalytic precious metal dissolution. Reaction conditions: solution (20 mL, CH₃CN: CH₂Cl₂ = 3:1), 50 mg catalyst, 50 mg Au/SiO₂, 500 rpm/min, 365 nm LED lamp (800 mW/cm²) in air condition.

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  Figure 4  Photocatalytic dissolution diagram of precious metals from (a) CPU board, (b) Pt/SiO₂ and (c) Pd/molecular sieve. Photographs of retrieving precious metals from (d) CPU board, (e) Pt/SiO₂ and (f) Pd/molecular sieve before and after reaction. (g) Cycle stability test of 4% P-TiO₂. Reaction conditions: Solution (60 mL, CH₃CN: CH₂Cl₂ = 3:1), 1.0 g catalyst, 1.0 g precious metal material, 500 rpm/min, 365 nm LED lamp (800 mW/cm²) in air condition.

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