Faqi
Zhan
*a,
Ruixin
Li
a,
Haiyan
Zhao
a,
Dalin
Chen
b,
Yisi
Liu
c,
Min
Zhu
a,
Yuehong
Zheng
a,
Peiqing
La
*a and
Jie
Li
d
aState Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals, School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou, 730050, China. E-mail: zhanfaqi@lut.edu.cn; pqla@lut.edu.cn; Fax: +860931-2976688; Tel: +860931-2976688
bState Key Laboratory of Nickel and Cobalt Resources Comprehensive Utilization, Jinchang, 737100, China
cInstitute of Advanced Materials, Hubei Normal University, Huangshi, 415000, China
dSchool of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China
First published on 16th April 2025
The efficient extraction and recycling of noble metal resources have become an inevitable trend for sustainable socio-economic development. Traditional methods suffer from lengthy processes, high energy consumption, and serious environmental hazards, necessitating the development of new, environmentally friendly, cost-effective, and efficient methods for noble metal extraction. This study primarily employs a photochemical method to reduce and enrich low-concentration Au/Pt noble metal solutions on the surface of the WO3 semiconductor, followed by alkali leaching and acidification steps to achieve Au/Pt extraction and WO3 recycling. It can complete the enrichment of noble metals within 20 min, with a deposition efficiency of over 99% and a total recovery efficiency of over 93%, suitable for noble metal concentrations ranging from 0.01 to 10 mM. The extracted Pt particles have a diameter of approximately 5 nm, and the Au particles have a diameter of about 60 nm, both with a purity of 99.9%. The acid-regenerated WO3 semiconductor can be reused cyclically, maintaining stable performance. This study establishes a novel technology for the recycling and recovery of noble metals from lean solutions based on semiconductor photochemistry, holding significant scientific and practical value for noble metal resource recycling.
In traditional catalytic industries, noble metals are frequently deposited onto semiconductor surfaces through photochemical methods to enhance photocatalytic performances.20–27 Naphaphan et al.28 prepared Au/TiO2 composite photocatalysts by reducing Au ions with TiO2 under UV irradiation, achieving a reduction time of less than 10 min and an Au ion concentration in the precursor of just 50 mg L−1. This study demonstrated that photocatalysts can reduce low concentrations of Au ions in solution through a process that is both environmentally friendly and efficient. Joseph et al.29 utilized ZnO as a substrate and sequentially deposited minute quantities of Au and Pt onto its surface. Their experiment indicated that semiconductor catalysts can recover various noble metals such as Au and Pt, highlighting the environmental benefits of photocatalysis. Zheng et al.30 reduced Au ions in a 0.1 mM HAuCl4 solution using UV light, depositing them onto ZnO, and prepared ZnO/Au materials within 10 min, effectively reducing Au ions at very low concentrations. According to the aforementioned studies, photodeposition methods can efficiently deposit noble metals onto semiconductor surfaces under low concentration conditions, and this technology has matured in the preparation of noble metal-modified semiconductor catalysts.31 However, to achieve efficient separation of photogened electrons and holes, the loading amount typically remains low. When applied to noble metal extraction, this technology's extraction efficiency and maximum loading capacity remain unknown.
Posachayanan et al.32 used TiO2-AC as a photocatalyst to recover noble metal Ag from electroplating wastewater under UV conditions, recovering 94% of the Ag in the wastewater within 45 min. Muscetta et al.33 used ZnO to deposit Pd from wastewater under UV conditions, completing the deposition of 0.1–1 mM Pd ions within 90 min. The loading of Pd can further enhance photocatalytic performance, and subsequent acidification was used to separate Pd–ZnO. It is worth noting that the reduction potentials of TiO2 and ZnO are relatively negative, approximately −0.3 V (vs. NHE).34 Theoretically, if non-noble metals are present in the solution system, they will also be reduced, ultimately leading to the extracted noble metals' purity being compromised. Moreover, ZnO is unstable under acidic conditions, while waste liquids in actual industrial production are mostly acidic. Although the aforementioned methods offer valuable insights for our research, catalysts like TiO2 and ZnO are not suitable for recovering noble metals from acidic waste solutions. The reduction potential of WO3 is approximately 0.7 V (vs. NHE),35 situating it between noble metals and non-noble metals. Theoretically, it can reduce noble metals such as Au without affecting non-noble metals like Fe, Co, and Ni. WO3 is relatively stable under acidic conditions (pH < 3),36 facilitating effective Au deposition. It readily dissolves under alkaline conditions (pH > 8),37 enabling the separation and purification of noble metals from WO3, while also allowing for its recycling and regeneration.
In this study, WO3 was selected as the catalyst, and nanoscale WO3 was prepared via the calcination method. The photoreduction method was employed to extract Au ions from acidic solutions, investigating the applicable concentration range and the kinetics of Au ion photodeposition. Through conditional experiments, the effects of varying WO3 content, pH levels, and ethanol concentrations on the photodeposition effect were studied, and the optimal process parameters for Au recovery were determined. Additionally, the effectiveness of recovering and extracting Au/Pt mixed ions and the repeated recovery performance of WO3 for Au/Pt were evaluated. Finally, the recovered samples underwent alkaline leaching to purify the noble metals. The regeneration and recycling of WO3 were achieved through the acidification-calcination method. An economic assessment of the entire experimental process and technical methods demonstrated significant economic advantages of the technology used in this study. Compared to traditional methods, this technology offers efficient, green, and low-cost recovery and extraction of noble metals, which is significant for the recycling of noble metal resources in China.
After determining the optimal conditions for the photocatalytic reaction of Au ions, a mixed solution containing Pt and Au ions, each at a concentration of 0.1 mM, was prepared. Ethanol was added to achieve a concentration of 0.2 M, followed by the addition of 0.25 g of WO3 nanoparticles, and the pH was adjusted to 1. Following photodeposition reaction, the samples were filtered, washed, and subsequently dried in a vacuum oven at 70 °C for 6 h.
Fig. 1a illustrates the relationship between absorbance and wavelength for standard solutions with 5 different initial Au ion concentrations. The Au ion concentrations were 0.001 mM, 0.005 mM, 0.01 mM, 0.05 mM, and 0.1 mM, respectively. Absorption peaks were observed at both 227 nm and 313 nm, but the correlation at 227 nm was lower (R2 = 0.97). Therefore, 313 nm was chosen as the characteristic peak. Fig. 1b depicts the relationship between time and absorbance at 313 nm for an Au ion concentration of 0.001 mM. The results indicated that the absorbance remained constant over time, suggesting that UV light had no effect on the sample during testing. This confirms the feasibility of using spectrophotometry to measure Au ion concentration. Fig. 1c illustrates the relationship between different initial Au ion concentrations and absorbance. The results demonstrated a linear relationship between absorbance and Au ion concentration, with an equation of y = 5.6906x + 0.0075 and a correlation coefficient of R2 = 99.91%, indicating a very high correlation. In the experiment, the Au ion concentration at a specific time can be calculated based on the absorbance of the solution at 313 nm.
Fig. 2a illustrates the relationship between absorbance and wavelength for standard solutions with 5 distinct initial Au ion concentrations: 0.001 mM, 0.005 mM, 0.01 mM, 0.05 mM, and 0.1 mM. Absorption peaks are observed at 205 nm and 262 nm, corresponding to the characteristic peaks of Pt ions and Au ions, respectively, in the coexisting ion solution. Fig. 2b shows the relationship between time and absorbance at an Au ion concentration of 0.001 mM and a wavelength of 262 nm. The graph demonstrates a relatively stable curve, indicating that UV light exerted no effect on the test solution during the experiment, thereby confirming the feasibility of spectrophotometry for determining ion concentration. Fig. 2c shows the standard curve of Pt ion concentration versus absorbance in the Pt/Au ion coexisting solution. The equation is y = 26.3035x + 0.0175, with a variance of R2 = 99.94%. Fig. 2d illustrates the standard curve of Au ion concentration versus absorbance in the Pt/Au ion coexisting solution. The equation is y = 41.7405x + 0.1926, with a variance of R2 = 97.53%.
It should be noted that all the metal ion concentrations measured by spectrophotometers had also been verified by ICP test, indicating the feasibility and reliability of the spectroscopy method.
The second method for WO3 regeneration: Take 1 g of the sample, add 20 mL of ammonia solution to dissolve, and stir at room temperature for 4 h to form solution A. Dissolve 2.0 g of PVP in 15.0 mL of deionized water to form solution B. Add solution B to solution A at room temperature, and stir at room temperature for 2 h. Add 2.0 g of PEG 1000 to the solution and stir at room temperature for 4 h. Pour the sol into a porcelain boat, dry at 80 °C, and then heat-treat it at 600 °C for 2 h to obtain the regenerated WO3 nanoparticles, marking as RA-WO3.
The regenerated WO3 nanoparticles were used for photodeposition to enrich and extract Au. The whole experimental procedure is shown in Fig. 3.
The impact of varying ethanol addition amounts on the photodeposition of Au under initial conditions of 0.1 mM Au concentration, pH = 1.0, and 0.5 g L−1 WO3 concentration is shown in Fig. 4f. The total photoreduction time for 0.1 M ethanol is 20 min, whereas for 0.2 to 0.5 M ethanol, the time is 10 min. Ethanol acts as a hole scavenger, preventing electron–hole pair recombination and significantly accelerating the efficiency of electron photoreduction of Au.38 The concentration of ethanol has a significant impact on the photodeposition reaction. At low ethanol concentrations, holes are not fully captured, resulting in partial recombination of electron–hole pairs and a reduced photodeposition efficiency. As ethanol concentration increases, the hole capture efficiency accelerates, thereby increasing the photodeposition efficiency. In industrial production, however, balancing economic efficiency requires optimal conditions that ensure complete hole capture while maintaining a high photodeposition efficiency. Therefore, the optimal ethanol concentration is determined to be 0.2 M.
The influence of varying pH levels on the photodeposition of Au is shown in Fig. 4g. At pH = 1.0, the total photoreduction time is 10 min, whereas it increases to 20 min at pH = 0.5 or 2. The pH value affects the surface charge properties of the photocatalyst WO3, thus influencing the adsorption and reduction of Au ions. At pH = 1.0, both adsorption and reduction processes are optimal. Furthermore, pH also influences the state of Au ions in the solution. The pH value influences the surface charge properties of the photocatalyst WO3, thereby affecting its adsorption and reduction of Au ions. According to the Nernst equation (Formula (1)) and the relationship between pH value and H+ concentration (Formula (2)),39 the pH value influences the surface potential of WO3. As the pH value increases, the surface potential decreases, leading to fewer photo-generated electrons and reduced photodeposition efficiency. However, within the pH range of 0.5 to 2.0, variations in pH have minimal impact on photodeposition efficiency.
E = EΘ + 0.059![]() | (1) |
pH = −log[H+] | (2) |
![]() | ||
Fig. 5 Potential–pH diagram of Au–H2O.40 |
Fig. 4h illustrates the impact of varying WO3 concentrations on the photodeposition of Au. For catalyst concentrations of 0.25 g L−1 and 1.0 g L−1, the time required for complete photoreduction is 20 min. However, a concentration of 0.5 g L−1 results in a reduction time of only 10 min, yielding the most favorable results. At low catalyst concentrations, the photodeposition process is inefficient, conversely, excessively high catalyst amounts can obstruct light, impairing light absorption and consequently diminishing the photodeposition efficiency. Under optimal conditions, the reduction efficiency of Au achieved 97.5%. When the WO3 catalyst content is low, the number of photoexcited electrons is insufficient to efficiently complete the photodeposition process. Conversely, when the catalyst content is high, it can block some light, affecting light absorption and thus reducing the photodeposition efficiency. However, within the range of 0.25 to 1.0 g L−1, the amount of WO3 has little effect on the photodeposition efficiency.
Fig. 4i and j presents photodeposition kinetics curve for Pt/Au at an initial concentration of 0.1 mM. Upon initiation of the photoreaction, the reaction efficiency increases rapidly. After 10 min of photoreaction, Pt ions are nearly completely photoreduced, reducing the concentration in the solution to 0.001 mM, corresponding to a reduction efficiency of 99%. After 2 h, the Pt ion concentration further decreases to 0.0004 mM, a reduction efficiency of 99.6%. For Au, after 10 min of photoreaction, the ions continue photoreduction at a slower efficiency compared to Pt ions. After 20 min, Au ions are nearly completely photoreduced, reducing the concentration to 0.001 mM, with a reduction efficiency of 99%. After 2 h, the Au ion concentration further decreases to 0.0005 mM, achieving a reduction efficiency of 99.5%. When Pt and Au ions coexist at low concentrations, the aforementioned experimental method still achieves high efficiency.
As described in Fig. 6c, the XRD characterization results of Pt/Au/WO3 obtained after photodeposition a Pt/Au solution with an initial concentration of 0.01 mM. The diffraction peaks corresponding to standard WO3 (PDF#20-1324) and standard Au (PDF#04-0784) are evident in this figure. Elemental Au primarily crystallizes along the (111) plane. No diffraction peaks corresponding to Pt are observed, mainly due to the low concentration of Pt in the solution and the small size and good dispersion of the Pt particles post-deposition. As presented in Fig. 6d, the SEM image of WO3 nanoparticles after Au deposition, while Fig. 6e shows the SEM image after Pt/Au deposition. The samples are uniformly dispersed and exhibit a granular morphology, with the size of the deposited Au particles estimated to be below 20 nm. In contrast, the Pt particles are even smaller, and agglomeration is more pronounced in their presence.
Fig. 6f shows the EDS surface scan distribution of Au deposited on WO3 nanoparticles, revealing a W content of 29.0 wt%, an O content of 70.8 wt%, and an Au content of 0.2 wt%. Despite the low Au content, it is uniformly distributed across the surface of WO3. Fig. 6g illustrates the EDS surface scan distribution following the photodeposition of Pt/Au, with a W content of 40.2 wt%, an O content of 57.5 wt%, a Pt content of 1.1 wt%, and an Au content of 1.2 wt%. Both Pt and Au are uniformly distributed on the surface of WO3, indicating successful incorporation of Pt and Au onto the WO3 particles.
Fig. 7a presents the transmission electron microscopy (TEM) characterization of WO3 nanoparticles following Au deposition. The image demonstrates that the particle sizes are heterogeneous, with a maximum dimension of approximately 80 nm and the smallest particles measuring less than 20 nm. Fig. 7b displays a high-resolution transmission electron microscopy (HRTEM) image, illustrating smaller particles deposited onto larger ones. The HRTEM characterization depicted in Fig. 7b reveals the crystal structure of WO3 with deposited Au. In the fast Fourier transform (FFT) image of the smaller nanoparticles, illustrated in inset Fig. 3, the corresponding [0] zone axis of Au exhibits crystal planes of (
1
) and (1
). In regions ① and ②, the corresponding zone axis of WO3 is [001], associated with crystal planes of (200) and (0
0), where a limited number of impurity particles are observed. In region ④, the corresponding zone axis of WO3 is [1
0], featuring crystal planes of (002) and (110). Fig. 7b confirms the presence of Au following the photodeposition reaction, with diffraction results from region ③ verifying that the smaller particles are indeed Au, indicating that Au is deposited onto the surface of the larger WO3 particles. The average grain size of WO3 is measured at 75 nm, while the minimum grain size of Au is 10 nm.
Fig. 7c presents a transmission electron microscopy (TEM) image following the deposition of Pt/Au, revealing small particles aggregating at one end of the larger particles. Fig. 7d displays a high-resolution transmission electron microscopy (HRTEM) image of the sample. The HRTEM image in Fig. 7d characterizes the crystal structure of WO3 with the deposited Pt/Au. The fast Fourier transform (FFT) patterns of the larger nanoparticles, as illustrated in inset patterns of the larger nanoparticles, as illustrated in inset Fig. 7⑥ and ⑦, correspond to the [110], [12], and [6
] zone axes of WO3, with the associated crystal planes being (002), (2
0), (112), (0
1), (130), and (
02). The region in inset Fig. 7⑤ illustrates the zone axes of WO3 and Au, where the zone axes of WO3 are [
30] and [
10], corresponding to the crystal planes (120), (002), (002), and (322), while the zone axis of Au is [210], corresponding to the crystal planes (02
) and (1
). The region in inset Fig. 7⑧ corresponds to the [310] zone axes of Pt and Au, with the associated crystal planes being (002), (131), (1
), and (00
). Additionally, a small quantity of impurity particles is present, with their specific composition and elemental characteristics pending further investigation. Fig. 7d effectively demonstrates the presence of Pt/Au following the photodeposition reaction. Diffraction analysis of region 8 indicates that the composition comprises pure Pt and Au, with no formation of solid solutions or, and is supported on larger WO3 particles.
Fig. 7e shows the TEM mapping distribution after the photodeposition of Au, with yellow regions indicating the presence of Au. Au is uniformly distributed across the mapping area and grows on the WO3 surface, occasionally showing epitaxial growth at the edges of WO3. The small yellow regions in panel (a) represent growing Au nanoparticles, corresponding to inset Fig. 7③. Fig. 7f presents the TEM mapping distribution after the photodeposition of Pt/Au, with yellow regions indicating Au and showing a uniform overall distribution. The purple sections indicate the presence of Pt, with some areas appearing in an agglomerated form, consistent with the EDS mapping results and corresponding to inset Fig. 7⑧.
Fig. 7g presents the XPS spectrum of WO3 nanoparticles following the deposition of Au. In the sample, only the elements O, W, and Au were detected, along with a C peak used for calibration, indicating the absence of other impurities. Fig. 7h displays the Au 4f spectrum, where the two peaks corresponding to Au 4f7/2 and 4f5/2 are observed at 84.5 eV and 88.2 eV, respectively, thereby confirming the presence of elemental Au.41–43 The research findings indicate that Au predominantly exists as elemental Au in the Au/WO3 composite material, with no other chemical states identified. In Fig. 7i, the W4f7/2 and W4f5/2 peaks are observed at 36.1 eV and 38.1 eV, respectively, indicating the presence of W6+, which denotes hexavalent tungsten in WO3. Additionally, characteristic energy loss peaks of WO3 are also observed. Fig. 7j presents the O 1s peak, which represents O2− and corresponds to the oxygen in WO3. At the O 1s peak of 532.8 eV, smaller peaks corresponding to water molecules and hydroxyl groups are observed.44 The binding energy of W 4f increased by approximately 0.4 eV when compared to the normal peak position. In contrast, the binding energy of O 1s increased by 0.3 eV. This observation suggests that the presence of Au results in a slight increase in the binding energies of both W and O.
Fig. 7k presents the XPS survey spectrum of WO3 nanoparticles following the deposition of Pt/Au. In addition to the calibration peak C 1s, only the elements W, O, Pt, and Au were detected, with no other impurities identified. Fig. 7l illustrates the Pt 4f spectrum, with the two peaks of Pt 4f7/2 and Pt 4f5/2 observed at 71.6 eV and 74.9 eV, respectively. Due to the small size of the Pt particles, some of the Pt is oxidized to form PtO, resulting in corresponding peaks at 73.1 eV and 76.6 eV. Fig. 7m shows the Au 4f7/2 and Au 4f5/2 peaks, which are located at 84.5 eV and 88.2 eV, respectively,41–43 given the significant and stable size of Au particles, no compounds other than the elemental form were detected. The results demonstrate that WO3 nanoparticles effectively deposited Pt/Au onto their surface.
The photodeposition kinetic curves for an initial Pt/Au concentration of 10.0 mM are presented in Fig. 8e and f. For Pt ions, the kinetic curves from the first three depositions indicate a progressive increase in reaction efficiency. This occurs because, during the reaction, the active sites transition from WO3 to Pt/Au/WO3, which enhances hole capture capacity and accelerates the reaction efficiency. However, in the fourth and fifth depositions, the reaction efficiency decreases due to the saturation of active sites, impeding the photocatalytic reaction. For Au ions, the kinetic curves from the first three depositions also indicate an increasing reaction efficiency. Compared to pure Au ions, the Pt/Au/WO3 active sites inhibit Au aggregation, promote dispersed growth, reduce the size of Au flakes, and improve light utilization and reaction efficiency. However, during the fourth and fifth depositions, the reaction efficiency decreases because the increased Au content expands the Au flake area, leading to greater light reflection, reduced light utilization, and ultimately impeding the photocatalytic reaction.
Based on the changes in the concentration of noble metal ions during each deposition, the cumulative deposition amounts of Au and Pt on WO3 over 5 depositions were calculated, as shown in Table 1. The concentrations of Pt and Au ions were both 10 mM, with each deposition lasting 8 h. After the first deposition, the deposition amounts of Au and Pt were 3.14 g g−1 WO3 and 3.89 g g−1 WO3, respectively. The amount of noble metals was more than three times that of the catalyst. After the fifth deposition, the deposition amounts of Au and Pt were 15.70 g g−1 WO3 and 19.47 g g−1 WO3, respectively. This demonstrates that even after multiple cycles, WO3 still has a strong deposition capacity for noble metals.
Noble metals | The cumulative loading amount of noble metals on WO3 (g per g WO3) | ||||
---|---|---|---|---|---|
1 st | 2 nd | 3 rd | 4 th | 5 th | |
Au | 3.14 | 6.28 | 9.42 | 12.56 | 15.70 |
Pt | 3.89 | 7.79 | 10.88 | 15.57 | 19.47 |
Solid samples | Concentrations of various elements in the solution after alkaline leaching | ||
---|---|---|---|
Pt (g L−1) | Au (g L−1) | W (g L−1) | |
WO3 nanoparticles with deposited Au | — | 0.0001 | 1.9872 |
WO3 nanoparticles with deposited Pt–Au | 0.0110 | 0.0027 | 2.3000 |
The Au recovered and extracted after alkali leaching is shown in Fig. 9a, and the sample appears golden yellow. Fig. 9c shows the XRD spectrum of the Au recovered and extracted after alkali leaching. From the figure, it can be seen that the measured peaks correspond to standard Au (PDF#04-0784), corresponding to the (111), (200), (220), and (311) crystal planes, confirming the sample as pure Au, with the peak of the (111) crystal plane being the strongest. According to the Debye–Scherrer formula,48 the grain sizes of the crystal planes are approximately 49.3 nm for (111), 39.4 nm for (200), 40.9 nm for (220), and 36.9 nm for (311). Fig. 9b shows the Pt/Au recovered after alkali leaching, appearing gray and indicating a mixture of Pt and Au. Fig. 9d displays the XRD pattern of Au and Pt recovered after alkali leaching. The measured peaks correspond to standard Au (PDF#04-0784) for the (111), (200), (220), and (311) crystal planes, confirming the elemental form of Au. Three sets of Pt peaks (PDF#87-0646) appear, corresponding to the (111), (200), and (220) crystal planes, indicating a higher Pt content compared to Au. Both Pt and Au predominantly grow on the (111) crystal plane. The calculated grain sizes reveal that the Au (111) crystal plane in the mixture is approximately 43.7 nm, while the Pt (111) crystal plane is approximately 11.8 nm.
Fig. 9e displays the X-ray photoelectron spectroscopy (XPS) of elemental Au extracted after alkaline leaching, with peaks corresponding to Au 4f7/2 and Au 4f5/2, having binding energies of 84.5 eV and 88.2 eV, respectively. This confirms that the product extracted by alkaline leaching is elemental Au and indicates the absence of other oxidation states. Fig. 9f and g present the XPS spectra after alkaline leaching extraction of Pt/Au, with Fig. 9f representing Au 4f and Fig. 9g representing Pt 4f. The peaks in Fig. 9f correspond to Au 4f7/2 and Au 4f5/2, with binding energies of 84.5 eV and 88.2 eV, confirming the presence of elemental Au, The peaks in Fig. 9g correspond to Pt 4f7/2 and Pt 4f5/2, with binding energies of 71.6 eV and 74.9 eV, confirming the presence of elemental Pt.41,49,50 Due to the extremely small size of Pt particles, a portion of the Pt surface oxidizes to form platinum oxide (PtO), which has a binding energy of 72.6 eV.
Fig. 10a and b display scanning electron microscope (SEM) images of the recovered Au elements following alkaline leaching. The Au elements appear as relatively uniform spherical particles, though they exhibit adhesion and agglomeration. Fig. 10c and d present SEM images of the recovered Pt/Au mixture following alkaline leaching. The images reveal significant agglomeration. Due to the smaller size of Pt particles, their agglomeration is more pronounced than that of Au particles. In the high-magnification images, two particle sizes can be observed: the larger particles are likely Au, while the smaller ones are Pt. Fig. 10e and f shows the energy-dispersive X-ray spectroscopy (EDS) mapping and overall spectra of the recovered Pt/Au mixture following alkaline leaching. The figure indicates the presence of Pt, Au, W, and O elements, with Pt and Au having higher contents of 83.24 wt% and 16.54 wt% respectively, and W and O having lower contents of 0.09 wt% and 0.03 wt% respectively. Because the Pt content is higher than that of Au, both elements exhibit agglomeration in certain areas, leading to uneven content distribution in the mapped region. Consequently, the Pt content is higher, and the Au content is lower, resulting in an overall Pt/Au mixture proportion of 99.88%.
The TEM images of Au and Pt/Au, recovered after alkali leaching, are presented in Fig. 10g and i, while the SAED images are shown in Fig. 10h and j. Au exists as nanoparticles with a grain size of approximately 40 nm, exhibiting uniform size and distribution. When Pt and Au coexist, they appear aggregated, making them difficult to distinguish clearly. Fig. 10h and j are the SAED images of Au and Pt/Au, respectively. Fig. 10g corresponds to different crystal planes of Au, specifically (111), (200), (220), (311), (222), and (400), which are consistent with the XRD results of Au. No W or other impurities were detected, indicating that the sample is relatively pure. The FFT image in Fig. 10j corresponds to different crystal planes of Pt, while the crystal planes of WO3 are (110) and (200). The crystal zone axis of Pt is [0], with the corresponding crystal planes being (
1
) and (1
). No Au was observed in the field of view, possibly due to its uneven distribution and agglomerated growth. At this stage, WO3 still exists in the recovered Pt/Au mixture, likely due to the incomplete dissolution of WO3 during the alkali leaching process, leaving a small amount of WO3 residue on the sample surface.
The efficiency of alkaline leaching is determined by dividing the mass of the element after leaching by its mass before leaching and then multiplying by 100%. The mass of the element before leaching is obtained by subtracting 0.25 g of WO3 from the initial mass prior to alkaline leaching. When calculating for the Pt/Au mixture, the remaining WO3 after alkaline leaching must also be deducted. As shown in Fig. 10f, the purity of the Pt/Au mixture is 99.88%. To calculate the alkaline leaching efficiency, the extraction efficiency of the Pt/Au mixture must be adjusted by its purity of 99.88%. Table 3 shows that the alkaline leaching efficiencies for Au and Pt/Au are 95.49% and 93.73%, respectively, indicating high leaching efficiency. Table 4 indicates that the overall extraction efficiency of the products is the product of photodeposition efficiency and alkaline leaching efficiency. Following photocatalytic reduction and alkaline leaching, the overall extraction efficiencies for Au and Pt/Au are 93.10% and 93.26%, respectively, with total efficiencies exceeding 93%, demonstrating high extraction efficiency.
Extracted products | Before alkaline leaching (g) | After alkaline leaching (g) | Alkaline leaching efficiency |
---|---|---|---|
Au | 1.1348 | 0.8449 | 95.49% |
Pt/Au | 3.3663 | 2.9244 | 93.73% |
Extracted products | Photodeposition efficiency | Alkaline leaching efficiency | Overall efficiency |
---|---|---|---|
Au | 97.5% | 95.49% | 93.10% |
Pt/Au | 99.5% | 93.73% | 93.26% |
Fig. 11b and c display the EDS surface scan and overall spectrum of WO3 after photodeposition of Au, produced by ammonia water calcination and direct calcination of H2WO4, respectively. The Au appears in an aggregated form. In Fig. 11b, the W content is 24.2 wt%, the O content is 71.3 wt%, and the Au content is 4.5 wt%. In Fig. 11c, the W content is 20.2 wt%, the O content is 75.1 wt%, and the Au content is 4.7 wt%. After the photodeposition of RA-WO3, the surface Au content is slightly higher. As observed in Fig. 11b and c RA-WO3 is larger and more irregular, appearing in flaky, granular, and bulk forms with strong aggregation. In contrast, R-WO3 is smaller and more regular, primarily consisting of granular and flaky forms. There is no significant difference in the photodeposition of Au on WO3 prepared by these two methods.
Fig. 11d and e display the TEM and HRTEM images of WO3 produced by calcination with ammonia water, whereas Fig. 11f and g show the TEM and HRTEM images of WO3 produced by direct calcination of H2WO4. As illustrated in Fig. 11d, the morphology of WO3 is irregular, predominantly comprising spherical, rod-like, and block-like shapes. Fig. 11e reveals a measured lattice fringe spacing of 0.335 nm, corresponding to the (120) plane spacing of WO3 (PDF#20-1324). As shown in Fig. 11f, WO3 primarily consists of blocky and triangular shapes, with grain sizes around 100 nm. Fig. 11g shows a measured lattice fringe spacing of 0.241 nm, corresponding to the (103) plane spacing of WO3 (PDF#20-1324).
Fig. 11h illustrates the photodeposition kinetics curves of WO3 prepared by different methods. All three types of WO3 were fully reduced after 20 min of reaction. After 10 min of photodeposition, the reaction efficiency of R-WO3 nanoparticles and RA-WO3 were relatively high, with slightly higher reduction efficiency, whereas the reaction efficiency of R-WO3 was slower, exhibiting a slightly lower reduction efficiency. After 20 min of reaction, the reaction efficiency of WO3 prepared by the three methods converged to essentially the same value. After 60 min of reaction, the concentration of Au ions in the solution decreased to 0.0025 mM, representing a reduction efficiency of 97.5%, indicating that almost all Au ions were reduced. RA-WO3 did not exhibit any significant advantage over R-WO3.
![]() | ||
Fig. 12 Mechanism diagram of (a) Au photodeposition on WO3 nanoparticles, and (b) Pt/Au photodeposition on WO3 nanoparticles. |
The mechanism of photo-deposition of Au on WO3 nanoparticles can be primarily divided into three parts. First, upon light excitation, WO3 undergoes an electron transition, resulting in the formation of holes in the valence band, as illustrated in eqn (3).
![]() | (3) |
During a reduction reaction in the solution, Au ions react with electrons to form elemental Au, as illustrated in eqn (4). WO3 reacts elemental Au to form a catalyst, Au/WO3. Subsequently, Au ions interact with Au/WO3 to regenerate elemental Au, as depicted in eqn (5).
![]() | (4) |
![]() | (5) |
![]() | (6) |
![]() | (7) |
The overall reaction is represented in eqn (8). Under light irradiation and with WO3 as the catalyst, Au ions as the raw material and ethanol as the hole scavenger ultimately produce elemental Au, CO2, and O2.
![]() | (8) |
The model and formula for photodeposition of Au on WO3 nanoparticles elucidate the mechanisms of electron transition and Au ion reduction during the photodeposition process, clearly demonstrating that the photodeposition method can extract noble metal ions from the solution, producing only CO2 and O2 as by-products, and without generating impurities, thereby ensuring the purity of the extracted noble metal Au is maintained.
Fig. 12b illustrates the mechanism of photo-deposition of Pt and Au on WO3 nanoparticles. When light irradiates the semiconductor WO3, electrons in the valence band are excited and transition to the conduction band. The excited electrons reduce Pt and Au ions to elemental Pt and Au, respectively, which deposit on the surface of WO3, forming new active sites Pt/WO3 and Au/WO3. At the Pt/WO3 active sites, electrons from WO3 combine with holes on Pt, enabling more electrons on Pt to participate in the reaction, thereby reducing Au ions to Au and Pt ions to Pt, promoting Pt cluster growth. Compared to elemental Pt, the presence of Au hinders Pt aggregation, weakening its tendency to aggregate and resulting in Pt–Au or Pt–Pt growth modes.55,56 At the Au/WO3 active sites, electrons from WO3 combine with holes on Au, enabling more electrons on Au to participate in the reaction, thereby reducing Pt ions to Pt and Au ions to Au, promoting Au cluster growth. Compared to elemental Au, the presence of Pt hinders Au aggregation, weakening its tendency to aggregate and resulting in Au–Pt or Au–Au growth modes. Ethanol in the solution acts as a hole scavenger, reacting with holes in WO3 to produce CO2 and H2O, thereby preventing electron–hole recombination, allowing more electrons to participate in the reduction reaction and enhancing electron utilization efficiency.57
WO3 + NaOH → Na2WO4 + H2O | (9) |
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Fig. 13 Potential–pH diagram of W-H2O.58 |
During the alkaline leaching process, the alkaline substances used are NaOH and NH4·H2O. Experiments indicate that noble metals dissolve when NH4·H2O is used for alkaline leaching, but do not dissolve when NaOH is used. Consequently, NaOH is selected as the reagent for alkaline leaching.
Materials | Unit price (¥) | Usage | Price (¥) |
---|---|---|---|
Anhydrous ethanol | 12/500 mL | 60 mL | 1.44 |
Ammonium metatungstate | 217/100 g | 0.37 g | 0.81 |
Polyvinylpyrrolidone | 55/250 g | 0.5 g | 0.11 |
Polyethylene glycol | 38/500 g | 0.5 g | 0.04 |
Sodium hydroxide | 68/500 g | 0.2 g | 0.03 |
Filter paper | 22/50 sheets | 1 sheet | 0.50 |
Centrifuge tube | 32/200 pieces | 5 pieces | 0.80 |
Electricity cost | 0.6/degree | 2 degrees | 1.20 |
Total | — | — | 4.93 |
Table 6 presents a comparison of various literature regarding the recycling of precious metals using different methods. Techniques such as solvent extraction and displacement are associated with relatively high costs and lack environmental friendliness, whereas methods like adsorption exhibit relatively low efficiency. By contrast, the photocatalytic method demonstrates both economic viability and high efficiency.
Methods | Materials | Precious metals | Initial concentration | Times | Efficiency | Cost | Ref. |
---|---|---|---|---|---|---|---|
Displacement | Si | Pt/Pd/Au | 1 mM | 5 min | 99.9% | Medium | 60 |
Cu | Au | 100 mg L−1 | 7 min | 93% | Medium | 61 | |
Extraction | Diethyl carbonate | Au | 100 mg L−1 | 5 min | 73% | Higher | 62 |
4-HAP | Au | 200 mg L−1 | 2 min | 99.7% | Medium | 63 | |
Adsorption | Activated carbon | Au | 50 mg L−1 | 24 h | 97% | Lower | 64 |
Biomass | Pt/Pd/Au | 100 mM | 50 h | 95% | low | 65 | |
Electrolysis | — | Au | 0.125 mM | 2 h | 94.2% | Lower | 66 |
Zn | Au | 10 mg L−1 | 30 min | 99.59% | Medium | 67 | |
Photocatalysis | ZnO | Pd | 0.3 mM | 15 min | >99% | Lower | 33 |
ZnO | Ag | 0.15–1.2 mM | <30 min | >90% | Lower | 68 | |
WO3 | Au | 0.1 mM | 10 min | 99% | Lower | This work |
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