Recovery of Palladium from Spent Activated Carbon-Supported Palladium Catalysts
Recovery of Palladium from Spent Activated Carbon-Supported Palladium Catalysts
Precipitation by sodium borohydride allows high grade recovery of palladium
Activated carbon-supported palladium catalysts are liable to progressive deactivation even in the absence of any gaseous contaminants during the oxidation of hydrogen under ambient conditions. The high value of palladium coupled with environmental considerations means that new, efficient and cost effective methods for the quantitative recovery of palladium from such materials are required. In the present study, a process for extracting precious metals from spent catalyst or inorganic waste was developed. Palladium was extracted from the spent catalyst with an acid solution containing dilute hydrochloric acid and hydrogen peroxide at leaching temperatures of around 90°C. Palladium in the leached solution was then precipitated by use of sodium borohydride solution. The effectiveness of the method for recovery of precipitated palladium was investigated by ultraviolet-visible (UV-vis) spectrophotometry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The recovered metallic palladium was of a sufficient grade to manufacture fresh activated carbon-supported palladium catalysts.
Hydrogen is prone to leakage and can form an explosive atmosphere due to its wide range of flammability limits. Methods of hydrogen elimination before accumulation of a detonable mixture in confined spaces are thus very important. Catalytic combustion of hydrogen is the most common method used in commercial hydrogen detection and elimination systems. Palladium is frequently used as the active component of the catalysts for such systems (1–4). These systems can respond to hydrogen when maintained at temperatures of 80°C, and in some cases less (5). However, for reasons which remain unclear, the catalyst is liable to progressive deactivation even in the absence of any gaseous contaminants (2).
The high value of palladium makes its recovery economically desirable: for example, in 2011 22% of the palladium market was supplied by recovery from autocatalysts and jewellery scrap (6). In addition environmental considerations mean that effective recovery of the residual precious metals from spent catalysts is of paramount importance for a process to be environmentally acceptable (7, 8).
Spent organic-based catalysts which contain precious metals have traditionally been treated by incineration to recover the precious metal content. However, palladium is recovered in oxide form, which is not ideal for the production of fresh catalyst (9).
Hydrometallurgical techniques are also widely used for the recovery and separation of precious metals from spent catalysts. The processes can be categorised into two types: support dissolution and precious metal dissolution (10). In the first of these, the support is dissolved with a nonoxidising acid or base, after which the precious metals remain as a residue. A major disadvantage is that this process requires large quantities of reagents.
In the second type of process the precious metals are extracted from the support by an acidic oxidant solution, leaving the bulk of the support undissolved. Different leaching mixtures such as sulfuric acid, hydrochloric acid or chloride salts in the presence of oxidising agents such as nitric acid, bromine or chlorine have been used for palladium recovery (11–15). Disadvantages of this technique are the environmental impact of the reagents, the high cost of nitric acid and the potential for evolution of hazardous nitric oxides. In addition, after leaching, nitric acid must be completely removed from the solution, adding to the complexity and cost. Separation of Pd from solution has also been carried out by reduction in phosphoric acid solution by formalin at 150°C (16); in hydrochloric acid by formic acid (17); by polyoxometallates (18); by bioreduction (19); and by reduction using aluminium powder (20).
Lastly, the development of new innovative and sustainable chemical processes with high energy efficiency and ideally 100% selectivity will require a new generation of catalytic materials with tailored functionality at the surface. Since the material properties change at the nanoscale, nanosized catalytic materials are expected to enable highly complex catalytic processes to be created on the basis of a controlled sequence of surface reactions and active sites. Nanostructured catalysts show great promise in the field of environmentally friendly, flexible and efficient materials processing (21). The kinetics of adsorption/desorption of hydrogen on the catalytic surface is closely related with its diffusion rate, and it is possible to improve these diffusion rates by use of nanoscale catalytic surfaces. The ability to obtain recovered palladium in a form which can be effectively re-used to generate fresh catalysts will be an advantage in this regard.
In the present work, new process conditions have been tested and optimised for the recovery of palladium from spent carbon-supported catalyst. Palladium was extracted from a spent catalyst matrix with a mixture of dilute hydrochloric acid and hydrogen peroxide and then precipitated from the leached solution by sodium borohydride. NaBH4 was selected as a much milder reducing agent which is safer to handle than the conventional alternatives. It has the further advantage that it can be used in aqueous or alcohol solutions. UV-vis spectrophotometry, XRD and SEM were used to monitor the effectiveness of the chosen recovery method and the resultant solid residue was shown to be suitable for manufacturing fresh activated carbon-supported palladium catalysts.
Commercial activated carbon-supported palladium catalysts containing a polytetrafluoroethylene (PTFE) binder in pellets of 100 g were used. The catalyst, sold for hydrogen elimination in confined spaces, was in the shape of rectangle with dimensions of 160 mm × 76 mm. The thickness of the catalyst plate was 1 mm. The palladium loading of the catalyst was determined to be 10% by weight. (Note that due to commercial confidentiality the supplier name and the detailed compositional analysis of the catalyst material have been withheld).
Hydrochloric acid (37%) and hydrogen peroxide (30%) were used for the preparation of the leaching solution. Sodium borohydride (99%) was used as a reducing agent for palladium. An aqueous solution of 5% sodium hydroxide was used to prevent the self-oxidation of sodium borohydride. All chemicals were of high grade for laboratory usage. Distilled water was used for dilutions.
2.3 Ageing of the Palladium Catalyst
The thermal conductivity of both activated carbon and PTFE is around 0.25 W m−1 K−1 (22) while palladium has a thermal conductivity of about 75 W m−1 K−1 (23). The thermal conductivity of gases is generally much lower at around 0.02 ± 0.005 W m−1 K−1 (24). The thermal conductivity of a porous solid such as found in a catalyst support can approach that of gases provided the porosity is high. For this reason, the catalyst body may be classified as a non-thermally conductive material (24, 25). As a result the transport of thermal energy out of a fixed bed catalytic reactor generally proceeds slowly, which can cause an increase in temperature in the catalyst bed at a level approaching the adiabatic temperature rise.
When the surface temperature of the catalyst exceeds 300°C, the PTFE binder of the catalyst starts to melt, agglomerates the active palladium species and finally leads to the deactivation of the catalyst. For this reason, the normal operating conditions of a hydrogen removal catalyst require a maximum hydrogen concentration in the order of hundreds or thousands of ppmv in air at 25°C and 1 atm. The maximum allowable effluent gas temperature has been determined to be between 150°C and 200°C to maintain the catalyst surface temperature below 300°C (26). Under normal operating conditions, the lifetime of the catalyst is at least two years. To quickly age the catalyst for the present study, a high fuel content of 3% hydrogen was intentionally chosen. Fresh catalyst was chopped and exposed to 3% hydrogen in air as described previously (26). The completion of ageing was monitored by the loss of catalytic activity with time-on-stream. An effluent gas temperature of ~246°C was reached indicating that the ageing condition had been met. The deactivated catalyst was subsequently tested for the recovery of palladium.
2.4 Method for the Recovery of Palladium
100 g of the spent catalyst was crushed, ground and sieved below 500 μm. The finely powdered spent catalyst was put into a jacketed 250 ml reactor. The leaching solution of hydrochloric acid and hydrogen peroxide was added slowly to the reactor under magnetic stirring. The reaction was carried out for 180 min at 90°C. At the end of the leaching process, the mixture was filtered via a filtration funnel under vacuum at 90°C. The Pd concentration of the leached solution was measured using inductively coupled plasma emission spectrometry (ICPES). The leached solution was used as a stock solution for the separation of palladium.
In the separation phase, in order to reduce the leached palladium species to the form of the palladium salt, alkaline sodium borohydride solution was added drop by drop onto 10 g of the stock solution in a 100 ml conical flask. To optimise the precipitation conditions, different concentrations of borohydride solution and different process temperatures were applied. The solution was then filtered off, and the precipitated palladium powder was washed, dried at 105°C and weighed to calculate the reduction efficiency. Figure 1 shows the process diagram for the recovery of palladium from the spent Pd/activated carbon (AC) catalyst.
Elemental analysis of the filtrate solution received after leaching was carried out by ICPES using a Thermo Jarrell Ash Atomscan 25. UV-vis absorption spectra of the samples were taken in the range 200 nm to 400 nm using a Shimadzu UV-1650PC UV-vis spectrophotometer. Wide-angle XRD patterns of the spent catalysts before and after leaching were recorded on a Shimadzu XRD-6000 diffractometer with CuKα radiation (λ of 0.154178 nm at 40 kV and 30 mA). XRD scans were performed at ambient temperature with a 2θ range between 10° and 60° at a scanning rate of 0.048° min−1. The morphology of the spent catalysts before and after leaching was investigated by SEM using a JEOL 6335F FEG-SEM field emission gun. Additionally, the average particle size of recovered palladium was measured using a Malvern Zetasizer Nano ZSP characterisation system.
3. Results and Discussion
3.1 Effect of HCl and H2O2 Concentrations
Treatment of activated carbon-supported palladium catalysts with a mixture of hydrochloric acid and hydrogen peroxide gives palladium chloride according to the Reaction (i):
The effects of the hydrogen peroxide and hydrochloric acid concentrations were studied by changing the concentration of hydrogen peroxide while keeping the concentration of hydrochloric acid constant or vice versa. In the first case, hydrogen peroxide concentration was increased from 0 to 15% in the presence of 5% hydrochloric acid. The results are plotted in Figure 2(a). It can be seen that the percentage recovery of palladium steadily increased with H2O2 concentration up to 5%, reaching a maximum palladium recovery of ~85%. For H2O2 concentrations above 5%, there was no further increase in palladium recovery. In the second case, HCl concentration was increased from 0 to 20% in the presence of 5% H2O2. The results are shown in Figure 2(b). The percentage recovery of palladium gradually increased with increasing HCl concentration, reaching a maximum of 99% recovery at 10% HCl. Thus a mixture of 10% HCl and 5% H2O2 was concluded to be effective for extracting palladium from activated carbon-PTFE organic matrix. Similar results have been reported for spent Pd/Al2O3 catalyst by Barakat et al. (20).
3.2 Effects of Temperature and Time
The effects of leaching time and reaction temperature on palladium recovery were studied over the temperature range 30°C to 100°C with leaching times of 1 h, 2 h or 3 h. Spent catalyst was treated with an acid/peroxide mixture having HCl and H2O2 concentrations of 10% and 5%, respectively. As seen in Figure 3, the percentage recovery of palladium increased with reaction temperature for each of the treatment time periods. The treatment time was seen to significantly affect the maximum percentage recovery of palladium, and a treatment time of 1 h was not sufficient to extract the highest levels of palladium. The results were closer for the treatment times of 2 h and 3 h. The maximum palladium recovery of >98% was obtained after treatment with an acid/peroxide mixture of 10% of HCl and 5% of H2O2 at 90°C for 3 h.
3.3 Effect of Reducing Agent
Sodium borohydride was used as the reducing agent to precipitate metallic palladium from the stock solution after filtration. The precipitation of palladium via reduction proceeds according to the following reactions:
The overall reaction is:
NaBH4 + 2H2O + 4H2PdCl4 → NaBO2 + 4Pd0 + 16HCl (iv)
In the absence of any reducible species in the solution, sodium borohydride decomposes into sodium metaborate and hydrogen gas according to Reaction (v):
To optimise the precipitation conditions, different concentrations of alkaline sodium borohydride solution and different process temperatures were applied for a fixed duration of 20 minutes. Figure 4 shows the change in percentage recovery of palladium versus the concentration of NaBH4 at different reduction temperatures. It can be seen that the percentage recovery of palladium increased both with temperature, as also verified by Barakat et al. (20), and with sodium borohydride concentration. The percentage recovery increased steadily with temperature, while the effect of NaBH4 concentration disappeared above 7% for the reduction temperature of 90°C. 100% palladium recovery was accomplished with a NaBH4 concentration and a reduction temperature of 7% and 100°C, respectively.
3.4 UV Results
Because of ligand-to-metal charge transfer, the transition of PdCl42− to H2PdCl4 exhibits two absorption bands at 209 nm and 237 nm, as reported by Yang et al. (27). These two strong UV absorption bands can be used to measure the reduction of H2PdCl4 to metallic palladium. To monitor the reduction upon treatment with different concentrations of NaBH4 at 100°C, UV-vis spectra of treated solutions were taken. Leached and alkaline sodium borohydride solutions were used in equimolar proportions. Figure 5 shows the results. It can be seen that the two absorption bands at 209 nm and 237 nm decreased with increasing concentrations of NaBH4. When the concentration of NaBH4 reached 10%, these absorption bands disappeared. This indicates that the H2PdCl4 precursor has been completely reduced to metallic palladium.
3.5 XRD and SEM Results
To prove the completeness of palladium recovery from the spent activated carbon-supported palladium catalyst, XRD measurements were taken on the solid residue before and after leaching (Figures 6(a) and 6(b)). The XRD pattern of the final reduced palladium sample is also shown (Figure 6(c)). In the spectra of the spent catalyst before leaching, the sharp and narrow peaks at 2θ = 40.1° and 46.6° were attributed to the presence of crystalline palladium in the form of large crystals, in line with findings reported by A. Drelinkiewicz et al. (28) and J. Y. Ying et al. (29). These crystalline palladium peaks were clearly separated from the relatively broad ‘halo’ centred shoulder of activated carbon at around 2θ = 26° (Figure 6(a)). After the leaching process, the characterisation peaks of Pd at 2θ = 40.1° and 46.6° were not detected indicating the completeness of the leaching process (Figure 6(b)). Upon reduction of the H2PdCl4 precursor to metallic Pd by NaBH4, the solid reside exhibited only the peaks at 40° and 47° (Figure 6(c)) indicating complete reduction. The unstrained Pd phases should exhibit an XRD pattern according to the powder diffraction files (PDF) database as follows: Pd(1 1 1) (39.8°), Pd(2 0 0) (46.3°) and Pd(3 1 1) (81.6°) (30). This confirms the crystal structure of bulk palladium. The peak at 81.6° is not shown as the XRD scans were performed at ambient temperature with a 2θ range between 10° and 60°.
Scanning electron micrographs for the samples before and after leaching are shown in Figures 7(a) and 7(b), respectively. The morphology of the spent catalyst samples before and after leaching was compared, confirming the presence of small discrete particulates before the leaching, as seen in Figure 7(a). These micrometre scale zero-valent Pd metal particles disappeared after the leaching process as shown in Figure 7(b). Along with the results obtained by XRD, this confirms that 100% recovery of palladium from the spent Pd/AC catalyst was successfully achieved.
Figure 7(c) shows the SEM image of metallic palladium after recovery. The morphology of the submicron crystallites can be clearly seen. The presence of very tiny particles in the range 250 nm to 550 nm was also verified by Zetasizer measurements. The recovered palladium is therefore demonstrated to be suitable for preparing fresh nanostructured activated carbon-supported catalyst.
A process for the recovery of palladium from spent activated carbon-supported palladium catalysts was described. Palladium was extracted from the matrix of the spent catalyst with an acid mixture of dilute hydrochloric acid and hydrogen peroxide. Palladium was precipitated from the leached solution by NaBH4. For leaching, a mixture of 10% HCl and 5% H2O2 was seen to be sufficiently effective to extract all palladium from the activated carbon-PTFE organic matrix. The maximum palladium recovery was obtained after treatment at 90°C for 3 h. Complete precipitation of palladium was accomplished using a NaBH4 concentration of 7% and a reduction temperature of 100°C.
Characterisation by UV-vis, XRD and SEM was used to prove the effectiveness of the developed process. It was shown by these studies that all palladium was recovered from the spent catalyst. The XRD study verified the metallic form of recovered palladium. The reduction of H2PdCl4 to metallic palladium was monitored by UV-vis, showing that the optimum concentration of NaBH4 in alkaline solution was 10% to completely reduce the H2PdCl4 precursor. The production of metallic nanoparticles by treatment with sodium borohydride was verified by SEM imaging and Zetasizer measurements. The metallic palladium recovered using the method described here is believed to have the required specification to manufacture fresh activated carbon-supported palladium catalysts.
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Şerife Sarioğlan is a Chief Senior Researcher at TÜBİTAK Marmara Research Center Chemistry Institute, Turkey. She completed her PhD in Polymer Chemistry at Istanbul Technical University, Turkey. Prior to taking part in industrial projects on recycling of oxidation catalysts and their preparation for end-use, she was primarily involved in inorganic after-treatment and shaping of minerals. Her research interests include heterogeneous catalysis for the petrochemical industry and various niche applications.