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Rhodium (Rh) catalyst has played an indispensable role in many important industrial and technological applications due to its unique and valuable properties. Currently, Rh is considered as a strategic or critical metal as the scarce high-quality purity can only be supplemented by refining coarse ores with low content (2–10 ppm) and is far from meeting the fast-growing market demand. Nowadays, exploring new prospects has already become an urgent issue because of the gradual depletion of Rh resources, incidental pressure on environmental protection, and high market prices. Since waste catalyst materials, industrial equipment, and electronic instruments contain Rh with a higher concentration than that of natural minerals, recovering Rh from scrap not only offers an additional source to satisfy market demand but also reduces the risk of ore over-exploitation. Therefore, the recovery of Rh-based catalysts from scrap is of great significance. This review provides an overview of the Rh metal recovery from spent catalysts. The characteristics, advantages and disadvantages of several current recovery processes, including pyrometallurgy, hydrometallurgy, and biosorption technology, are presented and compared. Among them, the hydrometallurgical process is commonly used for Rh recovery from auto catalysts due to its technological simplicity, low cost, and short processing time, but the overall recovery rate is low due to its high remnant Rh within the insoluble residue and the unstable leaching. In contrast, higher Rh recovery and less effluent discharge can be ensured by a pyrometallurgical process which therefore is widely employed in industry to extract precious metals from spent catalysts. However, the related procedure is quite complex, leading to an expensive hardware investment, high energy consumption, long recovery cycles, and inevitable difficulties in controlling contamination in practice. Compared to conventional recovery methods, the biosorption process is considered to be a cost-effective biological route for Rh recovery owing to its intrinsic merits, e.g., low operation costs, small volume, and low amount of chemicals and biological sludge to be treated. Finally, we summarize the challenges and prospect of these three recovery processes in the hope that the community can gain more meaningful and comprehensive insights into Rh recovery.

Hydrogenated carboxyl nitrile rubber (HXNBR) is endowed with superior mechanical performance and heat–oxygen aging resistance via emulsion hydrogenation of its precursor, i.e., carboxyl nitrile rubber (XNBR). Herein, a ruthenium-based catalyst was prepared to achieve the direct catalytic hydrogenation of XNBR latex. The effects of a series of hydrogenation conditions, such as catalyst dosage, solid content and reaction temperature, as well as the hydrogen pressure, on the hydrogenation reaction were investigated in detail. We found that the hydrogenation rate fell upon increasing the solid content of the XNBR latex, with an XNBR conversion rate of 95.01 mol% in 7 h with 11.25 wt% solid content. As the reaction temperature was increased, the hydrogenation rate first increased and then decreased. The fastest reaction hydrogenation rate was reached at 140 °C, with an XNBR conversion of 95.10 mol% in 5 h. The hydrogenation rate was positively related with the hydrogen pressure employed in the reactor. In view of the safety and cost, a pressure rate of 1300 psi was considered optimal. Similarly, the hydrogenation rate can also be enhanced by adding more catalyst. When 0.05 wt% catalyst was added, the fastest hydrogenation rate was achieved. In summary, the following optimum hydrogenation conditions were determined by using a synthesized ruthenium-based catalyst: 11.25 wt% solid content of XNBR latex, 140 °C of reaction temperature, 1300 psi of hydrogen pressure and 0.05 wt% catalyst. The vulcanization, mechanical performance, aging resistance and oil resistance of the produced HXNBR under the above reaction conditions were systematically investigated.