GrafeoPlad-Palladium: Converting Palladium into an Exceptionally Stable Catalyst

Palladium is a key catalytic metal widely employed both in synthetic organic chemistry and in environmental remediation catalysis. The metal is rare and expensive, whereas increasing demand coupled to increasingly difficult supply is driving both price volatility and substantial price increase. The metal cost $588/t in August 2016. Ten years later its price doubled to $1,290/t. The most commonly employed palladium catalyst, Pd/C consisting of Pd nanoparticles dispersed on the outer surface of charcoal, suffers from relatively quick deactivation [1]. Clearly, the development of a catalytically stable form of nanoparticulate Pd would be highly desirable, though prolonged lack of achievements of practical relevance makes the Pd/C catalyst still the catalytic material of choice for most heterogeneously catalyzed processes. To ensure economic viability of Pd catalysis, catalyst suppliers usually retrieve the spent Pd/C catalyst from customers for reactivation usually by extracting the metal in a suitable media and subsequent re-impregnation on carbon support followed by reduction [1]. In 2023, we anticipated the discovery of a completely new class of functional materials, dubbed “GrafeoPlad-Palladium”, for designating palladium doped with 3D entrapped graphene oxide (GO@nPd) [2]. Employed in the Suzuki–Miyaura cross-coupling reaction between phenylboronic acid and aryl halogenides conducted in methanol, catalyst reusability tests showed unprecedented stability of the new material toward sintering and deactivation [3]. Clustering to form palladium black is the common deactivation mechanism of most cross-coupling reactions mediated by both homogeneous [4] and heterogeneous [5] Pd catalysts. GrafeoPlad-Pd is significantly more stable than Pd black, showing that the entrapment of GO molecules in the nanoparticle lattice largely improves both its catalytic activity and stability against catalytic deactivation. In this lecture, I will discuss the outcomes of experimental and computational investigations of the metal-organic alloy GO@(nPd) hybrid material and the stabilizing role of the 3D-entrapped graphene moiety of the GO molecules against sintering, confirmed by DFT calculations [3]. XPS surface investigation prior and after employment in hydrogenation catalysis [6] indicate involvement in catalysis of both metallic Pd and PdO phases. These findings make the discovery of GrafeoPlad-Pd of direct relevance to the global fine chemical industry wherein the increasingly costly use of palladium in catalytic reactions is ubiquitous. This new class of metal-organic alloys (MORALs) [7] opens practically relevant new avenues in many areas of today's material science and technology.