Advances in Matrix-Supported Palladium Nanocatalysts for Water Treatment

Abstract

Advanced catalysts are crucial for a wide range of chemical, pharmaceutical, energy, and environmental applications. They can reduce energy barriers and increase reaction rates for desirable transformations, making many critical large-scale processes feasible, eco-friendly, energy-efficient, and affordable. Advances in nanotechnology have ushered in a new era for heterogeneous catalysis. Nanoscale catalytic materials are known to surpass their conventional macro-sized counterparts in performance and precision, owing it to their ultra-high surface activities and unique size-dependent quantum properties. In water treatment, nanocatalysts can offer significant promise for novel and ecofriendly pollutant degradation technologies that can be tailored for customer-specific needs. In particular, nano-palladium catalysts have shown promise in degrading larger molecules, making them attractive for mitigating emerging contaminants. However, the applicability of nanomaterials, including nanocatalysts, in practical deployable and ecofriendly devices, is severely limited due to their easy proliferation into the service environment, which raises concerns of toxicity, material retrieval, reusability, and related cost and safety issues. To overcome this limitation, matrix-supported hybrid nanostructures, where nanocatalysts are integrated with other solids for stability and durability, can be employed. The interaction between the support and nanocatalysts becomes important in these materials and needs to be well investigated to better understand their physical, chemical, and catalytic behavior. This review paper presents an overview of recent studies on matrix-supported Pd-nanocatalysts and highlights some of the novel emerging concepts. The focus is on suitable approaches to integrate nanocatalysts in water treatment applications to mitigate emerging contaminants including halogenated molecules. The state-of-the-art supports for palladium nanocatalysts that can be deployed in water treatment systems are reviewed. In addition, research opportunities are emphasized to design robust, reusable, and ecofriendly nanocatalyst architecture.

Keywords: hybrid catalyst-support systems; palladium nanocatalyst; water purification.

Figure 1

Schematic illustration of different types of support for palladium nano-catalyst.

Figure 2

Transmission electron microscope (TEM) images of Pd cluster on silica core−shell nanospheres after removal of polymer particles: (a) Z-contrast; and (b) bright-field. “Reproduced with permission from [50], American Chemical Society, 2014”.

Figure 3

TEM images of ALD Pd nanoparticles supported on the AC undergoing HNO₃ etching and then calcined at 800 °C. “Reproduced with permission from [103]. American Chemical Society, 2015”.

Figure 4

Basswood decorated with Pd NPs for water treatment. The zoom-in images show: (a) mesoporous structure of the wood perpendicular to the growth direction; (b) in-situ formed Pd NPs in wood where lignin acts as the reducing agent. “Reproduced with permission from [138]. American Chemical Society, 2017”.

Figure 5

Schematic of hierarchical structure, where carpet-like arrays of CNT are strongly attached to a porous carbon substrate. Carbon substrate can be flexible carbon fiber cloth (right-top), reticulated vitreous carbon foam (right-middle) or micro cellular carbon foam (right-bottom).

Figure 6

Supported PdNPs on CNT grafted carbon foam and carbon fabric. (a–d) aligned CNT carpet on RVC foam at different magnification, (a) low magnification overview, (b–d) CNT arrays entanglement level increase through the height of carpet, (b) root, (c) middle, (d) top [154]). (e–g) CNT carpet covered carbon fiber cloth. (e) low magnification micrograph of the CNT arrays. (f) aligned CNT arrys filling the spaces between the carbon fiber filament. (g) transmisison mode of CNT showing multiwall CNT features with uniform diameter distribution. “Reproduced with permission from [148]. Elsevier, 2021”.

Figure 6

Supported PdNPs on CNT grafted carbon foam and carbon fabric. (a–d) aligned CNT carpet on RVC foam at different magnification, (a) low magnification overview, (b–d) CNT arrays entanglement level increase through the height of carpet, (b) root, (c) middle, (d) top [154]). (e–g) CNT carpet covered carbon fiber cloth. (e) low magnification micrograph of the CNT arrays. (f) aligned CNT arrys filling the spaces between the carbon fiber filament. (g) transmisison mode of CNT showing multiwall CNT features with uniform diameter distribution. “Reproduced with permission from [148]. Elsevier, 2021”.

Figure 7

(a,b) SEM microstructure of PdNPs well attached on the CNT with fine distribution “Reproduced with permission from [155]. Elsevier, 2012”.

Figure 8

Palladium nanoparticles attached to CNT carpets: (a) scanning transmission electron microscopy (STEM) image showing microstructure of Pd nanoparticles attached on CNT; (b) Atomistic model of core-shell type architecture of palladium based nanocatalyst; (c) Comparative Atrazine degradation profile with different nanocatalyst “Reproduced with permission from [156]. Elsevier, 2012”.

Figure 9

Dechlorination profile of TCE using various catalysts. It must be noted that the nanoparticles of Pd and PdO attached to CNT-Foam show rapid and complete degradation that is not seen in any of the controls, including isolated Pd (loose nanoparticles) “Reproduced with permission from [157]. MDPI, 2016”.

Figure 10

TCS catalytic degradation in aqueous environment, nitrogen balanced hydrogen (5% H₂) mixture was supplied into the reactor, and the pressure was kept at a constant of 1 atm. “Reproduced with permission from [158]. Elsevier, 2021”.