Accelerating Catalytic Oxyanion Reduction with Inert Metal Hydroxides

Abstract

Adding Cr III or Al III salts into the water suspension of platinum group metal (PGM) catalysts accelerated oxyanion pollutant reduction by up to 600%. Our initial attempts of adding K 2 Cr VI O 4 , K 2 Cr VI 2 O 7 , or KCr III (SO 4 ) 2 into Pd/C enhanced BrO 3 – reduction with 1 atm H 2 by 6-fold. Instrument characterizations and kinetic explorations collectively confirmed the immobilization of reduced Cr VI as Cr III (OH) 3 on the catalyst surface. This process altered the ζ-potentials from negative to positive, thus substantially enhancing the Langmuir–Hinshelwood adsorption equilibrium constant for BrO 3 – onto Pd/C by 37-fold. Adding Al III (OH) 3 from alum at pH 7 achieved similar enhancements. The Cr–Pd/C and Al–Pd/C showed top-tier efficiency of catalytic performance (normalized with Pd dosage) among all the reported Pd catalysts on conventional and nanostructured support materials. The strategy of adding inert metal hydroxides works for diverse PGMs (palladium and rhodium), substrates (BrO 3 – and ClO 3 – ), and support materials (carbon, alumina, and silica). This work shows a simple, inexpensive, and effective example of enhancing catalyst activity and saving PGMs for environmental applications.

Enhancing Aqueous Chlorate Reduction Using Vanadium Redox Cycles and pH Control

“…All aqueous solutions were prepared with 18.2 MΩ cm –1 Milli-Q water. The Pd/C catalyst was prepared using our recently developed all- in situ method, which has been validated in our previous studies. ,,, Specifically, a magnetic stir bar, 40 mg of activated carbon, 400 mL of DI water, and the desired amount of Na 2 PdCl 4 (dissolved in a stock solution) were sequentially placed in a 500 mL round-bottom flask. The flask was capped with a rubber stopper and sonicated for 1 min.…”

“…Previously, we have enhanced the reduction of oxyanions by incorporating Group 6–8 transition metals into PGM catalysts. − Each secondary metal imparts novel and unique functionalities. For instance, Cr­(OH) 3 formed in various PGM catalysts (e.g., Pd or Rh nanoparticles supported by carbon, alumina, or silica) remarkably enhanced the adsorption of oxyanions .…”

“…Previously, we have enhanced the reduction of oxyanions by incorporating Group 6–8 transition metals into PGM catalysts. − Each secondary metal imparts novel and unique functionalities. For instance, Cr­(OH) 3 formed in various PGM catalysts (e.g., Pd or Rh nanoparticles supported by carbon, alumina, or silica) remarkably enhanced the adsorption of oxyanions . The reduced Mo and Re oxide species on the PGM catalysts substantially accelerated oxyanion reduction via Mo IV/VI and Re V/VII redox cycles, respectively. , The immobilization of Ru 0 nanoparticles onto Pd/C or Rh/C initiated a synergistic working mode, in which Ru rapidly reduces ClO 3 – while Pd or Rh scavenges the Ru-passivating chlorine intermediates .…”

“…Our research has investigated various metals in Groups 5 (V), 6 (Cr and Mo), 7 (Re), and 8 (Ru) for enhanced oxyanion reduction with supported PGM catalysts. Each metal follows unique mechanisms through enhancing oxyanion adsorption (e.g., Cr 28 ), creation of novel reaction pathways (e.g., V, Mo, 27 and Re 30 ), or synergistic work (e.g., Ru 29 ). Notably, the reaction pathways created by V, Mo, and Re differ remarkably.…”