Ambient base-free glycerol oxidation over bimetallic PdFe/SiO2 by in situ generated active oxygen species

Abstract

Low temperature oxidation of alcohols over heterogeneous catalysts is exceptionally challenging, particularly under neutral conditions. Herein, we report on an efficient, base-free method to oxidise glycerol over a 0.5%Pd-0.5%Fe/SiO2 catalyst at ambient temperature in the presence of gaseous H2 and O2. The exceptional catalytic performance was attributed to the in situ formation of highly reactive surface-bound oxygenated species, which promote the dehydrogenation on the alcohol. The PdFe bimetallic catalyst was determined to be significantly more active than corresponding monometallic analogues, highlighting the important role both metals have in this oxidative transformation. Fe leaching was confirmed to occur over the course of the reaction but sequestering experiments, involving the addition of bare carbon to the reactions, confirmed that the reaction was predominantly heterogeneous in nature. Investigations with electron paramagnetic resonance spectroscopy suggested that the reactivity in the early stages was mediated by surface-bound reactive oxygen species; no homogeneous radical species were observed in solution. This theory was further evidenced by a direct H2O2 synthesis study, which confirmed that the presence of Fe in the bimetallic catalyst neither improved the synthesis of H2O2 nor promoted its decomposition over the PdFe/SiO2 catalyst.

The Direct Synthesis of Hydrogen Peroxide Over Supported Pd-Based Catalysts: An Investigation into the Role of the Support and Secondary Metal Modifiers

“…Additionally, it should be noted that the weak signals in the Pd3d XPS spectra for both the 0.5%Pd-0.5%Au/SiO 2 and 0.5%Pd-0.5%Au/Al 2 O 3 catalysts preclude a reliable determination of Pd speciation, although it is possible to infer that Pd 2+ is the dominant ). In recent years a growing focus has been placed on the use non-precious metal containing Pd-based alloys for the direct synthesis of H 2 O 2 including, but not limited to Pb [48], Sn [26,32], Ni [28], Zn [34], Ga [30], Cu [27], Co [49] and Fe [7,10,50]. With these previous studies in mind and with an aim to reduce financial costs associated with catalyst synthesis, we subsequently investigated the efficacy of alloying Pd with a range of abundant transition metals.…”

A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols

“…Hydroxylation of oxides, through interaction with water for example, can substantially increase solubility. Another common source of leaching is the chelation of supported metals with reaction products, particularly products possessing multiple acidic components [28a,55] . Inductively coupled plasma mass spectrometry (ICP‐MS) and microwave plasma atomic emission spectrometry (MP‐AES) are incredibly effective at quantifying catalyst leaching, through examination of post reaction effluent.…”

“…Given that the performance of supported metal catalysts in alcohol oxidation are evidently highly dependent on their defined structure and morphology, their stability under reaction conditions must be assessed. Numerous deactivation mechanisms have been proposed in the literature, which include support or supported metal leaching,[ 7 , 13 , 28a , 51 ] product inhibition, [52] active metal sintering and even over oxidation of the active metal component. [53] Many of these deactivation mechanisms are dependent on the reaction conditions used and the specific transformation under investigation.…”

“…Another common source of leaching is the chelation of supported metals with reaction products, particularly products possessing multiple acidic components. [ 28a , 55 ] Inductively coupled plasma mass spectrometry (ICP‐MS) and microwave plasma atomic emission spectrometry (MP‐AES) are incredibly effective at quantifying catalyst leaching, through examination of post reaction effluent. It is exceptionally difficult to suppress leaching, without making changes to the catalyst (through changing the support to induce a stronger metal‐support interaction) or by changing the reaction conditions (through addition of additives such as − OH).…”

The degradation of phenol via in situ H2O2 production over supported Pd-based catalysts

“…8,11,12 However, due to limited selectivity there is often a need to alloy Pd with a range of precious metals, [13][14][15][16][17] although in recent years, a growing attention has been placed on the use of more abundant transition metals as promoters for Pd. [18][19][20][21][22][23] Indeed, we have recently demonstrated the ability of bi-functional Pdbased bimetallic nanoparticles to catalyse the direct synthesis of H 2 O 2 in addition to the selective oxidation of a range of substrates [24][25][26] as well as the remediation of microorganisms through radical pathways. 27 With these studies in mind, we now investigate the efficacy of supported Pd-based bimetallic catalysts that combine the H 2 O 2 synthesising activity of Pd and the ability of a range of transition metals to generate reactive oxygen species through Fenton's pathways for the degradation of phenol.…”