Facile Synthesis of Catalytic AuPd Nanoparticles within Capillary Microreactors Using Polyelectrolyte Multilayers for the Direct Synthesis of H₂O₂

Abstract

Microreactors present innovative solutions for problems pertaining to conventional reactors and therefore have seen successful application in several industrial processes. Yet, its application in heterogeneously catalyzed gas–liquid reactions has been challenging, mainly due to the lack of an easy and flexible methodology for catalyst incorporation inside these reactors. Herein, we report a facile technique for obtaining small (<2 nm) and well-distributed catalytic nanoparticles on the walls of silica-coated capillaries, that act as micro(channel) reactors. These particles are formed in situ on the reactor walls using polyelectrolyte multilayers (PEMs), built by layer-by-layer self-assembly. Manipulating the PEMs’ synthesis condition gives easy control over metal loading, without compromising on particle size. Both monometallic (Au and Pd) and bimetallic (AuPd) nanoparticles were successfully obtained using this technique. Finally, these catalytic microreactors were found to exhibit exceptional activity for the direct synthesis of hydrogen peroxide from H2 and O2.

Recent Advances in the Direct Synthesis of H2O2

“…Hutchings and co‐workers, and others have extensively studied the synergistic effect achieved through the combination of Au and Pd. With the development of Au core‐ PdO shell nanoparticle morphology upon calcination often reported as key for improvement in catalytic selectivity, with Cybula et al.…”

“…[103] Edwards et al [104] were the first to report increased catalytic activity towards H 2 O 2 formation when alloying Au and Pd, building on these initial findings subsequent investigation of Pd-based, bi-and tri-metallic catalysts for the direct synthesis reaction with numerous combinations of catalysts since reported, including; PdÀ Pt, [105][106][107][108] PdÀ Ru, [109] PdÀ Rh, [59,110] Pd-Ir [111] and AuÀ Pd-Pt. [112,113] A comprehensive study by Deguchi Hutchings and co-workers [77,[114][115][116][117] and others [118][119][120][121] have extensively studied the synergistic effect achieved through the combination of Au and Pd. With the development of Au core-PdO shell nanoparticle morphology upon calcination often reported as key for improvement in catalytic selectivity, with Cybula et al following the effect of calcination temperature on AuÀ Pd nanoparticles via HAADF microscopy combined with elemental mapping [122] (Figure 4) and Tiruvalam comparing the nature of AuÀ Pd nanoparticle structure and morphology for the direct production of H 2 O 2 .…”

The Direct Synthesis of H2O2 Using TS‐1 Supported Catalysts

“…were the first to demonstrate that high selectivity towards H 2 O 2 can be achieved through combination of Pd with Au, with the development of a Au core‐PdO shell often reported as a key factor in the enhancement of catalytic selectivity ,. Indeed, these bimetallic Au−Pd catalysts have received significant attention by both experimentalists and theoreticians, with a combination of electronic, structural and isolation effects likely to be responsible for the synergy observed in Au−Pd systems.…”

Towards coupling direct activation of methane with in situ generation of H2O2

“…Fused silica capillaries (320 μm ID) containing a 4 μm precoated layer of SiO 2 on the wall (CP-SilicaPLOT, Agilent) were used as the microchannel reactor. Adopting the catalyst synthesis and coating from our previous work, 15 the silica layer was first treated by a 1 M ammonium nitrate solution which was flushed through the capillary using a syringe pump. The capillaries were then dried and calcined at 120 and 300°C, respectively.…”

“…This study aims to address this limitation by coupling the in situ generation of H 2 O 2 with the direct methane conversion in an inherently safe washcoated microchannel reactor which offers the opportunity to explore the process under a wide range of concentrations. This catalytic washcoated microchannel reactor, which showed an outstanding performance for the direct H 2 O 2 synthesis in our previous work, 15 is obtained by embedding Au-Pd nanoparticles on the silicacoated walls of a microcapillary. In this context, the effects of CH 4 , O 2 and H 2 partial pressures, O 2 /H 2 molar ratio, gas-toliquid ratio and residence time on product yields were investigated.…”