Aqueous Methanol to Formaldehyde and Hydrogen on Pd/TiO₂ by Photocatalysis in Direct Sunlight: Structure Dependent Activity of Nano-Pd and Atomic Pt-Coated Counterparts

Abstract

In the present investigation, facet-controlled Pd nanoparticles with nanocube (PdNC) and truncated octahedron (PdTO) morphologies, and their counterparts with half-a-monolayer of atomic Pt coated (0.5θPt-PdNC and 0.5θPt-PdTO) surfaces were prepared. All of them were characterized and evaluated as cocatalyst after supporting them on commercial titania (P25) (PdNC/P25, PdTO/P25, 0.5θPt-PdNC/P25, and 0.5θPt-PdTO/P25) under direct sunlight and/or one sun conditions for the oxidation of methanol to formaldehyde along with solar hydrogen production. PdNC/P25 shows higher activity for hydrogen generation compared to PdTO/P25; however, activity reversal occurs with the above cocatalysts, but, after Pt-coating with further enhanced activity. The highest conversion of methanol (0.2 μmol/h.mg) to 100% selective formaldehyde was observed with 0.5θPt-PdTO/P25, while other catalysts show significantly lower methanol conversion in the following order: 0.5θPt-PdTO/P25 > 0.5θPt-PdNC/P25 > PdNC/P25 > PdTO/P25. Pt-coated on (111) facets of PdTO simulates the activity associated as that of Pt(111) facets and demonstrating the highest and facet dependent activity. The present study is truly in resonance with exploiting the surface properties for heterogeneous catalysis, and highlights that less than a monolayer of Pt is sufficient to simulate the activity as that of bulk Pt. It is worth exploring this concept to other metals and substrates too.

Selective and Generic Photocatalytic Oxidation of Alcohol with Pd−TiO2 Thin Films: Butanols to Butanal/Butanone with Different Morphologies of Pd and 0.5θPt‐Pd Counterparts

“…This observation underscores that Ptcoating on Pd enhances hydrogen generation; it is also notable that irrespective of catalysts, thin film form shows higher H 2 production than powder form and these results are consistent with our earlier reports. [21,31,33] While light scattering decreases the effective light absorption in the suspension form, efficient light absorption and internal light scattering within the thin film form enhance the rate of reaction in thin film form; [22] efficient light absorption in turn promotes more charge carriers generation and thus improves the photocatalytic activity.…”

“…Detailed procedure is given in our earlier publication. [21] As prepared Pd-nanoparticles is further cleaned with NaBH 4 to remove any absorbed impurities, such as PVP, before Pt-coating. 60 mg NaBH 4 in 5 ml water was mixed with 5 ml of nanoparticle solution and heated to 85 °C and kept at the same temperature for 3 h under constant stirring.…”

“…Although heterogeneous photocatalysis has been occasionally employed in the last decades, a renewed interest is clear from the increasing number of reports, especially due to environmental concerns and climate change. [21][22][23] Recently, Mohammadi et al reported the photooxidation of 2-butanol using multiwalled carbon nanotubes/cobalt phthalocyanine composites. [24] Gandarias et al reported the oxidation of butanol to butanal by using oxygen and stable Pt-based nanoparticles.…”

Non‐Classical Conversion of Methanol to Formaldehyde

“…Nalajala et al [95] report on the production of a costeffective photocatalyst and its performance under (artificial and real) sunlight to produce CH 2 O and H 2 from CH 3 OH. As Pt has excellent H 2 adsorption/desorption characteristics, but is more expensive than other cocatalysts, they coated only half a monolayer of Pt on Pd nanoparticles supported on TiO 2 and achieved enhanced activity.…”

A stable and recyclable Z-scheme g-C3N4/rGO/BiVO4 heterojunction photocatalyst for site-selective C-3 formylation of indoles with methanol as a formyl source under visible light

“…Delightfully, recent studies indicated that dehydrogenation of methanol to formaldehyde could also be achieved in heterogeneous photocatalytic systems under mild conditions. 27 These heterogeneous photocatalysts could be easily isolated and reused compared with homogeneous catalysts. 28 Moreover, the photoexcited electrons and holes (charge carriers) could bypass the external redox reagent to complete the chemical conversion.…”