Precisely Controlled Metal Oxide Overcoating On Pt Catalysts For Improved Catalytic Performance In Hydrogenolysis Of Glycerol

Bifunctional catalysts which contain both metal and acidic functions have been widely employed in fossil hydrocracking and renewable biomass conversions.The distance between the two types of sites plays a crucial role in the bi-functionality,which governs the catalytic performance including both activity and selectivity.Production of biofuels and chemicals from renewable biomass has been receiving increased interest.However,the metal-acid proximity effect has been much less explored in biomass conversions compared to fossil hydrocracking.Glycerol,an abundant and inexpensive by-product(?10 wt%)of biodiesel production,could be converted into value-added chemicals such as 1,2-propanediol(1,2-PD)and 1,3-propanediol(1,3-PD)via hydrogenolysis.Glycerol hydrogenolysis involves sequential dehydration and hydrogenation steps on acid and metal sites,respectively.It was proposed that Lewis acid site attacks the terminal OH of glycerol to form acetol as an intermediate,while Bronsted acid site attacks the internal OH to form 3-hydroxypropanal;next,hydrogenation of acetol or 3-hydroxypropanal takes place on metal sites to produce 1,2-PD and 1,3-PD,respectively.Supported noble metal catalysts have been used for glycerol hydrogenolysis.Adding acidic function to metal catalyst could remarkably promote the catalytic activity as the dehydration step requires acid sites.A number of studies have demonstrated that incorporating acid sites onto either metal NPs or support,or even a simply physical mixture of solid acids and metal catalysts,could be effective for achieving activity improvement.However,to the best of our knowledge,the metal-acid proximity effect has not been explored.Therefore,we selected the supported Pt catalyst for glycerol hydrogenolysis as a platform to investigate the closest metal-acid proximity effect on glycerol hydrogenolysis via depositing the acidic porous alumina overcoat onto Pt catalyst.In this work,we precisely deposited porous Al2O3 overcoat onto Pt/Al2O3 catalyst using atomic layer deposition(ALD)by taking advantage of its self-limiting surface reaction feature to tune the activity and selectivity in glycerol hydrogenolysis.The acidic porous Al2O3 overcoat deposited on the Pt nanoparticles improves the proximity between the Pt metal and alumina acid sites by forming more metal-acid interfaces.The main research results are as the following:(1)At first we synthesized the supported Pt/Al2O3 catalyst by wet impregnation method.Then,the porous alumina film was grown onto the surface of Pt nanoparticles by ALD.The Lewis acidic site and its intimacy effect with metal site were which improves the bi-functionality,thus enhanced the activity and selectivity to 1,2-propanediol in hydrogenolysis of glycerol.The Pt/Al2O3 sample coated with 30 cycles of alumina ALD(denoted as 30Al/Pt/Al2O3)performed the highest activity and selectivity to 1,2-propanediol.and the thickness of alumina overcoat was about 3.6 nm under high resolution transmission electron microscopy(HRTEM).After the reaction.the alumina overcoat on the Pt nanoparticles partially peeled off due to the instability of the amorphous alumina under harsh reaction conditions.However,the in-situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS)measurements of CO chemisorption confirmed that there's still some porous alumina remained on the surface of Pt nanoparticles while there was more Pt-alumina interface formed.With the control experiment of hydrogenation of acetol.the key reaction intermediate in glycerol hydrogenolysis,we confirmed that the observed activity improvement by alumina overcoat in hydrogenolysis of glycerol could be attributed to the enhancement of the dehydration reaction step,where the proximity between Pt hydrogenation site and alumina Lewis acid site plays a crucial role in.Besides,we found the selectivity to 1,2-propanediol could be enhanced by reducing Pt particle size.Therefore,the increase of 1.2-propanediol selectivity by alumina overcoat might be due to the geometric effect,where the surface of Pt nanoparticles is divided into small ensembles by the porous alumina overcoat,similar to decrease of Pt particle size.(2)We deposited different cycles of FeOx on commercial Pt/C catalyst via ALD process.The results showed us that trace amounts of FeOx can significantly improve the activity of the catalyst.Three cycles of FeOx deposition showed the best activity improvement,and the conversion of the glycerol hydrogenolysis reaction was 3 folds that of the Pt/C catalyst in same reaction time.