Kinetics of Hydrogenation Reactions on Supported Platinum and Platinum-Tin Catalysts

The overarching goal of this research project is to develop insights into the characterization of supported platinum and platinum-tin catalysts, not only from a materials science standpoint but more extensively with respect to their catalytic activity for hydrogenation reactions, employing probe reactions such as carbon monoxide methanation and vapor phase acetone hydrogenation, to help unveil the nature and quantity of active sites in supported mono- and bimetallic systems. Catalyst synthesis plays an important role in this research project, as the focus has been on the development and implementation of a synthesis method capable to afford better control over the displacement of Sn onto the Pt/γ-Al2O3 materials, generating materials with enhanced homogeneity, that are ultimately suitable for more meaningful kinetic studies. The method, namely sequential strong electrostatic adsorption (SEA) was used to synthesize Pt/γ-Al2O3 and PtSn/γ-Al2O3 materials whose performances were compared with mono- and bimetallic materials synthesized by the standard impregnation method, using SiO2 and γ-Al2O3 as supports. For carbon monoxide methanation, clearly distinct performances were observed between the SEA-synthesized bimetallic materials and the ones synthesized by the high throughput method (incipient wetness), where the SEA materials presented platinum-like performances, and the ones synthesized by incipient wetness presented considerably decreased rates. For the SEA bimetallic samples, the chemisorption uptakes of carbon monoxide and hydrogen were proportionally lowered to the increasing Sn/Pt atomic ratios, as well as their macroscopic rates of methanation reaction, whereas the oxygen chemisorption uptakes presented the opposite behavior. We were able to show with the XPS-derived Sn oxidation splits between Sn(0) and Sn(II,IV), that for that system, about half of the tin is present over the platinum catalyst surface, as Sn(0), interacting with platinum, forming local complexes which are inactive for chemisorption, and therefore methanation, and that the other half is dispersed across the support, either deposited onto the alumina acid sites or in the interfaces between the support and the catalyst particles. We estimated that for every two Sn(0) atoms on the surface, one Pt hydrogenation site was poisoned and that for every two partially reduced tin atoms, one active site for oxygen chemisorption was created. The poisoning seemed to have occurred without clear impacts on the bulk of the platinum crystals, as the leftover platinum atoms still kept their platinum-like performance, judging by unaltered apparent kinetic parameters (barriers and orders), equivalent in-situ FTIR CO adsorption character, and near-equivalent SSITKA-derived surface coverages for reacting species. For the incipient wetness samples, the same material homogeneity was not present, and the same type of analysis was not as fruitful. Moving on to probing the interaction of the carbonyl with the catalyst materials, in the acetone hydrogenation experiments, while comparing monometallic platinum catalysts with distinct crystallite sizes, dispersions, synthesis methods, and supports, it was found that the alumina-supported materials consistently presented a rate of reaction per active site (STY) about five times bigger than the silica-supported counterparts. We were able to show with a set of in-situ titration experiments, that the Pt/γ-Al2O3 catalysts were bifunctional, in essence, presenting two types of active sites; the platinum hydrogenation sites, and the alumina acidic sites, and proposed that the acceleration of the reaction was caused by a change in the reaction mechanism, where the alumina support would work as a reservoir for the surface reactive acetone enolates, and that the platinum sites were mainly responsible for hydrogen dissociative chemisorption, and spillover to the support

Hard Carbon Derived from Avocado Peels as a High-Capacity, High-Performance Anode Material for Sodium-Ion Batteries

Deriving battery grade materials from natural sources is a key element to establishing sustainable energy storage technologies. In this work, we present the use of avocado peels as a sustainable source for conversion into hard carbon based anodes for sodium ion batteries. The avocado peels are simply washed and dried then proceeded to a high temperature conversion step. Materials characterization reveals conversion of the avocado peels in high purity, highly porous hard carbon powders. When prepared as anode materials they show to the capability to reversibly store and release sodium ions. The hard carbon-based electrodes exhibit excellent cycling performance, namely, a reversible capacity of 352.55 mAh/g at 0.05 A/g, rate capability up to 86 mAh/g at 3500 mA/g, capacity retention of >90%, and 99.9% coulombic efficiencies after 500 cycles. This study demonstrates avocado derived hard carbon as a sustainable source that can provide excellent electrochemical and battery performance as anodes in sodium ion batteries