Exploring the Impact of Active Site Structure on the Conversion of Methane to Methanol in Cu-Exchanged Zeolites.

In the past, Cu-oxo or -hydroxy clusters hosted in zeolites have been suggested to enable the selective conversion of methane to methanol, but the impact of the active site's stoichiometry and structure on methanol production is still poorly understood. Herein, we apply theoretical modeling in conjunction with experiments to study the impact of these two factors on partial methane oxidation in the Cu-exchanged zeolite SSZ-13. Phase diagrams developed from first-principles suggest that Cu-hydroxy or Cu-oxo dimers are stabilized when O2 or N2O are used to activate the catalyst, respectively. We confirm these predictions experimentally and determine that in a stepwise conversion process, Cu-oxo dimers can convert twice as much methane to methanol compared to Cu-hydroxyl dimers. Our theoretical models rationalize how Cu-di-oxo dimers can convert up to two methane molecules to methanol, while Cu-di-hydroxyl dimers can convert only one methane molecule to methanol per catalytic cycle. These findings imply that in Cu clusters, at least one oxo group or two hydroxyl groups are needed to convert one methane molecule to methanol per cycle. This simple structure-activity relationship allows to intuitively understand the potential of small oxygenated or hydroxylated transition metal clusters to convert methane to methanol.

Combining Computational Modeling with Reaction Kinetics Experiments for Elucidating the In Situ Nature of the Active Site in Catalysis.

Microkinetic modeling based on density functional theory (DFT) derived energetics is important for addressing fundamental questions in catalysis. The quantitative fidelity of microkinetic models (MKMs), however, is often insufficient to conclusively infer the mechanistic details of a specific catalytic system. This can be attributed to a number of factors such as an incorrect model of the active site for which DFT calculations are performed, deficiencies in the hypothesized reaction mechanism, inadequate consideration of the surface environment under reaction conditions, and intrinsic errors in the DFT exchange-correlation functional. Despite these limitations, we aim at developing a rigorous understanding of the reaction mechanism and of the nature of the active site for heterogeneous catalytic chemistries under reaction conditions. By achieving parity between experimental and modeling outcomes through robust parameter estimation and by ensuring coverage-consistency between DFT calculations and MKM predictions, it is possible to systematically refine the mechanistic model and, thereby, our understanding of the catalytic active site in situ.Our general approach consists of developing ab initio informed MKM for a given active site and then re-estimating the energies of the transition and intermediate states so that the model predictions match quantities measured in reaction kinetics experiments. If (i) model-experiment parity is high, (ii) the adjustments to the DFT-derived energetics for a given model of the active site are rationalized within the errors of standard DFT exchange-correlation functionals, and (iii) the resultant MKM predicts surface coverages that are consistent with those assumed in the DFT calculations used to initialize the MKM, we conclude that we have correctly identified the active site and the reaction mechanism. If one or more of these requirements are not met, we iteratively refine our model by updating our hypothesis for the structure of the active site and/or by incorporating coverage effects, until we obtain a high-fidelity coverage-self-consistent MKM whose final kinetic and thermodynamic parameters are within error of the values derived from DFT.Using the catalytic reaction of formic acid (FA, HCOOH) decomposition over transition-metal catalysts as an example, here we provide an account of how we applied this algorithm to study this chemistry on powder Au/SiC and Pt/C catalysts. For the case of Au catalysts, on which the FA decomposition occurred exclusively through the dehydrogenation reaction (HCOOH → CO2+H2), our approach was used to iteratively refine the model starting from the (111) facet until we found that specific ensembles of Au atoms present in sub-nanometer clusters can describe the active site for this catalysis. For the case of Pt catalysts, wherein both dehydrogenation (HCOOH → CO2 + H2) and dehydration (HCOOH → CO + H2O) reactions were active, our approach identified that a partially CO*-covered (111) surface serves as the active site and that CO*-assisted steps contributed substantially to the overall FA decomposition activity. Finally, we suggest that once the active site and the mechanism are conclusively identified, the model can subsequently serve as a high-quality basis for designing specific goal-oriented experiments and improved catalysts.

A comparative evaluation of the efficacy of probiotic and chlorhexidine mouthrinses on clinical inflammatory parameters of gingivitis: A randomized controlled clinical study.

BACKGROUND: The aim of our clinical trial was to assess and compare the antiplaque and anti-inflammatory potential of a probiotic mouthwash with 0.2% chlorhexidine and saline. MATERIALS AND METHODS: A randomized parallel group study was designed for a period of 4 weeks on 45 systemically healthy subjects between 20 and 30 years having chronic gingivitis. The study population was divided into three groups. Group A - 15 subjects were advised experimental (probiotic) mouthwash. Group B - 15 subjects were advised positive control (chlorhexidine) mouthwash and Group C - 15 subjects into a negative control group (normal saline). Oral prophylaxis was done for all groups at baseline. After the proper oral hygiene instructions, all the three groups were instructed to rinse their mouth with 10 ml of their respective mouthrinse, undiluted for 1 min twice daily, 30 min after brushing. Clinical parameters such as plaque index (PI), gingival index (GI), and oral hygiene index simplified (OHI-S) were assessed at baseline, 2 weeks and 4 weeks, respectively. RESULTS: At day 28, the PI, GI, and OHI-S were significantly reduced by all treatment modalities ranking probiotic and chlorhexidine is greater than saline. CONCLUSION: The probiotic mouthrinses tested was effectively used as an adjunct to mechanical plaque control in the prevention of plaque and gingivitis. Thus, the probiotic mouthrinse has a great therapeutic potential.

Periodontal microsurgery: A case report.

The purpose of this article is to limelight the benefit of periodontal microsurgery in the surgical disciplines. It reviews the benefits and potential applications of magnification and microsurgery in the specialty of periodontics and a case report on microsurgical approach for free gingival graft surgery in the treatment of gingival recession. The increased demand for mucogingival esthetics has required the optimization of periodontal procedures. Microsurgery is a minimally invasive technique that is performed with the surgical microscope and adapted instruments and suture materials. Although this hardware and knowledge of various operations are necessary to achieve patient esthetic expectations, clinicians must be willing to undergo an extended period of systematic training to become familiar with novel operating procedures and instruments. This article describes the application of the surgical microscope to provide enhanced perioplastic treatment.