Investigation Of Gallium Promoted Pd Tio2 Catalysts For The Selective Hydrogenation Of Acetylene In The Presence Of Ethylene

Author : Madelyn Rose Baltus Ball
Publisher :
ISBN 13 :
Total Pages : 136 pages
Book Rating : 4.:/5 (123 download)

DOWNLOAD NOW!

Download or read book Insights Into Active Site Structures for Pd-based Bimetallic Heterogeneous Catalysts written by Madelyn Rose Baltus Ball and published by . This book was released on 2019 with total page 136 pages. Available in PDF, EPUB and Kindle. Book excerpt: Catalytic reactions play a large role in the production of energy and products used in our modern society. In order to improve the efficiency of existing processes and make the conversion of new feedstocks, such as biomass and natural gas, possible, there exists the need for improved catalytic materials. Many active catalysts have complex active sites containing multiple components; therefore, fundamental understanding of active sites and structure-performance relationships is needed to rationally design new catalysts. The work in this dissertation is focused on catalytic synthesis of well-controlled bimetallic catalysts in order to develop structure-activity and structure-selectivity relations for different chemical transformations. A controlled surface reaction synthesis approach is used to deposit Pd onto Au, Ag, or Cu parent catalysts. These Pd bimetallic catalysts are then studied for reactions where selectivity is a main challenge, and the selective Pd structure is identified. First, AuPd/TiO2 catalysts are studied for the gas phase amination of 1-hexanol using ammonia. It was determined that there are no monometallic Pd structures on the catalysts. The Pd atoms are well diluted in the Au nanoparticles at low loadings and as the Pd loading increases, more contiguous Pd sites are formed. From reactivity studies, it is determined that both the Au and Pd contribute to the overall hexanol conversion, however the activity in enhanced for the bimetallic catalysts compared to the monometallic catalysts. This rate improvement is attributed to a geometric effect where surface intermediates are not stabilized by hydrogen bonding to adsorbed ammonia in close proximity. Furthermore, the selectivity to primary amines is increased as the loading of Pd increases in the AuPd catalysts, while lower Pd loadings favor more substituted products. The selectivity trends are hypothesized to be related to both geometric and electronic effects present in the AuPd catalyst system. Next, AgPd catalysts were studied for the hydrodechlorination of 1,2-dichloroethane. The role of the support in the formation of well-controlled bimetallic nanoparticles was investigated and it was determined that well-mixed AgPd particles were formed on a TiO2 support, while some monometallic Pd particles were also formed on C and SiO2 supports. Again, using chemisorption, XAS, and CO-FTIR, it was determined that isolated Pd atoms within the Ag nanoparticles are formed on the TiO2 support at low loadings. On the SiO2 supported catalysts, contiguous Pd species are formed. The isolated Pd species are highly selective to ethylene while the contiguous Pd species are selective to the undesired ethane. Thus, the support was determined to play an important role in tuning the AgPd structure and the resulting selectivity for hydrodechlorination. The rate of 1,2-dichloroethane conversion was determined to be related to the selectivity, with the ethylene selective catalysts exhibiting the highest turnover frequencies. Finally, AgPd and CuPd catalysts were investigated for the selective hydrogenation of acetylene. Both TiO2 and SiO2 supported catalysts were studied, and it was determined by EDS analysis that the CuPd nanoparticle composition distribution was more uniform on the SiO2 support than on the TiO2 support. IR spectra of the catalyst surface after being exposed to reaction conditions indicate the presence of numerous spectator species and it is determined that there are less sites covered by spectators for the bimetallic catalysts compared to a monometallic Pd catalyst. On the TiO2 support, the bimetallic catalysts exhibit the highest rates per gram of Pd and high ethylene selectivites above 99%. The monometallic catalysts are less than 95% selective to ethane, indicating that the alloy formation improves the ethylene selectivity; the enhanced selectivity is attributed to electronic effects in the bimetallic nanoparticles. This work enabled the development of detailed active site hypotheses for different Pd based bimetallic catalysts and provided insights into the desired structures to inform rational catalyst design. The controlled surface reaction synthesis approach and general characterization methodology can be applied to a broad range of catalyst and reaction systems to improve chemical transformations for improved production of chemicals and fuels for modern society.