Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Catalyst”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Ethanol Decomposition on Pd–Au Alloy Catalysts

The mechanisms of ethanol (EtOH) decomposition via C–C or C–O bond cleavage on alloy surfaces are currently not well understood. Here, we model EtOH decomposition on close-packed Pd–Au catalytic surfaces using density functional theory (DFT) calculations and derived Brønsted–Evans–Polanyi (BEP) relationships. Three characteristic Pd–Au surfaces are considered, Pd 1 Au 2 (111), Pd 2 Au 1 (111), and a Pd monolayer (ML), Pd ML (111), on a Au substrate. We show that, on close-packed Pd–Au surfaces, the C–C bond is easier to cleave than C–O, indicating that the formation of CH 4 and CO is favored as the products of EtOH decomposition. Interestingly, we find that, though the C–C and C–O activation barriers on Pd ML (111) are generally lower than those on the other two surfaces, it is less active for EtOH decomposition due to a slow release of H 2 and possible carbon coking. Pd 2 Au 1 (111), on the other hand, has a higher theoretical reaction rate due to facile H 2 evolution from the surface and less carbon coking. A comparison of the surface d-band with the activation energy barriers shows that there is a trade-off between the barriers for C–C bond cleavage and H 2 association, with Pd 2 Au 1 (111) having the best performance. Temperature-programmed desorption experiments of EtOH on Pd/Au surfaces show significant C–C bond cleavage and both CH 4 and CO production on surfaces with Pd–Au interface sites. Furthermore, neither Auger electron spectroscopy nor EtOH reflection–adsorption infrared spectroscopy provide evidence of C–O bond cleavage. Finally, the experimental reaction rate for methane production from C–C bond cleavage was higher on surfaces with more Au present due to minimal carbon contamination and the promotion of product desorption. This combined theoretical and experimental study shows that, though Au is catalytically inactive for EtOH decomposition, it can dramatically promote the surface activity for EtOH steam reforming due to the existence of active Pd–Au surface ensemble sites.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Water Promotion (or Inhibition) of Condensation Reactions Depends on Exposed Cerium Oxide Catalyst Facets

Nanoparticles with well-defined facets enable quantitative correlations between surface features and catalytic activity. In this work, we explore the role of geometric and acid–base properties in the mechanism of aldol condensation catalyzed by ceria nanoshapes. In the crucial C–C coupling step, the two carbonyl adsorbates are bound to two adjacent cations. On the CeO 2 (110) plane, all atoms lie on the same layer, favoring the interaction between adsorbates and facilitating the bimolecular C–C coupling. Thus, the initial unimolecular deprotonation is rate-limiting. In contrast, the (100) and (111) planes have an open structure with O on the top layer and Ce on the layer below. The former O layer interferes between adsorbates linked to Ce sites, making the C–C coupling rate-limiting. Water helps in overcoming this spatial hindrance by remote bond polarization via H-bonds. Therefore, water promotion is only observed on those planes for which the bimolecular step is rate-limiting.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗