Chemisorption

About: Chemisorption is a research topic. Over the lifetime, 16298 publications have been published within this topic receiving 554989 citations.

Surface Composition and Selectivity of Alloy Catalysts

Abstract: Publisher Summary Alloying of a catalytically active metal with an inert component changes the selectivity in hydrocarbon reactions such that C–C bond fission is suppressed compared with C–H bond breaking. Upon alloying, the selectivity for cracking reactions decreases. This chapter shows that the surface composition of macroscopic alloys often differs from the bulk composition. Selectivity changes can be understood in terms of the primary ensemble effect—that is, the dilution of the metal surface with inactive atoms diminishes the probability for metal-adsorbate complexes containing several neighboring metal atoms as required for cracking. The concept of an ensemble of metal atoms, crucial to the understanding of selectivity patterns on alloys, implies that the metal atoms in the surface of the alloy keep their individuality and are only influenced by their immediate environment. The chapter discusses the secondary ensemble effect that ascribes changes in heat of chemisorption of multiply bonded atoms to a decrease in the coordination of these atoms to the surface metal atoms. This effect leads to a decrease in heat of adsorption upon alloying.

Surface Modification of Colloidal Gold by Chemisorption of Alkanethiols in the Presence of a Nonionic Surfactant

Abstract: Surface modification of colloidal gold with 11-mercaptoundecanoic acid or 16-mercaptohexadecanoic acid was performed in the absence or in the presence of the nonionic surfactant polyoxyethylene (20) sorbitan monolaurate (Tween 20). The stability of the colloidal systems was assessed with optical absorption spectroscopy. The surface-modified nanoparticles were stable only within a narrow range of intermediate pH values when chemisorption of alkanethiols was performed in the absence of Tween 20. This was explained in terms of partial ionization of the surface carboxylic groups and charge neutralization at high pH values by counterions present in the buffer solutions. Formation of a physisorbed monolayer of Tween 20 onto the nanoparticles prior to chemisorption of alkanethiols resulted in surface-modified colloidal gold that was stable over a broader range of pH values. Parallel experiments demonstrated that self-assembled monolayers could form on flat substrates in the presence of Tween 20. Therefore, possi...

Nanoscale metal oxide particles/clusters as chemical reagents. Unique surface chemistry on magnesium oxide as shown by enhanced adsorption of acid gases (sulfur dioxide and carbon dioxide) and pressure dependence

Abstract: Surface adsorptive properties of nanoscale MgO particles have been compared with more conventional samples. Morphologically the nanoparticles (autoclave prepared = AP-MgO) are unique and very different from the conventional samples (conventionally prepared = CP-MgO), and AP-MgO possesses more defects, edge and corner sites, higher surface area and more higher index surfaces. The number of residual surface −OH groups/nm2 is similar for both types of samples. Differences in adsorptivity of SO2 and CO2 at relatively low pressure (20 Torr) were determined by gravimetric means. Much larger quantities were adsorbed by AP-MgO. This process of chemisorption was dynamic, and oxygen scrambling occurred when SO2 and Mg18O nanoparticles were in contact. These results, complying with FTIR studies, are rationalized as due to higher intrinsic surface reactivity coupled with higher concentrations of lower coordination ions on the nanoparticle. Pressure studies showed, however, that as 100 Torr of SO2 or CO2 was reached, ...

Surface science studies of the electronic and chemical properties of bimetallic systems

Abstract: Recent studies dealing with the electronic, chemical, and catalytic properties of well-defined bimetallic surfaces (prepared by vapor-depositing one metal onto a crystal face of a second metal) are discussed. The result show that a metal atom supported on a dissimilar metal can be electronically perturbed and this perturbation can dramatically alter the chemical and catalytic properties of both constituents of the bimetallic system. In many cases, the metal adatoms exhibit properties toward the chemisorption of H{sub 2}, O{sub 2}, CO, and CO{sub 2} and reactivities toward small hydrocarbons that are significantly different from those seen for the pure metal. For supported monolayers of Ni, Cu, and Pd a correlation is observed between shifts in surface core-level binding energies and changes in the desorption temperature of CO from the metal adlayers. The shifts in core-level binding energies and CO desorption temperatures are a consequence of (1) electronic interactions between the metal overlayer and metal substrate and (2) variations that occur in the admetal-admetal interactions when the admetal adopts the lattice parameters of the substrate. Examples are provided which demonstrate the relevance of single-crystal studies for modeling the behavior of high surface area supported bimetallic catalysts. The coupling of an apparatus formore » the measurement of reaction kinetics at elevated pressures with an ultrahigh-vacuum system for surface analysis allows detailed studies of structure sensitivity, the effects of surface composition on catalytic activity, and, in certain cases, identification of reaction intermediates by postreaction analysis. The roles of ensemble' and ligand' effects in mixed-metal catalysts are discussed in the light of data obtained by using well-defined bimetallic surfaces.« less