Subtle interaction between Ag and \(\hbox {O}_{2}\): a near ambient pressure UV photoelectron spectroscopy (NAP-UPS) investigations

  • Manoj Kumar Ghosalya
  • Kasala Prabhakar Reddy
  • Ruchi Jain
  • Kanak Roy
  • Chinnakonda S Gopinath
Regular Article
  • 98 Downloads

Abstract

The Ag-\(\hbox {O}_{2}\) interaction, which is at the center-stage of Ag-catalyzed partial oxidation reactions, is studied with NAP-UPS up to 0.2 mbar \(\hbox {O}_{2}\) pressure between 295 and 550 K. Three temperature regimes were identified for distinct Ag-\(\hbox {O}_{2}\) interaction, which are (a) 295–390 K, where mainly dissociative chemisorption of \(\hbox {O}_{2}\) happens, (b) 390–450 K, where diffusion of O-atoms into the sub-surfaces of Ag is prominent, and (c) >450 K, where metastable oxide forms on polycrystalline Ag surfaces. The work function (WF) of Ag changed from 4.95 (\(\le \)390 K) to 5.30 eV (390–450 K), and then to 5.7 eV (\(\ge \)450 K) at 0.1 mbar \(\hbox {O}_{2}\) pressure. Oxygen population in the sub-surfaces imparts crucial modifications to Ag at 390–450 K; it makes the surface to be electron-deficient that relates to the change in the WF of Ag and facilitates the formation of space charge layer on Ag surface. Oxygen adsorbed on such modified Ag-surfaces is electrophilic in nature, and this appears at a higher binding energy in core level XPS than the chemisorbed oxygen on metallic Ag. This is supported by angle-dependent NAP-XPS studies. The subsurface population of oxygen in Ag no longer persists at >410 K when the \(\hbox {O}_{2}\) supply is removed. A high ratio of antibonding/bonding O 2p bands suggests the unique silver-oxygen interaction under the measurement conditions.

Graphical Abstract

Synopsis Mildly oxidized Ag-surface is identified to exhibit different electronic structure between 390 and 450 K and at 0.1 mbar \(\hbox {O}_{2}\). Intensity ratio of Ag 4d/O 2p \(\approx \) 3 indicating metallic-Ag character under the above conditions underscores the unique and subtle Ag-\(\hbox {O}_{2}\) interaction. Space charge layer created offers the electrophilic oxygen, to interact with electron-rich molecules.

Keywords

Epoxidation heterogeneous catalysis surface science electronic structure 

Notes

Acknowledgements

MKG and KPR, and RJ are thankful to the UGC, New Delhi, and CSIR, New Delhi, respectively, for research fellowships. We thank SERB (SR/S1/PC-16/2012) and CSC-0404 for partial funding of this research project.

Supplementary material

12039_2018_1434_MOESM1_ESM.pdf (244 kb)
Supplementary material 1 (pdf 243 KB)

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Copyright information

© Indian Academy of Sciences 2018

Authors and Affiliations

  1. 1.Catalysis DivisionNational Chemical LaboratoryPuneIndia
  2. 2.Centre of Excellence on Surface SciencePuneIndia

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