The effect of sub-surface oxygen on the catalytic properties of silver catalysts

Mechanism and active sites for the partial oxidation of methanol to formaldehyde over an electrolytic silver catalyst

“…'Pinhole' formation has also been observed after sequential exposure of polycrystalline silver foils to O 2 , then H 2 , at high temperatures [51]. The preferential development of 'pinholes' on dislocations in the silver structure noted here supports a well-established theory that such sites provide low resistance paths for oxygen and hydrogen diffusion into the silver bulk [10,11,15,23,24,31,45,48,52]. The importance of oxygen diffusion along grain boundaries and related structural faults to the operation of silver catalysts for formaldehyde manufacture is discussed below.…”

“…This led us to conclude that the 640 cm −1 band was associated with an O ␤ species formed on or in the vicinity of grain boundaries. Grain boundaries are known to provide low resistance paths for oxygen diffusion into the silver bulk [10,11,15,16,23,24,31,45]. Faceted surfaces created by the oxygen-induced restructuring of silver substrates at high temperatures are also characterised by a Raman band at 640 cm −1 [12,26,27,31].…”

“…O ␤ is identified by an O 1s binding energy between 530.0 and 531.0 eV [12,26,29,30,40,48], a Ag-O stretching frequency of 640 cm −1 when incorporated between the top two layers of a reconstructed Ag(1 1 1) surface [12,23,26,27,31], and gives a broad O 2 desorption signal between 680 and 800 K [13,16,17,28,31]. Grain boundaries or related dislocations in the silver structure facilitate the formation of the O ␤ [10,11,15,16,23,24,31,45]. At temperatures above 773 K, O ␤ formation is accompanied by bulk restructuring into facets of (1 1 1) texture [12,[14][15][16][17]19,[26][27][28][29][30][31][46][47][48].…”

Methanol Oxidation on a Copper Catalyst Investigated Using in Situ X-ray Photoelectron Spectroscopy

“…The experimental observation that all Ag and Cu catalysts require activation under oxidizing conditions, and the extensive studies of the deactivation of silver due to an increase of the structural perfection of the metal [57] support the view that this metastable, bulk-dissolved oxygen has the role of an enabling species for the facile adaptation of the metal-oxygen system to the structures that are dictated by thermodynamics. In this view the observation that a very small average lattice expansion of Cu correlates well with the catalytic activity indicates the importance of lattice defects as "exchange sites" for the surface-to-subsurface migration of oxygen species.…”

Restructuring and Redispersion of Silver on SiO2 under Oxidizing/Reducing Atmospheres and Its Activity toward CO Oxidation

“…It has been shown that metal particles annealed in a hydrogencontaining atmosphere, including silver particles, exhibit a morphology having rounded edges and corners. 29,30 Auroux and Gravelle observed that the reduced silver catalyst liberates high heats when interacting with oxygen, and they postulated that the surface of a freshly reduced silver particle contains a large number of lattice defects. 31 Furthermore, water vapor was also reported to be capable of increasing the surface roughness of a silver particle.…”