Concentration-Modulation Excitation DRIFT Spectroscopy Study of Carbonaceous Intermediates of the WGS Reaction on Pt/ceria

JULIA VECCHIETTI; ADRIAN BONIVARDI; SEBASTIAN COLLINS

Deuville

Congreso; 5th International Congress on Operando Spectroscopy; 2015

Caen U.

Reducible oxideshave been shown to greatly improve the activity of water gas shift (WGS)catalysts. The precise mechanism for this effect is a matter of intense debate.We investigate the relationship between the catalytic activity and thereducibility of the support (oxygen vacancies) using well characterizedplatinum catalysts supported on pure and gallium-doped ceria [1-4]. Pt/CeO2and Pt/CeGaOx catalysts present a remarkable reducibility, enhancedby the incorporation of Ga3+ cations [1,3] and a very stable metaldispersion. An inverse correlation was found between the catalytic activity toWGS and the amount of oxygen vacancies. Combining in situ time resolved X-raydiffraction, mass spectrometry and diffuse reflectance infrared spectroscopy(DRIFT) it was found that the oxygen vacancy filling by water is always fasteither in Pt/CeO2 or Pt/CeGa, suggesting that the activation ofwater molecule in the WGS mechanism is not the rate limiting step in thesesystems.Concentration-modulation spectroscopy (c-MES) inDRIFT mode under WGS reaction conditions allows the selective detection of keyreaction intermediates: monodentate formate (m-HCOO) and carboxylate (CO2d-) species (Fig. 1). Conversely, carbonate,bicarbonate and bridged formate groups behave like spectators. The carbonaceousreactive intermediates are formed by the reaction of CO adsorbed on Pt siteswith adjacent Ce3+-OH. It is proposed that m-HCOO/CO2d- species are stabilized in themetal/support(vacancy) interface, which indicates the prevalence of a carboxyl(HOCO) mechanism. DFT calculation provides molecular insights to understand thepathway of the WGS reaction at the metal?oxide interface sites.