Concentration-modulation excitation DRIFT spectroscopy study of carbonaceous intermediates of the WGS reaction on Pt/ceria

JULIA VECCHIETTI; ADRIAN BONIVARDI; MÓNICA CALATAYUD; SEBASTIÁN COLLINS

DEAUVILLE

Congreso; 5TH INTERNATIONAL CONFERENCE ON OPERANDO SPECTROSCOPY; 2015

Reducible oxides have 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 the reducibility of the support (oxygen vacancies) using well characterized platinum catalysts supported on pure and gallium-doped ceria [1-4]. Pt/CeO2 and Pt/CeGaOx catalysts present a remarkable reducibility, enhanced by the incorporation of Ga3+ cations [1,3] and a very stable metal dispersion. An inverse correlation was found between the catalytic activity to WGS and the amount of oxygen vacancies. Combining in situ time resolved X-ray diffraction, mass spectrometry and diffuse reflectance infrared spectroscopy (DRIFT) it was found that the oxygen vacancy filling by water is always fast either in Pt/CeO2 or Pt/CeGa, suggesting that the activation of water molecule in the WGS mechanism is not the rate limiting step in these systems.Concentration-modulation spectroscopy (c-MES) in DRIFT mode under WGS reaction conditions allows the selective detection of key reaction intermediates: monodentate formate (m-HCOO) and carboxylate (CO2-) species. Conversely, carbonate, bicarbonate and bridged formate groups behaves like spectators. The carbonaceous reactive intermediates are formed by the reaction of CO adsorbed on Pt sites with adjacent Ce3+-OH. It is proposed that m-HCOO/CO2- species are stabilized in the metal/support(vacancy) interface, which indicates the prevalence of a carboxyl (HOCO) mechanism. DFT calculation provides molecular insights to understand the pathway of the WGS reaction at the metal?oxide interface sites.