Interactive Surface Chemistry of CO₂ and NO₂ on Metal Oxide Surfaces: Competition for Catalytic Adsorption Sites and Reactivity

Abstract

Interactive surface chemistry of CO 2 and NO 2 on BaO x /Pt(111) model catalyst surfaces were investigated via X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) techniques with a particular emphasis on the competition between different adsorbates for the catalytic adsorption sites and adsorbateinduced morphological changes. After NO 2 adsorption, nitrated BaO x /Pt(111) surfaces do not reveal available adsorption sites for CO 2 at 323 K, irrespective of the presence/absence of exposed Pt sites on the surface. Although NO 2 adsorption on thick BaO x (>10MLE)/ Pt(111) overlayers at 323 K leads to the formation of predominantly nitrate species, NO 2 adsorption on the corresponding carbonated surface leads to the formation of coexisting nitrates and nitrites. The presence of carbonates on BaO x /Pt(111) overlayers does not prevent NO 2 uptake. Carbonated BaO x (1.5 MLE)/Pt(111) surfaces (with exposed Pt sites) obtained via CO 2 adsorption can also further interact with NO 2 , forming surface nitrate/nitrite species, accompanied by the transformation of surface carbonates into bulk carbonate species. These results suggest that the nitrate formation process requires the presence of two adjacent unoccupied adsorption sites. It is apparent that in the presence of both NO 2 and CO 2 , carbonate species formed on Lewis base (O 2− ) sites enable the formation of nitrites on Lewis acid (Ba 2+ ) sites. Thermal aging, nitration, and carbonation have a direct impact on the morphology of the two-/three-dimensional (2D/3D) BaO x aggregates on Pt(111). While thermal aging in vacuum leads to the sintering of the BaO x domains, nitration and carbonation results in redispersion and spreading of the BaO x domains on the Pt(111) substrate. ■ INTRODUCTIONMost of the heterogeneous catalytic reactions rely on the consecutive or simultaneous adsorption of reactants on the catalytically active sites for the generation of products. Along these lines, it is not uncommon for reactants, intermediates, and/or products bearing similar chemical structures to compete for similar catalytically active adsorption sites in heterogeneous catalytic processes. Thus, molecular level understanding of the competition phenomena occurring during the adsorption of reactants/intermediates/products on surfaces is a fundamentally crucial aspect for the elucidation of heterogeneous catalytic reaction mechanisms. Automotive exhaust emission catalysts are not an exception to this subject, where multiple catalytic pathways proceed in a parallel fashion in the presence of a large variety of reactants/intermediates/products. For instance, during the operation of the NO x storage-reduction (NSR) catalysts, 1,2 oxygen-rich exhaust gases of lean burn engines are treated in two different alternating operational cycles called lean (abundant in oxygen) and rich (abundant in hydrocarbon) cycles, where toxic NO x gases are initially oxidized/trapped in the solid state and then successively reduced to harmless N 2 .Despite considerable...

NOx storage and reduction pathways on zirconia and titania functionalized binary and ternary oxides as NOx storage and reduction (NSR) systems

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Surface chemistry of carbon dioxide revisited

“…This was resolved from the decomposition of the interaction energy into electrostatic, polarization and charge transfer contributions [155]. The formation of BaCO 3 on a pristine BaO surface after saturating it with CO 2 was confirmed through XPS analysis [161]. Typical peaks that were associated with BaCO 3 were observed, e.g.…”

NOx Storage on BaTi0.8Cu0.2O3 Perovskite Catalysts: Addressing a Feasible Mechanism

“…Both studies concluded that CO 2 reacts with the intermediate Ba 2 TiO 4 at high temperatures leading to the formation of barium carbonate and barium titanate as final products. Considering the competition between CO 2 and NO 2 for the basic oxide sites of a NSR model catalyst [ 42 , 43 ], despite different acidities of CO 2 and NO 2 , it may be assumed that the NOx storage follows the previously proposed Reaction (4).…”