AVS2010 Session SS1-MoM: Reactivity and Selectivity on Catalytic Surfaces

The study of adsorption and reactivity of oxygenated molecules on model oxide catalysts is of great interest to gain a better understanding of the mechanism of industrial reactions. Cerium oxide is commonly used in three-way auto exhaust and WGS catalysts. Our experimental design is intended to find the conditions where UHV experiments could mimic the real catalytic processes and from these results explain the reaction pathway from the atomistic level.

From the surface science point of view, previous studies of alcohols, ketones and aldehydes adsorbed on the surface of cerium oxide model catalysts have shown a variety of pathways for decomposition and reactivity. In the present work we have examined these pathways for two ethers, diethyl ether (DEE) and dimethyl ether (DME). Both ethers adsorb on fully oxidized CeO₂(111) and highly reduced CeOx(111) under UHV conditions but only at low temperature (~150 K) and they do not decompose. If the catalyst surface is pre-covered by hydroxyls, by adsorbing water at room temperature on the reduced CeOx(111) surface, then the adsorption geometry of the ether on this hydroxylayted surface changes, indicating interaction with OH groups. Regarding their reactivity towards decomposition, the two ethers behave differently when exposed to hydroxylated CeOx(111) at 300-400 K. In the case of DEE, this molecule promptly reacts by breaking the C-O bond presenting a very interesting chemistry. By means of Reflection Absorption Infrared Spectroscopy (RAIRS) and soft X-Ray Photoelectron Spectroscopy (sXPS), we could detect ethoxide and possibly carboxylate species as adsorbed intermediates for the reaction. However, when the hydroxylated CeOx(111) is exposed to DME at same conditions, the ether shows no reactivity, indicating the importance of H on a carbon atom β to oxygen. Possible explanations for the role of hydroxyls in helping break the C-O bond of ether molecules will be given.

The formation of palladium oxide (PdO) is thought to be responsible for the exceptional activity of supported Pd catalysts toward the complete oxidation of alkanes under oxygen-rich conditions. In this talk, I will discuss our recent investigations of the adsorption and activation of n-alkanes on a PdO(101) thin film that is prepared by oxidizing Pd(111) in ultrahigh vacuum (UHV) using an oxygen atom beam. We find that alkanes adsorb relatively strongly on the PdO(101) surface by forming σ-complexes along the rows of coordinatively unsaturated Pd atoms. The formation of σ-complexes causes the alkane binding energies on PdO(101) to exceed those of alkanes physisorbed on Pd(111). We also find that the alkane σ-complexes on PdO(101) act as precursors for initial C-H bond cleavage; propane and n-butane undergo facile C-H bond cleavage below about 215 K in UHV. Finally, I will discuss density functional theory calculations which show that dative bonding between the alkane and cus-Pd atoms weakens the coordinated C-H bonds, thereby lowering energy barriers for C-H bond scission by as much as 100 kJ/mol. These findings provide insights for understanding the role of molecularly chemisorbed precursors in the initial activation of alkanes on PdO(101) and possibly other transition-metal oxide surfaces.

The high Miller index surfaces of single crystal metals have chiral structures and interact enantiospecifically with chiral adsorbates. The enantiospecificity of surface reaction kinetics is dictated by the enantiospecificity of surface reaction energetics but can also be influences by reaction order. Recently, it has been shown that tartaric acid decomposes with explosive reaction kinetics on Cu(110) surfaces. Tartaric acid is chiral and on naturally chiral Cu surfaces such as Cu(643), Cu(653), Cu(17,5,1), and Cu(531) the non-linear nature of the reaction kinetics leads to extremely high enantiospecificity. The explosive reaction kinetics originte with the need for empty surface sites for the decomposition process. At high coverages of tartaric acid the limited availablity of empty sites contrains the decomposition process. Once nucleated, however, the reaction proceeds autocatalytically and occurs over a very narrow temperature range during heating. The decomposition kinetics of R,R- and S,S-tartaric acid depend on the handedness of the Cu substrate and the decomposition features observed during temperature programmed reaction spectroscopy are completely resovled. Peak enantiospecificities in the decomposition reaction rates can reach a factor of fifty.

The selective catalytic reduction (SCR) of NO_x by hydrocarbons on noble metals is critically important to the implementation of leaner burning more fuel efficient processes. Understanding the reaction mechanisms and pathways is essential for designing an effective catalytic system. As one small part of this effort, we focus on the interaction of nitrogen atoms and simple alkenes such as ethylene on the Pt(111) surface under UHV conditions in an effort to understand the potential intermediates in NO_x reduction. The adsorption of ethylene on a nitrogen pre-covered Pt(111) surface has been investigated using reflection absorption infrared spectroscopy (RAIRS) and thermal desorption spectroscopy (TDS) as well as density functional theory calculations. Three interesting observations have been made. First, we observed the presence of π-bonded ethylene below 220 K, indicating a switch in the preferred binding site for ethylene on N-Pt (111) as compared to the clean surface. Second, above 500 K, CN coupling occurs as indicated by the desorption of HCN and the identification of CNH₂ with RAIRS. Third, the formation of ammonia is observed through NH₃ desorption as well as by observation of the d_s (NH₃) mode of NH₃ at ~1170 cm^-1 with RAIRS. The appearance of ammonia is believed to be the result of a reaction between N atoms (or NH) with coadsorbed ethylidyne.