AVS2009 Session SS3-TuA: Catalysis: Mechanisms & Morphology

The kinetics of the reverse water gas shift (RWGS) reaction (CO₂ + H₂ → CO + H₂O) have been measured over clean and annealed Pt powder at a variety of conditions, and the surface coverage of adsorbed intermediates was verified by transient techniques. Under conditions were the surface coverage is very low and the reaction is occurring on essentially adsorbate-free Pt, the RWGS rate is 1000-fold faster than the rate of dissociative CO₂ adsorption (CO₂ → CO + O_ad), estimated from measurements of the reverse rate and equilibrium constant for this elementary step. This proves that the dominant mechanism for the RWGS reaction is not via dissociative CO₂ adsorption. The energetics estimated from DFT calculations of potential alternate pathways on Pt(111) suggests that CO₂ is instead activated by reaction with H_ad to make a COOH_ad intermediate, which dissociates to make CO_ad and OH_ad. A microkinetic model based on these DFT energetics reproduces well the measured absolute rate per Pt atom, its activation energy, and its dependences on CO₂ and H₂ partial pressures. This offers strong support for this alternate “carboxylate” pathway as dominating the mechanism. Estimates of the degrees of rate control of this reaction’s elementary steps and intermediates will also be discussed.

• Work supported by DOE-OBES Chemical Sciences Division.

The synthesis of methanol from CO₂ and H₂ (CO₂+3H₂→CH₃OH+H₂O) has attracted considerable attention. It is not only environmentally important due to its application in the conversion of greenhouse gas, CO₂, it is also of great industrial significance because the product, methanol, can serve as a raw material for the synthesis of other organic compounds, besides being used as a liquid fuel. Commercially, the reaction is performed on a catalyst containing Cu, ZnO and Al₂O₃ at a high temperate (220-240°C) and high pressure (50-100 bar). There is a need to understand the reaction mechanism in order to develop more active and selective catalysts even though the mechanism is still controversial. In this study, density functional theory (DFT) was employed to elucidate the reaction mechanism on the Cu(111) surface and the promotion effect of nano-size Cu compared to bulk Cu.

Cu₂₉ nanoparticles, exposing a combination of (100) and (111) faces in a pyramidal structure, is the model we used to study the reaction pathways on the Cu nanoparticles. For comparison, the reaction mechanism on the Cu(111) surface is also studied. It was found out that on both systems, the reaction undergoes via formate (HCOO) and dioxomethylene (H₂COO). The reaction rate is controlled by hydrogenation of HCOO and H₂COO. In accordance with experimental findings, our results show that the Cu₂₉ nanoparticles display a superior activity over Cu(111). The better behavior of Cu₂₉ is associated with the low-coordinated Cu sites, which provide a reasonably strong binding to the intermediates.

A key role in the activity of transition metal catalysts is the degree of interaction of metal d-bands with the adsorbed atoms/molecules [1]. It has been suggested that d-band center is a single effective measure of this interaction [2]. In this work we demonstrate how strain induced electronic structure changes can be used to tune the catalytic activity of the Oxygen Reduction Reaction (ORR). The limiting factor in the performance of the Pt-based PEM fuel cells is the low rate of the ORR taking place at the cathode. It has recently been shown that electrochemically leached PtCu catalysts, which have strained Pt rich shell due to dissolution of Cu, exhibit uniquely high reactivity for this reaction [3] . Using a surface science approach we have investigated the electronic structure effect in the enhanced ORR activity using Pt monolayers epitaxially grown on Cu(111) as a model system. We show that compressive strain and host substrate-induced changes in the Pt d-band center are responsible for the changes in chemisorption strength of adsorbed oxygen. Electronic nature of chemisorbed oxygen atoms on strained Pt monolayers has been investigated by probing oxygen projected density of states below and above the Fermi level using X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS), respectively. Combined oxygen K-edge XAS and XES results of oxygen on strained Pt monolayers show lowering of the adsorbate projected density of states and a complete filling up of antibonding states above Fermi level indicative of weakened metal-oxygen bond relative to that of oxygen on Pt(111). The weakening of the metal-oxygen bond is correlated with the broadening and lowering of the Pt d-band probed by valence band X-ray photoemission spectroscopy (XPS). The observed results are explained in terms of the d-band model.

References

[1] B. Hammer and J. K. Norskov, Theoretical surface science and catalysis—calculations and concepts, Adv. Catal. 45, 71 (2000).

[2] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis (John Wiley & Sons, New York, 1994).

[3] S. Koh and P. Strasser, Electrocatalysis on bimetallic surfaces: Modifying catalytic reactivity for oxygen reduction by voltammetric surface de-alloying, J. Am. Chem. Soc., 129 (42), 12624-12625 (2007).

In this study we focus on the questions related to the electronic band structure of the substrate. We chose a model system of evaporated Au nanoparticles on a rutile TiO2(110) single crystal substrate. The morphology of the evaporated Au nanoparticles were studied by STM, and it turned out to be strongly dependent on the pre-treatment of the TiO2 surface.[2]

For traditional surface science, the pressure gap between the studies in ultra high vacuum (UHV) and the industrial relevant reaction conditions is an important challenge. To overcome this problem, we have used in situ X-ray Photoelectron Spectroscopy (in situ XPS)[3] to study the evolution of the adsorbed chemical species and the electronic band structure of the rutile TiO2(110) during the individual steps in the reaction under pressures up to 1 Torr.

However, XPS on semiconductor substrates is challenging especially in the presence of gases. We have designed novel samples and test experiments to overcome these drawbacks. These results show that most in situ experiments on semiconductor substrates require extraordinary precautions. Now, we are able to avoid the newly discovered pitfalls, and we are able to present decisive results on the previously debated problems, e.g. the ongoing discussion about the charge state of gold and the role of oxygen vacancies.

[2] D. Matthey, J. Wang, S. Wendt, J. Matthiesen, R. Schaub, E. Laegsgaard, B. Hammer, and F. Besenbacher, “Enhanced bonding of gold nanoparticles on oxidized TiO2(110),” SCIENCE, vol. 315, Mar. 2007, pp. 1692-1696.

[3] D. Ogletree, H. Bluhm, G. Lebedev, C. Fadley, Z. Hussain, and M. Salmeron, “A differentially pumped electrostatic lens system for photoemission studies in the millibar range,” REVIEW OF SCIENTIFIC INSTRUMENTS, vol. 73, Nov. 2002, pp. 3872-3877.

Catalysts for NO_x storage and reduction (NSR) are being developed to reduce the NO_x emission from gasoline based internal combustion engines. BaO is considered to be potential NSR catalysts because of its strong interaction and an effective trapping of NO₂. BaO is more reactive when BaO is present as non-stoichiometric BaO clusters rather than bulk BaO. In order to understand the reactivity, BaO was grown on YSZ(111) and CeO₂(111)/YSZ(111) substrates by molecular beam epitaxy. In-situ reflection high-energy electron diffraction, ex-situ x-ray diffraction, atomic force microscopy and x-ray photoelectron spectroscopy have confirmed that the BaO grows as nanoclusters on YSZ(111) . During and following the growth under UHV conditions, BaO remains in single phase. The reaction of NO₂ with the BaO nanoclusters in different sizes on YSZ(111) and CeO₂(111)/YSZ(111) substrates was investigated using in situ high-resolution x-ray photoelectron spectroscopy (XPS). The adsorption of NO₂ on the BaO nanoclusters at room temperature and the formation of Ba(NO_x)₂ species at room temperature and above were probed. In addition the in situ XPS data collected from the BaO nanocluesters prior to and following the reaction with NO₂ were utilized to understand the morphology of BaO nanoclusters and the formation of Ba(NO_x)₂ species using Quases-Tougaard V5.1 software. These results are compared with the reported data from NO₂ reaction with BaO film deposited onto an Al₂O₃/NiAl(110) substrate.