AVS2017 Session HC+SA+SS-ThM: Mechanisms and Reaction Pathways in Heterogeneously Catalyzed Reactions

Recently it was shown that a Cu(111) surface will reconstruct to form nanoclusters when exposed to 0.1 – 100 Torr of CO.¹ We present the use of polarization dependent – reflection absorption infrared spectroscopy (PD-RAIRS) to monitor the Cu(111) restructuring in real time. Under 10 Torr of CO, PD-RAIR spectra display a peak for CO on top of Cu atoms. Scans were taken periodically and displayed new peaks related to CO bound to the nanoclusters that grow over a period of 30 minutes. Spectra obtained at 10 Torr and 300 K show that the creation of the Cu nanoclusters is correlated with an increase in intensity of these C-O vibrational features, which are only visible due to removal of gas phase CO features from the RAIR spectra. Dissociation of H₂O in UHV occurs on the nanoclusters, which is negligible on unreconstructed Cu(111). Previously the splitting of H₂O was thought to be a geometric effect caused by the nanoclusters as under UHV conditions CO does not adsorb on Cu(111) at room temperature. However, after exposing Cu(111) to 10 Torr of CO at room temperature for 30 minutes strong C-O vibrations are observed upon evacuation of the IR cell. In UHV, the H₂O partial pressure is increased in the IR cell to 2 × 10^-8 Torr and flowed over the reconstructed Cu(111) crystal. The RAIR spectra indicates there is a reactive CO species that interacts with H₂O to create formaldehyde. This is further confirmed by observing formaldehyde with temperature programmed desorption following H₂O exposure. Auger electron spectroscopy confirms the presence of oxygen on the Cu(111) surface after water exposures in the IR cell. Detailed interpretation of the data requires consideration for the formation of Fe-carbonyls, which can be present in the CO bottle, or produced in the reaction cell. The possibility of Fe as the cause for the CO bound in UHV will be presented.

1. Eren, B.; Zherebetskyy, D.; Patera, L. L.; Wu, C. H.; Bluhm, H.; Africh, C.; Wang, L.-W.; Somorjai, G. A.; Salmeron, M. Activation of Cu(111) Surface by Decomposition into Nanoclusters Driven by CO Adsorption. Science 2016, 351, 475 LP - 478.

Endothermic catalytic reactions require operation at elevated temperatures. The heating required is usually obtained by combustion of hydrocarbons and contributes to CO₂ emission. Instead electricity obtained in a sustainable should drive the reaction. In addition, it is desirable that the energy transfer involved is done in a bond specific manner. Plasma excitation and dissociation of molecules can serve this purpose. In plasma, all molecular degrees of freedom are not in equilibrium and dissociation of CO₂ can be realized much more efficiently than in thermodynamic equilibrium. There is a preferential vibrational excitation of CO₂.

In Chengdu we use a plasma chemical reactor with mass spectroscopy, infrared spectroscopy, optical emission spectroscopy and a Langmuir probe to study the characteristics of the plasma, reaction products and the catalyst. In Amsterdam we use a thermal reactor and gas chromatography to study reaction products. The reactions are carried out at pressures of several hundreds of Pa up to atmospheric in Ar buffergas. Catalysts are prepared in the usual way.

The simplest reaction studied in the plasma reactor is the dissociation of CO₂ into CO and O₂. We find energy efficiencies higher than 45%, indicating that the system is not in thermodynamic equilibrium and plasma favors vibrational excitation to translational heating. Adding a catalyst like AgO or NiO on Al₂O₃ does not enhance the yield. However, a purely metallic catalyst does significantly enhance the yield.

Optical emission spectroscopy shows that the radiofrequency (RF) and microwave (MW) plasma behave quite different. In the MW plasma predominantly emission from the C₂ Swann-band is seen, whereas the RF plasma shows mainly chemiluminescence from excited CO. This is due to a different electron excitation mechanism.

In the case of dry reforming of CH₄ with CO₂ in the plasma reactor we find that addition of an oxidic catalyst does not enhance the yield of CO + H₂. In the case of dry reforming of butane (C₄H₁₀) to yield butene (C₄H₈), plasma reforming with or without catalyst does shows only small conversion. Mainly cracking of butane into C₂H_x is seen and polymerization. However, running the same reaction under high temperature conditions in a thermal reactor yields a satisfactory conversion. A Co based catalyst has the best performance.

These studies allow us to obtain mechanistic information on the conversion of simple molecules, pretreated by plasma, on various catalysts. We are exploring to what extend direct Eley-Rideal reactions are relevant in the plasma reactor. This reaction mechanism is very unlikely under thermal conditions.

First, we shall discuss how surface science and mass-selected nanoparticles can be used to make efficient model systems for heterogeneous catalysts. We shall demonstrate how mass-selected nanoparticles of CuZn alloys can be used to elucidate the dynamics of the methanol synthesis catalysts. The produced nanoparticles will be compared to the conventional CuZnAl at 1 bar for synthesizing methanol from CO₂ and H₂ [1, 2, 3]. The methanol synthesis on CuZn will also be discussed with respect to our recent findings of using alloys of NiGa for methanol synthesis [4]. The use of mass-selected nanoparticles will be further demonstrated for electrochemical Oxygen Reduction Reaction, which is really the limiting reaction in Proton Exchange Membrane Fuel Cells. Here we have found entirely new classes of electro-catalysts by alloying Pt with early transition metals [5] or the lanthanides [6]. We have also shown that it is possible to make mass-selected nanoparticles of these alloys with very good activities [7] and PtGd alloys [8]. Finally, we shall also discuss how planar surface science can be used to identify new catalysts for ammonia oxidation. We shall demonstrate how Copper deposited on Ruthenium can enhance the activity substantially and give rational explanations for this enhancement which also can be transferred to high area catalysts used for diesel exhaust treatment [9].

References

1. S. Kuld,....... I. Chorkendorff and J. Sehested, Angew. Chemie 53 (2014) 1.

2. C. Holse,....., I. Chorkendorff,S. Helveg, J. H. Nielsen, J. Chem. Phys. 119 (2015) 2804-2812.

3. S. Kuld,..... I. Chorkendorff,J. Sehested, SCIENCE 352 (2016) 969-974.

4. F. Studt, ......, I. Chorkendorff, and J. K. Nørskov, Nature Chemistry. 6 (2014) 320.

5. J. Greeley, ..... I. Chorkendorff, J. K. Nørskov, Nature Chemistry. 1 (2009) 522.

6. M. Escudero-Escribano, ...., I. Chorkendorff, SCIENCE 352 (2016) 73-76.

7. P. Hernandez-Fernandez, ..... I. Chorkendorff, Nature Chemistry 6 (2014) 732-8.

8. A. Velázquez-Palenzuela,.....I. Chorkendorff, J. Catal. 328 (2015) 297-307.

9. D. Chakraborty, ....., I. Chorkendorff, Accepted Angew. Chemie. (2017).

Understanding the interactions of hydrogen with IrO₂ surfaces is central to improving applications of electrocatalysis as well as exploiting the high-reactivity of IrO₂ for promoting methane activation. In this talk, I will discuss our recent investigations of the dissociative chemisorption and oxidation of H₂ on stoichiometric and oxygen-rich IrO₂(110) surfaces. We find that H₂ dissociation is highly facile on s-IrO₂(110), with more than 90% of a saturated H₂ layer dissociating below 225 K during temperature-programmed reaction spectroscopy (TPRS). We observe only H₂O desorption in a broad TPRS peak from about 400 to 780 K after generating low H₂ coverages on s-IrO₂(110) at about 90 K. At high H₂ coverages, we also observe small H₂ desorption peaks at 200 and 530 K which we attribute to molecular and recombinative desorption processes, respectively. We present evidence that H₂ dissociation on IrO₂(110) occurs through a mechanism wherein H₂ σ-complexes adsorbed on the coordinatively-unsaturated (cus) Ir atoms serve as precursors for H₂ dissociation. We show that oxygen atoms adsorbed on the cus-Ir sites, so-called on-top O-atoms, hinder H₂ dissociation on IrO₂(110), while also facilitating H₂O desorption and promoting H-atom transfer from bridging O-atoms to on-top O-atoms. I will also discuss the results of density functional theory calculations of H₂ dissociation and initial steps of H₂O formation on stoichiometric and O-rich IrO₂(110).