AVS2018 Session HC+SS-TuA: A Tale of Two Scales: Catalytic Processes and Surface Science

Oxidative transformation of aldehydes to corresponding esters is a reaction of significant importance. Catalytic oxidative esterification of aldehydes and alcohols in the presence of heterogeneous catalysts is an attractive method for production of esters. Reaction can be carried out in the presence of palladium or platinum often with co-components to improve performance.

The influence of 4d and 5d metal promoters on Pd based catalysts was investigated using a suite of structure probing techniques including x-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and X-ray Absorption Spectroscopy (XAS). Results on supported Pd, Pd-Bi and Pd-Sb catalysts allow new insight into active Pd structure and role of the promoters on Pd structure. We present formation of bimetallic phase in both Bi and Sb promoted Pd catalysts and discuss implication on catalytic performance.

The traditional route to solid catalyst materials involves solution phase deposition or liquid-surface reactions. Examples include impregnation, deposition-precipitation, and solution phase grafting of molecular precursors. Atomic Layer Deposition (ALD) is a gas-solid deposition methodology having enormous potential for the synthesis of advanced heterogeneous catalysts with control of composition and structure at the atomic scale. The ability of ALD to produce conformal oxide coatings on porous, high-surface area materials can provide completely new types of catalyst supports. At the same time ALD can achieve highly uniform catalytically active metal and oxide phases with (sub-) nanometer dimensions. This lecture will provide examples from the laboratories at Northwestern University and Argonne National Laboratory of ALD used to synthesize oxide supports, catalytic oxide overlayers, single-site catalysts, metal nanoparticles, and new porous structures.

Aromatic aldehydes constitute a significant weight fraction of bio-oil.Transition metal catalysts can hydrogenate these aldehydes using either gaseous H₂ or organic donors to produce valuable chemicals that may replace conventional petroleum derivatives. Here, we study selective decarbonylation of aromatic aldehydes (furfural and benzaldehyde) over Ru(0001) and P_x-Ru(0001) to determine how phosphorus introduces new reaction pathways, such as catalytic transfer hydrogenation (CTH) steps between organic reactants. The catalytic properties of Ru(0001) and P_x-Ru(0001) were probed with temperature programmed reaction (TPR) of furfural, benzaldehyde, and isotopically labeled forms of furfural under ultra-high vacuum conditions with Ru(0001) single crystal. P_x-Ru(0001) is formed by exposing Ru(0001) to 2.5 L of PH₃ at 300 K followed by flash annealing to 1400 K. The treatment produces a surface with an atomic ratio of P: Ru of ~0.4, determined by Auger electron spectroscopy.

On P_0.4-Ru(0001), ~68% of furfural adsorbed at 100 K decarbonylates to furan and CO, whereas on Ru(0001), furfural decomposes completely to CO, H₂, and C-atoms. Similarly, benzaldehyde decarbonylates to benzene with a selectivity that is 12-fold greater over P_0.4-Ru(0001) than on Ru(0001). Together, these results suggest that, P-modification of Ru(0001) results in selective decarbonylation of aromatic aldehydes. Charge transfer from Ru to P results in reduced electron back donation from Ru to the adsorbates, and causes adsorbates to interact more weakly with P_0.4-Ru(0001) than with Ru(0001). These electronic modifications reduce the extent of dissociative reactions leading to selective decarbonylation of aromatic aldehydes, although ensemble effects may also contribute.

TPR of furfural on P_0.4-Ru(0001), pre-covered with D* atoms, yields five times more per-hydrogenated furan (C₄H₄O) than mono-deuterated furan (C₄H₃DO), which demonstrates that the CTH does not involve chemisorbed H*-atoms. On P_0.4-Ru(0001), TPR of isotopically labeled furfural (C₄H₃O-CDO) forms two furan isotopologues (C₄H₄O, and C₄H₃DO). In addition, C₄H₃DO formed desorbs at a temperature 20 K higher than C₄H₄O, which indicates that intermolecular H-transfer determines the rate of furan formation. The comparisons of labeled furan products show that these critical H-atoms originate from the furfural ring and the carbonyl group of furfural. Hence, P_0.4-Ru(0001) is more selective for decarbonylation of aromatic aldehydes over Ru(0001), and the addition of phosphorus atoms facilitates CTH steps that do not occur on metallic Ru(0001).

Nonadiabatic electronic excitation in exothermic chemical reactions leads to the flow of energetic electrons with an energy of 1-3 eV which is called “hot electrons”. Direct detection of hot electron flow and observation of its role in catalytic reactions are important for understanding metal-oxide heterogeneous catalysis [1, 2]. Using Pt/n-type TiO₂ Schottky nanodiode, we show the production of hot electron flow generated by methanol oxidation (P_methanol 4 Torr and P_oxygen at 760 Torr) on Pt thin film, and detect as steady-state hot electron current (chemicurrent) which is generated by exothermic chemical reactions on Pt catalyst surface. Under methanol oxidation, methanol can be converted to CO₂ by full oxidation or methyl formate by partial oxidation of methanol. We show that the activation energy of chemicurrent is quite close to that of turnover frequency, indicating that the chemicurrent was originated from the catalytic reaction on Pt thin film. In addition, the dependence of the partial pressure on the chemicurrent was investigated by varying partial pressure of methanol (1-4 Torr). The result shows that the selectivity toward methyl formate formation is well correlated with the chemicurrent. For fundamental understanding of correlation between selectivity and chemicurrent, we carried out the DFT calculation on the thermodynamic energy for each step, and found that the energy gain for partial oxidation reaction was higher than that of the full oxidation reaction, which is responsible for the higher flux of hot electron under methyl formate formation. We discuss the role of metal-oxide interfaces in determining the catalytic selectivity and chemicurrent yield.

Reference

1. Park, J. Y.; Baker, L. R.; Somorjai, G. A., Role of Hot Electrons and Metal–Oxide Interfaces in Surface Chemistry and Catalytic Reactions. Chem. Rev. 2015, 115 (8), 2781-2817.

2. Park, J. Y.; Kim, S. M.; Lee, H.; Nedrygailov, I. I., Hot-Electron-Mediated Surface Chemistry: Toward Electronic Control of Catalytic Activity. Acc. Chem. Res. 2015, 48 (8), 2475-2483.

Oxidative coupling of methane (OCM) is a catalytic partial oxidation process that converts methane directly to valuable C₂ products (ethane and ethylene). The main difficulties from further investigation of this reaction are due to the nature of its high temperature and reaction exothermicity. In this work, a specially designed online characterization setup is applied for this reaction, which achieved both precise bed temperature control and real time product measurement. The setup combines a micro reactor and realtime mass spectroscopy. The reaction was performed under simulated industrial condition. For the first time, the Arrhenius plots of the major OCM products (CO₂, ethane and ethylene) were obtained, and their temperature dependence as well as the respective activation energy barriers were clearly differentiated, over a recently reported high performance nanorod La₂O₃ catalyst. Different from general expectation, CO_2, the fully oxidized carbon species, dominates all the products in the lower temperature region, and less oxidized C₂ species are only formed at much higher temperatures. Further analysis of the Arrhenius plots indicates that selectivity and apparent activation energy for both CO_x and C₂ products are strongly influenced by the oxygen concentration and temperature. Combined with density functional theory calculations and additional experimental measurements, significant insights are brought to this high temperature reaction of wide interest. Furtheranalysis specially focusing on this temperature region, applying XPS surface studies with in-situ high pressure cell and XRD bulk structure with operando reactor, revealed that there are both intermediates and poisoning species formation. With these new experiment results with distinguished lights-off products temperature provide new insights for understanding OCM reaction.

Direct synthesis of H₂O₂ (H₂ + O₂ → H₂O₂) could enable on-site, and even in situ, H₂O₂ production, which motivates searches for highly selective catalysts and process conditions. H₂O₂ formation rates and selectivities depend sensitively on the addition of other transition metals, adsorption of halides, and solvent identity. The reasons for these changes are not completely understood and are difficult to explain mechanistically.

Rate measurements, X-ray absorption spectroscopy, and computation were conducted for Pd and Pd-based bimetallic clusters to determine the mechanism of this reaction and to understand the reasons why alloying Pd often increases H₂O₂ selectivities. In aqueous alcohols, the change in H₂O₂ and H₂O formation rates with H₂ and O₂ pressures are not consistent with a Langmuirian mechanism, but instead suggest O₂* species react in steps mediated by the solvent. In addition, H₂O₂ formation rates in protic solvents are 10³ larger than those measured in aprotic liquids and large kinetic isotope effects (k_H/k_D > 7) strongly suggest that alcohols serve as reactants in the kinetically relevant steps for H­₂O₂ formation. In parallel, O-O bonds within chemisorbed intermediates cleave to form H₂O with rates that are less sensitive to the solvent identity. Persistent organic surface residues introduce low barrier reaction pathways to reduce O₂* and increase those for O-O dissociation relative to reaction pathways in pure water. These results show that long-standing observations that H₂O₂ forms in greater yields within alcoholic solvent are not explained by simple differences in the solubility of H₂ in the liquid-phase.

Similar rate laws and solvent requirements indicate that these reactions proceed by the same pathways in the presence of strongly binding halide adsorbates and acids. These modifications change barriers for the formation of H₂O (significantly) with lesser effects on barriers for steps that lead to H₂O₂, and are consistent with electronic modifications of Pd active sites by intra-atomic orbital rehybridization or by charge transfer from Pd atoms, respectively. Overall, this work presents evidence for the mechanism for H₂O₂ formation and explains the roles of solvent identity and surface modification strategies on H₂O₂ selectivities.