Surface Conditions That Constrain Alkane Oxidation on Perovskites

Abstract

The crystal structure of perovskites can incorporate a wide variety of cations, which makes this class of materials so interesting for studies of links between solid-state chemistry and catalysis. Perovskites are known as typical total combustion catalysts in hydrocarbon oxidation reactions. The fundamental question that we investigate here is whether surface modifications of perovskites can lead to the formation of valuable reaction products in alkane oxidation. We studied the effect of segregated two-dimensional surface nanostructures on selectivity to propene in the oxidative dehydrogenation of propane. Manganese-based perovskites AMnO 3 (A = La, Sm) were prepared by combustion and hydrothermal synthesis. Bulk and surface structures were investigated by X-ray diffraction, temperature-programmed reduction, aberration-corrected scanning transmission electron microscopy (STEM), multiwavelength Raman, and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) in combination with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Surface oxygen species responsible for C−H activation were distinguished by AP-XPS on the basis of a rigorous in situ analysis of the O 1s spectra recorded under a broad range of reaction conditions. Signals at 529.2, 530.1, 530.9, 531.2, and 531.8 eV were attributed to lattice O, defect-affected O, surface O, oxygen in carbonates, and hydroxyl groups, respectively. Operando AP-XPS revealed critical surface features, which occur under catalyst operation. The catalyst performance depends on the synthesis technique and the reaction conditions. In presence of a two-dimensional MnO x surface phase, addition of steam to the feed resulted in an increase in selectivity to the partial oxidation product propene to practically relevant values. The selectivity increase is related to the presence of Mn in a low oxidation state (2+/3+), an increased concentration of hydroxyl groups, and a higher abundance of adsorbed activated oxygen species on the catalyst surface. The surface analysis of a working catalyst highlights the importance of the termination layer of polycrystalline perovskites as a genuine property implemented by catalyst preparation. Such a termination layer controls the chemical properties and reactivity of perovskites. The information provides input for the development of realistic models that can be used by theory to predict functional properties.

Surface Reconstruction and Phase Transition on Vanadium–Cobalt–Iron Trimetal Nitrides to Form Active Oxyhydroxide for Enhanced Electrocatalytic Water Oxidation

“…[ 11c,30 ] The N atom possibly accelerates the surface reconstruction to the (oxy)hydroxide species. [ 11c,31 ] Consequently, a negative shift of the lattice metal–O in the O 1s XPS spectrum is observed after the OER test, [ 7a,26,32 ] which is located at 529.6 eV (O L ) (Figure 4d), due to the V etching and the formation of metal (oxy)hydroxide on the surface that result in charge transfer and the increase of the electron density around lattice O. This phenomenon is especially prevalent in alkali metal oxides where extensive charge transfer to lattice metal–O induces the shift of O 1s peak to lower binding energy.…”

The Influence of the Chemical Potential on Defects and Function of Perovskites in Catalysis

“…In addition to the increasing interest in perovskites as catalysts for oxygen reduction and oxygen evolution reactions (Suntivich et al, 2011;Gupta et al, 2016;Zhu et al, 2019), this class of materials is also widely used in photocatalysis (Grabowska, 2016;Tasleem and Tahir, 2020) or total oxidation to remove volatile organic and hazardous compounds (Gil et al, 2004;Liu et al, 2018a;Liu et al, 2018b;Liu et al, 2019). However, perovskites are also suitable catalysts for selective oxidations at low temperatures (Polo-Garzon and Wu, 2018;Kamata, 2019), especially for the oxidative dehydrogenation of propane (Koch et al, 2020). The catalytic properties can be influenced by various factors, such as the surface composition (Fierro and Tejuca, 1987;Koch et al, 2020), the concentration of defects, or the partial reduction of metal ions (Evarestov et al, 2005;Mierwaldt et al, 2014;Ignatans et al, 2019;Koch et al, 2020).…”

“…However, perovskites are also suitable catalysts for selective oxidations at low temperatures (Polo-Garzon and Wu, 2018;Kamata, 2019), especially for the oxidative dehydrogenation of propane (Koch et al, 2020). The catalytic properties can be influenced by various factors, such as the surface composition (Fierro and Tejuca, 1987;Koch et al, 2020), the concentration of defects, or the partial reduction of metal ions (Evarestov et al, 2005;Mierwaldt et al, 2014;Ignatans et al, 2019;Koch et al, 2020). Oxygen species at the catalyst surface are responsible for the activation of the propane molecule.…”

“…X-ray Photoelectron Spectroscopy (XPS) allows the identification of different oxygen species in the surface region of perovskites (Mierwaldt et al, 2014;Stoerzinger et al, 2014;Stoerzinger et al, 2015). Furthermore, the dynamics of these species can be studied as a function of the chemical potential of the gas phase under reaction conditions of the selective oxidation of propane using ambient pressure (AP) XPS (Koch et al, 2020).…”

A combined computational and experimental study of methane activation during oxidative coupling of methane (OCM) by surface metal oxide catalysts

“…Moreover, according to this correlation between C-H activation barrier and reducibility of the surface metal oxide site, one of the roles of Na-promotion is to increase the reducibility of surface WO x sites, as evidenced by lowering of peak temperature It is known that MnO x based catalysts form excellent partial and total oxidation catalysts owing to their enhanced reducibility. Examples of Mn-oxide based catalysts used for oxidation reactions include: Mn 2 O 3 for complete oxidation of methane,61 Mn-oxide-CeO 2 catalysts for formaldehyde oxidation,66 Spinel CoMn 2 O 4 for toluene oxidation,67 mesoporous Mn-oxide catalysts for water oxidation,68 alkane oxidation over Mn-perovskites,69 CeO 2 -supported Mn-oxide for propane oxidation,70 In fact, analogous to the CH 4 dissociation pathway over Mn 2 O 5 moiety envisioned in the present study, operando FTIR spectroscopy elsewhere71 confirmed the formation of Mn-OH surface species by abstraction of hydrogen atoms by nucleophilic oxygen atoms (Mn-O-Mn) from propane during propane oxidation, providing support for the DFT modelling results and chemical insights presented herein. Recent state-of-the-art understanding regarding the role of Mn-and Na-promoters in Mn 2 O 3 -Na 2 WO 4 /SiO 2 OCM catalysts puts chemical insights provided in the present work in perspective.It was recently shown via experimental CH 4 +O 2 temperature-programmed-surface-reaction of well-defined single site catalysts that Na 2 WO 4 surface sites could effectively and selectively activate CH 4 in absence of any Mn-promoter to form C 2 products, making the role of Mn-promoter unclear 22.…”