In situ/operando studies for the production of hydrogen through the water-gas shift on metal oxide catalysts

Abstract

In this perspective article, we show how a series of in situ techniques {X-ray diffraction (XRD), pair-distribution-function analysis (PDF), X-ray absorption fine structure (XAFS), environmental transmission electron microscopy (ETEM), infrared spectroscopy (IR), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS)} can be combined to perform detailed studies of the structural, electronic and chemical properties of metal oxide catalysts used for the production of hydrogen through the water-gas shift reaction (WGS, CO + H2O → H2 + CO2). Under reaction conditions most WGS catalysts undergo chemical transformations that drastically modify their composition with respect to that obtained during the synthesis process. Experiments of time-resolved in situ XRD, XAFS, and PDF indicate that the active phase of catalysts which combine Cu, Au or Pt with oxides such as ZnO, CeO2, TiO2, CeOx/TiO2 and Fe2O3 essentially involves nanoparticles of the reduced noble metals. The oxide support undergoes partial reduction and is not a simple spectator, facilitating the dissociation of water and in some cases modifying the chemical properties of the supported metal. Therefore, to optimize the performance of these catalysts one must take into consideration the properties of the metal and oxide phases. IR and AP-XPS have been used to study the reaction mechanism for the WGS on metal oxide catalysts. Data of IR spectroscopy indicate that formate species are not necessarily involved in the main reaction path for the water-gas shift on Cu-, Au- and Pt-based catalysts. Thus, a pure redox mechanism or associative mechanisms that involve either carbonate-like (CO3, HCO3) or carboxyl (HOCO) species should be considered. In the last two decades, there have been tremendous advances in our ability to study catalytic materials under reaction conditions and we are moving towards the major goal of fully understanding how the active sites for the production of hydrogen through the WGS actually work.

Structuring Pt/CeO2/Al2O3 WGS catalyst: Introduction of buffer layer

“…A significant enhancement of the water splitting process on partially reduced ceria has been already reported [38,[54][55][56][57].Recently, Anarifard et al [53] found for Pt/CeO2…”

X-ray absorption fine structure spectroscopy in nanomaterials

“…Their studies aim to obtain a systematic understanding of the structural, electronic, and chemical properties of these catalysts under the operando conditions, in order to reveal the nature of the active phase in metal/oxide WGS catalysts and the WGS reaction mechanism. Some of these studies have been reviewed in subsection of "Oxide nanoparticles" in this paper, and interested readers are referred to a few review articles [108,109] on the capabilities, challenges, and opportunities of the in-situ/operando catalytic studies.…”

Gold Clusters and Nanoparticles Stabilized by Nanoshaped Ceria in Catalysis

“…Water dissociation was found to be the rate-limiting step and the authors suggested that the reduced ceria support facilities H 2 O dissociation. Following this line, the authors performed extensive work on oxidesupported metals to show the importance of the metal-support interface in the WGS reaction [76]. It is worthwhile to mention that for Cu {111}, Gokhale et al explored the same mechanism, except for the COOH dissociation step: instead of direct dissociation of H to the Cu surface, COOH dissociates H to a OH surface intermediate [77].…”

“…This will generate a surface oxygen vacancy neighboring to Au. Such oxygen vacancy between the Au adatom and ceria surface plays an important role in the water activation step [76,80]. Water adsorbs at the interface between Au and the defective ceria surface with an adsorption energy of 61 kJ mol −1 (Figure 19) [81].…”