The surface and bulk reduction characteristics of bare ceria and ceria with supported cobalt nanoparticles were investigated under ethanol steam reforming conditions using AP-XPS and XANES techniques. Ceria particles were prepared in two different particle sizes, one in nano and the other in micron size (termed CeO2-NP and CeO2-MP), with average particle sizes of 4 and 120 nm, respectively. It was found that particle size affects surface reducibility of ceria particles; smaller particle size leads to a higher extent of surface reduction. Supported cobalt nanoparticles have a significant effect on the surface reducibility of both CeO2-NP and CeO2-MP. Compared to bare ceria particles, the presence of fully oxidized cobalt nanoparticles on the surface of ceria support retards surface reducibility of ceria since reduction of the cobalt oxide phases (Co3O4 and CoO) takes precedence over that of ceria. The degree of reduction of the cobalt phase during ethanol steam reforming determines the effect of cobalt on the reduction process of ceria, i.e., whether it retards or facilitates the reduction of ceria support. AP-XPS studies show that the surface of cobalt nanoparticles consists of both metallic Co and CoOx. The reduction of the surface region of CoOx to metallic Co forms a metallic Co-based shell and CoOx-based core. The anchor of metallic Co on CoOx make metallic Co shell well dispersed on CeO2 without sintering. In addition, the reforming reaction takes place primarily at the interface of metallic Co and CeO2. The much larger difference between Co/CeO2-NP and Co/CeO2-MP than the difference between CeO2-NP and CeO2-MP suggests the significance of metallic Co in catalyzing the reforming reaction, although bare ceria support shows some dehydration activity in its own right.
Most materials and devices typically operate under specific environmental conditions, many of them highly reactive. Heterogeneous catalysts, for example, work under high pressure of reactants or in acidic solutions. The relationship between surface structure and composition of materials during operation and their chemical properties needs to be established in order to understand the mechanisms at work and to enable the design of new and better materials. Although studies of the structure, composition, chemical state, and phase transformation under working conditions are challenging, progress has been made in recent years in the development of new techniques that operate under a variety of realistic environments. With them, new chemistry and new structures of materials that are only present under reaction conditions have been uncovered.
The bimetallic catalyst has been one of the main categories of heterogeneous catalysts for chemical production and energy transformation. Isolation of the continuously packed bimetallic sites of a bimetallic catalyst forms singly dispersed bimetallic sites which have distinctly different chemical environment and electronic state and thus exhibit a different catalytic performance. Two types of catalysts consisting of singly dispersed bimetallic sites Pt1Com or Pd1Con (m and n are the average coordination numbers of Co to a Pt or Pd atom) were prepared through a deposition or impregnation with a following controlled calcination and reduction to form Pt1Com or Pd1Con sites. These bimetallic sites are separately anchored on a nonmetallic support. Each site only consists of a few metal atoms. Single dispersions of these isolated bimetallic sites were identified with scanning transmission electron microscopy. Extended X-ray absorption fine structure spectroscopy (EXAFS) revealed the chemical bonding of single atom Pt1 (or Pd1) to Co atoms and thus confirmed the formation of bimetallic sites, Pt1Com and Pd1Con. Reduction of NO with H2 was used as a probing reaction to test the catalytic performance on this type of catalyst. Selectivity in reducing nitric oxide to N2 on Pt1Com at 150 °C is 98%. Pd1Con is active for reduction of NO with a selectivity of 98% at 250 °C. In situ studies of surface chemistry with ambient-pressure X-ray photoelectron spectroscopy and coordination environment of Pt and Pd atoms with EXAFS showed that chemical state and coordination environment of Pt1Com and Pd1Con remain during catalysis up to 250 and 300 °C, respectively. The correlation of surface chemistries and structures of these catalysts with their corresponding catalytic activities and selectivities suggests a method to develop new bimetallic catalysts and a new type of single site catalysts.
A metal catalyst supported on an inert substrate could consist of both metal nanoparticles and singly dispersed metal atoms. Whether these singly dispersed metal atoms are active and how different their catalytic mechanism could be in contrast to a supported metal catalyst are fundamentally important for understanding catalysis on a supported metal or oxide. By taking reduction of NO with CO on singly dispersed Rh atoms anchored on an inert support SiO2 as a probe system (Rh1/SiO2), here we demonstrated how singly dispersed metal atoms on an inert support could perform a complex multi-step catalytic cycle through a mechanism distinctly different from that for a supported metal nanoparticle with continuously packed metal sites. These singly dispersed Rh1 atoms anchored on SiO2 are active in reducing nitric oxide with carbon monoxide through two reaction pathways that are different from those of Rh nanoparticles. In situ IR studies show that a CO molecule and a NO molecule coadsorb on a singly dispersed Rh atom, Rh1 anchored on SiO2, and couple to form an N atom to adsorb on the surface and a CO2 molecule to desorb. The adsorbed N atom further couples with another CO molecule in the gas phase to form an intermediate −NCO on Rh1; this intermediate can directly couple with an NO molecule adsorbed on the same Rh1 to form N2 and CO2. In another pathway, the adsorbed N atom can couple with a coadsorbed NO on the same Rh1 to form N2O; N2O further reacts with adsorbed CO on the same Rh1 to form N2 and CO2 through a high activation barrier that can be overcome at a high temperature. Our studies show that the singly dispersed metal atoms on an inert support have great potential to perform selective transformation of chemicals. The confirmed catalysis with a singly dispersed Rh1 on SiO2 through a mechanism different from a metal nanoparticle supported on the same substrate suggests the significance of taking the single-atom catalysis (SAC) into fundamental studies of catalysis of a supported metal catalyst, since metal nanoparticles and singly dispersed metal atoms likely coexist on the inert support of many supported catalysts.
Hydrogenation of carbon dioxide (CO 2 ) is a promising route to utilize CO 2 to produce value-added chemicals. Ru metal is highly active for this reaction. CO 2 hydrogenation on Ru-based high surface area catalyst was studied with ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In this chapter the AP-XPS study of CO 2 hydrogenation on Ru nanoparticles supported on Co 3 O 4 , designated as (Co 0.95 Ru 0.05 ) 3 O 4 , was taken as one example for CO 2 hydrogenation on high surface area catalysts. Other than the confirmed role of Ru in the significant promotion of activity and selectivity for production of CH 4 through hydrogenation of CO 2 , Co is a necessary element in having the high conversion of CO 2 .
Search of catalysts made of nonprecious metal for reduction of nitric oxide is important for having a sustainable environment. Catalytic performances of α-MnO2 nanorods in reduction of nitric oxide and nitrous oxide by carbon monoxide were investigated. Surface chemistry of α-MnO2 catalyst during catalysis was tracked with ambient pressure X-ray photoelectron spectroscopy. Correlation between catalytic performance and the corresponding in situ surface chemistry during catalysis revealed that Mn3+ ions and oxygen vacancies are active catalytic sites. Bulk phase of α-MnO2 nanorods below the catalyst surface is restructured to Mn3O4 in catalysis. Kinetics studies suggested that the reduction of nitric oxide with CO is performed through the formation of the intermediate N2O with a followed dissociation to N2. This study suggested restructuring of transition metal oxides can tune catalytic performance and even develop catalysts.
A new in-house ambient pressure XPS (AP-XPS) was designed for the study of surfaces of materials under reaction conditions and during catalysis. Unique features of this in-house AP-XPS are the use of monochromated Al Kα and integration of a minimized reaction cell, and working conditions of up to 500 °C in gases of tens of Torr. Generation of oxygen vacancies on ceria and filling them with oxygen atoms were characterized in operando.
CO hydrogenation to higher alcohols (C2+OH) provides a promising route to convert coal, natural gas, shale gas, and biomass feedstocks into value-added chemicals and transportation fuels. However, the development of nonprecious metal catalysts with satisfactory activity and well-defined selectivity toward C2+OH remains challenging and impedes the commercialization of this process. Here, we show that the synergistic geometric and electronic interactions dictate the activity of Cu0–χ-Fe5C2 binary catalysts for selective CO hydrogenation to C2+OH, outperforming silica-supported precious Rh-based catalysts, by using a combination of experimental evidence from bulk, surface-sensitive, and imaging techniques collected on real and high-performance Cu–Fe binary catalytic systems coupled with density functional theory calculations. The closer is the d-band center to the Fermi level of Cu0–χ-Fe5C2(510) surface than those of χ-Fe5C2(510) and Rh(111) surface, and the electron-rich interface of Cu0–χ-Fe5C2(510) due to the delocalized electron transfer from Cu0 atoms, facilitates CO activation and CO insertion into alkyl species to C2-oxygenates at the interface of Cu0–χ-Fe5C2(510) and thus enhances C2H5OH selectivity. Starting from the CHCO intermediate, the proposed reaction pathway for CO hydrogenation to C2H5OH on Cu0–χ-Fe5C2(510) is CHCO + (H) → CH2CO + (H) → CH3CO + (H) → CH3CHO + (H) → CH3CH2O + (H) → C2H5OH. This study may guide the rational design of high-performance binary catalysts made from earth-abundant metals with synergistic interactions for tuning selectivity.
Catalysis science has emerged as one of the crucial fields in energy science responsible for a sustainable energy world. Fundamental study of surface structures of catalysts at atomic scale under reaction conditions or during catalysis is critical in understanding catalytic mechanisms because a single catalysis event is performed on a specific site comprising one or several atoms with appropriate geometric and electronic structures. Ambient pressure high temperature scanning tunneling microscopy (APHT-STM) can identify structural details of catalyst surfaces at nano or atomic level when catalysts are under reaction conditions or during catalysis. By using APHT-STM, structures of the step edge of Pt(111) and the surface of Ni(557) were studied under reaction conditions. For Pt(111) model catalyst in a CO environment at a pressure of 0.1 Torr or higher, Pt atoms at step edges exhibit a dynamic restructuring. They form kink sites at 0.1 Torr and create nanoclusters near the step edges at 1–10 Torr within 1–2 min. A pressure-dependent restructuring of step edges of Pt(111) was revealed. In in situ studies of the vicinal surface, Ni(557) shows a pressure-dependent restructuring in CO, resulting from reorganization of all surface atoms. Nickel nanoclusters are formed on the whole surface, consistent with the increased coverage of CO chemisorbed on the Ni surface at a relatively higher pressure. Restructuring of atoms at a step edge of terraces of a flat surface and all atoms of a vicinal surface suggest the dynamic nature of model metal catalyst surfaces. Essentially, the surface structure of a metal catalyst in a reactive environment is determined by its reaction or catalysis condition.
X-ray photoelectron spectroscopy (XPS) is one of the main analytical techniques in catalysis, surface science, materials science, and energy science. X-ray is produced through bombardment of surface of a metallic solid or a liquid metal in some unusual case with high-energy electron beam. Upon receiving energy of X-ray photons, an electron in a subshell of an atom of a sample can be excited to leave its home atom by overcoming its binding to the nucleus of its home atom. By collecting the emitted photoelectrons and measuring their kinetic energies, binding energy (BE) of these photoelectrons is then known. Interpretation of the chemical shift must be based on a profound understanding of initial state and final state effects. Typically, an initial state effect of BE of electrons on a subshell of an atom is attributed to the bonding between this atom and its surrounding atoms.
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