Direct observation of oxygen vacancy formation and migration over ceria surface by in situ environmental transmission electron microscopy

The extremely high structural tolerance of ceria to oxygen vacancies(Ov)has made it a desirable catalytic material for the hydrocarbon oxidation to chemicals and pharmaceuticals and the reduction of gaseous pollutants.It is proposed that the formation and diffusion of Ov originate from its outstanding reduction property.However,the formation and diffusion process of Ov over the surface of ceria at the atomic level is still unknown.Herein,the structural and valence evolution of CeO_(2)(111)surfaces in reductive,oxidative and vacuum environments from room temperature up to 700℃was studied with in situ aberration-corrected environmental transmission electron microscopy(ETEM)experiments.Ov is found to form under a high vacuum at elevated temperatures;however,the surface can recover to the initial state through the adsorption of oxygen atoms in an oxygen-contained environment.Furthermore,in hydrogen environment,the step-CeO_(2)(111)surface is not stable at elevated temperatures;thus,the steps tend to be eliminated with increasing temperature.Combined with first-principles density function calculations(DFT),it is proposed that O-terminated surfaces would develop in a hypoxic environment due to the dynamic diffusion of Ov from the outer surface to the subsurface.Furthermore,in a reductive environment,H2 facilitates the formation and diffusion of Ov while Ce-terminated surfaces develope.These results reveal dynamic atomic-scale interplay between the nanoceria surface and gas,thereby providing fundamental insights into the Ov-dependent reaction of nano-CeO_(2) during catalytic processes.

Atomic dispersion of bulk/nano metals to atomic-sites catalysts and their application in thermal catalysis

Supported metal-group materials are commonly utilized as state-of-the-art catalysts in industry.Atomic-sites catalysts(ASCs)have attracted increasing attention in catalysis owing to their 100%atom efficiency and unique catalytic performances toward various reactions.In particular,atomic dispersion of bulk and nano metals has become the focus of research and development in the synthesis of ASCs.Over the past decade,burgeoning interests have been paid to atomic dispersion in ASCs and their applications in catalysis.However,to the best of our knowledge,the systematic summary and analysis of atomic dispersion were rarely reported.In this review,recently developed ASCs by atomic dispersion were discussed in terms of synthetic atmosphere,driving force,applications in thermal catalytic reactions.Perspectives related to challenges and directions as well as design strategies of ASCs in atomic dispersion were also provided.

An insight into the enhanced mechanism of Ru–MoO_(2)interfacial chemical bonding for hydrogen evolution reaction in alkaline media

An effective strategy was proposed to control the formation of the interfacial bonding between Ru and molybdenum oxide support to stabilize the Ru atoms with the aim to enhance the hydrogen evolution reaction(HER)activity of the resultant catalysts in alkaline medium.The different interfacial chemical bonds,including Ru–O,Ru–O–Mo,and mixed Ru–Mo/Ru–O–Mo,were prepared using an induced activation strategy by controlling the composition of reducing agents in the calcination process.And the regulation mechanism of the interfacial chemical bonds in molybdenum oxide supported Ru catalysts for optimizing HER activity was investigated by density functional theory(DFT)and experimental studies.We found that a controlled interfacial chemical Ru–O–Mo bonding in Ru-MoO_(2)/C manifests a 12-fold activity increase in catalyzing the hydrogen evolution reaction relative to the conventional metal/metal oxide catalyst(Ru-O-MoO_(2)/C).In a bifunctional effect,the interfacial chemical Ru-O-Mo sites promoted the dissociation of water and the production of hydrogen intermediates that were then adsorbed on the nearby Ru surfaces and recombined into molecular hydrogen.As compared,the nearby Ru surfaces in Ru–Mo bonding have weak adsorption capacity for the generation of these hydrogen intermediates,resulting in a 5-fold increase HER activity for Ru-Mo-MoO_(2)/C catalyst compared with Ru-O-MoO_(2)/C.

Unraveling the advantages of Pd/CeO_(2)single-atom catalysts in the NO+CO reaction by model catalysts

Selective catalytic reduction of NO by CO is challenging in environmental catalysis but attractive owing to the advantage of simultaneous elimination of NO and CO.Here,model catalysts consisting of Pd nanoparticles(NPs)and single-atom Pd supported on a CeO_(2)(111)film grown on Cu(111)(denoted as Pd NPs/CeO_(2)and Pd_(1)/CeO_(2),respectively)were successfully prepared and characterized by synchrotron radiation photoemission spectroscopy(SRPES)and infrared reflection absorption spectroscopy(IRAS).The NO+CO adsorption/reaction on the Pd_(1)/CeO_(2)and Pd NPs/CeO_(2)catalysts were carefully investigated using SRPES,temperature-programmed desorption(TPD),and IRAS.It is found that the reaction products on both model catalysts are in good agreement with those on real catalysts,demonstrating the good reliability of using these model catalysts to study the reaction mechanism of the NO+CO reaction.On the Pd NPs/CeO_(2)surface,N_(2)is formed by the combination of atomic N coming from the dissociation of NO on Pd NPs at higher temperatures.N_(2)O formation occurs probably via chemisorbed NO combined with atomic N on the surface.While on the single-atom Pd_(1)/CeO_(2)surface,no N_(2)O is detected.The 100%N_(2)selectivity may stem from the formation of O-N-N-O^(*)intermediate on the surface.Through this study,direct experimental evidence for the reaction mechanisms of the NO+CO reaction is provided,which supports the previous density functional theory(DFT)calculations.

Single-atom site catalysts for environmental catalysis

In recent decades,the environmental protection and long-term sustainability have become the focus of attention due to the increasing pollution generated by the intense industrialization.To overcome these issues,environmental catalysis has increasingly been used to solve the negative impact of pollutants emission on the global environment and human health.Supported platinum-metal-group(PGM)materials are commonly utilized as the state-of-the-art catalysts to eliminate gaseous pollutants but large quantities of PGMs are required.By comparison,single-atom site catalysts(SACs)have attracted much attention in catalysis owing to their 100%atom efficiency and unique catalytic performances towards various reactions.Over the past decade,we have witnessed burgeoning interests of SACs in heterogeneous catalysis.However,to the best of our knowledge,the systematic summary and analysis of SACs in catalytic elimination of environmental pollutants has not yet been reported.In this paper,we summarize and discuss the environmental catalysis applications of SACs.Particular focus was paid to automotive and stationary emission control,including model reaction(CO oxidation,NO reduction and hydrocarbon oxidation),overall reaction(three-way catalytic and diesel oxidation reaction),elimination of volatile organic compounds(formaldehyde,benzene,and toluene),and removal/decomposition of other pollutants(Hg0 and SO3).Perspectives related to further challenges,directions and design strategies of single-atom site catalysts in environmental catalysis were also provided.

Use of rare earth elements in single-atom site catalysis:A critical review——Commemorating the 100th anniversary of the birth of Academician Guangxian Xu

Rare earth metals are strategic resources with potential applications in optics,metallurgy and catalysis.In recent years,single-atom site catalysts(SASCs) have attracted increasing attention owing to their 100%atom efficiency and unique catalytic performances.Over the past decade,rare earth elements,including rare earth metals and their oxides,have shown great potential in SASCs.However,systematic analyses of data are still handful.In this mini-review,the use of rare earth metals and their oxides in SASCs was summarized and the results are discussed.A particular focus was paid to the synthetic strategies,characterization of rare earth-containing SASCs,and applications as catalysis supports,promoters and active sites.Current issues faced by rare-earth metals and their oxides in SASCs,as well as future prospects were also provided.

In-situ DRIFT assessment on strengthening effect of cerium over FeO_(x)/TiO_(2)catalyst for selective catalytic reduction of NO_(x) with NH_(3)

Fe-based catalysts have a great potential to be used for selective catalytic reduction(SCR)of NO_(x)with NH3 reaction due to their low cost,nontoxicity and excellent catalytic activity.The aim of this paper is to investigate Ce doping effect on activity of NH_(3)-SCR over the FeO_(x)/TiO_(2)catalyst.In-situ diffuse reflectance infrared fourier transform(DRIFT)technology was utilized to verity the adsorbed species on the surface of FeO_(x)/TiO_(2)and FeO_(x)-CeO_(2)/TiO_(2)catalysts.With respect to the obtained results,among the four catalysts studied,the FeO_(x)-CeO_(2)/TiO_(2)with the FeO_(x)/CeO_(2)ratio of 3/8 shows the best NO conversion more than 98%in the temperature range of 230—350℃,The active centers for NH_(3)adsorption and activation are assigned to Lewis acid sites over the FeO_(x)-CeO_(2)/TiO_(2)and monodentate nitrates can act as the key intermediate in the NH3-SCR.Moreover,both of Langmuir-Hinshelwood and Eley-Rideal mechanisms are observed over the FeO_(x)-CeO_(2)/TiO_(2)catalysts in the SCR.