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Catalytic ethane dehydrogenation (EDH) to ethylene over Pt-based catalysts has received increasing interests in recent years as it is a potential alternative route to conventional steam cracking. However, the catalysts used in this reaction often suffer from rapid deactivation due to serious coke deposition and metal sintering. Herein, we reported the effects of Zn modification on the stability of Pt/Al2O3 for EDH. The Zn-modified sample (PtZn2/Al2O3) exhibits stable ethane conversion (20%) with over 95% ethylene selectivity. More importantly, it exhibits a significantly low deactivation rate of only 0.003 h−1 at 600 °C for 70 h, which surpasses most of previously reported catalysts. Detailed characterizations including in situ FT-IR, ethylene adsorption microcalorimetry, and HAADF-STEM etc. reveal that Zn modifier reduces the number of Lewis acid sites on the catalyst surface. Moreover, it could modify Pt sites and preferentially cover the step sites, which decrease surface energy and retard the sintering of Pt particle, then prohibiting the further dehydrogenation of ethylene to ethylidyne. Consequently, the good stability is realized due to anti-sintering and the decrease of coke formation on the PtZn2/Al2O3 catalyst.

Furfural is one of the most important biomass-derived chemicals. Its large-scale availability calls for the exploration of new transformation methods for further valorization. Here we report on the direct, one-step conversion of furfural into 1,4-pentanediols (1,4-PeDs) using a combination of Amberlyst-15 and Ru-FeOx/AC catalysts. It is interesting to find that the introduction of a suitable amount of FeOx results in a great improvement in the dispersion of Ru and a decrease in the Lewis acidity. Both XPS and H2-TPR show that there is electron transfer from Ru to Fe, and the electronic interaction facilitates the reduction of both Ru and Fe species. When used in combination with Amberlyst-15, the Ru-6.3FeOx/AC catalyst afforded the best performance with a 1,4-PeD yield of 86%; by contrast, Ru/AC free of FeOx only gave levulinic acid as the major product, demonstrating the key role of the acid/metal balance in the one-pot conversion of furfural to 1,4-PeD. Moreover, such a dual catalyst exhibited excellent durability within 175 h time-on-stream.

Rh1/CeO2 single-atom catalyst is highly active for CO oxidation and has the potential to serve as a multifunctional catalyst to save the usage of other noble metals in three-way catalysts. The high activity is achieved on single-atom active sites via the Mars-van Krevelen mechanism, thus avoiding CO poisoning at low temperatures.

Monodisperse gold particles (ca. 2 nm) were prepared and deposited on various supports (SiO2, Al2O3, HAP, MgAl2O4 and MgO). The acid/base properties of supports were characterized by NH3 and CO2 sorption. The size of the gold nanoparticles spans in the 1.7–6.5 nm mean diameter range after calcination as determined from TEM measurements. The amounts of accessible surface sites were estimated by binary concentration pulse chromatography of CO with Kr adsorption. The data are in agreement with the results of CO adsorption obtained by DRIFT spectroscopy. The activities of the catalysts were compared in the oxidation of benzyl alcohol in stirred batch reactors under two different conditions: in xylene solvent with atmospheric oxygen at 60 °C (in presence and in absence of K2CO3), and in a solvent-free mixture at elevated pressure and temperature (5 bar O2, 150 °C, 5 h). The activities of catalysts in benzyl alcohol conversion are described in two variants, namely related to (i) active catalytic sites (ASNA), and (ii) number of Au atoms on the geometric surface of particles (GSNA). The activities of catalysts in xylene solvent at 60 °C were excellent, with 0.28–1.11 s−1 characteristic GSNAini values (initial reaction rates related to surface Au atoms, Ausurf) in presence of K2CO3. The observed order of activities under these conditions is Au/SiO2

Selective methanation of CO (SMET) is promising for deep removal of low-concentration CO in excess H2. However, the development of suitable catalysts applied in practical conditions remains a great challenge. Here we report a general strategy to develop highly efficient SMET catalysts by using hydrogenated TiO2 (H-TiO2) as support. The H-TiO2 supported Ru catalysts can remove CO to below 10 ppm with greater than 50 % selectivity in a wide temperature range of 200−265 °C, with a good long-term stability. Detailed characterizations including operando spectroscopy and in-situ XPS combined with DFT calculation revealed that the lowered Ru valance on H-TiO2 strengthens CO adsorption on both metal surface and metal-support interface which improved CO activation and inhibited CO2 hydrogenation, respectively, leading to enhancement of both activity and selectivity.

Identification of size effects at an atomic level is essential for designing high-performance metal-based catalysts. Here, the performance of a series of FeOx-supported Pt catalysts with Pt as nanoparticles (Pt-NP) or single atoms (Pt-SAC) are compared for the low-temperature water-gas shift (WGS) reaction. A variety of characterization methods such as adsorption microcalorimetry, H2-TPR, in situ DRIFTS, and transient analysis of product tests were used to demonstrate that Pt nanoparticles exhibit much higher adsorption strength of CO; the adsorbed CO reacts with the OH groups, which are generated from activated H2O, to form intermediate formates that subsequently decompose to produce CO2 and H2 simultaneously. On the other hand, Pt single atoms promote the formation of oxygen vacancies on FeOx which dissociate H2O to H2 and adsorbed O that then combines with the weakly adsorbed CO on these Pt sites to produce CO2. The activation energy for the WGS reaction decreases with the downsizing of Pt species, and...

Although single-atom catalysts (SACs) that bridge homogeneous and heterogeneous catalysis exhibit excellent performance in various reactions, only a few examples have reported the use of SACs in electrocatalysis, especially in new types of photovoltaics. This work focused on the association between SAC Pt1/FeOx and the electrocatalysis in hybrid photovoltaics, with the role of single-Pt atom in facilitating triiodide (I3-) catalytic reduction and enhancing the conversion efficiency of dye-sensitized solar cells. Even with an extremely low dispersion density of one Pt atom per 100 nm2 (the atomic ratio between Pt and Fe is 1:12214), the conversion efficiency could be enhanced by 69.3% compared to bare FeOx. DFT calculation indicated that ionization potential (IP), which was responsible for the rate-determining step, decreased with the anchor of single-Pt atoms on an oxygen-terminated Fe2O3(001) slab, thereby the electron-donating ability of catalysts was enhanced. The interaction between I- and O3- terminated Pt1/Fe2O3(001) showed that charge transfer occurred mainly between I and Pt atoms. Single atom Pt played a powerful role in triiodide (I3-) catalytic reduction, since its 5d orbital interacted with the support Fe2O3, accompanied with much more concentrated electronic states and higher density of the occupied states of Pt1/Fe2O3(001) around the Fermi energy.

Homogeneous catalysts generally possess superior catalytic performance compared to heterogeneous catalysts. However, the issue of catalyst separation and recycling severely limits their use in practical applications. Single-atom catalysts have the advantages of both homogeneous catalysts, such as "isolated sites", and heterogeneous catalysts, such as stability and reusability, and thus would be a promising alternative to traditional homogeneous catalysts. In the hydroformylation of olefins, single-atom Rh catalysts supported on ZnO nanowires demonstrate similar efficiency (TON≈40000) compared to that of homogeneous Wilkinson's catalyst (TON≈19000). HAADF-STEM and infrared CO chemisorption experiments identified isolated Rh atoms on the support. XPS and XANES spectra indicate that the electronic state of Rh is almost metallic. The catalysts are about one or two orders of magnitude more active than most reported heterogeneous catalysts and can be reused four times without an obvious decline in activity.

Low-temperature CO oxidation is important for both fundamental studies and practical applica-tions. Supported gold catalysts are generally regarded as the most active catalysts for low-temperature CO oxidation. The active sites are traditionally believed to be Au nanoclusters or nanoparticles in the size range of 0.5-5 nm. Only in the last few years have single-atom Au catalysts been proved to be active for CO oxidation. Recent advances in both experimental and theoretical studies on single-atom Au catalysts unambiguously demonstrated that when dispersed on suitable oxide supports the Au single atoms can be extremely active for CO oxidation. In this mini-review, recent advances in the development of Au single-atom catalysts are discussed, with the aim of illustrating their unique catalytic features during CO oxidation.

The discovery that gold catalysts could be active for CO oxidation at cryogenic temperatures has ignited much excitement in nanocatalysis. Whether the alternative Pt group metal (PGM) catalysts can exhibit such high performance is an interesting research issue. So far, no PGM catalyst shows activity for CO oxidation at cryogenic temperatures. In this work, we report a sub-nano Rh/TiO2 catalyst that can completely convert CO at 223 K. This catalyst exhibits at least three orders of magnitude higher turnover frequency (TOF) than the best Rh-based catalysts and comparable to the well-known Au/TiO2 for CO oxidation. The specific size range of 0.4-0.8 nm Rh clusters is critical to the facile activation of O2 over the Rh-TiO2 interface in a form of Rh-O-O-Ti (superoxide). This superoxide is ready to react with the CO adsorbed on TiO2 sites at cryogenic temperatures.

Preferential oxidation of CO (PROX) in H2-rich stream is critical to the production of clean H2 for the H2-based fuel cells, which provide clean and efficient energy conversion. Development of highly active and selective PROX catalysts is highly desirable but proved to be extremely challenging. Here we report that CeO2-supported Au single atoms (Au1/CeO2) are highly active, selective, and extremely stable for PROX at the PEMFC working temperature (∼80 °C) with >99.5% CO conversion over a wide temperature window, 70–120 °C (or 50–100 °C, depending on the Au loading). The high CO conversion realized at high temperatures is attributed to the unique property of single-atom catalysts that is unable to dissociatively adsorb H2 and thus has a low reactivity toward H2 oxidation. This strategy is proven in general and can be extended to other oxide-supported Au atoms (e.g., Au1/FeOx), which may open a new window for the efficient catalysis of the PROX reaction.

Ceria (CeO2) nanofibers with high porosity are fabricated using an approach involving sol–gel, electrospinning, and calcination. Specifically, cerium(III) acetylacetonate and polyacrylonitrile (PAN) are dissolved in N,N-dimethylformamide (DMF) and then electrospun into nanofibers. The PAN matrix plays a critical role in stabilizing the porous structure from collapse during calcination in air up to 800 °C. CeO2 porous nanofibers comprising an interconnected network of single crystalline and fully oxidized CeO2 nanoparticles about 40 nm in size are obtained. The hierarchically porous structure of the CeO2 nanofibers enables the facile deposition of Pt nanoparticles via heterogeneous nucleation in a photochemical method. When conducted in the presence of poly(vinyl pyrrolidone) (PVP) and 4-benzyolbenzoic acid, uniform Pt nanoparticles with an average diameter of 1.7 nm are obtained, which are evenly dispersed across the entire surface of each CeO2 nanofiber. The high porosity of CeO2 nanofibers and the uniform distribution of Pt nanoparticles greatly improve the activity and stability of this catalytic system toward the water-gas shift reaction. It is believed that this method could be extended to produce a variety of catalysts and systems sought for various industrial applications.

Au/γ-Fe2O3 and Au/α-Fe2O3 catalysts with identical size of Au nanoparticles, chemical state of Au species, and amount of surface OH– group were prepared. The Au/γ-Fe2O3 catalyst exhibited exceptionally high activity, regardless of the heat treatments. The CO-TPR, sequential pulse reaction, and in situ Raman spectra demonstrate that the much higher activity of Au/γ-Fe2O3 originated from its higher redox property at low temperature. Systematic study shows that this higher-redox-property-based higher activity could be extended to γ-Fe2O3-supported Pt-group metals and to other reactions that follow Mars–Van Krevelen mechanism. This finding may provide a new avenue for catalyst improvement or development by choosing the suitable crystal phase of the oxide support.

The catalytic activities of titanium alkoxides and alkali metal alkoxides for hydrosilylation of unfunctionalized olefins have been studied. Titanium(IV) alkoxides showed excellent catalytic activity, while alkali metal alkoxides have low catalytic activity for the hydrosilylation of olefins. However, by using titanocene dichloride as an additive, alkali metal alkoxides showed also excellent catalytic property for hydrosilylation. In comparison with titanium alkoxides, no α-adduct was obtained by using alkali metal alkoxides/Cp2TiCl2 as catalysts.

Hydrosilylation of olefins catalyzed by Cp2TiCl2/Sm (Cp = cyclopentadienyl) under solvent free conditions have been investigated. By using Cp2TiCl2/Sm as catalyst system, β-adducts and hydrogenation products were detected. Hydrosilylation of olefins catalyzed by Cp2TiCl2/LiAlH4 under room temperature has also been studied. The influence of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) on Cp2TiCl2/Sm and Cp2TiCl2/LiAlH4, respectively, indicated that hydrosilylation of olefins catalyzed with Cp2TiCl2/Sm went through a free radical reaction pathway while a coordination mechanism was applied for Cp2TiCl2/LiAlH4 catalyst system.

Fe(oh)(x)-supported noble metal catalysts exhibited good performance in low-temperature co oxidation or co preferential oxidation (prox), which usually resulted from the high reducibility of fe(oh)(x,) however, we found here that the use of fe(oh) x promoted the formation of oh species during prox over ir/fe(oh)(x) catalysts, which not only greatly lowered the temperature for 100% co conversion, even to room temperature, but also improved the stability. these oh species originated from the reaction between the adsorbed o on fe2+ sites and the adsorbed h on ir sites. they changed the reaction route for the oxidation of co through adsorbed co and oh with lower activation energy (ea: similar to 5.2 kj/mol) rather than through adsorbed co and o (ea: similar to 15.4 kj/mol). with further time-resolved mass spectroscopy and diffuse reflectance infrared spectroscopy, the oh species, prior to the adsorbed 0, were proved to react with co directly. (c) 2014 elsevier inc. all rights reserved.

The single Pt1 and Au1 atoms stabilized by lattice oxygen on ZnO{1010} surface for methanol steam reforming is reported. Density functional theory calculations reveal that the catalysis of the single precious metal atoms together with coordinated lattice oxygen stems from its stronger binding toward the intermediates, lowering reaction barriers, changing on the reaction pathway, enhancing greatly the activity. The measured turnover frequency of single Pt1 sites was more than 1000 times higher than the pristine ZnO. The results provide valuable insights for the catalysis of the atomically dispersed precious metals on oxide supports.

Pt single atoms and small clusters were dispersed on iron oxides by a facile coprecipitation method. These catalysts, with or without calcination at elevated temperatures, show excellent activity a...

La-doped γ-Al2O3 supported Au catalysts show high activity and selectivity for the PROX reaction under PEMFC operation conditions. The superior performance is attributed to the formation of LaAlO3, which suppresses H2 oxidation and strengthens CO adsorption on Au sites, thereby improving competitive oxidation of CO at elevated temperature.

Proton-exchange membrane fuel cells have been regarded as one of the most promising candidates for efficient energy conversion. However, the CO poisoning of the anode and the scarcity of the Pt metal are two major barriers to their commercialization. Development of CO-tolerant Pd electrocatalysts is considered to be a plausible approach to overcome these problems. Herein we report that for the CO-tolerant H-2 oxidation, sub-2-nm size Pd clusters as well as single atoms, supported on FeOx nanocrystallites, are highly active and durable at a wide temperature range of 20-120 degrees C. Experimental data demonstrated that CO preferentially adsorbs on these small Pd clusters or single atoms linearly, resulting in weakened CO binding and enhanced H-2 competitive adsorption. Therefore, FeOx-supported small Pd clusters or single atoms provide a higher CO-tolerant performance. This finding may offer a new strategy to develop CO-tolerant Pd-based electrodes.

Supported metal nanostructures are the most widely used type of heterogeneous catalyst in industrial processes. The size of metal particles is a key factor in determining the performance of such catalysts. In particular, because low-coordinated metal atoms often function as the catalytically active sites, the specific activity per metal atom usually increases with decreasing size of the metal particles. However, the surface free energy of metals increases significantly with decreasing particle size, promoting aggregation of small clusters. Using an appropriate support material that strongly interacts with the metal species prevents this aggregation, creating stable, finely dispersed metal clusters with a high catalytic activity, an approach industry has used for a long time. Nevertheless, practical supported metal catalysts are inhomogeneous and usually consist of a mixture of sizes from nanoparticles to subnanometer clusters. Such heterogeneity not only reduces the metal atom efficiency but also frequently leads to undesired side reactions. It also makes it extremely difficult, if not impossible, to uniquely identify and control the active sites of interest. The ultimate small-size limit for metal particles is the single-atom catalyst (SAC), which contains isolated metal atoms singly dispersed on supports. SACs maximize the efficiency of metal atom use, which is particularly important for supported noble metal catalysts. Moreover, with well-defined and uniform single-atom dispersion, SACs offer great potential for achieving high activity and selectivity. In this Account, we highlight recent advances in preparation, characterization, and catalytic performance of SACs, with a focus on single atoms anchored to metal oxides, metal surfaces, and graphene. We discuss experimental and theoretical studies for a variety of reactions, including oxidation, water gas shift, and hydrogenation. We describe advances in understanding the spatial arrangements and electronic properties of single atoms, as well as their interactions with the support. Single metal atoms on support surfaces provide a unique opportunity to tune active sites and optimize the activity, selectivity, and stability of heterogeneous catalysts, offering the potential for applications in a variety of industrial chemical reactions.

FeOx-supported gold nanocatalyst is one of the most active catalysts for low-temperature CO oxidation. However, the origin of the high activity is still in debate. In this work, using a combination of surface-sensitive in situ FT-IR, Raman spectroscopy, and microcalorimetry, we provide unambiguous evidence that the surface lattice oxygen of the FeOx support participates directly in the low-temperature CO oxidation, and the reaction proceeds mainly through a redox mechanism. Both the presence of gold and the ferrihydrite nature of the FeOx support promote the redox activity greatly. Calcination treatment has a detrimental effect on the redox activity of the Au/FeOx, which in turn decreases greatly the activity for low-temperature CO oxidation. The gold-assisted redox mechanism was also extended to other metal-supported FeOx catalysts, demonstrating the key role of the FeOx support in catalyzing the CO oxidation reaction.

Co-oxidation of CO and H 2 or oxidation of H 2 in the presence of CO was first achieved at low temperatures (0–20 °C) over single-component catalysts (Pd/FeO x ) at a space velocity of 15 , 000 ml g cat - 1 h - 1 . A systematical study of FeO x - and Al 2 O 3 -supported Pd catalysts is carried out by means of STEM, TEM, XRD, XPS, Mossbauer spectra, and time-resolved CO titration. Highly dispersed Pd nanoparticles with size 1–1.8 nm over 5.4 wt% Pd/FeO x are confirmed by HAADF-STEM and HRTEM. Time-resolved CO titration, Mossbauer spectra, and XRD and XPS results suggest FeO x support supplying active oxygen is involved in the oxidations. Particle size effect and oxygen supply from FeO x support are speculated to be the main reasons for the excellent performance of Pd/FeO x in co-oxidation of CO and H 2 or oxidation of H 2 in the presence of CO.

Ferric hydroxide supported Pd catalyst prepared by a simple co-precipitation method without calcinations at elevated temperatures and only reduced at 50 °C possessed unexpectedly higher activity for CO oxidations even compared with that of supported Au catalysts. XRD and TEM results showed that the support was mixture of Fe(OH)x and Fe3O4 and Pd was highly dispersed on it. XPS results showed that Pd existed as mixture of oxidation and metal state. The large amount of OH group on the supports may play an important role in O adsorption and activation.

1-Ethyl-3-methylimidazolium dicyanamide EMImN(CN)2 and S-ethyltetrahydrothiophene dicyanamide EThN(CN)2 physically confined into mesoporous silica gel with pore sizes of 6–8 nm (IL-sg) were synthesized according to a proper sol–gel process. Greatly enhanced fluorescence emissions of dicyanamide based ILs after being confined were exhibited.

Ferric hydroxide supported Au catalysts prepared with co-precipitation method at room temperature without any heat treatment hereafter exhibited high catalytic activity and selectivity for CO oxidation in air and CO selective oxidation in the presence of H2. With calcination temperature rising, both activity and selectivity decreased. X-ray Photoelectron Spectra (XPS) indicated that Au existed as Au0 and Au+ in the catalyst without heat treatment and even after being calcined at 200 °C, while after being calcined at 400 °C, Au existed as Au0 completely. X-ray Diffraction (XRD) and High Resolution Transmission Electron Microscopic (HRTEM) investigations indicated that both the supports and Au species were highly dispersed as nano or sub-nano particles even after being calcined at 200 °C, but after being calcined at 400 °C the supports transformed to crystal Fe2O3 with typical diameter of 30 nm and Au species aggregated to nano-particles with typical diameter of 2–4 nm. HRTEM investigations also suggested that the supports calcined at 200 °C were composed of amorphous ferric hydroxide and crystal ferric oxide. Results of computer simulation (CS) showed that O2 was adsorbed on Au crystal cell and then were activated, which should be the key factor for the subsequent reaction. It also suggested that O2 species were more easily adsorbed on Au+ than on Au0, indicating that higher positive charge of the Au species possessed the higher activity for CO oxidation.

A FeOx supported Pt single-atom catalyst (Pt-SAC) exhibited much higher NO conversion and selectivity to N2 than the supported Pt nanocatalyst (Pt-Nano). This better performance was attributed to not only the stronger NO adsorption and easier dissociation of the N–O bond but also the presence of more oxygen vacancies on the Pt-SAC.

Supported Au catalysts Au-Au + -Cl x /Fe(OH) y ( x y ≤ 3) and Au-Cl x /Fe 2 O 3 prepared with co-precipitation without any washing to remove Cl − and without calcining or calcined at 400 °C were studied. It was found that the presence of Cl − had little impact on the activity over the unwashed and uncalcined catalysts; however, the activity for CO oxidation would be greatly reduced only after Au-Au + -Cl x /Fe(OH) y was further calcined at elevated temperatures, such as 400 °C. XPS investigation showed that Au in catalyst without calcining was composed of Au and Au + , while after calcined at 400 °C it reduced to Au 0 completely. It also showed that catalysts precipitated at 70 °C could form more Au + species than that precipitated at room temperatures. Results of XRD and TEM characterizations indicated that without calcining not only the Au nano-particles but also the supports were highly dispersed, while calcined at 400 °C, the Au nano-particles aggregated and the supports changed to lump sinter. Results of UV–vis observation showed that the Fe(NO 3 ) 3 and HAuCl 4 hydrolyzed partially to form Fe(OH) 3 and [AuCl x (OH) 4− x ] − ( x = 1–3), respectively, at 70 °C, and such pre-partially hydrolyzed iron and gold species and the possible interaction between them during the hydrolysis may be favorable for the formation of more active precursor and to avoid the formation of Au–Cl bonds. Results of computer simulation showed that the reaction molecular of CO or O 2 were more easily adsorbed on Au + and Au 0 , but was very difficultly absorbed on Au − . It also indicated that when Cl − was adsorbed on Au 0 , the Au atom would mostly take a negative electric charge, which would restrain the adsorption of the reaction molecular severely and restrain the subsequent reactions while when Cl − was adsorbed on Au + there only a little of the Au atom take negative electric charge, which resulting a little impact on the activity.

High specific activity and cost effectiveness of single-atom catalysts hold practical value for water gas shift (WGS) reaction toward hydrogen energy. We reported the preparation and characterization of Ir single atoms supported on FeO(x) (Ir1/FeO(x)) catalysts, the activity of which is 1 order of magnitude higher than its cluster or nanoparticle counterparts and is even higher than those of the most active Au- or Pt-based catalysts. Extensive studies reveal that the single atoms accounted for ∼70% of the total activity of catalysts containing single atoms, subnano clusters, and nanoparticles, thus serving as the most important active sites. The Ir single atoms seem to greatly enhance the reducibility of the FeO(x) support and generation of oxygen vacancies, leading to the excellent performance of the Ir1/FeO(x) single-atom catalyst. The results have broad implications on designing supported metal catalysts with better performance and lower cost.

The proton-exchange membrane fuel cell (PEMFC) has been regarded as one of the most promising candidates for the efficient use of hydrogen energy. However, small amounts of CO (0.3–1%) in the H2 stream from reforming processes must be selectively removed because CO is highly poisonous to the Pt anode of a PEMFC. The preferential oxidation of CO in a H2-rich gas (PROX) is presently the most effective approach to address this problem. Oxide-supported Au catalysts are highly active for the PROX reaction even at room temperature, but the lower stability and sensitivity to CO2 constrain their practical applications. Supported Pt catalysts, on the other hand, are less active and only a few have shown reasonable activity for conversion of CO at temperatures lower than 60 8C. Therefore, it is highly desirable to develop improved catalysts with better catalytic performance for the PROX reaction at lower temperatures. Ir has a higher melting point and surface energy than other metals with 5f orbitals, such as Pt and Au, and Ir can be well-dispersed on and strongly interact with the support. However, compared to Ptand Au-based catalysts, Ir-based catalysts have limited applications in heterogeneous catalysis and are rarely investigated for the PROX reaction, most probably because of its inferior activity. Although much effort has been made to improve the activity of Ir-based catalysts and remarkable progress has been achieved, their activities for the PROX reaction are still low at low temperatures. In fact, there is no report so far claiming that Ir-based catalysts can show high activity at temperatures below 80 8C; thus it remains a formidable challenge to utilize Ir-based catalysts for the PROX reaction at ambient temperatures. One basic task of modern catalysis is to rationally design catalysts based on the fundamental understanding of their reaction mechanisms. Especially, the contribution of support materials to the performance of the final catalysts should be taken into account. For the PROX reaction, the strong binding of CO to Ir poisons the surface so that O2 cannot competitively adsorb on the Ir surface and be activated at low temperatures, thereby prohibiting the conversion of CO to CO2. Therefore, weakening the adsorption strength of CO and/or promoting the activation of O2 at lower temperatures have become the crucial steps. Ferric oxide has proven effective for O2 activation and has been used extensively as an additive to Pt-based catalysts. Recently, we have designed a bimetallic catalyst by adding FeOx to a supported Ir catalyst, and the activity for the PROX reaction was improved. Further study of the catalytic reactions showed that the reaction rate of CO oxidation correlated well with the presence and amount of Fe, suggesting that Fe sites were indeed the active sites for O2 activation. [13] The coordinatively unsaturated Fe center was also recently identified as the site to activate O2, which helped the design of a highly active FeOx/Pt/SiO2 catalyst to totally convert CO at room temperature. All of these studies suggest that the presence of low-valent Fe (Fe) played a decisive role in improving the PROX activity, thus providing a clue for obtaining a highly effective Ir-based catalyst by incorporating materials containing, or easily forming, Fe species. Ferric hydroxide (Fe(OH)x) is a novel support material which has recently been adopted to stabilize various types of metal species for CO oxidation. It possesses a large surface area and a large amount of OH groups; these unique properties make Fe(OH)x a good candidate for generating highly dispersed metal clusters or even single-atom catalysts. Furthermore, the longer Fe O bonds in Fe(OH)x (compared to those in Fe2O3) make it easier to form Fe 2+

Platinum-based heterogeneous catalysts are critical to many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the surface active-site atoms are used. Catalysts with single-atom dispersions are thus highly desirable to maximize atom efficiency, but making them is challenging. Here we report the synthesis of a single-atom catalyst that consists of only isolated single Pt atoms anchored to the surfaces of iron oxide nanocrystallites. This single-atom catalyst has extremely high atom efficiency and shows excellent stability and high activity for both CO oxidation and preferential oxidation of CO in H-2. Density functional theory calculations show that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms, which help to reduce both the CO adsorption energy and the activation barriers for CO oxidation.

The Au/FeO(x)-hydroxyapatite composite prepared by a simple deposition-precipitation method is not only highly active and stable for CO oxidation at low temperatures, but also strongly sintering-resistant for calcination at as high as 600 °C.

Ferric hydroxide supported Au prepared with co-precipitation without heat treatment could be a very effective catalyst for selective CO oxidation in the presence of H2 at lower temperatures.