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2017 Volume 38 Issue 9
2017, 38(9):
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2017, 38(9): 1431-1431
doi: 10.1016/S1872-2067(17)62895-X
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2017, 38(9): 1432-1442
doi: 10.1016/S1872-2067(17)62886-9
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An account of recent work on supported single-atom catalyst design is given here for reactions as diverse as the low-temperature water-gas shift, methanol steam reforming, selective ethanol dehydrogenation, and selective hydrogenation of alkynes and dienes. It is of fundamental interest to investigate the intrinsic activity and selectivity of the active metal atom site and compare them to the properties of the corresponding metal nanoparticles and sub-nm clusters. It is also important to understand what constitutes a stable active metal atom site in the various reaction environments, and maximize their loadings to allow us to design robust catalysts for industrial applications. Combined activity and stability studies, ideally following the evolution of the active site as a function of catalyst treatment in real time are recommended. Advanced characterization methods with atomic resolution will play a key role here and will be used to guide the design of new catalysts.
An account of recent work on supported single-atom catalyst design is given here for reactions as diverse as the low-temperature water-gas shift, methanol steam reforming, selective ethanol dehydrogenation, and selective hydrogenation of alkynes and dienes. It is of fundamental interest to investigate the intrinsic activity and selectivity of the active metal atom site and compare them to the properties of the corresponding metal nanoparticles and sub-nm clusters. It is also important to understand what constitutes a stable active metal atom site in the various reaction environments, and maximize their loadings to allow us to design robust catalysts for industrial applications. Combined activity and stability studies, ideally following the evolution of the active site as a function of catalyst treatment in real time are recommended. Advanced characterization methods with atomic resolution will play a key role here and will be used to guide the design of new catalysts.
2017, 38(9): 1443-1453
doi: 10.1016/S1872-2067(17)62839-0
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2017, 38(9): 1454-1459
doi: 10.1016/S1872-2067(17)62878-X
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2017, 38(9): 1460-1472
doi: 10.1016/S1872-2067(17)62900-0
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2017, 38(9): 1473-1480
doi: 10.1016/S1872-2067(17)62882-1
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2017, 38(9): 1481-1488
doi: 10.1016/S1872-2067(17)62880-8
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2017, 38(9): 1489-1497
doi: 10.1016/S1872-2067(17)62799-2
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2017, 38(9): 1498-1507
doi: 10.1016/S1872-2067(17)62872-9
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The idea that single metal atoms dispersed on a solid support can act as an efficient heterogeneous catalyst was raised in 2011 when single Pt atoms on an FeOx surface were reported to be active for CO oxidation and preferential oxidation of CO in H2. The last six years have witnessed tremendous progress in the field of single-atom catalysis. Here we introduce the major achievements on this topic in 2015 and 2016. Some particular aspects of single-atom catalysis are discussed in depth, including new approaches in single-atom catalyst (SAC) synthesis, stable gold SACs for various reac-tions, the high selectivity of Pt and Pd SACs in hydrogenation, and the superior performance of non-noble metal SACs in electrochemistry. These accomplishments will encourage more efforts by researchers to achieve the controllable fabrication of SACs and explore their potential applications.
The idea that single metal atoms dispersed on a solid support can act as an efficient heterogeneous catalyst was raised in 2011 when single Pt atoms on an FeOx surface were reported to be active for CO oxidation and preferential oxidation of CO in H2. The last six years have witnessed tremendous progress in the field of single-atom catalysis. Here we introduce the major achievements on this topic in 2015 and 2016. Some particular aspects of single-atom catalysis are discussed in depth, including new approaches in single-atom catalyst (SAC) synthesis, stable gold SACs for various reac-tions, the high selectivity of Pt and Pd SACs in hydrogenation, and the superior performance of non-noble metal SACs in electrochemistry. These accomplishments will encourage more efforts by researchers to achieve the controllable fabrication of SACs and explore their potential applications.
2017, 38(9): 1508-1514
doi: 10.1016/S1872-2067(17)62903-6
Abstract:
Noble single-atom catalysts have rapidly been attracting attention due to their unique catalytic properties and maximized utilization. Atomic layer deposition (ALD) is an emerging powerful tech-nique for large-scale synthesis of stable single atom. In this review, we summarize recent develop-ments of single atom synthesized by ALD as well as explore future research direction and trends.
Noble single-atom catalysts have rapidly been attracting attention due to their unique catalytic properties and maximized utilization. Atomic layer deposition (ALD) is an emerging powerful tech-nique for large-scale synthesis of stable single atom. In this review, we summarize recent develop-ments of single atom synthesized by ALD as well as explore future research direction and trends.
2017, 38(9): 1515-1527
doi: 10.1016/S1872-2067(17)62782-7
Abstract:
This review summarizes a variety of experimentally identified gas-phase catalytic cycles, all of which are mediated by atomic metal ions, bare metal clusters, metal oxide clusters or metal com-plexes. Emphasis is placed on the latest advances in the unique catalytic reactivity of clus-ter-confined single noble metal atoms. The cycles discussed in this paper cover a wide range of inorganic and organic molecules. The use of start-of-the-art mass spectrometric instrumentation in conjunction with quantum chemistry calculations is also reported, as these techniques have deter-mined the mechanistic details of the elementary steps of such catalytic cycles. The important role of gas-phase data in guiding the rational design of better-performing catalysts in related condensed phase reactions is also examined. In particular, this review focuses on the following three topics:(1) the catalytic oxidation of carbon monoxide, (2) the catalytic functionalization of methane, and (3) catalytic decarboxylation.
This review summarizes a variety of experimentally identified gas-phase catalytic cycles, all of which are mediated by atomic metal ions, bare metal clusters, metal oxide clusters or metal com-plexes. Emphasis is placed on the latest advances in the unique catalytic reactivity of clus-ter-confined single noble metal atoms. The cycles discussed in this paper cover a wide range of inorganic and organic molecules. The use of start-of-the-art mass spectrometric instrumentation in conjunction with quantum chemistry calculations is also reported, as these techniques have deter-mined the mechanistic details of the elementary steps of such catalytic cycles. The important role of gas-phase data in guiding the rational design of better-performing catalysts in related condensed phase reactions is also examined. In particular, this review focuses on the following three topics:(1) the catalytic oxidation of carbon monoxide, (2) the catalytic functionalization of methane, and (3) catalytic decarboxylation.
2017, 38(9): 1528-1539
doi: 10.1016/S1872-2067(17)62770-0
Abstract:
Supported and colloidal single-atom catalysts (SACs), which possess excellent catalytic properties, are particularly important in both fundamental studies and practical applications. The progress made in the preparation methods, characterization, catalytic performances and mechanisms of SACs anchored to metal oxides, two-dimensional materials and the surface of metal nanoclusters (NCs) are reviewed. The different techniques for SAC fabrication, including conventional solution methods based on co-precipitation, incipient wetness co-impregnation, and the chemical vapor deposition method, as well as the newer atom layer deposition (ALD) and galvanic replacement methods, are summarized. The main results from experimental and theoretical studies of various catalytic reac-tions over SACs, including oxidation reactions, hydrogenation, water gas shift, photocatalytic H2 evolution and electrochemical reactions, are also discussed. Moreover, the electronic properties of the single atoms and their interactions with the supports are described to assist in understanding the origin of the high catalytic activity and selectivity of SACs. Finally, possible future research di-rections of SACs and their applications are proposed.
Supported and colloidal single-atom catalysts (SACs), which possess excellent catalytic properties, are particularly important in both fundamental studies and practical applications. The progress made in the preparation methods, characterization, catalytic performances and mechanisms of SACs anchored to metal oxides, two-dimensional materials and the surface of metal nanoclusters (NCs) are reviewed. The different techniques for SAC fabrication, including conventional solution methods based on co-precipitation, incipient wetness co-impregnation, and the chemical vapor deposition method, as well as the newer atom layer deposition (ALD) and galvanic replacement methods, are summarized. The main results from experimental and theoretical studies of various catalytic reac-tions over SACs, including oxidation reactions, hydrogenation, water gas shift, photocatalytic H2 evolution and electrochemical reactions, are also discussed. Moreover, the electronic properties of the single atoms and their interactions with the supports are described to assist in understanding the origin of the high catalytic activity and selectivity of SACs. Finally, possible future research di-rections of SACs and their applications are proposed.
2017, 38(9): 1540-1548
doi: 10.1016/S1872-2067(17)62847-X
Abstract:
Cu-alloyed Pd single-atom catalysts exhibit excellent catalytic performance for the semi-hydrogenation of acetylene; however, the limit of the Cu/Pd atomic ratio for forming the al-loyed Pd single-atom catalyst is ambiguous. Herein, silica-supported Cu-Pd bimetallic catalysts with fixed Pd content and varied Cu loadings were synthesized using an incipient wetness co-impregnation method. The X-ray absorption spectroscopy results indicated that Pd formed an alloy with Cu after reduction at 250℃ and that the Pd atoms were completely isolated by Cu for Cu/Pd atomic ratios ≥ 40/1. Notably, increasing the reduction temperature from 250 to 400℃ hardly affected the catalytic performances of the Cu-Pd/SiO2 catalysts. This finding can be attribut-ed to the similar chemical environments of Pd demonstrated by the X-ray absorption spectroscopy results.
Cu-alloyed Pd single-atom catalysts exhibit excellent catalytic performance for the semi-hydrogenation of acetylene; however, the limit of the Cu/Pd atomic ratio for forming the al-loyed Pd single-atom catalyst is ambiguous. Herein, silica-supported Cu-Pd bimetallic catalysts with fixed Pd content and varied Cu loadings were synthesized using an incipient wetness co-impregnation method. The X-ray absorption spectroscopy results indicated that Pd formed an alloy with Cu after reduction at 250℃ and that the Pd atoms were completely isolated by Cu for Cu/Pd atomic ratios ≥ 40/1. Notably, increasing the reduction temperature from 250 to 400℃ hardly affected the catalytic performances of the Cu-Pd/SiO2 catalysts. This finding can be attribut-ed to the similar chemical environments of Pd demonstrated by the X-ray absorption spectroscopy results.
2017, 38(9): 1549-1557
doi: 10.1016/S1872-2067(17)62899-7
Abstract:
We have dispersed individual Pd atoms onto ZnO nanowires (NWs) as single-atom catalysts (SACs) and evaluated their catalytic performance for several selected catalytic reactions. The Pd1/ZnO SAC is highly active, stable, and selective towards CO2 for steam reforming of methanol to produce hy-drogen. This catalyst system is active for oxidation of CO and H2 but performs poorly for preferential oxidation of CO in hydrogen-rich stream primarily due to the strong competitive oxidation of H2 on ZnO supported Pd1 atoms. At ambient pressure, reverse water-gas-shift reaction occurs on the Pd1/ZnO SAC. This series of tests of catalytic reactions clearly demonstrate the importance of se-lecting the appropriate metal and support to develop SACs for catalytic transformation of molecules.
We have dispersed individual Pd atoms onto ZnO nanowires (NWs) as single-atom catalysts (SACs) and evaluated their catalytic performance for several selected catalytic reactions. The Pd1/ZnO SAC is highly active, stable, and selective towards CO2 for steam reforming of methanol to produce hy-drogen. This catalyst system is active for oxidation of CO and H2 but performs poorly for preferential oxidation of CO in hydrogen-rich stream primarily due to the strong competitive oxidation of H2 on ZnO supported Pd1 atoms. At ambient pressure, reverse water-gas-shift reaction occurs on the Pd1/ZnO SAC. This series of tests of catalytic reactions clearly demonstrate the importance of se-lecting the appropriate metal and support to develop SACs for catalytic transformation of molecules.
2017, 38(9): 1558-1565
doi: 10.1016/S1872-2067(17)62829-8
Abstract:
We examined the water adsorption and dissociation on ceria surfaces as well as ceria-supported Au single-atom catalysts using density functional theory calculations. Molecular and dissociative water were observed to coexist on clean CeO2 and reduced Au1/CeO2-x surfaces because of the small dif-ference in adsorption energies, whereas the presence of dissociative water was highly favorable on reduced CeO2-xand clean Au1/CeO2 surfaces. Positively charged Au single atoms on the ceria surface not only provided activation sites for water adsorption but also facilitated water dissociation by weakening the intramolecular O-H bonds. In contrast, negatively charged Au single atoms were not reactive for water adsorption because of the saturation of Au 5d and 6s electron shells. This work provides a fundamental understanding of the interaction between water and single-atom Au cata-lysts.
We examined the water adsorption and dissociation on ceria surfaces as well as ceria-supported Au single-atom catalysts using density functional theory calculations. Molecular and dissociative water were observed to coexist on clean CeO2 and reduced Au1/CeO2-x surfaces because of the small dif-ference in adsorption energies, whereas the presence of dissociative water was highly favorable on reduced CeO2-xand clean Au1/CeO2 surfaces. Positively charged Au single atoms on the ceria surface not only provided activation sites for water adsorption but also facilitated water dissociation by weakening the intramolecular O-H bonds. In contrast, negatively charged Au single atoms were not reactive for water adsorption because of the saturation of Au 5d and 6s electron shells. This work provides a fundamental understanding of the interaction between water and single-atom Au cata-lysts.
2017, 38(9): 1566-1573
doi: 10.1016/S1872-2067(17)62879-1
Abstract:
An FeOx-based Pt single-atom catalyst (SAC), Pt1/FeOx, has stimulated significant recent interest owing to its extraordinary activity toward CO oxidation. The concept of SAC has also been success-fully extended to other FeOx supported transition metal systems both experimentally and theoreti-cally. However, the FeOx substrate itself (denoted by Fe1/FeOx following the same nomenclature of Pt1/FeOx) as a typical transition metal oxide possesses a very low catalytic activity toward CO oxida-tion, although it can be viewed as Fe1/FeOx SAC. Here, to understand the catalytic mechanism of FeOx-based SACs for CO oxidation, we have performed density functional theory calculations on Pt1/FeOx and Fe1/FeOx for CO oxidation to address the differences between these two SACs in terms of the catalytic mechanism of CO oxidation and the chemical behavior of the catalysts. Our calcula-tion results indicated that the catalytic cycle of Fe1/FeOx is much more difficult to accomplish than that of SAC Pt1/FeOx because of a high activation barrier (1.09 eV) for regeneration of the oxygen vacancy formed when the second CO2 molecule desorbs from the surface. Moreover, density of states and Bader charge analysis revealed differences in the catalytic performance for CO oxidation by the SACs Fe1/FeOx and Pt1/FeOx. This work provides insights into the fundamental interactions between the single-atom Pt1 and FeOx substrate, and the exceptional catalytic performance of this system for CO oxidation.
An FeOx-based Pt single-atom catalyst (SAC), Pt1/FeOx, has stimulated significant recent interest owing to its extraordinary activity toward CO oxidation. The concept of SAC has also been success-fully extended to other FeOx supported transition metal systems both experimentally and theoreti-cally. However, the FeOx substrate itself (denoted by Fe1/FeOx following the same nomenclature of Pt1/FeOx) as a typical transition metal oxide possesses a very low catalytic activity toward CO oxida-tion, although it can be viewed as Fe1/FeOx SAC. Here, to understand the catalytic mechanism of FeOx-based SACs for CO oxidation, we have performed density functional theory calculations on Pt1/FeOx and Fe1/FeOx for CO oxidation to address the differences between these two SACs in terms of the catalytic mechanism of CO oxidation and the chemical behavior of the catalysts. Our calcula-tion results indicated that the catalytic cycle of Fe1/FeOx is much more difficult to accomplish than that of SAC Pt1/FeOx because of a high activation barrier (1.09 eV) for regeneration of the oxygen vacancy formed when the second CO2 molecule desorbs from the surface. Moreover, density of states and Bader charge analysis revealed differences in the catalytic performance for CO oxidation by the SACs Fe1/FeOx and Pt1/FeOx. This work provides insights into the fundamental interactions between the single-atom Pt1 and FeOx substrate, and the exceptional catalytic performance of this system for CO oxidation.
2017, 38(9): 1574-1580
doi: 10.1016/S1872-2067(17)62784-0
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Atomically dispersed catalysts have shown promising prospects in catalysis studies. Among all of the developed methods for synthesizing atomically dispersed catalysts, the photochemical approach has recently aroused much attention owing to its simple procedure and mild preparation conditions involved. In the present study, we demonstrate the application of the photochemical method to synthesize atomically dispersed Pd catalysts on (001)-exposed anatase nanocrystals and commer-cial TiO2 (P25). The as-prepared catalysts exhibit both high activity and stability in the hydrogena-tion of styrene and catalytic oxidation of CO.
Atomically dispersed catalysts have shown promising prospects in catalysis studies. Among all of the developed methods for synthesizing atomically dispersed catalysts, the photochemical approach has recently aroused much attention owing to its simple procedure and mild preparation conditions involved. In the present study, we demonstrate the application of the photochemical method to synthesize atomically dispersed Pd catalysts on (001)-exposed anatase nanocrystals and commer-cial TiO2 (P25). The as-prepared catalysts exhibit both high activity and stability in the hydrogena-tion of styrene and catalytic oxidation of CO.
2017, 38(9): 1581-1587
doi: 10.1016/S1872-2067(17)62768-2
Abstract:
Pd-based catalysts are widely used in hydrogenation reactions, and it is essential to improve the selectivity of these catalysts to give the desired products, especially at high conversions. However, improvements in selectivity have generally been achieved at the expense of catalytic activity. Here, we report that deposition of FeOx onto a Pd/Al2O3 catalyst using atomic layer deposition with pre-cise, near atomic control provides a remarkable improvement in both activity and butene selectivity in the selective hydrogenation of 1,3-butadiene under mild conditions. Diffuse reflectance infrared Fourier transform spectroscopy for CO chemisorption measurements illustrate that FeOx preferen-tially nucleates on Pd (111) facets and divides the Pd surface atoms into small ensembles. X-ray photoelectron spectroscopy measurements revealed that the Pd became electron deficient after FeOx deposition owing to the strong Pd-FeOx interaction. Our results suggest that a geometric effect, that is, the formation of small Pd ensembles, is the main contributor to the improvement in butene selectivity, whereas the enhancement in hydrogenation activity may be attributed to both electronic effects and the newly generated Pd-FeOx interface.
Pd-based catalysts are widely used in hydrogenation reactions, and it is essential to improve the selectivity of these catalysts to give the desired products, especially at high conversions. However, improvements in selectivity have generally been achieved at the expense of catalytic activity. Here, we report that deposition of FeOx onto a Pd/Al2O3 catalyst using atomic layer deposition with pre-cise, near atomic control provides a remarkable improvement in both activity and butene selectivity in the selective hydrogenation of 1,3-butadiene under mild conditions. Diffuse reflectance infrared Fourier transform spectroscopy for CO chemisorption measurements illustrate that FeOx preferen-tially nucleates on Pd (111) facets and divides the Pd surface atoms into small ensembles. X-ray photoelectron spectroscopy measurements revealed that the Pd became electron deficient after FeOx deposition owing to the strong Pd-FeOx interaction. Our results suggest that a geometric effect, that is, the formation of small Pd ensembles, is the main contributor to the improvement in butene selectivity, whereas the enhancement in hydrogenation activity may be attributed to both electronic effects and the newly generated Pd-FeOx interface.
2017, 38(9): 1588-1596
doi: 10.1016/S1872-2067(17)62778-5
Abstract:
Top-down synthesis has been used to prepare catalytic materials with nanometer sizes, but fabri-cating atomically dispersed metal catalysts remains a challenge because surface single metal atoms are prone to aggregation or coalescence. A top-down strategy is used to synthesize atomically dis-persed metal catalysts, based on supported Ag nanoparticles. The changes of the geometric and electronic structures of the Ag atoms during the top-down process are studied using the in situ syn-chrotron X-ray diffraction technique, ex situ X-ray absorption spectroscopy, and transmission elec-tron microscopy. The experimental results, coupled with the density functional theory calculations, demonstrate that the electronic perturbation of the Ag frontier orbitals, induced by the Ag-O inter-actions at the perimeter of the metal-support interface, is the driving force of the top-down process. The top-down synthesis has two important functions:to increase the number of catalytic active sites and to facilitate the study of complex reaction mechanisms (e.g., formaldehyde oxidation) by developing single-site model catalysts.
Top-down synthesis has been used to prepare catalytic materials with nanometer sizes, but fabri-cating atomically dispersed metal catalysts remains a challenge because surface single metal atoms are prone to aggregation or coalescence. A top-down strategy is used to synthesize atomically dis-persed metal catalysts, based on supported Ag nanoparticles. The changes of the geometric and electronic structures of the Ag atoms during the top-down process are studied using the in situ syn-chrotron X-ray diffraction technique, ex situ X-ray absorption spectroscopy, and transmission elec-tron microscopy. The experimental results, coupled with the density functional theory calculations, demonstrate that the electronic perturbation of the Ag frontier orbitals, induced by the Ag-O inter-actions at the perimeter of the metal-support interface, is the driving force of the top-down process. The top-down synthesis has two important functions:to increase the number of catalytic active sites and to facilitate the study of complex reaction mechanisms (e.g., formaldehyde oxidation) by developing single-site model catalysts.
2017, 38(9): 1597-1602
doi: 10.1016/S1872-2067(16)62571-8
Abstract:
The active sites for hydrogenation over Ru/SBA-15 catalysts were identified using in situ Fouri-er-transform infrared spectroscopy.The amount of active sites was proportional to the interfacial circumference of the Ru particles.In contrast,the rate of hydrogen spillover from Ru to the support was inversely proportional to the size of the Ru metal particles.Consequently,a catalyst with small Ru metal particles has a high rate of hydrogen spillover but a low density of active sites,whereas one with large Ru particles has a low rate of hydrogen spillover but a high density of active sites. The formation of these active sites is probably an intermediate step in hydrogen spillover.
The active sites for hydrogenation over Ru/SBA-15 catalysts were identified using in situ Fouri-er-transform infrared spectroscopy.The amount of active sites was proportional to the interfacial circumference of the Ru particles.In contrast,the rate of hydrogen spillover from Ru to the support was inversely proportional to the size of the Ru metal particles.Consequently,a catalyst with small Ru metal particles has a high rate of hydrogen spillover but a low density of active sites,whereas one with large Ru particles has a low rate of hydrogen spillover but a high density of active sites. The formation of these active sites is probably an intermediate step in hydrogen spillover.
2017, 38(9): 1603-1612
doi: 10.1016/S1872-2067(17)62842-0
Abstract:
The catalytic oxidation of toluene over Ag/SBA-15 synthesized under different pretreatment condi-tions, including O2 at 500℃ (denoted O500), H2 at 500℃ (H500), and O2 at 500℃ followed by H2 at 300℃ (O500-H300) was studied. The pretreated samples were investigated by N2 physisorption, X-ray diffraction, and ultraviolet-visible diffuse reflectance. The pretreatment atmosphere greatly influences the status of the Ag and O species, which in turn significantly impacts the adsorption and catalytic removal of toluene. Ag2O and amorphous Ag particles, as well as a large amount of subsur-face oxygen species, are formed on O500, and the subsurface oxygen enhances the interaction be-tween Ag species and toluene, so O500 shows good activity at higher temperature. However, its activity at lower temperature is not as high as expected, with a reduced presence of Ag2O and lower adsorption capacity for toluene. H2 pretreatment at 500℃ is conducive to the formation of large Ag particles and yields the largest adsorption capacity for toluene, so H500 exhibits the best activity at lower temperatures; however, because of poor interaction between Ag and toluene, its activity at higher temperature is modest. The O500-H300 sample exhibits excellent catalytic activity during the whole reaction process, which can be attributed to the small and highly dispersed Ag nanoparti-cles as well as the existence of subsurface oxygen.
The catalytic oxidation of toluene over Ag/SBA-15 synthesized under different pretreatment condi-tions, including O2 at 500℃ (denoted O500), H2 at 500℃ (H500), and O2 at 500℃ followed by H2 at 300℃ (O500-H300) was studied. The pretreated samples were investigated by N2 physisorption, X-ray diffraction, and ultraviolet-visible diffuse reflectance. The pretreatment atmosphere greatly influences the status of the Ag and O species, which in turn significantly impacts the adsorption and catalytic removal of toluene. Ag2O and amorphous Ag particles, as well as a large amount of subsur-face oxygen species, are formed on O500, and the subsurface oxygen enhances the interaction be-tween Ag species and toluene, so O500 shows good activity at higher temperature. However, its activity at lower temperature is not as high as expected, with a reduced presence of Ag2O and lower adsorption capacity for toluene. H2 pretreatment at 500℃ is conducive to the formation of large Ag particles and yields the largest adsorption capacity for toluene, so H500 exhibits the best activity at lower temperatures; however, because of poor interaction between Ag and toluene, its activity at higher temperature is modest. The O500-H300 sample exhibits excellent catalytic activity during the whole reaction process, which can be attributed to the small and highly dispersed Ag nanoparti-cles as well as the existence of subsurface oxygen.
Effect of the degree of dispersion of Pt over MgAl2O4 on the catalytic hydrogenation of benzaldehyde
2017, 38(9): 1613-1620
doi: 10.1016/S1872-2067(17)62815-8
Abstract:
One of the central tasks in the field of heterogeneous catalysis is to establish structure-function relationships for these catalysts, especially for precious metals dispersed on the sub-nanometer scale. Here, we report the preparation of MgAl2O4-supported Pt nanoparticles, amorphous aggre-gates and single atoms, and evaluate their ability to catalyze the hydrogenation of benzaldehyde. The Pt species were characterized by N2 adsorption, X-ray diffraction (XRD), aberration-corrected transmission electron microscopy (ACTEM), CO chemisorption and in situ Fourier transform infra-red spectroscopy of the chemisorbed CO, as well as by inductively coupled plasma atomic emission spectroscopy. They existed as isolated or neighboring single atoms on the MgAl2O4 support, and formed amorphous Pt aggregates and then nanocrystallites with increased Pt loading. On the MgAl2O4 support, single Pt atoms were highly active in the selective catalytic hydrogenation of ben-zaldehyde to benzyl alcohol. The terrace atoms of the Pt particles were more active but less selec-tive; this was presumably due to their ability to form bridged carbonyl adsorbates. The MgAl2O4-supported single-atom Pt catalyst is a novel catalyst with a high precious atom efficiency and excellent catalytic hydrogenation ability and selectivity.
One of the central tasks in the field of heterogeneous catalysis is to establish structure-function relationships for these catalysts, especially for precious metals dispersed on the sub-nanometer scale. Here, we report the preparation of MgAl2O4-supported Pt nanoparticles, amorphous aggre-gates and single atoms, and evaluate their ability to catalyze the hydrogenation of benzaldehyde. The Pt species were characterized by N2 adsorption, X-ray diffraction (XRD), aberration-corrected transmission electron microscopy (ACTEM), CO chemisorption and in situ Fourier transform infra-red spectroscopy of the chemisorbed CO, as well as by inductively coupled plasma atomic emission spectroscopy. They existed as isolated or neighboring single atoms on the MgAl2O4 support, and formed amorphous Pt aggregates and then nanocrystallites with increased Pt loading. On the MgAl2O4 support, single Pt atoms were highly active in the selective catalytic hydrogenation of ben-zaldehyde to benzyl alcohol. The terrace atoms of the Pt particles were more active but less selec-tive; this was presumably due to their ability to form bridged carbonyl adsorbates. The MgAl2O4-supported single-atom Pt catalyst is a novel catalyst with a high precious atom efficiency and excellent catalytic hydrogenation ability and selectivity.
2017, 38(9): 1621-1628
doi: 10.1016/S1872-2067(17)62760-8
Abstract:
An efficient and economical oxygen evolution reaction (OER) catalyst is critical to the widespread application of solar energy to fuel conversion. Among many potential OER catalysts, the metal oxy-hydroxides, especially FeOOH, show promising OER reactivity. In the present work, we performed a DFT + U study of the OER mechanism on the γ-FeOOH (010) surface. In particular, we established the chemical potential of the OH- and hole pair and included the OH- anion in the reaction pathway, accounting to the alkaline conditions of anodic OER process. We then analyzed the OER pathways on the surface with OH-, O-and Fe-terminations. On the surface with OH-and O-terminations, the O2 molecule could form from either OH reacting with the surface oxygen species (-OH* and -O*) or the combination of two surface oxygen species. On the Fe-terminated surface, O2 can only form by adsorbing OH on the Fe sites first. The potential-limiting step of the oxygen evolution with different surface terminations was determined by following the free-energy change of the elementary steps along each pathway. Our results show that oxygen formation requires recreating the surface Fe sites, and consequently, the condition that favors the partially exposed Fe sites will promote oxygen formation.
An efficient and economical oxygen evolution reaction (OER) catalyst is critical to the widespread application of solar energy to fuel conversion. Among many potential OER catalysts, the metal oxy-hydroxides, especially FeOOH, show promising OER reactivity. In the present work, we performed a DFT + U study of the OER mechanism on the γ-FeOOH (010) surface. In particular, we established the chemical potential of the OH- and hole pair and included the OH- anion in the reaction pathway, accounting to the alkaline conditions of anodic OER process. We then analyzed the OER pathways on the surface with OH-, O-and Fe-terminations. On the surface with OH-and O-terminations, the O2 molecule could form from either OH reacting with the surface oxygen species (-OH* and -O*) or the combination of two surface oxygen species. On the Fe-terminated surface, O2 can only form by adsorbing OH on the Fe sites first. The potential-limiting step of the oxygen evolution with different surface terminations was determined by following the free-energy change of the elementary steps along each pathway. Our results show that oxygen formation requires recreating the surface Fe sites, and consequently, the condition that favors the partially exposed Fe sites will promote oxygen formation.
2017, 38(9): 1629-1641
doi: 10.1016/S1872-2067(17)62798-0
Abstract:
A series of catalysts consisting of three-dimensionally ordered macroporous (3DOM) x-CeO2/Al2O3-supported Au nanoparticles (x=2, 10, 20, and 40 wt%) were successfully synthesized using a reduction-deposition method. These catalysts were characterized using scanning electron microscopy, the Brunauer-Emmett-Teller method, X-ray diffraction, transmission electron micros-copy, ultraviolet-visible spectroscopy, and temperature-programmed reduction by H2. Au nanopar-ticles of mean particle size 5 nm were well dispersed and supported on the inner walls of uniform macropores. The 3DOM structure improved the contact efficiency between soot and the catalyst. An Al-Ce-O solid solution was formed in the multilayer support, i.e., x-CeO2/Al2O3, by the incorporation of Al3+ ions into the CeO2 lattice, which resulted in the creation of extrinsic oxygen vacancies. Strong interactions between the metal (Au) and the support (Ce) increased the amount of active oxygen species, and this promoted soot oxidation. The catalytic performance in soot combustion was eval-uated using a temperature-programmed oxidation technique. The presence of CeO2 nanolayers in the 3DOM Au/x-CeO2/Al2O3 catalysts clearly improved the catalytic activities in soot oxidation. Among the prepared catalysts, 3DOM Au/20%CeO2/Al2O3 showed high catalytic activity and stabil-ity in diesel soot oxidation.
A series of catalysts consisting of three-dimensionally ordered macroporous (3DOM) x-CeO2/Al2O3-supported Au nanoparticles (x=2, 10, 20, and 40 wt%) were successfully synthesized using a reduction-deposition method. These catalysts were characterized using scanning electron microscopy, the Brunauer-Emmett-Teller method, X-ray diffraction, transmission electron micros-copy, ultraviolet-visible spectroscopy, and temperature-programmed reduction by H2. Au nanopar-ticles of mean particle size 5 nm were well dispersed and supported on the inner walls of uniform macropores. The 3DOM structure improved the contact efficiency between soot and the catalyst. An Al-Ce-O solid solution was formed in the multilayer support, i.e., x-CeO2/Al2O3, by the incorporation of Al3+ ions into the CeO2 lattice, which resulted in the creation of extrinsic oxygen vacancies. Strong interactions between the metal (Au) and the support (Ce) increased the amount of active oxygen species, and this promoted soot oxidation. The catalytic performance in soot combustion was eval-uated using a temperature-programmed oxidation technique. The presence of CeO2 nanolayers in the 3DOM Au/x-CeO2/Al2O3 catalysts clearly improved the catalytic activities in soot oxidation. Among the prepared catalysts, 3DOM Au/20%CeO2/Al2O3 showed high catalytic activity and stabil-ity in diesel soot oxidation.
