Complexes with neighboring metal centers and their analogues on surfaces are drawing increasing a... more Complexes with neighboring metal centers and their analogues on surfaces are drawing increasing attention as catalysts. These include molecular homogeneous catalysts incorporating various ligands; enzymes; and solids that include pairs of metal atoms mounted on supports. Catalysts in this broad class are active for numerous reactions and offer unexplored opportunities to address challenging reactions, such as oxidation of methane and oxidation of water in artificial photosynthesis. The subject of supported metal pair-site catalysts is in its infancy, facing challenges in (a) precise synthesis, (b) structure determination at the atomic scale, and (c) stabilization in reactive atmospheres. In this Perspective, we summarize key characteristics of molecular and enzymatic catalysts that incorporate neighboring metal centers and build on this foundation to assess the emerging literature of metal pair-site catalysts on various supports. The supported catalysts include those synthesized by anchoring molecular dinuclear precursors to support surfaces and those synthesized by selective formation of dinuclear surface species from mononuclear surface species. Examples of metals in this class are rhodium and iridium, and
Journal of the American Chemical Society, Dec 9, 2021
Atomically dispersed noble metal catalysts have drawn wide attention as candidates to replace sup... more Atomically dispersed noble metal catalysts have drawn wide attention as candidates to replace supported metal clusters and metal nanoparticles. Atomic dispersion can offer unique chemical properties as well as maximum utilization of the expensive metals. Addition of a second metal has been found to help reduce the size of Pt ensembles in bimetallic clusters; however, the stabilization of isolated Pt atoms in small nests of nonprecious metal atoms remains challenging. We now report a novel strategy for the design, synthesis, and characterization of a zeolitesupported propane dehydrogenation catalyst that incorporates predominantly isolated Pt atoms stably bonded within nests of Zn atoms located within the nanoscale pores of dealuminated zeolite Beta. The catalyst is stable in long-term operation and exhibits high activity and high selectivity to propene. Atomic resolution images, bolstered by X-ray absorption spectra, demonstrate predominantly atomic dispersion of the Pt in the nests and, with complementary infrared and nuclear magnetic resonance spectra, determine a structural model of the nested Pt.
The reaction pathways on supported catalysts can be tuned by optimizing the catalyst structures, ... more The reaction pathways on supported catalysts can be tuned by optimizing the catalyst structures, which helps the development of efficient catalysts. Such design is particularly desired for CO 2 hydrogenation, which is characterized by complex pathways and multiple products. Here, we report an investigation of supported cobalt, which is known for its hydrocarbon production and ability to turn into a selective catalyst for methanol synthesis in CO 2 hydrogenation which exhibits good activity and stability. The crucial technique is to use the silica, acting as a support and ligand, to modify the cobalt species via CoO -SiO n linkages, which favor the reactivity of spectroscopically identified *CH 3 O intermediates, that more readily undergo hydrogenation to methanol than the CO dissociation associated with hydrocarbon formation. Cobalt catalysts in this class offer appealing opportunities for optimizing selectivity in CO 2 hydrogenation and producing high-grade methanol. By identifying this function of silica, we provide support for rationally controlling these reaction pathways.
Many metal organic frameworks (MOFs) incorporate metal oxide clusters as nodes. Node sites where ... more Many metal organic frameworks (MOFs) incorporate metal oxide clusters as nodes. Node sites where linkers are missing can be catalytic sites. We now show how to dial in the number and occupancy of such sites in MIL-53 and MIL-68, which incorporate aluminum-oxide-like nodes. The methods involve modulators used in synthesis and postsynthesis reactions to control the modulatorderived groups on these sites. We illustrate the methods using formic acid as a modulator, giving formate ligands on the sites, and these can be removed to leave μ 2-OH groups and open Lewis acid sites. Methanol dehydration was used as a catalytic reaction to probe these sites, with infrared spectra giving evidence of methoxide ligands as reaction intermediates. Control of node surface chemistry opens the door for placement of a variety of ligands on a wide range of metal oxide cluster nodes for dialing in reactivity and catalytic properties of a potentially immense class of structurally well-defined materials.
A well-defined aluminium-bound hydroxyl group on the surface of mesoporous SBA-15, [(≡Si-O-Si≡) (... more A well-defined aluminium-bound hydroxyl group on the surface of mesoporous SBA-15, [(≡Si-O-Si≡) (≡Si-O)2 Al-OH], 3 was obtained by reacting di-isopropyl aluminium hydride with SBA-15 treated at 700 °C. The resulting surface [(≡Si-O-Si≡) (≡Si-O) 2 Al (isobutyl) fragment undergoes β-H elimination at 400 ᴼC leading to [(≡Si-O-Si≡)(≡Si-O-)2Al-O) Al-H]. Further oxidation of this Al-hydride with N2O leads to 3. This acidic support was used to create a well-defined surface organo-tungsten fragment [(≡Si-O-Si≡)(≡Si-O-)2Al-O-W(≡CtBu)(CH2tBu)2] by reacting 3 with W(≡Ct-Bu)(CH2-tBu)3. A further reaction with hydrogen under mild conditions afforded the tungsten carbyne bis-hydride [(≡Si-O-Si≡)(≡Si-O-)2Al-O-W(H)2(≡C-tBu)]. The performance of each of the W-supported catalysts was assessed for propane metathesis in a flow reactor at 150 °C. [(≡Si-O-Si≡)(≡Si-O-)2 Al-O-W(≡CtBu)(H)2] was found to be a single-site catalyst, giving the highest turnover number (TON = 800) and the highest reported selectivity for butane (45%) vs. ethane (32%) known for oxide-supported tungsten complex catalysts (with the supports being silica, silica-alumina, and alumina). The results demonstrate that modification of the oxide ligands on silica via the creation of Al Lewis acid center as an anchoring site for organometallic complexes opens up new catalytic properties, markedly enhancing the catalytic performance of supported organo-tungsten species.
Sol-gel synthesis was used to prepare oxides of aluminum and magnesium from mixtures of Al O-s-Bu... more Sol-gel synthesis was used to prepare oxides of aluminum and magnesium from mixtures of Al O-s-Bu and 3 Ž. Mg O-Et , with the atomic ratio of Al to Mg being 4. The hydrolysis and condensation reactions were controlled by acetic 2 Ž w x w Ž. x. acid in the absence of added water; the ratio R of acetic acid to aluminum alkoxide R s CH COOH r Al O-s-Bu was 3 3 varied from 1 to 6. The products were characterized by differential thermal analysis, X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and atomic force microscopy. For R) 1, gelatinous precipitates formed as relatively large amounts of acetate species in the form of crystalline layered materials, which upon calcination lost their acetate ligands and formed a crystalline material composed of a mixture of amorphous alumina and crystalline spinel syn MgAl O. The excess 2 4 alumina is inferred to have remained amorphous and highly dispersed in this relatively ordered material. For R s 1, however, a gel formed, and the material was less crystalline than the gelatinous precipitates because of the relatively low concentration of acetate. When the gel was calcined, a mixture of crystalline spinel syn MgAl O and amorphous alumina 2 4 formed, with the alumina being separate from the spinel and not highly dispersed so that, after calcination at temperatures higher than 7508C, a heterogeneous mixture of crystalline phases of magnesium-aluminum oxide and aluminum oxide formed.
... JOSEPH J. RAFALKO,'' PATRICIA M. WATSON," DANIEL H. MALUEG,Ib RAYM... more ... JOSEPH J. RAFALKO,'' PATRICIA M. WATSON," DANIEL H. MALUEG,Ib RAYMOND E. DAVIS,*Ib and BRUCE C. GATES ... characterized by the monoclinic space group PZl/c with unit cell constants (I = 12.574 (5) A, b = 10.917 (3) A, c = 21.606 (5) A, fi = 104.84 (3)" and Z = 4. Full ...
ABSTRACT The conversion of eugenol (4-allyl-2-methoxyphenol), a compound derived from the lignin ... more ABSTRACT The conversion of eugenol (4-allyl-2-methoxyphenol), a compound derived from the lignin in woody biomass, was catalyzed by HY zeolite at 573 K and atmospheric pressure. The main products were isoeugenol and guaiacol, formed by isomerization and by deallylation, respectively. Substituted guaiacols with saturated side-chains (4-methylguaiacol, 4-ethylguaiacol, and 4-propylguaiacol) were also formed, by hydrogen transfer and alkylation reactions. The pseudo-first-order rate constant for the overall disappearance of eugenol was found to be 12.4 L (g of catalyst)/h. When the catalyst was Pt/γ-Al2O3 used in the presence of H2, significant hydrogenation of the propenyl side-chain took place, accompanied by isomerization, and hydrodeoxygenation. Under similar operating conditions, the reaction catalyzed by Pt/γ-Al2O3 in the presence of H2 gave a higher eugenol conversion (X = 0.70) than the reaction catalyzed by HY zeolite (X = 0.11), primarily because of the dominant hydrogenation observed with the former catalyst. In the absence of H2 as a co-reactant, the acidic γ-Al2O3 support in Pt/γ-Al2O3 evidently catalyzed all the classes of reactions catalyzed by HY zeolite. Graphical Abstract .
A closed Ir 4 carbonyl cluster, 1, comprising a tetrahedral metal frame and three sterically bulk... more A closed Ir 4 carbonyl cluster, 1, comprising a tetrahedral metal frame and three sterically bulky tert-butyl-calix [4]arene(OPr) 3 (OCH 2 PPh 2) (Ph ¼ phenyl; Pr ¼ propyl) ligands at the basal plane, was characterized with variable-temperature 13 C NMR spectroscopy, which show the absence of scrambling of the CO ligands at temperatures up to 313 K. This demonstration of distinct sites for the CO ligands was found to extend to the reactivity and catalytic properties, as shown by selective decarbonylation in a reaction with trimethylamine N-oxide (TMAO) as an oxidant, which, reacting in the presence of ethylene, leads to the selective bonding of an ethyl ligand at the apical Ir site. These clusters were supported intact on porous silica and found to catalyze ethylene hydrogenation, and a comparison of the kinetics of the singlehydrogenation reaction and steady-state hydrogenation catalysis demonstrates a unique single-site catalyst-with each site having the same catalytic activity. Reaction orders in the catalytic ethylene hydrogenation reaction of approximately 1/2 and 0 for H 2 and C 2 H 4 , respectively, nearly match those for conventional noble-metal catalysts. In contrast to oxidative decarbonylation, thermal desorption of CO from silica-supported cluster 1 occurred exclusively at the basal plane, giving rise to sites that do not react with ethylene and are catalytically inactive for ethylene hydrogenation. The evidence of distinctive sites on the cluster catalyst leads to a model that links to hydrogen-transfer catalysis on metals-involving some surface sites that bond to both hydrocarbon and hydrogen and are catalytically engaged (so-called "*" sites) and others, at the basal plane, which bond hydrogen and CO but not hydrocarbon and are reservoir sites (so-called "S" sites).
Well-defined single-site silica-supported hafniaaziridine complex [(≡Si-O-)Hf(η 2 π-MeNCH 2)(η 1-... more Well-defined single-site silica-supported hafniaaziridine complex [(≡Si-O-)Hf(η 2 π-MeNCH 2)(η 1-NMe 2)(η 1-HNMe 2)] was prepared using surface organometallic chemistry. Upon thermal treatment under high vacuum, the grafted species was converted into an unprecedented hafnium imido, bis-amido, complex [(≡Si-O-)Hf(=NMe)(η 1-NMe 2)]. The surface complexes were characterized by elemental analysis and the following spectroscopic techniques: infrared, solid-state single and multiple quantum NMR, advanced DNP-SENS, and extended X-ray absorption fine structure. [(≡Si-O-)Hf(=NMe)(η 1-NMe 2)] catalyzed imine metathesis under mild conditions, and characterization of the reactivity showed that the imido exchange with N-(4phenylbenzylidene)benzylamine yielded [(≡ Si-O-)Hf (=NCH 2 Ar) (η 1-NMe 2)], demonstrating a kind of 2+2 mechanism involving the imine and the imido, proposed reaction mechanism is also supported by DFT calculations.
This research project, now in its third year, is an investigation of the synthesis, structure, an... more This research project, now in its third year, is an investigation of the synthesis, structure, and bonding of a family of metal complexes, clusters, and particles on the surfaces of high-area •metal oxide supports. The focus is the structure of the metal-support interface. Surface species have been prepared by synthetic organometallic chemistry on the support surfaces. The organometallic precursors are complexes of W, Re, Os, lr, and Pt, including W(CO)6, HRe(CO)5, Re2(CO)I0,/'I3Re3(CO)12, H2Os(CO)4, Ir(r13-C3Hs)3, and Pt(rl3-C3Hs)2. The supports are primarily MgO and y-AI203. 2"he surface species have been characterized by infrared and EXAFS spectroscopies, among other techniques (Table 1). The major results have been obtained for Re and Ir. A significant fraction of the effort has been devoted to finding conditions for preparation of nearly unique ("molecular") organometallic and metallic species on the support surfaces. We have found conditions for preparation of apparently unique subcarbonyls of W, Re, Os on support surfaces (Table 1). Iridium and platinum are more difficult to prepare in the form of stable subcarbonyls, being easily converted into the metallic form, giving structurally nonuniform crystallites on the supports; our results with iridium and platinum subcarbonyls are still preliminary (Table 1).
The 2016 Faraday Discussion on the topic "Designing new Heterogeneous Catalysts" brought together... more The 2016 Faraday Discussion on the topic "Designing new Heterogeneous Catalysts" brought together a group of scientists and engineers to address forefront topics in catalysis and the challenge of catalyst design-which is daunting because of the intrinsic nonuniformity of the surfaces of catalytic materials. "Catalyst design" has taken on a pragmatic meaning which implies the discovery of new and better catalysts on the basis of fundamental understanding of catalyst structure and performance. The presentations and discussion at the meeting illustrate rapid progress in this understanding linked with improvements in spectroscopy, microscopy, theory, and catalyst performance testing. The following essay includes a statement of recurrent themes in the discussion and examples of forefront science that evidences progress toward catalyst design. Catalysis and catalyst design Catalysis is the key to control of chemical change, in processes ranging from the biological to the technological. It is used to make products including chemicals, fuels, materials, food, beverages, and personal care products, and together these have a value of roughly 5-10 trillion dollars (US) per year worldwide. Catalysis is also essential for the removal of environmental pollutants such as those generated in motor vehicles and fossil fuel-fired power plants. Thus, the science underlying catalytic technology is essential. Catalysis science is also challenging, because almost all large-scale industrial catalysts are solids. These work at their surfaces-and these surfaces are notoriously nonuniform in both composition and structure, often being substantially different from simple terminations of the bulk material-and they undergo changes when exposed reactants.
Complexes with neighboring metal centers and their analogues on surfaces are drawing increasing a... more Complexes with neighboring metal centers and their analogues on surfaces are drawing increasing attention as catalysts. These include molecular homogeneous catalysts incorporating various ligands; enzymes; and solids that include pairs of metal atoms mounted on supports. Catalysts in this broad class are active for numerous reactions and offer unexplored opportunities to address challenging reactions, such as oxidation of methane and oxidation of water in artificial photosynthesis. The subject of supported metal pair-site catalysts is in its infancy, facing challenges in (a) precise synthesis, (b) structure determination at the atomic scale, and (c) stabilization in reactive atmospheres. In this Perspective, we summarize key characteristics of molecular and enzymatic catalysts that incorporate neighboring metal centers and build on this foundation to assess the emerging literature of metal pair-site catalysts on various supports. The supported catalysts include those synthesized by anchoring molecular dinuclear precursors to support surfaces and those synthesized by selective formation of dinuclear surface species from mononuclear surface species. Examples of metals in this class are rhodium and iridium, and
Journal of the American Chemical Society, Dec 9, 2021
Atomically dispersed noble metal catalysts have drawn wide attention as candidates to replace sup... more Atomically dispersed noble metal catalysts have drawn wide attention as candidates to replace supported metal clusters and metal nanoparticles. Atomic dispersion can offer unique chemical properties as well as maximum utilization of the expensive metals. Addition of a second metal has been found to help reduce the size of Pt ensembles in bimetallic clusters; however, the stabilization of isolated Pt atoms in small nests of nonprecious metal atoms remains challenging. We now report a novel strategy for the design, synthesis, and characterization of a zeolitesupported propane dehydrogenation catalyst that incorporates predominantly isolated Pt atoms stably bonded within nests of Zn atoms located within the nanoscale pores of dealuminated zeolite Beta. The catalyst is stable in long-term operation and exhibits high activity and high selectivity to propene. Atomic resolution images, bolstered by X-ray absorption spectra, demonstrate predominantly atomic dispersion of the Pt in the nests and, with complementary infrared and nuclear magnetic resonance spectra, determine a structural model of the nested Pt.
The reaction pathways on supported catalysts can be tuned by optimizing the catalyst structures, ... more The reaction pathways on supported catalysts can be tuned by optimizing the catalyst structures, which helps the development of efficient catalysts. Such design is particularly desired for CO 2 hydrogenation, which is characterized by complex pathways and multiple products. Here, we report an investigation of supported cobalt, which is known for its hydrocarbon production and ability to turn into a selective catalyst for methanol synthesis in CO 2 hydrogenation which exhibits good activity and stability. The crucial technique is to use the silica, acting as a support and ligand, to modify the cobalt species via CoO -SiO n linkages, which favor the reactivity of spectroscopically identified *CH 3 O intermediates, that more readily undergo hydrogenation to methanol than the CO dissociation associated with hydrocarbon formation. Cobalt catalysts in this class offer appealing opportunities for optimizing selectivity in CO 2 hydrogenation and producing high-grade methanol. By identifying this function of silica, we provide support for rationally controlling these reaction pathways.
Many metal organic frameworks (MOFs) incorporate metal oxide clusters as nodes. Node sites where ... more Many metal organic frameworks (MOFs) incorporate metal oxide clusters as nodes. Node sites where linkers are missing can be catalytic sites. We now show how to dial in the number and occupancy of such sites in MIL-53 and MIL-68, which incorporate aluminum-oxide-like nodes. The methods involve modulators used in synthesis and postsynthesis reactions to control the modulatorderived groups on these sites. We illustrate the methods using formic acid as a modulator, giving formate ligands on the sites, and these can be removed to leave μ 2-OH groups and open Lewis acid sites. Methanol dehydration was used as a catalytic reaction to probe these sites, with infrared spectra giving evidence of methoxide ligands as reaction intermediates. Control of node surface chemistry opens the door for placement of a variety of ligands on a wide range of metal oxide cluster nodes for dialing in reactivity and catalytic properties of a potentially immense class of structurally well-defined materials.
A well-defined aluminium-bound hydroxyl group on the surface of mesoporous SBA-15, [(≡Si-O-Si≡) (... more A well-defined aluminium-bound hydroxyl group on the surface of mesoporous SBA-15, [(≡Si-O-Si≡) (≡Si-O)2 Al-OH], 3 was obtained by reacting di-isopropyl aluminium hydride with SBA-15 treated at 700 °C. The resulting surface [(≡Si-O-Si≡) (≡Si-O) 2 Al (isobutyl) fragment undergoes β-H elimination at 400 ᴼC leading to [(≡Si-O-Si≡)(≡Si-O-)2Al-O) Al-H]. Further oxidation of this Al-hydride with N2O leads to 3. This acidic support was used to create a well-defined surface organo-tungsten fragment [(≡Si-O-Si≡)(≡Si-O-)2Al-O-W(≡CtBu)(CH2tBu)2] by reacting 3 with W(≡Ct-Bu)(CH2-tBu)3. A further reaction with hydrogen under mild conditions afforded the tungsten carbyne bis-hydride [(≡Si-O-Si≡)(≡Si-O-)2Al-O-W(H)2(≡C-tBu)]. The performance of each of the W-supported catalysts was assessed for propane metathesis in a flow reactor at 150 °C. [(≡Si-O-Si≡)(≡Si-O-)2 Al-O-W(≡CtBu)(H)2] was found to be a single-site catalyst, giving the highest turnover number (TON = 800) and the highest reported selectivity for butane (45%) vs. ethane (32%) known for oxide-supported tungsten complex catalysts (with the supports being silica, silica-alumina, and alumina). The results demonstrate that modification of the oxide ligands on silica via the creation of Al Lewis acid center as an anchoring site for organometallic complexes opens up new catalytic properties, markedly enhancing the catalytic performance of supported organo-tungsten species.
Sol-gel synthesis was used to prepare oxides of aluminum and magnesium from mixtures of Al O-s-Bu... more Sol-gel synthesis was used to prepare oxides of aluminum and magnesium from mixtures of Al O-s-Bu and 3 Ž. Mg O-Et , with the atomic ratio of Al to Mg being 4. The hydrolysis and condensation reactions were controlled by acetic 2 Ž w x w Ž. x. acid in the absence of added water; the ratio R of acetic acid to aluminum alkoxide R s CH COOH r Al O-s-Bu was 3 3 varied from 1 to 6. The products were characterized by differential thermal analysis, X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and atomic force microscopy. For R) 1, gelatinous precipitates formed as relatively large amounts of acetate species in the form of crystalline layered materials, which upon calcination lost their acetate ligands and formed a crystalline material composed of a mixture of amorphous alumina and crystalline spinel syn MgAl O. The excess 2 4 alumina is inferred to have remained amorphous and highly dispersed in this relatively ordered material. For R s 1, however, a gel formed, and the material was less crystalline than the gelatinous precipitates because of the relatively low concentration of acetate. When the gel was calcined, a mixture of crystalline spinel syn MgAl O and amorphous alumina 2 4 formed, with the alumina being separate from the spinel and not highly dispersed so that, after calcination at temperatures higher than 7508C, a heterogeneous mixture of crystalline phases of magnesium-aluminum oxide and aluminum oxide formed.
... JOSEPH J. RAFALKO,'' PATRICIA M. WATSON," DANIEL H. MALUEG,Ib RAYM... more ... JOSEPH J. RAFALKO,'' PATRICIA M. WATSON," DANIEL H. MALUEG,Ib RAYMOND E. DAVIS,*Ib and BRUCE C. GATES ... characterized by the monoclinic space group PZl/c with unit cell constants (I = 12.574 (5) A, b = 10.917 (3) A, c = 21.606 (5) A, fi = 104.84 (3)" and Z = 4. Full ...
ABSTRACT The conversion of eugenol (4-allyl-2-methoxyphenol), a compound derived from the lignin ... more ABSTRACT The conversion of eugenol (4-allyl-2-methoxyphenol), a compound derived from the lignin in woody biomass, was catalyzed by HY zeolite at 573 K and atmospheric pressure. The main products were isoeugenol and guaiacol, formed by isomerization and by deallylation, respectively. Substituted guaiacols with saturated side-chains (4-methylguaiacol, 4-ethylguaiacol, and 4-propylguaiacol) were also formed, by hydrogen transfer and alkylation reactions. The pseudo-first-order rate constant for the overall disappearance of eugenol was found to be 12.4 L (g of catalyst)/h. When the catalyst was Pt/γ-Al2O3 used in the presence of H2, significant hydrogenation of the propenyl side-chain took place, accompanied by isomerization, and hydrodeoxygenation. Under similar operating conditions, the reaction catalyzed by Pt/γ-Al2O3 in the presence of H2 gave a higher eugenol conversion (X = 0.70) than the reaction catalyzed by HY zeolite (X = 0.11), primarily because of the dominant hydrogenation observed with the former catalyst. In the absence of H2 as a co-reactant, the acidic γ-Al2O3 support in Pt/γ-Al2O3 evidently catalyzed all the classes of reactions catalyzed by HY zeolite. Graphical Abstract .
A closed Ir 4 carbonyl cluster, 1, comprising a tetrahedral metal frame and three sterically bulk... more A closed Ir 4 carbonyl cluster, 1, comprising a tetrahedral metal frame and three sterically bulky tert-butyl-calix [4]arene(OPr) 3 (OCH 2 PPh 2) (Ph ¼ phenyl; Pr ¼ propyl) ligands at the basal plane, was characterized with variable-temperature 13 C NMR spectroscopy, which show the absence of scrambling of the CO ligands at temperatures up to 313 K. This demonstration of distinct sites for the CO ligands was found to extend to the reactivity and catalytic properties, as shown by selective decarbonylation in a reaction with trimethylamine N-oxide (TMAO) as an oxidant, which, reacting in the presence of ethylene, leads to the selective bonding of an ethyl ligand at the apical Ir site. These clusters were supported intact on porous silica and found to catalyze ethylene hydrogenation, and a comparison of the kinetics of the singlehydrogenation reaction and steady-state hydrogenation catalysis demonstrates a unique single-site catalyst-with each site having the same catalytic activity. Reaction orders in the catalytic ethylene hydrogenation reaction of approximately 1/2 and 0 for H 2 and C 2 H 4 , respectively, nearly match those for conventional noble-metal catalysts. In contrast to oxidative decarbonylation, thermal desorption of CO from silica-supported cluster 1 occurred exclusively at the basal plane, giving rise to sites that do not react with ethylene and are catalytically inactive for ethylene hydrogenation. The evidence of distinctive sites on the cluster catalyst leads to a model that links to hydrogen-transfer catalysis on metals-involving some surface sites that bond to both hydrocarbon and hydrogen and are catalytically engaged (so-called "*" sites) and others, at the basal plane, which bond hydrogen and CO but not hydrocarbon and are reservoir sites (so-called "S" sites).
Well-defined single-site silica-supported hafniaaziridine complex [(≡Si-O-)Hf(η 2 π-MeNCH 2)(η 1-... more Well-defined single-site silica-supported hafniaaziridine complex [(≡Si-O-)Hf(η 2 π-MeNCH 2)(η 1-NMe 2)(η 1-HNMe 2)] was prepared using surface organometallic chemistry. Upon thermal treatment under high vacuum, the grafted species was converted into an unprecedented hafnium imido, bis-amido, complex [(≡Si-O-)Hf(=NMe)(η 1-NMe 2)]. The surface complexes were characterized by elemental analysis and the following spectroscopic techniques: infrared, solid-state single and multiple quantum NMR, advanced DNP-SENS, and extended X-ray absorption fine structure. [(≡Si-O-)Hf(=NMe)(η 1-NMe 2)] catalyzed imine metathesis under mild conditions, and characterization of the reactivity showed that the imido exchange with N-(4phenylbenzylidene)benzylamine yielded [(≡ Si-O-)Hf (=NCH 2 Ar) (η 1-NMe 2)], demonstrating a kind of 2+2 mechanism involving the imine and the imido, proposed reaction mechanism is also supported by DFT calculations.
This research project, now in its third year, is an investigation of the synthesis, structure, an... more This research project, now in its third year, is an investigation of the synthesis, structure, and bonding of a family of metal complexes, clusters, and particles on the surfaces of high-area •metal oxide supports. The focus is the structure of the metal-support interface. Surface species have been prepared by synthetic organometallic chemistry on the support surfaces. The organometallic precursors are complexes of W, Re, Os, lr, and Pt, including W(CO)6, HRe(CO)5, Re2(CO)I0,/'I3Re3(CO)12, H2Os(CO)4, Ir(r13-C3Hs)3, and Pt(rl3-C3Hs)2. The supports are primarily MgO and y-AI203. 2"he surface species have been characterized by infrared and EXAFS spectroscopies, among other techniques (Table 1). The major results have been obtained for Re and Ir. A significant fraction of the effort has been devoted to finding conditions for preparation of nearly unique ("molecular") organometallic and metallic species on the support surfaces. We have found conditions for preparation of apparently unique subcarbonyls of W, Re, Os on support surfaces (Table 1). Iridium and platinum are more difficult to prepare in the form of stable subcarbonyls, being easily converted into the metallic form, giving structurally nonuniform crystallites on the supports; our results with iridium and platinum subcarbonyls are still preliminary (Table 1).
The 2016 Faraday Discussion on the topic "Designing new Heterogeneous Catalysts" brought together... more The 2016 Faraday Discussion on the topic "Designing new Heterogeneous Catalysts" brought together a group of scientists and engineers to address forefront topics in catalysis and the challenge of catalyst design-which is daunting because of the intrinsic nonuniformity of the surfaces of catalytic materials. "Catalyst design" has taken on a pragmatic meaning which implies the discovery of new and better catalysts on the basis of fundamental understanding of catalyst structure and performance. The presentations and discussion at the meeting illustrate rapid progress in this understanding linked with improvements in spectroscopy, microscopy, theory, and catalyst performance testing. The following essay includes a statement of recurrent themes in the discussion and examples of forefront science that evidences progress toward catalyst design. Catalysis and catalyst design Catalysis is the key to control of chemical change, in processes ranging from the biological to the technological. It is used to make products including chemicals, fuels, materials, food, beverages, and personal care products, and together these have a value of roughly 5-10 trillion dollars (US) per year worldwide. Catalysis is also essential for the removal of environmental pollutants such as those generated in motor vehicles and fossil fuel-fired power plants. Thus, the science underlying catalytic technology is essential. Catalysis science is also challenging, because almost all large-scale industrial catalysts are solids. These work at their surfaces-and these surfaces are notoriously nonuniform in both composition and structure, often being substantially different from simple terminations of the bulk material-and they undergo changes when exposed reactants.
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