Nonreducible Metal Oxide Research Articles

One-pot synthesis of stable cationic gold species highly active in the CO oxidation confined into mordenite-like zeolite

The presence of cationic gold species is a key factor for the CO oxidation over the supported gold catalysts based on non-reducible metal oxides. The deactivation of such catalysts is mainly associated with the Au nanoparticles (AuNPs) sintering and the concomitant reduction of cationic Au species. Here we report the high catalytic stability at the CO oxidation of the cationic Au species confined into a mordenite-like zeolite matrix prepared by a one-pot synthesis method. The sample with lowest Au loading (0.32 % wt.) exhibited the higher catalytic performance and stability during CO oxidation. Detailed FTIR analysis showed that the CO oxidation on sample with low Au content (0.32 wt%) occurred preferentially via CO interaction with the cationic interface Au-hydroxyl (Au1+–OH−) species to produce CO2(g) via decarboxylation. In contrast, the sample with a high Au content (1.56 wt%) displayed lower CO oxidation ability, which was associated with a low content and less stability of cationic gold species. Proposed synthesis method permits to obtain in a one-pot way stable cationic gold species highly effectively for the CO oxidation.

Deploying radical inter-transition from [rad]OH to supported NO3[rad] on Mono-dentate NO3--modified ZrO2 to sustain fragmentation of aqueous contaminants

To advance aqueous pollutant degradation using OH, H2O2 (OH carrier) should be cleaved homolytically on a non-reducible metal oxide (ZrO2) rather than heterolytically on a reducible counterpart (MnO2), given the merits of H2O2 homolysis such as improved OH productivity, unnecessity to recover H2O2 activators (Lewis acidic metals; LA) via electron reduction, and minute LA leaching. This paper presents a methodology to exploit H2O2 homolysis with the rate-determining step of endothermic OH desorption, thereby proposing the coupling of H2O2 homolysis and exothermic radical inter-conversion of OH → NO3SUP (supported NO3) to create the overall OH → NO3SUP route. ZrO2 was modified with NO3− functionalities (NO3SUP precursors) to form ZrO2-N, where NO3-SUP species were located close to Zr4+ (LA) and Brönsted acidic -OH (BA) sites, whose acidic strengths must be elevated to facilitate OH desorption for reducing the energy barrier (EBARRIER) of the overall OH → NO3SUP route. NO3-SUP species were bound to the ZrO2 surface via mono-dentate configuration only, thereby avoiding LA loss (rate in a per-gram↑), escalating LA/BA strengths (EBARRIER↓), and imparting two free oxygens available to OH → NO3SUP (rate in a per-site↑). Moreover, NO3SUP species extract electrons from contaminants via electron transfer to recover NO3-SUP species used for recurring OH → NO3SUP, while sustaining pollutant fragmentation efficiency by circumventing surface poison accumulation. Hence, NO3SUP on ZrO2-N revealed higher efficiencies in fragmenting bisphenol A or recycling phenol degradation than OH evolved from ZrO2. In addition, ZrO2 outperformed MnO2 in exploiting NO3SUP species, thus showing greater recyclability in mineralizing textile wastewater, while leaching a negligible amount of Zr.

Hydrogen spillover and substrate-support hydrogen bonding mediate hydrogenation of phenol catalyzed by palladium on reducible metal oxides.

Substrate-support interactions play an important role in the catalytic hydrogenation of phenolic compounds by ceria-supported palladium (Pd/CeO2). Here, we combine surface contrast solution NMR methods and reaction kinetic assays to investigate the role of substrate-support interactions in phenol (PhOH) hydrogenation catalyzed by titania-supported palladium (Pd/TiO2). We show that PhOH adsorbs on the catalyst via a weak hydrogen-bonding interaction between the -OH group of the substrate and one oxygen atom on the support. Interestingly, we observe that the addition of 20 mM inorganic phosphate results in a ∼2-fold destabilization of the PhOH-support interaction and a corresponding ∼2-fold inhibition of the catalytic reaction, suggesting an active role of the PhOH-TiO2 hydrogen bond in catalysis. A comparison of the data measured here with the results previously reported for a Pd/CeO2 catalyst indicates that the efficiency of the Pd-supported catalysts is correlated to the amount of PhOH hydrogen bonded to the metal oxide support. Since CeO2 and TiO2 have similar ability to uptake activated hydrogen from a noble metal site, these data suggest that hydrogen spillover is the main mechanism by which Pd-activated hydrogens are shuttled to the PhOH adsorbed on the surface of the support. Consistent with this hypothesis, Pd supported on a non-reducible metal oxide (silica) displays negligible hydrogenation activity. Therefore, we conclude that basic and reducible metal oxides are active supports for the efficient hydrogenation of phenolic compounds due to their ability to hydrogen bond to the substrate and mediate the addition of the activated hydrogens to the adsorbed aromatic ring.

Evaluating metal oxide support effects on the RWGS activity of Mo2C catalysts

Mo2C supported on nonreducible metal oxides shows increased activity for the reverse water gas shift reaction compared to reducible oxides.

Dynamic chemical processes on ZnO surfaces tuned by physisorption under ambient conditions

The catalytic properties of non-reducible metal oxides have intrigued continuous interest in the past decades. Often time, catalytic studies of bulk non-reducible oxides focused on their high-temperature applications owing to their weak interaction with small molecules. Hereby, combining ambient-pressure scanning tunneling microscopy (AP-STM), AP X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations, we studied the activation of CO and CO2 on ZnO, a typical non-reducible oxide and major catalytic material in the conversion of C1 molecules. By visualizing the chemical processes on ZnO surfaces at the atomic scale under AP conditions, we showed that new adsorbate structures induced by the enhanced physisorption and the concerted interaction of physisorbed molecules could facilitate the activation of CO and CO2 on ZnO. The reactivity of ZnO towards CO could be observed under AP conditions, where an ordered (2 × 1)–CO structure was observed on ZnO(101¯0). Meanwhile, chemisorption of CO2 on ZnO(101¯0) under AP conditions was also enhanced by physisorbed CO2, which minimizes the repulsion between surface dipoles and causes a (3 × 1)–CO2 structure. Our study has brought molecular insight into the fundamental chemistry and catalytic properties of ZnO surfaces under realistic reaction conditions.

Rapid Interchangeable Hydrogen, Hydride, and Proton Species at the Interface of Transition Metal Atom on Oxide Surface.

Hydrogen spillover is the phenomenon where a hydrogen atom, generated from the dissociative chemisorption of dihydrogen on the surface of a metal species, migrates from the metal to the catalytic support. This phenomenon is regarded as a promising avenue for hydrogen storage, yet the atomic mechanism for how the hydrogen atom can be transferred to the support has remained controversial for decades. As a result, the development of catalytic support for such a purpose is only limited to typical reducible oxide materials. Herein, by using a combination of in situ spectroscopic and imaging technique, we are able to visualize and observe the atomic pathway for which hydrogen travels via a frustrated Lewis pair that has been constructed on a nonreducible metal oxide. The interchangeable status between the hydrogen, proton, and hydride is carefully characterized and demonstrated. It is envisaged that this study has opened up new design criteria for hydrogen storage material.

The effect of phase composition and crystallite size on activity and selectivity of ZrO2 in non-oxidative propane dehydrogenation

A series of bare ZrO2 materials composed either of monoclinic or tetragonal phase with the size of crystallites varying between 3.7 and 43.4 nm were prepared by precipitation, hydrothermal or thermal calcination methods. The samples were characterized by XRD, BET, CO-TPR, NH3-TPD, electrical conductivity measurements and operando UV–vis spectroscopy. Density functional theory calculations provided molecular insights into the kind of active site and individual steps of propane dehydrogenation to propene and hydrogen. Regardless of the phase composition, two Zr cations located at an oxygen vacancy, i.e. coordinatively unsaturated Zr cations (Zrcus sites) having, nevertheless, different chemical environment in tetragonal and monoclinic ZrO2, were concluded to be responsible for homolytic breaking of CH bonds in propane. Monoclinic ZrO2 showed, however, higher rate of propene formation and higher propene selectivity than ZrO2 stabilized in the tetragonal phase. Both the activity and the selectivity to propene increased with a decrease in the size of crystallites. The effects of phase composition and crystallite size on the PDH performance were related to the ability of ZrO2 to release lattice oxygen upon reductive catalyst treatment and thus to create Zrcus sites. The knowledge derived can be used for further optimizing PDH performance of ZrO2-based catalysts and also extended to other non-reducible metal oxides.

Correlation of Methane Activation and Oxide Catalyst Reducibility and Its Implications for Oxidative Coupling

We investigate methane activation over a range of metal-oxide surfaces. Density functional theory calculations are used to correlate the C–H bond activation energy to the surface reducibility (oxygen vacancy formation energy, work function). The correlation includes several reducible and nonreducible metal-oxides, doped CeO2, doped TiO2, ZnO, and doped MgO, and also holds for various oxidation states of TbOx, different surface facets of TiO2, and variation of Hubbard U parameter for CeO2. We find a linear correlation between the C–H activation reaction energy, ·CH3 adsorption energy, and the oxygen vacancy formation energy of pure/doped metal-oxides, making surface reducibility a descriptor for predicting catalyst activity and selectivity against further oxidation of the ·CH3 radical.

Unusual dissociative adsorption of H2 over stoichiometric MgO thin film supported on molybdenum

The dissociation of a hydrogen molecule on perfect MgO(001) films deposited on Mo(001) surface is investigated systematically using periodic density-functional theory (DFT) method. The unusual adsorption behavior of heterolytic dissociative hydrogen molecule at neighboring surface oxygen and surface magnesium, is clarified here. To our knowledge, this heterolytic dissociative state has never been found before on bulk MgO(001) or metal supported perfect MgO(001) surfaces (without low coordination sites). The results confirm that, in all cases, the heterolytic dissociation is much more favorable that homolytic dissociation both energetically and kinetically. The energy differences between two dissociative states are very large, in the range of 1.1eV–1.5eV for Mo supported 1ML–3ML oxide films, which inhibits, to a great extent, the homolytic dissociation in the respect of reaction thermodynamics. The energy barriers of heterolytic dissociation are about 0.5eV, much lower that the barrier of homolytic dissociation. The transformation reaction on thick films will be more endothermic. Passing through heterolytic dissociation state has significantly lowered the reaction heat and the energy barrier for obtaining homolytic dissociative structure, which makes the homolytic splitting of H2 easier on 2ML oxide films. The results provide a useful strategy for enhancing the reactivity of the nonreducible metal oxide.

Reactions of methyl groups on a non-reducible metal oxide: The reaction of iodomethane on stoichiometric α-Cr2O3(0001)

The reaction of iodomethane on the nearly stoichiometric α-Cr2O3(0001) surface produces gas phase ethylene, methane, and surface iodine adatoms. The reaction is initiated by the dissociation of iodomethane into surface methyl fragments, −CH3, and iodine adatoms. Methyl fragments bound at surface Cr cation sites undergo a rate-limiting dehydrogenation reaction to methylene, =CH2. The methylene intermediates formed from methyl dehydrogenation can undergo coupling reactions to produce ethylene via two principle reaction pathways: (1) direct coupling of methylene and (2) methylene insertion into the methyl surface bond to form surface ethyl groups which undergo β-H elimination to produce ethylene. The liberated hydrogen also combines with methyl groups to form methane. Iodine adatoms from the dissociation of iodomethane deactivate the surface by simple site blocking of the surface Cr3+ cations.

Methylene migration and coupling on a non-reducible metal oxide: The reaction of dichloromethane on stoichiometric α-Cr2O3(0001)

The reaction of CH2Cl2 over the nearly-stoichiometric α-Cr2O3(0001) surface produces gas phase ethylene, methane and surface chlorine adatoms. The reaction is initiated by the decomposition of CH2Cl2 into surface methylene and chlorine. Photoemission indicates that surface cations are the preferred binding sites for both methylene and chlorine adatoms. Two reaction channels are observed for methylene coupling to ethylene in temperature-programmed desorption (TPD). A desorption-limited, low-temperature route is attributed to two methylenes bound at a single site. The majority of ethylene is produced by a reaction-limited process involving surface migration (diffusion) of methylene as the rate-limiting step. DFT calculations indicate the surface diffusion mechanism is mediated by surface oxygen anions. The source of hydrogen for methane formation is adsorbed background water. Chlorine adatoms produced by the dissociation of CH2Cl2 deactivate the surface by simple site-blocking of surface Cr3+ sites. A comparison of experiment and theory shows that DFT provides a better description of the surface chemistry of the carbene intermediate than DFT+U using reported parameters for a best representation of the bulk electronic properties of α-Cr2O3.

Activation of Molecular Oxygen and the Nature of the Active Oxygen Species for CO Oxidation on Oxide Supported Au Catalysts

Although highly dispersed Au catalysts with Au nanoparticles (NPs) of a few nanometers in diameter are well-known for their high catalytic activity for several oxidation and reduction reactions already at rather low temperatures for almost 30 years, central aspects of the reaction mechanism are still unresolved. While most studies focused on the active site, the active Au species, and the effect of the support material, the most crucial step during oxidation reactions, the activation of molecular oxygen and the nature of the resulting active oxygen species (Oact), received more attention just recently. This is topic of this Account, which focuses on the formation, location, and nature of the Oact species present on metal oxide supported Au catalysts under typical reaction conditions, at room temperature and above. It is mainly based on quantitative temporal analysis of products (TAP) reactor measurements, which different from most spectroscopic techniques are able to detect and quantify these species even at the extremely low concentrations present under realistic reaction conditions. Different types of pulse experiments were performed, during which the highly dispersed, realistic powder catalysts are exposed to very low amounts of reactants, CO and/or O2, in order to form and reactively remove Oact species and gain information on their formation, nature, and the active site for Oact formation. Our investigations have shown that the active oxygen species for CO oxidation on Au/TiO2 for reaction at 80 °C and higher is a highly stable atomic species, which at 80 °C is formed only at the perimeter of the Au-oxide interface and whose reactive removal by CO is activated, but not its formation. From these findings, it is concluded that surface lattice oxygen represents the Oact species for the CO oxidation. Accordingly, the CO oxidation proceeds via a Au-assisted Mars-van Krevelen mechanism, during which surface lattice oxygen close to the Au NPs is removed by reaction with CO, resulting in a partially reduced TiO2 surface, which is subsequently reoxidized by O2. We demonstrate that this is the dominant reaction pathway for Au catalysts based on reducible metal oxides in general, at typical reaction temperatures, while for less active Au catalysts based on nonreducible metal oxides, this reaction pathway is not possible and the remaining activity must arise from another pathway, most probably a Au-only mechanism. At lower reaction temperature, reactive removal of Oact becomes increasingly inhibited, leading to a change in the dominant reaction pathway.

Hydrogen Spillover to Nonreducible Supports

Density functional theory (DFT) calculations show that the interaction between a hydrogen atom and the surface of MgO is weak, the interaction between a hydrogen atom and the surface of SiO2 is repulsive, and the migration of H atoms from a metal particle to a nonreducible support is endothermic. As a consequence, transition-state theory estimates that the migration of a hydrogen atom from a metal particle to the surface of a nonreducible support is too slow to explain the observed hydrogenation of molecules adsorbed on the support by spillover of H atoms. On the other hand, H atoms bind strongly to the surface of WO3, because the H atoms decompose into electrons, which reduce the W6+ cations, and protons, which bind strongly to the oxygen anions. Hydrogen spillover to defect-free surfaces of nonreducible metal oxides cannot take place, but spillover to defects is possible, as is spillover from a metal particle to an oxygen-containing group on a carbon surface. Defects and carbonaceous deposits may theref...

Influence of the Support and the Size of Gold Clusters on Catalytic Activity for Glucose Oxidation

Not all that glitters…︁ The activity of supported gold nanoparticles depends on the method used for their preparation. Gold clusters of about 2 nm in diameter were deposited on nonreducible metal oxides and carbon materials by solid grinding of a volatile organogold complex in a ball mill and subsequent calcination (see scheme). Au/ZrO2 and Au/Al2O3 prepared in this way were extremely efficient catalysts for the aerobic oxidation of glucose.

Fischer–Tropsch synthesis: support, loading, and promoter effects on the reducibility of cobalt catalysts

Temperature programmed reduction (TPR) and hydrogen chemisorption combined with reoxidation measurements were used to define the reducibility of supported cobalt catalysts. Different supports (e.g. Al 2O 3, TiO 2, SiO 2, and ZrO 2 modified SiO 2 or Al 2O 3) and a variety of promoters, including noble metals and metal cations, were examined. Significant support interactions on the reduction of cobalt oxide species were observed in the order Al 2O 3>TiO 2>SiO 2. Addition of Ru and Pt exhibited a similar catalytic effect by decreasing the reduction temperature of cobalt oxide species, and for Co species where a significant surface interaction with the support was present, while Re impacted mainly the reduction of Co species interacting with the support. For catalysts reduced at the same temperature, a slight decrease in cluster size was observed in H 2 chemisorption/pulse reoxidation with noble metal promotion, indicating that the promoter aided in reducing smaller Co species that interacted with the support. On the other hand, addition of non-reducible metal oxides such as B, La, Zr, and K was found to cause the reduction temperature of Co species to shift to higher temperatures, resulting in a decrease in the percentage reduction. For both Al 2O 3 and SiO 2, modifying the support with Zr was found to enhance the dispersion. Increasing the cobalt loading, and therefore the average Co cluster size, resulted in improvements to the percentage reduction. Finally, a slurry phase impregnation method led to improvements in the reduction profile of Co/Al 2O 3.

Selective oxidation of light alkanes: interaction between the catalyst and the gas phase on different classes of catalytic materials

Some aspects concerning the chemistry of oxidative transformation of paraffins catalyzed by different classes of materials are examined. Attention is given mainly to: (i) the role of the redox properties of transition metal oxide-based systems, and (ii) the contribution of radical-type, homogeneous and heterogeneously-initiated homogeneous reactions over nonreducible metal oxide catalysts. Technological solutions aimed at the control of oxidizing properties of the catalyst are also examined.

Near Absence of Support Effects in Toluene Hydrogenation Catalyzed by MgO-Supported Iridium Clusters

Iridium cluster catalysts were prepared by reaction of [Ir4(CO)12] with MgO that had been calcined at 300, 500, or 700°C to vary to degree of hydroxylation, followed by removal of the CO ligands from the iridium by treatment in He. The catalysts were tested for toluene hydrogenation at 60, 80, and 100°C and 1 atm. Extended X-ray absorption fine structure (EXAFS) spectra of each catalyst after 120 h of continuous operation in the absence of deactivation showed that they had changed little, if any, nearly retaining the Ir4tetrahedra of the precursor [Ir4(CO)12]. The catalyst with the most highly hydroxylated support (calcined at 300°C) evidently underwent slight aggregation, giving iridium clusters with nuclearities of about 6±2. Thus the data allow a determination of the influence of the degree of hydroxylation of the support on the catalytic activity in the near absence of metal cluster size effects. The turnover frequency at 60°C decreased from 7.5×10−4to 6.2×10−4s−1as the MgO calcination temperature increased from 300 to 700°C. The change is negligible in view of the experimental errors.These results, combined with similar values of the turnover frequency for toluene hydrogenation catalyzed by γ-Al2O3-supported clusters that were also nearly Ir4, indicate that the effects of changes in nonreducible metal oxide supports are very small. The Ir4clusters are regarded as quasi-molecular in character, with the metal oxide supports being multidentate ligands; the ligand effects are inferred to be similar for the γ-Al2O3and MgO with various densities of surface O and OH groups.

Selective Catalytic Reduction of NO with NH3over Supported Vanadia Catalysts

The selective catalytic reduction (SCR) of NO with NH3was systematically investigated over a series of supported vanadia catalysts to obtain additional insight into this important industrial reaction. The influence of surface vanadia coverage, promoters (surface tungsten oxide, niobium oxide, and sulfate species), and the specific oxide support (TiO2, Al2O3, and SiO2) was examined. The molecular structures of the surface metal oxide species were determined byin situRaman spectroscopy, and the corresponding surface acidity properties were monitored with infrared spectroscopy employing pyridine adsorption. The redox properties of the surface metal oxide species were probed with the sensitive methanol oxidation reaction and temperature-programmed reduction. The SCR reactivity of the various catalysts was determined over a wide temperature range. The current findings suggest that a dual-site (a surface vanadia redox site and an adjacent nonreducible metal oxide site) mechanism is required for the efficient selective catalytic reduction of NO with NH3over supported vanadia catalysts. The SCR reaction is sensitive to the immediate environment of the surface vanadia species: overall surface coverage of the metal oxide overlayer (factor of 5 in turnover frequency), nature of adjacent surface metal oxide species (factor of 10 in turnover frequency) and oxide support ligands (factor of 3 in turnover frequency). The SCR reaction, however, does not appear to depend on the specific structure of the surface vanadia species. The SCR selectivity toward N2formation also varies with the immediate environment of the surface vanadia species. The selectivity depends on the specific oxide support (TiO2>Al2O3>SiO2), temperature (decreases at higher temperature due to oxidation of NH3and NO to N2O), and surface concentration of redox sites (decreases with the concentration of pairs of redox sites). The SCR reaction is not related to the properties of the terminal V=O bond sincein situRaman studies during SCR, employing V=18O, demonstrate that this bond is relatively stable under reaction conditions (possessing a lifetime that is ∼10 times the characteristic reaction time). Thus, the bridging V–O–support bond appears to be involved in the rate-determining step.

Process for recycling spent potliner and electric arc furnace dust into commercial products using oxygen enrichment

This paper discusses the Enviroscience, Inc (ESI) process for recycling hazardous wastes by extractive metallurgy followed by vitrification of non-reducible metal oxides. In this process, volatile and reducible metals are reclaimed as their respective oxides and elements. Non-reducible metal oxides are converted into a man-made vitreous fiber identical in chemical content to mineral wool fiber known as rock or slag wool. The project discussed here involved aluminum plant and steel plant recycling of potliner and EAF dust into commercial products. Potliner (K088) is produced by primary aluminum plants. The annual waste generation rate is 130,000 tons/year. Electric Arc furnace dust (K061) is produced by 76 steel companies and the annual generation rate is estimated to be 600,000 tons/year. An 18-inch I.D. (pilot plant scale) countercurrent mineral wool type shaft furnace was used in the project. The feed was spent potliner and electric arc furnace dust. The pilot plant results were 1) cyanide consumption as a reductant: 96.89% to 99.55%, 2) fluoride utilization: 69.2% to 89.50%, 3) zinc recovery: 96.54% to 97.76%, 4) cadmium recovery: 99.47% to 99.99%, 5) lead recovery: 97.98% to 99.41%, 6) slag (fiber) recovery: 49.33% to 64.31%, 7) iron recovery: 95.12% to 96.93% in the iron and slag (fiber) products. Oxygen enrichment tests conducted by Airco increased the furnace melt rate by 44.8% and reduced coke consumption (energy use) by 41.4%. No detectable levels of NO 2, N 2O 3, H 2S, SO, or SO 2 were found in limited emissions test. Oxygen enrichment resulted in 66.3% fiber production. Analytical results on the enriched zinc oxide iron and fiber products are comparable to prior tests conducted by Enviroscience. As a result of these tests, extractive metallurgy combined with fiber production (vitrification) has now been proven to be a cost-effective recycle process when compared to thermal treatment, secure landfill disposal or standard HTMR processes. The process provides for the production of commercial products from select inorganic liquid, semi-solid, and solid hazardous wastes.

Nature of the metal-support interface in supported metal catalysts: Results from X-ray absorption spectroscopy

X-ray absorption spectra characterizing the metal-support interface in supported metal complexes and supported metal catalysts are summarized and evaluated. Single-metal-atom transition metal complexes on non-reducible metal oxide supports are bonded with metal-oxygen bonds with metal-oxygen distances of approximately 2.15 A; the bonding distance is only weakly sensitive to the oxidation state of the metal. Nearly this same metal-oxygen distance is characteristic of the metal-support interface in metal-oxide-supported metal clusters following high temperature reduction in H2 (HTR:T > 450 °C). The metals at the interface may be polarized sufficiently that they bond with the oxygen of the support much as the cations in mononuclear complexes bond with it. When the supported metals are treated in H2 at low temperatures (LTR:T < 350 °C) or are prepared under He with partially hydroxylated supports, a longer metal-support oxygen distance is observed, typically 2.5–2.7 A. This distance is suggested to characterize interactions between zero-valent metals and support oxygen. Changes in the performance of supported metal catalysts resulting from differences in the temperature of pretreatment in H2 are attributed to changes in the electronic properties and/or morphology of the metal clusters, which are suggested to be related to the concomitant changes in the structure of the metal-support interface.