Highly recyclable Ti0.97Ni0.03O1.97 catalyst coated on cordierite monolith for efficient transformation of arylboronic acids to phenols and reduction of 4-nitrophenol.

A stable Ni2+ substituted TiO2 catalyst (Ti0.97Ni0.03O1.97) has been synthesized by a solution combustion method with an average crystallite size of 7.5 nm. Ti1-xNixO2-x (x = 0.01-0.06) crystallizes in the TiO2 anatase structure with Ni2+ substituted in Ti4+ ion sites and Ni taking a nearly square planar geometry. This catalyst is found to be highly active in the transformation of diverse arylboronic acids to the corresponding phenols. The catalyst coated cordierite monolith can even be recycled for up to 20 cycles with a cumulative TOF of 1.8 × 105 h-1. In scale-up reactions, various phenols are synthesized by employing a single cordierite monolith. It also shows high performance in the reduction of 4-nitrophenol.

Direct evidence of redox interaction between metal ion and support oxide in Ce(0.98)Pd(0.02)O(2-δ) by a combined electrochemical and XPS study.

A combined electrochemical method and X-ray photo electron spectroscopy (XPS) has been utilized to understand the Pd(2+)/CeO(2) interaction in Ce(1-x)Pd(x)O(2-δ) (x = 0.02). A constant positive potential (chronoamperometry) is applied to Ce(0.98)Pd(0.02)O(2-δ) working electrode which causes Ce(4+) to reduce to Ce(3+) to the extent of ~35%, while Pd remains in the +2 oxidation state. Electrochemically cycling this electrode between 0.0-1.2 V reverts back to the original state of the catalyst. This reversibility is attributed to the reversible reduction of Ce(4+) to Ce(3+) state. CeO(2) electrode with no metal component reduces to CeO(2-y) (y~0.4) after applying 1.2 V which is not reversible and the original composition of CeO(2) cannot be brought back in any electrochemical condition. During the electro-catalytic oxygen evolution reaction at a constant 1.2 V for 1000 s, Ce(0.98)Pd(0.02)O(2-δ) reaches a steady state composition with Pd in the +2 states and Ce(4+): Ce(3+) in the ratio of 0.65:0.35. This composition can be denoted as Ce(4+)(0.63)Ce(3+)(0.35)Pd(0.02)O(2-δ-y) (y~0.17). When pure CeO(2) is put under similar electrochemical condition, it never reaches the steady state composition and reduces almost to 85%. Thus, Ce(0.98)Pd(0.02)O(2-δ) forms a stable electrode for the electro-oxidation of H(2)O to O(2) unlike CeO(2) due to the metal support interaction.

Sonochemical synthesis of Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) nanocrystallites: oxygen storage material, CO oxidation and water gas shift catalyst.

Nanocrystalline Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) of ~4 nm sizes were synthesized by a sonochemical method using diethyletriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and transmission electron microscopy (TEM). Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) crystallize in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of smaller Fe(3+) ion in CeO(2), lattice oxygen is activated and 33% Fe substituted CeO(2)i.e. Ce(0.67)Fe(0.33)O(1.835) reversibly releases 0.31[O] up to 600 °C which is higher or comparable to the oxygen storage capacity of CeO(2)-ZrO(2) based solid solutions (Catal. Today 2002, 74, 225-234). Due to interaction of redox potentials of Pd(2+/0)(0.89 V) and Fe(3+/2+) (0.77 V) with Ce(4+/3+) (1.61 V), Pd ion accelerates the electron transfer from Fe(2+) to Ce(4+) in Ce(0.65)Fe(0.33)Pd(0.02)O(1.815), making it a high oxygen storage material as well as a highly active catalyst for CO oxidation and water gas shift reaction. The activation energy for CO oxidation with Ce(0.65)Fe(0.33)Pd(0.02)O(1.815) is found to be as low as 38 kJ mol(-1). Ce(0.67)Fe(0.33)O(1.835) and Ce(0.65)Fe(0.33)Pd(0.02)O(1.815) have also shown high activity for the water gas shift reaction. CO conversion to CO(2) is 100% H(2) specific with these catalysts and conversion rate was found to be as high 27.2 µmoles g(-1) s(-1) and the activation energy was found to be 46.4 kJ mol(-1) for Ce(0.65)Fe(0.33)Pd(0.02)O(1.815).

Structure of Ce(1-x)Sn(x)O(2) and its relation to oxygen storage property from first-principles analysis.

CeO(2)-SnO(2) solid solution has been reported to possess high oxygen storage/release property which possibly originates from local structural distortion. We have performed first-principles based density functional calculations of Ce(1-x)Sn(x)O(2) structure (x=0, 0.25, 0.5, 1) to understand its structural stability in fluorite in comparison to rutile structure of the other end-member SnO(2), and studied the local structural distortion induced by the dopant Sn ion. Analysis of relative energies of fluorite and rutile phases of CeO(2), SnO(2), and Ce(1-x)Sn(x)O(2) indicates that fluorite structure is the most stable for Ce(1-x)Sn(x)O(2) solid solution. An analysis of local structural distortions reflected in phonon dispersion show that SnO(2) in fluorite structure is highly unstable while CeO(2) in rutile structure is only weakly unstable. Thus, Sn in Ce(1-x)Sn(x)O(2)-fluorite structure is associated with high local structural distortion whereas Ce in Ce(1-x)Sn(x)O(2)-rutile structure, if formed, will show only marginal local distortion. Determination of M-O (M=Ce or Sn) bond lengths and analysis of Born effective charges for the optimized structure of Ce(1-x)Sn(x)O(2) show that local coordination of these cations changes from ideal eightfold coordination expected of fluorite lattice to 4+4 coordination, leading to generation of long and short Ce-O and Sn-O bonds in the doped structure. Bond valence analyses for all ions show the presence of oxygen with bond valence approximately 1.84. These weakly bonded oxygen ions are relevant for enhanced oxygen storage/release properties observed in Ce(1-x)Sn(x)O(2) solid solution.

Noble metal ionic catalysts.

Because of growing environmental concerns and increasingly stringent regulations governing auto emissions, new more efficient exhaust catalysts are needed to reduce the amount of pollutants released from internal combustion engines. To accomplish this goal, the major pollutants in exhaust-CO, NO(x), and unburned hydrocarbons-need to be fully converted to CO(2), N(2), and H(2)O. Most exhaust catalysts contain nanocrystalline noble metals (Pt, Pd, Rh) dispersed on oxide supports such as Al(2)O(3) or SiO(2) promoted by CeO(2). However, in conventional catalysts, only the surface atoms of the noble metal particles serve as adsorption sites, and even in 4-6 nm metal particles, only 1/4 to 1/5 of the total noble metal atoms are utilized for catalytic conversion. The complete dispersion of noble metals can be achieved only as ions within an oxide support. In this Account, we describe a novel solution to this dispersion problem: a new solution combustion method for synthesizing dispersed noble metal ionic catalysts. We have synthesized nanocrystalline, single-phase Ce(1-x)M(x)O(2-delta) and Ce(1-x-y)Ti(y)M(x)O(2-delta) (M = Pt, Pd, Rh; x = 0.01-0.02, delta approximately x, y = 0.15-0.25) oxides in fluorite structure. In these oxide catalysts, Pt(2+), Pd(2+), or Rh(3+) ions are substituted only to the extent of 1-2% of Ce(4+) ion. Lower-valent noble metal ion substitution in CeO(2) creates oxygen vacancies. Reducing molecules (CO, H(2), NH(3)) are adsorbed onto electron-deficient noble metal ions, while oxidizing (O(2), NO) molecules are absorbed onto electron-rich oxide ion vacancy sites. The rates of CO and hydrocarbon oxidation and NO(x) reduction (with >80% N(2) selectivity) are 15-30 times higher in the presence of these ionic catalysts than when the same amount of noble metal loaded on an oxide support is used. Catalysts with palladium ion dispersed in CeO(2) or Ce(1-x)Ti(x)O(2) were far superior to Pt or Rh ionic catalysts. Therefore, we have demonstrated that the more expensive Pt and Rh metals are not necessary in exhaust catalysts. We have also grown these nanocrystalline ionic catalysts on ceramic cordierite and have reproduced the results we observed in powder material on the honeycomb catalytic converter. Oxygen in a CeO(2) lattice is activated by the substitution of Ti ion, as well as noble metal ions. Because this substitution creates longer Ti-O and M-O bonds relative to the average Ce-O bond within the lattice, the materials facilitate high oxygen storage and release. The interaction among M(0)/M(n+), Ce(4+)/Ce(3+), and Ti(4+)/Ti(3+) redox couples leads to the promoting action of CeO(2), activation of lattice oxygen and high oxygen storage capacity, metal support interaction, and high rates of catalytic activity in exhaust catalysis.

Pt metal-CeO2 interaction: direct observation of redox coupling between Pt0/Pt2+/Pt4+ and Ce4+/Ce3+ states in Ce(0.98)Pt(0.02)O2-delta catalyst by a combined electrochemical and x-ray photoelectron spectroscopy study.

Pt ions-CeO(2) interaction in Ce(1-x)Pt(x)O(2-delta) (x=0.02) has been studied for the first time by electrochemical method combined with x-ray diffraction and x-ray photoelectron spectroscopy. Working electrodes made of CeO(2) and Ce(0.98)Pt(0.02)O(2-delta) mixed with 30% carbon are treated electrochemically between 0.0-1.2 V in potentiostatic (chronoamperometry) and potentiodynamic (cyclic voltametry) mode with reference to saturated calomel electrode. Reversible oxidation of Pt(0) to Pt(2+) and Pt(4+) state due to the applied positive potential is coupled to simultaneous reversible reduction of Ce(4+) to Ce(3+) state. CeO(2) reduces to CeO(2-y) (y=0.35) after applying 1.2 V, which is not reversible; Ce(0.98)Pt(0.02)O(2-delta) reaches a steady state with Pt(2+):Pt(4+) in the ratio of 0.60:0.40 and Ce(4+):Ce(3+) in the ratio of 0.55:0.45 giving a composition Ce(0.98)Pt(0.02)O(1.74) at 1.2 V, which is reversible. Composition of Pt ion substituted compound is reversible between Ce(0.98)Pt(0.02)O(1.95) to Ce(0.98)Pt(0.02)O(1.74) within the potential range of 0.0-1.2 V. Thus, Ce(0.98)Pt(0.02)O(2-delta) forms a stable electrode for oxidation of H(2)O to O(2) unlike CeO(2). A linear relation between oxidation of Pt(2+) to Pt(4+) with simultaneous reduction in Ce(4+) to Ce(3+) is observed demonstrating Pt-CeO(2) metal support interaction is due to reversible Pt(0)/Pt(2+)/Pt(4+) interaction with Ce(4+)/Ce(3+) redox couple.

Noble metal ionic catalysts: correlation of increase in CO oxidation activity with increasing effective charge on Pd ion in Pd ion substituted Ce1-xMxO2-delta (M = Ti, Zr and Hf).

Pd ion substituted Ce1-xMxO2-delta (M = Ti, Zr, Hf) have been prepared by a single step solution combustion method. Two atom% Pd ion substitution in the title compounds is confirmed by X-ray diffraction (XRD) and Pd ion charge state and redox properties have been determined by X-ray photoelectron spectroscopy (XPS) and H2/TPR studies. While Pd ion in CeO2 (Ce0.98Pd0.02O2-delta) showed higher catalytic activity for CO oxidation than Pd metal impregnated over CeO2, further increase in the catalytic activity is observed with Pd ion in Ce1-xTi(Hf)xO2 and a decrease in the activity with Pd ion in Ce1-xZrxO2. Effective charge on Pd ion could be varied by its substitution in these solid solutions Ce1-xMxO2 (M = Ti, Zr & Hf) and also in TiO2 compared to Pd ion in PdO. Effective positive charge on Pd ion is determined from the core level binding energy shift of Pd(3d5/2) peak with respect to Pd metal. Rate of CO oxidation increased and activation energy decreased with increase in effective charge on the Pd ion in the Pd ion substituted fluorites.

Magneto-structural study of a Cr-doped CaRuO(3).

Temperature-dependent neutron powder diffraction, magnetization and XPS studies were carried out on an optimally Cr-doped CaRuO(3), i.e. CaRu(0.85)Cr(0.15)O(3) (CRC-15). XPS data revealed that Cr exist in 3+ and 6+ oxidation states. The charge dissociation preserves the overall 4+ nominal charge of the Ru site. Although ferromagnetic correlations develop around 100 K, the system exhibits a large coercive field below 50 K. The unit cell volume exhibits negative thermal expansion below 50 K since the lattice expansion due to the magnetostrictive effect outweighs the thermal contraction due to the phonon-driven mechanism.

Local structure of Pt and Pd ions in Ce(1-x)TixO2: x-ray diffraction, x-ray photoelectron spectroscopy, and extended x-ray absorption fine structure.

Ce(1-x-y)Ti(x)Pt(y)O(2-delta) (x=0.15; y=0.01) and Ce(1-x-y)Ti(x)Pd(y)O(2-delta) (x=0.25; y=0.02 and 0.05) are found to be good CO oxidation catalysts [T. Baidya et al., J. Phys. Chem. B 110, 5262 (2006); T. Baidya et al., J. Phys. Chem. C 111, 830 (2007)]. A detailed structural study of these compounds has been carried out by extended x-ray absorption fine structure along with x-ray diffraction and x-ray photoelectron spectroscopy. The gross cubic fluorite structure of CeO(2) is retained in the mixed oxides. Oxide ion sublattice around Ti as well as Pt and Pd ions is destabilized in the solid solution. Instead of ideal eight coordinations, Ti, Pd, and Pt ions have 4+3, 4+3, and 3+4 coordinations creating long and short bonds. The long Ti-O, Pd-O, and Pt-O bonds are approximately 2.47 A (2.63 A for Pt-O) which are much higher than average Ce-O bonds of 2.34 A.

Oxygen-release/storage properties of Ce0.5M0.5O2 (M = Zr, Hf) oxides: interplay of crystal chemistry and electronic structure.

Fluorite-type Ce0.5Zr0.5O2 and Ce0.5Hf0.5O2 have been synthesized by a solution combustion route, and their oxygen release and reduction have been investigated up to 850 degrees C. On reduction, the zirconium system forms two pyrochlore phases, Ce2Zr2O7 (pyrochlore-I) and Ce2Zr2O6.2 (pyrochlore-II), while the hafnium system forms only a disordered fluorite phase with the composition Ce0.5Hf0.5O1.77, under the same experimental conditions. The crystal structures of the reduction products have been characterized by powder X-ray diffraction and Rietveld refinement, and their electronic structures have been investigated by photoelectron spectroscopy and electrical conductivity measurements. Pyrochlore-I (a = 10.6727(4) A) is a semiconductor, while pyrochlore-II (a = 10.6463(8) A) is a good conductor (with a nearly temperature independent resistivity of approximately 2.5 ohm.cm in the 400-1000 K range). X-ray photoelectron spectroscopy (XPS) shows an admixture of Ce(5d,6s) with Zr(4d) and O(2p) and a significant density of states near EF in the highly reduced pyrochlore-II phase. The changes have been rationalized in terms of a qualitative energy band scheme that brings out the special role of zirconium vis-à-vis hafnium in the reduction/oxygen release properties of Ce0.5Zr0.5O2 and Ce0.5Hf0.5O2.

Enhanced reducibility of Ce1-xTixO2 compared to that of CeO2 and higher redox catalytic activity of Ce1-x-yTixPtyO2-delta compared to that of Ce1-xPtxO2-delta.

Nanocrystalline Ce(1)(-)(x)Ti(x)O(2) (0 < or = x < or = 0.4) and Ce(1-)(x)(-)(y)Ti(x)Pt(y)O(2)(-)(delta) (x = 0.15, y = 0.01, 0.02) solid solutions crystallizing in fluorite structure have been prepared by a single step solution combustion method. Temperature programmed reduction and XPS study of Ce(1)(-)(x)Ti(x)O(2) (x = 0.0-04) show complete reduction of Ti(4+) to Ti(3+) and reduction of approximately 20% Ce(4+) to Ce(3+) state compared to 8% Ce(4+) to Ce(3+) in the case of pure CeO(2) below 675 degrees C. The substitution of Ti ions in CeO(2) enhances the reducibility of CeO(2). Ce(0.84)Ti(0.15)Pt(0.01)O(2)(-)(delta) crystallizes in fluorite structure and Pt is ionically substituted with 2+ and 4+ oxidation states. The H/Pt atomic ratio at 30 degrees C over Ce(0.84)Ti(0.15)Pt(0.01)O(2)(-)(delta) is 5 and that over Ce(0.99)Pt(0.01)O(2)(-)(delta) is 4 against just 0.078 for 8 nm Pt metal particles. Carbon monoxide and hydrocarbon oxidation activity are much higher over Ce(1-)(x)(-)(y)Ti(x)Pt(y)O(2) (x = 0.15, y = 0.01, 0.02) compared to Ce(1)(-)(x)Pt(x)O(2) (x = 0.01, 0.02). Synergistic involvement of Pt(2+)/Pt degrees and Ti(4+)/Ti(3+) redox couples in addition to Ce(4+)/Ce(3+) due to the overlap of Pt(5d), Ti(3d), and Ce(4f) bands near E(F) is shown to be responsible for improved redox property and higher catalytic activity.

Various strategies to tune the ionic/electronic properties of electrode materials.

This Perspective highlights, through several snapshot examples, the importance of electrochemically-driven redox reactions in tuning the electronic/ionic as well as magnetic properties of 3d-metal-based inorganic compounds through a careful control of the metal oxidation state. Although such redox reactions usually imply the electron-ionic duality, they can be extended to insulating compounds (LiFePO(4)) or semiconductors (CoO) as long as we can combine electrochemistry at the nanoscale to reduce diffusion and migration limitations, and provide the compounds with electrons through metallic coating techniques. A thorough investigation of the composition-structure-property relationships of the Li(x)CoO(2) system, through the assembly of LiCoO(2)/Li electrochemical cells has led to the identification of the CoO(2) phase, whose property and stability are discussed in terms of cationic-anionic redox competition, thus bearing some similarity with the high T(c) cuprate superconductors. Such a d-sp redox competition could have structural and electronic consequences. Encouraged by the recently reported superconductivity in Na(x)CoO(2);yH(2)O phase, the room temperature Li(x)CuO(2) phase diagram was reinvestigated through Li-driven electrochemical reactions. A solid solution domain was unravelled but superconductivity was not evident. With Cu-based materials such as Cu(2.33)V(4)O(11), we have shown the feasibility of a new reversible Li electrochemically-driven copper extrusion/insertion process, owing to the enhanced copper diffusion within the structure.

Photocatalytic degradation of organic compounds over combustion-synthesized nano-TiO2.

The photocatalytic degradation of various organics such as phenol, p-nitrophenol, and salicylic acid was carried out with combustion-synthesized nano-TiO2 under UV and solar exposure. Under identical conditions of UV exposure, the initial degradation rate of phenol with combustion-synthesized TiO2 is 2 times higher than the initial degradation rate of phenol with commercial Degussa P-25 TiO2. The intermediates such as catechol (CC) and hydroquinone (HQ) were not detected during the degradation of phenol with combustion-synthesized TiO2, while both the intermediates were detected when phenol was degraded over Degussa P-25. This indicates that the rates of secondary photolysis of CC and HQ occur extremely faster than the rates at which they are formed from phenol and further implies that the primary hydroxylation step is rate limiting for the combustion-synthesized TiO2 aided photodegradation of phenol. The degradation rates of salicylic acid and p-nitrophenol were also investigated, and the rates were higher for combustion-synthesized titania compared to Degussa P-25 TiO2. Superior activity of combustion-synthesized TiO2 toward photodegradation of organic compounds can be attributed to crystallinity, higher surface area, more surface hydroxyl groups, and optical absorption at higher wavelength.

Synthesis and structure of nanocrystalline TiO2 with lower band gap showing high photocatalytic activity.

Nanocrystalline TiO2 was synthesized by the solution combustion method using titanyl nitrate and various fuels such as glycine, hexamethylenetetramine, and oxalyldihydrazide. These catalysts are active under visible light, have optical absorption wavelengths below 600 nm, and show superior photocatalytic activity for the degradation of methylene blue and phenol under UV and solar conditions compared to commercial TiO2, Degussa P-25. The higher photocatalytic activity is attributed to the structure of the catalyst. Various studies such as X-ray diffraction, Raman spectroscopy, Brunauer-Emmett-Teller surface area, thermogravimetric-differential thermal analysis, FT-IR spectroscopy, NMR, UV-vis spectroscopy, and surface acidity measurements were conducted. It was concluded that the primary factor for the enhanced activity of combustion-synthesized catalyst is a larger amount of surface hydroxyl groups and a lowered band gap. The lower band gap can be attributed to the carbon inclusion into the TiO2 giving TiO(2-2x)C(x) VO2**.

GaAs Clusters in the Quantum Size Regime: Growth on High Surface Area Silica by Molecular Beam Epitaxy.

Molecular beam epitaxy has been used to grow microcrystalline clusters of gallium arsenide (GaAs) in the size range from 2.5 to 60 nanometers on high-purity, amorphous silica supports. High-resolution transmission electron microscopy reveals that clusters as small as 3.5 nanometers have good crystalline order with a lattice constant equal to that of bulk GaAs. Study of the microcrystallite surfaces by x-ray photoelectron spectroscopy shows that they are covered with a shell (1.0 to 1.5 nanometers thick) of native oxides of gallium and arsenic (Ga(2)O(3) and As(2)O(3)), whose presence could explain the low luminescence efficiency of the clusters. Optical absorption spectra of the supported GaAs are consistent with the blue-shifted band edge expected for semiconductor microcrystallites in the quantum size regime.