N.M. Gupta

Temperature-programmed reduction and CO oxidation studies over Ce–Sn mixed oxides

The reduction behaviour of Ce–Sn mixed oxides has been studied by a temperature-programmed hydrogen reduction technique and compared with that of pure SnO2 and CeO2. The mixed oxides were found to reduce at lower temperature as compared to that of individual oxides. Carbon monoxide oxidation studies showed that mixed oxides have better activity for CO oxidation reaction than the constituent oxides, which is in conformity with their surface reduction behaviour. The improved oxidation activity is attributed to a synergetic effect existing in these mixed oxides.

Interfacial and physico-chemical properties of polymer-supported CdS·ZnS nanocomposites and their role in the visible-light mediated photocatalytic splitting of water

Nano-composite CdSZnS moieties coated over polyester strip were found to exhibit better visible-light-mediated photo-activity for splitting of water, as compared to corresponding pure CdS or ZnS containing coupons. This increase in activity depended upon the mol ratio of the two component sulphides in a particular sample. HRTEM experiments revealed the presence of 1-3 nm size CdS particles embedded over larger size ZnS clusters, the composite samples thus functioning as a highly dispersed guest-host system. In the case of CdS and ZnS dispersed individually over polyester, average crystallite size was found to be around 5 and 15 nm, respectively. A blue shift was observed in the UV-vis absorption spectrum of CdS on addition of ZnS, in conformation with the quantum size effects. Powder XRD, electron diffraction and XPS studies showed that the nanocomposites were comprised of the face-centered cubic (alpha) phases of both CdS and ZnS in a close contact with each other. At the same time, certain solid solution phases, i.e. Cd(1-x)Zn(x)S, were generated at the interfaces of these two semiconductors. Our study demonstrates that the increase in the number of reaction sites due to smaller size of CdS particles and the micro-structural properties associated with the nanostructured CdS or CdS/ZnS interfaces may together play a vital role in the augmented catalytic activity of CdSZnS composite photocatalysts.

FTIR spectroscopic study of the interaction of CO2 and CO2 + H2 over partially oxidized Ru/TiO2 catalyst

At least three distinct linearly bound carbonyl species are identified in the adsorption of CO2 or CO2 + H2 over RuRuOxTiO2 catalyst. The relative concentration and the growth of these species depend on metal oxidation state, presence of hydrogen, reaction temperature, and duration of exposure. The presence of preadsorbed or coadsorbed hydrogen promotes formation of x173-1 type species, the RuOx-(CO)ad species develop only on prolonged exposure to a dose of CO2 or CO2 + H2. The oxygen or the hydrogen ligand bonded to ruthenium facilitates CO bond scission. The widely reported lower temperature requirement for the CO2 methanation reaction as compared to that of CO is attributed to the high reactivity of nascent carbonyl species which give methane directly via “active” carbon formation. As shown earlier (Gupta et al., J. Catal. 137, 437 (1992)), the CO methanation requires multistep transformations, making the process energy intensive, particularly in the 300–450 K temperature range. The studies using 2H and 13C labeled adsorbates helped in the identification of oxygenated surface species having vibrational bands in the 1000–1800 cm−1 region. These species are regarded as inactive side products formed on the support and/or at the Ru-support interfaces.

The entrapment of UO22+ in mesoporous MCM-41 and MCM-48 molecular sieves

A method based on direct template-ion-exchange was employed for the entrapment of UO22+ ions in MCM-41 and MCM-48 molecular sieves via swapping of cetyltrimethylammonium cations present in the mesoporous channels by the UO22+ ions in an aqueous solution. The samples were characterized by XRD, FT-IR, and ICP-AES techniques. The entrapment of UO22+ ions is facilitated by the large pore size vis-a-vis the high surfactant content in the as-synthesized host materials. A higher loading of UO22+ ions was achieved in MCM-48 as compared to MCM-41, which could be attributed to its three-dimensional pore system and higher surfactant-to-silica ratio. FT-IR results provide an evidence of a strong binding of UO22+ groups with the defect silica sites of mesoporous molecular sieves.

The role of nanosized gold particles in adsorption and oxidation of carbon monoxide over Au/Fe2O3 Catalyst

The presence of gold is found to promote the development of weakly bonded (CO)ad species over the surface of Au/Fe2O3 catalyst during interaction with carbon monoxide (CO) or a mixture of carbon monoxide and oxygen. The concentration of these species and the nature of the bonding depend on the gold particle size. No such species are formed for gold particles larger than ∼11 nm or over gold-free iron oxide. The bulk carbonate-like species, formed in the process with the involvement of the hydroxy groups of the support, are merely side products not responsible for the low temperature activity of this catalyst.Thermochemical measurements reveal that the oxidation of carbon monoxide on both Fe2O3 and Au/Fe2O3 occurs via similar redox mechanisms, involving the abstraction and replenishment of lattice oxygen, where the presence of nanosize gold particles promotes these processes. This is attributed to their capacity to adsorb carbon monoxide because of their inherent defective structural sites. It is suggested that the energy that evolves during chemisorption of CO is responsible for the surge in temperature at the Au-Fe2O3 interfaces, which in turn serve as sites for the accelerated reaction between CO and the support. The role of gold particle size is discussed in terms of the effect of geometry of surface metal atoms in the nanosize clusters.

Layered-Double-Hydroxide-Supported Pd(TPPTS)2Cl2: A New Heterogeneous Catalyst for Heck Arylation of Olefins

The Pd(TPPTS)2Cl2 (TPPTS: triphenylphosphine trisulfonate, sodium salt) complex is immobilized on layered double hydroxides by the ion-exchange method. This heterogeneous catalyst is successfully used in Heck arylation of olefins. The catalyst can be recycled several times with almost consistent activity.

Synthesis, characterization, and redox behavior of mixed orthovanadates La1-xCexVO4

A series of mixed orthovanadates with general formula La1−xCexVO4 (0<x<1) were synthesized and characterized using powder XRD and FTIR techniques. The monoclinic phase of LaVO4 was retained for the samples with x≤0.2, while the tetragonal phase of CeVO4 was stabilized in x≥0.5 compositions. On the other hand, the mixed phases of LaVO4 and La0.5Ce0.5VO4 existed for the values of 0.2<x<0.5. The lattice parameters, deduced by indexing of XRD patterns, were found to decrease with the increasing cerium content in the case of both the monoclinic and the tetragonal phases of substituted orthovanadates having single-phase compositions. This trend can be attributed to the presence of Ce in +3 oxidation state and to its smaller ionic size compared to that of La3+. The temperature-programmed reduction/oxidation studies showed that, compared to CeVO4 or LaVO4, the single-phase mixed orthovanadates exhibited a better reproducibility during the repeated cycles of reduction/oxidation.

Synthesis, characterisation, TPR/TPO and activity studies on LaMnxV1-xO4-δ catalysts

Abstract LaMnxV1−xO4−δ samples (0≤x≤1) were synthesised and characterised using XRD and FTIR techniques, while their oxygen content, reducibility, stability and ionic mobility were investigated by TPR, TPO and TPD methods. All mixed oxides were found to be of single phase; they were stable in air and exhibited reproducible TPR/TPO patterns over consecutive six to seven cycles. The doping of Mn resulted in lowering of reduction and oxidation temperatures. The substitution of B cations with Mn4+ also facilitated CO oxidation reaction in a lower temperature range of 200–500°C, while LaVO4 or LaMnO4−δ showed very poor catalytic activity. The XRD powder patterns of all the Mn-doped samples revealed a redox behaviour, i.e. conversion of LaVO4 to LaVO3 and back to LaVO4, on subjecting a sample to H2-reduction and re-oxidation treatments. Our results substantiate the important role of oxygen vacancies generated in the crystal lattice on introduction of Mn4+, such vacancies in turn lead to improved catalytic activity because of a greater oxygen mobility in the lattice. An ionic redox mechanism, where the re-oxidation step is a limiting process, is applicable to CO oxidation at temperatures above 200°C.

The Role of Nanosize Particles of Uranium Oxide in the Adsorption/Reaction of Methanol Over U3O8/MCM-48: FTIR Study

The surface species formed over MCM-48, U3O8 and U3O8/ MCM48 catalysts during the adsorption/reaction of methanol were monitored using FTIR spectroscopy, in order to get an insight into the high catalytic activity exhibited by the nanosize crystallites of uranium oxide dispersed in MCM-48. The results of this in situ study revealed that the title catalysts exhibited a distinct behavior for adsorption and subsequent reaction of methanol. Thus, while the room temperature adsorption over bulk U3O8 resulted in the formation of formate complex and oxymethylene species, the interaction over MCM-48 resulted in simultaneous and instant formation of surface methoxy groups and dimethyl ether. On the other hand, the exposure of methanol over U3O8/MCM-48 under similar conditions resulted in the appearance of intense IR bands due to surface-adsorbed (–OCH2)n species, where n≥1, in addition to those of formate complexes, oxymethylene and methoxy groups. The role of the above-mentioned intermediate species in the formation of different reaction products is discussed in brief.

Effect of hydrogen reduction on the CO adsorption and methanation reaction over Ru/TiO2 and Ru/Al2O3 catalysts

The C–O stretch vibrational bands developed over Ru/TiO2 and Ru/Al2O3 catalyst surfaces during adsorption of CO at different temperatures were investigated as a function of H2 pretreatment given to a sample in the temperature range of 575–875 K. While the reduction at temperatures below 675 K had no significant effect, the higher temperature H2 pretreatment resulted in the progressive annihilation of νCO bands in the 2050–2145 cm−1 region, identified with the multicarbonyl species bonded to Ru sites of different oxidation states. The removal of these bands showed a parallelism with the loss of catalyst activity at reaction temperatures below 500 K, whereas the activity at the temperatures above 550 K remained almost unaffected. We conclude that the Ru(CO)n species are formed over highly dispersed metal surfaces and are responsible to the low temperature catalytic activity. On the other hand, the catalytic activity at higher reaction temperatures is attributed to certain monocarbonyl moieties, the formation of which is rather independent of metal dispersion. Our results also reveal the occurrence of the reductive surface agglomeration of Ru metal at high temperatures and the reformation of the smaller crystallites on subsequent exposure of the catalyst to O2.