Wen-Sheng Xia

Structures and thermal properties of strontium and barium 1,3-propanediaminetetraacetates

In neutral and acidic solution, homonuclear s-block metal complexes [Sr2(1,3-pdta)(H2O)3.5] n (1), {[Ba2(1,3-pdta)(H2O)6]·H2O} n (2), {[Sr(1,3-H2pdta)]·(H2O)} n (3), and [Ba(1,3-H2pdta)(H2O)3] n (4) {1,3-H4pdta=1,3-propanediamine-N,N,N′,N′-tetraacetic acid, CH2[CH2N(CH2CO2H)2]2} were isolated. In 1 and 2, hexadentate 1,3-pdta joins two metal ions via the diamine chain. In 3 and 4, the nitrogens of 1,3-H2pdta were protonated and show no coordination. There is no coordinated water in 3, unusual coordination for strontium. In 4, the coordination number is nine and there are three coordinated waters for each barium. One carboxy group of pdta is free without coordination. Thermal decomposition shows that temperatures of ligand elimination start at 408, 423, 298, and 250 °C for 1–4, respectively. Acidic condition is favorable for preparation of metal oxide at low temperature.

Effects of Polyvinylpyrrolidone on the Preparation of Supported La2O3 Catalysts by a Modified Impregnation Method for the Oxidative Coupling of Methane

The direct transformation of methane, such as oxidative coupling of methane (OCM), is a long‐standing challenge in methane utilization. Keeping this objective in mind, we explored the dispersion and acid–base effects of the catalysts on OCM. Herein, we prepared a series of La2O3/MgO catalysts by using a modified impregnation method with polyvinylpyrrolidone added to impregnated solutions. The BET, XRD, SEM, and TEM characterizations of the samples confirm that La2O3 dispersion on MgO increases with the addition of polyvinylpyrrolidone and the particles become smaller in size and more uniform in distribution. High catalytic performances (e.g., the C2 yield ≈16 %) of the well‐dispersed La2O3/MgO catalysts for OCM are demonstrated at a lower temperature (e.g., 550 °C) and a lower loading (e.g., 1.9 wt %). In addition, with the modification of acid–base properties of the supports, we find that base properties of the supports are responsible for C2 formation and acidic properties of the supports are responsible for COx formation. These properties establish a good correlation of the dispersion and basicity of the catalysts with the catalytic performances of the catalysts for OCM.

DEVELOPING SELECTIVE OXIDATION CATALYSTS OF LIGHT ALKANES: FROM FUNDAMENTAL UNDERSTANDING TO RATIONAL DESIGN

Selective oxidation of light alkanes remains to be a great challenge for the wider use of alkanes as feedstocks. To achieve high activity and at the same time high selectivity, some key issues have to be addressed: (1) the stability of the desired products with respect to the reactants; (2) the roles of the active components in the catalysts, the structure and the functionality of the active centers; (3) the reducibility of the metal cations, the Lewis acid sites and their synergic effects with the basic sites of the lattice oxygen anions; (4) spatial isolation of the active centers; and (5) the mechanisms for the formation and transformation of the intermediates and their kinetic controls. In this contribution, we took selective oxidation of propane to acrolein as our target reaction, and reviewed mainly our own work, trying to provide some thinking and answers to these five questions.

In Situ Raman and Pulse Reaction Study on the Partial Oxidation of Methane to Synthesis Gas over a Pt/Al2O3 Catalyst

Catalytic partial oxidation of methane (POM) to synthesis gas (syngas) over Pt/Al(2)O(3) was investigated by in situ microprobe Raman and pulse reaction methods with attention focused on the mechanism of syngas formation in the oxidation zone (i.e., the catalyst zone in which O(2) was still available in the reaction feed). It was found that the amount of platinum oxide in the catalyst under POM conditions was below the detection level of Raman spectroscopy. Raman bands of carbon species that originated from methane dissociation were detected at the entrance of the catalyst bed under working conditions. The results of the pulse reaction study on POM as well as steam and CO(2) reforming of methane at 700 °C with a contact time of less than 1 ms over the catalyst suggest that pyrolysis of methane on reduced platinum sites followed by coupling of two surface hydrogen atoms to H(2) and partial oxidation of surface carbon species to CO are the major reactions responsible for syngas formation in the oxidation zone. Under the experimental conditions, steam and CO(2) reforming of methane play only a minor role in syngas formation in the same reaction zone. The contribution of the last two reactions increases with increasing contact time.

Pathways between superoxide and peroxide species on small La-O clusters

National Basic Research Program of China (973 Program) [2010CB732303]; National Natural Science Foundation of China [21033006, 21373169, 20373054]; Program for Changjiang Scholars and Innovative Research Team in the University [IRT1036]

Highly dispersed CoOx in layered double oxides for oxidative dehydrogenation of propane: guest–host interactions

Co–Al layered-double-oxides (LDOs) derived from inorganic anion (nitrate, carbonate, sulfate and phosphate) pillared Co–Al layered-double-hydroxides (LDHs) were synthesized. These LDO samples exhibited impressive catalytic performances in oxidative dehydrogenation of propane (ODHP) at low temperatures. Characterization techniques such as XRD, TEM, H2-TPR and O2-TPD were used to measure the physicochemical properties of the samples. The results showed that the LDO samples consisted of homogeneously mixed cobalt spinel oxides (Co2AlOx) containing both Co(III) and Co(II). Co(III) instead of Co(II) was linked with the active sites for ODHP at low temperatures. The sulfate and phosphate anions were preserved in the corresponding LDO samples, unlike the nitrate and carbonate pillars. Sulfate (phosphate) around the cobalt decreased the size of the spinel oxide particles more relative to nitrate and carbonate, and so the dispersion of CoOx in the LDO and the selectivity to propylene in ODHP over these catalysts were improved. Both surface adsorbed oxygen species and surface lattice oxygen species were significantly found over the phosphate-pillared LDO, but only lattice oxygen species were available over the sulfate-pillared LDO. Different mechanisms of propane activation were exhibited over these LDO samples for ODHP.

18O isotopic study of photo-induced formation of peroxide species on cubic Nd2O3

Abstract Photo-induced formation of peroxide species on cubic Nd 2 O 3 was studied by in situ microprobe Raman spectroscopy using 18 O as a tracer and a 325-nm laser as an excitation source. The results confirmed that the peroxide ions were formed through photooxidation of the lattice oxygen species in neodymium sesquioxide by molecular oxygen species. Under UV excitation (λ = 325 nm), the reaction between O 2 and O 2− could take place at room temperature. A fast oxygen exchange between the peroxide ions and the lattice oxygen species in Nd 2 O 3 took place under the experimental conditions studied. Also, bulk lattice oxygen species in Nd 2 O 3 could migrate to the surface layer and participate in the formation of peroxide ions. The migration of lattice oxygen species and the oxygen exchange between lattice oxygen and peroxide ions were promoted by UV laser irradiation.