Meijun Li

Probing defect sites on CeO2 nanocrystals with well-defined surface planes by Raman spectroscopy and O2 adsorption.

Defect sites play an essential role in ceria catalysis. In this study, ceria nanocrystals with well-defined surface planes have been synthesized and utilized for studying defect sites with both Raman spectroscopy and O(2) adsorption. Ceria nanorods ({110} + {100}), nanocubes ({100}), and nano-octahedra ({111}) are employed to analyze the quantity and quality of defect sites on different ceria surfaces. On oxidized surfaces, nanorods have the most abundant intrinsic defect sites, followed by nanocubes and nano-octahedra. When reduced, the induced defect sites are more clustered on nanorods than on nanocubes, although similar amounts (based on surface area) of such defect sites are produced on the two surfaces. Very few defect sites can be generated on the nano-octahedra due to the least reducibility. These differences can be rationalized by the crystallographic surface terminations of the ceria nanocrystals. The different defect sites on these nanocrystals lead to the adsorption of different surface dioxygen species. Superoxide on one-electron defect sites and peroxide on two-electron defect sites with different clustering degree are identified on the ceria nanocrystals depending on their morphology. Furthermore, the stability and reactivity of these oxygen species are also found to be surface-dependent, which is of significance for ceria-catalyzed oxidation reactions.

Synthesis of silica supported AuCu nanoparticle catalysts and the effects of pretreatment conditions for the CO oxidation reaction

Supported gold nanoparticles have generated an immense interest in the field of catalysis due to their extremely high reactivity and selectivity. Recently, alloy nanoparticles of gold have received a lot of attention due to their enhanced catalytic properties. Here we report the synthesis of silica supported AuCu nanoparticles through the conversion of supported Au nanoparticles in a solution of Cu(C(2)H(3)O(2))(2) at 300 °C. The AuCu alloy structure was confirmed through powder XRD (which indicated a weakly ordered alloy phase), XANES, and EXAFS. It was also shown that heating the AuCu/SiO(2) in an O(2) atmosphere segregated the catalyst into a Au-CuO(x) heterostructure between 150 °C to 240 °C. Heating the catalyst in H(2) at 300 °C reduced the CuO(x) back to Cu(0) to reform the AuCu alloy phase. It was found that the AuCu/SiO(2) catalysts were inactive for CO oxidation. However, various pretreatment conditions were required to form a highly active and stable Au-CuO(x)/SiO(2) catalyst to achieve 100% CO conversion below room-temperature. This is explained by the in situ FTIR result, which shows that CO molecules can be chemisorbed and activated only on the Au-CuO(x)/SiO(2) catalyst but not on the AuCu/SiO(2) catalyst.

Support Shape Effect in Metal Oxide Catalysis: Ceria-Nanoshape-Supported Vanadia Catalysts for Oxidative Dehydrogenation of Isobutane

The support effect has long been an intriguing topic in catalysis research. With the advancement of nanomaterial synthesis, the availability of faceted oxide nanocrystals provides the opportunity to gain unprecedented insights into the support effect by employing these well-structured nanocrystals. In this Letter, we show by utilizing ceria nanoshapes as supports for vanadium oxide that the shape of the support poses a profound effect on the catalytic performance of metal oxide catalysts. Specifically, the activation energy of VOx/CeO2 catalysts in oxidative dehydrogenation of isobutane was found to be dependent on the shape of ceria support, rods < octahedra, closely related to the surface oxygen vacancy formation energy and the numbe of defects of the two ceria supports with different crystallographic surface planes.

Ultra-thin PtFe-nanowires as durable electrocatalysts for fuel cells

Ultra-thin Pt(x)Fe(y)-nanowires (Pt(x)Fe(y)-NWs) with a diameter of 2-3 nm were successfully prepared through a solution-phase reduction method at Pt-Fe compositions from 1:1 to 2:1. The carbon supported Pt(x)Fe(y)-NWs (Pt(x)Fe(y)-NWs/C) demonstrated higher oxygen reduction reaction (ORR) activity and better electrochemical durability than conventional Pt/C catalyst. After 1000 cycles of 0-1.3 V (versus RHE), the relative electrochemical surface area (ECSA) of Pt(2)Fe(1)-NW/C dropped down to 46%, which was two times better than Pt/C catalyst, and the mass activity at 0.85 V (versus RHE) for Pt(1)Fe(1)-NW/C was 39.9 mA mg(-1)-(Pt), which is twice that for Pt/C (18.6 mA mg(-1)-(Pt)).

Phase transformation in the surface region of zirconia and doped zirconia detected by UV Raman spectroscopy

The phase evolution of yttrium oxide and lanthanum oxide doped zirconia (Y2O3–ZrO2 and La2O3–ZrO2, respectively) from their tetragonal to monoclinic phase has been studied using UV Raman spectroscopy, visible Raman spectroscopy and XRD. UV Raman spectroscopy is found to be more sensitive at the surface region while visible Raman spectroscopy and XRD mainly give the bulk information. For Y2O3–ZrO2 and La2O3–ZrO2, the transformation of the bulk phase from the tetragonal to the monoclinic is significantly retarded by the presence of yttrium oxide and lanthanum oxide. However, the tetragonal phase in the surface region is difficult to stabilize, particularly when the stabilizer’s content is low. The phase in the surface region can be more effectively stabilized by lanthanum oxide than yttrium oxide even though zirconia seemed to provide more enrichment in the surface region of the La2O3–ZrO2 sample than the Y2O3–ZrO2 sample, based on XPS analysis. The surface structural tension and the enrichment of the ZrO2 component in the surface region of ZrO2–Y2O3 and ZrO2–La2O3 might be the reasons for the striking difference between the phase change in the surface region and the bulk. Accordingly, the stabilized tetragonal surface region can significantly prevent the phase transition from developing into the bulk when the stabilizer’s content is high.

A Raman Spectroscopic Study of the Speciation of Vanadia Supported on Ceria Nanocrystals with Defined Surface Planes

Vanadia (VOx) supported on ceria (CeO2) nanocrystals with defined surface planes, which includes rods, cubes and octahedra, was synthesized and used to explore the effect of support surface structure on the speciation of surface vanadia. The vanadia structures on these ceria “nanoshapes” were identified by in situ visible and UV Raman spectroscopy as a function of loading and calcination temperature, and they include monomeric, dimeric, trimeric, polymeric vanadia, and eventually crystalline V2O5 and CeVO4 as vanadia loading increases. As expected, the faceted ceria nanocrystals provide a rather homogeneous platform for anchoring the vanadia. At low vanadia surface density, only monomeric vanadia exists on the ceria nanoshapes, in contrast to vanadia supported on polycrystalline CeO2 in which multiple vanadia species coexist. Formation of CeVO4 from the reaction between surface vanadia and ceria upon high temperature calcination was compared for the three ceria nanoshapes with similar surface vanadia density (≈1/4 monolayer). It was found that both the surface structure and the amount of defect sites on the ceria nanoshapes play major roles in the production of CeVO4. The easier formation of CeVO4 on ceria rods, compared with cubes or octahedra, is attributed to the rods’ lowest surface oxygen vacancy formation energy and largest amount of defect sites.

The visible luminescent characteristics of ZnO supported on SiO2 powder

In situ laser-induced luminescence spectroscopy is used to study the visible luminescent characteristics of ZnO during the preparation process of ZnO supported on SiO2 by the pyrolysis of different Zn precursors in N2 or O2 atmosphere. The excitation source is 325 nm light, which is above the band gap (3.37 eV) of ZnO. In N2 atmosphere, it is shown that green (centered at ca. 520 nm), yellow (centered at ca. 580 nm) and orange (centered at ca. 640 nm) luminescence bands appear for ZnO produced from zinc acetate, zinc hydroxide and zinc nitrate, respectively. After these samples are treated by O2, green band is changed into yellow band and yellow band is changed into orange band. On the other hand, it is also found that the laser irradiation on the sample could alter the luminescent behavior of ZnO produced at the beginning decomposition temperature of the Zn precursors. While this sample is irradiated, the orange band is gradually changed to a yellow band, the luminescent intensity finally increases more than 30 times that at the beginning of irradiation. However, irradiation hardly affects the luminescent properties of ZnO after calcination above 160 °C. The results indicate that the visible luminescence from ZnO is associated with the oxygen vacancies in ZnO, and the electronic state levels responsible for the visible luminescence bands are changing with the density of oxygen vacancies in ZnO. The green, yellow and orange bands are ascribed to the state of ZnO with high density of oxygen vacancies, with moderate density of oxygen vacancies and with less oxygen vacancies, respectively.

Unusual calcination temperature dependent tetragonal–monoclinic transitions in rare earth-doped zirconia nanocrystals

The dependence of tetragonal-monoclinic (t→m) transitions in the hydrothermally prepared (ZrO2)0.98(RE2O3)0.02 (RE=Sc, Y) nanocrystals upon the calcination between 400–1400°C were explored by means of X-ray diffraction (XRD), high temperature X-ray diffraction (HTXRD), near infrared Fourier transform (FT) Raman and ultraviolet (UV) Raman spectroscopies. XRD and FT Raman scattering analysis indicated that the monoclinic fraction varied discontinuously with increasing the temperature from 400 to 1400°C, which is unusual and is mainly ascribed to the microstrain effect. HTXRD analyses (from room temperature to 1400°C) revealed that the as-prepared samples underwent tetragonal→monoclinic (t→m) transformation at 500°C for (ZrO2)0.98(Sc2O3)0.02 and at 200°C for (ZrO2)0.98(Y2O3)0.02 during the cooling process. UV Raman spectra suggested that the t→m transformation initially occurred at the surface region of the calcined samples. Hence, the monoclinic phase was enriched on the surface of the zirconia grains. Finally, the size effect of the stabilizers (Y3+ and Sc3+) on the critical crystallite size and the metastability of the tetragonal phase were also discussed.

Effect of Dopants on the Adsorption of Carbon Dioxide on Ceria Surfaces

High-surface-area nanosized CeO2 and M-doped CeO2 (M=Cu, La, Zr, and Mg) prepared by a surfactant-templated method were tested for CO2 adsorption. Cu, La, and Zr are doped into the lattice of CeO2, whereas Mg is dispersed on the CeO2 surface. The doping of Cu and La into CeO2 leads to an increase of the CO2 adsorption capacity, whereas the doping of Zr has little or no effect. The addition of Mg causes a decrease of the CO2 adsorption capacity at a low Mg content and a gradual increase at a higher content. The CO2 adsorption capacity follows the sequence Cu-CeO2 >La-CeO2 >Zr-CeO2 ≈CeO2 >Mg-CeO2 at low dopant contents, in line with the relative amount of defect sites in the samples. It is the defect sites on the surface, not in the bulk of CeO2, modified by the dopants that play the vital role in CO2 chemisorption. The role of surface oxygen vacancies is further supported by an in situ IR spectroscopic study of the surface chemistry during CO2 adsorption on the doped CeO2.

Epoxidation of cyclohexene on Ti/SiO2 catalysts prepared by chemical grafting TiCl4 on deboronated silica xerogel

Abstract Ti/SiO 2 (or Ti/de[B]SiO 2 ) catalysts were prepared by grafting deboronated silica xerogel with gaseous TiCl 4 . Using TBHP as oxidant, the Ti/de[B]SiO 2 catalyst shows both catalytic activity and selectivity in epoxidation of cyclohexene better than 80%, and the activity can be comparable with that of Ti-β. The catalytic activity of Ti/de[B]SiO 2 strongly depends on the content of B of support precursor, and the pretreatment temperature of the support. IR studies show that the sites in the deboronated silica xerogel to react with TiCl 4 are not only the silanol nests, but also the defect sites produced during the deboronation.