The effect of preparation methods on the thermal and chemical reducibility of Cu in Cu-Al oxides

The influence of the preparation methods on the physicochemical properties of Cu and Al oxides was studied. Four different preparation methods were used to synthesize the oxides (catalysts): co-precipitation by the hydrotalcite precursor route (CuAl-HT-c), the solid-state reaction route (CuAl-s), the physical mixing of the oxides (CuAl-o), and individual oxide precipitation followed by physical mixing (CuAl-p). Chemical analysis, X-ray diffraction and textural analysis results of the precursor CuAl-HT-c showed that the respective preparation method is efficient to obtain the hydrotalcite phase with high purity as well as generating a mixed oxide with the greatest surface area among all of the catalysts in this study. The efficacy of this method should be highlighted as it is very difficult to obtain high purity hydrotalcites with copper and aluminum. Furthermore, through the evolution of the FWHM with temperature, this method provided a structurally and texturally more stable mixed oxide. The thermal reducibility of the CuAl-HT-c catalyst was also much higher than that observed in the Cu and Al oxides prepared by other methods. Under an He flow, the CuAl-HT-c catalyst showed a significant reduction of CuO starting at 440 °C, with the formation of Cu2O and metallic Cu, confirmed by XRD and XANES, both in situ. When investigating the effect of the pretreatment gas for this catalyst, H2 reduction occurs faster than with CO, and with less Cu+ formation. Thus, Cu and Al mixed oxides obtained from a precursor with the hydrotalcite structure showed advantages over other methods, presenting unique textural and reducibility properties.

Fourier transform infrared study of low-temperature CO adsorption on CuMgAl-hydrotalcite

Single-phase CuMgAl ternary hydrotalcite with (Cu+Mg)/Al atomic ratio of 3.0 and Cu/Mg atomic ratio of 1.0 was synthesized by coprecipitation. Thermoanalytical studies of this sample showed transformations in three stages in the temperature range up to ca. 900 K yielding mainly CuO phase. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic measurements showed the presence of carbonates even after calcination of the sample at 973 K. The genesis of Cu+ sites during thermal treatment in vacuo at different temperatures for this sample was followed by IR spectroscopy of CO adsorbed at low temperature. Essentially no Cu+ sites are present on a sample calcined at 723 K, consistent with X-ray photoelectron spectroscopic (XPS) data. However, sample subjected to activation (1 h of O2 treatment at 723 K followed by 1 h of evacuation at the same temperature) upon CO adsorption at 85 K unambiguously showed the presence of Cu+ sites. 12CO-13CO coadsorption studies confirmed the presence of dicarbonyls, which are converted to linear Cu+-CO species during evacuation at 85 K. Concentration of the accessible Cu+ sites increased with the increase in activation temperature up to 873 K and decreased with a further temperature rise. The copper sites on the sample are heterogeneously distributed and their distribution depends on the activation temperature. Two routes of reduction of Cu2+ to Cu+ are proposed: (i) autoreduction during evacuation and (ii) reduction by CO.

The role of synchrotron-based studies in the elucidation and design of active sites in titanium-silica epoxidation catalysts

X-ray absorption fine structure (XAFS) measurements unambiguously establish the structure of the active centers in high-performance Ti-SiO2 epoxidation catalysts. Subtleties in structural changes in proceeding from the dispersed titanocene dichloride precursor to the anchored Ti(IV) centers are charted, in situ, by both near-edge (XANES) and extended-edge (EXAFS) measurements. The active centers are tripodally attached titanol groups: (Si-O)3-TiOH. These permit facile expansion of the coordination shell (also monitored by in situ XAFS studies) during catalysis, and thereby lead to the formulation of a plausible mechanism of epoxidation of alkenes that is consonant with recent DFT computations. Previously proposed mechanisms, based on a three-coordinated titanyl group (as well as possible five- or six-coordinated active sites), are discounted. Comparable XAFS studies of soluble catalytically active four-coordinated Ti(IV)-containing molecular entities establish that the heterogeneous Ti-SiO2 catalyst has "single-site" active centers. Equipped with such information, minute changes in hydrophobicity and local composition (e.g., replacement of one Si by a Ge) can be wrought so as to boost the performance of the catalyst. The merits of using combined in situ X-ray absorption spectroscopic and X-ray diffractometric measurements are also highlighted.