Roman M. Kenzhin

CO Oxidation over Pd/ZrO2 Catalysts: Role of Support′s Donor Sites

A series of supported Pd/ZrO2 catalysts with Pd loading from 0.2 to 2 wt % was synthesized. The ZrO2 material prepared by a similar technique was used as a reference sample. The samples have been characterized by means of transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), testing reaction of ethane hydrogenolysis (HGE), N2 adsorption, and electron paramagnetic resonance (EPR) spectroscopy. 1,3,5-trinitrobenzene was used as a probe molecule for the EPR spin probe method. The catalytic performance of samples was tested in the model reaction of CO oxidation. It was shown that the concentration of donor sites of support measured by EPR spin probe correlates with catalytic behavior during light-off tests. Low concentration of donor sites on a support’s surface was found to be caused by the presence of the specific surface defects that are related to existence of coordinately unsaturated structures.

Effect of Alumina Phase Transformation on Stability of Low-Loaded Pd-Rh Catalysts for CO Oxidation

Bimetallic Pd-Rh catalysts with precious metal loading of 0.2 wt% was prepared by incipient wetness impregnation of the support (γ-Al2O3 or δ-Al2O3) with dual complex salt [Pd(NH3)4]3 [Rh(NO2)6]2. Monometallic Pd and Rh catalysts as well as its mechanical mixture were used as the reference samples. All samples were exposed for in situ prompt thermal aging procedure, and characterized by EPR spectroscopy, UV–Vis diffuse reflectance spectroscopy and photoluminescence spectroscopy. The nature of the support was found to have strong effect on high temperature stability of the samples. δ-Al2O3 having non-uniform phase structure due to presence of θ-Al2O3 and α-Al2O3 traces causes the concentrating of rhodium near the interphase boundary, thus changing the mechanism of Rh3+ bulk diffusion if compare with γ-Al2O3. No noticeable anchoring effects were observed for bimetallic Pd-Rh samples neither in terms of Rh bulk diffusion nor with regard to the Pd sintering. It has been found experimentally that phase transformation of γ-Al2O3 at high temperatures does not play dramatic role for the deactivation of bimetallic Pd-Rh active species anchored to the electron-donor site of the support.

Carbon nanoreactor for the synthesis of nanocrystalline high-temperature oxide materials

The use of nanocrystalline oxides as precursors for the synthesis of new nanomaterials in which the initial nanoparticle size is preserved is of considerable interest. Here, the major problem is the sintering and growth of the initial nanoparticles at high temperature. One method for solving this problem is the deposition of a coating on the surface of nanoparticles so that it would prevent the sintering of the nanoparticles without hindering their interactions with the molecules of the gas phase and the solid-state transformations inside the shell. This study demonstrates that a carbon coating deposited on the surface of nanocrystalline oxides can be permeable for gaseous reagents and is able to function as a rather firm shell for the nanoreactor, so that the nanoparticles of the oxides under the shell can undergo transformations into nanomaterials of different chemical origins or different phase compositions. The carbon coating prevents the nanoparticles of the solid-state reaction product from sintering and makes it possible to synthesize new nanomaterials, the particle sizes of which are similar to those of the initial nanooxide precursors. This approach was shown to be efficient for the synthesis of finely dispersed oxide materials based on TiO2 and Al2O3.

Nanocrystalline carbon coated alumina with enhanced phase stability at high temperatures

A comparative investigation of the phase stability at high temperatures of nanocrystalline Al2O3 and carbon-coated Al2O3@C systems was performed using a set of physicochemical and spectroscopic methods. The obtained data demonstrate that the carbon coating hinders the sintering of the δ-Al2O3 phase and its transformation to the α-Al2O3 phase at 1250 °C. Without the carbon coating, the δ-Al2O3 sinters and becomes completely converted to corundum at noticeably lower temperatures. The stabilization of the nanosized oxide particles in the Al2O3@C system was shown to be the decisive factor preventing their transformation to the α-Al2O3 phase. The thermal stability of the Al2O3@C samples calcined within a range of 1180–1250 °C in an argon atmosphere followed by the calcination in air to remove the carbon coating was found to exceed that of pure δ-Al2O3. Such samples are characterized by the presence of carbon–alumina interfaces, when carbon is encapsulated in small amounts at the places of contact between the oxide nanoparticles. Such interfaces hinder the sintering of alumina nanoparticles. It is important that the active sites present on the surface of the oxide core in Al2O3@C samples calcined in air are similar to those known for pure alumina. The high concentration of such sites after thermal treatment at elevated temperatures makes this class of materials promising for use as catalysts or catalyst supports capable of operating at high temperatures.

Dehydrochlorination of 1-Chlorobutane Over Nanocrystalline MgO: The Role of Electron-Acceptor Sites

Nanocrystalline MgO aerogels possess high surface areas and outstanding reactivity with respect to variety of harmful organic substances. The catalytic activity of MgO aerogels in dehydrochlorination of 1-chlorobutane substantially increases with time due to partial MgO conversion to MgCl2. This catalytic activity increase and surface area decrease indicate that more active catalytic sites are formed on the surface during this reaction. The catalytic activity was found to increase significantly during the 1-chlorobutane dehydrochlorination reaction, which is accompanied by the MgO modification with chloride ions. The activation energy of this reaction over partially chlorinated MgO was found to be equal to 135 kJ/mol at T < 225 °C and 63 kJ/mol at T > 225 °C. For the first time the concentrations of electron-acceptor sites were measured during a catalytic reaction. No electron-acceptor sites were observed on the surface of initial AP–MgO samples. They appeared only during the reaction. A good correlation was observed between the catalytic activity and the concentration of weak electron-acceptor sites tested using perylene. The obtained results indicate that they may be the active sites accounting for 1-chlorobutane dehydrochlorination in the active state of the catalyst.