C.J.G. van der Grift

Effect of the reduction treatment on the structure and reactivity of silica-supported copper particles

Abstract Silica-supported copper particles of high thermostability have been subjected to oxidation-reduction treatments after which the metal particle size, the surface structure, and the catalytic hydrogenolysis of methyl acetate were investigated. The metal particle size was assessed from the dissociative adsorption of nitrous oxide, X-ray line broadening, and transmission electron microscopy. The surface structure of the copper particles was derived from infrared spectra of adsorbed carbon monoxide. The hydrogenolysis of methyl acetate was used as a structure-sensitive test reaction to illustrate the effect of the surface structure on the activity of the catalyst. The copper particle size is not affected by reduction treatments up to 873 K, whereas the surface structure of the copper particles and thereby the oxygen uptake during dissociative adsorption of nitrous oxide and the activity of the catalyst in the hydrogenolysis of methyl acetate strongly depend upon the temperature and duration of the reduction treatment. Without a change of the copper particle size, prolonged reduction of the catalyst results in more densely packed copper surfaces that are more susceptible to penetration of oxygen during passivation with nitrous oxide and less active in the hydrogenolysis of methyl acetate. The rearrangement of the surface structure of the copper particles is reversible upon repeated oxidation-reduction cycles.

Preparation of silica-supported copper catalysts by means of deposition-precipitation

Abstract Silica-supported copper catalysts have been prepared by means of deposition-precipitation under both atmospheric and hydrothermal conditions. Characterization of the catalyst precursor indicates the formation of a highly dispersed copper hydrosilicate with structural properties similar to the mineral chrysocolla. Increasing the metal loading from 5 to 40 wt.-% causes the specific surface area of the catalyst precursors to rise from 210 to 520 m2/g. The precipitation conditions affect both the reduction behavior and the texture of the catalyst precursors. Hydrothermal synthesis gives rise to formation to chrysocolla-like catalyst precursors, which exhibit a decreased ease of reduction and a higher pore volume, Due to the formation of a copper hydrosilicate of a high specific surface area, this preparation method facilitates the formation of highly dispersed copper-on-silica particles in the reduced catalyst, even at elevated metal loadings. After reduction, the catalysts show a mean metal particle size gradually increasing from 3 to 8 nm as the metal loading increases.

Characterization of silica-supported copper catalysts by means of temperature-programmed reduction

Abstract The effect of the preparation procedure on the reduction behavior of silica-supported copper catalysts was studied. The reduction process was monitored by measurement of the hydrogen consumption during temperature-programmed reduction (TPR). The catalysts were prepared by a variety of methods, viz. impregnation, ion-exchange, homogeneous deposition-precipitation, and co-precipitation. The results show that the reduction behavior of the catalysts strongly depends on the preparation method as well as on the thermal pretreatment. Comparison of the TPR profiles of the catalysts with results obtained on mineral copper hydrosilicates and bulk copper oxide facilitated identification of the copper precursor species present in the catalysts.

Characterization of the surface of a Cu/SiO2 catalyst exposed to NO and CO using IR spectroscopy

Abstract The IR absorbance of adsorbed CO is a suitable probe for evaluating the extent of oxidation of small supported copper particles. The residual absorbance of the main band at about 2120 cm-1 after evacuation of the gas phase and the presence of additional CO absorbance bands indicate the extent of oxidation. Exposure of a reduced catalyst to NO brings about the oxidation of the copper particles. With increasing extent of oxidation the intensity of bands due to adsorbed NO and to adsorbed nitrito and nitrato like species increases. Adsorbed NO already desorbs upon room-temperature evacuation, the nitrito and nitrato like species decompose at temperatures between 423 and 473 K. The formation of N2O (during exposure of NO to a catalyst which is not completely oxidized) and the formation of isocyanate (during subsequent exposure to CO) indicate the removal of weakly adsorbed nitrogen, originating from dissociated NO, from the surface of the copper particles. Upon simultaneous exposure of the reduced catalyst to equimolar amounts of NO and CO at room temperature, a surface reaction proceeds: the copper particles are not severely oxidized and the formation of CO2, N2O and isocyanate is observed. The agreement of the results from IR spectroscopy with those from XPS and anticipated from single-crystal results is good.

THE REDUCTION BEHAVIOUR OF SILICA-SUPPORTED COPPER CATALYSTS PREPARED BY DEPOSITION-PRECIPITATION

Abstract The reduction behaviour of silica-supported copper catalysts prepared by homogeneous deposition-precipitation has been studied by temperature-programmed reduction (TPR), differential thermogravimetry (DTG), in-situ diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy, and evolved gas analysis (EGA). Preparation by homogeneous deposition-precipitation produces catalyst precursors in which the copper ions are highly dispersed over the silica support, and which show reduction behaviour similar to that of the mineral copper hydrosilicate, chrysocolla. Calcination of the catalyst precursors at 700 K leads to decomposition of the copper hydrosilicate, causing subsequent reduction to occur at a lower temperature. During initial reduction of the chrysocolla-like precursors, the consumption of hydrogen and the release of water are not synchronous. Water formed by the reduction is retained in the catalyst, probably due to participation of water in a reorganization of the silica support. During the second reduction stage of silica-supported copper oxide particles, formed by previous reduction and re-oxidation of the chrysocolla-like precursor, the consumption of hydrogen and the release of water do occur simultaneously. Finally, the influence of the composition of the reducing gas atmosphere on the reduction behaviour of the catalysts has been investigated. Increasing the hydrogen concentration in the reducing gas mixture from 5 to 50 vol.% does not affect the on-set of the reduction reaction, whereas addition of about 2.4 vol.% H2O to the reducing gas mixture leads to significantly higher on-set temperatures. Once initiated, the reduction proceeds more rapidly at higher hydrogen partial pressures.

Characterization of copper-silica catalysts by means ofin Situ diffuse reflectance infrared Fourier transform spectroscopy

Copper silica catalyst precursors have been characterized by diffuse reflectance infrared Fourier transform spectroscopy. The measurements were performed both under atmospheric conditions and under controlled gasatmospheres at elevated temperatures. Two reflection bands at 3615 and 690 cm−1 have been attributed to hydroxyl vibrations associated with the presence of copper ions highly dispersed over the silica surface. Calcination of a catalyst precursor at 800 K led to the disappearance of the copper-related surface hydroxyl groups.

The preparation and characterization of silica-supported PtNi catalysts

Abstract In this work we have utilized a relatively simple procedure to produce silica-supported bimetallic Ni Pt catalysts. During deposition-precipitation nickel reacts with the support to form nickel hydrosilicate and simultaneously platinum is adsorbed onto the reacting support. Platinum appears to facilitate the hydrogen reduction of the nickelhydrosilicate. Electron micrographs of reduced and passivated catalysts show the presence of small particles with diameters ranging from 2 to 8 nm depending on the metal loading, a relatively narrow particle size distribution, and crystallites densely and homogeneously dispersed over the support. By infrared experiments we have shown that reduction results in the formation of alloy particles. It is established that the surface composition of the reduced alloy particles is not too different from the bulk deposition. Oxidation leads to a surface enrichment in nickel. The alloys are found to be stable in an oxidizing atmosphere.

The sol—gel preparation of porous catalyst spheres

Abstract A silica sol-gel process was used to prepare porous catalyst spheres. A separately prepared suspension of a silica-supported copper catalyst was added to a silica sol. Droplets of the mixture thus obtained, were sprayed into hot paraffin to facilitate sol—gel conversion. By this procedure porous copper-on-silica hydrogel spheres were prepared. A hydrothermal treatment was used to establish the final texture. The copper-on-silica suspension is homogeneously distributed through the silica gel matrix. The texture of the spheres can be adapted by varying the hydrothermal conditions. Temperature-programmed reduction shows the reduction behavior to depend upon both the pore-size distribution and copper particle size. It appears that upon hydrothermal treatment the copper surface is (partly) covered by silica, which slows down the reduction.

The preparation of silica-supported Pt-Ni alloys by controlled surface reactions

Abstract In this work we have investigated a preparation method for the production of bimetallic platinum-nickel catalysts. Alloy formation is brought about by a reaction between supported nickel or a nickel precursor and a platinum complex in solution. Thus, platinum ions can be reduced by either supported Ni(OH) 2 or by supported metallic nickel. Alloy formation is entirely controlled by the selective reduction reaction, provided conditions are used that eliminate adsorption of the platinum complex onto the bare support. The platinum content of the bimetallic catalyst is determined by the chemical nature and the dispersion of the parent nickel catalyst. Thus variations in the bulk composition of the prepared bimetallic catalysts are expected to be small. Application of the preparation procedure to other bimetallic systems seems possible without great difficulties.

REACTIONS OF SILICA-SUPPORTED COPPER OXIDE AS A REGENERABLE SORBENT FOR FLUE GAS DESULPHURISATION

Abstract Silica-supported copper oxide can be used as a regenerable sorbent for flue gas desulphurisation. At 700 K, interaction of sulphur dioxide and molecular oxygen with copper (oxide) leads to the formation of copper sulphate, while upon regeneration of the sorbent metallic copper is formed with simultaneous release of water and sulphur dioxide. The sulphur dioxide present in the flue gas at low concentrations can be trapped and made available at high concentrations by alternating the reaction with copper oxide in the presence of oxygen with the subsequent reduction of the copper sulphate thus formed. Temperature-programmed sulphation, regeneration and oxidation experiments have been used to study the reactivity of two copper-on-silica sorbents exhibiting different copper particle size distributions. A high dispersion of the copper particles is a prerequisite for a high absorption rate. After a small initial decrease of the SO 2 -absorption rate, the CuU20 sorbent containing small silica-supported copper particles shows a stable activity, whereas the CuI20 sorbent containing large copper particles shows hardly any activity for SO 2 -absorption. As demonstrated by XRD, sulphation results in the formation of anhydrous bulk copper sulphate. Upon regeneration, the bulk of the sulphate particles reacts to metallic copper. Measurements of the total hydrogen consumption during regeneration and subsequent temperature-programmed oxidation suggest the retention of sulphur, probably as a surface cuprous sulphide. The retention of sulphur in the regenerated sorbent decreases the absorption rate and the absorption capacity, whereas the consumption of reducing agent is increased. Some reactions are proposed to explain the formation of a surface copper sulphide.