F. Coloma

Crotonaldehyde hydrogenation over bimetallic PtSn catalysts supported on pregraphitized carbon black. Effect of the preparation method

Three bimetallic PtSn/C catalysts have been prepared by successive impregnation of pregraphitized carbon black with an aqueous solutions of hexachloroplatinic acid and tin(II) chloride. One monometallic Pt/C sample was also prepared and studied for comparison. All catalysts were characterized by hydrogen and carbon monoxide chemisorption at room temperature and X-ray photoelectron spectroscopy and their behaviour in the gas phase hydrogenation of crotonaldehyde, at atmospheric pressure, determined. The amount of surface platinum is greatly reduced by the addition of tin, as deduced from chemisorption measurements and XPS. Both Pt0 and PtII are detected by XPS in the fresh bimetallic catalysts; after reduction in flowing hydrogen at 623 K platinum is completely reduced to the metallic state and, although a high proportion of tin remains in an oxidized state, a relatively important amount is reduced to Sn0, this allowing the possibility of PtSn alloys formation. The catalytic activity in the gas phase hydrogenation of crotonaldehyde is greatly improved by the presence of tin, in spite of the fact that the amount of surface platinum is reduced. Tin has also a very important effect on the selectivity towards the hydrogenation of the CO bond, increasing the production of crotyl alcohol in respect to the hydrogenation of the CC bond that would lead to the production of butyraldehyde. The observed results are explained on the basis of a promoting effect of oxidized tin species for the hydrogenation of the CO group, whereas the formation of a PtSn alloy or the dilution of surface platinum by metallic tin would hinder the hydrogenation of the olefinic CC bond.

Gas phase hydrogenation of crotonaldehyde over Pt/Activated carbon catalysts. Influence of the oxygen surface groups on the support

Abstract Three activated carbons were prepared with different content of oxygen surface complexes and impregnated with aqueous solutions of [Pt(NH 3 4 ]Cl 2 . The catalysts were characterized by H 2 and CO chemisorption at room temperature, temperature-programmed decomposition (TPD) and X-ray photoelectron spectroscopy, and their catalytic behavior in the vapor phase hydrogenations of benzene and crotonaldehyde ( trans -2-butenal) was determined. Metal dispersion is highly dependent on the degree of support oxidation, being lower for the catalyst support containing the highest amount of surface acidic complexes. This is attributed to the decomposition of the surface complexes, which act as anchoring centers for the platinum precursor, upon the reduction treatments at which the catalysts are subjected. The specific catalytic activity in the gas phase hydrogenation of crotonaldehyde is higher as the starting support is more oxidized, and the activity per gram of platinum increases with reduction temperature. The selectivity for the hydrogenation of the carbonilic CO bond instead of the olefinic CC bond is also improved when an oxidized support is used, and the production of the unsaturated alcohol, crotyl alcohol, is enhanced when catalysts are reduced at higher temperature, especially those prepared with oxidized supports.

Determination of the nature and reactivity of copper sites in Cu–TiO2 catalysts

XPS, temperature programmed reduction (TPR) and FTIR spectroscopy of adsorbed CO and the hydrogenation of crotonaldehyde were used to study the nature of the surface species present for two Cu–TiO2 catalysts (2 and 5% Cu) subjected to reduction in hydrogen at 523, 623 and 723 K. Despite TPR and XPS evidence for complete reduction, IR studies gave clear evidence for the presence of surface Cu(II) ions and two distinct types of Cu(I) sites. The large absorption coefficient for CO on the latter meant that spectra were dominated by bands due to CO on Cu+ even though these ions made up less than an estimated 10% of the total exposed copper sites. Exposed Cu+ remained after re-oxidation treatment, at 623 K in oxygen but this treatment, when followed by 523 K reduction, did not recover Cu(0) sites lost by high temperature reduction, indicating that sintering rather than encapsulation by TiOx was responsible for loss of copper surface area.

Crotonaldehyde hydrogenation over alumina- and silica-supported Pt–Sn catalysts of different composition. In situ DRIFT study

Alumina- and silica-supported bimetallic Pt–Sn catalysts with different Pt/Sn atomic ratios have been prepared by using the bimetallic complex cis-[PtCl(SnCl3)(PPh3)2], the monometallic complex cis-[PtCl2(PPh3)2] and SnCl2 as metal precursors. Catalysts have been characterised by X-ray diffraction and transmission electron microscopy techniques, including energy-dispersive X-ray microanalysis, electron diffraction experiments and lattice-fringe microanalysis. The phase composition of the different catalysts has been related to the catalytic behaviour in the vapour phase hydrogenation of crotonaldehyde (but-2-enal). Results show that catalysts containing platinum–tin alloy phases with large particles are more selective towards the hydrogenation of the carbonyl bond to yield crotyl alcohol. The presence of oxidised tin species on the surface of the large alloy particles promotes the hydrogenation of the carbonyl bond. In situ diffuse reflectance infrared spectroscopy studies of crotonaldehyde adsorption under He or H2 reveal differences between bimetallic catalysts and monometallic, and these differences have been related to the different catalytic behaviour of the catalysts.

Gas phase hydrogenation of crotonaldehyde over platinum supported on oxidized carbon black

Abstract The effect of a previous oxidation treatment of the carbonaceous support on the catalytic behavior of platinum in the gas phase hydrogenation of crotonaldehyde (trans-2-butenal) over Pt/carbon black catalysts has been analyzed. Two heat-treated carbon blacks, one of them subjected to an oxidizing treatment with hydrogen peroxide, have been used as supports for platinum, which was introduced using H2PtCl6 as metal precursor. The catalysts were characterized by H2 chemisorption at room temperature and temperature-programmed desorption, and their catalytic behavior in the gas phase hydrogenation of crotonaldehyde was determined under differential conditions. Both the specific catalytic activity and the selectivity towards the formation of the unsaturated alcohol (crotyl alcohol) in the title reaction have been found to be higher for the catalyst with the oxidized support, and they are enhanced after reduction at increasing temperatures.

Improvement of the selectivity to crotyl alcohol in the gas-phase hydrogenation of crotonaldehyde over platinum/activated carbon catalysts

Abstract Pt/activated carbon catalysts were prepared by using [Pt(NH3)4]Cl2 as the metal precursor, and characterized by a number of techniques. The selectivity of these catalysts towards crotyl alcohol in the gas-phase hydrogenation of crotonaldehyde was considerably increased when the activated carbon support was slightly oxidized before the metal incorporation.

Nitrogen complexes formation during NO–C reaction at low temperature in presence of O2 and H2O

Oxygen and nitrogen complexes formation in coal char due to its reaction with NO at 100 °C in presence or absence of O2 and/or H2O was studied by XPS. Formation of NO2 and pyridinic complexes and a considerable increment of oxygen complex was observed. The NO2 complexes can be formed in all cases, even when there is no molecular oxygen present in the gas mixture. The presence of O2 and/or H2O promotes their formation. Apparently, this complex can be formed by the presence of C(O) complexes next to the site capable of chemisorbing NO.

Structural and photoelectrochemical properties of porous TiO2 nanofibers decorated with Fe2O3 by sol-flame

The hybrid structure of Fe2O3 nanoparticles/TiO2 nanofibers (NFs), combines the merits of large surface areas of TiO2 NFs and absorption in ultraviolet light–visible light range. This structure can be used for many applications such as photoelectrochemical water splitting and photo-catalysis. Here, a sol-flame method is used for depositing Fe2O3 on TiO2 NFs that were prepared by hydrothermal on Ti sheets. The obtained materials were characterized by XRD, SEM, UV/Vis diffuse reflectance, Raman, and XPS. The results revealed the formation of rutile and anatase crystalline phases together with Fe2O3. This process moves the absorption threshold of TiO2 NFs support into visible spectrum range and enhances the photocurrent in comparison to bare TiO2 NFs, although no hole scavenger was used. The impedance measurement at low and high frequencies revealed an increase in series resistance and a decrease in resistance of charge transfer with sol-flame treatment time. A mechanism for explaining the charge transfer in these TiO2 NFs decorated with Fe2O3 nanoparticles was proposed.

Influence of the metal precursor on the catalytic behavior of Pt/Ceria catalysts in the preferential oxidation of CO in the presence of H2 (PROX)

The effect of the metal precursor (presence or absence of chlorine) on the preferential oxidation of CO in the presence of H2 over Pt/CeO2 catalysts has been studied. The catalysts are prepared using (Pt(NH3)4)(NO3)2 and H2PtCl6, as precursors, in order to ascertain the effect of the chlorine species on the chemical properties of the support and on the catalytic behavior of these systems in the PROX reaction. The results show that chloride species exert an important effect on the redox properties of the oxide support due to surface chlorination. Consequently, the chlorinated catalyst exhibits a poorer catalytic activity at low temperatures compared with the chlorine-free catalyst, and this is accompanied by a higher selectivity to CO2 even at high reaction temperatures. It is proposed that the CO oxidation mechanism follows different pathways on each catalyst.

Preparation and characterization of carbon-supported Pt-CeO2 catalysts

Abstract Activated carbon-supported cerium dioxide (CeO2/C) has been prepared by impregnation of the support with a solution of cerium (III) nitrate and further decomposition in helium. Then, platinum has been deposited on CeO2/C by impregnation with solutions containing H2PtC16 or [Pt(NH3)4](NO3)2. Sample characterisation has been carried out by thermogravimetry (TG), temperature-programmed decomposition (TPD) and reduction (TPR), X-Ray diffraction and temperature-programmed desorption of adsorbed hydrogen (H2-TPD). The interaction between cerium oxide and platinum after different reduction treatments has been checked in two test reactions carried out in the gas phase: hydrogenation of benzene and hydrogenation of crotonaldehyde (2-butenal). Results show that ceria is very well dispersed on the carbon support, this enhancing its interaction with the active metal after high temperature reduction treatments (773 K).