C. Salinas-Martínez de Lecea

The effect of oxygen surface groups of the support on platinum dispersion in Pt/carbon catalysts

Abstract A series of Pt catalysts supported on carbon black has been prepared; the original support was treated in such a way as to ensure carbons with the same porous texture but with an increasing amount of oxygen surface groups. Pt catalysts (about 1% loading) were prepared with these carbons using the same experimental procedure and the metal dispersion was calculated from hydrogen chemisorption measurements. Both dispersion and resistance to sintering were found to be a function of the number of oxygen surface groups of the support.

Metal-support interaction in Pt/C catalysts. Influence of the support surface chemistry and the metal precursor

The influence of support surface chemistry and metal precursor species on the properties of Pt/C catalysts has been analyzed. The char of a phenolformaldehyde polymer is the carbon source for obtaining four different supports: three with different degree of surface oxidation and another one modified by ion-exchanged calcium. These supports have been impregnated with aqueous solutions of two platinum precursors with different ionic character: chloroplatinic acid and tetraaminplatinum chloride. The study of the system consists of a preliminary characterization of the supports (surface chemistry and textural properties), an EXAFS analysis of the fresh impregnated catalysts and the determination of platinum dispersion. The platinum precursor-support interaction, established after the impregnation step, and the platinum precursor distribution have been related to the surface chemistry of the supports and the platinum precursor used. The effect of these parameters in the final metal dispersion has also been investigated. The results obtained show that the degree of support surface oxidation has a strong influence on the distribution of the metal precursor on the support and, consequently, on the final platinum dispersion. The surface oxidation of the supports seems to have a negative effect on the platinum dispersion, independently of the platinum precursor used. Thus, the lower the number of surface oxygen complexes, the higher the metal dispersion. The reduction of precursor H2PtCl6 by interaction with the carbon has also been found to depend on the surface chemistry of the supports.

Factors controling the SO2 removal by porous carbons: relevance of the SO2 oxidation step

Activated carbons (AC) and activated carbon fibres (ACF) with different surface chemistry and porosity have been studied to analyse the SO2 retention in presence of O2 at room temperature. Samples surface chemistry was studied using temperature programmed desorption (TPD) and H2 temperature programmed reaction experiments. The porous texture was determined by CO2 and N2 adsorption isotherms at 273 K and 77 K, respectively. SO2 adsorption experiments were performed at 313 K on fresh and heat treated (N2 1173 K) samples using gas mixtures of 2000 ppm SO2 in N2 or 2000 ppm SO2/5% O2 in N2. Adsorption experiments on heat treated samples show that the presence of surface oxygen complexes impedes the SO2 adsorption and its oxidation to SO3. Additionally, no correlation has been found between the amount of SO2 adsorbed and the number of active sites created by the evolution of oxygen complexes during heat treatment. The results obtained have been explained using the fundamentals of gas adsorption for microporous solids. The SO2 uptake is analyzed considering the SO2 oxidation to SO3 as a new variable that is strongly affected by the pore size distribution. An optimum pore size exists (i.e. pore size of about 7A)in which the oxidation of SO2 to SO3 is favoured. A pore width enlargement decreases the conversion of SO2 to SO3 and, thus, the total amount of SO2 retained by the carbon sample.

Effect of carbon support and mean Pt particle size on hydrogen chemisorption by carbon-supported Pt ctalysts

Abstract A series of carbon-supported platinum catalysts were prepared and characterized; the only variable during catalyst preparation was the support used. The influence of different carbon properties on the platinum dispersion is described. The results show that there is not a single carbon property that can be related to dispersion, each one playing a more or less important role. However, the surface area outside the micropores may control the Pt dispersion if the chemical nature of the support is kept constant and vice versa. The large range of platinum particle sizes used in this work—obtained from different carbon supports and sintering treatments—has allowed us to observe that a relationship between platinum particle size and weakly chemisorbed hydrogen exists. The uptake of this kind of hydrogen increases with particle size.

Catalytic NOx reduction by carbon supporting metals

Abstract Catalytic NOx reduction by carbon supporting transition metals (Fe, Co, Ni, Cu) and potassium has been studied. The effect of oxygen on the catalytic properties of the metals has been analyzed. Temperature-programmed reactions and isothermal reactions have been conducted in a fixed bed flow reactor. Temperature-programmed reduction in hydrogen, XRD and XPS have been used to characterize the catalysts. All the metals studied catalyze the NOx reduction by carbon in the presence of oxygen, but also the O2–carbon reaction. Metal catalytic activity is the result of two factors, the tendency of the metal to be oxidized by NO and the easiness of the resulting oxide to be reduced by carbon. Among the metals studied, nickel exhibits the highest selectivity for NOx reduction. The results of this study strengthen the possible benefit of the lack of a gaseous reducing agent (such as ammonia or hydrocarbons) since the reduction of NOx is performed by the carbon support itself.

States of Pt in Pt/C catalyst precursors after impregnation, drying and reduction steps

Abstract The characterization of Pt/C catalysts after impregnation and activation steps has been carried out by using different techniques: TPD; TPR; XPS; EXAFS and H 2 chemisorption. Furthermore, the catalytic activity of the samples has been tested in the cyclohexane dehydrogenation. The catalysts were prepared from a purified peach pit derived carbon and two H 2 O 2 -functionalized supports obtained from it. Results showed that the interaction of H 2 PtCl 6 with the carbon implies a redox process in which after impregnation and drying steps, the metal complex is stabilized as Pt 2+ on the carbon surface. The functionalization treatment seems to have no effect on the resulting state of platinum. During the activation step (thermal treatment in H 2 ), apart from the reduction of Pt species to Pt 0 , catalyzed and non-catalyzed reactions involving CO and CO 2 desorbed from the carbon surface, take place. It was also found that the Pt 2+ species are reduced to zerovalent Pt even by thermal treatment of the dried samples with He. The electronic state of reduced platinum is not modified by the differences in the support surface chemistry.

On the Structure Sensitivity of deNOx HC-SCR over Pt-Beta Catalysts

Abstract In this paper, the integration of activity results for the selective catalytic reduction of NO x with C 3 H 6 with a thorough characterization of Pt-beta catalysts with different platinum dispersion has been accomplished. The parent zeolite NH 4 -beta (Si/Al = 11.4) was ion-exchanged with a Pt(II) precursor and activated by calcination and reduction in H 2 before reaction. The evolution and nature of the Pt phase during the different preparation stages and after catalytic tests were investigated by CO chemisorption, TEM, XRD, and XPS. By using different heating rates during activation of the ion-exchanged material and upon the deNO x HC-SCR, the average particle size of platinum in the final catalyst was varied in the range of 2–25 nm. It has been clearly shown that deNO x HC-SCR conditions produce a decrease in the metal dispersion of the catalysts by sintering of Pt particles. The increase in average particle size has a positive effect on the activity of the catalysts. Thus, the larger the platinum particles, the higher the NO x conversion and the lower the operation temperature. The XPS results show that both Pt(0) and Pt(II) species are present in the calcined samples, after H 2 reduction and during reaction. Coke deposits, formed during reaction on the zeolite support, were studied by XPS, DRIFT, and TPO-TPD/MS. The structure sensitivity of the lean deNO x reaction toward the platinum phase has been confirmed by the direct correlation established between platinum particle size and TOF. Based on previous results on a single crystal, it seems that the key steps are the NO dissociation on Pt(100) planes and PtO clean off which will be easily performed on large Pt particles. On the other hand, the independence of Pt(100)/Pt(111) ratios with particle size explains the similar N 2 and N 2 O selectivity values presented along all the samples.

CO2 chemisorption to characterize calcium catalysts in carbon gasification reactions

CO2 chemisorption, at different temperatures, has been studied in CaO and in the CaO-carbon system by means of simultaneous TG-DTA, MS, and XRD techniques. As a whole the different techniques used show that CO2 chemisorbs in an irreversible manner and is mainly restricted to the surface of the CaO particles, provided that the chemisorption temperature is lower than 573 K. CO2 chemisorption at 573 K can be used as a tool to measure the external surface area of CaO particles. Selective CO2 chemisorption at 573 K has been applied to the CaO-carbon system in the field of catalytic carbon gasification. Several examples are given to show the usefulness of the catalyst surface area determined by CO2 chemisorption in interpreting the catalytic activity of calcium in carbon gasification.

The effects of hydrogen on thermal desorption of oxygen surface complexes

Abstract Temperature programmed desorption/reduction techniques have been applied to an investigation of the interaction of hydrogen with oxygen surface complexes and reactive sites on the surface of a resin char during thermal desorption. It is shown that the presence of hydrogen has a number of significant effects on the evolution patterns of the oxides of carbon and water vapor. A number of conclusions regarding the influence of hydrogen on oxidized carbon surfaces are presented, based upon analyses of the resultant TPD/TPR spectra for these samples. These results suggest that the interaction of hydrogen with oxygen surface complexes during temperature programmed experiments can be a useful “tool” in probing the nature and origin of surface species.

Platinum catalysts supported on graphitized carbon black—I: Characterization of the platinum by titrations and differential calorimetry

Abstract A graphitized carbon black and samples resulting from its oxidation in air at 798 K over different periods of time have been used as supports for platinum catalysts. Platinum dispersion and mean diameter of platinum particles have been determined by O 2 -H 2 , H 2 -O 2 and O 2 -CO titrations and by differential calorimetry of the O 2 -H 2 reaction. Comparison of titration results does not permit an unambiguous choice of Pt O or Pt CO ratios to be used in the titration stoichiometries. However, results from the calorimetric study suggest that the Pt O ratio should be taken as one. Dispersion increases with increasing burn-off of the support, the surface area of the support being the principal factor conditioning the platinum dispersion, although there is also a smaller contribution due to the increase in surface heterogeneity.