Characterization of mordenite-supported Pd, Pt, and Ir determined by CO adsorption microcalorimetry and dehydrogenation reaction of C3 alkanes

Transcription

1 Characterization of mordenite-supported Pd, Pt, and Ir determined by CO adsorption microcalorimetry and dehydrogenation reaction of C3 alkanes Sao Pedro, Brazil 15 th April 2010 Juan Carlos Moreno Piraján Grupo de Sólidos S Porosos y Calorimetría Universidad de los Andes Bogota, Colombia

2 OUTLINE Summary Introduction Methodology Results Conclusions

3 Summary Techniques such as adsorption microcalorimetry and the dehydrogenation of alkanes are presented to measure the differential heats of adsorption on reactive catalyst surfaces. An adsorption microcalorimeter built specifically to determine adsorption heats is employed. CO was employed as the probe molecule in this study and was synthesized with the following catalysts: Pd/mordenite, Pd-Pt/mordenite, and Pd-Ir/mordenite. The results show that differential heat adsorption was between kj/mol. The adsorption heats decrease with an increase in CO for all catalysts. The best conversion for alkene studies was the Pd- Ir/mordenite, which was close to 70%.

4 Introduction

5 Introduction Adsorption microcalorimetry has been widely used to measure the strength with which probe molecules adsorb on a solid catalyst surface NH 3 By contrast, microcalorimetry has been used, to a lesser extent, for probing sites on metal-based catalysts, since reduced metal surfaces might be more susceptible to poisoning by trace oxygenates (e.g. O 2, H 2 O, CO and CO 2 ) present in high-vacuum systems. /?p= rface/research/index To probe the strength of acid sites

6 Introduction New advances in adsorption microcalorimetry have made it possible to measure differential heats of adsorption on metal single crystals, polycrystalline metal films, and metal powders and metal-based catalysts that are reactive towards oxygenates.

7 Introduction In this work, CO was employed as the probe molecule for adsorption studies of mordenite-supported Pd, Pd-Pt and Pd-Ir catalysts by means of microcalorimetry, and attempts have been made to correlate the initial differential heats of adsorption, CO coverage and the changes of CO differential heats with the dehydrogenation performance of C3 alkanes over these catalysts.

8 Experimental

9 Preparation of catalyst* Support: Morderite, specific surface:439 m 2 /g The Pd/mordenite catalyst was prepared by impregnating the mordenite support with an HCl solution of PdCl 2 Dried and calcined * Synthesized by Catalysis group of Chemical Engineering of Universidad Nacional de Colombia

10 Preparation of catalyst Support: Morderite, specific surface:439 m 2 /g The Pd-Pt/mordenite catalyst was prepared by the coimpregnation method. An HCl solution of PdCl 2 and an aqueous solution of PtCl 2 were mixed under an N 2 atmosphere to form the Pd-Pt complex. This Pd-Pt complex was then impregnated onto the mordenite support. Dried and calcined.

11 Preparation of catalyst Support: Morderite, specific surface:439 m 2 /g The Ir-Pt/mordenite catalyst was prepared by the coimpregnation method. An HCl solution of IrCl 3 and an aqueous solution of IrCl 3 were mixed under an N 2 atmosphere to form the Pd-Ir complex. This Pd-Ir complex was then impregnated onto the mordenite support. Dried and calcined.

12 Preparation of catalyst The Pd-Pt/mordenite catalyst precursors were first prepared by impregnating the mordenite support with a PtCl 2 or IrCl 3 aqueous solution, which was then Dried and calcined. In all of the Pd-containing samples, the Pd content was wt%. In the synthesized catalysts, the ratios of Pt (Ir) to Pd are atomic ratios.

13 Measurement of differential heats of CO adsorption The differential heats of CO adsorption were measured using a Tian-Calvet heat flow microcalorimeter built in our laboratory. 1. Precision valves 2. Calibration volume 3. Transducter of full pressure 4. Cold trap 5. Injection gases 6. High-vacuum pump 7. Adsorption microcalorimeter. * Moreno-Piraján Juan Carlos, Giraldo Liliana, Adsorption Science and Technology, ISSN: Vol.27(3), The microcalorimeter is capable of operation at temperatures from 77 K to 573 K.

14 Measurement of differential heats of CO adsorption * Rodriguez Giovanny, Giraldo Liliana, Moreno-Piraján Juan Carlos, Applied Surface Science, ISSN: doi /j.apsusc in press This microcalorimeter is connected, by means of a specially designed set of calorimeter cells, to a volumetric system equipped with a vacuum system (dynamic vacuum of 10 6 Atm), a gas handling system with probe molecule reservoir, and a calibrated dosing volume employing Pfeiffer transducer manometers (± Atm).

15 Measurement of differential heats of CO adsorption The cell calorimetric containing the sample as well as two diffusers is placed along each of the cell stems to minimise convective air currents within the transducer well. The upper portion of cells are fitted with a MDC bellowssealed linear motion feed through fixed to the top of the cells using standard, copper-gasketed, 0.5 inch outside diameter vacuum flanges (MDC). A system of precision valves allows the dosing of the respective amounts of gases.

16 Measurement of differential heats of CO adsorption Before CO adsorption, the catalysts were reduced at 480 C C by H 2 (ordinary pressure, flow rate 30 ml/min) for 1 h. After, the catalyst sample was transferred to the calorimeter, the t system was evacuated to 10-6 Atm. Then 0.65 atm H 2 was admitted into the system. The cell was heated to 480 C, 2 h to reduce the catalyst. The gas in the cell was evacuated and replaced with fresh hydrogen two times during the reduction. Following reduction, the catalyst was outgassed at 480 C, 2 h. The calorimeter thermal block was subsequently raised around the cell l and the system was allowed to equilibrate overnight.

17 Measurement of differential heats of CO adsorption In a typical experiment, a measured mass of sample (typically g) is loaded into the calorimetric cell, followed by treatment with the following gases and reduction to the metallic state in hydrogen at elevated temperatures (e.g. 723 K). After the completion of the treatment cycle, the sample is purged with helium at an elevated temperature to remove the adsorbed gases, and subsequently evacuated to Atm. Then the calorimetric cell is immersed in the isothermal block.. The cells are evacuated to 10 6 Atm and allowed to reach thermal equilibrium with the calorimeter (five to six hours), at which point a stable differential heat response (baseline) is achieved.

18 Measurement of differential heats of CO adsorption The microcalorimetric measurements are initiated when dosages of the adsorbent (dosages between µmol) are sequentially admitted to the sample until it becomes saturated. The resulting heat response for each dose is recorded as a function of time and subsequently integrated to determine the amount of heat generated (mj). The amount of gas adsorbed (µmol/g) is determined volumetrically from dose and equilibrium pressures, and the system volumes and temperatures. The differential heat (kj/mol), is then calculated for each dose by dividing the heat generated by the amount of gas adsorbed.

19 Dehydrogenation of C3 alkanes The dehydrogenation of C3 alkanes (isopropane:: 75 mol%; propane: 25 mol%) was carried out to study the dehydrogenation performance of the catalysts. After reduction at 480 C C for 1 h, the catalysts were used for dehydrogenation reactions under conditions of ordinary pressure at 580 C: H2/C3 alkanes = 1: 1 (mole ratio). The reaction products were analysed by a Perkin Elmer chromatograph equipped with mass spectrometer and thermal conductivity detector and column design specially.

20 Results

21 Differential heats in kj/mol Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite CO covergae in micromol/g Initial heats of kj/mol were obtained for CO adsorption on the 0.85 wt% Pt/SiO2 catalyst, showing excellent agreement with the heats of 140 kj/mol reported in previous research for CO adsorption at 403 K on the Pt/SiO2 catalyst containing 1.2, 4.0 and 7.0 wt% platinum. This figure shows the differential heats of CO adsorption at 403 K on 0.85% Pt/SiO 2 when is employed as standard microcalorimetry.

22 Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite Differential heats in kj/mol The differential heats decreased with adsorbate coverage for both calorimetric methods until the saturation coverage of 35 µmol.g was reached for CO CO covergae in micromol/g This figure shows the differential heats of CO adsorption at 403 K on 0.85% Pt/SiO 2 when is employed as standard microcalorimetry.

23 Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite Differential heats in kj/mol CO covergae in micromol/g The calorimetric technique presented here provided results equivalent to those obtained using standard calorimetric methods for samples that are not particularly sensitive to the trace components typically present in high-vacuum systems. This figure shows the differential heats of CO adsorption at 403 K on 0.85% Pt/SiO 2 when is employed as standard microcalorimetry.

24 Differential heat/kj/mol Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite 140 Pd-Ir/mordenite 120 Pd-Pt/mordenite 100 Pd/mordenite CO coverage/micromol CO/g catalyst The initial differential heat of CO adsorption on the Pd/mordenite catalyst is 140 kj/mol. The differential heat of CO adsorption decreased strongly at the beginning,, and then began to decrease in a buffered manner when the differential heat of CO adsorption was close to 60 kj/mol. The figure show the differential heats of CO adsorption on Pd/mordenite, Pd- Pt/mordenite and Pd-Ir/mordenite at 303 K with 3:1 atomic ratios.

25 Differential heat/kj/mol Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite 140 Pd-Ir/mordenite 120 Pd-Pt/mordenite 100 Pd/mordenite This might be because of the adsorption of CO on weaker sites and/or interaction between adsorbed species. At higher coverages,, the significant decrease in the differential heat of adsorption indicated saturation at the surface CO coverage/micromol CO/g catalyst The figure show the differential heats of CO adsorption on Pd/mordenite, Pd- Pt/mordenite and Pd-Ir/mordenite at 303 K with 3:1 atomic ratios.

26 Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite 140 Pd-Ir/mordenite 120 Pd-Pt/mordenite The addition of Pt and Ir had no significant effects on the initial differential heat. Differential heat/kj/mol CO coverage/micromol CO/g catalyst Pd/mordenite However, the sites corresponding to higher differential heats of CO adsorption were increased,, as manifested by the linear increase of the differential heat of CO adsorption on the Pd- Pt/mordenite catalyst. The figure show the differential heats of CO adsorption on Pd/mordenite, Pd- Pt/mordenite and Pd-Ir/mordenite at 303 K with 3:1 atomic ratios.

27 Differential heat/kj/mol Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite 140 Pd-Ir/mordenite 120 Pd-Pt/mordenite 100 Pd/mordenite CO coverage/micromol CO/g catalyst For the Pd-Ir/mordenite catalyst, the portion of strong CO adsorption present increased at first with increasing of CO coverage, and then decreased remarkably when the differential heat of CO adsorption was lower than 110 kj/mol. The saturation coverage of CO changed dramatically with the adding of Ir to the Pd/mordenite catalyst. The figure show the differential heats of CO adsorption on Pd/mordenite, Pd- Pt/mordenite and Pd-Ir/mordenite at 303 K with 3:1 atomic ratios.

28 Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite The distribution of differential heats of CO adsorption for the Pd-Pt/mordenite and Pt-Ir/mordenite catalysts were different from that of the Pd/mordenite catalyst, showing differential heat/mol and a higher main distribution of differential heats of CO adsorption. These results show that the addition of Pt and Ir caused the adsorption sites of medium differential heat to increase,, with the catalyst prepared by the coimpregnation method,, which resulted in a homogeneous distribution of the surface adsorption energies of Pd.

29 Differential heat, kj/mol Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite Pd-Ir/mordenite Pd-Pt/mordenite The heat adsorption of Pd- Ir/mordenite is more higher that the other catalyst. Pd/mordenite The addition of Pt and Ir had a significant influence on the initial differential heat CO coverage, micromol CO/ g catalyst Differential heats of CO adsorption on Pd/mordenite, Pd-Pt/mordenite and Pd- Ir/mordenite at 303 K with 2:1 atomic ratios

30 Differential heat, kj/mol Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite Pd-Ir/mordenite 80 Pd-Pt/mordenite 60 Pd/mordenite CO coverage, micromol CO/ g catalyst The saturation coverage of CO changed significantly after adding Ir to the Pd/mordenite catalyst. For the Pd-Pt/ Pt/mordenite catalyst with a Pd/Pt atomic ratio of 2:1, 60% of the adsorbed CO gave differential heats of kj/mol, with the maximum 110 kj/mol, and the number of adsorption sites was higher than that on the Pd/mordenite catalyst. Differential heats of CO adsorption on Pd/mordenite, Pd-Pt/mordenite and Pd- Ir/mordenite at 303 K with 2:1 atomic ratios

31 Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite The addition of Pt and Ir to the Pd/mordenite catalyst caused the main distribution of differential heats of CO adsorption to fall into the medium range of differential heats of adsorption. This implies that the Pt and Ir components weakened the strong CO adsorption sites on the Pd surface, and there was a strong interaction between Pt and Ir of the Pd-Pt/ Pt/mordenite.

32 Results: Differentials heats of CO adsorption over Pd/, Pd-Pt/, Pt/, and Pd-Ir/mordenite CO adsorption numbers corresponding to kj/mol differential heats on catalyst Catalyst with different Adsorption numbers /μmol CO/g catalyst Pt(Ir)/Pd atomic ratios 0:1 2:1 6:1 10:1 Pd/mordenite 2.6 Pt-Pd/mordenite 2.6 4, Ir-Pd/mordenite 2.6 4,

33 Results: Relationship between the performance of the dehydrogenation of C3 alkanes and results of CO adsorption calorimetry Conversion C3 alkanes (%) a (Pt)Ir/Pd in atomic ratios Relationship between the performance of the dehydrogenation of C3 alkanes and the Pd surface adsorption sites corresponding to kj/mol differential heats of CO adsorption on Pd-Pt/mordenite and Pd-Ir/mordenite catalysts. (a) Dehydrogenation of C3 alkanes (120 min). (b) Pd surface adsorption sites with kj/mol differential heats of CO. Pd-Pt/mordenite Pd-Ir/mordenite Number of CO adsorption sites b Pt(Ir)/Pd in atomic ratios Pd-Pt Pd-Ir

34 Results: Relationship between the performance of the dehydrogenation of C3 alkanes and results of CO adsorption calorimetry The stabilities of the Pd-Ir/mordenite catalyst first increased, and then decreased with the increasing of the Ir/Pd ratios. Moreover, their stabilities were all better than that of the Pd/mordenite catalyst. The dehydrogenation stability was best when Ir/Pd = 6:1. Pd adsorption sites that yielded kj/mol of differential heats of CO adsorption are the unique catalytic sites for the dehydrogenation of alkanes.

35 Conclusions

36 Conclusions CO adsorption microcalorimetry was employed in the study of mordenite supported Pd, Pd-Pt and Pd-Ir catalysts. The surface adsorption energy was changed by adding Pd or Ir to Pd/mordenite. The distribution of differential heat of CO adsorption on the Pd-Ir/mordenite catalyst was broad and homogeneous. These results indicate that the surface Ir centers with differential heats of kj/mol for CO adsorption possess superior activity for the dehydrogenation of alkanes.

37 Conclusions From the above conclusions we may say that microcalorimetry of CO adsorption is a valuable tool to determine the distribution of surface sites and adsorption modes on metal-supported catalysts.

38 Acknowledgments General San Luis Symposium Organizing Committee: Professor Francisco Zaera (University of California, Riverside, USA) Professor Wilfred T. Tysoe (University of Wisconsin, Milwaukee, USA) Professor Giorgio Zgrablich (Universidad Nacional de San Luis, Argentina) Local Organizer: Professor Pedro A. P. Nascente - Brazilian representative, Universidad Federal de San Carlos, Brazil

39 STUDENTS OF GROUP Juan F. González Diego Blanco Vanessa García Paola Rodríguez Manuel Monroy Diana Vargas Yesid Murillo

40 THE PROFESSORS!!!!!

41 COLOMBIA

42 Cartagena de Indias

43 Cartagena de Indias

44 BOGOTA: 2640 METROS SOBRE EL NIVEL DEL MAR

45 O MAS CAFE SUAVE DO MUNDO

46 Universidad de Los Andes

47 ALBERTO MAGNO

48 THANKS FOR YOUR ATTENTION