characterization of mordenite-supported pd, pt, and ir...

Characterization of Characterization of mordenitemordenite--supported Pd, supported Pd, Pt, and Pt, and IrIr determined by CO adsorption determined by CO adsorption microcalorimetrymicrocalorimetry and dehydrogenation and dehydrogenation

reaction of C3 reaction of C3 alkanesalkanes

Juan Carlos Moreno Juan Carlos Moreno PirajPirajáánnGrupo de SGrupo de Sóólidos Porosos y Calorimetrlidos Porosos y Calorimetrííaa

Universidad de los AndesUniversidad de los AndesBogota, ColombiaBogota, Colombia

Prof. Juan Carlos Moreno-Piraján GGrupo de Investigación en Sólidos Porosos y Calorimetría

Sao Pedro, Brazil 15th April 2010

OUTLINEOUTLINE

SummarySummary

IntroductionIntroduction

MethodologyMethodology

ResultsResults

ConclusionsConclusions



SummarySummaryTechniques such as adsorption microcalorimetry and the dehydrogenation of alkanes are presented to measure thedifferential 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 between50–150 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%.

IntroductionIntroductionAdsorption Adsorption microcalorimetrymicrocalorimetry has been widely used to measure the has been widely used to measure the strength with which probe molecules adsorb on a solid catalyst strength with which probe molecules adsorb on a solid catalyst surface surface

NHNH33

http://www.iap.tuwien.ac.at/www/surface/research/index

http://jcwinnie.biz/wordpress/?p=1935

To probe the strength of acid sites

By contrast, microcalorimetryhas 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. O2, H2O, CO and CO2) present in high-vacuum systems.



IntroductionIntroductionNew advances in adsorption New advances in adsorption microcalorimetrymicrocalorimetry have have made it possible possible to measure differential heats of adsorption on metal single crysto measure differential heats of adsorption on metal single crystals, tals, polycrystalline metal films, and metal powders and metalpolycrystalline metal films, and metal powders and metal--based based catalysts that are reactive towards oxygenates.catalysts that are reactive towards oxygenates.


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.


ExperimentalExperimental


Preparation of catalyst*Preparation of catalyst*

SupportSupport: : MorderiteMorderite, , specificspecific surfacesurface::439 m439 m22/g/g

The Pd/mordenite catalyst was prepared byimpregnating the mordenite support with an HClsolution of PdCl2

Dried and calcined

* Synthesized by Catalysis group of Chemical Engineering of Universidad Nacional de Colombia



Preparation of catalystPreparation of catalystSupportSupport: : MorderiteMorderite, , specificspecific surfacesurface::439 m439 m22/g/g

The Pd-Pt/mordenite catalyst was prepared by the co-impregnation method. An HCl solution of PdCl2 and an aqueous solution of PtCl2 were mixed under an N2

atmosphere to form the Pd-Pt complex. This Pd-Pt complex was then impregnated onto the mordenitesupport.

Dried and calcined.

Preparation of catalystPreparation of catalystSupportSupport: : MorderiteMorderite, , specificspecific surfacesurface::439 m439 m22/g/g

The Ir-Pt/mordenite catalyst was prepared by the co-impregnation method. An HCl solution of IrCl3 and an aqueous solution of IrCl3 were mixed under an N2atmosphere to form the Pd-Ir complex. This Pd-Ircomplex was then impregnated onto the mordenitesupport.


Dried and calcined.

Preparation of catalystPreparation of catalystThe Pd-Pt/mordenite catalyst precursors were first prepared by impregnating the mordenite support with a PtCl2 or IrCl3 aqueous solution, which was then

Dried and calcined.

In all of the Pd-containing samples, the Pd content was 0.765 wt%. In the synthesized catalysts, the ratios of Pt (Ir) to Pd are atomic ratios.



Measurement of differential heats of CO Measurement of differential heats of CO adsorptionadsorption

The differential heats of CO adsorption were measured using aThe differential heats of CO adsorption were measured using aTian-Calvet heat flow microcalorimeter built in our laboratorybuilt in our laboratory. .

1. Precision valves2. Calibration volume3. Transducter of full pressure4. Cold trap5. Injection gases6. High-vacuum pump7. Adsorption microcalorimeter.

The microcalorimeter is capable of operation at temperatures from 77 K to 573 K.

* Moreno-Piraján Juan Carlos, Giraldo Liliana, Adsorption Science and Technology, ISSN: 0263-6174.2009 Vol.27(3), 255-265


This microcalorimeter is connected, by means of a specially designed set ofcalorimeter cells, to avolumetric system equipped with a vacuum system(dynamic vacuum of 10–6

Atm), a gas handling systemwith probe moleculereservoir, and a calibrated dosing volume employing Pfeiffer transducer manometers (± 1 × 10–4 Atm).


* Rodriguez Giovanny, Giraldo Liliana, Moreno-Piraján Juan Carlos, “Applied Surface Science, ISSN: 0169-4332. doi. 10.1016/j.apsusc.2009.12.107. in press 2010.


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 bellows-sealed 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.




Before CO adsorption, the catalysts were Before CO adsorption, the catalysts were reduced reduced at 480at 480°°C by HC by H22(ordinary pressure, flow rate 30 (ordinary pressure, flow rate 30 mLmL/min) for 1 h. /min) for 1 h.

After, the catalyst sample was transferred to the calorimeter, tAfter, the catalyst sample was transferred to the calorimeter, the system he system waswas evacuated to 10to 10--6 6 Atm.Atm.

Then 0.65 Then 0.65 atmatm HH22 waswas admitted into the system. into the system.

The cell wasThe cell was heated heated to 480to 480°°C, 2 hC, 2 h to reduceto reduce the catalyst.the catalyst.

The gas in the cell was evacuated and replaced with fresh hydrogThe gas in the cell was evacuated and replaced with fresh hydrogen two en two times during the reduction. times during the reduction.

Following reduction, the catalyst wasFollowing reduction, the catalyst was outgassed at 480at 480°°C, 2 h. The C, 2 h. The calorimeter thermal block was subsequently raised around the celcalorimeter thermal block was subsequently raised around the cell and l and the system was allowed to equilibrate overnight.the system was allowed to equilibrate overnight.


In a typical experiment, a measured mass of sample (typically 0.5–2.0 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 purgedwith helium at an elevated temperature to remove the adsorbedgases, and subsequently evacuated to 1 × 10-6 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.



The microcalorimetric measurements are initiated when dosages of the adsorbent (dosages between 10–30 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 volumetricallyfrom dose and equilibrium pressures, and the system volumes and temperatures.

The differential heat (kJ/mol), is then calculated for each dose bydividing the heat generated by the amount of gas adsorbed.


Dehydrogenation of C3 Dehydrogenation of C3 alkanesalkanes

TheThe dehydrogenation of C3 of C3 alkanesalkanes ((isopropaneisopropane: 75 mol%; : 75 mol%; propane: 25 mol%) was carried out to study the dehydrogenation propane: 25 mol%) was carried out to study the dehydrogenation performance of the catalysts. performance of the catalysts.

After reduction at 480After reduction at 480°°C for 1 h, the catalysts were used forC for 1 h, the catalysts were used fordehydrogenation reactions under conditions of ordinary under conditions of ordinary pressure at 580pressure at 580˚̊C: H2/C3 C: H2/C3 alkanesalkanes = 1: 1 (mole ratio).= 1: 1 (mole ratio).

The The reaction products were analysed by a Perkin Elmer were analysed by a Perkin Elmer chromatograph equipped with mass spectrometer and thermal equipped with mass spectrometer and thermal conductivity detector and column design specially.conductivity detector and column design specially.


ResultsResults



Results: Differentials heats of CO adsorption Results: Differentials heats of CO adsorption over Pd/, Pdover Pd/, Pd--Pt/, and PdPt/, and Pd--Ir/mordeniteIr/mordenite

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CO covergae in micromol/g

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This figure shows the differential heats of CO adsorption at 403 K on 0.85% Pt/SiO2 when is employed as standard microcalorimetry.

InitialInitial heats of 137.67 heats of 137.67 kJ/molkJ/mol were obtained for were obtained for CO adsorption on the 0.85 CO adsorption on the 0.85 wt% Pt/SiO2 catalyst, wt% Pt/SiO2 catalyst, showing excellent showing excellent agreement with theagreement with the heats of heats of 140 kJ/mol140 kJ/mol reported in reported in previous research for CO previous research for CO adsorption at 403 K on the adsorption at 403 K on the Pt/SiO2 catalyst containing Pt/SiO2 catalyst containing 1.2, 4.0 and 7.0 wt% 1.2, 4.0 and 7.0 wt% platinum. platinum.



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TheThe differential heats differential heats decreaseddecreased with with adsorbateadsorbatecoverage for both coverage for both calorimetric methods until calorimetric methods until the saturation coverage of the saturation coverage of 35 35 µµmol.gmol.g––1 1 was reached for was reached for CO.CO.



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The calorimetric technique The calorimetric technique presented herepresented here provided provided results equivalent to those results equivalent to those obtained using standard obtained using standard calorimetric methodscalorimetric methods for for samples that are not samples that are not particularly sensitive to the particularly sensitive to the trace components typically trace components typically present in highpresent in high--vacuum vacuum systems.systems.


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.

The initial differential heat The initial differential heat of CO adsorption on the of CO adsorption on the Pd/Pd/mordenitemordenite catalyst is 140 catalyst is 140 kJ/mol.kJ/mol.


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Pd/mordenite

The differential heat of CO The differential heat of CO adsorption adsorption decreased strongly at the beginning, and then , and then began to decrease in a began to decrease in a buffered manner when the differential heat of CO adsorption was of CO adsorption was close to 60 kJ/mol.close to 60 kJ/mol.





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Pd/mordenite

This might be because of the This might be because of the adsorption of CO on weaker adsorption of CO on weaker sites and/or interaction sites and/or interaction between adsorbed species.between adsorbed species.

At higher At higher coveragescoverages, the , the significant decrease in the significant decrease in the differential heat of adsorption differential heat of adsorption indicatedindicated saturation at the surface.





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TheThe addition of Pt and Irhad no significant effects on on the initial differential heat.the initial differential heat.

However, the sites However, the sites corresponding tocorresponding to higher differential heats of CO adsorption were increased, as , as manifested by the linear manifested by the linear increase of the differential heat increase of the differential heat of CO adsorption on the of CO adsorption on the Pd-Pt/mordenite catalyst.catalyst.





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Pd/mordenite

For theFor the Pd-Ir/mordenitecatalyst, the portion of strong catalyst, the portion of strong CO adsorption presentCO adsorption presentincreased at first with increasing of CO coverage, , and then decreased and then decreased remarkably when the remarkably when the differential heat of CO differential heat of CO adsorption was lower than 110 adsorption was lower than 110 kJ/mol. kJ/mol.

TheThe saturation coverageof CO changed dramatically of CO changed dramatically with thewith the adding of Ir to the Pd/mordenite catalyst.catalyst.

The The distribution of differential heats of CO adsorption for the of differential heats of CO adsorption for the Pd-Pt/mordenite and and Pt-Ir/mordenite catalysts were different catalysts were different from that of the Pd/from that of the Pd/mordenitemordenite catalyst, showing catalyst, showing differential differential heat/molheat/mol and aand a higher main distribution of differential heats of CO adsorption.

These results show that theThese results show that the addition of Pt and Ir caused thecaused theadsorption sites of medium differential heat to increase, with the catalyst prepared by the, with the catalyst prepared by the co-impregnation method, which resulted in a , which resulted in a homogeneous distribution of the surface adsorption of the surface adsorption energies of Pd. energies of Pd.




Differential heats of CO adsorption on Pd/mordenite, Pd-Pt/mordenite and Pd-

Ir/mordenite at 303 K with 2:1 atomic ratios

The heat adsorption of PdThe heat adsorption of Pd--Ir/mordeniteIr/mordenite is more higher is more higher that the other catalystthat the other catalyst..


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CO coverage, micromol CO/ g catalyst

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Pd/mordenite The addition of The addition of Pt and Pt and IrIrhad a significant influence had a significant influence on the on the initial differential initial differential heat.heat.


Differential heats of CO adsorption on Pd/mordenite, Pd-Pt/mordenite and Pd-

Ir/mordenite at 303 K with 2:1 atomic ratios


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Pd/mordenite

TheThe saturation coverage of CO changed of CO changed significantly after adding significantly after adding IrIrto the to the Pd/Pd/mordenitemordenitecatalyst.catalyst.

For the For the PdPd--Pt/Pt/mordenitemordenitecatalyst with a catalyst with a Pd/Pt Pd/Pt atomic atomic ratio of 2:1, 60% of the ratio of 2:1, 60% of the adsorbed CO gave adsorbed CO gave differential heats of 65differential heats of 65––110 110 kJ/mol, with the maximum kJ/mol, with the maximum 110 kJ/mol, and the number 110 kJ/mol, and the number of adsorption sites was of adsorption sites was higher than that on the higher than that on the Pd/Pd/mordenitemordenite catalyst. catalyst.

The addition of Pt and The addition of Pt and IrIr to the Pd/to the Pd/mordenitemordenite catalyst catalyst caused the main distribution of differential heats of CO caused the main distribution of differential heats of CO adsorption to fall into the medium range of differential adsorption to fall into the medium range of differential heats of adsorption. This implies that the heats of adsorption. This implies that the Pt and Pt and IrIrcomponentscomponents weakened the strong CO adsorption sites on weakened the strong CO adsorption sites on the Pd surface, and there was a strong interaction the Pd surface, and there was a strong interaction between Pt and between Pt and IrIr of the Pdof the Pd--Pt/Pt/mordenitemordenite..



CO adsorption numbers corresponding to 50-130 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 4,2 5.0 2.1 Ir-Pd/mordenite 2.6 4,5 5.6 1.7




Results: Relationship between the performance of the Results: Relationship between the performance of the dehydrogenation of C3 dehydrogenation of C3 alkanesalkanes and results of CO and results of CO

adsorption adsorption calorimetrycalorimetry

Relationship between the performance of the dehydrogenation of C3 alkanes and the Pd surface adsorption sites corresponding to 50–130 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 50–130 kJ/mol differential heats of CO.

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Pd-Ir/mordenite

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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 50–130 kJ/mol of differential heats of CO adsorption are the unique catalytic sites for the dehydrogenation of alkanes.

Results: Relationship between the performance of the Results: Relationship between the performance of the dehydrogenation of C3 dehydrogenation of C3 alkanesalkanes and results of CO and results of CO

adsorption adsorption calorimetrycalorimetry

ConclusionsConclusions



ConclusionsCO 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 60-110 kJ/mol for CO adsorption possess superior activity for the dehydrogenation of alkanes.

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.


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



STUDENTS OF GROUP

Diego BlancoVanessa García

Juan F. González

Diana Vargas Yesid Murillo

Paola Rodríguez

Manuel Monroy


THE PROFESSORS!!!!!


COLOMBIA

Cartagena de Indias



BOGOTA: 2640 METROS SOBRE EL NIVEL DEL MAR


O MAS CAFE SUAVE DO MUNDO¡

Universidad de Los Andes


ALBERTO MAGNO


THANKS FOR YOUR ATTENTION

characterization of mordenite-supported pd, pt, and ir...

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