Ivelisse Ortiz

In Situ Investigation of Solid-Liquid Catalytic Interfaces by Attenuated Total Reflection Infrared Spectroscopy

MELANIE SCHAAL

YARITZA LOPEZ

Background

Ivelisse Ortiz received a bachelors degree at the Chemical Engineering Department at the University of Puerto Rico in May 2000.  Her first research experience was on the REU program at USC in 1998.  She worked under professor Amiridis in the Synthesis and testing of ABO3 type perovskites for NO_x reduction in the presence of C₃H₆ and O₂.  She worked under the supervision of Dinyar Captain and Vijay Rajagopalan.  In the summer of 1999 she participated of the MURF – SURF program at the California Institute of Technology.  She worked on the Selective Permeability of CO₂ over CH₄ using Sol-gel Silica films on a macroporous Al₂O₃ rod substrate (Mass transfer area).  In her senior year she started working under the SLOAN program for undergraduates and worked in the areas of thermodynamics and bioengineering.  Ivelisse joined the PhD. Program in August of 2000 as a SLOAN scholar.  She joined Williams group and started working on her thesis topic entitled In Situ Characterization of Solid-Liquid Catalytic Interfaces using Attenuated Total Reflection Infrared (ATR-IR) Spectroscopy.  Her work has been focus in the development of the ATR-IR technique and the testing of multiple catalytic systems.  So far, she has studied CO adsorption and formaldehyde dissociation on 5% Pt/Al₂O₃. Currently, she is working on the study of Nitrile Hydrogenation on 5%Pt/Al₂O₃ using hexane as a solvent. The data is being analyzed by using a Multivariate Analysis Program which allows us to discern species that otherwise were hidden due to bulk phase interference. In her free time, she likes to sing, cook, walk at parks and go to the movies.  She is part of the choir in her church.

Research

Introduction

In recent years there has been an increased desire to implement heterogeneous catalysis in fine chemicals and pharmaceuticals industries.^1-3 One of the main obstacles in the establishment of heterogeneous catalysis in these industries is the difficulty in reproducing homogeneous catalyst properties, such as selectivity and product quality.  Monitoring surfaces of solid catalysts in the liquid phase can provide much-needed insight into the reaction mechanisms that govern these reactions.  One approach that has not been utilized extensively for surface studies of heterogeneous catalysts is attenuated total reflection infrared (ATR-IR) spectroscopy.  Nevertheless, ATR-IR has been used to analyze different types of solid-liquid interfaces related to heterogeneous catalysts.⁴  In my dissertation work, we extend the ATR-IR approach to examine adsorption and dissociation of several probe molecules  (CO, CH₂O, C₂H₅OH, C₄H₇N) in various liquid solvents (H₂O, C₂H₅OH, C₆H₁₄) onto a Pt/Al₂O₃ powder catalyst. 

Results and Discussion

The catalyst samples consist of 5wt% Pt/g-Al₂O₃ prepared using standard wet (aqueous) impregnation with H₂PtCl₆ as the precursor.  The support is g-Al₂O₃ powder from Alfa Aesar with a mean particle size of 37 nm and a surface area of 45 m²/g.  All spectra were acquired using a Nicolet 670 FTIR spectrometer with a liquid nitrogen-cooled MCT detector.  A horizontal ATR accessory (Spectra Tech) was used in conjunction with a home-built aluminum flow cell.  A thin catalyst film was deposited in a germanium 60º waveguide from SpectraTech by coating with a suspension consisting of 100 mgs of catalyst in 20 mL of ethanol.  The catalyst films were saturated with solvent (e.g., water, ethanol).  Then, they were pretreated by saturating the solvent with oxygen while flowing followed by reduction with hydrogen while flowing.  Figure 1 shows a schematic of the experimental set-up.

The applicability of this technique for studying adsorption and reaction of molecules on supported Pt catalysts has been tested using several probe molecules (carbon monoxide, formaldehyde, ethanol, butyronitrile) and solvents (water, ethanol, hexane).  For example, examination of adsorption of CO from aqueous and ethanolic solutions reveals that CO resides in both atop and bridged configurations on the catalyst surface in both solvents.  Results for CO adsorption in water and in ethanol are shown in figure 2.  A 10-fold decrease in the oxidation rate of adsorbed CO in ethanol was observed.  This is attributed both to the lower solubility of O₂ in ethanol compared to water and the likely presence of trace ethanol dissociation products that may block O₂ adsorption.  The adsorption of formaldehyde and ethanol in water was studied by following the formation of adsorbed CO which is a well-known dissociation product for both molecules.  The extent of dissociation appeared considerably larger for formaldehyde than for ethanol as determined by comparing absorption intensities and peak frequencies. 

A computer program has been written, which allows for the removal of experimental and spectroscopic artifacts to be performed as well as the determination of the main sources of variation in the spectroscopic data that correlate to catalyst activity.  The multivariate program combines the concept of partial least squares (PLS) and classical least squares (CLS).  CLS allows for the elimination of artifacts e.g. water, carbon dioxide, water vapor, ice formation, and equipment drift.  With PLS the corrected data is analyzed as a combination of vectors and scores.  The scores describe how the data is changing with respect to the concentration of the different variables i.e. time, H₂, O₂, reactant concentration.  With the vectors and scores we can determine how the reactants are interacting.

This program is being used for the intepretation of the data obtained with formaldehyde and nitrile adsorption.  Currently we are studying the adsorption of butyronitrile and acetonitrile from hexane.  The most commonly proposed surface species are shown on Figure 3.  From those species, only the σ-bonded and π-bonded species have been seen under UHV conditions.  Butyronitrile and acetonitrile were found to adsorb on the catalyst by σ-bonding of the CN group with the platinum.  We also found that acetonitrile adsorbs via a π-bonded species with a characteristic frequency at 1640 cm^-1 (Figure 4).  The appearance of this species suggests the presence of imine intermediate as shown on figure 3 (C=N).⁵  Further studies are being performed by using multiple concentrations on one film.  We hope that with these studies we can shed some light on the role of these surface species play in nitrile hydrogenation.

Figure 1.  Schematic of the automated experimental set-up which allows for the study using multiple concentrations.

Figure 2.  I.  ATR-IR spectra of adsorption and oxidation of CO on Pt/Al₂O₃ in ethyl alcohol.  Spectra were acquired a) after O₂/H₂ pretreatment, b) after bubbling CO through the solution, and c) after purging with He and flowing O₂ for 3 hrs.  II.  ATR-IR spectra of adsorption and oxidation of CO on Pt/Al₂O₃ in water.  Spectra were acquired a) after O₂/H₂ pretreatment, b) after bubbling CO through the solution, c) after purging with He and flowing O₂ for 30 min, d) after purging with He and flowing H₂ for 30 min, and e) after re-adsorption of CO.

References

1.   Carpenter, K. J. Chem.Eng. Sci. 2001, 56, 305.

2.   Mills, P. L.; Chaudhari, R.V. Catal. Today 1997, 37, 367.

3.   Sheldon, R.A.; Downing, R. S. Appl. Catal. A:. General 1999, 189, 163.

4.   See review articles: a) Hind, A. R.; Bhargava, S. K.; McKinnon, A. Adv. Coll. Inter. Sci.,­ 2001, 93, 91.  b) B.W. Johnson, J. Bauhofer, K. Doblhofer and B. Pettinger Electrochimca Acta, 1992 37 (12), 2321

5.   See articles:  a) Hubbard, A.T.; Cao, E.Y.; Stern, D.A. Pergamon, 1994, 39(8/9), 1007. ba) Ou, E.C.; Young, P.A.; Norton, P.R. Surf. Sci., 1992, 277 123. 

1. "In Situ Investigation of Solid-Liquid Catalytic Interfaces by Attenuated Total Reflection Infrared Spectroscopy", I. Ortiz-Hernandez and C. T.  Williams, Langmuir, 19(7), 2956-2962 (2003).

2. "Multivariate Analysis of ATR-IR Spectroscopic Data: Applications to the Solid-Liquid Catalytic Interface", I. Ortiz-Hernandez, D. J. Owens, M. R. Strunk and C. T. Williams, Langmuir, 22(6), 2629-2639 (2006).