Simon Podkolzin - Acidity-Basicity of La Surfaces

Surface Acidity and Basicity of La₂O₃, LaOCl, and LaCl₃ Characterized by IR Spectroscopy, TPD, and DFT Calculations.

Olga V. Manoilova,¹ Simon G. Podkolzin,² Balarishna Tope,³ Johannes Lercher,³ Eric E. Stangland,² Jean-Michel Goupil,⁴ and Bert M. Weckhuysen¹

Journal of Physical Chemistry B 108(40), 15770-15781 (2004)

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Abstract

Adsorption sites of La₂O₃, LaOCl, and LaCl₃ catalysts were characterized with probe molecules using infrared spectroscopy, temperature-programmed desorption (TPD), and density-functional theory (DFT) calculations. Surface acid sites were probed with CO, pyridine, and 2,6-dimethylpyridine (DMP), and basic sites were probed with CO₂. Shifts of the CO vibrational frequency at low surface coverage at 77 K suggest that the strength of Lewis acid sites increases with the concentration of Cl in the material; i.e., La₂O₃ < LaOCl < LaCl₃. DFT estimates for CO adsorption energies and LUMO energies were consistent with this ranking. On the basis of a downward shift of the surface OH stretching bands interacting with CO, pyridine, and DMP spectra at room temperature (RT) and TPD results, and confirmed by DFT calculations, the strength of Brønsted acid sites was concluded to increase in the same order. Additional DFT calculations with a frequency analysis were used to elucidate CO₂ adsorption modes. DFT calculations and IR spectra of CO₂ adsorbed on LaOCl suggest that CO₂ forms coupled bridged species. Proton affinity calculations were used to rank the basicity strength of surface O and Cl sites. The amount of CO₂ adsorbed on LaCl₃ was negligibly small, confirming the requirement of lattice O adsorption sites. IR spectra of CO₂ adsorbed on La₂O₃ at RT were similar to those of bulk La₂(CO₃)₃ and, accordingly, were assigned to the formation of polydentate and bulk carbonates. CO₂ evolution from La₂O₃ in TPD experiments closely matched the reported thermal stability of La₂(CO₃)₃.

Address:

[1] Departement Anorganische Chemie en Katalyse, Debye Instituut, Universiteit Utrecht, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands

[2] Core Research and Development, The Dow Chemical Company, Midland, Michigan 48674, USA

[3] Institute for Chemical Technology, Technische Universität München, Lichtenbergstrasse 4, D-85747 Garching, Germany

[4] Laboratory "Catalyse et Spectrochimie", UMR CNRS, ENSICAEN, Caen University, 6 bd. Maréchal Juin, 14050 Caen Cedex, France

Publisher:

American Chemical Society

CODEN: JPCBFK, ISSN: 1520-6106, CAN 141:401451, AN 2004:749178

Evaluation of Brønsted acid sites on La₂O₃, LaOCl and LaCl₃

	La₂O₃	LaOCl	LaCl₃
Δn(OH) experimental at high CO coverage, cm^-1	-34	-58	-107

Computational model surface	La₂O₃(001)	LaOCl(001)	LaCl₃(100)
Computational model surface	La₂O₃(001)	Cl-only	LaCl₃(100)
Δn(OH) calculated with H-bonded CO, cm^-1	No comparable surface OH model	-38	-107
ΔE for OH^- (surf) = O^2-(surf) + H⁺ at 0.25 ML, kJ/mol	No comparable surface OH model	1702	1524


CO on an OH surface site of a LaOCl(001) model: cif, xsd, pdb		CO on an OH surface site of a LaCl₃(100) model: cif, xsd, pdb

Evaluation of Lewis acid sites on La₂O₃, LaOCl and LaCl₃

	La₂O₃		LaOCl		LaCl₃
n(CO) experimental at low coverage, cm^-1	2178		2180		2184
n(CO) experimental at high coverage, cm^-1	2167, 2175(sh.*)		2166, 2173(sh.)		2166, 2179

Computational model surface	La₂O₃(001)	LaOCl(001)				LaCl₃(100)
		Cl-only		mixed O-Cl
n(CO) calculated at 0.25 ML, cm^-1	2153	2157		2175		2170
ΔE for CO adsorption at 0.25 ML, kJ/mol	-23	-28		-36		-37
LUMO energy, kJ/mol. Lower energy indicates stronger Lewis acidity	-177	-208		-249		-297


CO on a lanthanum surface site of a La₂O₃(001) model: cif, xsd, pdb		CO on a lanthanum surface site of a LaOCl(001) model: cif, xsd, pdb		CO on a lanthanum surface site of a LaCl₃(100) model: cif, xsd, pdb

Identification of surface carbonate species - CO₂ adsorption on Lewis base sites

La₂O₃ surfaces

FTIR spectra of CO₂ adsorbed at room temperature on La₂O_3.with increasing coverage.

Calculated monodentate

	0.25 ML	Schematic: Structure files: cif, xsd, pdb
n_as(COO), cm^-1	1691
n_s(COO), cm^-1	1258
n(C-O), cm^-1	842
C-O bonds, Å	1.26 1.26 1.43
DE ads, kJ/mol	-49

Calculated polydentate

	0.25 ML	High coverage	Schematic: Structure files: cif, xsd, pdb
n(C-O), cm^-1	n/a	1413
n_as (CO₃), cm^-1	1401	1470
n_s (CO₃), cm^-1	1037	1051
C-O bonds, Å	1.30 1.30 1.30	1.28 1.30 1.30
DE ads, kJ/mol	-56

FTIR spectra of CO₂ adsorbed at room temperature on LaOCl_.with increasing coverage.

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Surface Acidity and Basicity of La2O3, LaOCl, and LaCl3 Characterized by IR Spectroscopy, TPD, and DFT Calculations.

Olga V. Manoilova,1 Simon G. Podkolzin,2 Balarishna Tope,3 Johannes Lercher,3 Eric E. Stangland,2 Jean-Michel Goupil,4 and Bert M. Weckhuysen1

Journal of Physical Chemistry B 108(40), 15770-15781 (2004)

Abstract

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Evaluation of Brønsted acid sites on La2O3, LaOCl and LaCl3

LaOCl

Evaluation of Lewis acid sites on La2O3, LaOCl and LaCl3

Identification of surface carbonate species - CO2 adsorption on Lewis base sites