Gas adsorption microcalorimetry: probing energetics of ...mms.technologynetworks.net/posters/0154.pdfhelium and measurement of BET surface area of the sample by nitrogen adsorption;

UC DAVISUC DAVISThermochemistry

Facility

Sergey V Ushakov and Alexandra Navrotsky ltanavrotskyucdavisedugt

Thermochemistry Facility and NEAT ORU University of California at Davis

Gas adsorption microcalorimetry probing energetics of oxide surfaces Gas adsorption microcalorimetry probing energetics of oxide surfaces

MethodMethod

0 200 400 600

wmicro

Time min

0002 0004 0006 00080

Relative Pressure PP0

Adsorbed volumes of H O 2

dosed with surface area analyzer

As the dimensions decrease surface properties become an overriding feature controlling reactivity and phase transformations in nanophases The control of properties of solid surfaces is of paramount importance in a number of fields ranging from heterogeneous catalysis and fuel cells to microchip fabrication and corrosion-resistive coatings

Gas adsorption microcalorimetry is an unsurpassed method for providing information on surface energetics and reactivity

C It uses well established technology for volumetric dosing implemented in BET instruments

C Small total volume of the system allows fast equilibration

C High sensitivity of Calvet-type microcalorimeter allows small samples enthalpies of adsorption were measured on samples with

2total surface area less than 05 m

C The temperature range of the microcalorimeter allows one to study a wide range of gas-solid interactions from -100 to about 800 degC

Micromeritics ASAP2020 is used as a dosing system Setaram Calvet-DSC 111 operated in isoperibol mode used as calorimetric detector for heat of adsorption The adsorbent is placed into one side of the quartz fork-type sample tube which fits into the twin calorimeter cells The other side is left empty and serves as a reference In these conditions measured experimental heats are in fact enthalpies of adsorption Experiments include three steps

i) sample degassing in vacuum at the desired temperature

ii) measurement of free volume of the sample tube with helium and measurement of BET surface area of the sample by nitrogen adsorption

iii) final evacuation of the system and measurement of the heats of gas adsorption in a series of small dosing steps with the calorimeter at 25 degC Dose amounts and equilibration times are set up with the surface analyzer controls and the adsorption isotherm is measured simultaneously

Novel setup for gas adsorption microcalorimetry

unique to UC Davis Thermochemistry Facility

Novel setup for gas adsorption microcalorimetry

unique to UC Davis Thermochemistry Facility

Applications examplesApplications examples

Determination of energies of anhydrous surfaces from solution calorimetry on hydrated phases

dilute solution in oxide melt at 700 degC

DHds coarse DHds nano

DHds coarse-to-nano

nano coarse(= gmiddotSA)

If adsorbed water present one needs to account for DH to estimate energies des

of anhydrous surfaces

Enthalpies of FeOOH phases relative to bulk hematite and liquid water vs surface area Values are corrected for adsorbed water assuming DH = -DH Slope of the lines des ads

corresponds to average surface enthalpy for each phase

0 10000 20000 30000

SA m2mol of FeOOH

goethite

lepidocrocite

akaganeite

L Mazeina SW Deore A Navrotsky L Mazeina A Navrotsky et al In preparation

Chem Mater 2006 18 1830-8

Determination of the surface site energy distribution

0 1 2 3

m-ZrO2

t-ZrO2

ZrO2 surface coverage H

2O per nm

SV Ushakov and A Navrotsky Applied Physics Letters 87 164103 (2005)

Bulk phase dependence

Differential enthalpies of water adsorption on monoclinic and tetragonal zirconia phases after degassing at 800 degC

HfO2 surface coverage H2O per nm20 1 2 3

m-HfO2 800degC (gp) (pr)

Synthesis route dependence

Differential enthalpies of water adsorption on monoclinic hafnia Commercial Alfa Aesar samples compared with synthesized by precipitation (pr) and condensation from gas phase (gp) samples after degassing at 800 degC

Particle size dependence

-400 1 2 3 4 5 6 7 8

2 nm 125 C7 nm 125 C7 nm 150 C30 nm 150 C

FeOOH surface coverage H2O per nm2

Differential enthalpies of water adsorption on goethite particles of different size

L Mazeina and A Navrotsky Clays and Clay Minerals 53 (2) 113 (2005)

Dopant dependence

0 25 5 750

gg-Al2O3

Mg-doped Zr-doped

-Al O surface coverage H O per nm2 3 2

Differential enthalpies of water adsorption on pure gamma alumina compare 1 Zr and 3 Mg doping

0 10 000 20 0000

Mg-doped

Zr-doped

Oa -Al2 3

gg-Al2O3

Enthalpies of pure and doped g-Al O wrt to bulk a-Al2O3 2 3

vs surface area Values are corrected for adsorbed water assuming DH = -DH Slope of the lines corresponds to des ads

average surface enthalpy for each phaseRHR Castro SV Ushakov L Gengembre D Gouvecirca and A Navrotsky Chem Mater 2006 18 1867-72NSF and DOE funding is gratefully acknowledgedNSF and DOE funding is gratefully acknowledged

One Shields Avenue Davis CA 95616 Presented at NEAT ORU meeting February 14 2006 Davis CA

H O2 N2

Volumetric dosing systemMicromeritics ASAP2020

Calvet-type microcalorimeter DSC111

RHR Castro SV Ushakov L Gengembre D Gouvecirca and A Navrotsky Chem Mater2006 18 1867-72

Documents

Gas adsorption microcalorimetry: probing energetics of ...mms.technologynetworks.net/posters/0154.pdfhelium and measurement of BET surface area of the sample by nitrogen adsorption;