Supported nanosized gold catalysi the influence of support morphology and reaction mechanism 2

In atransmission electron microscope, high energy electron beam is collimated bymagnetic lenses and allowed to pass through the specimen under high vaccum.. Imaging resolution is limited

Chapter 2 Experimental Section

2.1 Catalyst Preparation

The method of preparation strongly influences the particle size.1-4, which is believed

to be one of the important factors that can influenc catalysts activity 5-8 For most ofthe reactions, only the catalysts with gold particles smaller than 5 nm lead to highactivity; this is especially true for the oxidation of carbon monoxide 9,10 There aretwo kinds of chemical preparation methods that are widely used in the preparation ofgold nano particles supported on metal oxide support

The first kind is termed as co-precipitation, in which the support and the goldprecursor are formed at the same time Co-precipitation is the carrying down by aprecipitate of substances normally soluble under the conditions employed.11There arethree main mechanisms of co-precipitation: inclusion, occlusion, and adsorption.12Aninclusion occurs when the impurity occupies a lattice site in the crystal structure of thecarrier, resulting in a crystallographic defect; this can happen when the ionic radiusand charge of the impurity are similar to those of the carrier An adsorbate is animpurity that is weakly bound (adsorbed) to the surface of the precipitate Anocclusion occurs when an adsorbed impurity gets physically trapped inside the crystal

as it grows Besides its applications in chemical analysis and in radiochemistry, precipitation is also "potentially important to many environmental issues closelyrelated to water resources, including acid mine drainage, radionuclide migration infouled waste repositories, metal contaminant transport at industrial and defense sites,metal concentrations in aquatic systems, and wastewater treatment technology".13Co-precipitation is also used as a method of magnetic nanoparticle synthesis.14 However

co 29 co

-the co-precipitation method tends to be difficult to control and to reproduce.Nucleation and growth can easily occur in the solution rather than on the carrier,which results in undesirably large metal particles (low dispersion) and aninhomogeneous metal distribution on the carrier,

The second kind of methods includes impregnation, ion adsorption, precipitation and colloid-based methods In these methods the gold precursor isapplied to the preformed support Among these methods, deposition-precipitation andcolloid-based method were utilized in the preparation of gold nanoparticles supported

deposition-on irdeposition-on oxide support

Deposition-precipitation is a modification of the precipitation methods in solution Itconsists of the conversion of a highly soluble metal precursor in another substance oflower solubility, which specifically precipitates onto a support and not in solution.The conversion into the low soluble compound, and then into the precipitate, isusually achieved by raising the pH of the solution This can also be done bydecreasing the pH, or by changing the valence state of the metal precursor throughelectrochemical reactions or by using a reducing agent, or by changing theconcentration of a complexing agent The key point for a successful deposition-precipitation is, therefore, the gradual addition of the precipitating agent to avoid localrise of concentration above the solubility curve, which would cause a rapid nucleation

of the precipitate in solution Deposition-precipitation is widely used in thepreparation of nanosized gold catalysts, but G.C Bond mentioned in his book that theterm deposition-precipitation employed here is not so accurate 15 It might be moresuitable to be described as grafting or ion adsorption, because deposition-precipitationhints the forming of hydroxide or hydrated oxide for the deposition onto the surface of

the metal oxide support, while precipitation involves a nucleation by the support andall the active phase attached to the support.

Colloidal-based method has the advantage of small mean particle size and narrow sizedistribution under appropriate conditions, and the influence of the support is almostignorable This method was first used in preparing nanosized gold particles by JohnTurkevich and his associates in 1950s.16,17 They used sodium citrate as the reductionagent for the AuCl4- ion Many other reducing agents have been used ever since forthe reduction of gold related ions, including phosphorus, sodium thiocyanate, polyethylene-imine, tetrakis phosphonium chloride and sodium borohydride Theadvantage of using the colloidal route for preparing supported gold catalysts lies inthe way that condition of preparation can be manipulated to give particles having anarrow size distribution about the desired mean

In this thesis Au/iron oxide catalysts were prepared by various methods, including precipitation (CP), deposition-precipitation (DP), and colloids-based methods, usingHAuCl4 (sigma-aldrich) and Fe(NO3)3·9H2O (sigma-aldrich) as precursors In thecase of the co-precipitation (CP) method, an aqueous mixture of the HAuCl4 andFe(NO3)3 precursors was poured into an aqueous solution of Na2CO3 (0.25M) whichwas maintained at 70oC under vigorous stirring (500 rpm) The precipitate waswashed, dried, and calcined in air at 110oC for 12 hrs This co-precipitation sample iscoded AuCP In the deposition-precipitation method, Au nanoparticles weredeposited on iron oxide support by keeping the pH value of the aqueous solution ofHAuCl4 at pH = 8 using 0.1M NaOH The Fe2O3support was generated, prior to the

co-DP process, from 1.0 M Fe(NO3)3 solution Excessive amount of 1.0M NaOHsolution was added to the Fe(NO3)3solution drop-wisely till all the iron ions in the

31

-solution were deposited Then the mixed -solution was thoroughly washed using DIwater by centrifugation The slurry after centrifuge was dried in 110oC oven for 48hours The above prepared sample was then calcined at 500oC for 5 hour The as-prepared iron oxide was mainly presented in -Fe2O3 phase, with small amount of γ-

Fe2O3phase detectable by XRD This self-prepared iron oxide sample was used as thesupport for the AuDP catalyst (The deposition-precipitation sample is coded AuDP).Two other samples, AuCH and AuCM were prepared using colloid-based methodwith assistance of the ultrasound irradiation18 The support used for AuCH wascommercial Fe2O3 (hematite, Sigma-Aldrich), while that for AuCM was commercial

Fe3O4 (Magnetite, Sigma-Aldrech) In colloid-based method L-lysine was added as acapping agent, which has better control on gold particle size compared toconventional DP method used in literature HAuCl4 (1mM) was reduced by NaBH4

(0.1M) During the reduction period, colloid-based method was applied The nano-Auparticles were deposited on iron oxide supports The slurry was dried at 70ºC aftercentrifuge four times using DI water As chloride ions is a poison to the catalyticreaction and may affect the activity of catalyst, the addition of capping agent andreduction agent and the followed washing procedure are able to remove almost ofchlorine in the solution

2.2 X-ray Photoelectron Spectroscopy

X-ray Photoelectron Spectroscopy (XPS) is widely used to investigate the chemicalcompositions and oxidation state of surfaces Its surface specificity, applicability tonearly all elements, and sensitivity to chemical state give XPS great potential forcontributing to the understanding of a wide variety of catalyst problems.19The X-rayspenetrate far into the solid (1~10 m) but the mean free path for the escape of a 100-

1500 eV electron without energy loss is only 1-8 nm Secondary electron emission

and inelastic losses account for much of the background in the spectrum, but theinformation carried in the spectral peaks applies to a thin surface layer because of therelatively short electron mean free path Thus, XPS is inherently a surface technique.Surface analysis by XPS is accomplished by irradiating a sample with mono energeticsoft X-rays (usually Mg K (1253.6 eV) or Al K (1486.6 eV)) under ultra-highvacuum (UHV) conditions, causing electrons to be emitted from the surface region bythe photoelectric effect and analyzing the energies of the detected electrons Theemitted photoelectrons have measured kinetic energies given by.20

KE h BE  s (2.1)

where h is the energy of the photon, BE is the binding energy of the atomic orbital

from which the electron originates, and sis the spectrometer work function Thebinding engery which is theoretically equivalent to the ionization energy of theelectron can also be regarded as the energy difference between the initial and finalstates after the photoelectron has left the atom Thus each element has a unique set ofbinding energies The binding energies of the core-level electrons are sufficientlyaffected by differences in the chemical potential and polarizability of the neighboringcompounds This would cause a detectable photoelectron energy shift, which rangesfrom 0.1 up to 10 eV This kind of shift is termed as chemical shift Several types ofpeaks are observed in XPS spectra Photoelectron lines, Auger lines and shake-uplines are the mostly encountered feature in our research The dominant features in anXPS spectrum are photoelectron lines Most intense photoelectron lines are relativelysymmetrical and have typically the narrowest FWHM of 1-2 eV Less intensephotoelectron lines at higher binding energies are usually 1-4 eV wider than the lines

33

-at lower binding energies All of the photoelectron lines of insul-ating solids are of theorder of 0.5 eV wider than photoelectron lines of conductors Auger lines areobservable in XPS spectra, due to relaxation of the excited ions left afterphotoemission The Auger electron possesses the kinetic energy equal to thedifference between the energies of the initial ion and the doubly charged final ion.Shake-up losses are final state effects which arise when the photoelectron impartsenergy to another electron of the atom This electron ends up in a higher unoccupiedstate (shake-up) This results in a satellite peak a few eV higher in BE than the mainpeak

During our XPS analysis, charging often occurs because our nano gold on conductor (CuO or TiO2) samples might lack of delocalized electrons to neutralize thepositively charged macroscopic clusters created by the ejection of photoelectronsand/or Auger electrons As a result, a positive potential builds up at the surface of thesample, which retards the outgoing electrons This retardation appears in thespectrum as an additional positive shift in binding energies Referring to the BE ofC1s (284.60.4 eV) contributed from the adventitious carbon is the commonly usedprocedure to solve the charging problem Any shift from this value is taken as ameasure of the steady-state static charging

semi-Quantification of XPS spectra is quite straightforward The elemental composition ofthe interested sample determined as followed:

%X = (A

x/S

x) / Σ

N i=1(A

x/S

x) (2.2)

where X is the element, A

xthe area under the peak of element X in the spectrum, andS

xis the relative atomic sensitivity factor %X is the fractional atomic concentration

In our research, the S

xdata are provided by the instrument manufacture VG ScientificLtd

2.3 Secondary ion mass spectrometry(SIMS)

Secondary ion mass spectrometry (SIMS) is a technique used to analyze thecomposition of solid surfaces and thin films by sputtering the surface of the specimenwith a focused primary ion beam and collecting and analyzing ejected secondary ions

It is particularly useful in detecting hydrogen species on surfaces While only chargedsecondary ions emitted from the material surface through the sputtering process areused to analyze the chemical composition of the material, these represent a smallfraction of the particles emitted from the sample There are two different modes of

analytical SIMS application: static and dynamic SIMS In static SIMS, 21,22 theinformation on the composition of the uppermost monolayer is generated virtuallywithout disturbing the composition and structure of the surface This is achieved byvery low primary ion current densities For an ion current density of 10-9 Acm-2

thelife-time of a monolayer is in the order of some hours Essentially static SIMSprovides a mass spectrum of the surface The mix of elemental and cluster ions in thespectrum can generate a rich store of information regarding the chemistry of thesurface layer Hence static SIMS is a technique of great potential for understandingthe chemical behavior and structure of surfaces

In dynamic SIMS, high primary ion-current densities up to some Acm-2

is applied,thus resulting in a short lifetime of a monolayer down to the 10-3sec range This fastsurface erosion continuously moves the surface into the bulk material, thus supplying

is caused by a "collision cascade" which is initiated by the primary ion impacting thesample surface The emitted secondary ions are extracted into the TOF analyzer byapplying a high voltage potential between the sample surface and the mass analyzer.

TOF-SIMS spectra are generated using a pulsed primary ion source (very short pulses

of <1 ns) Secondary ions travel through the TOF analyzer with different velocities,depending on their mass to charge ratio (ke=½mv2)

2.4 X-ray Powder Diffraction (XRD)

X-ray diffraction is accomplished by beaming X-ray photons onto a sample andrecording the diffracted X-ray beam The diffraction would take place only at thoseangles which fulfill the Bragg condition.24

whereis the wavelength of the X-ray, d inter-planar spacing of crystal lattice planes,

 the glancing angle of incidence of the primary beam relative to the atomic plane,

and n an integer The X-ray detector moves at an angular velocity twice that of the

rotating sample during a scan This will ensure that the X-ray glancing angle ofincidence is equal to the diffracted beam glancing angle Whenever the Braggcondition is fulfilled, X-ray is reflected to the detector The diffracted beam intensity

is recorded and plotted against 2by a computer

Information about the structure, composition as well as state of polycrystallinematerials can get from XRD analysis A few typical applications include identification

of unknown structures based on the crystalline peaks, variable temperature studies,precise measurement of lattice constants and residual strains as well as refinement ofatomic coordinates

Bruker XRD D8 advance with Cu Kα radiation was used for examination of the

crystalline structure (see Figure 2.1) The intensity data were collected over a 2 θ

range of 20˚-70˚ with a scan speed of 5˚/min and a scan step of 0.02˚

37

-Figure 2.1 Bruker XRD D8 advance

2.5 Transmission Electron Microscopy (TEM)

The transmission electron microscope was first developed in the 1930’s In atransmission electron microscope, high energy electron beam is collimated bymagnetic lenses and allowed to pass through the specimen under high vaccum

25

Thisresults in the diffraction pattern that contained transmitted beam and a number ofdiffracted beams, which will be then imaged on a fluorescent screen below thespecimen In this case, one can obtain the lattice spacings and symmetry informationfor the structure under consideration

25

The transmitted beam or one of the diffracted beams can also form magnified image

of the sample on the viewing screen One can obtain the high-resolution image thatcontains information on the atomic structural of the materials if the transmitted beamand one or more diffracted beams are to recombine

TEM offer much help in the study of local structure, morphology and chemistry ofmaterial in extremely high resolution Nonetheless, TEM requires preparation ofsamples that can be time consuming In addition, there are some materials especially

37

-Figure 2.1 Bruker XRD D8 advance

2.5 Transmission Electron Microscopy (TEM)

The transmission electron microscope was first developed in the 1930’s In atransmission electron microscope, high energy electron beam is collimated bymagnetic lenses and allowed to pass through the specimen under high vaccum

25

Thisresults in the diffraction pattern that contained transmitted beam and a number ofdiffracted beams, which will be then imaged on a fluorescent screen below thespecimen In this case, one can obtain the lattice spacings and symmetry informationfor the structure under consideration

25

The transmitted beam or one of the diffracted beams can also form magnified image

of the sample on the viewing screen One can obtain the high-resolution image thatcontains information on the atomic structural of the materials if the transmitted beamand one or more diffracted beams are to recombine

TEM offer much help in the study of local structure, morphology and chemistry ofmaterial in extremely high resolution Nonetheless, TEM requires preparation ofsamples that can be time consuming In addition, there are some materials especially

37

-Figure 2.1 Bruker XRD D8 advance

2.5 Transmission Electron Microscopy (TEM)

The transmission electron microscope was first developed in the 1930’s In atransmission electron microscope, high energy electron beam is collimated bymagnetic lenses and allowed to pass through the specimen under high vaccum

25

Thisresults in the diffraction pattern that contained transmitted beam and a number ofdiffracted beams, which will be then imaged on a fluorescent screen below thespecimen In this case, one can obtain the lattice spacings and symmetry informationfor the structure under consideration

25

The transmitted beam or one of the diffracted beams can also form magnified image

of the sample on the viewing screen One can obtain the high-resolution image thatcontains information on the atomic structural of the materials if the transmitted beamand one or more diffracted beams are to recombine

TEM offer much help in the study of local structure, morphology and chemistry ofmaterial in extremely high resolution Nonetheless, TEM requires preparation ofsamples that can be time consuming In addition, there are some materials especially

polymers that lose their crystallinity and mass upon interaction with strong electronbeam Imaging resolution is limited to about 0.2nm because of the great difficulties inmanipulating the magnetic field to get a higher resolution.

25

2.6 Scanning Electron Microscopy (SEM)

JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM), shown in

Figure 2.2, was used to observe the particle shape, size and morphology.

In SEM, electron beam is used to bombard on a sample, which generates secondaryelectrons (that reveals surface morphology), backscattered electrons (that revealscomposition contrast), characteristic X-ray (use in elemental analysis), etc All thesignals generated are detected simultaneously by the individual detectors that arecurrently mounted on the JSM-6700F SEM

Figure 2.2 JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM)

2.7 BET Measurement

The surface area of the catalysts was measured by the Brunauer-Emmett-Teller (BET)

polymers that lose their crystallinity and mass upon interaction with strong electronbeam Imaging resolution is limited to about 0.2nm because of the great difficulties inmanipulating the magnetic field to get a higher resolution

25

2.6 Scanning Electron Microscopy (SEM)

JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM), shown in

Figure 2.2, was used to observe the particle shape, size and morphology.

In SEM, electron beam is used to bombard on a sample, which generates secondaryelectrons (that reveals surface morphology), backscattered electrons (that revealscomposition contrast), characteristic X-ray (use in elemental analysis), etc All thesignals generated are detected simultaneously by the individual detectors that arecurrently mounted on the JSM-6700F SEM

Figure 2.2 JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM)

2.7 BET Measurement

The surface area of the catalysts was measured by the Brunauer-Emmett-Teller (BET)

polymers that lose their crystallinity and mass upon interaction with strong electronbeam Imaging resolution is limited to about 0.2nm because of the great difficulties inmanipulating the magnetic field to get a higher resolution

25

2.6 Scanning Electron Microscopy (SEM)

JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM), shown in

Figure 2.2, was used to observe the particle shape, size and morphology.

In SEM, electron beam is used to bombard on a sample, which generates secondaryelectrons (that reveals surface morphology), backscattered electrons (that revealscomposition contrast), characteristic X-ray (use in elemental analysis), etc All thesignals generated are detected simultaneously by the individual detectors that arecurrently mounted on the JSM-6700F SEM

Figure 2.2 JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM)

2.7 BET Measurement

The surface area of the catalysts was measured by the Brunauer-Emmett-Teller (BET)