[newbie!]

i need help with the following questions:

Explain very briefly how the RAIRS technique achieves surface sensitivity in measuring infrared spectra of adsorbed molecules.


List the major pieces of experimental evidence for nitrogen adsorption being the rate limiting step in the ammonia synthesis reaction over iron catalysts.


What techniques in surface science do the acronyms SIMS, XPS, LEED and HREELS stand for?


Define selectivity in a catalytic reaction.


i have no workings coz i dont know the amswers.
is q3 even to do with surface chem!?

[DevaDevil]

i need help with the following questions:

Explain very briefly how the RAIRS technique achieves surface sensitivity in measuring infrared spectra of adsorbed molecules.


List the major pieces of experimental evidence for nitrogen adsorption being the rate limiting step in the ammonia synthesis reaction over iron catalysts.


What techniques in surface science do the acronyms SIMS, XPS, LEED and HREELS stand for?


Define selectivity in a catalytic reaction.


i have no workings coz i dont know the amswers.
is q3 even to do with surface chem!?

google would give you the acronyms as well, so I will give them here:

Secondary Ion Mass Spectroscopy
X-Ray Photoelectron Spectroscopy
Low Energy Electron Diffraction
High Resolution Electron Energy Loss Spectroscopy

These are all surface specific analyzing techniques. Learn them, they are VERY udeful to know and use in heterogeneous Catalysis.

The last question is just something you need to learn. You have activity and selectivity in a catalytic reaction. What do you think both would mean?

[lauren wind]

Vibrational spectroscopy provides the most definitive means of identifying the surface species generated upon molecular adsorption and the species generated by surface reactions. In principle, any technique that can be used to obtain vibrational data from solid state or gas phase samples (IR, Raman etc.) can be applied to the study of surfaces - in addition there are a number of techniques which have been specifically developed to study the vibrations of molecules at interfaces (EELS, SFG etc.).

There are, however, only two techniques that are routinely used for vibrational studies of molecules on surfaces - these are :

   1. IR Spectroscopy (of various forms, e.g. RAIRS, MIR)
   2. Electron Energy Loss Spectroscopy ( EELS )

 
IR Spectroscopy

There are a number of ways in which the IR technique may be implemented for the study of adsorbates on surfaces.

For solid samples possessing a high surface area :

Transmission IR Spectroscopy : employing the same basic experimental geometry as that used for liquid samples and mulls. This is often used for studies on supported metal catalysts where the large metallic surface area permits a high concentration of adsorbed species to be sampled. The solid sample must, of course, be IR transparent over an appreciable wavelength range.

Diffuse Reflectance IR Spectroscopy ( DRIFTS ) : in which the diffusely scattered IR radiation from a sample is collected, refocused and analysed. This modification of the IR technique can be employed with high surface area catalytic samples that are not sufficiently transparent to be studied in transmission.

For studies on low surface area samples (e.g. single crystals) :

Reflection-Absorption IR Spectroscopy ( RAIRS ) : where the IR beam is specularly reflected from the front face of a highly-reflective sample, such as a metal single crystal surface.

Multiple Internal Reflection Spectroscopy ( MIR ) : in which the IR beam is passed through a thin, IR transmitting sample in a manner such that it alternately undergoes total internal reflection from the front and rear faces of the sample. At each reflection, some of the IR radiation may be absorbed by species adsorbed on the solid surface - hence the alternative name of Attenuated Total Reflection (ATR).

[neon flash!]

thx.

got a few more

q13) briefly outline how the aount of physisorbed gas may be determined using a dynamic method.

a) volumentric, gravimetric, pulse chemisorption - are these dynamic methods?



Q16) The Kelvin equation states that the vapour pressure above a curved surface, such as a water droplet, is greater than that above a planar surface. Would you expect the larger or smaller pores to empty first when the adsorbent pressure over a porous solid is reduced?

lecturer said something about P>Po and miniscus is concave but i dont know would it be the smaller or larger pores to empty 1st?



Q17)  Why can the method of “pulsing” to measure gas uptake be described as a non-equilibrium procedure?



Q18)  Why does exposure of a surface to nitrogen at 78 K give rise to what is often described as “non-specific adsorption”?

- lecturer said "adsorbs onto everything"  which i get but i feel its lacking as an answer? i though the question was more focussed on why exposing a surface to nitrogen at 78K gives rise to this non-specific adsorbtion, rather than just explaining what non specific adsorption is?



20) For a saturated solution of a surfactant molecule, draw a graph of surfactant concentration against temperature showing the expected behavior around the Krafft point.

- no idea