6f Flashcards
what are the main deviations in the langmair isotherm
- all sites are identical and independent is incorrect,,, there can be attractive and repulsive forces between adsorbants
- adsorbants must hit an empty space and cannot slide into one,,, if they could slide into them,, the rate of adsorbance would increase.
- adsorbants can only form one monolayer is also incorrect,, they can grow layer on layer or island growth.
what things were made to overcome the attractive and repulsive adosrbancts in close proximity sites that go against adsorbance sites being identical and independent
u have the alternative and empirical isotherms,, which we changed to fit data.
- freudlich: developed to deal with the repulsive interactions where enthalpy of adsorption increases as coverage increases. (bc if lots of things are repelling,, theyre not gonna want to adsorb onto the surface making the enthalpy large)
- frenkin + slygin: developed to deal with the attractive interactions where enthalpy of adsorbance decreases as coverage increases. ( bc they like eachother so if favourable for more coverage to occur)
describe the 2 graphs where u have the langmuir isotherm prediction and the one that goes agsinst its rule that adsorbants cant slide into sites
- okay so obvs if the adsorbant can slide into the sites,, instead of falling straight into it,, the rate of adsorption will increase.
so on graphs of theta // coverage against pressure there will be a steap r and a less steep r to show how coverage is much faster when the adsorbant can slide into sites. rather than it having to fit into a site.
when ur observing the rate of adsorbance,, why is one axis theta and one pressure
theta bc thats the coverage, aka particles adsorbing to a surface
and pressure bc adsorption is a gas particle going onto the surface!!! obvs more pressure means more particles able to adsorb onto it.
what is temperature programmed desorption
its how we find out what is happening on the surface!!!
okay describe the process of temperature programmed desorption
u have ur sample and heater in a vacuum environment!!!
u also have mass spec
and then u heat ur sample and use mass spec to figure out what is desorbing from the mass change.
when u have the temp programmed desorption,, what do u plot on the graph axis to get beta and what is beta
u plot
Temp in kelvin on y axis
and time in seconds on the x axis
and the heating rate = beta
ad beta needs to be between 5. to 20 ks-1
the mass spec signal is directly proportional to what
the mass spec signal is directly proportional to the rate of desorption
how does heating rate effect desorption and mass spec signal
the faster the rate of heating == the faster the desorption = the stronger the mass spec signalll!!!!
okay now lets talk about the kinetics of temperature programmed desorption,, whats the rate equation
okay so the gen equation is:
A(a) –k—> A(g)
where u have A adsorbed and then A in gas phase.
the equation =
rate = k [A(a)] ^n
and we write n bc we dk if this is an elementary step or not,,,, so we just write n for the order just to be sure.
what whats the different rate equation for desorption
theta^n V e^-Ed/RT
where theta^n is the coverage and the order its in
then u have V which is the preexponential factor for K,, normally around 10^13 s-1
and Ed is the activation enegry for desorption,,, aka the Ea for desorption
okay when we look at the kinetics of the temperature programmed desorption,, what is plotted on the axis
u have the mass spec signal on the y axis and this is directly proportional to the rate of desorption
then u have Temp in kelvin on the x axis
and u get 3 different looking peaks depening on what the order of theta,, aka the coverage is.
but u cant keep increasing the temp bc ur gonna run out of adsorbate
whats richard lambest all about
its about how he used oxygen to form an epoxide using ethene however when he used a diff molecule he didnt get an epoxide but CO2 and H2O,, which makes us wonder if the O2 dissociates on the metal catalyst or not.
aka is oxygen active state molecular or atomic
what do we need to do when we are doing the richard stuff
we put a mix of 18O2 and 16O2 on a Ag(|||) crystal and then look at the mass spec
we get a graph of temp on the x axis in kelvin and we get 3 peaks per line.
2 peaks occur at 180 and have the weight of 32 and 36, which correspond to the 16 and 18 O2.
then u have a peak at 300K which has a weight of 34 which is obvs due to one 18O and one 16O.
the humps show where desorption occurs
and bc theres desorption occuring at 180 and 300,, we conclude that there still has to be oxygen present after the first desorption for it to occur again at 300k.
it tells us that at temps above 180 there are O O separate but on the surface of the Ag,,, and then at 300k,, the oxygen come together and desorb at O2.
and this is a reaction limited desorption meaning O2 forms in the process of desorbing
whats a reaction limited desorption
its a eaction where O2 forms from 2O in the process of desorbing
aka they come together at higher temps and then desorb as O2.
when we say we want to determine the kinetic parameters from temperature programmed desorption,, what do we mean by that
we want to figure out the
order of reaction
activation energy
pre exponential factor
and with all this we know what will happen to the. reaction no matter the temp
in these next exampled,, what does Tp mean
its the peak temperature!!!
how do we determine the order of a TPD incorrectly // wrong way
we look at the shape of the graph
0 = peak with a very sudden drop
1 = peak with a more gentle drop
2 = symmetric peak!!!
whats the better way of determining the order of reaction
by looking at the behaviour of coverage by redoing the experiment a few times with different initial coverage.
describe the way of finding the order of reaction using diff initial coverage : aka looking at the behaviour of coverage by redoing it a bunch of times using diff initial coverage
u obvs chnage the coverage and u get diff peaks in diff places.
0th order: more coverage peak will be larger and to the RHS of the previous one. as u increase coverage,, the Tp also increases (aka the temp the peak is at)
1st order: more coverage peak will be larger and above the previous peak. aka increasing coverage keeps the Tp constant.
2nd order: the more coverage peak will be on the LHS of the less coverage peak. aka increasing coverage lowers the Tp.
when ur finding the order of the reactions, whats the axis
u have rate n the y axis
u have temp in kelvin on the x axis
how do u determine the activation energy of desorption
u have the rate of desorption // the mass spec signal on the y axis and temp in kelvin on the x axis
u have a peak of order of some sort of TPD.
the area under the TPD peak curve is proportional to the initial coverage.
u pick a temp and draw a line straight down and give this the name T1.
to the lhs = amount desorbed
to the rhs = amount of coverage still on the surface.
arrhenium equation and its straight line form to help us remember what to plot and wherw
k = A e -Ea/RT
ln(k)= ln(A) -Ea/RT
y = c - mx
y = ln(k)
x = 1/T
m = -Ea/R = gradient
c = ln(A) = y intercept
what happens when we plot arrhenius equation aka what do we need in order to plot it
we need the diff k values for diff temperatures but we keep the concentration constant
why cant the arrhenius thing be applied onto the desorption gradient in TPD in order to help us find the Ea of the desorption
bc conc is changing and so is temp,, as coverage changes.
the curve does tell u the rate of desorption but yhh,, not only temp is chnaging,, conc is too :(.
what can we do to apply the arrhenius equation onto the desorption TPD graph to help us find its Ea energy!!
we can do the same thing we do to arrhenius
by running the experiment at several different initial coverage values
starting a low coverage and building ur way up
describe finding the Ea of the TPD using different initial coverage values
u have ur graph with axis y being rate // mass spec signal
the x axis is temp in kelvin
the graph we used as the example is 1st order but basically ur gonna just keep on getting curves slightly above eachother
and ur baically gonna make a table of theta, time and rate
u do thsis by taking a temperature and measuring the area after that time ,, u measure the area to the rhs of the time line to measure the amount of coverage still there.
u keep repeating to get diff rates at specific temperatures that all have the same coverage. this is how we calculate the Ea and pre exponential factor.
