Dear Adhidesh Kumawat,

In a first approach I would state that thermodynamics provide the necessary condition for the stability and thus decomposition of metal oxides. However, I would guess that for individual metal oxides the kinetics of decomposition will appear relevant. At this point, experiments will enter on stage. So, good luck! 

Dear Prof. Van,

It would be really helpful if you could share some literature related to the above phenomenon. Thanks for your response.

Dear Adhidesh Kumawat,

The stability of metal oxides can be obtained from standard thermodynamic data. Being retired I have no direct access to library resources. My point is that the kinetics of decomposition in case the metal oxide is thermodynamically instable, will play a part. I guess that this often implies case study of the decomposition behaviour of the metal oxide concerned.  

Greetings from P. van Mourik

Dear Prof. Van,

From ellingham diagram we can come to know that PbO reduction to Pb is not possible directly i.e. by using just P and T conditions, but is it possible that PbO is reduced by X ray irradiations that it come across during XPS analysis..??

Dear Adhidesh Kumawat,

X-rays initiating PbO-decomposition would certainly not be my first guess, so I would advise you to check the total assembly of your experimental conditions/circumstances. The oxygen atoms cannot disappear?

Dear Prof. Van, 

PbO catalyst was investigated for Xray diffraction and only PbO and no Pb was detected in the results obtained, but in XPS analysis of the same catalyst, I obtained peaks of both Pb and PbO.

This is the element of confusion,

First, I thought PbO might get reduced due to vacuum and low pressure conditions, but on observing "Elingham diagram" i found that this option was not possible.

Second guess deterioration of PbO to Pb due to high energy X ray beams, but again energy of Xrays in XRD is multiple times higher than those used in XPS and hence that possibility is also negated.

I want to know if there is any other possibility also that I am not able to realize to cause PbO transformation to Pb..?

Dear Adhidesh Kumawat,

1. PbO-decomposition is not compatible with the thermodynamical condition: there is no transformation at all from PbO to Pb according to the evidence as yet available.

2. Mass conservation is maintained for Pb as well as O.

3. X-Ray diffraction and XPS are different investigation techniques: is it possible that XPS 'sees' some Pb-peaks originating from PbO as single Pb peaks?

4. In the chemical formula of PbO lead en oxygen are equivalent: are you quite sure that this equivalency holds in your case? Otherwise stated, the formula might be less stoichiometric, like PbO1-x.

PS: my first name is 'Pieter' and my family name is 'van Mourik'

Dear Prof. Van Mourik,

Sincere apologies for misspelling of the name.  :(

This doubt of appearance of Pb peaks in PbO has become more troublesome after I have taken more points in to account:

1. XRD observes surface depth upto 200 nm and XPS observes till about 10 nm. So, if XPS detects Pb then it has to inevitably detected in XRD.

2. XRD uses hard XRays (10 keV) whereas XPS uses soft Xrays (600eV) for analysis, hence again if the transformation is due to irradiation, then again XRD has to show higher degree of conversion than XPS.

3. As per JCPDS database, for (x>0), no compound of Pb1-x have been reported, instead all the compounds reported are either PbO or PbO1+x.

I am really appreciating the experience, knowledge and time that you are  spending to reply to my doubts and answer the queries giving it a further direction.

P.S.: Apologies again for misspelling. :(

Dear Adhidesh Kumawat,

1. "From ellingham diagram we can come to know that PbO reduction to Pb is not possible directly i.e. by using just P and T conditions", but are you performing your experiment on P,T-conditions? Suppose that your high vacuum is such that the partial oxygen pressure is lower than the equilibrium oxygen pressure given by the decomposition chemical reaction, then it would be possible that PbO decomposes. However, my guess would be that such a high vacuum is rather difficult to achieve, which is confirmed by your statement.

2. "XRD observes surface depth upto 200 nm and XPS observes till about 10 nm. So, if XPS detects Pb then it has to inevitably detected in XRD". This 'jumping to a conclusion', neglects the large difference in exposure depth yielding possibly different ratios in peak/background radiation.

3. XPS observes upto 10 nm depth; clearly, surface effects may play a role, e.g. due to stresses compensating adhesive substrate stresses that may yield shifts of some of the PbO-peaks. Of course, this suggestion should be taken with caution, but PbO-decomposition must be excluded on thermodynamical grounds.

4. At the moment I see no other possible explanations as such surface effects; hence I must apologize for leaving this discussion by wishing you good luck!

Dear Prof. Van Mourik,

Thank you for your suggestions and precious time to guide me through the doubt.

Regards,

Adhidesh