Hi Friends,
I have prepared Pd impregnated carbon based catalysts using HCHO and NaBH4 as the reducing agents. From XPS, it has been observed that the surface of the catalysts contains both Pd0 and Pd+2 and the % of Pd+2 for HCHO and NaBH4 reduced catalyst are 70 % and 74% respectively. But, the H2 chemisorption analysis shows a higher H2 consumption value for HCHO reduced catalyst (0.091 mmol/g) and a less value for NaBH4 reduced catalyst (0.061 mmol/g), although the % of Pd+2 is less in the case of HCHO reduced catalyst. How can I justify this observation? Can any body suggest what could be the reason for the same?
Thanking you...
ANSARI P
So your samples are
A: 70% Pd2+ and 30% Pd0 (more active)
B: 74% Pd2+ and 26% Pd0 (less active)
Correct?
I mean, I don't have too much overview in the field, but it doesn't seem counterintuitive to me that neutral Pd has the stronger catalytic activity. Of course your catalyst particles may also have differently covered surfaces, there are plenty of options.
Dear Jurgen,
You are correct. The catalyst with 30% Pd0 is more active. But the point is, it is having less % of Pd+2 compared to catalyst B. Hence the volume of hydrogen consumed during H2 chemisorption should be less as the amount of hydrogen required to reduce Pd+2 is less, am I correct. Instead, it is giving high value.
Dear Ansari Palliyarayil ,
The problem with H2 chemisorption in this case is that it is possible that the support is also adsorbing H2, so you may be having an erroneous measurement, I recommend that you replace H2 with another probe molecule.
Thank you for your reply dear Jhonatan. But the thing is, for both the catalysts, the support used is same. In that case, effect will be same for both, am I correct?
What is the dispersion of Pd in the samples? And how were the samples prepared prior the chemisorption? I guess that You had a reduction step at about 300-400 C before the measurement? I so all (close to) the surface Pd sould be as Pd0 . There might be some spillover of H2 into the bulk of Pd and also into the support.
The problem is that you are including more uncertainty to your measure since as Henrik says, with H2 a spillover effect can be produced and in the end you will not know how much is adsorbed by the Pd and how much is adsorbed by the support. I suggest you use CO or another probe molecule to do the chemisorption experiment.
Dear Henrik,
Thank you for your response. The percentage metal dispersion obtained from H2 chemisorption is 23 % for NaBH4 reduced catalyst (Pd particle size - 4.9 nm) and 26% for HCHO reduced one (particle size - 4.3 nm). And yes you are correct, prior to analysis, the samples were subjected to reduction with H2 at 300 deg C for 1 hr. Will there be any effect from the reducing agent used, I mean, formaldehyde?
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