Formic acid can reduce H2PtCl6 in solution. So if I put a piece of carbon paper in the solution, the Pt will firstly be reduced on the carbon paper. And we can obtain short Pt nanorods.
Is it because carbon particles can provide the nucleation sites? I read some literature and the explanation about the nanorods is the formic acid will block or reduce the growth of the other crystal phase so it is 1 D structure.
Then some nano Au particles were embedded on the carbon paper by sputtering coating. Then I did the same as above: leave the carbon paper in the solution. What happens next is the Pt reduced by formic acid was different. The morphology of it changed and the Pt is more like nanowires or long nanorods on the carbon paper. Also, it was more uniform and less agglomeration was observed.
Does anyone have any idea why the Au affected the reduction of the Pt/growth of Pt and how I can prove it by experiment? Or there is any literature about it. Thanks a lot in advance.
This is how the Au induced Pt nanorods look like:
Yichang Yan
An interesting set of observations. Activated C is used as a support for many PGM catalysts e.g. 5% Pd/C used in hydrogenation of organic compounds.
As I see it your H2PtCl6.xH2O is absorbed on the high surface area carbon paper and (easily) reduced in situ there. There are specific active sites that may lead to preferential absorption in one place. At room temperature then Au and Pt are probably fully miscible (see attached phase diagram from JM, who sponsored my Ph.D by the way) - my experience in my Ph.D was with Pd-Au systems (which are fully miscible over the entire composition range). I would be co-impregnating the paper with HAuCl4.yH2O rather than sputtering and seeing if you get alloy formation by XRD. Both Au and Pt compounds are easily reduced around room temperature - H2 reduction is also worth looking at to see if you get a change in structure. In an earlier study I was able to absorb HAuCl4.yH2O on PTFE and these were reduced easily at room temperature.
The concentration of components together (plus conditions such as temperature) with the chemistry governs what will happen. These are the main considerations. One thing I learned early on was to avoid borohydride reduction - everything gets contaminated with intractable B which acts as both a contaminant and catalyst poison for many reactions.
So, in your Pt-Au system, look at alternate co-impregnation techniques which are more favored to lead to alloy formation (shown by XRD: peak symmetry and Vegard's rule is close to being obeyed) and no additional peaks. And, of course, an alloy is likely to have different properties than the pure components. This is certainly true in the Pd-Au and Pd-Ag systems that I studied.
Please use the edit button (bottom left) to edit the typos in your question: mechanism not 'Mechasim', for example...
Alan F Rawle
Really appreciate your answer.
And you are right about the alloy. By adding HAuCl4 to the solution, we can obtain Pt-Au alloy and with AgNO3, we can even get AuAgPt alloy. However, the problem is these catalysts do not help with the ORR. This is why I tried Au sputtering.
Meanwhile, by XRD, there is no peak of Au observed. Another thing is if I use Pd rather than Au sputtering, the morphology of Pt will change too. It will be smaller and the diameter will also decrease more like needles. But just have no idea how the Au and Pd affect the Pt growth on the carbon.
Yichang Yan
By ORR, do you mean overall reaction rate? If so, then this is a function of composition. Normally one looks at TOF - turnover factor or frequency. Look for a paper by Allison & Bond from 1972 on Pd-Au Alloys. I’ll look it up when I am off my hang glider.
Yichang Yan
OK, down on land again. The reference is: E. G. Allison & G. C. Bond (1972) The Structure and Catalytic Properties of Palladium-Silver and Palladium-Gold Alloys, Catalysis Reviews, 7:2, 233-289, DOI: 10.1080/01614947208062259
This shows maximum rate approximately at Pd60:Au40 composition for reactions such as butadiene hydrogenation. I worked on isoprene hydrogenation for similar catalysts I made.
Geoff Bond was my Ph.D supervisor. I note that the citation refers to Brunei, and not Brunel, University (when running your cursor over the author's names)... Tells you something about Taylor & Francis, I guess...
TOF gets discussed in (self-citation!): Geoffrey C. Bond, Alan F. Rawle, Catalytic hydrogenation in the liquid phase. Part 1. Hydrogenation of isoprene catalysed by palladium, palladium-gold and palladium-silver catalysts, Journal of Molecular Catalysis A: Chemical, Volume 109, Issue 3, 1996,
Pages 261-271, ISSN 1381-1169, https://doi.org/10.1016/1381-1169(96)00027-1.
Yichang Yan
In terms of no Au XRD peak then the low loading combined with the small size is the reason. I needed 24 hours under the x-ray beam for my 5% Au/SiO2 catalysts.
Alan F Rawle
Thanks a lot for your help. Yes, the loading of Au on the GDL is really low. I checked it last week and it was 0.5 wt% of the Pt, which is about 1 ug/cm2. Another reason might be that the Au on the GDL is amorphous as the intensities of C peaks decrease with the increasing thickness.
Sorry, forgot to explain the ORR. ORR means oxygen reduction reaction which happens at the cathode of a fuel cell. Now the probable story is that Au somehow influences the speed of formic acid reducing H2PtCl6 thus we got Pt nanorods.
The first image is Pt reduced by formic acid only. The second one is Pt reduced by formic acid on an Au-sputtered GDL.
But have not figured out a method to directly prove this.
Yichang Yan
Read the Allison/Bond paper. There is a maximum in reaction rate for a bunch of differing reactions with a maximum at ~ Pd60Au40 (I'm aware that this isn't your system, but there can be similarities, and useful examples to glean). Possible reasons:
- The Pd (active) is diluted, the atoms more separated, and catalyst poisoning could be more difficult
- Different crystal faces get exposed as the proportions of elements change (a classic example is ppm of ferrocyanide doping causing NaCl to crystallize in the octahedral rather than cubic shape)
Now Pd and Au form a set of completely miscible solutions over the whole composition range and Vegard's rule is close to the observed behavior. This isn't the case for Pt and Au. Look up the phase diagram in this article: Article Thermodynamics of nanoalloys
You could speculate that there will be a change in the proportion of exposed crystal faces over the composition range. One to investigate. Also be aware of chemisorption induced segregation - are you using XPS? If not then you should be. If you have 0.5% Au in Pt then virtually zero of it could be exposed on the surface (the oxygen in the air prefers the Pt and draws it to the surface). XRD has a penetration range (depending on the elements) and XPS is 5 - 10 atomic layers. These may be reasons for weak or non-existent peaks. However, I'd be looking at several 10's of hours of x-ray exposure to see if you can get a Au peak (amorphous or crystalline - I still suspect the latter at small sizes). My earlier comment of 24 hours exposure for my Pd alloy catalysts (nominally 5% total metal loading) still applies I beleive.
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