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Fireside Chat – Catalytic Resonance Theory – Dr. Omar Abdelrahman

Summary
Dr. Omar Abdelrahman, Assistant Professor at the University of Massachusetts – Amherst, and Dr. Andrew Jones at Activated Research Company discuss Catalytic Resonance Theory – a dynamic catalysis approach involving oscillating between potentials to balance kinetic and thermodynamic needs for better catalytic performance.

Topics

1.[00:00]Prelude

2.[02:30]Introduction

3.[04:41]Dynamic Catalysis

4.[05:26]Results and Ramifications

5.[10:09]Disruption

6.[11:04]Poisons

7.[12:22]Analytics + Jetanizer™

8.[14:44]Future Directions

9.[16:26]Last Act

Prelude[00:00]

I loved reading this work. I loved the Catalytic Resonance Theory. It’s one of those things where I need to sit down and do the math, so it makes more sense to me – but I understand it. I’ve got your paper right here. It was a good read.

Glad you enjoyed it.

The easiest way to explain it is you’re changing these potentials, which is like changing temperature and thermal catalysis. You’re trying to balance your kinetic rate constant and your coverages. You can never find one condition that properly balances them both, so you’re oscillating between conditions that satisfy kinetics and thermodynamics at a time scale which lets you maximize both.

I think in our case, we knew there was a lot of complexity associated with understanding these dynamic catalyst systems. We wanted to do it with a precious metal like platinum because our goal was to show that through dynamic oscillations – you can exceed some of the best static catalysts – so that’s why we chose platinum. To avoid complexity, we wanted to work with bulk platinum to avoid support effects and things along those lines. The challenge with that was you have very low surface area of exposed platinum, so if you were to use something like a thermal conductivity detector to quantify the CO2 that’s formed in the course of formic acid oxidation, which we chose as a model reaction, it would be almost impossible, if not just highly inaccurate, to try and quantify CO2 with the thermal conductivity detector given the low surface area of bulk platinum. We wanted to avoid supports which could disperse the platinum because we didn’t yet understand what role the support was going to have.

When you say bulk platinum this was like a mesh of platinum?

Even simpler – just a thin platinum wire.

Oh, that makes sense.

Let’s go ahead and get started from the beginning.

Introduction[02:30]

Hello everyone. I’m here today with Dr. Omar Abdelrahman talking about his work on Resonance Promoted Formic Acid Oxidation by a Dynamic Electro Catalytic Modulation. That’s a lot of words Omar – what does it mean?

The basic idea is when you apply a potential to facilitate a Faraday chemistry, it’s hard to balance both your kinetic needs and your thermodynamic needs for a catalyst. The idea is instead of trying to find one voltage, or one set of material, that can satisfy both – we would instead oscillate between two distinct potentials. One which favors kinetics and another which favors thermodynamics. As a result, you get a pseudo-steady state that satisfies both of those needs better than just a static applied potential.

Interesting. When you say thermodynamics, you’re talking about the absorption step of the formic acid coming onto the platinum?

That’s right – the absorption step and the coverage of other relevant reactive intermediates. You want to be able to have sufficient coverage of them to get a higher rate of reaction but doing that tends to sacrifice the value of your kinetic rate constitute.

Okay. So, you go to these low potentials which basically “sucks up” the formic acid which gets the surface coverage you want – and then you ratchet up the voltage to push the reaction forward from those surface intermediates. Is that more. or less, correct?

In a sense, yes, that’s exactly it.

Awesome. I understand this dynamic catalysis was invented at the University of Minnesota while you were there – you and Dr. Paul Dauenhauer. It’s a pretty new field – what made you choose to go in this direction?

Dynamic Catalysis[04:41]

In the field of heterogeneous catalysis – there’s been a long-term challenge of “How can we design more effective catalysts?” The focus has mainly been on material design – but we’re facing an upper bound on what material design alone can do. So, out of necessity, we’ve been trying to accelerate the rate of many important catalytic reactions and have realized there is an unexplored dimension which is Time.

Yeah, it’s pretty cool. What did you conclude from the work?

Results and Ramifications[05:26]

We concluded that even with inexpensive, off-the-shelf catalysts that don’t require any exotic synthesis – one can increase the rate of a reaction, by orders of magnitude, by applying energetic oscillations. Moving away from steady state operation – at the catalyst layer only – applying oscillations creates a manageable form of reactor operation that significantly improves the rate of an existing catalytic system.

Wow, this has ramifications in every area of catalysis. Do you think this could be applied in all areas – or is there a need for oscillating some electronic property?

On the surface, it seems that – for any heterogeneously catalyzed reaction – Can you find an energetic stimulus that your catalytic chemistry is sensitive to? Can you apply it to the catalyst surface on a time scale that is comparable to the catalytic chemistry? If the answer is yes, it’s definitely worth exploring whether you can dynamically operate your system.

That time scale piece is pretty important! You’re oscillating at a time scale that’s relevant to the turnover frequency. Is that it – is it to the adsorption events?

It’s a little bit of both. You want to oscillate your surface at a frequency that is faster than what your catalytic surface can re-equilibrate. If you go with too slow of an oscillation, your surface says, “I don’t really care about what you’re trying to do – I can re-equilibrate and hit those same steady state coverages.” if you go with too fast of an oscillation, it’s like a hummingbird flapping its wings. The human eye can’t process the image fast enough and it looks static. There’s a sweet spot in the middle where you’re oscillating at a frequency that matches the natural turnover frequencies of the catalytic material.

I see. I assume you’ve still got to worry about mass transfer. You can improve the catalyst a lot, but in many of these reactions, you might still need to worry about mass transfer limitations.

Yes, absolutely. As you start to increase the rate of reaction, you must worry if your products can diffuse out of the porous network fast enough, or if you can replenish the catalyst surface with reactants fast enough on the bulk phase. That’s part of the reason why on the first experimental demonstration of this work – we chose to go with a non-important system. We focused entirely on what’s happening on the catalyst surface and would later add the complexity of a real catalyst.

I love the simplicity. You chose a platinum wire, can oscillate the voltage, and look at formic acid decomposition. That’s very cool. Could you take an iron wire, which I assume is a thousand times cheaper than platinum, and repeat the same experiment and maybe get rates that are similar to platinum at steady state?

You’ve hit the nail on the head. That’s exactly where we want to go. We’re currently working toward using cheaper metal catalysts – iron, or maybe a coinage metal like copper – and seeing if we can get them to catalyze reactions at a rate comparable to precious metal catalysts. That would be revolutionary as we wouldn’t need as many precious metals as we have in the past.

I see a lot of ramifications. Ammonia synthesis, hydrogen synthesis, or any of the large reactions that we do with either precious metals or at conditions that are expensive. Do you think this could disrupt a lot of that?

Disruption[10:09]

Absolutely, especially in cases where you have high pressure reactions which can be a killer from any process technologies – particularly when you attempt to scale them down. If we could increase the rate of reaction by virtue of these energetic oscillations – at a cost that is significantly cheaper than the cost of pressurization – both on the capital and the operating side, we think this could make a significant impact in those areas.

Have you looked at poisons? I assume if you had a sulfur bound to a catalyst surface you could change the voltage to push that sulfur off? I’m thinking about hydro desulfurization, or other sulfur sensitive reactions. Could this approach be used in those cases?

Poisons[11:04]

I think you could. You can find an energetic stimulus that can affect the strength with which sulfur is binding to the metal surface – or how fast one can oxidize, or reduce, the sulfur off the surface. That can definitely be applied. In the case of formic acid, we have an analogous situation where for the longest time – formic acid decomposing to carbon monoxide in these electric catalytic systems was considered a poison because you couldn’t quite get it off the surface, even though you could decompose it to carbon monoxide quite rapidly. We found that if you oscillate the surface in terms of its applied potential, you’re actually accessing that route of formic acid completely decomposing to CO2, but through the carbon monoxide route which was previously considered a poison because you couldn’t complete the turnover.

Interesting. You’re essentially accessing a new transition state.

Yes. It’s steering the pathway selectivity towards the same end product.

Yeah, wow. Since we’re an analytical company, tell me a bit about how you did the analytical piece of this. How you measured your components, and how you decided what to do there.

Analytics + Jetanizer™[12:22]

The major challenge we faced was the desire to work with the platinum wire. You have a very small number of active sites, and that translates into relatively low concentrations of our product, in this case carbon dioxide, which we needed to quantify. Traditional analytical techniques like gas chromatography equipped with a thermal conductivity detector, or even a methanizer – were challenging to use due to sensitivity. Instead, we realized we could simply use ARC’s Jetanizer™, which was appealing because it was cost effective, very easy to add on to an existing GC system and gave us very high sensitivity towards carbon dioxide. By virtue of having a flame ionization detector that could see the product – that really allowed us to quantify TPM, and at times PPD, levels of CO2. It made it possible to get that chemical, or analytical chemistry, certainty when quantifying our products.

You were using GC coupled with a Jetanizer™ FID to measure CO, CO2 with some sort of carbon column. What was your column choice, do you remember?

We were using a PLOT Q column. We were trying to get anything that could hold on to it for quite some time and separate on CO and CO2. It worked great. We wanted a capillary column but didn’t want to dabble too much into micro packed or cryogenic separation, so that worked out nicely for us and we could quantify CO and CO2 and the added advantage of the Jetanizer™ is we realized they both had the same calibration factor since we were turning into methane which – made our life on the analytical side much easier.

Easier than TCD for sure. What’s the follow-on with this work? Where are you going to go next? Analytically, are there any choices you would change in the future?

Future Directions[14:44]

Where we’re going next is working with more complex reactions. Complex in the sense we now have the challenge of product selectivity, not just pathway selectivity. We’re looking at more relevant electrochemical oxidation technologies so we can try to control the oxidation selectivity toward value-added products. We’re going to have a much more complicated mixture of products, where we’re going to be using something like a Polyarc® device because we have more than just CO and CO2 to consider.

That’s awesome. You’ll have to start looking into hydrogen electrolysis too. See if you can do it – and then call me when you make your first billion dollars.

That’s the dream. That we can get those water electrolyzers to speed up without having to use something like iridium oxide or platinum for your cathode and your anode. That would be the dream.

Any work in that area from your group, or is that future?

We hope to get into that space in the future – maybe not just with applied potential oscillations. There are other strategies – other forms of energetic stimulus we’re looking into that we think can make a difference. In particular, we’re looking into helping on the cathodic side, or anodic side, of reactions.

That’s smart.

Last Act[16:26]

Well, this has been fantastic Omar. It’s always fun to learn more about this topic and see where the future is headed. You’re forging a pretty cool path here. I’m excited to see what comes out of it.

Thank you very much. It’s always great talking with you, Andrew.

Thank you, Omar. Good luck this semester and don’t be a stranger.

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