Scratching the Surface: Subsurface Hydrogen Aids CO Hydrogenation to Methanol With Nickel

Title: Hydrogenation of CO to Methanol on Ni(110) through Subsurface Hydrogen

Authors: Adam P. Ashwell, Wei Lin, Michelle S. Hofman, Yuxin Yang, Mark A. Ratner, Bruce E. Koel, and George C. Schatz

Year: 2017

Journal: Journal of the American Chemical Society, DOI: 10.1021/jacs.7b09914

Scratching the Surface: Subsurface Hydrogen Aids CO Hydrogenation to Methanol With Nickel

In today’s Chembite we explore some remarkable mechanistic aspects of the hydrogenation of CO at a nickel surface. The paper covered gives the first account of catalytic methanol and formaldehyde production from CO by Ni. But to explain why we need to go deeper than the surface…

How do reactants from the gas phase combine to form products on the surface of a solid catalyst? It is quite a fundamental question. It is also important for the chemical industry – hydrogenation (the addition of hydrogen) of CO affords compounds such as methane, formaldehyde and methanol, which are useful products and building blocks for the chemical industry that produces medicines, fuels and chemicals for agriculture. As such, chemists and physicists have worked on this problem for many decades.

Even if you have not heard of the Eley-Rideal or Langmuir-Hinshelwood mechanisms by name, you may be familiar with them. Proposed in the first half of the 20th century, they describe possible ways by which two molecules from the gas phase can react on a surface. In the Eley-Rideal mechanism, reactant ‘A’ adheres to a reactive site on the surface (it is said to ‘adsorb’ to the surface, S) and then reacts with a gas-phase reactant ‘B’ to form products. In the Langmuir-Hinshelwood mechanism, both gases must adsorb before they react. The mechanisms are shown below.

Eley-Rideal mechanism:

A(g) + S(s) ⇌ AS(s)

AS(s) + B(g) → Products

Langmuir-Hinshelwood mechanism:

A(g) + S(s) ⇌ AS(s)

A(g) + S(s) ⇌ BS(s)

AS(s) + BS(s) → Products

Although these mechanisms are a century old and seem quite simplistic, they are so fundamental and powerful that they are still referred to in the modern literature. What has changed since then is our level of understanding about what goes on just underneath the surface of the catalyst, which brings us to the research paper.

As the authors point out in the paper, nickel is not the only metal capable of hydrogenating CO. But change the metal and you also change which products you get. Copper surfaces favor methanol production, whilst cobalt favors methane. Just as it sits between Cu and Co in the periodic table, Ni sits in the center ground. Prior to this paper, Ni had been known to produce methane but theoretical work has suggested methanol could also be produced using a Ni catalyst. The methane production arose from a different type of surface structure on the Ni, this paper studies the more unsaturated ‘step-edge’ surface, which is a quite exposed and reactive surface.

The authors used a theoretical method called density functional theory, in which energies are obtained from electron densities. The authors calculated the energies of several possible reaction pathways for both methane and methanol formation. They discovered that pathway 1 (figure 1) wasthe most favorable pathway (i.e. providing the lowest energy barriers to reaction) for methanol production. At every step, they found that hydrogenation, which leads to methanol, was more favorable than the C-O bond breaking that would lead to methane.

Figure 1: Possible hydrogenation mechanisms investigated in the paper.

Interestingly, the authors then entertained the effect of hydrogen atoms, one of the reactants, actually going into the layer beneath the catalyst surface. How do these atoms get below the surface? They either hit the surface with so much energy that they overcome the large barrier to entry, or they are hammered into the surface when a high-energy gas molecule crashes into an adsorbed H atom.

It turns out that, when the H atoms hydrogenating the CO start off under the catalyst surface (as so-called ‘subsurface’ H atoms) the energetics of methanol formation become much more favorable. Although there is quite a high energy barrier a H atom getting out of the subsurface, those that have a high enough kinetic energy to escape are it extremely reactive and capable of overcoming energy barriers. Overall, the energy of the system is lowered considerably when the subsurface atoms re-emerge to react. It is interesting how the energy input to the reaction can be tracked, from the H atoms being plunged into the subsurface to their return to the surface primed for reaction.

Additionally, the paper also includes simulations of the Eley-Rideal-type mechanism in which CO adsorbed to the surface of the Ni was bombarded with H atoms. The results suggested that product formation was highly unlikely in these encounters, allowing this mechanism to be ruled out.

Finally, moving to an experimental approach, the authors were subsequently able to show that their step-edge-like surface did, as their calculations suggested, produce both methanol and formaldehyde. This was done by measuring the masses of the products using a mass spectrometer. Going back to pathway 1, it is worth pointing out that formaldehyde is an intermediate and could desorb before it reacts. The authors note that further hydrogenation is slightly more favorable energetically but the result suggests that a fraction of the molecules can desorb before they react again.

This is the first report of a Ni catalyst producing either methanol or formaldehyde from CO reduction, and indicates that Ni is able to produce different products depending on the type of surface used. It goes to show that even in heterogeneous catalysis it is important to look deeper than the surface!

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