The Catalyst Review June 2024 - 8

SPECIAL FEATURE
This optimal loading led to a 2.5-fold increase in the methanol space time yield or a more than double increase in the amount of
methanol produced per unit of time in a given volume of reactor at 543 K compared to undoped Cu/ZnO catalyst. Other research
has looked at bimetallic catalysts incorporating noble metals such as the aforementioned palladium and gold in addition to
copper. Positive effects observed from the use of both metals include higher activity and improved hydrogen spillover (Salmi
2023).
Another class of promoting agents being researched are alkali metals such as potassium and caesium. Their low ionization energy
requirement allows these metals to donate electrons to the copper surface easily, thereby improving the electronic structure of
the catalyst surface and encouraging the dissociation of hydrogen gas into hydrogen atoms. This improves catalyst activity and
selectivity for methanol production as it leads to stronger adsorption of hydrogen and higher dissociation. In addition to alkali
metals, transition metals such as titanium, cerium (IV) oxide and zirconium dioxide have been found to improve performance
by increasing the oxygen storage capacity of the catalyst, and therefore increase the reducibility of the catalyst. A more highly
reduced catalyst means there will be more oxygen vacancies or defects in the zinc oxide lattice. These defects are thought to
create higher availability of active sites, thereby increasing the hydrogen spillover. These promoters, similarly, to alkali metals, can
improve the electronic surface of the catalyst and enhance the hydrogen spillover mechanism (Salmi 2023).
As previously mentioned, the reduction of oxides in the catalyst has a large impact on the hydrogen spillover mechanism. Further
benefits include supposed activation of CO2
molecules, which encourages them to react with the hydrogen atoms. These oxygen
vacancies are also thought to enhance diffusion and mobility of hydrogen atoms as the reduced catalyst surface has a higher
surface area. Lastly, the reduction of oxides makes byproduct carbon monoxide (CO) less likely to bind to the catalyst. This is
important as CO is a catalyst poison; when it binds to the catalyst, the catalyst is deactivated. Due to these benefits, the reduction
of a newly installed catalyst charge is a very important step in ensuring good catalyst lifetime and activity. There are several
methods by which this can be done, including calcining, hydrogen reductions and plasma reductions (Salmi 2023).
As alluded to earlier, the presence of compounds such as oxygen and carbon monoxide can have negative impacts on the
process. Having excess oxygen present has a negative impact on the hydrogen spillover process by leading to the undesirable
formation of surface oxides on the catalyst and lowering the concentration of active sites. Carbon monoxide similarly prevents
hydrogen from adsorbing from the catalyst surface by competing for and adsorbing to active sites. Optimizing the amount of
oxygen and carbon monoxide present in the process and enhancing the ability of the catalyst to be reduced are areas of interest in
helping mitigate the impact of these compounds on the hydrogen spillover mechanism. Another compound that can influence the
process is water. Water's role is more nuanced. Its presence can promote the spillover effect by improving the mobility of hydrogen
atoms, but it can also decrease spillover by occupying active sites and dissociating into hydroxyl groups which adsorb to these
active sites in lieu of hydrogen (Salmi 2023).
Studying the Hydrogen Spillover Mechanism
It is clear that the hydrogen spillover mechanism is of great importance to the production of methanol. As such, research efforts
to clarify and optimize the amount of hydrogen spillover occurring in the methanol production process are ongoing. There are
several cutting-edge methodologies being used to do this. The scavenger method is one of these. First, a stream of hydrogen
gas is passed over the catalyst. This is followed by a scavenger molecule stream such as carbon monoxide or nitrous oxide. The
scavenger molecules react with the hydrogen atoms that did not spillover from the copper surface to the zinc oxide surface.
The amount of hydrogen that was scavenged can then be measured via infrared spectroscopy and compared to the amount
of hydrogen that was introduced to the process initially. This method has been used to show that hydrogen spillover impacts
catalytic activity, and it has also been used to measure the kinetics and thermodynamics of the spillover mechanism (Salmi 2023).
Second, there are the isotope methods in which different isotopes of hydrogen are used to study the movement of atoms on the
catalyst. In the hydrogen labeling variety of these methods, hydrogen gas is enriched with either deuterium or tritium atoms that
then become incorporated into the methanol product. The differences in the respective amounts of isotope in the process gas and
product can be measured and compared via mass spectrometry. A second isotope method called hydrogen exchange involves
first exposing the catalyst to deuterium gas, and then CO2
gas. The deuterium atoms exchange with hydrogen atoms, and the
amount of deuterium in the product is measured. This method has shown that chemisorbed hydrogen needs to be present for
there to be effective CO2 conversion to methanol and has helped elucidate the ways in which the hydrogen spillover mechanism
works. Another isotope method is temperature-programmed desorption. In this method, a hydrogen isotope gas stream is passed
over the catalyst at a low temperature, the temperature is then increased, and the desorption of the isotope atoms that have not
spilled over to the zinc oxide surface is measured as a function of temperature. Lastly, there is the isotopic exchange method.
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The Catalyst Review
June 2024

The Catalyst Review June 2024

Table of Contents for the Digital Edition of The Catalyst Review June 2024