The Catalyst Review June 2024 - 7

SPECIAL FEATURE
SPECIAL FEATURE
Catalytic Mechanism Overview
In this section, we will introduce the hydrogen
spillover mechanism, which is key to the
efficacy of the methanol production process.
The hydrogen spillover mechanism involves
the migration of hydrogen atoms from their
initial adsorption location on the copper
surface of methanol synthesis catalyst to the
adjacent zinc oxide surface. Its importance
lies in the fact that once the hydrogen atoms
are adsorbed onto the zinc oxide surface,
they are available to react with CO2
, and
kickstart the chain of reactions that eventually
produce methanol. The formate formed in
the initial reaction of the hydrogen atom
and CO2
is thought to then either dissociate
Figure 2 Applications of Methanol. Source: IRENA 2021
Factors Impacting the Hydrogen Spillover Mechanism
Many factors have been found to impact this mechanism, and therefore impact methanol production efficiency. This mechanism
has become the subject of numerous research and development efforts in recent years with the number of publications discussing
Cu/ZnO-based catalysts and their chemistry increasing
steadily every year since 2000. More than 1,200 papers were
published in the years 2020 and 2021 alone (Salmi 2023).
Figure 3 (a) Methanol selectivity; (b) CO2
conversion; (c)
methanol STY; and (d) CO STY in methanol synthesis from CO2
hydrogenation over a series of catalysts. Source: Salmi 2023
The Catalyst Review
One such factor is the metal-oxide interface along which
the hydrogen atoms diffuse to the active oxide sites. Higher
temperature also impacts the hydrogen spillover mechanism
by increasing the mobility of the hydrogen atoms. Other
important factors are the metal loading of the catalyst
and use of promoting agents. For example, higher metal
loading typically leads to the presence of more active sites
for hydrogen atoms to adsorb onto within a given volume
of catalyst. There is a limit to the benefit, however, leading
to the need for research on optimization. Increased copper
loading leads to higher hydrogen dissociation and spillover
but can also lead to increased sintering of the catalyst.
Promoting agents such as the noble metal palladium have
been extensively researched to find optimal concentrations,
as well. Figure 3 shows the results of one such study in
which various parameters were studied leading to the
conclusion that optimal palladium loading seems to be 1%.
into carbon monoxide and hydrogen gas or
follow the desired synthesis reactions via
further hydrogenation reactions forming
dioxomethylene, formaldehyde, methoxy, and,
finally, methanol. Studying and optimizing
this crucial part of the methanol production
process is therefore of great interest (Salmi
2023).
June 2024
7

The Catalyst Review June 2024

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