The Catalyst Review October 2024 - 13

Experimental Abstracts
Hydrogen Spillover Mechanisms on Copper on Zinc Oxide-Based Catalysts for
Carbon Dioxide Hydrogenation to Methanol
Processes for converting CO2
well-established approach involves the hydrogenation of CO2
into valuable chemicals are seen as an effective means to mitigate climate change. One
to produce methanol using Cu/ZnO-based catalysts.
In this process, zinc oxide facilitates the splitting of hydrogen molecules, while copper provides sites for methanol
formation. The catalysts' effectiveness is further enhanced by hydrogen spillover - the transfer of hydrogen atoms from
zinc oxide to copper, a process that reduces the energy barrier for methanol production. Herein, the author reviews
critical factors that influence hydrogen spillover in Cu/ZnO-based catalysts and discusses the various methods used to
observe and analyze this phenomenon.
Chemisorption of hydrogen is a critical step in the hydrogen spillover mechanism on Cu/ZnO-based catalysts,
including the industrial catalyst Cu/ZnO/Al2
O3
used for CO2
hydrogenation to methanol. Hydrogen is first adsorbed
onto the copper surface of the catalyst as hydrogen molecules (physisorption). The hydrogen molecules then dissociate
into hydrogen atoms on the copper surface, which are strongly bound to the surface by chemical bonds chemisorption).
These chemisorbed hydrogen atoms can then migrate from the copper surface to the neighboring zinc oxide surface
via a process called spillover.
Observation of hydrogen spillover includes techniques such as hydrogen temperature-programmed reduction (H2
-
TPR) and temperature-programmed desorption (TPD), as well as in-situ characterization techniques like lowtemperature
scanning tunneling microscopy (LT-STM), which can be used to visualize atomic hydrogen spillover.
Other methods include in situ spectroscopy and X-ray absorption spectroscopy. Together, these methodologies help
to identify the types of active sites involved in the spillover process.
There are a number of
factors that can influence the
hydrogen spillover mechanism,
including the
metal-oxide
interface, metal loading, and
temperature. The metaloxide
interface is a critical
factor because it is where the
hydrogen atoms diffuse from
the metal to the oxide. Higher
metal loading usually means
a higher
active
site
density,
which results in more adsorbed
hydrogen atoms for spillover.
Temperature also affects the
hydrogen spillover mechanism,
as higher temperatures will
increase the mobility of the
hydrogen atoms. In addition,
some researchers are exploring
various metal loading strategies
to optimize hydrogen spillover
performance in the Cu/ZnO/
Al2
O3
are
catalyst, while
others
introducing additional
metals or promoters to the
catalyst to enhance its activity
and selectivity and reduce
deactivation (Figure 1).
The Catalyst Review
Figure 1. (a) Methanol selectivity; (b) CO2
synthesis from CO2
conversion; (c) methanol space time yield (STY); and (d) CO STY in methanol
hydrogenation over a series of catalysts
October 2024
13

The Catalyst Review October 2024

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