# Nature and Dynamics of Active Sites in Cu-Based Catalysts for the CO2 Hydrogenation to Methanol

## TL;DR

This paper explores how copper-based catalysts work for converting CO2 into methanol, focusing on the role of active sites and how reaction conditions affect their performance.

## Contribution

The paper introduces a combined experimental and computational approach to identify active sites and understand dynamic changes in Cu-based catalysts during CO2 hydrogenation.

## Key findings

- Metal/oxide interfaces and alloying processes significantly influence catalytic activity and selectivity.
- Ab initio molecular dynamics and metadynamics help model dynamic changes in catalysts under reaction conditions.
- The oxygen chemical potential affects the stability and nature of active sites in Cu-based catalysts.

## Abstract

Catalytic CO2 hydrogenation
to methanol
is among the
most attractive routes in CO2 conversion, as methanol is
a chemical feedstock and a relevant energy carrier for the sustainable
methanol economy. Cu-based catalysts are the typical choice for this
reaction, and Cu-ZnO-Al2O3, the industrial reference
material for hydrogenating CO (in the presence of CO2),
has also been shown to perform well for CO2 hydrogenation.
Adding other elements to Cu NPs as promoters (Zn, Ga, In, etc.) and
using specific supports (ZrO2, Al2O3) enhance the catalytic activity and selectivity of Cu toward methanol,
often minimizing the undesired competitive Reverse Water–Gas
Shift and methanation reactions. However, these materials are complex,
showing a delicate interplay between metal–metal and metal–support
interactions in driving the overall selectivity toward methanol. Besides,
the reactive gas-phase atmosphere (CO/CO2/H2/H2O in various ratios), in other words, the chemical
potential, significantly affects the catalyst states, in terms of
both structures and dynamics; this additional complexity often precludes
the identification of the active sites, hampering the design of better
catalytic materials based on structure–activity relationships
derived from simple descriptors.

In this Account, we show how
combining experiments and atomistic
calculations provides detailed information on how interfaces, alloying,
and dynamics play a crucial role in CO2 hydrogenation by
stabilizing specific adsorbates from CO2 to key reaction
intermediates, i.e., formate or methoxy. Specifically, we discuss
the role played by metal/oxide interfaces and alloying/dealloying
processes in driving catalytic activity (and selectivity); we also
highlight how reaction conditions that define the chemical potential
alter the stability and dynamics of the reactive states of catalysts.
All of these aspects are crucial and interconnected, hence a challenge
for both experimental and theoretical approaches.

This Account
discusses these challenges and exemplifies their importance,
focusing on the following: (i) How benchmarking catalytic models against
experimental data is crucial in obtaining reliable computational models
of the active sites; (ii) How specific surfaces/interfaces are particularly
suited to stabilize key catalytic intermediates such as activated
CO2, formate, and methoxy species; (iii) How dynamic changes
in the systems can be accounted for via ab initio molecular dynamics
combined with metadynamics, confronted with in situ X-ray absorption
spectroscopy; (iv) How the “oxygen chemical potential”
defined by the applied reaction conditions (e.g., H2/CO2 ratio) may affect the nature and stability of catalysts by
using ab initio atomistic thermodynamics.

Finally, we provide
an outlook on ongoing methodological developments
that are needed to refine our understanding of the properties of these
fascinating and dynamic materials.