Topics in Current Chemistry (2019) 377:5
1 3
1 Introduction
Generally, catalysts consist of non-consumable chemicals capable of providing a
favorable energy landscape for mediating steps of chemical reactions. Catalysts
can be found everywhere in nature, as is the case of the enzymes in the living
world, which are extremely selective [1]. In the case of inorganic catalysts, the
efficiency almost always needs to be improved [2]. In heterogeneous catalysis,
catalyst materials very often are solids—such as oxides or metals on a support—
and the reactants are found in the fluid phase—as is the gaseous or liquid phase.
The mechanism through which the reactions proceed on a catalyst surface is complex, involving many reaction intermediates, and it is worth mentioning that many
parameters, which can be intrinsic and extrinsic to the catalyst surface, influence
its overall efficiency [3]. The efficiency of catalysts can be (and had historically
been [2]) improved by trial-and-error, but this strategy is very time-consuming
and does not allow pushing the limits of the efficiency of the catalyst in terms of
both activity and selectivity (and also long-term stability). The factors underlying
the efficiency in heterogeneous electrocatalysis only can be reached if in-depth
knowledge about the molecular relationships between the catalyst surface structure and the reactants/reactions taking place on it is successfully achieved. In this
regard, from the theoretical viewpoint, it has been proposed that energy-scaling
relations, i.e., for catalyzed reactions on the surface of some transition metals, the
binding energies of adsorbed intermediates correlate with each other and these
scaling relations likely limit the catalysis efficiency [3, 4]. This theoretical perception of catalysis has been extremely influential in the interpretation of heterogeneous catalysis at the solid/gas interface [5].
In electrocatalysis, catalysts are immersed in an electrolyte with which they
interact, so that the catalyst surfaces are never unoccupied or “clean” as in ultrahigh vacuum (where pressure below ~ 10
−9
torr is required so the surface is kept
“clean” for up to about 1 h [6]). In this sense, beyond a parameter such as the
external applied potential, also absent in ordinary heterogeneous catalysis, the
catalyst surface in electrocatalysis permanently interacts also with, generally, a
water-based electrolyte and its ions (and, not to say, the surface impurities coming from the electrolyte). This means that, during the attachment of the reactants
to the catalyst surface, competitive reactions always take place. However, as in
heterogeneous catalysis, the rate of reaction and the selectivity in electrocatalysis may be tactically tuned, playing with characteristics intrinsic to the electrode/
catalyst surface, as the structure of the surfaces, as the shape of nanocrystals and
step site density [7–9]. In addition, the catalytic activity can be modified based on
parameters extrinsic to the catalyst surface, as is the electrolyte composition, the
effect/influence of anions and, as more recently pointed out, cations (still not well
understood) and pH of the solution [10, 11].
In controlled experiments, a structure’s sensitivity is successfully assessed by
controlling the specific atomic configuration on the catalyst surfaces [12]. Despite
surface-structure-sensitivity relationships being easily pointed out, the determination of the exact structure of the active sites involved in a specific reaction
Reprinted from the journal
80
1 3
1 Introduction
Generally, catalysts consist of non-consumable chemicals capable of providing a
favorable energy landscape for mediating steps of chemical reactions. Catalysts
can be found everywhere in nature, as is the case of the enzymes in the living
world, which are extremely selective [1]. In the case of inorganic catalysts, the
efficiency almost always needs to be improved [2]. In heterogeneous catalysis,
catalyst materials very often are solids—such as oxides or metals on a support—
and the reactants are found in the fluid phase—as is the gaseous or liquid phase.
The mechanism through which the reactions proceed on a catalyst surface is complex, involving many reaction intermediates, and it is worth mentioning that many
parameters, which can be intrinsic and extrinsic to the catalyst surface, influence
its overall efficiency [3]. The efficiency of catalysts can be (and had historically
been [2]) improved by trial-and-error, but this strategy is very time-consuming
and does not allow pushing the limits of the efficiency of the catalyst in terms of
both activity and selectivity (and also long-term stability). The factors underlying
the efficiency in heterogeneous electrocatalysis only can be reached if in-depth
knowledge about the molecular relationships between the catalyst surface structure and the reactants/reactions taking place on it is successfully achieved. In this
regard, from the theoretical viewpoint, it has been proposed that energy-scaling
relations, i.e., for catalyzed reactions on the surface of some transition metals, the
binding energies of adsorbed intermediates correlate with each other and these
scaling relations likely limit the catalysis efficiency [3, 4]. This theoretical perception of catalysis has been extremely influential in the interpretation of heterogeneous catalysis at the solid/gas interface [5].
In electrocatalysis, catalysts are immersed in an electrolyte with which they
interact, so that the catalyst surfaces are never unoccupied or “clean” as in ultrahigh vacuum (where pressure below ~ 10
−9
torr is required so the surface is kept
“clean” for up to about 1 h [6]). In this sense, beyond a parameter such as the
external applied potential, also absent in ordinary heterogeneous catalysis, the
catalyst surface in electrocatalysis permanently interacts also with, generally, a
water-based electrolyte and its ions (and, not to say, the surface impurities coming from the electrolyte). This means that, during the attachment of the reactants
to the catalyst surface, competitive reactions always take place. However, as in
heterogeneous catalysis, the rate of reaction and the selectivity in electrocatalysis may be tactically tuned, playing with characteristics intrinsic to the electrode/
catalyst surface, as the structure of the surfaces, as the shape of nanocrystals and
step site density [7–9]. In addition, the catalytic activity can be modified based on
parameters extrinsic to the catalyst surface, as is the electrolyte composition, the
effect/influence of anions and, as more recently pointed out, cations (still not well
understood) and pH of the solution [10, 11].
In controlled experiments, a structure’s sensitivity is successfully assessed by
controlling the specific atomic configuration on the catalyst surfaces [12]. Despite
surface-structure-sensitivity relationships being easily pointed out, the determination of the exact structure of the active sites involved in a specific reaction
Reprinted from the journal
80