1 3
Topics in Current Chemistry (2019) 377:5
pathway is not trivial and requires a great number of high-quality experiments
in fundamental electrocatalysis. For electrocatalyzed reactions, this subject has
been addressed in two review articles a few years ago [9, 13]. With regard to the
surface structure, it is well established that studies of single crystal surfaces have
historically played a prominent role in advancing understanding of molecular factors in heterogeneous catalysis at both solid/gas [6] and solid/liquid electrified
interfaces [14]. The suitability of studies with single crystal surfaces—as a step
in the direction for understanding the structure-activity relationships—is because
the single crystal surfaces simplify enormously the number of variables of the
catalytic process, and the single crystal, at a certain level, mimics the facets of the
catalyst’s nanoparticles used in real applications.
The identification of active sites requires the design of specific experiments for
the description and investigation of the causes of the reactivity, catalytic activity
and selectivity in heterogeneous electrocatalysis [15, 16]. In this article, we discuss
efforts devoted to the establishment of relationships between reactivity and surface
reaction pathways for the following electrochemical reactions: hydrogen adsorption/desorption reaction, electro-oxidation of CO, methanol, ammonia and glucose
on platinum single crystal electrodes. It is worth noticing that the significance of
these reactions in the field of electrocatalysis is because they are surface structuresensitive reactions. The hydrogen adsorption/desorption and oxidation of carbon
monoxide reactions can serve as surface model reactions. The oxidation of methanol
and ammonia can serve as examples of potential fuel in low-temperature fuel cells.
Moreover, the study of ammonia oxidation has a great appeal for environmental
issues. Electro-oxidation of glucose is the best-documented case of enantiospecific
interaction between a chiral molecule and an intrinsically chiral surface.
2 The Structure of the Catalytic Substrate
In heterogeneous electrocatalysis, the catalytic events, i.e., all the elementary reaction steps into the catalytic cycle, take place on the topmost layer of the atoms at the
catalyst. The atomic ensemble at the surface where reaction takes place, involving
the reaction intermediates and transition states, combines the effect of both electronic and geometric nature and is simplified under the name of surface active sites.
In this sense, the active sites are far from being atoms, but structures that eventually
may have some level of synergy. Historically, from a surface structure point of view,
the insight on the active sites is attributed to Taylor and back to 1925 [17]. Taylor
reviewed the mechanism of the catalytic action in solid surfaces, and he concluded
that the occurrence of the reactions on surfaces was restricted to occur on unsaturated surface atoms [17]. In fact, in the real world, the surfaces of the catalyst consist
of non-periodical arrangement of atoms, resulting in different local chemical environments [18]. As an example, the different configurations of atoms at the catalyst
surfaces can be represented by a crystal surface model, as illustrated in Fig. 1 for
crystals of the fcc (face-centered cubic) lattices.
The surface in Fig. 1 contains terrace, step edge, kink, ad-atom, ad-island
and hole atoms. Under this framework, it is usually considered that the surface
Reprinted from the journal
81
Topics in Current Chemistry (2019) 377:5
pathway is not trivial and requires a great number of high-quality experiments
in fundamental electrocatalysis. For electrocatalyzed reactions, this subject has
been addressed in two review articles a few years ago [9, 13]. With regard to the
surface structure, it is well established that studies of single crystal surfaces have
historically played a prominent role in advancing understanding of molecular factors in heterogeneous catalysis at both solid/gas [6] and solid/liquid electrified
interfaces [14]. The suitability of studies with single crystal surfaces—as a step
in the direction for understanding the structure-activity relationships—is because
the single crystal surfaces simplify enormously the number of variables of the
catalytic process, and the single crystal, at a certain level, mimics the facets of the
catalyst’s nanoparticles used in real applications.
The identification of active sites requires the design of specific experiments for
the description and investigation of the causes of the reactivity, catalytic activity
and selectivity in heterogeneous electrocatalysis [15, 16]. In this article, we discuss
efforts devoted to the establishment of relationships between reactivity and surface
reaction pathways for the following electrochemical reactions: hydrogen adsorption/desorption reaction, electro-oxidation of CO, methanol, ammonia and glucose
on platinum single crystal electrodes. It is worth noticing that the significance of
these reactions in the field of electrocatalysis is because they are surface structuresensitive reactions. The hydrogen adsorption/desorption and oxidation of carbon
monoxide reactions can serve as surface model reactions. The oxidation of methanol
and ammonia can serve as examples of potential fuel in low-temperature fuel cells.
Moreover, the study of ammonia oxidation has a great appeal for environmental
issues. Electro-oxidation of glucose is the best-documented case of enantiospecific
interaction between a chiral molecule and an intrinsically chiral surface.
2 The Structure of the Catalytic Substrate
In heterogeneous electrocatalysis, the catalytic events, i.e., all the elementary reaction steps into the catalytic cycle, take place on the topmost layer of the atoms at the
catalyst. The atomic ensemble at the surface where reaction takes place, involving
the reaction intermediates and transition states, combines the effect of both electronic and geometric nature and is simplified under the name of surface active sites.
In this sense, the active sites are far from being atoms, but structures that eventually
may have some level of synergy. Historically, from a surface structure point of view,
the insight on the active sites is attributed to Taylor and back to 1925 [17]. Taylor
reviewed the mechanism of the catalytic action in solid surfaces, and he concluded
that the occurrence of the reactions on surfaces was restricted to occur on unsaturated surface atoms [17]. In fact, in the real world, the surfaces of the catalyst consist
of non-periodical arrangement of atoms, resulting in different local chemical environments [18]. As an example, the different configurations of atoms at the catalyst
surfaces can be represented by a crystal surface model, as illustrated in Fig. 1 for
crystals of the fcc (face-centered cubic) lattices.
The surface in Fig. 1 contains terrace, step edge, kink, ad-atom, ad-island
and hole atoms. Under this framework, it is usually considered that the surface
Reprinted from the journal
81