Intermetallic Compounds in Catalysis

Abstract

In catalysis, intermetallic compounds play an important role as precursors for skeletal catalysts or can be formed under reaction conditions and influence the catalytic properties significantly. They can also be applied as catalysts in an unsupported state in a large number of reactions. Intermetallic compounds are structurally more complex than elemental metals or substitutional alloys and thus offer a broad range of different crystal and electronic structures. In addition, the often partly covalent bonding in the compounds increases their stability under reaction conditions and enables an innovative realization of geometric and electronic concepts. This article summarizes the work in the field, aiming at presenting the different ideas and not at giving a full literature review. Special emphasis is put on the knowledge‐based development, which becomes feasible using unsupported and in situ stable intermetallic compounds as catalysts.

Formation of Intermetallic Compounds by Reactive Metal–Support Interaction: A Frequently Encountered Phenomenon in Catalysis

“…Bi‐ or multimetallic systems, and among them intermetallic compounds, represent an interesting class of materials in catalysis due to their structural and electronic properties, rendering them promising catalysts for a range of reduction or oxidation reactions 1. 2 A multimetallic system might be either a structurally well‐defined intermetallic compound (defined as a chemical compound of two or more metals with a crystal structure different from the constituting metals; characteristic is the presence of covalent and/or ionic bonds) or a substitutional alloy (defined by a material composed of at least two elements as a single‐ or multiphase solid solution), depending on the corresponding phase diagram 1.…”

“…Bi‐ or multimetallic systems, and among them intermetallic compounds, represent an interesting class of materials in catalysis due to their structural and electronic properties, rendering them promising catalysts for a range of reduction or oxidation reactions 1. 2 A multimetallic system might be either a structurally well‐defined intermetallic compound (defined as a chemical compound of two or more metals with a crystal structure different from the constituting metals; characteristic is the presence of covalent and/or ionic bonds) or a substitutional alloy (defined by a material composed of at least two elements as a single‐ or multiphase solid solution), depending on the corresponding phase diagram 1. Whereas recent experimental and quantum mechanical studies focus on the knowledge‐based matching of intermetallic compounds and catalytic reactions,1 a thorough literature review reveals that intermetallic compounds are much more frequently encountered than previously assumed (for a comprehensive overview of the use of intermetallic compounds in catalysis, we refer to Refs.…”

“…Whereas recent experimental and quantum mechanical studies focus on the knowledge‐based matching of intermetallic compounds and catalytic reactions,1 a thorough literature review reveals that intermetallic compounds are much more frequently encountered than previously assumed (for a comprehensive overview of the use of intermetallic compounds in catalysis, we refer to Refs. 1, 2). This comprises instances of intermetallic compounds deliberately prepared to exploit distinct structural or electronic properties (e.g., with the well‐known preparation of the supported intermetallic compound ZnPd for CO 2 ‐selective steam reforming3–6) and materials in which unintended intermetallic compound formation occurs as a result of their preparation or application under catalytic reaction conditions 7–12.…”

“…Notably, the latter case represents the overwhelming majority of intermetallic compounds discussed in the catalytic community. Supported intermetallic compounds are prepared deliberately either through 1) promotion or modification of an oxide‐supported metal (particle) with a second metal1 by reaction with a highly reactive reagent or 2) the oxide support itself taking part in the formation during the preparation of the intermetallic compound 1. It is the latter example, which often creates the structurally and electronically most complex scenarios, on which this Review is focused.…”

“…There are many examples of this kind in the literature with seemingly endless combinations of noble metal and oxide support 1. 13–158 The chemical properties of the supporting oxide usually determine the requirements for intermetallic compound formation under given reduction conditions and are in most cases the source of complexity 1. 13–15, 159 This is mainly due to the general presence of metal–support interactions and especially “strong metal–support interactions” (SMSI) 160–164.…”