TY - JOUR
T1 - Beyond Ordered Materials
T2 - Understanding Catalytic Sites on Amorphous Solids
AU - Goldsmith, Bryan R.
AU - Peters, Baron
AU - Johnson, J. Karl
AU - Gates, Bruce C.
AU - Scott, Susannah L.
N1 - S.L.S. and B.P. acknowledge support from the Catalysis Science Initiative of the U.S. Department of Energy (DOE), Basic Energy Sciences (BES) via grant DE-FG02-03ER15467. B.C.G.
acknowledges support from DOE BES via grant DE-FG02-04ER15513. J.K.J. acknowledges support from DOE BES via grant DE-FG02-10ER16165. B.R.G. acknowledges support from the Alexander von Humboldt-Foundation via a Postdoctoral Fellowship. B.R.G. also thanks Markus Heyde at the Fritz Haber Institute of the Max Planck Society for fruitful discussions. B.P. thanks Richard Sear for stimulating discussions.
PY - 2017/11/3
Y1 - 2017/11/3
N2 - Amorphous materials are widely used as components of solid catalysts and have been the subject of much applied research. In some instances, their catalytic performance is demonstrably superior to that of their crystalline counterparts, due in part to their greater flexibility. Amorphous or disordered phases can also be generated from crystalline phases under reaction conditions, and thus, ex situ observations of long-range order may provide an incomplete or misleading picture. Until recently, theorists and experimentalists have mostly neglected these important materials in fundamental studies, preferring instead to study "well-defined" (often crystalline) catalysts that are potentially more tractable and amenable to computational modeling of their structure-activity relationships. The amorphous materials were assumed to be simply nonuniform versions of compositionally similar materials with long-range order, having the same key features at short and medium length scales. In this Perspective, shortcomings of this assumption are discussed, as well as challenges inherent in tackling amorphous catalysts more directly, namely, identifying and describing the active sites (especially under reaction conditions), discerning how subtle structural variations modulate site activity, and building atomically detailed models of amorphous catalysts. Three important classes of amorphous catalytic materials are highlighted to illustrate key issues: amorphous oxides, metal ions atomically dispersed on amorphous supports, and supported metal clusters. Amorphous and disordered silicas, aluminas, and silica-aluminas are discussed in terms of challenges and progress toward identifying how their local structural disorder and surface heterogeneity may impact the behavior of active sites. Promising models of amorphous materials with atomistic detail and increased fidelity to experiment are becoming available. However, for reactions in which small fractions of sites dominate the total activity, computational estimates of the observed kinetics will require statistical sampling methods, even for the most detailed catalyst models. Further developments in in situ and operando characterization techniques and computational modeling will advance our understanding of amorphous catalytic materials and the impact of structural disorder.
AB - Amorphous materials are widely used as components of solid catalysts and have been the subject of much applied research. In some instances, their catalytic performance is demonstrably superior to that of their crystalline counterparts, due in part to their greater flexibility. Amorphous or disordered phases can also be generated from crystalline phases under reaction conditions, and thus, ex situ observations of long-range order may provide an incomplete or misleading picture. Until recently, theorists and experimentalists have mostly neglected these important materials in fundamental studies, preferring instead to study "well-defined" (often crystalline) catalysts that are potentially more tractable and amenable to computational modeling of their structure-activity relationships. The amorphous materials were assumed to be simply nonuniform versions of compositionally similar materials with long-range order, having the same key features at short and medium length scales. In this Perspective, shortcomings of this assumption are discussed, as well as challenges inherent in tackling amorphous catalysts more directly, namely, identifying and describing the active sites (especially under reaction conditions), discerning how subtle structural variations modulate site activity, and building atomically detailed models of amorphous catalysts. Three important classes of amorphous catalytic materials are highlighted to illustrate key issues: amorphous oxides, metal ions atomically dispersed on amorphous supports, and supported metal clusters. Amorphous and disordered silicas, aluminas, and silica-aluminas are discussed in terms of challenges and progress toward identifying how their local structural disorder and surface heterogeneity may impact the behavior of active sites. Promising models of amorphous materials with atomistic detail and increased fidelity to experiment are becoming available. However, for reactions in which small fractions of sites dominate the total activity, computational estimates of the observed kinetics will require statistical sampling methods, even for the most detailed catalyst models. Further developments in in situ and operando characterization techniques and computational modeling will advance our understanding of amorphous catalytic materials and the impact of structural disorder.
KW - amorphous oxides
KW - atomic-level characterization
KW - dispersed metal ions
KW - first-principles modeling
KW - heterogeneity
KW - noncrystalline catalysts
KW - supported metal clusters
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U2 - 10.1021/acscatal.7b01767
DO - 10.1021/acscatal.7b01767
M3 - Article
AN - SCOPUS:85032993785
SN - 2155-5435
VL - 7
SP - 7543
EP - 7557
JO - ACS Catalysis
JF - ACS Catalysis
IS - 11
ER -