Dynamic Processes on Gold-Based Catalysts Followed by Environmental Microscopies

catalysts Review Dynamic Processes on Gold-Based Catalysts Followed by Environmental Microscopies Eric Genty 1,*, Luc Jacobs 1, Thierry Visart de Bocarmé 1,2,* and Cédric Barroo 1,2 1 Chemical Physics of Materials and Catalysis, Université libre de Bruxelles, CP243, 1050 Brussels, Belgium; [email protected] (L.J.); [email protected] (C.B.) 2 Interdisciplinary Center for Nonlinear Phenomena and Complex Systems (CENOLI), Université libre de Bruxelles, 1050 Brussels, Belgium * Correspondence: [email protected] (E.G.); [email protected] (T.V.d.B.); Tel.: +32-2-650-5711 (E.G.); +32-2-650-5724 (T.V.d.B.) Academic Editors: Leonarda F. Liotta and Salvatore Scirè Received: 30 March 2017; Accepted: 20 April 2017; Published: 1 May 2017 Abstract: Since the early discovery of the catalytic activity of gold at low temperature, there has been a growing interest in Au and Au-based catalysis for a new class of applications. The complexity of the catalysts currently used ranges from single crystal to 3D structured materials. To improve the efficiency of such catalysts, a better understanding of the catalytic process is required, from both the kinetic and material viewpoints. The understanding of such processes can be achieved using environmental imaging techniques allowing the observation of catalytic processes under reaction conditions, so as to study the systems in conditions as close as possible to industrial conditions. This review focuses on the description of catalytic processes occurring on Au-based catalysts with selected in situ imaging techniques, i.e., PEEM/LEEM, FIM/FEM and E-TEM, allowing a wide range of pressure and material complexity to be covered. These techniques, among others, are applied to unravel the presence of spatiotemporal behaviours, study mass transport and phase separation, determine activation energies of elementary steps, observe the morphological changes of supported nanoparticles, and finally correlate the surface composition with the catalytic reactivity. Keywords: gold catalysis; Au-based catalysts; environmental microscopies; in situ microscopies; surface science; FEM; FIM; PEEM; LEEM; E-TEM 1. Introduction In the current technology era, an increasing amount of restrictions is imposed on industrial applications, e.g., fine chemical industry and automotive industry, to safeguard the environment. Moreover, the development of novel, economically cheap and green processes in the chemical industry requires the use of catalytic materials. According to the current estimations, the major part of chemical processes uses a catalytic substance in at least one of the production steps (90% of the processes). Among these, the dominant part uses heterogeneous catalytic materials (solid catalysts principally), while the homogeneous catalysts and biocatalysts are used in only 20% of the catalytic processes. One important and challenging role of any catalyst is to increase the selectivity towards the desired products. This is achieved by providing an alternative reaction pathway, of lower activation energy, for the reaction towards the desired products. In the specific case of heterogeneous catalysis, the materials used can be divided into two types: the noble-metal based catalysts (Rh, Pd, Pt, Ru, Au. ) supported on oxides (Al2O3, ZrO2, CeO2, SiO2, TiO2. ) or zeolites [1–8]; and the metal-oxide catalysts [9–18]. Regarding the first class of catalytic materials, the deposition method of the active species, principally in case of noble metals, allows highly dispersed nanoparticles (NPs) to be obtained [19,20] resulting in an important number of active sites available for catalytic reactions. These catalysts include mainly Catalysts 2017, 7, 134; doi:10.3390/catal7050134 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 134 2 of 45 Catalysts 2017, 7, 134 2 of 44 platinum, palladium and rhodium supported on oxide supports and play key roles in many important chemicalThese processes. catalysts include For instance, mainly platinum-basedplatinum, palladium catalysts and rhodium are useful supported in hydro-treatment on oxide supports and completeand oxidationplay key processes, roles in whereasmany important palladium chemical and rhodiumprocesses. areFor crucialinstance, components platinum-based in the catalysts abatement are of automotiveuseful in emissions hydro-treatment [20–23 and]. Indeed, complete the oxidation coexistence processes, of these whereas three palladium noble metals and rhodium in the catalyticare converterscrucialcorresponds components in to the abatement optimum ofof catalyticautomotive activity emissions at low [20–23]. temperature Indeed, the for thecoexistence NOx selective of these three noble metals in the catalytic converters corresponds to the optimum of catalytic activity catalytic reduction and CO and hydrocarbon oxidation reactions. One of the main problems during this at low temperature for the NOx selective catalytic reduction and CO and hydrocarbon oxidation process concerns the emissions of pollutants when the system has not reached its working temperature, reactions. One of the main problems during this process concerns the emissions of pollutants when also calledthe system cold has start not period.reached its The working majority temperature, of the emissions also calledare cold produced start period. after The themajority starting of the of the vehicleemissions engine are when produced the temperature after the starting in the of the exhaust vehicle system engine when is lower the temperature than 425 K. in For the theexhaust catalytic oxidationsystem of is carbon lower than monoxide, 425 K. theFor Pdthe orcatalytic Pt-based oxidation catalysts of carbon do not monoxide, bring atotal the Pd conversion or Pt-based at this temperaturecatalysts [ 24do, 25not]. bring Understanding a total conversion how the at this catalyst temperature works in[24,25]. order Unders to improvetanding its how catalytic the catalyst activity at low temperatureworks in order is to thus improve of great its catalytic importance. activity At at the low end temperature of the 1980s, is thus Haruta of great and importance. co-workers At showedthe an outstandingend of the catalytic1980s, Haruta activity and for co-workers the CO oxidation showed at an low outstanding temperature catalytic (200 K)activity with thefor usethe ofCO highly dispersedoxidation gold at nanoparticles low temperature on (200 reducible K) with support. the use of The highly deposition dispersed of gold nanoparticles nanoparticles with on reducible diameters of support. The deposition of nanoparticles with diameters of 2 to 5 nm on reducible support such as 2 to 5 nm on reducible support such as Fe2O3, Co3O4, and NiO exhibits the highest reactivity [26–29]. Fe2O3, Co3O4, and NiO exhibits the highest reactivity [26–29]. Together with the research performed Together with the research performed by the Hutchings group on the hydro-chlorination of acetylene, by the Hutchings group on the hydro-chlorination of acetylene, Haruta’s discovery triggered a great Haruta’sscientific discovery interest triggered for catalysis a great by gold scientific [30–32]. interest This is forshown catalysis in Figure by gold1 representing [30–32]. Thisthe growing is shown in Figuretrend1 representing in term of thepublications growing trendper year in term involvin of publicationsg gold-based per catalytic year involving materials gold-basedand the use catalytic of materialsenvironmental and the use microscopies. of environmental microscopies. FigureFigure 1. Increasing 1. Increasing number number of of published published papers papers from 1980 1980 to to 2017 2017 inin the the Scopus Scopus Scholar Scholar database database usingusing “gold-based “gold-based catalysts” catalysts” and and “environmental “environmental microscopy”microscopy” as as search search keywords keywords (March (March 2017). 2017). Regarding the evolution of the number of studies using gold-based catalysts, the major part of Regardingthe scientific the publications evolution concern of the the number methods of studiesused for usingthe deposition gold-based of gold catalysts, nanoparticles the major with part of thevery scientific narrow and publications well-defined concern sizes [33–47]. the methods Besides conflicting used for papers the deposition on the influence of gold of the nanoparticles support with verymaterials, narrow the effect and well-definedof the size on the sizes reactivity [33–47 ].of Besidesgold catalysts conflicting for environmental papers on thereactions influence is well of the supportestablished. materials, The the small effect size of of the gold size nanoparticles on the reactivity induces of a modification gold catalysts of the for electronic environmental structure reactions of is wellthese established. nanoparticles. The In smalladdition size to this, of gold a decrease nanoparticlesd size of the induces particles a increases modification the dispersion, of the electronic and structureaccordingly of these the nanoparticles. number of edges In additionand kinks toon this, the metallic a decreased particles: size such of the highly particles uncoordinated increases the dispersion,surface andsites could accordingly correspond the to number the preferential of edges ad andsorption kinks sites, on and the their metallic increasing particles: number such could highly then be responsible for the increasing reactivity of the catalytic materials. During the last decade, the uncoordinated surface sites could correspond to the preferential adsorption sites, and their increasing

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