Review
`Heterogeneous Catalysis on Metal Oxides
`
`Jacques C. Védrine ID
`Laboratoire de Réactivité de Surface, Université P. & M. Curie, Sorbonne Université, UMR-CNRS 7197,
`4 Place Jussieu, F-75252 Paris, France; jacques.vedrine@upmc.fr; Tel.: +33-1-442-75560
`
`Received: 8 October 2017; Accepted: 27 October 2017; Published: 10 November 2017
`
`Abstract: This review article contains a reminder of the fundamentals of heterogeneous catalysis and
`a description of the main domains of heterogeneous catalysis and main families of metal oxide
`catalysts, which cover acid-base reactions, selective partial oxidation reactions, total oxidation
`reactions, depollution, biomass conversion, green chemistry and photocatalysis. Metal oxide catalysts
`are essential components in most refining and petrochemical processes. These catalysts are also critical
`to improving environmental quality. This paper attempts to review the major current industrial
`applications of supported and unsupported metal oxide catalysts. Viewpoints for understanding the
`catalysts’ action are given, while applications and several case studies from academia and industry are
`given. Emphases are on catalyst description from synthesis to reaction conditions, on main industrial
`applications in the different domains and on views for the future, mainly regulated by environmental
`issues. Following a review of the major types of metal oxide catalysts and the processes that use these
`catalysts, this paper considers current and prospective major applications, where recent advances in
`the science of metal oxide catalysts have major economic and environmental impacts.
`
`Keywords: metal oxide catalysts; heterogeneous catalysis; main catalytic reactions; main catalytic processes
`
`1. Introduction to Catalysis
`
`Catalysis is an important domain of chemistry [1–3]. Berzelius first used the word “catalysis”
`in 1836, taken from the Greek word “καταλεινν” (=loose down, dissolve) by analogy to the word
`“analysis” in order to rationalise well-known experimental observations such as wine and beer
`fermentation, soap and sulfuric acid (oil of vitriol) synthesis, starch transformation to sugar by
`acids, decomposition of H2O2 by metals, ethanol oxidation to acetic acid on Pt, etc. By definition,
`catalysis is a process by which a reaction rate is enhanced by a small amount of the so-called catalyst,
`which supposedly does not undergo any change during the reaction, at variance with surface or
`stoichiometric reactions. However, as any manager of industrial plants knows, this is a very optimistic
`definition as real catalysts change in structure, activity and selectivity with time on stream (activation
`step) and deactivate more or less rapidly. Some 60 years after Berzelius, Oswald established the kinetic
`nature of this phenomenon and gave in 1895 the definition: “a catalyst is a substance that changes the
`rate of a chemical reaction without itself appearing in the products”. According to IUPAC (1976), a catalyst
`is a substance that, being present in small proportions, increases the rate of attainment of chemical
`equilibrium without itself undergoing chemical change.
`It has been recognised that the catalyst acts by reducing the energy necessary to proceed along
`the reaction pathway, i.e., the activation energy Ea that needs to be surmounted to yield products.
`This activation energy is the energy required to overcome the reaction barrier and determines how
`fast a reaction occurs. The lower the activation barrier, the faster the reaction will be. Note that the
`thermodynamics of the reaction remains unchanged under catalyst action and that the main effect is
`that the catalyst influences the reaction rate.
`Heterogeneous catalysis (gas or liquid phase and solid catalyst) proceeds via adsorption of one or
`two reactant molecule(s) on the solid surface, enhancing the reactant(s) concentration on the surface
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`Catalysts 2017, 7, 341; doi:10.3390/catal7110341
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`catalysts
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`and favouring its (their) activation. The first step of the reaction is thus the reactant(s) adsorption,
`whereas the reaction energy includes the activation barrier energies of adsorbed reactants (Aads),
`of adsorbed intermediates (Iads) and of desorption of products (Pads). In other words, the degree
`of catalytic efficiency gained in following a given path is governed by the energetics of the various
`intermediates, which encompass adsorbed reactant, the activation energy required to convert the
`bound reactant into a surface intermediate and finally to a product and its desorption.
`Catalysis plays a prominent role in modern life. The majority of actual industrial chemical
`processes, involving the manufacturing of commodity-, petro-, pharmaceutical- and fine-chemicals,
`clean fuels, etc., as well as pollution abatement technologies, have a common catalytic origin. It is well
`recognised that 85–90% of industrial chemical processes involve at least one catalytic step. The main
`objectives of using a catalyst are to get high activity, i.e., high conversion of reactants and overall
`high selectivity to a desired product, the latter property avoiding or limiting separation/purification
`procedure, which involves important steps to take into account, particularly for economical and overall
`environmental issues.
`Solid catalysts are classified as (i) conductors, (metals and alloys); (ii) semiconductors (oxides and
`sulphides); and (iii) insulators (metal oxides and solid acids or bases, including heteropolyacids, natural
`clays, silica–alumina and zeolites). Oxidation reactions are catalysed by oxides, while desulphurisation
`reactions occur on a sulphide catalyst. Insulator oxides catalyse dehydration, and acid/base solids act
`in processes with carbocationic/carbanionic intermediates. These features have led to the idea that
`there is a kind of compatibility between the catalyst and the reactant molecules. We will discuss this
`point when mentioning the structure sensitivity of catalytic reactions on metal oxides and describing
`some active sites (vide infra).
`The major domains of heterogeneous catalysis, applied industrially, concern:
`
`•
`•
`•
`•
`•
`
`Oil refining, energy and transport,
`Bulk chemicals,
`Polymers & materials and detergents & textiles,
`Fine chemicals, pharmaceutical & medical chemicals and food & feed,
`Plant design/engineering and realisation, catalyst design, subsequent development of catalysts
`and of catalytic processes,
`•
`Commercial production of catalysts in sufficient quantities,
`• Monitoring and control of chemical reactions and plant operations,
`•
`Environmental issues.
`
`2. Introduction to Metal Oxide Catalysts
`
`Among the different fields of heterogeneous catalysis, catalysis by metal oxides is one of the most
`important, as it covers the majority of processes and of catalyst families used industrially, such as silica,
`alumina, clays, zeolites, TiO2, ZnO, ZrO2, porous and mesoporous metal oxides, polyoxometallates
`(POMs) of Keggin or Dawson type, the phosphates family (e.g., VPO, FePO4, silica phosphoric acid
`(SPA)), multicomponent mixed oxides (molybdates, antimonates, tungstates, MoVTe(Sb)Nb-O, etc.),
`perovskites, hexaaluminates, etc.
`Metal oxides became prominent in the mid-1950s when they were found to effectively
`catalyse a wide variety of reactions, in particular oxidation and acid-base reactions. They are
`involved in many petrochemicals, intermediates, fine and pharmaceutical chemicals and biomass
`transformation reactions. They are also the basis for metallic (mono- or pluri-metallic) catalysts, for
`hydrodesulphurisation catalysts (CoMoO4-, NiMoO4-, NiWO4-based), for deNOx, deSOx, and bulk
`single or mixed metal oxide catalysts. The main catalytic domains cover oxidation (selective or total),
`acid and base catalyses, photocatalysis, depollution and biomass conversion, as described below.
`This paper, being devoted to gas–solid heterogeneous catalysis, considers that the reaction occurs
`at the interface of two media, namely the solid catalyst and the medium of the reactants and products,
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`and thus involves fluid-phase transport of reactants to and products away from the catalyst surface.
`In heterogeneous catalysis the catalysts are in the form of powders, pellets or extrudates, in order to
`permit the reactant flow to cross the full catalytic bed in the reactor. For model catalysts, films or single
`crystals may also be considered.
`Metal oxides [4,5] constitute a class of inorganic materials that have peculiar and various properties and
`applications as sensors, catalysts, fuel cells, etc. Oxide surfaces terminate by oxide O2− anions, as their size is
`much larger than that of Mn+ cations. It follows that the symmetry and coordination of Mn+ cations are lost
`at the surface. Moreover, the surface of an oxide may contain different types of defects and environments
`(kinks, steps, terraces), which play a determining role in the catalytic phenomenon [6,7]. This surface
`unsaturation is usually compensated for by a reaction with water vapour, leading to the formation of surface
`hydroxyls according to: O2− + H2O → 2OH−. OH groups are conjugated acids of lattice oxygen ions O2−,
`which are strong bases and conjugated bases of water molecules.
`Among the solid state of metal oxides [8], important parameters or features act on catalytic properties.
`One may distinguish point and extended defect structures, the atomic composition and structure of a crystalline
`phase, and electronic defects, which correspond to the probability that an electron occupies an energy state,
`given by the Fermi–Dirac function: F(E) = ne/N = 1/(exp[(E − EF)/RT], where ne is the number of electrons,
`N is the number of available energy states, and EF is the Fermi level energy. In an intrinsic semiconductor,
`the number of electrons in the conduction band equals the number of holes in the valence band and is
`given by: n = ne = nh = n0 exp (−EG/2kT), where n0 is constant and EG is the energy gap. Given that
`most oxides have a large band gap and often high contents of impurities, most electronic defects are
`extrinsic. Extrinsic defects can introduce carriers into localised energy levels within the band gap and, in such
`cases, are electrically active. An electronic defect in an energy level just below the conduction band edge is
`considered a donor as it may give electrons to the conduction band and increase the n-type conductivity.
`For instance, when a P atom of +5 oxidation state is added to silicon in silica where Si is at +4 oxidation state,
`the additional electron for charge compensation introduces a donor state (exciting the electron necessitates
`ED rather than EG), while introducing a Ga ion at +3 oxidation state creates a hole in the valence band and
`can accept an electron, increasing the p-type conductivity. For a donor-doped material, the total number
`of the charge carrier equals: ntotal = ne (dopant) + ne (intrinsic) + nh (intrinsic) = n0D exp(−ED/kT) + 2n0
`exp(−EG/2kT). This aspect is illustrated in Figure 1.
`
`Figure 1. Creation of a donor band below the conduction band CB (left) (exciting the electron
`necessitates ED) and acceptor band (exciting the electron necessitates EA) above the valence band
`VB (right) upon doping. EG = energy gap.
`
`As heterogeneous catalysis occurs at the interface of the two media, the fluid-phase transport
`of reactants to and of products away from the catalyst has to be considered. In the case of solid
`metal oxide catalysts, solid-phase transport of defects, oxide anions and electronic conductivity
`should also be taken into consideration. Important properties of a metal oxide, such as electrical
`conductivity, lattice oxygen anions’ mobility, atoms/ions’ diffusion acting on sintering or phase
`separation, catalytic activity, melting point and various optical properties, depend on the presence of
`defects vs. an ideal ionic crystal. There are different types of defects: electrons and positive holes, excitons,
`vacant lattice sites (designated V), interstitial atoms, impurity atoms at interstitial or substitution
`location, dislocations and stacking faults. The subscript denotes the site that the defect occupies
`(lattice atom site or interstitial atom), the dot • denotes a positive charge, and x represents neutrality.
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`Point defects are missing, substituted, or interstitial ions. They are electronically charged and can
`be intrinsic (thermally generated in a crystal) or extrinsic (impurity or dopant). The most common
`point defects are Schottky-type, i.e., vacant cationic or anionic lattice site, or Frenkel-type, i.e., cations
`or anions displaced to interstitial sites. One also has electronic defects (electrons e− and electron holes
`h• influencing electric, ionic and protonic conductivities) and non-stoichiometry in oxides, such as
`perovskites with an unfilled 3d electron shell and cation and lattice oxygen anion deficiency or excess.
`Single or complex metal oxides based on the first transition series present a wide variety of
`non-stoichiometric phenomena, which originate from the unfilled 3d electron shell. For instance,
`Fe sites can be vacant in FeO of rocksalt framework (Fe1−xO with 0.05 < x < 0.18).
`In ABO3
`perovskite oxides, non-stoichiometry comes from a cation deficiency in A or B sites (AxWO3, A being
`an alkali ion) or oxygen anions excess. Cation A has a great influence on electrical properties.
`For instance, WO3 is an insulator, while AxWO3 is a semiconductor at low x and metallic at
`high x. Oxygen-deficient perovskites have attracted much attention because of their oxygen storage
`ability and their redox properties, quite useful for total oxidation reactions.
`In fully oxidised
`CaMnO3−δ and CaFeO3−δ perovskites, Mn and Fe are at +4 oxidation state and δ = 0, the material can
`accommodate up to 17% oxygen vacancies without losing its structure. The Sr1−xLaxCo1−yFeyO3−δ
`series, with brownmillerite-type oxygen defects, exhibits high electronic/oxygen ion mobility. In some
`perovskites a small oxygen excess can be accommodated by the formation of cation vacancies
`at A- or B-sites, leaving the oxygen sub-lattice intact. Electronic defects may be created upon
`reduction and oxidation of metal cations at different oxygen partial pressures (pO2). At low pO2,
`the material loses oxygen, which generates electrons, enhancing the n-type conductivity, according
`to: O0 ↔ (1/2) O2 + VO
`•• + 2e−. At high pO2, oxygen is incorporated into an oxygen vacancy and
`takes two electrons from the valence band, leading to holes contributing to the p-type conduction,
`•• ↔ O0 + 2h•.
`according to (1/2) O2 + VO
`A consequence of the trapping of electronic defects is that the solid becomes insulating at low
`temperatures. Defect associations between oxygen vacancies and acceptor dopants are observed in
`•• are trapped by YZr
`• defects leading to a drastic decrease in oxide ion conductivity.
`Y-ZrO2, where VO
`Protonic defects, associated with acceptor dopants, limit protonic conduction in metal oxides. As all
`metal oxides contain impurities, the defect mechanism of “pure” oxides is similar to that of doped
`oxides. For instance, strontium titanate, SrTiO3, has a similar defect chemistry to acceptor-doped SrTiO3,
`because the impurities are ions at lower oxidation states such as Fe3+, Al3+, Mg2+, Na+, etc., and their
`concentrations, typically ca. 102–103 ppm, are sufficient to dominate the defect chemistry, especially at
`high pO2. All these aspects should be taken into account when considering the reaction mechanisms and
`kinetics on metal oxide catalysts, for instance in selective or total oxidation reactions on mixed metal
`oxides, where electrons and oxygen anions mobilities and lattice oxygen diffusion or storage capacity
`are important, for instance in a redox catalytic oxidation reaction. Lattice defects can lead to an electron
`transfer from the solid toward an adsorbed molecule (anionic chemisorption) or, by contrast, from the
`adsorbed species toward the solid (cationic chemisorption) in the case of non-stoichiometric oxides or
`of oxides doped with an ion of different valence. In the first case and for n-type semi-conduction, one
`observes a decrease in conductivity and the reverse for semi-conductors of p-type, while in the second
`case, conductivity increases for n-type semiconductors and decreases for p-type semiconductors.
`The development of descriptors that may correlate the activity of catalysts and their physical
`properties is an important goal in catalysis. For instance, it has been shown [9] that the apparent
`activation energy for propene oxidation to acrolein over scheelite-structured, multicomponent, mixed
`metal oxides (Bi3FeMo2O12, Bi2Mo2.5W0.5O12, Bi1−x/3V1−xMoxO4, with 0 ≤ x ≤ 1) is related to the
`band gap of the catalyst measured at reaction temperature, as supported by theoretical calculations.
`As we have seen above, catalysis starts with chemisorption, which should be not too strong to permit
`intermediate formation, and the intermediate desorption not too weak to permit adsorbed species to be
`activated, which is known as Sabatier’s principle. Later, Taylor suggested [10] that activation energy was
`a key factor during the adsorption stage. He has also suggested that preferential adsorption on a catalyst
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`surface should take place at those atoms or ensemble of atoms, depending on the reaction and reactant size,
`situated at the peaks, fissures, kinks, edges, etc., of a crystallite rather than on flat surfaces. The notion of
`active sites or active centres was then suggested to be the locus of catalytic conversion.
`However, depending on the reactant molecule size and reaction, more than one surface atom may
`be involved in the reaction. This is what was defined for metals by Boudart as a structure-sensitive
`reaction [11,12]. In the case of metal oxides, the active sites are often composed of surface ensembles of
`atoms to permit the reaction to occur. For instance, for oxidation reactions where both the reactant
`molecule and the gaseous oxygen have to be activated, the surface active site should be large enough
`to accommodate the reactant molecule and the oxygen from the lattice to diffuse through the solid
`catalyst to permit oxidation. This diffusion should be fast enough to renew lattice oxygen anions,
`leaving the surface with the product in a Mars and van Krevelen mechanism but not too high to
`avoid over-oxidation to CO2 [13]. This is illustrated in Figures 2 and 3 for butane oxidation to maleic
`anhydride on a vanadyl pyrophosphate catalyst, designated as a VPO catalyst [14,15], and corresponds
`to the structure sensitivity of metal oxide for selective oxidation reactions, as demonstrated by Volta [16]
`for MoO3 and extended to other metal oxides [17,18].
`
`Figure 2. Site isolation by structure and composition: schematic of (VO)2P2O7 surface structure. Facile
`exchange of surface oxygens exists within domains (represented by arrows), but not between domains.
`Site isolation established between domains by picket fence of pyrophosphate groups posing oxygen
`diffusion barriers. Schematic representation of a surface structure of a type of polytype of (VO)2P2O7.
`The arrows represent the facile pathways for surface oxygen mobility. This illustrates the site isolation
`principle by surface P2O7 entities groups that constitute a barrier to oxygen diffusion. Reproduced
`with permission from [13]. Copyright Elsevier, 2014.
`
`Figure 3. Adsorption of n-butane on (100) and (021) (VO)2P2O7. Anchoring of butane on (100) and one
`possible way of adsorption of butane on (021). Medium shaded and small black circles are C and H, respectively
`(carbon, oxygens and lattice at scale). Reproduced with permission from [14]. Copyright Springer, 2000.
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`Taylor [10] has suggested that chemisorption may be an activated process, and may occur slowly.
`The idea that adsorbed species and intermediates in catalysis may require ensembles composed of
`several adjacent free-surface atoms was first expressed by Balandin [17,19], who suggested that
`a reacting molecule may be simultaneously adsorbed on several atoms (the multiplets theory).
`The typical concentration of active sites has been evaluated to be in the domain of 1019 m−2 for
`metals and 1015 m−2 for metal oxides.
`The above description considers active sites as rather static. However, a metal oxide catalyst
`surface has been observed to change under working conditions. Moreover, it is worth recalling that
`oscillations on the surface occur during a catalytic reaction, which is important for kinetics studies;
`a theory was proposed [20]. It was shown [21] that stationary heterogeneous catalytic reactions
`involve a gas/catalyst equilibrium, which acts on the rate-determining step (RDS) of the reaction.
`Moreover, during stationary catalytic processes, the surface is covered with many two-dimensional
`chemisorbed crystal-like islands of different sizes rather than with isolated chemisorbed molecules.
`Depending on the chemical nature of the catalyst and of the reacting mixture, and on the ambient
`conditions, these islands can be of the same or of two or more chemical compositions. Each ensemble
`of these islands of any one chemical composition is considered a two-dimensional chemically adsorbed
`phase (chemadphase, ChPh), which oscillates, i.e., moves relative to other islands the surface of
`a crystal in such a way that, under stationary catalysis, the total surface area of each ChPh remains
`unchanged. Most of the two-dimensional ChPh crystals consist of a great number of chemadmolecules
`and represent an element of surface metalloinorganic or metalloorganic entities that are connected with
`the solid. Following such an approach, a revisited view of the active sites in heterogeneous catalysis on
`metal oxides is given in [22] and an example of active sites composed of four dimers for VPO catalyst
`is given above in Figure 4.
`Another important aspect of the surface of metal oxides is the appearance of a segregated phase
`on the surface as a consequence of the inability of the surface to homogeneously accommodate all
`cations in its lattice and to be altered when exposed to an external environment. This particularly
`holds true for complex mixed oxides (e.g., multicomponent catalysts in selective oxidation reactions)
`and depends on the preparation method and activation medium and procedure, and often occurs
`during a catalytic reaction, as illustrated in Figure 4 for MoVTeNb-O (so-called M1 phase) catalyst.
`In the case of supported oxides, this is even more drastic as mass transport on the surface or overall,
`within the pores, limits regular deposition far from equilibrium, leading to a concentration gradient of
`the deposited oxide within the pores.
`In a study using microcalorimetry, Schlögl et al. have shown [23] that, on an activated mixed
`oxide MoVTeNb-O catalyst, active for propene and propane selective oxidation and ammoxidation to
`acrylic acid and acrylonitrile, respectively, the adsorption energy of propene and propane are about
`constant vs. coverage (57 kJ·mol−1). However, after reaction the energy decreases and its distribution
`in strength becomes heterogeneous, as illustrated in Figure 4.
`The first part of this curve is thus characteristic of a “single-site heterogeneous catalyst” (SSHC site,
`as defined by Thomas [24]), i.e., with an active site of similar activity. However, under reaction
`conditions the surface is modified and the sites are heterogeneous in strength (no longer satisfying
`the SSHC principle). Moreover, in situ X-ray photoelectron spectroscopy (XPS) analysis has shown
`that depletion of Mo and V enrichment did occur during the reaction. This observation characterises
`element segregation or the formation of a sublayer of different composition than the bulk, as shown by
`Millet et al. [25]. It was observed [26] that the surface composition of the M1 phase differs significantly
`from the bulk, implying that the catalytically active sites are not part of the M1 crystal structure and
`rather part of all terminating planes. It then appeared that the active sites are formed under propane
`oxidation conditions and are embedded in a thin layer enriched in V, Te, and Nb on the surface of the
`stable self-supporting M1 phase [23]. This example shows how complex a catalytic surface can be and
`how the surface structure and chemical composition may change under reaction (working) conditions.
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`Figure 4. (A) Adsorption of propene on MoVTeNb oxide (so called M1 phase) before (filled circles)
`and after propane oxidation (open circles; reaction conditions of propane oxidation: 400 ◦C,
`C3/O2/H2O/N2 = 3/6/40/51); (B) adsorption of propane on activated (fresh) MoVTeNb oxide
`(M1 phase), (filled squares) and after adsorption and subsequent desorption of propene at 40 ◦C
`(open triangles). The differential heat of adsorption is plotted as a function of coverage normalised to
`the specific surface area. Reproduced with permission from [23]. Copyright Elsevier, 2012.
`
`This aspect of changes and reconstructions in the surface composition of metal oxide catalysts
`under reaction conditions is well known. For instance, for propane oxidative dehydrogenation on
`3− units scattered
`VMgO catalyst, the active surface was shown to be amorphous and composed of VO4
`over magnesia and polymeric VOx species. During the reaction a completely reversible phenomenon of
`order/disorder of the V overlayer was observed in connection with the redox state of the surface [27].
`This aspect is therefore recognised as occurring very often and shows that characterising a catalyst
`surface under working conditions with analysis of reactants and products on line (so-called real in situ
`or operando) is compulsory for a reliable description of active sites in metal oxide catalysts.
`
`3. Theoretical Approach and Calculations on Metal Oxide-Based Heterogeneous Catalysts
`
`The description of chemical catalytic reactions occurring at the surface of a solid catalyst has
`led to many studies by chemists and physicists all over the world since the 1960s, favoured by
`the huge increase of computational power [28–30]. A wide set of theoretical approaches has been
`developed from configuration interactions [31] to coupled cluster methods [32] and quantum chemistry
`Monte Carlo [33]. At present, the method of choice is the density functional theory (DFT) [34] to
`describe adsorption, surface properties and reactions at surfaces of metals, semi-conductors and
`insulator surfaces. The theory permits us to compute and predict reaction enthalpies and entropies,
`transition state structures and identify reaction mechanisms and surface properties [35]. For oxide
`catalysts, one needs an acceptable description of the electronic structure of the oxide, the energy of its
`defects and localised states, and the position of the valence and conduction bands to obtain a reliable
`prediction of the thermodynamic and kinetic aspects of the reaction. It is important to determine
`theoretically the oxide band gap and alignment of occupied and unoccupied levels vs. vacuum
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`level for redox properties and calculation of the reaction energies at the oxide surface. Standard
`implementations of DFT are based on the Kohn–Sham (KS) equation [36] and on the use of local
`density or a generalised gradient approximation for the exchange correlation functional. In a study of
`a catalytic system, a crucial problem is to determine the band gap of an oxide as the positions of the top
`of the valence band (VB) and bottom of the conduction band (CB) determine the redox properties of the
`catalyst, as they are electronic levels involved in charge transfer from or to the catalyst [9]. The KS band
`gap is the difference between eigenvalues of CB minimum and VB maximum. Also, the role of oxygen
`vacancies is important, as discussed above, and can be clarified with theoretical calculations [37].
`We have seen above (paragraph 2) that one can modify an oxide surface by substituting some
`metal atoms with another metal, M. The presence of the other metal atom changes the bonding in
`the oxide, and then either the oxygen atoms near M become chemically active or M becomes reactive
`and able to activate oxygen. It may also happen that both M and O are activated and participate in
`reactions in which an incoming molecule is dissociatively adsorbed.
`However, it has been shown that DFT may have serious troubles when it is used to study catalysis
`by some oxides [34]. It was shown that the energies of the Kohn–Sham orbitals are not correct, as there
`is a large difference between the molecular orbitals and the excitation energy. One has to consider the
`shortcomings of gradient approximation and density function theory (GGA-DFT) when one extends
`it to doped oxides [38]. For instance, for Li-doped MgO, some of the Mg atoms in the surface layer
`are replaced with Li, which creates an electron deficit in the oxide, which allows it to be a catalyst
`for methane, ethane, or propane activation. GGA-DFT calculations show that the electron deficit is
`distributed evenly on the surrounding oxygen atoms, and B3LYP and higher-quality methods find that
`it is localised on one oxygen atom.
`
`4. Metal Oxide Catalyst Preparation
`
`The objective of this section is to address the broad topic of metal oxide syntheses for
`heterogeneous catalysis by considering the main preparation procedures for bulk and supported
`metal oxide catalysts [39–42]. It deals with three main areas: (1) synthesis of bulk simple metal oxides;
`(2) synthesis of bulk mixed oxides; and (3) elaboration of supported metal oxides.
`For simple oxides, it is important to control the following key parameters: the nature of the
`polymorph, the morphology, the textural properties (surface area, porosity), as well as the thermal,
`chemical and mechanical stability. Besides purely thermal methods, which do not give access to fine
`control of these parameters, three main families of usual preparation routes based on the reactions
`between water or an organic solvent and inorganic precursors are usually considered, namely gas-phase
`polymerisation, aqueous-phase precipitation, and sol-gel (hydrolytic and non-hydrolytic) chemistry.
`Solvothermal syntheses in the presence of organic molecules and templating approaches yield
`materials with controlled morphology and textural properties, in particular organised microporous,
`mesoporous, macroporous and hierarchically organised oxides. Some novel preparation routes, such as
`complexation and solvent-free activated reactive synthesis, allow one to overcome the limited surface
`area reached after crystallisation at high temperature. Organisation of the porosity by hard or colloidal
`templating is a recent approach to maximise the specific surface area, leading to mesoporous and
`hierarchical porosity materials.
`For mixed oxides, the most common technique is co-precipitation by mixing two metal salts in
`a solution and forcing them to precipitate using a suitable base at a given pH. The obtained precipitate
`is then calcined under specific atmosphere depending on the catalyst used.
`Bulk catalysts mainly comprise active substances, but some inert binder is often added to facilitate
`the catalyst particles’ formation and limit attrition, particularly in moving bed reactors. However,
`in some cases, bulk catalysts are used as prepared, without binder, when prepared by high-temperature
`fusion, as for the iron-based catalysts used in ammonia synthesis.
`
`Opiant Exhibit 2322
`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 8
`
`

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`Catalysts 2017, 7, 341
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`9 of 25
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`The preparation of mixed-oxide catalysts involves co-precipitation of both components at a give