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Periodic Variations in the Catalytic Properties of Metals the INFLUENCE of SOLID STATE PARAMETERS on ADSORPTION and CATALYSIS

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Periodic Variations in the Catalytic Properties of Metals the INFLUENCE of SOLID STATE PARAMETERS on ADSORPTION and CATALYSIS Periodic Variations in the Catalytic Properties of Metals THE INFLUENCE OF SOLID STATE PARAMETERS ON ADSORPTION AND CATALYSIS By G. C. Bond, D.s~.,F.R.I.C. Research Laboratories, Johnson Matthey & Co Limited (2) The first row Group VIII metals (base Of the one hundred or so elements in the metals): Fe, Co, Ni. Periodic Table no less than spventy-jive (3) The second and third row Group VIII are metals, but of these only the twelve in metals (platinum group metals) : Ru, Groups VIIl and IB have important Rh, Pd, Os, Ir, Pt. catalytic properties. The marked changes in these properties on proceed- Comparison of Groups VIII and IB ing either certically or horizontally There are dramatic differences between the through the groups is discussed in terns catalytic properties of the Group IB metals of the periodic variation of the solid- and those metals lying adjacent in Group state properties of the mijtals. VIII : these differences are particularly marked with hydrogenation and similar It is now well established that the occur- reactions. Silver and gold are totally unable rence of significant and useful catalytic to activate hydrogen and hence to catalyse properties is largely confined to the elements reactions involving hydrogen, while their lying in Groups VIII and IB of the Periodic immediate neighbours to the left, palladium Table. Other elements of the transition series and platinum, are among the best known are able to act catalytically, but because of the hydrogenation catalysts. This simple ob- great thermochemical difficulty of reducing servation constitutes the most vivid possible them to the metallic state, and of keeping demonstration of the existence of an electronic them reduced, they do not find any practical factor in catalysis, since geometrically speak- applications. Metals which have either no ing there is very little difference between d-electrons or which have only filled d- palladium and silver, or between platinum electron shells, while sometimes thermo- and gold (see Table I). Their electronic pro- chemically suitable, are generally catalytically perties (magnetic susceptibility, electrical inactive, and catalysis (particularly of hydro- conductivity, etc.) are of course substantially genation and related reactions) has therefore different; the Group VIII metals having come to be particularly associated with the incomplete d-electron bands are paramagnetic, presence of unpaired d-electrons in thc metal. while the Group IB metals having no un- There are, however, many large and im- paired d-electrons are diamagnetic (see Table portant differences between the twelve metals TI). The presence of unfilled d-electrons is constituting Groups VIII and IB : to facilitate essential to hydrogen chemisorption, as they discussion we will subdivide the twelve stabilise an intermediate weakly-held form metals as follows: without which dissociation cannot occur. (I) The Group IB metals (coinage metals) : A similar but less marked difference in Cu, Ag, Au. catalytic properties exists between nickel Platinum Metals Rev., 1968, 12, (3), 100-105 100 and copper. Although nrckel is generally much Table I superior, copper is not without some modest Crystal Structure, Metallic Radius and properties as a hydrogenation catalyst, being Density of the Elements of Groups VIII particularly attractive for the selective hydro- and IB genation of alkynes and diolefins (I). The b.c.c. =body-centred cubic reason for the slight activity of copper has c.p.h. =close packed hexagonal been a matter of debate for many years and f.c.c. =facecentred cubic no firm conclusion has been arrived at. It is Fe co Ni cu possible that the d-electrons in copper are h.c.c. f.c.c. f.c.c. f.c.c. 1.248 1.258 1.248 1.278 more readily thermally excited to sp-states 7.87 g/cc 8.90 g/cc 8.90 g/cc 8.96 g/cc than those of silver and gold, and that as Ru Rh Pd Ag before the resulting unpaired d-electrons are c.p.h. f.c.c. f.c.c. f.c.c. responsible for catalytic activity. I'.32lf r.348 1.37A 1.44A 1.90 g/CC 12.44 g/CC 12.02 g/CC 10.49 g/CC The Group IB metals are more active than their Group VIII neighbours in the rare 0s Ir Pt Au c.p.h. f.c.c. f.c.c. f.c.c. cases where the metal is required to act as an 1.338 1.358 1.388 1.448 electron-donor, as for example in the case of 2.48 g/Cc 22.5 g/CC 21.45 g/CC 19.3 g/CC the decomposition of hydrogen peroxide (2). Silver has an important role to play as a There is also a related sharp break between controlled oxidation catalyst, and is used on the first and second row elements with regard a large scale for the oxidation of ethylene to electronic properties. The base metals are to ethylene oxide and of methanol to ferromagnetic while the platinum group formaldehyde. Copper is too easily oxidised metals are paramagnetic (see Table 11): the to be a possible catalyst, and gold is so noble base metals are readily oxidised and corroded that it is unable even to chemisorb oxygen. (and their compounds are correspondingly The Group VIII metals usually give only more difficult to reduce to the metallic state), complete oxidation. and have lower electrode potentials, than the platinum group metals. This has important Comparison of the Base Metals practical consequences in catalyst preparation, with the Platinum Group Metals since for example nickel oxide is not easily It is now desirable to compare the differ- reduced to the metal while the oxides of ences in physical and catalytic properties palladium and platinum can be reduced to observed on moving vertically through each the corresponding metals by hydrogen under sub-group of Group VIII. The base metals ambient conditions. The significant differ- differ from the platinum group metals in ences in the chemistry of the elements of many of their physical and chemical pro- Group VIII also have repercussions on pre- perties. The lanthanide contraction is re- parative procedures for catalysts. Thus there sponsible for the close similarity in the atomic are no palladium or platinum analogues of the and metallic radii of vertically adjacent pairs simple nickel salts such as Ni(NO,), and of the platinum group metals, and this is NiSO,, while hydrated nickel chloride, substantially responsible for other similarities, freely water soluble, contrasts with palladium for example, in latent heats of sublimation chloride, which is anhydrous, polymeric in and in melting points (see Table 111) and in structure and virtually insoluble in water. mechanical properties. All the metals of the The differences between the catalytic second and third sub-groups are face-centred properties of the base metals and the platinum cubic in structure, but those in the first sub- group metals are too numerous and well- group are either body-centred cubic or close- known to warrant a detailed exposition. The packed hexagonal (see Table I). latter are generally superior to the former as Platinum Metals Rev., 1968, 12, (3) 101 Table II methylene groups in certain steroids is more Magnetic Character and Mass Magnetic stereoselective when iridium is the cataIyst Susceptibility (c.g.s. units x 106) of the than when platinum is used (4). As a final Elements of Groups VIII and 1B example, palladium is more active than rho- Fe CO Ni cu dium for the hydrogenolysis of substituents ferro-. ferro- ferro- dia- (for example, alkoxy groups and halogen magnetic magnetic magnetic magnetic atoms) from aromatic rings (5). Attempts to - - - -0.09 rationalise effects of this kind have only been made in very limited contexts, the difficulty para- para-, para- dia- being the lack of adequate knowledge concern- magnetic magnetic magnetic magnetic ing metals other than palladium and platinum. 9 +0.50 -+1.11 7 5.4 -0.20 It is instructive to seek the origin of these Ir Pt All para- para- para- dia- differences in catalytic behaviour. The differ- magnetic magnetic magnetic magnetic ent crystal habits (Table I) may have some effect, but it is probably not large. There has been much discussion over a protracted catalysts for hydrogenation and related re- period on the relation between the catalytic actions, although the cost differential (about properties and the bulk properties of metals 20 between nickel and palladium) sometimes (6, 7, 8). Although catalytic effects must counteracts the activity difference. For this result from the physico-chemical properties reason also iron is widely used in ammonia of surface atoms, there are grounds for hoping synthesis and decomposition, although the that these, themselves not easily measurable, more costly ruthenium is in fact more efficient. will reflect the more easily measurable bulk The superiority of platinum as a hydrogena- properties. tion-dehydrogenation catalyst, however, deter- An important effect in determining catalytic mines its selection for petroleum reforming activity is undoubtedly the strength of ad- operations, notwithstanding the cost factor. sorption of intermediate species. For optimum The platinum group metals are the undoubted catalytic performancc there is required choice for oxidation processes such as the maximum coverage by species of minimum hydrogen-oxygen reaction, complete hydro- adsorption energy. With metals to the left carbon combustion and ammonia oxidation, of Group VIII activity is lower because the where base metals would he rapidly oxidised adsorption energy is too high; to the right it and inactivated. is lower because the adsorption energy is too small to maintain adequate surface coverage. Catalytic Properties of the The precise position of the activity maximum Platinum Group Metals depends on the nature of the reaction, as is Although the metals of the platinum group demonstrated in Figs I and 2.
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