Catalysis Today 89(2004)
255–268
Review
Environmental catalysis
François Garin ∗
Laboratoire des Matériaux,Surfaces et Procédés pour la Catalyse (LMSPC),UMR 7515CNRS,ECPM,
ULP ,25Rue Becquerel,67087Strasbourg Cedex 2,France
Abstract
This review article was constructed around the first Algerian–French congress aimed on emerging materials which was held at Tamanrasset by the end of February 2003.The aim of this review is to point out that a lot of work has been done in heterogeneous catalysis to better understand the active sites responsible for the catalytic reactivity.Most of these researches were performed under reducible atmosphere,on metallic catalysts,to improve our knowledge about hydrocarbon reforming catalysts.Starting from this base which was recalled through various classes of important studies such
as:(i)the dilution of the active sites,(ii)the use of bimetallics,(iii)the use of well-crystallised surfaces and (iv)the influence of the metal–support interactions;a development and an opening is made on the three-way catalysis and the DeNO x reactions.The objectives being to point out the very important influence of the experimental conditions and of the gas phase compositions which may induce very strong surface modifications of the initial metallic aggregates.Moreover,it will depend on the isomerisation,oxidation,reduction where the active site may also be composed of,in addition to the metallic crystallites,the participation of the oxygen of the support.A tentative for a general interpretation of the observed results is given by the use of the variations of the local density of states and of the “d”band centre energy.
©2003Elsevier B.V .All rights reserved.
Keywords:Hydrocarbon reforming reactions;Skeletal rearrangement of alkanes;Particle size effects;Alloys and bimetallic effects;Support influence;Well-defined surfaces;Three-way catalysis;NO x reduction;DeNO x process
1.Introduction
Sometimes a title is so used,so omnipresent,that its meaning is very small;but its basic sense is so imp
ortant that we must not pass it under silence and say nothing.To this “Environmental Catalysis”is linked the notion of sustainable development.From the book edited by Janssen and van Santen [1]there is a good definition of sustainable development “which is a process of change in which the
exploitation of resources,the direction of investments,the orientation of technological development and institutional change are all in harmony and enhance both current and future potential to meet human needs and aspirations”.In this definition we find words as future,employment and technical development.With respect to this last is-sue catalysis plays an innovative role in the development of new technologies to prevent and reduce all types of emissions.
∗
Tel.:+33-3-90-24-27-37;fax:+33-3-90-24-27-61.E-mail address:garin@chimie.u-strasbg.fr (F.Garin).
Another aspect in the past few years is the huge increase in the interest in nanotechnology,a term that was virtu-ally unheard of a decade ago.In fact,the length scale of importance in heterogeneous catalysis has been known by researchers to be nanometer or smaller for many years [2].Catalysts represent the oldest commercial application of nanotechnology.
Finally in the development of catalyst-based technolo-gies the catalysts were mostly optimised for activity all through the 20th century.Catalysis research in the 21st cen-tury should focus on achieving 100%selectivity for the de-sired product in all catalyst-based processes [3,4].This way can achieve clean manufacturing without by-products.This eliminates the need for waste disposal,and provide environ-mental sound green catalysts-based chemical processes [4].Moreover,we know how important is pollution linked to transportation,hence,from all the points raised above it seems necessary to make a review about what was done concerning catalysis and automotive pollution control and to point out the influence of the active sites which have nanometric scales.
0920-5861/$–see front matter ©2003Elsevier B.V .All rights reserved.doi:10.1016/j.cattod.2003.12.002
局域态密度
投资
256F.Garin/Catalysis Today89(2004)255–268
I shall not develop this manuscript around new materials but only about those already used for years with a new look on the results.The intention being to develop new ideas from former results.
This review article will be divided into three parts devoted, respectively,to hydrocarbon reforming reactions,three-way catalysis and DeNO x catalysis.These three topics were de-veloped during thefirst Franco-Algerian meeting devoted to emerging materials which was held at Tamanrasset from23 to25February2003.
The quality of diesel or gasoline is thefirst step to take into account when you are concerned by automotive pollution control.Too often there is not a global approach between the quality of the mixture of petrol and the efficiency of the catalysts used for automotive gas emissions.This situation can be understood from an economical point of view as two different huge industries are concerned and their interests are opposed;but from a scientific point of view we have tofill this gap and to erect a bridge between these two industries.
2.Hydrocarbon reforming reactions
2.1.Introduction
Due to the gasoline engine process,to get the best yield, the chemical nature of the gasoline should have a low con-tent of double bonds,either aromatics or olefins;be almost free of heteroatoms except for oxygen and have a narrow boiling point distribution.It has a low-volatility and a high octane number.
Therefore highly branched paraffins with 8–10carbon atoms would best fulfil all the requirements. Isooctane,which has an octane number equal to100by definition,is the reference structure and it can be assumed as a model;other molecules should come as close as pos-sible[5].In other words it means that we have tofind catalysts able to give branched hydrocarbons.At the oppo-site,in the Section4,devoted to DeNO x reactions,we shall discuss about the quality of the Diesel fuel and the mean-ing of the cetane number,where linear hydrocarbons are favoured.
It is not the purpose of this article to review all the mechanisms of reactions undergone by the carbon skeletons of aliphatic and alicyclic hydrocarbons in the presence of metallic catalysts but we want to stress the influence of the dispersion of metal particles in skeletal isomerisation reac-tions as well as cyclisation,ring opening and hydrogenolysis reactions.
From the pioneer works of the group of Gault and cowork-ers[6,7],it has been clearly pointed out that catalysis by oxide-supported metals may take place on the metal surfaces alone,and more open surface sites with lower packing den-sity as stepped platinum surfaces have a greater reactivity in H–H,C–H and C–C bond breaking than low index crystal surfaces[8,9].In parallel to these studies the influence of the particle size was pointed out since1969.Boudart[10]de-fined two types of reactions:“structure-insensitive”or facile reactions and“structure-sensitive”or demanding reactions.
A facile reaction may be defined as one for which the spe-cific activity of the catalysts is practically independent of its preparation mode[11].From these observations extensive studies on the influence of particle size in reactivity of alka-nes have been undertaken for about half a century.Such an investigation is directly correlated to the concept of active centres which can be already found in Taylor’s1925paper in which he wrote:“there will be all extremes between the case in which all the atoms in the surface are active and that in which relatively few are so active”and“...the amount of surface which is catalytically active is determined by the reaction catalysed”[12].
All the experiments devoted to hydrocarbon reforming re-actions are usually performed under reductive atmospheres where a mix of hydrogen and hydrocarbon passes through the catalyst bed.In general these experiments take place un-der stationary conditions in reactant compositions,tempera-ture and gasflow velocity.The temperature range to perform such reactions is between150◦C and up to550◦C.Most of the studies which we are going to give the results of were made on metal-supported catalysts in which the metal(Pt, Pd or Ir)was deposited on a catalytically more or less in-ert carrier.Besides these model“industrial”catalysts,single crystals and stepped surfaces were also used to characterise the active sites.
2.2.Results and discussion about skeletal rearrangement of alkanes
Several questions at that stage have to be asked:
(a)Where does the catalytic reaction occur?
(b)What are the parameters which govern the catalytic re-
action;are they electronic and/or geometric effects? (c)Are catalytic properties governed by individual atoms
or by ensemble atoms?
(d)Do catalytic reactions,in the adsorption step,follow a
dissociative or an associative process?
We are going to answer these questions one after the other on the base of an ensemble of convergent experiments,as it is shown in Fig.1.These experiments were conducted since 1965up to1980to better understand the catalytic active sites.
2.2.1.Particle size effects[6,7]
Two basic mechanisms were proposed for the skele-tal isomerisation of hydrocarbons on metals.The bond shift mechanism(Fig.2a)explains the isomerisation of short molecules.When the carbon chain is long enough, another mechanism takes place,which involves dehy-drocyclisation to an adsorbed cyclopentane intermediate followed by ring cleavage and desorption of the products (Fig.2b).On most platinum catalysts,eitherfilms or sup-ported platinum with moderate degree of dispersion,both
reaction to a book or an article脂肪族
解吸
F.Garin/Catalysis Today89(2004)255–268257
USE OF BIMETALLICS,
since 1970
[13,14] DILUTION OF THE ACTIVE METAL,
since 1969
[6,7]
ACTIVE SITES ON A
CATALYST AS
“M/OXIDE”
INFLUENCE OF VARIOUS
SUPPORTS, since 1970
[15,16] and
STRONG METAL
SUPPORT INTERACTION
since 1978 [17,18]
STUDIES ON WELL
CRYSTALLIZED
SURFACES,
Since 1975
[9,19,20]
Fig.1.Convergent
studies to approach a better understanding of the active sites.
a) Bond Shift
Fig.2.(a)Bond shift(BS)and(b)cyclic mechanism(CM)for skeletal isomerisation of alkanes.
258F .Garin /Catalysis Today 89(2004)
255–268
Cyclic mechanism
METHYL MIGRATION
CHAIN LENGTHENING
CM only
Methyl Shift A
BS
Fig.3.Bond shift and cyclic mechanism for the isomerisation of 2-methylpentane to 3-methylpentane and n -hexane.Use of 13C
labelled hydrocarbons.
the cyclic and the bond shift mechanisms take place.The first problem which arises,then,is that of determining,in each case,the contribution of each mechanism.This problem may easily be solved by using tracer techniques [7].Fig.3shows how the use of 2-13C-2-methylpentane allows a distinction to be made between the cyclic and the bond shift mechanism in the case of the isomerisation of 2-methylpentane to 3-methylpentane.Similarly,2-13C and 4-13C-2-methylpentanes yield n -hexanes labelled on differ-ent positions according to whether the chain lengthening occurs by cyclic or bond shift mechanism.
The description of the isomerisation mechanisms as being of bond shift or cyclic mechanism is very rough;structural effects,especially those resulting from substitution of hydro-gen atoms in the reacting
molecules,have also been consid-ered.Such effects are very pronounced in the case of methyl-cyclopentane hydrogenolysis,one of the steps involved in the cyclic mechanism.Such a reaction takes place accord-ing to two different mechanisms,one selective and the other non-selective.For the former reaction only di-secondary –CH 2–CH 2–bonds are broken on catalyst of low disper-sion (10%Pt/alumina);at the opposite,for the latter reac-tion,an almost equal chance of rupturing any –CHR–CHR  –bond of the ring takes place on highly dispersed catalysts such as 0.2wt.%Pt/alumina;but breaking of cyclic C–C bonds containing a quaternary carbon atom never occurs [21].
Now we are going to correlate these reaction mechanisms with the size of the metal particles and more generally with the structure of the metal surface.One could expect that selective hydrogenolysis,favoured on large metal particles,requires a larger number of metal atoms than non-selective hydrogenolysis,which takes place on extremely dispersed catalysts.Similarly,isomerisation of 2-methylpentane to 3-methylpentane takes place predominantly according to a bond shift mechanism on catalysts of low dispersion and according to a cyclic mechanism on catalysts with very small metal particles;this again could imply a larger num-ber of metal sites for the former than for the latter reaction [21–24].It was shown [25]that the percentage of cyclic mechanism in the isomerisation of 2-methylpentane to 3-methylpentane as a function of metal dispersion remains roughly constant (ca.20%)
over a large dispersion range (0–50%)and increases above 50%dispersion.Careful de-termination by electron spectroscopy and SAXS of metal particle size distributions shows that there are no crystal-lites smaller than 1nm in the catalysts of low dispersion while they are present in increasing amounts with increas-ing dispersion in those catalysts for which an enhancement of the cyclic mechanism is observed.From these results,it was suggested that both types of isomerisation sites include edge atoms;and an upper limit of metal particle size around 2.5nm was defined below which selective hydrogenolysis is no longer possible.
These experiments were able to show the particle size effects in isomerisation and hydrogenolysis reactions.Other approaches were also undertaken to better understand the “nature”of the active sites.
丙基
甲基
F.Garin/Catalysis Today89(2004)255–268259
2.2.2.Alloys and bimetallics influence[13,14]
By the use of alloys the debate about the electronic and geometric effects was at its maximum and ver
y good arti-cles written by Ponec[14,26]clarified this point.Alloying of metals may result in important changes in their activity and selectivity in catalytic reactions.These changes are ex-perimentally well established but theoretically still difficult to understand as a lot of parameters have to be taken into account;among them,there are surface segregation and the thermodynamic of its formation.
When a metal which is active in a certain reaction is alloyed with an inactive one,two effects can be conceived
[27]:
(a)A“geometric”or“ensemble size”effect.By alloying,
the number of contiguous identical atoms is clearly de-creased.Catalytic reactions which require large ensem-bles of active atoms will then obviously be suppressed more strongly than reactions which require only small ensembles.
(b)An“electronic”or ligand effect.The electronic structure
of the metals may be changed by alloying.If so,then the bond strength of the adsorbed species and thereby their reactivity may change as well.
In spite of the difficulties to understand the“real”be-haviour of such systems,these studies always bring a huge amount of results which improve the knowledge of the global catalytic reactions.In fact,a large amount of alloys or bimetallics was studied since thefirst one prepared by Kluksdahl;it was a Pt–Re catalyst[28].
2.2.
3.Support influence[17,18]
The other approach to the understanding of active sites concerned the influence of the support on the intrinsic prop-erties of the supported metals.The phenomenon of“strong metal–support interaction”(SMSI)has attracted interest and has principally been interpreted since1984on the basis of decoration of the metal surface,partially or largely,by the support[18].When a SMSI effect takes place it was originally reported that the hydrogen uptake on platinum could be restored after oxidation at673K[17].Subsequent studies have found that the adsorptive properties of the metal could be partially restored even by oxygen expo-sure at room temperature[29]or by exposure to steam at 525K[30].Since the activity in hydrogenolysis reactions is affected strongly by the onset of SMSI,reactivity is a better probe than chemisorption for monitoring the reversal of SMSI.
After,around1990,this simple view has been questioned and the role of electron transfer between support and metal, originally proposed by Schwab and Pietsch[31]and Soly-mosi[32]has been revived.
Studies of Clarke et al.[33]have shown that high temper-ature reduced(HTR)Pt/TiO2catalysts exhibit SMSI as in-ferred from negligible hydrogen chemisorption take-up and moderate activity for skeletal reactions of alkanes.The in-terest in titania supports is heightened by their unique abil-ity to enhance the reactivity of metal in hydrogenation of CO[34]or molecules that have CO functional groups[35], while suppressing hydrogenolysis of hydrocarbons such as ethane[36]or n-butane[37].The SMSI effect appears to be prevalent on both small and large metal particles.
At that point we may underline that such SMSI may take place in automotive exhaust catalysts as they are forced to high temperatures and changes in gas composi-tions from reductive to oxidant as we shall see in the next section.
There is only one step jumping and to enter in the area of active supports as solid acid supports and substitutes of platinoids and their(induced)influence on the supported metals.On one hand,bifunctional catalysis operates either following the“classical”mechanism proposed by Mills et al.[38]which comprises dehydrogenation of alkanes on the metal surfaces,isomerisation of the protonated alkenes
on the acid sites,and hydrogenation of the isomerised alkenes on the metal surfaces,or the presence of a metal–proton adduct[H–(M m)(H+)x]x+site which combines metallic and acid sites and consequently the migration step occurring in the former mechanism between the two sites,metallic and acid,is suppressed[39,40].And,with the solid acid sup-port participation,it is generally agreed that acid-catalysed hydrocarbon conversion reactions proceed by way of highly reactive,positively charged intermediates,that are referred to carbocations.
On the other hand,the generation of new acid sites on mixed oxides wasfirst proposed by Thomas[41],further developed by Tanabe and Takeshita[42]and by Connel and Dumesic[43].The latter have studied the generation of new acid sites on a silica surface by addition of several kinds of dopant cations.There seems to be a common idea in these works that the generation of new acids sites is ascribed to the charge imbalance at locally formed M(1)–O–M(2)bond-ings,where M(1)is the host metal ions and M(2)the doped and/or mixed metal ions.The charge imbalance might be ex-pected even on single-component metal oxides consisting of small particles,since the electronic properties of small-sized metal or oxide particles are widely accepted to be somewhat different from those of the bulk materials[44].These dif-ferences are attributed to the surface imperfections,which can be metal or oxygen vacancies,causing the local charge imbalance.From the work done by Nishiwaki et al.[45] on Ti
O2catalysts with various particle sizes from around 5to25nm;they noticed that the highest acid strength in-creases with a decrease in the particle size,indicating the generation of new and strong acid sites on small-sized TiO2 particles.This is likely to be due in this case to the pres-ence of many oxygen vacancies existing on the surface of small-sized TiO2particles.The oxygen vacancies generate considerable numbers of dangling bonds,whose energy lev-els are located in the band gap region between the valence and the conduction bands.Electrons trapped in these levels may cause the local charge imbalances and hence the gener-

版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。