BH4在 Au和 Ag催化剂上的电氧化机理
Electrochimica Acta51(2006)
5459–5467
Kinetics of sodium borohydride direct oxidation and oxygen reduction in
sodium hydroxide electrolyte
Part I.BH4?electro-oxidation on Au and Ag catalysts
Marian Chatenet?,Fabrice Micoud,Ivan Roche,Eric Chainet1
Laboratoire d’Electrochimie et de Physico-chimie des Mat′e riaux et des Interfaces,LEPMI,UMR5631CNRS-INPG-UJF,ENSEEG,BP75,
38402Saint Martin d’H`e res Cedex,France
Received23November2005;received in revised form13February2006;accepted14February2006
Available online29March2006
Abstract
The direct oxidation of sodium borohydride in concentrated sodium hydroxide medium has been studied by cyclic and linear voltammetry, chronoamperometry and chronopotentiometry for silver and gold electrocatalysts,either bulk and polycrystalline or nanodispersed over high area carbon blacks.Gold and silver yield rather complete utilisation of the reducer:around7.5electrons are delivered on these materials,versus4at the most for platinum as a result of the BH4?non-negligible hydrolysis taking place on this latter material.The kinetic parameters for the direct borohydride oxidation are better for gold than for silver.A strong in?uence of the ratio of sodium hydroxide versus sodium borohydride is found: whereas the theoretical stoichiometry does forecast that eight hydroxide ions are needed for each borohydride ion,our experimental results prove that a larger excess hydroxide ion is necessary in quasi-steady state conditions.When the above-mentioned ratio is unity(1M NaOH and1M NaBH4),the tetrahydroborate ions direct oxidation is limited by the hydroxide concentration,and their hydrolysis is no longer negligible.The hydrolysis products are probably BH3OH?ions,for which gold displays a rather good oxidation activity.Additionally,silver,which is a weak BH4?oxidation electrocatalyst,exhibits the best activity of all the studied materials towards the BH3OH?direct oxidation.
Finally,carbon-supported gold nanoparticles seem promising as anode material to be used in direct borohydride fuel cells.
?2006Elsevier Ltd.All rights reserved.
Keywords:Borohydride;Direct oxidation;Gold;Silver;Concentrated sodium hydroxide
1.Introduction
In a recent past,many liquid fuels have been studied as a substitute to hydrogen for low-temperature fuel cells.How-ever,their electro-oxidation is generally slow and irreversible, like for methanol[1]or ethylene glycol[2].Sodium borohy-dride:NaBH4,may be an alternative,thanks to its high capacity (5.7Ah g?1)and energy density(9.3Wh g?1at1.64V).These values are much higher than for metal hydrides,https://www.sodocs.net/doc/0f4961054.html,Ni5or Fe–Ti alloys,for which the capacity does not exceed0.4Ah g?1. So,sodium borohydride,rather safe,non-toxic,chemically sta-ble and easy to transport in its dry state,appears as an attractive hydrogen source for fuel cells.However,its use undergoes a few drawbacks,the?rst of which being its ability to hydrolyse ?Corresponding author.Tel.:+33476826588;fax:+33476826777.
E-mail address:Marian.Chatenet@lepmi.inpg.fr(M.Chatenet).
1ISE member.quasi-spontaneously on a great variety of electrode materials [3].We note,that one way of using borohydride can precisely consist of taking advantage of the heterogeneous hydrolysis on a well-chosen decomposition catalyst,e.g.platinum[4]or ruthe-nium[4,5],in order to produce extremely clean hydrogen gas on the electrodes,hydrogen being subsequently electro-oxidized [6].Nevertheless,such use of borohydride is not optimized, since the faradic ef?ciency is50%at the most and the heteroge-neous hydrolysis reaction is dif?cult to control.The hydrolysis apparently proceeds in various steps,among which BH3OH?intermediate(rather stable)is produced and further hydrolysed, as summarized by Eqs.(1)and(2)[7]:
BH4?+H2O→BH3OH?+H2(1) BH3OH?+H2O→BO2?+3H2(2) However,alkaline solutions of pH higher than12hinder the borohydride hydrolysis reaction[8](but do not totally suppress
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5460M.Chatenet et al./Electrochimica Acta51(2006)5459–5467
it[9])and the tetrahydroborate anion BH4?can be oxidized directly,theoretically without any(quantitative)hydrolysis step. The reaction was?rst attempted in a fuel cell anode by Amendola [10].Provided the hydrolysis is totally blocked,the reaction involves eight electrons per tetrahydroborate ion:
BH4?+8OH?→BO2?+8e?+6H2O(3) The recent development of anion-exchange membranes[11,12] has drawn renewed interest in the direct borohydride fuel cell (DBFC)[13,14].From Indig and Snyder[15]or Jazinski[16]?rst attempts in the1960s,the direct oxidation of sodium or potassium borohydride has been studied in alkaline media for various metal electrocatalysts such as platinum,nickel,palla-dium,copper and gold[17,18]or nickel-based alloys such as NiZr[19,20]and Ni2B[16].However,due to the complex boro-hydride direct oxidation mechanism,involving not less than eight electrons,none of the previous studies provide a complete view of the reaction scheme and kinetics,most of the papers deal-ing with fuel cell tests[1,10,13,14,18–20].A probable reaction path has been proposed by Elder and Hickling[9]and Morris et al.[3]for platinum.Tetrahydroborate ion?rst hydrolyses, yielding BH3OH?stable intermediate,which undergoes par-tial oxidation(rate determining step)and subsequent hydrolysis. Such mechanism yields2–4electrons per tetrahydroborate ion. Elder and Hickling did conclude that only electrode materials exhibiting good activity to ionize hydrogen(Pt,Pd and Ni)could oxidize borohydride at a reasonable overvoltage.However all these materials are good borohydride hydrolysis catalysts and thus yield faradic ef?ciencies well below unity:between25 and50%for platinum[3,9]and50and75%for nickel[21] and palladium[18]respectively.We know now that gold,how-ever inactive for hydrogen oxidation,is able to electrocatalyse borohydride oxidation[8,17,22].The reaction seems to proceed through dual pathways:the direct oxidation of BH4?and the partial hydrolysis of BH4?into BH3OH?followed by the oxi-dation of that latter species.BH3OH?oxidation is much faster than that of BH4?and probably occurs at potentials around0.5V lower[8,22].
At the light of these data,which remain fuzzy in their conclu-sions,the present paper aims at presenting kinetics and mech-anistic results for the direct oxidation of sodium borohydride in sodium hydroxide solutions.As the classical hydrogen oxi-dation catalysts do not appear to be suited for tetrahydroborate ion oxidation(from a faradic ef?ciency point of view),we com-pared the selectivity and activity of gold and silver,either as bulk electrodes or carbon-supported electrocatalysts.
2.Experimental procedures
2.1.Electrode materials,apparatus and solutions
The sodium borohydride oxidation electrocatalysts were gold and silver,either bulk and polycrystalline(noted as Au and Ag) or nanodispersed over carbon(noted as Au/C and Ag/C).These supported materials all originate from E-Tek and consist of elec-trocatalysts with a10wt.%metal loading on Vulcan XC72.
The bulk materials were simply characterized after mirror-like polishing using diamond paste down to1?m,followed by subsequent washing for15min in ultrasonic baths of acetone, ethanol–water(1–1)and water successively in order to remove any trace of impurities.
The electrocatalysts nanoparticles were characterized as micron-size active layers,deposited on a glassy carbon elec-trode(diameter5mm),previously polished using the procedure described above.The active layer precursor was an ink contain-ing the tested electrocatalyst and PTFE as explained in former papers[23,24].After the solvent evaporation at room temper-ature,the layer(around3?m thick,with10wt.%PTFE)was sintered15min at180?C under an inert atmosphere in order to obtain suf?cient mechanical stability.
The direct oxidation of sodium borohydride was studied in 0.1–1M sodium hydroxide solutions(Normapur,PROLABO), with sodium borohydride(Merck,Suprapur)in the range10?2 to1M.In all experiments,the temperature was held at25±1?C by a RM6-Lauda water-circulation thermostat-cryostat.
The electrochemistry experiments were controlled using a numeric potentiostat,either PAR263(EG&G)or VMP2Z(Bio-logic),in a four-electrode cell with a PTFE bottom,as explained in previous papers[25,26].A gold grid and a gold sphere were respectively used as counter and auxiliary electrodes;indeed, using platinum would have resulted in harsh borohydride hydrol-ysis.The reference electrode was a classical mercury/mercuric oxide electrode(Radiometer analytical)in1M sodium hydrox-ide solution.However,all the potentials are expressed towards the normal hydrogen electrode(NHE).All the electrode materi-als have been characterized using a rotating disk electrode(EDT 101,Radiometer Copenhagen)setup.
2.2.Physical characterization of the carbon-supported materials
Due to the impossibility to determine the active area of Ag/C and Au/C via direct electrochemical measurement(hydrogen adsorption–desorption coulometry)the gold and silver parti-cle size has been investigated using Transmission Electron Microscopy(TEM),using a high-resolution TEM Jeol2010. For statistical relevance of the analysis,the counting takes into account at least500particles.
2.3.Sodium borohydride direct oxidation experiments
We characterized the electrocatalysts activity towards the sodium borohydride direct oxidation under argon atmosphere, following argon(Air Liquide,N45)bubbling into the solution. We used linear and cyclic voltammetry either in quasi-steady state(5mV s?1)or for rather fast scan rate(10–200mV s?1)to quantify the reaction kinetics.The potential range studied lied between?1.1V versus NHE and0.4V versus NHE depending on the solution composition and the studied electrocatalyst.In the quasi-steady state linear voltammetry experiments,the RDE rotation rate varied in the range41.9–262rad s?1depending on the experiments.Prior to each linear voltammetry,the potential was held for1min at the starting potential,which varied accord-
M.Chatenet et al./Electrochimica Acta 51(2006)5459–54675461
ing to the solution composition and the electrode material.The kinetic parameters were determined from the diffusion correc-tion from the raw current density/potential data.They consist of the Tafel slope (b (V dec ?1))and the exchange current density (i 0(A cm ?2))determined by the intercept of the Tafel slope with the zero NaBH 4oxidation overpotential axis.
In view of determining the number of electrons exchanged per tetrahydroborate ion,we used the classical Levich Eq.(4)[27]:
i lim =0.620nFD 2/3C ?ν?1/6?1/2
(4)
where i lim (A cm ?2)
is the limiting current density on the voltammogram,n the number of electrons involved in the reac-tion,D =1.6×10?5cm 2s ?1,the BH 4?diffusion coef?cient [18–28],C *(mol cm ?3)the BH 4?concentration in solution,ν=1.19×10?2cm 2s ?1,the NaOH solution kinematic viscos-ity [29]and ?(rad s ?1)is the RDE revolution rate.
Chronometric methods were attempted to con?rm the voltammetry results.We tried chronoamperometry,varying the electrode potential from its open circuit value to a value cor-responding to the limiting current region on the quasi-steady state voltammograms;the overall number of electrons was then calculated,assuming the only reaction occurring was the boro-hydride oxidation,from the well-known Cottrell equation [27]giving the relationship between the faradic current density i f and the solution parameters:
i f =nF
D C
?
(5)Chronopotentiometry has also been used from open-circuit conditions to oxidation current.The number of electrons involved in the reaction was derived from the following Sand equation [27]:
i f =nF
πD 2τC
?
(6)where τ(s)is the transition time.
Here again,we assumed no parasitic reaction occurs con-comitantly with the oxidation of tetrahydroborate ion;the valid-ity of this hypothesis will be discussed further.
We realize that the chronometric methods only give a good approximate of the overall number of electrons exchanged per tetrahydroborate anion,since they rely on the knowledge of parameters which are known with imperfect accuracy.C *is a priori known,but due to the spontaneous BH 4?hydrolysis,its value may non-negligibly decrease in the course of the exper-iment.However,as stated by Amendola et al.[30]using the formula (7)from Kreevoy and Jacobson [31]:log t 1/2=pH ?(0.034T ?1.92)
(7)
where t 1/2(min)is the half time (the time for one half of a NaBH 4solution to decompose)and T (K )is the solution temperature.NaBH 4solutions at pH 14and T =298K have a half time of 430days.At pH 13.8(the pH of 1M NaOH [32])t 1/2=270days at 298K.Our experiments lasted 5h at the most (between the solution preparation and the last measurements),so any
spontaneous BH 4?hydrolysis was negligible.Moreover,the reproducibility attempts we did undertake (immediately after the electrolyte introduction in the cell and at the end of the quasi-stationary measurements)showed the NaBH 4concentration was indeed stable during the course of the experiment,which is con-sistent with the use of gold counter and auxiliary electrode (cf.part 2.1).
3.Results and discussion
3.1.Physical characterization of the carbon-supported electrocatalysts (transmission electron microscopy)
The carbon-supported silver nanoparticles (10wt.%,E-Tek)are not homogeneous in size (Fig.1).Whereas most of the nanoparticles are around 5nm (Fig.1a),some aggregates are also detected,as well as nice round-shaped particles of size above 100nm.Thus,the average particle-size determination is very tricky and probably suffers high relative error:the mean diame-ter value of 4nm was however calculated from the histogram of Fig.1b (not taking into account the particles of diameter above 100nm).Such average diameter could be used to calculate the active area of silver,but with our broad particle-size distribu-tion,we cannot ensure that the TEM pictures strictly represent the overall population of nanoparticles.
As for silver,the gold nanoparticles did not exhibit a nar-row size distribution (Fig.2).If some small nanoparticles are detected (Fig.2a),most of them are of much bigger size (Fig.2
b).
Fig.1.TEM picture for Ag/C (10wt.%,E-Tek)(a)and corresponding particle-size histogram (b).
5462M.Chatenet et al./Electrochimica Acta51(2006)
5459–5467
Fig.2.HRTEM(a)and TEM(b)pictures for Au/C(10wt.%,E-Tek). Moreover,some particles seem to have undergone sintering and thus display a very irregular shape.As for Ag/C,the wide disper-sion in size for the gold particles,as well as their very irregular shape renders very dif?cult any average diameter calculation.
In consequence,the electrochemical parameters for Ag/C and Au/C are given in term of mass activity and not of speci?c activity.
3.2.Direct oxidation of sodium borohydride
The bibliographic data reveal that mainly materials exhibiting good electrochemical activity towards the oxidation of hydro-gen(Pt,Pd and Ni)have been tested for the oxidation of sodium (or potassium)borohydride so far.Despite Morris et al.’s con-clusions specifying that electrochemically active borohydride oxidation materials were necessarily also active for hydrogen oxidation[3],gold proved to be a good electrocatalyst[22].The main reason for such activity is gold relative inactivity towards the hydrolysis reaction.Moreover,if BH3OH?is produced from BH4?spontaneous hydrolysis in solution,it can be further oxi-dized electrochemically on gold electrodes.
We tried some unsuccessful experiments on active layers
made of carbon-supported platinum nanoparticles:the hydrol-
ysis is so fast on platinum that the hydrogen evolution dra-
matically deteriorates the active layer.In addition,only2–4
electrons can be recovered from the borohydride oxidation on
platinum[3].So,despite the rather high reaction rate[33]and
platinum good activity towards hydrogen oxidation reaction we
decided to focus on gold and silver as tetrahydroborate electro-
oxidation materials.Most experiments have been undertaken
in1M sodium hydroxide in the presence of10?2M sodium
borohydride,which should easily yield RDE diffusion limiting
current at high overpotential while slow down the spontaneous
tetrahydroborate hydrolysis,the rate of which is?rst order with
respect to BH4?concentration[3].
3.2.1.Silver electrocatalysts
3.2.1.1.Polycrystalline silver(Ag).Silver,which had not been
studied in the literature,shows some activity towards the boro-
hydride oxidation reaction.The open-circuit potential of a
freshly polished silver electrode lies around?0.5V versus NHE,
nearly0.75V higher than the theoretical equilibrium potential
(?1.24V versus NHE),resulting from the existence of a mixed
potential.It could follow tetrahydroborate ions non-negligible
hydrolysis(yielding hydrogen evolution)on silver and subse-
quent hydrogen oxidation and/or the reduction of silver native
surface oxides.The quasi-stationary voltammograms of Fig.3a
show that silver suffers very high overpotential(around1V)
for tetrahydroborate oxidation.The voltammograms exhibit the
classical limiting current at high overpotential(beyond0.4V
versus NHE),whereas the onset for BH4?oxidation is around ?0.2V versus NHE.Fig.3b shows the agreement between the Levich theory and our experimental data when ilim is measured
at0.4V versus NHE.The voltammetric wave is however not
classical,since a pseudo-wave is observed between the kinetic
and diffusion parts(centred at0.1V versus NHE).Such pseudo-
wave does not agree with the Levich criteria(Fig.3b),which
Fig.3.NaBH4oxidation voltammogram(5mV s?1)plotted on Ag in1M NaOH+0.01M NaBH4—argon atmosphere(a)and corresponding i lim vs.?1/2 plots;i lim pseudo-wave data are measured at0.13V vs.NHE;i lim plateau data are measured at0.4V vs.NHE(b).
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M.Chatenet et al./Electrochimica Acta 51(2006)5459–5467
5463
Fig.4.Benchmark voltammogram (5mV s ?1)plotted on Ag in 1M NaOH in the absence of sodium borohydride at 41.9rad s ?1,argon atmosphere.
expresses that the process occurring in the pseudo -wave is not limited by the diffusion–convection.The pseudo-wave could then follow the formation
of silver oxides,as shown in Fig.4:the ?rst silver oxide plateau (OH adsorbed layer)gently falls at the same potentials.Silver oxide (Ag 2O)formation onset is monitored around 0.2V versus NHE,which coincides with the increased tetrahydroborate oxidation current monitored on silver (Fig.3a).So,pristine silver would not exhibit a good borohy-dride oxidation activity,contrarily to silver oxides.Silver native oxide are probably reduced upon silver contact with the reducing electrolytic solution (10?2M BH 4?)for potentials lower than ?0.4V versus NHE.Holding the silver electrode at ?0.375,?0.1and 0.1V versus NHE indeed showed the appearance of a greyish oxidation layer at ?0.1V versus NHE.This layer is much thicker at 0.1V versus NHE,while it is not observed at ?0.375V versus NHE.Liu et al.also reported such effect of surface oxides for Ni and Pd electrocatalysts [18].
Plotting (i lim versus ?1/2),we determine that in average 7.5±0.2electrons are involved in the oxidation reaction (at 0.4V versus NHE),in agreement with the theory.Unfortunately,the tetrahydroborate oxidation onset (?0.2V versus NHE)signs the bad reaction kinetics and renders such good result in term of faradic ef?ciency of poor interest for any DBFC application,as also evidenced from silver kinetic parameters in Table 1.
The chronopotentiometry results of Fig.5reveal once again the in?uence of silver state of surface.As for the voltammetry experiments,there seem to be two waves on the chronopoten-tiograms.Based on the ?rst one,which is also the better de?ned,and using Eq.(6),we calculate 4.8±0.2electrons (averaged on four experiments)per tetrahydroborate anion.This low value probably ?nds its origin on the fact that the ?rst chronopo-tentiometry wave ends around ?0.1V versus NHE,a potential where the silver oxide layer is not complete.The fuzzy second chronopotentiometry wave for the two lowest currents (ending around 0.3V versus NHE,at potentials where silver oxides are stable)gives 7.5±0.5electrons,in agreement with the voltam-
Fig. 5.NaBH 4oxidation chronopotentiogram plotted on Ag in 1M NaOH +0.01M NaBH 4between the open circuit condition (?0.5V vs.NHE for 1min)and various oxidation currents—argon atmosphere;silver geometric area =0.283cm 2.
metry results.So,assuming the oxide formation is not altering too much the number of electron determination following Sand equation (which could probably be discussed),the agreement between the voltammetry and chronopotentiometry experiments is rather good.It shows that tetrahydroborate anion oxidize in an eight-electron process on silver oxide (Ag 2O),whereas OH-covered silver only partially oxidizes BH 4?.On the contrary,pristine silver is rather inactive for the reaction,and proba-bly activates the chemical hydrolysis.Such result agrees with the chronoamperometry experiments (not shown)for which 7.9electrons are found for a step from the open-circuit potential (?0.5V versus NHE)to 0.25V versus NHE (in the silver oxide potential zone).We emphasize the fact that such value pos-sibly suffers little inaccuracy due to the contribution of the oxide formation.However,based on the voltammograms from Figs.3and 4,which have been plotted in the same experimental conditions,the current relative to the silver oxide formation only contributes to approximately 4–10%of the overall tetrahydrob-orate oxidation current.
3.2.1.2.Carbon-supported silver nanoparticles (Ag/C).The quasi steady-state tetrahydroborate oxidation voltammograms on silver nanoparticles exhibit the same features than those for polycrystalline silver (Fig.6).As for the bulk silver electrode,the open circuit potential lies around ?0.5V versus NHE and a pseudo -wave is monitored.However,the pseudo -wave onset is at ?0.3V versus NHE,at potentials 0.1V more negative than for bulk silver,and it is better de?ned.This faster reaction on sil-ver nanoparticles than on bulk silver possibly follows a particle size effect.Such size effect had already been evidenced for the molecular oxygen reduction on platinum [23,34]and on silver nanoparticles [25].Anyway,even with the improved kinetics on Ag/C (Table 1),silver still exhibits poor tetrahydroborate anion oxidation properties.
Table 1
Kinetic parameters (b ,i 0and E 1/2)for silver and gold electrocatalysts and arithmetic average number of electrons based on the quasi steady-state voltammetry,chronopotentiometry and chronoamperometry results Electrocatalyst b (V dec ?1)i 0(A cm ?2)i 0(A g ?1)E 1/2(V vs.NHE)n Ag
0.11 3.6×10?10–
07.6Ag/C (10wt.%)0.13 2.1×10?8 1.9×10?3?0.13–Au
0.157.4×10?6–?0.167.4Au/C (10wt.%)
0.15
2.0×10?5
1.8
?0.2
8
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5464M.Chatenet et al./Electrochimica Acta51(2006)
5459–5467
Fig.6.NaBH4oxidation voltammogram(5mV s?1)plotted on Ag/C in1M NaOH+0.01M NaBH4,argon atmosphere.
3.2.2.Gold electrocatalysts
As stated in the introduction,gold has already been studied
with some success for the tetrahydroborate oxidation in alkaline
medium[8,17,22].However none of these studies concerns the
steady-state behaviour of gold electrocatalysts.
3.2.2.1.Polycrystalline gold(Au).The quasi-steady state voltammograms for Au presented in Fig.7show a shape which
dramatically differs to that for silver electrocatalysts.Only one
oxidation wave is monitored,showing the gold surface is prob-
ably not modi?ed during the potential sweep in the window
of the experiment.The onset for the oxidation wave is around ?0.5V versus NHE,about0.3V more negative than for Ag, showing gold is a far better material to perform the reaction.
This is also con?rmed by the lowest open-circuit potential value:?0.8V versus NHE for Au compared to?0.5V versus NHE for https://www.sodocs.net/doc/0f4961054.html,ing the Levich Eq.(4),the limiting current value gives7.0±0.2electrons per tetrahydroborate ion,very close to the ideal value(8)and literature[8].This value is also con-?rmed by the chronopotentiogram and the chronoamperogram analysis(Fig.8),which give7.3and7.8electrons per tetrahy-droborate anion using the Sand equation(5)and that of Cottrell (6),respectively.Fig.9shows that no gold oxidation occurs below0.2V versus NHE[35],con?rming pristine gold is the very material that oxidizes tetrahydroborate anions,contrarily to the case of silver.Our experimental conditions(quasi steady-state,low potential sweep rate:5mV s?1,rotation-induced elec-trolyte convection at the RDE)enable the reactant(BH4?)to be evenly provided at the electrode surface,and also permits the evacuation of the soluble reaction products(e.g.BH3OH?[3,9,22])away from the electrode vicinity.Such experimen-
tal conditions dramatically differ from those in the
literature,
Fig.7.NaBH4oxidation voltammogram(5mV s?1)plotted on Au in1M NaOH+0.01M NaBH4,argon
atmosphere.Fig.8.NaBH4oxidation chronopotentiogram plotted between the open circuit condition(?0.9V vs.NHE for1min)and oxidation currents(a)and chronoam-perogram plotted between the open circuit condition(?0.9V vs.NHE for1min) and BH4?oxidation potentials(b)for Au in1M NaOH+0.01M NaBH4,argon atmosphere;gold geometric area=0.0314cm2.
which either deal with testing in fuel-cell or three-electrode cell without stirring,both conditions being compatible with reaction product stagnation close to the electrode surface.In addition,the low borohydride concentration(10?2M NaBH4in 1M NaOH)induces very little borohydride spontaneous hydrol-ysis.So our quasi-stationary voltammetry mostly provide insight to the direct oxidation of tetrahydroborate anion and not to the oxidation of its reaction products.Thus,the nearly eight elec-trons monitored at the limiting current plateau means that all reaction intermediates for BH4?oxidation consist of adsorbed species on the gold surface,which agrees with Okinaka con-clusions[22]for gold,and those from Elder and Hickling[9] for platinum.It also somewhat contradicts the observations of Mirkin et al.,for whom the reaction intermediate,borine species (BH3),do not adsorb at the gold substrate and can diffuse into the solution.
3.2.2.2.Carbon-supported gold nanoparticles(Au/C).As for Ag/C,Au/C quasi stationary-voltammograms(not shown) reveal the onset for the oxidation wave is lower than for the bulk metal:?0.57V versus NHE for Au/C versus?0.5V
versus Fig.9.benchmark voltammogram(5mV s?1)plotted on Au in1M NaOH at 41.9rad s?1—argon atmosphere.
M.Chatenet et al./Electrochimica Acta51(2006)5459–54675465 NHE for Au.The improved kinetics in term of exchange current
density and half wave potential(Table1)could also follow a par-
ticle size effect.From Levich Eq.(4),we calculate that around
eight electrons are exchanged per tetrahydroborate anion.Such
high value is not easily con?rmed by the transient methods:the
BH4?concentration pro?le in the active layer(3?m thick)is
not simple,which somewhat alters the validity of the Sand and
Cottrell equations.So with these data,any conclusion about
the mechanism at the gold nanoparticles is risky.However,we
see no relevant reasons to think that different reaction pathways
should be proposed for gold nanoparticles compared to bulk
gold.
3.2.3.In?uence of the electrolyte composition
The papers reporting DBFC characterization and testing deal
with BH4?concentrations in the range of several moles per
litre.Moreover,eight hydroxide anions being involved in the
reaction(3),the ratio[OH?]/[BH4?]should also play a role.
So,we decided to study these parameters,keeping in mind that
the highest energy density would be required for the fuel used
to feed the DBFC.
3.2.3.1.In?uence of the sodium hydroxide concentration.We
varied the sodium hydroxide concentration at rather low borohy-
dride concentration(10?2M)so as to change the[OH?]/[BH4?]
ratio from10to50.The quasi-stationary voltammograms of
Fig.10reveal the great in?uence of the above mentioned ratio,
although it is always maintained beyond the theoretical value
of8.A ratio of10is not enough to optimize the reaction rate
and make it not depend on the hydroxide concentration.Thus,
the tetrahydroborate oxidation reaction kinetics highly depends
on the hydroxide concentration,even when enough hydrox-
ide species are theoretically provided.This probably means
that the reaction rate determining step involves OH?species.
We point out that the conductivity of the solution is enough,
even in the worst case(0.1M NaOH+10?2M NaBH4)and
that it does not bother the recorded voltammograms.The need
for[OH?]/[BH4?]ratios higher than10would detrimentally
decrease the energy density of the fuel and hinder any DBFC
application.
The experiments for[OH?]/[BH4?]=1,10or100in molar
sodium hydroxide,show once again the reaction limitation
by the hydroxide species and how increasing the BH4?con-
centration improves the reaction kinetics(Fig.11a).
When
Fig.10.NaBH4oxidation voltammogram(5mV s?1)plotted on Au/C in x M NaOH+0.01M NaBH4,argon
atmosphere.Fig.11.NaBH4oxidation voltammogram plotted at5mV s?1(a)and zoom on the kinetic part(b)for bulk Au(bold)or Ag(thin)in1M NaOH containing various concentration of NaBH4,argon atmosphere;168rad s?1.
[OH?]/[BH4?]=1,the oxidation reaction starts at much lower
potential(Fig.11b)and the open-circuit potential falls below ?1.2V versus NHE for Ag,and?1V versus NHE for gold. Such OCP values cannot be linked to H2oxidation,since
E eq(H+/H2)=?0.83V versus NHE in1M NaOH.Moreover,
a?rst oxidation wave is detected at?1.2V versus NHE and ?0.95V versus NHE for Ag and Au respectively with1M BH4?in solution,about0.8or0.5V below the onset of the principal oxidation wave when0.1M BH4?is in solution.A well-de?ned plateau quickly follows this?rst oxidation wave, showing that the species are oxidized in a fast reaction with a diffusion–convection control at fairly low overpotential.The very low potentials at which the?rst wave is observed both for Ag and Au can only be related to the BH3OH?direct oxidation,which was reported to occur around0.5V lower than that of BH4?on gold[22].So,in equimolar hydrox-ide and tetrahydroborate solution,BH4?would spontaneously hydrolyse in solution into BH3OH?species[3]which are driven to the electrode surface by the electrode rotation.Then, they probably adsorb at the electrode surface,as reported by Elder and Hickling[9]and follow subsequent oxidation [22].Now,the?rst wave plateaus show little current com-pared to the principal one(Fig.11)which agrees with Okinaka observations:the[BH3OH?]/[BH4?]ratio is generally below 2%.
The tetrahydroborate oxidation mechanism at gold or silver
electrodes would then differ in concentrated and diluted boro-
hydride solution.
In the latter case(low NaBH4concentration),the BH4?
species are more stable,leading to negligible BH3OH?pro-
duction.The oxidation reaction thus proceeds without the?rst
chemical hydrolysis step(Eq.(1))with a slow kinetics[3,8,22].
So,we think that the limiting step of the tetrahydroborate anion
oxidation is the?rst step involving BH4?.According to Mirkin
et al.[8],the?rst step is a reversible two-electron transfer yield-
5466M.Chatenet et al./Electrochimica Acta51(2006)5459–5467
ing to BH3in solution(Eq.(8)):
BH4?+OH??BH3+H2O+2e?(8) However,we showed the reaction intermediates are all adsorbed at the electrode surface.So,the borane(BH3)intermediate prob-ably adsorbs on the electrode before combining with OH?to give BH3OH?,as reported by Elder and Hickling for platinum [9].Thus,when negligible amounts of BH3OH?are present in solution,the reaction is probably initiated by the?rst electron transfer on BH4?or possibly BH4,ad?and the mechanism could start following the adsorption step(9)and electrochemical step (10),one of these steps being the rate determining step:
BH4?+M→BH4,ad?(9) BH4,ad?+OH?→BH3,ad+H2O+2e?(10) The two steps(9)and(10)differ from those proposed earlier for platinum[9].After these steps,the reaction could proceed according several additional steps involving adsorbed interme-diates.The overall eight-electron reaction rate slowness could follow a self-poisoning of the electrode surface by one adsorbed intermediate,or simply the dif?culty to adsorb BH4?species at the electrode surface.
When the[OH?]/[BH4?]ratio is around unity,BH4?anions are spontaneously hydrolysed in solution into BH3OH?accord-ing to Eq.(1).Then,BH3OH?species probably adsorb at the electrode surface and follow subsequent oxidation at a high reac-tion rate:the initial steps of the BH3OH?oxidation are probably much faster than the?rst steps of BH4?direct oxidation,in agreement with Okinaka conclusions[22].
4.Conclusion
The direct oxidation of sodium borohydride was studied in concentrated sodium hydroxide medium by cyclic voltamme-try,chronoamperometry and chronopotentiometry for silver and gold electrocatalysts.
Around7.5electrons are exchanged per BH4?anion on the quasi-stationary limiting current plateaus,in agreement with the results from the chronometric methods.Such rather com-plete faradic utilisation of the reducer probably means that the reaction proceeds through several steps all involving adsorbed intermediates.
The kinetic parameters for direct borohydride oxidation are better for gold than for silver,especially in term of oxidation wave onset potential.In addition,there seem to be a particle size effect for carbon-supported metals:the onset for the oxida-tion wave is at least0.1V lower for the nanoparticles than for the bulk metals.The state of the electrode surface also plays a role in the tetrahydroborate oxidation reaction.For silver elec-trodes,surface oxides need to be present to enable the reaction. This effect is not observed for gold electrodes,for which the borohydride oxidation is initiated well below(0.6V at least)the surface oxide formation.
The[OH?]/[BH4?]ratio greatly in?uences the reaction kinetics:whereas only eight hydroxide ions are theoretically needed for each borohydride ion,our experimental results show that a larger excess hydroxide ion is necessary(more than100 times OH?compared to BH4?)in quasi-steady state condi-tions.As a consequence,in1M NaOH–1M NaBH4solution,the tetrahydroborate ions direct oxidation is limited by the hydrox-ide concentration.
When the solution contains non-negligible amounts of BH3OH?,which basically happens at low[OH?]/[BH4?]ratio, the reaction proceeds through a dual pathway.The little amount of aqueous BH3OH?resulting from the spontaneous hydroly-sis of BH4?is oxidized at very low potential(
In conclusion,carbon-supported gold nanoparticles seem very interesting materials to be used in direct borohydride fuel cell anodes,but silver nanoparticles might be useful as co-catalyst to ef?ciently oxidize the tetrahydroborate ions hydrol-ysis by-product.
Acknowledgements
The authors would like to express their gratefulness to Jean-Franc?ois Fauvarque and Jean-Paul Diard for their kind advices. References
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PCB介电常数知识
1、我们常用的PCB介质是FR4材料的，相对空气的介电常数是4.2-4.7。这个介电常数是会随温度变化的，在0-7 0度的温度范围内，其最大变化范围可以达到20%。介电常数的变化会导致线路延时10%的变化，温度越高，延时越大。介电常数还会随信号频率变化，频率越高介电常数越小。100M以下可以用4.5计算板间电容以及延时。 2、一般的FR4材料的PCB板中内层信号的传输速度为180ps/inch(1inch=1000mil=2.54cm)。表层一般要视情况而定，一般介于140与170之间。 3、实际的电容可以简单等效为L、R、C串联，电容有一个谐振点，在高频时(超过这个谐振点)会呈现感性，电容的容值和工艺不同则这个谐振点不同，而且不同厂家生产的也会有很大差异。这个谐振点主要取决于等效串联电感。现在的比如一个100nF的贴片电容等效串联电感大概在0.5nH左右，ESR(等效串联电阻)值为0.1欧，那么在24M 左右时滤波效果最好，对交流阻抗为0.1欧。而一个1nF的贴片电容等效电感也为0.5nH(不同容值差异不太大)，E SR为0.01欧，会在200M左右有最好的滤波效果。为达好较好的滤波效果，我们使用不同容值的电容搭配组合。但是，由于等效串联电感与电容的作用，会在24M与200M之间有一个谐振点，在这个谐振点上有最大阻抗，比单个电容的阻抗还要大。这是我们不希望得到的结果。（在24M到200M这一段，小电容呈容性，大电容已经呈感性。两个电容并联已经相当于LC并联。两个电容的ESR值之和为这个LC回路的串阻。LC并联的话如果串阻为0，那么在谐振点上会有一个无穷大的阻抗，在这个点上有最差的滤波效果。这个串阻反倒会抑制这种并联谐振现象，从而降低LC谐振器在谐振点的阻抗）。为减轻这个影响，可以酌情使用ESR大些的电容。ESR相当于谐振网络里的串阻，可以降低Q值，从而使频率特性平坦一些。增大ESR会使整体阻抗趋于一致。低于24M的频段和高于200M的频段上，阻抗会增加，而在24M与200M频段内，阻抗会降低。所以也要综合考虑板子开关噪声的频带。国外的一些设计有的板子在大小电容并联的时候在小电容(680pF)上串几欧的电阻，很可能是出于这种考虑。(从上面的参数看，1nF的电容Q值是100nF电容Q值的10倍。由于手头没有来自厂商的具体等效串感和ESR的值，所以上面例子的参数是根据以往看到的资料推测的。但是偏差应该不会太大。以往多处看到的资料都是1nF和100nF的瓷片电容的谐振频率分别为100M和10M，考虑贴片电容的L要小得多，而又没有找到可靠的值，为讲着方便就按0.5nH计算。如果大家有具体可靠的值的话，还希望能发上来^_^) 介电常数（Dk, ε，Er）决定了电信号在该介质中传播的速度。电信号传播的速度与介电常数平方根成反比。介电常数越低，信号传送速度越快。我们作个形象的比喻，就好想你在海滩上跑步，水深淹没了你的脚踝，水的粘度就是介电常数，水越粘，代表介电常数越高，你跑的也越慢。 介电常数并不是非常容易测量或定义，它不仅与介质的本身特性有关，还与测试方法，测试频率，测试前以及测试中的材料状态有关。介电常数也会随温度的变化而变化,有些特别的材料在开发中就考虑到温度的因素.湿度也是影响介电常数的一个重要因素,因为水的介电常数是70,很少的水分,会引起显著的变化. 以下是一些典型材料的介电常数(在1Mhz下)：
催化原理
一、催化剂的定义与催化作用的特征 1．定义：凡能加速化学反应趋向平衡，而在反应前后其化学组成和数量不发生变化的物质。2．特征:①加快反应速率;②反应前后催化剂不发生化学变化(催化剂的化学组成--不变化物理状态---变化（晶体、颗粒、孔道、分散))③不改变化学平衡④同时催化正、逆反应。⑤对化学反应有定向选择性。 二、催化剂的评价指标 工业催化剂的四个基本指标：选择性、稳定性、活性、成本。 对工业催化剂的性能要求：活性、选择性、生产能力、稳定性、寿命、机械强度、导热性能、形貌和粒度、再生性。 1.活性催化剂使原料转化的速率：a=-(1/w)d(nA)/dt 2.生产能力--时空收率：单位体积（或单位质量）催化剂在单位时间内所生产的目的产物量Y v,t=n p/v.t or Y W,t=n p/w.t 3.选择性：目的产物在总产物中的比例S=Δn A→P/Δn A=(p/a).(n P/Δn A) =r P/Σr i 4.稳定性：指催化剂的活性随时间变化 5.寿命：是指催化剂从运行至不适合继续使用所经历的时间 三、固体催化剂催化剂的组成部分 主催化剂---活性组份：起催化作用的根本性物质，即催化剂的活性组分，如合成氨催化剂中的Fe。其作用是：化学活性，参与中间反应。 共催化剂：和主催化剂同时起作用的组分，如脱氢催化剂Cr2O3-Al2O3中的Al2O3。甲醇氧化的Mo-Fe催化剂。 助催化剂：它本身对某一反应无活性，但加入催化剂后（一般小于催化剂总量10%）能使催化剂的活性或选择性或稳定性增加。加助催化剂的目的：助活性组份或助载体。 载体：提高活性组份分散度，对活性分支多作用，满足工业反应器操作要求，满足传热传质要求。 四、固体催化剂的层次结构 初级粒子：内部具有紧密结构的原始粒子； 次级粒子：初级粒子以较弱的附着力聚集而成-----造成固体催化剂的细孔； 催化剂颗粒：次级粒子聚集而成-----造成固体催化剂的粗孔； 多孔催化剂的效率因子：η=K多孔/K消除内扩散=内表面利用率<1 五、催化剂的孔内扩散模型 物理吸附：分子靠范德华力吸附，类似于凝聚，分子结构变化不大，不发生电子转移与化学键破坏。 努森扩散（微孔扩散）：当气体浓度很低或催化剂孔径很小时，分子与孔壁的碰撞远比分子间的碰撞频繁，扩散阻力主要来自分子与孔壁的碰撞。散系数D K=9700R(T/M)0.5 式中：R是孔半径，cm; T是温度，K；M是吸附质相对分子量。 体相扩散（容积扩散）：固体孔径足够大，扩散阻力与孔道无关，扩散阻力是由于分子间的碰撞，又称分子扩散。体相扩散系数D K=νγθ/（3τ）式中ν、γ 分别是气体分子的平均速率和平均自由程；θ 固体孔隙率；τ 孔道弯曲因子，一般在2~7。 过渡区扩散：介于Knudsen扩散与体相扩散间的过渡区。分子间的碰撞及分之与孔道的碰撞都不可忽略 构型扩散：催化剂孔径尺寸与反应物分子大小接近，处于同一数量级时，分子大小发生微小变化就会引起扩散系数发生很大变化。例如：分子筛择形催化 六、催化过程的分类 均相催化：反应物和催化剂处于同一相
均相催化臭氧氧化设备处理染料废水技术
均相催化臭氧氧化设备处理染料废水技术 催化臭氧氧化设备是使催化剂和反应物作用, 形成不稳定的中间产物, 改变反应途径, 或加快氧化剂的分解并使之与水中有机物迅速反应, 在较短的时间内降解染料分子并提高氧化剂的利用效率的方法。而光电催化氧化技术根据催化剂的形态不同又分为均相催化臭氧化和非均相催化臭氧化。 催化臭氧氧化设备 1、均相催化臭氧氧化设备处理染料废水技术 前人多选用均相催化剂处理染料废水,虽然均相催化臭氧氧化可以达到令人满意的处=理效果, 但因为催化剂是以离子的形态分布在水中,无法与反应体系分离, 处理完毕后催化剂便同染料废水一起排放, 不仅造成催化剂的流失浪费, 同时也造成了水体的金属离子的二次污染。为了解决这一问题, 研究人员把具有催化作用的活性组分通过某些方法固定到一些载体上, 把负载了活性组分的固体催化剂投入到废水中在臭氧存在的条件下与废水反应, 进行非均相催化臭氧氧化反应。 2、非均相催化臭氧氧化设备处理染料废水技术 在非均相催化中, 催化剂是以固态存在, 主要有贵金属系、铜系和稀土系三大类。而贵金属因为价格昂贵其应用受到限制, 目前研究最多的是廉价金属及金属氧化物。非均相催化剂根据其制备工艺分为非负载型和负载型, 目前研究的重点在负载型非均相催化剂。负载型非均相催化剂由载体、活性组分和助剂三部分组成。常用的载体有Al2O3、沸石、活性炭纤维、分子筛等, 活性组分多为过渡金属。
为了进一步提高催化臭氧氧化的效果, 往往需要在单组分催化剂的基础上进行多元组分催化剂的研究, 根据催化剂的制备条件、各种活性组分的配比和助剂的选择来制备催化效率更高的催化剂。
甲醇制甲醛过程中催化剂失活的原因
甲醇制甲醛过程中催化剂失活的原因 以甲醇为原料，结晶银作催化剂制取甲醛，催化剂寿命短的原因很多，有外在因素，也有内在因素，根据生产经验，总结出主要的原因有以下几点： 1、反应温度高 结晶银催化制取甲醛，反应温度较高(一般控制在 630-650 ℃)，催化剂长期处于高温状态，导致催化剂的晶相、晶粒分解度逐渐发生变化，破坏了原有的组织和结构，这是结晶银催化剂寿命短的主要原因。有时反应器温度波动过大或出现超温运行，催化剂的物理结构便会逐渐发生变化，其孔隙率相应减少，温度再升高，就会出现催化剂选择性下降，副产物增多的问题，直接影响了催化剂的活性。 2 、有害杂质影响 结晶银催化剂由于受到原料气夹带的外来物质污染和反应 物结焦，其活性表面容易被覆盖，催化剂孔隙被堵塞。使催化剂粘聚在一起，造成床内局部阻力上升，反应气走短路，直接导致催化剂利用率降低，寿命缩短。比如原料气中含有挥发性硫、氯化物，会与结晶银生成硫化银和氯化银而使催化剂中毒，如含有醛、酮等有机物，则会因其树脂化作用而堵塞银粒表面的孔隙，导致催化剂活性的降低；如含有挥发性铁化合物，会在催化剂上分解成氧化铁，覆盖在表面而破坏其活性，而且催化剂表面覆盖
了氧化铁细粒，将会加快甲醇的完全燃烧反应，使尾气中CO2含量增加，同时放出大量热，使反应温度迅速升高甚至失控，从而影响触媒的选择性，导致副反应增多。因此反应原料气中硫、氯化物、醛、酮、铁杂质等有害杂质的存在可导致催化剂中毒。此外，如果电解银催化剂本身带有氯化物、铁等杂质，在反应条件下有可能与有效成分银作用，使催化剂的催化效能受到破坏，从而发生催化剂中毒现象。 3 、生产过程不稳定 甲醛生产中，由于各种因素的影响，生产的稳定性有可能会受到破坏。比如，工作不正常引起的临时停车；生产过程操作不得当，使蒸发温度或氧化反应温度产生较大的波动；蒸发器液位控制不好(过高或过低)等等都会对催化剂活性造成一定的影响，从而缩短其使用寿命。 4 、催化剂床层破坏 甲醛生产中，如果催化床层厚薄松紧不均，催化剂与氧化器器壁有缝隙存在或出现床层裂缝、塌陷都会加剧甲醛的深度氧化，从而影响催化剂的活性。 5、旧催化剂所含杂质 由催化剂失活的原因可以总结出旧催化剂所含的主要杂质 成分，如下： 1）催化剂床层底部为铜网，旧催化剂取出时会带出大量铜杂质。
催化臭氧技术
一、水处理催化臭氧技术 催化臭氧技术是基于臭氧的高级氧化技术，它将臭氧的强氧化性和催化剂的吸附、催化特性结合起来，能较为有效地解决有机物降解不完全的问题。催化臭氧化按催化剂的相态分为均相催化臭氧化和多相催化臭氧化，在均相催化臭氧化技术中，催化剂分布均匀且催化活性高，作用机理清楚，易于研究和把握。但是，它的缺点也很明显，催化剂混溶于水，导致其易流失、不易回收并产生二次污染，运行费用较高，增加了水处理成本。多相催化臭氧化法利用固体催化剂在常压下加速液相（或气相）的氧化反应，催化剂以固态存在，易于与水分离，二次污染少，简化了处理流程，因而越来越引起人们的广泛重视。 1催化臭氧化 对于催化臭氧化技术，固体催化剂的选择是该技术是否具有高效氧化效能的关键。研究发现，多相催化剂主要有三种作用。 一是吸附有机物，对那些吸附容量比较大的催化剂，当水与催化剂接触时，水中的有机物首先被吸附在这些催化剂表面，形成有亲和性的表面螯合物，使臭氧氧化更高效。 二是催化活化臭氧分子，这类催化剂具有高效催化活性，能有效催化活化臭氧分子，臭氧分子在这类催化剂的作用下易于分解产生如羟基自由基之类有高氧化性的自由基，从而提高臭氧的氧化效率。 三是吸附和活化协同作用，这类催化剂既能高效吸附水中有机污染物，同时又能催化活化臭氧分子，产生高氧化性的自由基，在这类催化剂表面，有机污染物的吸附和氧化剂的活化协同作用，可以取得更好的催化臭氧氧化效果[3]。在多 相催化臭氧化技术中涉及的催化剂主要是金属氧化物（Al 2O 3 、TiO 2 、MnO 2 等）、 负载于载体上的金属或金属氧化物(Cu/TiO 2 、Cu/Al 2 O 3 、TiO 2 /Al 2 O 3 等)以及具有 较大比表面积的孔材料。这些催化剂的催化活性主要表现对臭氧的催化分解和促进羟基自由基的产生。臭氧催化氧化过程的效率主要取决于催化剂及其表面性质、溶液的pH值，这些因素能影响催化剂表面活性位的性质和溶液中臭氧分解反应[4]。 1.1 （负载）金属催化剂 通过一定方式制备的金属催化剂能够促使水中臭氧分解, 产生具有极强氧
(推荐)臭氧催化氧化计算书
一、进水条件 当用于处理废水时，除要求布水布气均匀外，还要注意调查分析进水来源状况，特别注意是否含有对催化剂产生危害的物质。以下为部分重要的原水进水条件。 1.1pH 催化剂适宜的酸碱运行条件为pH=3～12，最佳的酸碱运行条件为pH=6-9，pH过低会影响催化剂寿命，并导致出水质量下降，pH过高会影响臭氧催化氧化的使用效果。 1.2温度 进水温度过高或者过低会影响臭氧的使用效果，也会对催化剂的催化效果产生影响，建议温度范围为10-30℃，最佳运行温度为25℃。 1.3氯化物 氯化物过高会对催化剂的使用效果产生影响，建议氯化物的浓度在5000mg/L以下，氯化物最佳浓度为500mg/L以下。 1.4臭氧投加方式 臭氧分子在水中的扩散速度与污染物的反应速度是影响去除效果的主要因素。 二、相关简图 1.1催化氧化填料 催化剂主要特点如下： (1) 选用碘值高、吸附能力强、耐磨强度好、质量稳定可靠的优质活性炭为载体，制备的催化剂具有很大的比表面积和合适的孔结构； (2) 在活性炭载体表面选择性的负载Fe、Mn等过渡金属活性组分及K、Na 等碱金属催化助剂，原位促进臭氧分解成羟基自由基并降解有机物； (3) 催化剂的制备采用机械混合、成型、炭化和活化的生产工艺，活性组分
在载体表面分散性
良好。 催化剂填料图片如下： 臭氧催化氧化填料 规格参数如下： 项目指标单位规格 外观指 标 吸水率%45% -55%粒径mm条形3-6堆积密度t/m30.45 -0.62耐磨强度%≥92% 压碎强度N/cm≧110碘值mg/g≧550 活性金属含量%3% -4% 性能指 标 COD去除率%40%-75% Rt（水力停留时间）min30-60寿命年3~5
材料的介电常数和磁导率的测量
无机材料的介电常数及磁导率的测定 一、实验目的 1. 掌握无机材料介电常数及磁导率的测试原理及测试方法。 2. 学会使用Agilent4991A 射频阻抗分析仪的各种功能及操作方法。 3. 分析影响介电常数和磁导率的的因素。 二、实验原理 1.介电性能 介电材料（又称电介质）是一类具有电极化能力的功能材料，它是以正负电荷重心不重合的电极化方式来传递和储存电的作用。极化指在外加电场作用下，构成电介质材料的内部微观粒子，如原子，离子和分子这些微观粒子的正负电荷中心发生分离，并沿着外部电场的方向在一定的范围内做短距离移动，从而形成偶极子的过程。极化现象和频率密切相关，在特定的的频率范围主要有四种极化机制：电子极化 (electronic polarization ，1015Hz)，离子极化 (ionic polarization ，1012～1013Hz)，转向极化 (orientation polarization ，1011～1012Hz)和空间电荷极化 (space charge polarization ，103Hz)。这些极化的基本形式又分为位移极化和松弛极化，位移极化是弹性的，不需要消耗时间，也无能量消耗，如电子位移极化和离子位移极化。而松弛极化与质点的热运动密切相关，极化的建立需要消耗一定的时间，也通常伴随有能量的消耗，如电子松弛极化和离子松弛极化。 相对介电常数（ε），简称为介电常数，是表征电介质材料介电性能的最重要的基本参数，它反映了电介质材料在电场作用下的极化程度。ε的数值等于以该材料为介质所作的电容器的电容量与以真空为介质所作的同样形状的电容器的电容量之比值。表达式如下： A Cd C C ?==001εε （1） 式中C 为含有电介质材料的电容器的电容量；C 0为相同情况下真空电容器的电容量；A 为电极极板面积；d 为电极间距离；ε0为真空介电常数，等于8.85×10-12 F/m 。 另外一个表征材料的介电性能的重要参数是介电损耗，一般用损耗角的正切（tanδ）表示。它是指材料在电场作用下，由于介质电导和介质极化的滞后效应
合金催化剂及其催化作用和机理
合金催化剂及其催化作用 金属的特性会因为加入别的金属形成合金而改变，它们对化学吸附的强度、催化活性和选择性等效应，都会改变。 （1）合金催化剂的重要性及其类型 炼油工业中Pt-Re及Pt-Ir重整催化剂的应用，开创了无铅汽油的主要来源。汽车废气催化燃烧所用的Pt-Rh及Pt-Pd催化剂，为防止空气污染作出了重要贡献。这两类催化剂的应用，对改善人类生活环境起着极为重要的作用。 双金属系中作为合金催化剂主要有三大类。第一类为第VIII族和IB族元素所组成的双金属系，如Ni-Cu、Pd-Au等；第二类为两种第IB族元素所组成的，如Au-Ag、Cu-Au等；第三类为两种第VIII族元素所组成的，如Pt-Ir、Pt-Fe等。第一类催化剂用于烃的氢解、加氢和脱氢等反应；第二类曾用来改善部分氧化反应的选择性；第三类曾用于增加催化剂的活性和稳定性。 （2）合金催化剂的特征及其理论解释 由于较单金属催化剂性质复杂得多，对合金催化剂的催化特征了解甚少。这主要来自组合成分间的协同效应（Synergetic effect），不能用加和的原则由单组分推测合金催化剂的催化性能。例如Ni-Cu催化剂可用于乙烷的氢解，也可用于环己烷脱氢。只要加入5%的Cu，该催化剂对乙烷的氢解活性，较纯Ni的约小1000倍。继续加入Cu，活性继续下降，但速率较缓慢。这现象说明了Ni与Cu之间发生了合金化相互作用，如若不然，两种金属的微晶粒独立存在而彼此不影响，则加入少量Cu后，催化剂的活性与Ni的单独活性相近。 由此可以看出，金属催化剂对反应的选择性，可通过合金化加以调变。以环己烷转化为例，用Ni催化剂可使之脱氢生成苯（目的产物）；也可以经由副反应生成甲烷等低碳烃。当加入Cu后，氢解活性大幅度下降，而脱氢影响甚少，因此造成良好的脱氢选择性。 合金化不仅能改善催化剂的选择性，也能促进稳定性。例如，轻油重整的Pt-Ir催化剂，较之Pt催化剂稳定性大为提高。其主要原因是Pt-Ir形成合金，避免或减少了表面烧结。Ir有很强的氢解活性，抑制了表面积炭的生成，维持和促进了活性。
甲醛合成催化剂的研究进展
甲醛合成催化剂的研究进展 摘要：对甲烷直接氧化法、甲醇空气氧化法等甲醛制备方法的特点进行了描述。介绍了各甲醛制备方法所用催化剂的研究进展。甲烷直接氧化法制甲醛需三个步骤：(1)甲烷进行水蒸气重整制合成气；(2)由合成气制甲醇；(3)甲醇进一步氧化为甲醛。该法的催化剂体系有Mo基催化剂、V基催化剂、含Fe的催化剂、其他催化剂。甲醇空气氧化法制备甲醛所用催化剂类型有Ag催化剂法和Fe-Mo催化剂法两类。二甲醚选择氧化制甲醛的催化剂中，负载型MoOx 和VOx的催化性能较好。开发二甲醚氧化法制甲醛的高效催化剂具有较好的潜在市场。 关键词：甲醛；催化剂；甲烷；甲醇；二甲醚 引言 甲醛是一种重要的有机化工原料，是甲醇最重要的衍生物之一，广泛用于生产脲醛、酚醛、三聚氰胺等树脂，也用于生产乌洛托品、季戊四醇和染料等，在农业上又可作为农药和消毒剂等使用。随着汽车工业、建筑业和装饰业的迅猛发展，甲醛作为传统的大宗化工原料已成为高增长消费产品，开发高技术下游产品前景十分广阔。本文对甲烷直接氧化法、甲醇空气氧化法及二甲醚氧化法制甲醛等方法进行了简要介绍，综述了各制备法所用催化剂的研究进展。 1 甲烷直接氧化法 目前工业上由甲烷合成甲醛共需3个步骤：(1)甲烷进行水蒸气重整制合成气； (2)由合成气制甲醇；(3)甲醇进一步氧化为甲醛。该三步法生产甲醛的过程需要高压设备，工艺流程复杂，技术要求高，能量消耗大且利用率低，成本相对高。与之相比，甲烷直接氧化法可简化流程设备，合理利用能量，是一条天然气开发利用的理想途径。甲烷直接氧化制甲醛的反应中甲醛只是甲烷氧化过程中的中间产物，极不稳定，易深度氧化为CO和CO2，致使申醛的收率很低，只有2%左右。近年来已研制出多种用于甲烷直接氧化制甲醛反应的催化剂，其中负载Mo或V的催化剂性能较好。 1.1 Mo基催化剂 王承学等以MoO3和V2O5为活性组分，考察了以SiO2，SnO2，Al2O3为载体时的催化性能，其中SiO2为载体最佳；也研究了一系列助剂对Mo/SiO2催化剂体系性能的影响，发现加入Fe2O3，V2O5，CuO，B2O3，La2O3，CeO2，MgO，SnO2都有助于提高甲烷的转化率，而加入P2O5，La2O3，Cr2O3，CeO2有利于提高甲醛的选择性。张昕等用ZrO2作载体，制备了一系列甲烷选择氧化制甲醛的Mo/ZrO2催化剂。Mo质量分数为12%时，在n(CH4)：n(O2)：n(N2)=10：1：3、气态空速12L/(g·h)、5.0MPa,400℃的条件下，甲烷转化率为8.3%，甲醛选择性为47.8%，甲醛收率为4.0%，时空收率为396 g/(kg·h)。研究表明，Mo/ZrO2催化剂中主要含有ZrO2和Zr(MoO4)2，催化剂的性能与zr(MoO4)2的性质密切相关。 1.2 V基催化剂 文献报道，V2O5/SiO2催化剂的性能与MoO3/SiO2催化剂相近。Nguyen等用一种新的制备方法将V的氧化物负载在SiO2上，V质量分数为2.2%时，在反应温度581℃、常压的反应条件下，甲烷的转化率为3.5%，甲醛的选择性为74.2%，甲醛的时空收率为2.435 kg/(kg·h)。催化剂的高活性可归结为V活性物种在载体上的高度分散，这些单核V活性物种包括1个V=O 键和3个桥式V-O-Si键，或1个-OH和2个桥式V-O-Si键，-OH和V-O-Si键有利于活性位的再生或甲烷的活化。文献报道了介孔分子筛(如MCM-41，MCM-48，SBA-15等)作为甲烷选择氧化反应的催化剂。Bemdt等用MCM-41和MCM-48分子筛负载VOx，用浸渍法制备了V质量分数为2.8%的V/MCM-41催化剂。采用该催化剂，甲醛的时空收率为2.255 kg/(kg·h)，但
常见物质介电常数汇总
Sir-20说明书普通材料的介电值和术语集 1
常见物质的相对介电常数值和电磁波传播速度(RIS-K2说明书)
------------------《探地雷达方法与应用》（李大心）
2007第二期勘察科学与技术
电磁波在部分常见介质中的传播参数 （The propagation parameters of the electromagnetic wave in the medium） 地球表面大部分无水的物质(如干燥的土壤和岩石等)的介电常数，实部一般介于1.7-6之间，水的介电常数一般为81，虚部很小，一般可以忽略不计。岩石和土壤的介电常数与其含水量几乎呈线形关系增长，且与水的介电常数特性相同。所以天然材料的电学特性的变化，一般都是由于含水量的变化所致。对于岩石和土壤含水量和介电常数的关系国内外进行了详细研究(P.Hoekstra, 1974; J.E.Hipp,1 974;J .L.Davis,1 976;G A.Poe,1 971;J .R.Wang,1 977;E .G.巧okue tal ,1 977)。在实验室内大量测量了不同粒度的土壤一水混合物介电常数，考虑到束缚水和游离水，提出了经验土壤介电常数混合模型(J.R.Wang, 1985)。实验室内用开路探头技术和自由空间天线技术测量干燥岩石的介电常数(F.TUlaby, 1990)。国内肖金凯等人(1984, 1988)测量了大量的岩石和土壤的介电常数，王湘云、郭华东(1999)研究了三大岩类中所含的矿物对其介电常数的影响。研究表明，土壤中
含水量的变化影响介电常数的实部，水溶液中含盐量的变化影响土壤的导电性，即介电常数的虚部。水与某些铁锰化合物具有高的介电常数，绝大多数矿物的介电常数较低，约为4--12个相对单位，由于主要造岩矿物与水的相对介电常数存在较大差异，所以，具有较大孔隙度岩石的介电常数主要取决于它的含水量，泥岩由于含有大量的弱束缚水，所以其相对介电常数可高达50--60,岩石含泥质较多时，它们的介电常数与泥质含量有明显的关系，很多火成岩的孔隙度只有千分之几，其相对介电常数主要取决于造岩矿物，一般变化范围为6--12，水的介电常数与其矿化度的关系较弱，与此相应，岩石孔隙中所含水的矿化度同样对其介电常数不应有大的影响，水的矿化度的增大只导致岩石介电常数的少许增加。 表1 常见介质的电性参数值 媒质电导率 / （S/m） 介电常 数（相对 值） 电磁波速度/ （m/ns） 空气0 1 0.3 水10-4～3х10-281 0.033 花岗岩（干）10-8 5 0.15 灰岩（干）10-97 0.11 灰岩（湿） 2.5х10-28～10 0.11～0.095 粘土（湿）10-1～1 8～12 0.11～0.087 混凝土10-9～10-86～15 0.12～0.077 钢筋∞∞
催化剂及其作用机理
1基本概念 金属氧化物催化剂常为复合氧化物（Complex oxides），即多组分氧化物。如VO5-MoO3，Bi2O3-MoO3，TiO2-V2O5-P2O5，V2O5-MoO3-Al2O3，MoO3-Bi2O3-Fe2O3-CoO-K2O-P2O5-SiO2（即7组分的代号为C14的第三代生产丙烯腈催化剂）。组分中至少有一种是过渡金属氧化物。组分与组分之间可能相互作用，作用的情况常因条件而异。复合氧化物系常是多相共存，如Bi2O3-MoO3，就有α、β和γ相。有所谓活性相概念。它们的结构十分复杂，有固溶体，有杂多酸，有混晶等。 就催化剂作用和功能来说，有的组分是主催化剂，有的为助催化剂或者载体。主催化剂单独存在时就有活性，如MoO3-Bi2O3中的MoO3；助催化剂单独存在时无活性或很少活性，但能使主催化剂活性增强，如Bi2O3就是。助催化剂可以调变生成新相，或调控电子迁移速率，或促进活性相的形成等。依其对催化剂性能改善的不同，有结构助剂，抗烧结助剂，有增强机械强度和促进分散等不同的助催功能。调变的目的总是放在对活性、选择性或稳定性的促进上。 金属氧化物主要催化烃类的选择性氧化。其特点是：反应系高放热的，有效的传热、传质十分重要，要考虑催化剂的飞温；有反应爆炸区存在，故在条件上有所谓“燃料过剩型”或“空气过剩型”两种；这类反应的产物，相对于原料或中间物要稳定，故有所谓“急冷措施”，以防止进一步反应或分解；为了保持高选择性，常在低转化率下操作，用第二反应器或原料循环等。 这类作为氧化用的氧化物催化剂，可分为三类：①过渡金属氧化物，易从其晶格中传递出氧给反应物分子，组成含2种以上且价态可变的阳离子，属非计量化合物，晶格中阳离子常能交叉互溶，形成相当复杂的结构。②金属氧化物，用于氧化的活性组分为化学吸附型氧物种，吸附态可以是分子态、原子态乃至间隙氧（Interstitial Oxygen）。③原态不是氧化物，而是金属，但其表面吸附氧形成氧化层，如Ag对乙烯的氧化，对甲醇的氧化，Pt对氨的氧化等即是。 金属硫化物催化剂也有单组分和复合体系。主要用于重油的加氢精制，加氢脱硫（HDS）、加氢脱氮（HDN）、加氢脱金属（HDM）等过程。金属氧化物和金属硫化物都是半导体型催化剂。因此由必要了解有关半导体的一些基本概念和术语。 2半导体的能带结构及其催化活性 催化中重要的半导体是过渡金属氧化物或硫化物。半导体分为三类：本征半导体、n-型半导体和p型半导体。具有电子和空穴两种载流子传导的半导体，叫本征半导体。这类半导体在催化并不重要，因为化学变化过程的温度，一般在300~700℃，不足以产生这种电子跃迁。靠与金属原子结合的电子导电，叫n-型（Negative Type）半导体。靠晶格中正离子空穴传递而导电，叫p-型（Positive Type）半导体。 属n-型半导体的有ZnO、Fe2O3、TiO2、CdO、V2O5、CrO3、CuO等，在空气中受热时失去氧，阳离子氧化数降低，直至变成原子态。属于p-型半导体的有NiO、CoO、Cu2O、PbO、Cr2O3等，在空气中
水处理催化臭氧技术 常用的3种催化剂总结
水处理催化臭氧技术常用的3种催化 剂总结 臭氧催化氧化技术是基于臭氧的高级氧化技术，它将臭氧的强氧化性和催化剂的吸附、催化特性结合起来，能较为有效地解决有机物降解不完全的问题。 臭氧催化氧化技术按催化剂的相态分为均相臭氧催化氧化技术和多相臭氧催化氧化技术，在均相臭氧催化氧化技术技术中，催化剂分布均匀且催化活性高，作用机理清楚，易于研究和把握。但是它的缺点也很明显，催化剂混溶于水，导致其易流失、不易回收并产生二次污染，运行费用较高，增加了水处理成本。多相臭氧催化氧化技术法利用固体催化剂在常压下加速液相（或气相）的氧化反应，催化剂以固态存在，易于与水分离，二次污染少，简化了处理流程，因而越来越引起人们的广泛重视。 对于臭氧催化氧化技术技术，固体催化剂的选择是该技术是否具有高效氧化效能的关键。研究发现，多相催化剂主要有三种作用： 一是吸附有机物，对那些吸附容量比较大的催化剂，当水与催化剂接触时，水中的有机物首先被吸附在这些催化剂表面，形成有亲和性的表面螯合物，使臭氧氧化更高效。 二是催化活化臭氧分子，这类催化剂具有高效催化活性，能有效催化活化臭氧分子，臭氧分子在这类催化剂的作用下易于分解产生如羟基自由基之类有高氧化性的自由基，从而提高臭氧的氧化效率。 三是吸附和活化协同作用，这类催化剂既能高效吸附水中有机污染物，同时又能催化活化臭氧分子，产生高氧化性的自由基，在这类催化剂表面，有机污染物的吸附和氧化剂的活化协同作用，可以取得更好的催化臭氧氧化效果的。 在多相臭氧催化氧化技术技术中涉及的催化剂主要是金属氧化物（Al2O3、TiO2、MnO2等）、负载于载体上的金属或金属氧化物（CuTiO2、CuAl2O3、TiO2AlO3等）以及具有较大比表面积的孔材料。这些催化剂的催化活性主要表现对臭氧的催化分解和促进羟基自由基的产生。臭氧催化氧化过程的效率主要取决于催化剂及其表面性质、溶液的pH值，这些因素能影响催化剂表面活性位的性质和溶液中臭氧分解反应。
臭氧催化氧化计算书电子教案
臭氧催化氧化计算书
一、进水条件 当用于处理废水时，除要求布水布气均匀外，还要注意调查分析进水来源状况，特别注意是否含有对催化剂产生危害的物质。以下为部分重要的原水进水条件。 1.1pH 催化剂适宜的酸碱运行条件为pH=3～12，最佳的酸碱运行条件为pH=6-9，pH过低会影响催化剂寿命，并导致出水质量下降，pH过高会影响臭氧催化氧化的使用效果。 1.2温度 进水温度过高或者过低会影响臭氧的使用效果，也会对催化剂的催化效果产生影响，建议温度范围为10-30℃，最佳运行温度为25℃。 1.3氯化物 氯化物过高会对催化剂的使用效果产生影响，建议氯化物的浓度在5000mg/L以下，氯化物最佳浓度为500mg/L以下。 1.4臭氧投加方式 臭氧分子在水中的扩散速度与污染物的反应速度是影响去除效果的主要因素。 二、相关简图 1.1催化氧化填料 催化剂主要特点如下： (1) 选用碘值高、吸附能力强、耐磨强度好、质量稳定可靠的优质活性炭为载体，制备的催化剂具有很大的比表面积和合适的孔结构； (2) 在活性炭载体表面选择性的负载Fe、Mn等过渡金属活性组分及K、Na 等碱金属催化助剂，原位促进臭氧分解成羟基自由基并降解有机物；
(3) 催化剂的制备采用机械混合、成型、炭化和活化的生产工艺，活性组分在载体表面分散性良好。 催化剂填料图片如下： 臭氧催化氧化填料 规格参数如下：
1.2进水方式 臭氧催化高级氧化进水工艺流程 上游出水进入臭氧催化高级氧化池，首先进入臭氧催化高级氧化池第一段，从原水取一定比例的水进行循环，在离心泵管道上设置射流溶气装置，通过溶气装置投加臭氧，达到提高臭氧气体的溶解效率，并有效减少臭氧投加量。溶解臭氧的污水，通过池底设置的二次混合设备，将含臭氧污水与原污水充分混合。含臭氧的污水，混合后的污水流经固定填充的固相催化剂表面，催化剂表面具有不平衡电位差，在催化剂的作用下，激发产生羟基自由基，羟基自有基的氧化还原电位为E0=2.8ev，在如此高的氧化电位的作用下大部分难降解的有机物发生断链反应形成短链的有机物或直接被氧化至CO2和H2O。第二段、第三段取水位置分别是第一段出水和第二段出水，同样采用高效臭氧溶气装置投加臭氧，原理与第一段相同。通过三段投加，污水中难降解有机物被充分降解，使污水达到设计标准。接触池内未溶解的臭氧需重新还原变为氧气，避免对大气环境造成污染。在臭氧接触池池顶上设置有臭氧尾气分解处理设施，设计采用热触媒式臭氧尾气处理装置进行处理，将空气中残留臭氧还原为氧气，使尾气处理装置出口处臭氧浓度低于0.1ppm。 相关工程案例平面简图如下：
络合催化剂及其催化作用机理
络合催化剂及其催化作用机理 1 基本知识 络合催化剂，是指催化剂在反应过程中对反应物起络合作用，并且使之在配位空间进行催化的过程。催化剂可以是溶解状态，也可以是固态；可以是普通化合物，也可以是络合物，包括均相络合催化和非均相络合催化。 络合催化的一个重要特征，是在反应过程中催化剂活性中心与反应体系，始终保持着化学结合（配位络合）。能够通过在配位空间内的空间效应和电子因素以及其他因素对其过程、速率和产物分布等，起选择性调变作用。故络合催化又称为配位催化。 络合催化已广泛地用于工业生产。有名的实例有： ①Wacker工艺过程： C2H4 + O2 CH3?CHO C2H4 + O2 + CH3?COOH CH3?COO C2H4 + H2O R?CH? (CHO) ?CH3R?CH2?CH2?CHO②OXO工艺过程： R?CH=CH2 + CO/H2 催化剂：HCo(CO)4，150℃，250×105Pa；RhCl(CO)(PPh3)2，100℃，15×105Pa ③Monsanto甲醇羰化工艺过程： CH3OH + CO CH3?COOH 催化剂：RhCl(CO)(PPh3)2/CH3I 从以上的几例可以清楚地看到，络合催化反应条件较温和，反应温度一般在100~200℃左右，反应压力为常压到20×105Pa上下。反应分子体系都涉及一些小分子的活化，如CO、H2、O2、C2H4、C3H6等，便于研究反应机理。主要的缺点是均相催化剂回收不易，因此均相催化剂的固相化，是催化科学领域较重要的课题之一。 2 过渡金属离子的化学键合 （1）络合催化中重要的过渡金属离子与络合物 过渡金属元素（.）的价电子层有5个(n - 1)d，1个ns和3个np，共有9个能量相近的原子轨道，容易组成d、s、p杂化轨道。这些杂化轨道可以与配体以配键的方式结合而形成络合物。凡是含有两个或两个以上的孤对电子或π键的分子或离子都可以作配体。过渡金属有很强的络合能力，能生成多种类型的络合物，其催化活性都与过渡金属原子或离子的化学特性有关，也就是和过渡金属原子（或离子）的电子结构、成键结构有关。同一类催化剂，有时既可在溶液中起均相催化作用，也可以使之成为固体催化剂在多相催化中起作用。 空的(n - 1)d轨道，可以与配体L（CO、C2H4…等）形成配键（M←:L），可以与H、R-Φ-基形成M-H、M-C型σ键，具有这种键的中间物的生成与分解对络合催化十分重要。由于(n - 1)d轨道或nd外轨道参与成键，故.可以有不同的配位数和价态，且容易改变，这对络合催化的循环十分重要。 大体趋势是：①可溶性的Rh、Ir、Ru、Co的络合物对单烯烃的加氢特别重要；②可溶性的Rh、Co的络合物对低分子烯烃的羰基合成最重要；③Ni络合物对于共轭烯烃的齐聚较重要；④Ti、V、Cr络合物催化剂适合于α-烯烃的齐聚和聚合；⑤第VIII族.元素的络合催化剂适合于烯烃的齐聚。这些可作为研究开发工作的参考。 （2）配位键合与络合活化 各种不同的配体与.相互作用时，根据各自的电子结构特征形成不同的配位键合，配位体本身得到活化，具有孤对电子的中性分子与金属相互作用时，利用自身的孤对电子与金属形成给予型配位键，记之为L→M，如:NH3、H2就是。给予电子对的L:称为L碱，接受电子对的M称为L酸。M要求具有空的d或p空轨道。H?，R?等自由基配体，与.相互作用，形成电子配对型σ键，记以L-M。金属利用半填充的d、p轨道电子，
催化剂及其基本特征
催化剂及其基本特征 Company Document number：WTUT-WT88Y-W8BBGB-BWYTT-19998
1、催化剂及其基本特征 催化剂是一种物质，它能够改变化学反应的速率，而不改变该反应的标准Gibbs自由焓变化；此过程称为催化作用，涉及催化剂的反应称为催化反应。 催化剂的基本特征 催化剂只能实现热力学可行的反应，不能实现热力学不可能的反应； 催化剂只能改变化学反应的速度，不能改变化学平衡的位置； 催化剂能降低反应的活化能，改变反应的历程； 催化剂对反应具有选择性。 2、催化剂的组成 主催化剂：催化剂的主要活性组分，起催化作用的根本性物质，如合成氨催化剂的铁，催化剂中若没有活性组分存在，那么就不可能有催化作用。 助催化剂：催化剂中具有提高活性组分的催化活性和选择性的组分，以及改善催化剂的耐热性、抗毒性，提高催化剂机械强度和寿命的组分。 催化剂载体：主要是负载催化活性组分的作用，还具有提高催化剂比表面积、提供适宜的孔结构、改善活性组分的分散性、提高催化剂机械强度、提高催化剂稳定性等多种作用 3、催化剂的稳定性 指催化剂的活性和选择性随反应时间的变化，催化剂的性能稳定性情况，通常以寿命表示。催化剂在反应条件下操作，稳定一定活性和选择性水平的时间称为单程寿命；每次性能下降后，经再生又恢复到许可水平的累计时间称为总寿命。催化剂稳定性包括热稳定性，抗毒稳定性，机械稳定性三个方面。 4、物理吸附与化学吸附的主要区别 物理吸附： 指气体物质（分子、离子、原子或聚集体）与表面的物理作用（如色散力、诱导偶极吸引力）而发生的吸附，其吸附剂与吸附质之间主要是分子间力（也称“van der Waals”力）。 化学吸附： 指在气固界面上，气体分子或原子由化学键力（如静电、共价键力）而发生的吸附，因此化学吸附作用力强，涉及到吸附质分子和固体间化学键的形成、电子重排等。 5、何谓B酸和L酸，及其简便的鉴定方法 能够给出质子的都是酸，能够接受质子的都是碱，Brnsted 定义的酸碱称为B酸（B碱），又叫质子酸碱。 能够接受电子对的都是酸，能够给出电子对的都是碱，所以Lewis定义的酸碱称为L酸（L碱），又叫非质子酸碱。 固体酸的类型有B酸和L酸两种，对固体酸类型最有效的区分方法是红外光谱法，它是通过研究NH3或吡啶在固体酸表面上吸附的红外光谱来区分B酸和L酸的。固体酸吸附吡啶的红外吸收谱带见表所示，通过这些谱带很容易的确定固体酸表面的B酸和L酸。 6、如何利用红外光谱法鉴定B酸和L酸 7、如何利用碱滴定法测定固体酸的酸量 就是把固体酸催化剂粉末悬浮于苯溶液中，其中加入指示剂，用正丁胺进行滴定，使用不同pKa值的各种指示剂，就可通过胺滴定来测定各种酸强度的酸量，这样测得的酸量为B酸和L酸的总和。对于有颜色的样品，可用分光光度计法或掺入已知酸强度的白色固体予以稀释，也可用胺量热滴定法来测定有色或黑色固体酸样品的酸量。 8、如何利用CO2吸附法测定固体碱的碱量 就是在TPD装置上将预先吸附了CO2的固体碱在等速升温，并通入稳定流速的载气条件下，检测一定温度下脱附出的酸性气体，得到TPD曲线。这种曲线的形状、大小及出现最高峰的温度值，都与固体碱的表面碱性有关，从而确定碱量。 9、简述固体酸催化剂的催化作用机理。 固体酸、碱催化剂，如硅铝胶、分子筛、MgO-SiO2等在烃类转化，包括裂解、异构化、烷基化、聚合反应中都有极好的活性。现普遍认为，固体酸催化反应与均相酸催化反应一样，都是按正碳离子机理进行的，与此相对应，烃类在固体碱催化剂作用下，反应按负碳离子机理进行的。所谓正碳离子和负碳离子相理，简单地说就在反应中，通过反应分子的质子化生成碳正离子，或从反应分子除去一个质子生成负碳离子，从而使反应分子得以活化的过程，并且是反应的控制步骤。 10、催化裂化反应有哪些规律 （1）新生成的伯正碳离子极不稳定，并迅速转化为仲正碳离子，然后再β处断裂，反应继续下去，直至成为不能再断裂的小正碳离子为止，并在反应过程中将H+ 传给催化剂变成烯烃。 （2）烯烃裂化时也首先形成正碳离子，并遵循β处断裂原则，生成一个较小的烯烃和一个伯正碳离子，伯正碳离子再重排，裂化为较小的烯烃。 （3）环烷烃裂化时形成的正碳离子的机理与烷烃一体，但由于存在大量仲碳离子和叔碳离子，所以环烷烃的反应能力很高，并能生成各种与烯烃裂化类似的产品，同时还存在一定的芳烃。
催化剂与催化作用试题 - 副本
名词解释（10~15分，4~6题）填空（10~15分，5~10题）简要回答问题（45~55分，6~8题）论述题（25~35，2~3题） 第1、2章复习思考题 1、催化剂是如何定义的？ 催化剂是一种能够改变化学反应速度而不能改变反应的热力学平衡位置，且自身不被明显消耗的物质。 2、催化剂在工业上的作用功能或者效果有哪些？ 1）使得原来难以在工业上实现的过程得以实现。 2）由过去常常使用的一种原料，可以改变为多种原料。 3）原来无法生产的过程，可以实现生产。 4）原来需要多步完成的，变为一步完成。 5）由原来产品质量低，能耗大，变为生产成本低，质量高 6）由原来转化率低，副产物多，污染严重，变为转化率高，产物单一，污染减少 3、载体具有哪些功能和作用？8 ①分散作用，增大表面积，分散活性组分；②稳定化作用，防止活性组分熔化或者再结晶；③支撑作用，使催化剂具备一定机械强度，不易破损；④传热和稀释作用，能及时移走热量，提高热稳定性； ⑤助催化作用，某些载体能对活性组分发生诱导作用，协助活性组分发生催化作用。 4、代表催化剂性能的重要指标是什么？ 催化剂的反应性能是评价催化剂好坏的主要指标，它主要包括催化剂的活性、选择性和稳定性。（1）催化剂的活性：指催化剂能加快化学反应的反应速度的程度 （2）催化剂的选择性：使反应向生成某一特定产物的方向进行。 （3）催化剂的稳定性：是指在使用条件下，催化剂具有稳定活性的周期 5、多相催化反应的过程步骤可分为哪几步？实质上可分为几步？ （1）外扩散—内扩散—化学吸附—表面反应—脱附—内扩散—外扩散 （2）物理过程—化学过程—物理过程 6、吸附是如何定义的？ 气体与固体表面接触时，固体表面上气体的浓度高于气相主体浓度的现象。 7、物理吸附与化学吸附的本质不同是什么？ 本质：二者不同在于其作用力不同，前者为范德华力，后者为化学键力，因此吸附形成的吸附物种也不同，而且吸附过程也不同等诸多不同。 不同的表现形式为：（后面） 8、为何说Langmuir吸附为理想吸附？基本假设是什么？ 模型假设：①吸附表面均匀，各吸附中心能量相同；②吸附分子间无相互作用；③单分子层吸附，吸附分子与吸附中心碰撞进行吸附，一个分子只占据一个吸附中心；④在一定条件下，吸附与脱附可建立动态平衡。 9、催化剂的比表面测定有哪些实验方法？ （1）BET法测比表面积 1）测定原理和计算方法 依据BET提出的多层吸附理论以及BET吸附等温曲线进行测定和计算的。利用BET方程进行作图，采用试验采集数据并利用图解法进行计算。 2）实验方法 测定表面积的实验方法通常有，低温氮吸附容量法、重量法和色谱法等，当表面积比较小时，采用氮吸附法。 （2）色谱法测定比表面积 色谱法测定比表面积时载气一般采用He或H2,用N2做吸附质，吸附在液氮温度下进行。 10、何为扩散？催化剂颗粒内部存在几种扩散形式？ （1）扩散：分子通过随机运动，从高浓度向低浓度进行传播的现象。 （2）1）普通扩散（分子扩散）：分子扩散的阻力来自分子间的碰撞，通常在大孔（孔径大于100nm）