Rozovskii Et Al. (2003) - Fundamentals of Methanol Synthesis and Decomposition

Fundamentals of methanol synthesis and decomposition

Alexander Ya. Rozovskii� and Galina I. Lin

Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninsky Prospect, 117912, Moscow, Russia

Fundamental studies of methanol synthesis and decomposition (mainly over Cu-based catalysts) have been carried out. Various

kinetic approaches, i.e. TPD study after various chemical treatments of catalyst, non-steady-state transformation of strongly

adsorbed species, tracer technique, and steady-state kinetics, have been used. The macroscopic mechanism and detailed reaction

scheme of methanol synthesis, as well as the kinetic description of the process have been established and proven. Methanol

synthesis over Cu-based catalysts was found to occur by CO2 hydrogenation only, which was coupled with the water-gas shift

reaction.

Methanol decomposition and steam reforming over Cu-based catalysts have been studied. It was shown that methanol

decomposed into a mixture of CO andH2 via methyl formate as an intermediate. Methanol transformation into the mixture of CO2

and H2 occurred by interaction of methanol and water. The reaction proceeded as the reverse methanol synthesis reaction,

accompanied by the reverse water-gas shift reaction.

KEY WORDS: reaction mechanism; kinetics; methanol synthesis; methanol decomposition; methanol dehydrogenation; methyl

formate synthesis; methyl formate decomposition; surface species; adsorption substitution

1. Introduction

Methanol is used as a feedstock in numerouschemical syntheses. It can be used also as an energycarrier for hydrogen storage and transportation. Forthese reasons, both methanol synthesis and methanoldecomposition are of great importance for industry,now and in the near future.

Methanol synthesis from syngas

COþ 2H2 ¼ CH3OH ð1Þ

has been commercially operated for many years (from1923, by BASF). Cu-based catalysts, since beingpioneered by ICI in the ‘60s, are used for the highlyselective process. In addition, methanol synthesis can beconsidered as a key reaction in the combined processesof one-step dimethyl ether (DME) and gasoline synth-eses from syngas.

Methanol transformation into gaseous mixturesenriched with hydrogen can be performed in two ways,by methanol decomposition

CH3OH ¼ COþ 2H2 ð2Þ

or by methanol steam reforming

CH3OHþH2O ¼ CO2 þ 3H2 ð3Þ

Cu-based catalysts can be used in all these reactions.Though very different, reactions (1)–(3) have one

common feature: their mechanisms are similar in thatthe products of these reactions are adsorbed strongly on

the active sites [1,2]. This adsorption is ‘‘irreversible’’from a kinetics viewpoint, which means the character-istic time of desorption of these substances from activesites is much greater than that of catalysis. Thus, almostall active sites are covered by strongly adsorbed speciesin the course of reaction. For this reason, the exchangebetween the surface and the gas phase occurs in suchprocesses mainly through the reactions of adsorptionsubstitution involving formation of a ‘‘two-particle’’intermediate on an active site [3]. This unusualmechanism shows itself in non-trivial kinetic patternsof the reactions above. An analogous evaluation(‘‘almost all the active sites are occupied by adsorptionspecies’’) was made by Matsumura and Tode in kineticstudies of methanol decomposition over silica-sup-ported Ni [4].

Reactions (1) and (2), when proceeding over Cu-based catalysts, have another distinction: their mechan-isms are quite unlike the ones that can be assumed fromreaction stoichiometry. Such is the case of methanolsynthesis from CO and H2; which proceeds through theintermediate step of CO2 formation by the water-gasshift reaction (WGSR)

COþH2O! CO2 þH2 ð4Þ

followed by CO2 hydrogenation to methanol

CO2 þ 3H2! CH3OHþH2O ð5Þ

Direct CO hydrogenation to methanol does not takeplace at all over Cu-based catalysts [1,2].

The reaction of methanol decomposition (2) pro-ceeds, strange as it may seem, through the intermediate

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E-mail: [email protected]

Topics in Catalysis Vol. 22, Nos. 3–4, April 2003 (# 2003) 137

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formation of methyl formate, which decomposes theninto CO and H2:

2CH3OH! CH3OCHO þ 2H2 ð6Þ

CH3OCHO! 2COþ 2H2: ð7Þ

These peculiarities will be considered below in detail.Methanol synthesis is paid slightly more attention tohere, because the mechanism of this reaction is offundamental importance also for methanol steamreforming. Although the fact that methanol synthesisover Cu-based catalysts proceeds only through CO2

hydrogenation by reaction (5) was proved a quarter ofcentury ago [5–7], and some of those results have beenreproduced later by Chinchen et al. [8,9], publicationsstill appear where methanol synthesis is proposedthrough CO or CO=CO2 hydrogenation (e.g., [10–14]).Therefore, this paper will deal with the most importantexperimental results concerning various peculiarities ofmethanol synthesis mechanisms that seem to escape theattention of many researchers.

Decomposition and steam reforming of methanol areless studied processes. The latter, though, can bedescribed as reverse methanol synthesis. It will beshown that data on methanol synthesis mechanismallow one to establish the detailed scheme of themethanol reforming mechanism.

As to methanol decomposition, it was noted abovethat in the case of the absence of water this reactionproceeds over Cu-based catalysts through the formationand the decomposition of methyl formate. The existenceof a free site adjacent to an active one is found to benecessary for methyl formate decomposition to proceed.

2. Methanol synthesis

Methanol synthesis over Cu-containing catalysts is awell-researched process. Various surface species weredetermined in studies of methanol synthesis [15].However, their respective roles in the reaction mechan-ism are still disputed. For example, Kilo et al. suggestedthat over Cu=ZrO2-based catalysts methanol is formedmainly from bidentate surface carbonate via adsorbedCO; �-bound formaldehyde and methoxy group [16]. Ina paper by Chen et al. [17] another mechanism isproposed: carbonate ! formate ! methoxy group !methanol.

Synergy between Cu and ZnO in the catalysis ofmethanol synthesis has been the subject of activediscussion during recent years. Even in this relativelysimple case there is no common opinion on both thenature of the effect and its quantitative expression.Fujitani and Nakamura reported on the anomalousZnO content dependence of Cu/ZnO catalyst activity(the dependence of activity on metallic copper surfacearea was linear, while the relationship between the

copper specific activity in methanol synthesis and ZnOcontent was volcano-like [18]). Waugh [19] assumed thatsuch a difference may occur due to the difference inspecific activity of various samples relating to theirdifferent morphologies. According to Spencer, thevaried results obtained by different groups concerningsynergy between ZnO and Cu in methanol synthesiscatalysts are a result of different experimental testconditions [20].

K. Futagami and others reported recently [14,21] thatreduction of the physical mixture of Cu=SiO2 andZnO=SiO2 in hydrogen atmosphere at 573–723Kincreased the yield of methanol produced by CO2

hydrogenation. At the same time, no such promotionwas observed for the reverse WGSR [21] and for COhydrogenation [14], indicating that the active sites forCO2 and CO hydrogenation are different. This conclu-sion contradicts to the data of Ma et al. who have foundthat ZnO and Cr2O3 improved the activities of skeletalcopper catalysts in all reactions: methanol synthesis,decomposition, and WGSR [22].

As to the nature of this synergy, it is also interpretedin different manners. Spencer suggested that synergy, ifit occurs, arises from H spillover from ZnO to metallicCu [20,23], while Schilke et al. proposed for the case ofCu=SiO2–Zr catalyst that after CO2 adsorption overZrO2 as bidentate HCO3–Zr; the latter species ishydrogenated by atomic hydrogen which is generatedover metallic Cu, i.e. hydrogen diffusion occurred in theopposite direction [24].

Overall, formate adsorbed over copper seems to bethe most probable participant in methanol synthesisfrom all surface compounds found by different authors.Of course, contradictions in various data do not endhere. For example, Fujitani et al. reported on the basisof in situ FTIR measurements that the active sites ofmethanol synthesis created on the Cu=SiO2 catalyst byinteraction with ZnO were found not to promote CO2

and H2 transformation into formate over Cu [25].Nomura et al., however, demonstrated by the samemethod that catalyst activity grew along with concen-tration of surface formate in methanol synthesis fromCO2 and H2 over Cu=TiO2 with various promotersincluding ZnO [26].

The key to understanding these patterns can befound probably in the work of Bando et al. [27], whereit is shown by in situ FTIR that, although formatespecies on copper easily form at CO2 adsorption overvarious Cu-based catalysts (Cu=TiO2; Cu=SiO2;Cu=Al2O3), they differ in stability: over Cu=TiO2 theycan be hydrogenated at as low a temperature as 150 �C;while over other catalysts these particles are stable at200 �C: Thus, the concentration of formate species isdefined by the difference in rates of their formation anddecomposition, which may be affected by variations incomposition of different catalysts and other factors,leading to ambiguity in conclusions. Even in situ

A.Y. Rozovskii, G.I. Lin/Methanol synthesis and decomposition138

measurements can be interpreted in various ways due tothese reasons.

Some conclusions may seem weak when the results ofdifferent authors are thoroughly compared; usually thisresults from the fact that the investigations of the natureof the surface species by methods of surface science areseparated from studies of reaction kinetics in mostworks. Nevertheless, the latter is the only method that,being combined with the results of surface studies,allows one to establish the sequence of the surfaceintermediates in the reaction mechanism.

2.1. Reaction mechanism

The methanol synthesis mechanism had been con-sidered partly [1] and recently in detail [2]. It issummarized in this section.

The first (and still the most important) questionunder discussion is whether carbon monoxide or carbondioxide is the source for methanol. From the chemist’spoint of view, sequential reduction of carbon oxides isthe most evident way for this reaction:

CO2! CO! CH3OH: ð8Þ

However, experimental results from both reactionkinetics studies at small contact time [5] and tracertechnique [6,7] have shown that the reaction proceedsvia an alternative pathway over Cu-based and Zn–Croxide catalysts as well:

CO! CO2! CH3OH: ð9Þ

Moreover, it is the only sequence of reactions that canbe observed in the reasonable range of temperatures(180–260 8C) and pressures (0.1–20MPa). It was shownin a special experiment [7] that a CO=H2 mixturecarefully cleaned from CO2 and water (which in turnproduces CO2 by WGSR) does not produce methanol.SNM-1, a Cu–ZnO–Al2O3 catalyst originating frommethanol industry of the former USSR, was used in thisexperiment.

Methanol synthesis from a CO2=H2 mixture over awide range of Cu-based catalysts has been studied indetail in our laboratory ([1]). In all experiments with aCO2=H2 mixture as the feedstock the initial rate ofmethanol synthesis was at its maximum value [1],[5].This agrees with the conclusion that methanol synthesisover Cu-based catalysts proceeds by reaction (9), not byreaction (8).

2.1.1. 14C label experimentsThe conclusion above has been supported by results

of experiments using 14C label [6,7]. The results ofsteady-state methanol synthesis from the mixture con-taining predominantly 12CO (30 vol%) and small addi-tions of 14CO2 (4 vol%) illustrate this point most

evidently. Let us consider some of these results [6,7].Methanol synthesis was performed under � 5MPapressure over Cu-based catalyst SNM-1.

The reaction proceeded in a circulating flow systemwhere methanol and water were removed from the flowby freezing and the total pressure slightly decreasedduring the run due to methanol synthesis. The label inCO2 continuously decreased during the run, the label inCO increased - due to direct and reverse WGSR.Methanol label was measured at the end of the run. Itwas, therefore, an integral average value for the runduration. For CO and CO2; initial and final 14Cfractions were measured, as well as those in an integralaverage gas mixture sample obtained by continuoussampling of gas mixture during the run. The average gassample can be considered as an equivalent to themethanol sample obtained by freezing, and their labelscan be compared directly.

Table 1 demonstrates the results of two runsperformed at 218 and 250 �C with syngas containingeither CO or CO2 in excess [6,7]. It is seen from table 1that when CO was in excess, the 14C fraction increasedin CO while decreasing in CO2 during the run due toisotopic exchange by WGSR. The methanol label wasmuch higher than that of CO, being close to the CO2

label. When CO2 was in excess and 14CO was added, themethanol label corresponded to the CO2 label just thesame. It is clear from these data that CO2 hydrogenationis the only pathway for methanol synthesis.

Analogous results were obtained by our researchgroup using tracer technique ð14CÞ in steady-statemethanol synthesis over a zinc–chromium catalyst([2]), as well as by Chinchen et al. [8,9] in methanolsynthesis over a Cu-based catalyst under steady-stateconditions.

The resulting conclusion can be made that methanolsynthesis over Cu-based catalysts occurs through CO2

hydrogenation ðCO2 þ 3H2 ¼ CH3OHþH2OÞ: Thisprocess is accompanied by WGSRðCOþH2O ¼ CO2 þH2Þ: It can be seen that thecombination of these two reactions is a molecularchain reaction where excess oxygen atoms migrate

Table 1

Specific radioactivity (in arbitrary units) of carbon-containing

species in methanol synthesis over industrial Cu�ZnO�Al2O3

catalyst SNM-1 under a pressure of �5MPa

Species CO CO2 CH3OH

Feed: 14CO2 (4 vol%) and 12CO (30 vol%), balance H2, 250 8CInitial label 0 5900 –

Final label 490 790 –

Integral average label 430 1480 1500

Feed: 12CO2 (19 vol%) and 14CO (1.1 vol%), balance H2, 218 8CInitial label 406 0 –

Final label 194 11 –

Integral average label – – 7

A.Y. Rozovskii, G.I. Lin/Methanol synthesis and decomposition 139

between CO2 and H2O; supporting the reaction chainfor methanol synthesis from a CO=H2 mixture even at asmall content of excess oxygen in CO2 or H2O:

2.1.2. 18O labelKlier et al. [28] studied label transfer of oxygen and

deuterium in steady-state methanol synthesis in a flowreactor using a mixture of syngas and D18

2 O as afeedstock. The flows of participating compounds weremeasured at the reactor outlet. Most of the 18O fromD18

2 O was found in the methanol flow. Combining theseresults with the data on D transfer (which will beconsidered later in this work), Klier et al. concluded thatCO and H2O are the precursors of methanol, whichdirectly conflicts with the conclusion above. However,calculations [1] using the data [28] show that in theseexperiments due to large contact times the isotopicfraction of 18O at the reactor exit became the same inwater (3.3%), CO2 (3.6%) and methanol (3.4%). Thus,both water and CO2 can be the source of oxygen inmethanol (not CO, for its content of 18O is 0.6%, i.e.more than 5 times lower than in water, carbon dioxideor methanol).

Less definitive results can be obtained from methanolsynthesis studies under non-steady-state conditions. Thework of Liu et al. [29] seems to be the most interestingone. A Cu–ZnO catalyst was treated here under steady-state conditions with an initial mixture containing noneof the labeled components. C18O2 was then introducedin the feed and initial formation rates were measured forall components. The conclusion made by the authorswas that both CO2 and CO were the source of methanoland that water retarded the CO2 route. However, as wassubsequently shown [1], the original data of Liu et al.resulted from the existence of an oxygen buffer on thecatalyst surface. Figure 1, reproduced from [1], demon-strates the dynamics of the 18O fraction in CO2 (the onlysource of 18O in the system) and in the producedmethanol, as calculated by data [29]. It is seen that the

fraction of 18O continuously decreased in CO2 whileincreasing at the same time in methanol. By the end of a15-minute run, these values became close. Therefore, therelaxation times of the chemisorbed layer exceeded theduration of the experiment markedly. It follows that anoxygen buffer does exist on the surface. Oxygen fromC18O2 is diluted there by oxygen from the surfacespecies. Only then does the oxygen atom with anaveraged label participate in the formation of amethanol molecule. Due to oxygen equilibration, thelabels in CO2; surface buffer and methanol graduallybecome closer. If CO is present in the feed, it alsoparticipates in oxygen exchange with the buffer,supplying 16O: Thus, the difference in the oxygenisotopic content exists between CO2 and surface bufferuntil the moment when the 18O fractions in CO and CO2

are equal. Taking into account that the methanol andthe CO2 labels converge (figure 1), it is clear that CO2 is,at least, the most important methanol precursor. Thereason for the existence of the oxygen buffer on thesurface will become clear from the later discussion of thedetailed mechanism of methanol synthesis.

2.1.3. TPD experiments and dynamics studyIn TPD experiments, as a rule, the following standard

procedure was used. A catalyst sample after reductionand vacuum pumping was treated with some of thereaction mixture components under various conditions,and then the TPD spectrum was measured. In someexperiments sequential treatment with different compo-nents was performed before TPD. The temperatureincrease rate in the TPD procedure was 25 degrees perminute. A mass spectrometer, MX-7301 (made inRussia), was used in these experiments [1].

As follows from TPD experiments with varioussamples [1,30–32], the surface of reduced catalyst iscovered with large amounts of strongly adsorbed waterðTm � 350 �CÞ and CO2ðTm � 300 �CÞ: The H2O peakmay relate to water or to surface hydroxyl. Pre-treatment with water and then with carbon dioxideresults in the appearance of another CO2 peakðTm � 180 �CÞ and its corresponding shoulder in thewater TPD spectrum (figure 2). Two similar CO2 peaksappear in the TPD spectrum after treatment of thesample with methanol [2], and also after steady-statemethanol synthesis over Cu/ZnO catalyst [33].

Upon the contact of the reduced catalyst with CO2 orwater, their peaks were observed in the TPD spectrumtogether with a diffused peak of hydrogen, which wasrelated in the cited works to reactive hydrogen,appearing as the result of the interaction of chemisorbedwater with reduced sites on the surface. This conclusionis favored by the following fact: after pre-treatment ofthe reduced catalyst with hydrogen at 250 8C, thehydrogen peak in the TPD spectrum becomes smallerinstead of growing.

Figure 1. Dynamics of 18O fraction in CO2 (1) and in forming

methanol (2), as calculated from data [29].


The dynamics of surface species transformations overCu-based industrial catalysts in methanol synthesis(SNM-1 and 51-2, the latter by ICI) have beenconsidered in detail [1,2]. The catalyst sample [1] wastreated with some of the reaction mixture components,then put into a helium flow, which was abruptlyswitched to gaseous reactant flow. Due to contact withgas components, various reactions took place with theparticipation of strongly adsorbed species. The progressof such reactions was judged by product concentrationsat the reactor outlet and usually took several dozens ofseconds. These studies have shown that CO, CO2 andH2O; strongly chemisorbed on the active sites, canundergo redox transformations and also participate inadsorption substitution reactions with molecules fromthe gas phase, for example:

ZH2Oþ CO2ðgÞ , ZH2O:CO2

, ZCO2 þH2OðgÞ; ð10Þ

where Z is a Cu-containing active site.It should be noted that the studies of the dynamics of

surface species transformations do not allow assignmentof observed species to definitive chemical structures.Thus, ZH2O can be chemisorbed water or two hydroxylsthat produce water under TPD conditions. The onlything known for sure is that an intermediate denotedhere as ZH2O:CO2 includes both H2O and CO2: It willbe referred to now on as a ‘‘two-particle’’ intermediate.

At low temperatures the intermediate ZH2O:CO2 isstable, and its decomposition can be observed by TPD(figure 2(a)). At the temperature of methanol synthesis itdecomposes quickly, leaving strongly adsorbed water orCO2. Although in these experiments water and hydroxylcannot be distinguished, considering that 100 8C is arelatively low temperature for substitution, it can beassumed that adsorbed water participates in thesubstitution reaction. Studies of substitution dynamicshave shown that this reaction proceeds with high rate[1]. Its characteristic time is less than that of methanolsynthesis, so this reaction is suitable for the purpose ofgetting CO2 molecule onto the active center during thesynthesis.

At the excess of CO in the reaction system two otherreactions are possible [34]:

ZH2OþCOðgÞ! ZH2O:CO! ZCO2 þH2ðgÞ ð11Þ

ZH2OþCOðgÞ! ZH2O:CO! Zþ CO2ðgÞ þH2ðgÞ: ð11aÞ

This reaction also proceeds through a ‘‘two-particle’’intermediate. Its exact route ((11) or (11a)) remainsunclear, though. Nevertheless, it should be noted that itis the reversible reaction (11) which can make possiblequick oxygen exchange between CO and CO2 observedin the experiments of Liu et al. [29]. Therefore, thisreaction can be considered for inclusion into themechanism scheme as the more probable.

Studies of the dynamics of surface species transfor-mations allow one to obtain information on chemicalproperties of the active sites on the surface of Cu-basedcatalysts of methanol synthesis. These sites are able toparticipate in redox reactions involving H2O; H2 andCO: Moreover, they are the only sites on the catalystsurface where adsorbed CO; CO2 and H2O moleculesare able to react with molecules from the gas phase. It isthese sites that are active in methanol synthesis andcatalytic activity is proportional to their number [1]. Fora set of fresh and partly deactivated samples ofindustrial catalysts (SNM and ICI 51-2), differing inactivity by about 5 times, specific activity per active site(turnover frequency) was the same: 0:47� 0:05 s�1 [1].

2.1.4. Mechanistic scheme

The absence of adsorbed species other than CO2 andwater on the active sites is one of the most important

Figure 2. TPD spectra of industrial Cu/Zn/Al oxide catalyst SNM-1

reduced by CO: (a) after exposure to H2O (0.67 kPa, 10min, 100 8C)followed by exposure to CO2 (0.67 kPa, 10min, 100 8C); (b) after

exposure to H2O (0.67 kPa, 10min, 100 8C) followed by exposure

to methanol (0.67 kPa, 10min, 30 8C). 1;H2O; 2;CO2; 3;CO2 directly

after reduction, 4;CO2; 5;H2; 6;H2O:


features in methanol synthesis. Another feature of thisprocess is that all active sites are covered with stronglyadsorbed species under methanol synthesis conditions,so there are almost no free active sites. This is whysubstitution reaction (10) is of the most importance,providing the way for methanol formation. Watermolecules form as the result of each cycle of thesynthesis, and the time of their desorption is dozens ofhours, i.e. markedly exceeding the characteristic time ofmethanol synthesis. It is clear that without such achannel of exchange with the gas phase as reaction (10)methanol synthesis could not proceed at all.

An additional channel for the release of stronglyadsorbed water from an active site or for waterreplacement with CO2 is provided by either reaction(11) or (11a). If reaction (11a) does proceed, then ZCO2

forms directly by CO2 adsorption on vacated sites. Ifreaction (11) occurs, then ZCO2 species cannot be directmethanol precursors. Otherwise 14C label would transferfrom CO into methanol, which is not supported byexperiment. Therefore, reaction (11) can be included inthe reaction mechanism scheme only if the step of quickexchange of ZCO2 with gas-phase CO2 is added.Ultimately, both pathways can be described by similartheoretical kinetic models, and the differences arealmost negligible. The role of reaction (11) or (11a)increases with increase of CO concentration.

The question remains open as to further transforma-tions of CO2-containing species on active sites. Twosuch species, ZH2O:CO2 and ZCO2; participate inreaction (10), which can be considered as an experimen-tally established reaction. In principle, methanol canform by hydrogenation of any of these species. It ispossible to expect that, in hydrogenation of the formerspecies, at least one hydrogen atom from H2O willtransfer to the CH3 group of methanol, and that inhydrogenation of the ZCO2 species, only hydrogen fromthe gas phase will be included in the methyl group. Thus,it is possible to discriminate between these two speciesby using D2O in methanol synthesis.

Such an experiment in steady-state methanol synth-esis was performed by Klier et al. [28]. As the CH2Dgroup was observed in methanol, it was evident thatZH2O:CO2 is the methanol precursor. Nevertheless,considering that the contact time was too high in theexperiments [28] ðGHSV � 104 h�1Þ; an additional studyof the effect of D2O addition on the methyl groupcomposition was performed in our work [35]. Methanolsynthesis [35] was conducted in a circulating flow andthe products were frozen out. At long contact times, asmentioned before, the results of Klier et al. [28] werereproduced. However, at small contact times(GHSV � 106 h�1; at which the reactor became closeto the differential one in relation to deuterium transfer)no label transfer from D2O to the methanol CH3 groupwas found. It can be seen from the data [35] presented infigure 3 that deuterium fraction in the methanol CH3

group varies in the range 1–4%, being equal to itsfraction in hydrogen and much less than its fraction inwater (70–85% [35]).

These data show that ZCO2 is the only methanolprecursor from all species on active sites. Its hydro-genation is a multi-step process due to the participationof three hydrogen molecules. The order of the stationaryreaction on hydrogen would be a useful value; nH2 ¼ 1from the data [1]. Because hydrogen does not participatein any of the previous steps, it follows that the firsthydrogenation step is the rate-limiting one and hydro-gen from the gas phase is involved. It should bementioned that gas-phase hydrogen appears to be thereactant also in hydrogenation of methyl formate intomethanol over Cu-based catalysts [36].

The results detailed above are sufficient for establish-ing the mechanism scheme and theoretical kinetic modelon the basis of this mechanism. The only remainingquestion is which surface species will be produced in thisreaction in the end.

Species formed after the limiting step are inequilibrium with reaction products and thereforeunreachable for kinetic analysis in the investigation ofdirect reaction. Nevertheless, it is known that amethanol molecule and water molecule will be producedin the end. Considering that both molecules can bestrongly bound to active sites, it can be assumed that atleast one of them remains on the surface.

If it is a methanol molecule that remains on an activesite, then the latter undergoes excessive reduction in thereaction involving this molecule, followed by itsdeactivation. Because the characteristic time of deacti-vation is much higher than that of methanol synthesis, itcan be assumed that the percentage of such occurrences

Figure 3. Dynamics of deuterium accumulation in hydrogen (open

points) and in the CH3 group of methanol (filled points) in methanol

synthesis at 240 8C over industrial Cu-based catalyst 51–2. Partial

pressures of the components (in MPa): 1; �CO; 0:85; CO2; 0:034;

H2; 3:6; D2O; 0:024; � ¼ 5:10�3 s; 2;CO; 0:037; CO2; 0:99; H2; 4:1;

D2O, 0:029; � ¼ 3:10�3 s:


is negligible. Therefore, it can be concluded that amethanol molecule in the gas phase and a watermolecule strongly bound to the active site are formedas the final result of ZCO2 hydrogenation. Interaction ofZCO2 species with a hydrogen molecule from the gasphase is the controlling step in the sequence ofhydrogenation steps.

On this basis two mechanism schemes can be built.Scheme 1 is based on the reaction sequence occurring atrelatively low CO content in a feed. Scheme 2 of a moregeneral kind, takes into account the reaction pathway(11).

The upper line in scheme 1 is the reaction ofadsorption substitution (10) which was observed in anindependent experiment. Its intermediate (‘‘two-parti-cle’’ complex) is present in the TPD spectrum at lowertemperature (100 8C). Participation of hydrogen fromthe gas phase occurs in the first hydrogenation step. It isalso the rate-limiting step, as follows from the first orderof the reaction of hydrogen. Thus, all steps in scheme 1except the final macro step are proved by experiments.

Scheme 2, which includes reaction (11), is lessadequate, because it includes the hypothetical step ofquick CO2 exchange between active surface and gasphase. Quick transfer of oxygen label from CO into CO2

observed in [29] becomes understandable in view of thisreaction. All other steps including the formation of

ZH2O:CO intermediate do proceed, as shown quitedefinitively by the data on chemisorbed H2O transfor-mation dynamics [1].

2.1.5. Surface intermediatesThe correct assignment of intermediates in the

schemes above to the real structures is relativelycomplicated, although surface species on Cu-basedcatalysts are studied in detail by various spectroscopicmethods (e.g., [16,17,21,24–28,33,37–44]). Additionalcomplications arise, though, as active centers of thecatalysis appear to be clusters of Cu atoms or ions. Aswas found by ESR spectroscopy [45], treatment of theCu–Zn–Al–oxide methanol synthesis catalysts withpyridine did not produce isolated pyridinate complexes.Moreover, three types of copper clusters were observed.In the case of Cu–ZnO system clusters up to 1 nm werefound by electron microscopy [46]. The complicated andunpredictable structure of Cu-based catalyst surfaces,which change in the process of reduction and catalysis,is also described [37,38]. Nevertheless, the specificactivity of Cu-based catalysts (turnover frequency) wasfound to be the same for two different industrialcatalysts (see the ending of section 2.1.3). Therefore,we can conclude that despite the initial differencebetween the surfaces of various catalysts, quite similarstructures may form in the course of catalysis.

Species on the surface of Cu-containing catalystshave been actively studied in recent years by variousmethods, including in situ methods. Nevertheless,information obtained on the role of various surfacespecies in methanol synthesis is confusing. Most authorsrefer the key role in methanol synthesis to surfaceformate on copper (e.g., [26,33,39–41]). However, someauthors [42,43] consider surface formaldehyde to be ofthe most importance. At present, surface carbonateseems to become more prominent in the mind of manyscientists as the first surface species to undergo hydro-genation [16,17,43,44]. In one paper [44] this step isdeemed as the rate-controlling step.

Considering these data for the use in assignment ofintermediates in schemes 1 and 2, let us take intoaccount that 14C label does not transfer from CO intomethanol. For this reason, the probability is low thatintermediate ZCO2 in scheme 1 has a formate structure,which easily forms with participation of CO from thegas phase, especially in the presence of water on thesurface. More likely, it has a carbonate structure. Thus,ZH2O:CO2 may be related to a combination (onadjacent sites) either of carbonate and water or ofbicarbonate and hydroxyl. Correspondingly, it can beassumed that the product of the first hydrogenation stephas the structure of formate on copper, which quicklytransforms into resulting products.

The ZH2O:CO intermediate in scheme 2, most likely,is also formate on copper, but of some other structurethan the product of the first step of hydrogenation,

Scheme 1. Simplified scheme of methanol synthesis mechanism over

Cu-based catalysts at low CO content in syngas.

Scheme 2. Scheme of methanol synthesis mechanism over Cu-contain-

ing catalysts considering reaction (11).


because 14C label does not transfer into methanol. Itsexistence is proved by experiments on non-steady-statekinetics of transformations of adsorbed water reactingwith CO from the gas phase [1]. Surface carbonates ofdifferent structure are referred to in scheme 2 as Z½ �CO2

and ZCO2½ �; only the latter species can be hydrogenatedinto methanol.

2.2. Reaction kinetics

Two specific features of the process should be takeninto account to establish the theoretical kinetic model:

(i) Under conditions of methanol synthesis activecenters are covered with various species, mainlywater and CO2: The reaction proceeds in theabsence of free centers.

(ii) All active centers are equal in their reactivity.

The latter conclusion directly follows from the data ondynamics of surface species transformations. It isillustrated by figure 4, where kinetic curves are givenfor adsorption substitution reaction where adsorbed COis substituted with water from the gas phase over afreshly reduced and partly deactivated industrial catalyst51-2 (ICI), according to data [1]. For these measure-ments the catalyst sample was reduced by CO followedby a gas flow switch to helium at the temperature of theexperiment, then to HeþH2O: The composition of thegas flow at the reactor outlet was monitored by GCusing a procedure similar to the ‘‘stop-flow’’ method.The increase of rate corresponding to the left branchesof the curves in figure 4 is due to the increase of H2Oconcentration in the gas flow. As to the right branches,the reaction proceeds at constant H2O concentration.Conversion curves (dependence of reaction rate onconversion of surface species) are given in the same plotby using average values of reaction rate. The linear formof their right branches allows one to make the definitiveconclusion on equal reactivity of active centers (andsurface species) participating in the process. Similarcurves with linear right branches were obtained for allstudied reactions of surface species over Cu-basedcatalysts.

A simplified kinetic model of methanol synthesisaccording to scheme 1 is given below. A number of non-linear terms in the denominator which become promi-nent near the equilibrium is not shown here.

r ¼k3pH2

ð1�pmpH2O

KpðmÞp3H2

pCO2

Þ

1þ K�2pH2O þ K�2pH2O=ðK1pCO2Þ;

where r is the reaction rate, ki and Ki the correspondingrate constant and equilibrium constant of step i; KpðmÞ

the methanol synthesis equilibrium constant and p thepartial pressures of components. WGSR can bedescribed in a similar manner.

Although this model is built for the set of activecenters that are equal in their reactivity, the denomi-nator of the equation contains non-linear terms, whichis surely non-trivial for the kinetics of reactions over auniform surface. Nevertheless, such a form of kineticequation is typical for the reactions with participation ofstrongly adsorbed species [3].

In scheme 2, the reactions of methanol synthesis andWGSR are combined, resulting in unusual kineticpatterns, for example, anomalous increase of effectivereaction rate along with contact time in the flow reactor.This is illustrated by figure 5 [2,47], where the contacttime dependence of methanol synthesis rate over anindustrial catalyst 51-2 is given for the mixture CO2=H2

and for the mixture where CO is in excess. The anomalycaused by conjugation can be seen clearly in the lattercase. A more detailed discussion of scheme 2 and thewhole kinetic model for this scheme is presentedelsewhere [47,48].

The results of computer modeling made in order tocheck the applicability of the proposed models to theobserved kinetics of methanol synthesis are illustrated intable 2 obtained from data [2].

Figure 4. Kinetic (a) and characteristic (b) curves of CO substitution

by water from the gas phase at 250 8C over industrial catalyst 51–2: 1,

fresh catalyst; 2, catalyst deactivated in methanol synthesis.


Simultaneous progress of the WGSR and methanolsynthesis was taken into consideration in these calcula-tions. Gas mixtures of quite different compositions, i.e.one with CO2 in excess and one with CO in excess, wereused. In this manner the applicability of the theoreticalkinetic model can be tested to the full. As seen from table2, for the mixtures with high CO2 content both kineticdescriptions match the experiment well; at high contacttimes scheme 2 matches better. When the synthesis isperformed from mixtures enriched with CO, conjugationof WGSR and methanol synthesis reveals itself moreevidently. It is seen that in the case of CO-enriched

mixtures scheme 1 cannot provide an effective kineticdescription. Scheme 2 again appears as the morereasonable one. It can be concluded that a descriptionconsidering the full scheme 2 would be important notonly for unusual cases of anomalous kinetics, but also forthe cases of practical importance. The applicability ofthis model is considered in detail elsewhere [48].

Summarizing the description above, it can be con-cluded that methanol synthesis studies allow one to buildthe scheme of the reaction mechanism and, on this basis,the theoretical kinetic model, which matches experimen-tal results well. It is shown that the mechanism scheme ofmethanol synthesis over Cu-based catalysts is ratherspecific. The strongly (‘‘irreversibly’’) chemisorbed spe-cies play an important role in this mechanism, givingadditional non-linearities in the reaction kinetic model.Specific properties of this type of heterogeneous catalyticreaction are considered elsewhere [3].

3. Methanol decomposition

3.1. Comments on reactions occurring

Methanol decomposition is of importance due to theproblem of hydrogen production, including the use inlow-temperature fuel cells. Due to the extreme sensitiv-ity of fuel cells to CO poisoning it is very important toresearch the reaction network and to use it in order tocontrol process selectivity.

The main products of methanol decomposition overCu-containing catalysts in the temperature range 150–250 8C are methyl formate (MF), CO, CO2 and H2 [49].Methanol decomposition dynamics were studied [50] by

Figure 5. Kinetics of methanol synthesis (1, 2) and WGSR (3, 4) at

240 8C under a pressure of 5MPa over industrial catalyst 51–2 (the

curves were calculated from kinetic model, the points correspond to

experiment). Feed composition (vol%): 1; 4; CO, 3:6; CO2; 21:8; H2,

74:3; N2; 0:3; 2; 3, CO; 18:3; CO2; 3:2; H2; 77; N2; 1:5.

Table 2

Computer simulation of methanol synthesis accompanied by WGSR using theoretical kinetic

models based on schemes 1 and 2 (industrial catalyst 51–2, 240 8C, 5.2MPa)

Contact time, sPCH3OH, atm PH2O, atm

ExperimentCalculated by

ExperimentCalculated by

Scheme 1 Scheme 2 Scheme 1 Scheme 2

Feed mixture in vol%: CO2, 23.5; CO, 0.1; H2 76.4

0.015 0.25 0.25 0.22 0.40 0.38 0.40

0.034 0.38 0.38 0.36 0.59 0.57 0.65

0.069 0.52 0.54 0.51 0.90 0.81 0.90

0.127 0.64 0.72 0.67 1.22 1.09 1.18

0.237 0.86 0.99 0.88 1.53 1.47 1.51

Feed mixture in vol%: CO2, 3.3; CO, 18.2; H2 77.9; N2, 0.6

0.020 0.59 0.45 0.63 0.16 0.19 0.15

0.029 0.78 0.60 0.88 0.17 0.19 0.16

0.057 1.56 1.08 1.56 0.19 0.20 0.17

0.126 3.06 2.20 2.84 0.20 0.21 0.19

0.240 4.39 3.89 4.24 0.21 0.23 0.22

0.428 5.44 6.21 5.73 0.24 0.26 0.25

0.457 5.95 6.52 5.91 0.24 0.26 0.25


sequential injections of measured amounts of methanolonto a Cu=ZnO=Al2O3 catalyst reduced by hydrogen.Decomposition products in the gas phase were deter-mined by GC.

Strongly adsorbed water, left on the surface afterreduction, was consumed in the reaction with methanolaffecting the composition of products. Figure 6 illus-trates the dynamics of product accumulation in the gasphase at 200 8C (in two sequential methanol injections)and at 300 8C [50]. As seen from the figure, the dynamicsof MF and CO accumulation reveal the classic illustra-tion for the case of the intermediate and the end productof consecutive reaction correspondingly. Consideringthat the initial rate of CO formation is close to zero(figure 6), the conclusion can be made that CO formsover the Cu-based catalyst as the result of the followingreactions:

2CH3OH! CH3OCHO þ 2H2 ð12Þ

CH3OCHO! 2COþ 2H2: ð13Þ

The second important result of the described experi-ment (figure 6) is that these reactions occur intensivelyonly after removal of bulk surface water by the reactionwith methanol:

ZH2Oþ CH3OH! ZCO2 þ 3H2: ð14Þ

Combination of reactions (14) and ð�10Þ gives thereverse reaction of methanol synthesis. Taking intoaccount that water adsorbs strongly on the active sites ofa Cu-based catalyst, it can be concluded that in thepresence of water in the gas phase the occurrence ofreactions (12) and (13) is, at least, of small probability.

Because the appearance of CO in the products ofmethanol decomposition over Cu-based catalysts iscaused by reaction (13), the features of this reactionare important for controlling the CO content inmethanol decomposition at moderate temperatures.

3.2. The patterns of methyl formate (MF) formation anddecomposition

The thermodynamics of methanol dehydrogenationinto MF (reaction (13)) is interpreted in figure 7.Although MF equilibrium yield increases along withthe temperature, the rate of MF decomposition into COand H2 increases faster than that of MF formation,resulting in low selectivity of methanol dehydrogenationinto MF at temperatures higher than 200 8C.

Kinetics of MF hydrogenation into methanol (areverse reaction of (12)) and MF decomposition into COand H2 were studied in order to obtain information onthe mechanism of methanol decomposition [36,52].Figure 8 illustrates the contact time dependence of MFconversion into CO and H2 by reaction (13) in a flowreactor at 167 8C, under atmospheric pressure and lowMF concentrations (up to 1%) in helium flow. Underthese conditions MF transforms with high selectivity

Figure 6. Dynamics of product accumulation in the gas phase in

methanol decomposition over Cu–ZnO–Al2O3 catalyst: (a) at 200 8C in

the first (open points) and the second (filled points) methanol

injections; (b) at 300 8C (methanol initial pressure 50 torr): 1, CO; 2,

MF [50].Figure 7. Equilibrium constants for MF synthesis (1) and decomposi-

tion (2), as calculated from data [51].


into CO and H2 as the only products. As seen from thefigure, points corresponding to differing (three-fold)values of the MF initial concentration belong to thesame kinetic curve. Thus, at low MF concentrations itsconversion in reaction (13) does not depend onconcentration, and it can be concluded that the reactionorder on MF is 1.

As shown in [36], the use of hydrogen instead ofhelium as carrier gas under the same conditions resultsin simultaneous MF decomposition and MF hydro-genation into methanol—up to a certain point, depend-ing on MF initial concentration. However, at higher MFconcentration (5.4 vol%), MF conversion was 46% andno MF decomposition into CO=H2 mixture wasdetected. The only product of MF conversion wasmethanol. The MF hydrogenation order on MF wasabout 1. This value did not change at high MFconcentrations, in contrast with MF decomposition.The MF hydrogenation order on hydrogen was alsoclose to 1.

The conclusion can be made from these data that inthe first step of MF hydrogenation a hydrogen moleculefrom the gas phase joins adsorbed MF, and this step isthe rate-limiting one.

These phenomena—the change in MF decompositionkinetics, i.e. suppression of MF decomposition causedby an increase of MF concentration, while MFhydrogenation kinetics remains the same—can beinterpreted in only one way: MF decomposition requiresan additional free center.

This suppression effect of MF decomposition can be

practically applied for selective synthesis of MF bymethanol dehydrogenation. Some of the resultsobtained in [52] are given in table 3.

These data show that even at relatively hightemperature, 240 8C, which is favorable to MF decom-position, under moderately high initial partial pressuresof methanol it is possible to achieve selectivity to MF ashigh as 95%, the conversion being almost at equilibrium(x=x1 ¼ 0:99).

3.3. Reactions proceeding in the course of methanoldecomposition into CO and H2 over Cu-containingcatalysts

Some additional information can be deduced fromthe data above. In particular, the results of MFhydrogenation into methanol allow one to concludethat the reverse reaction, methanol dehydrogenation,has at least two consecutive steps of hydrogen releaseinto the gas phase, the second step being rate-limiting.

2CH3OHþ 2Z$ IðþH2Þmacro step

! Z2CH3OCHOðþH2Þ; ð15Þ

where I is a relatively stable intermediate, which hasbeen proposed [53] as a surface compound with semi-acetal structure.

Because MF strongly adsorbs on the active site, itsspontaneous desorption during the time of reaction doesnot occur. MF exchange with a reacting methanoloccurs by an adsorption substitution reaction:

Z2CH3OCHO

þ CH3OH ��! ZCH3OH ��!þCH3OHþZ

IþH2: ð16Þ

Combination of reaction (16) and the second step ofreaction (15) corresponds to the kinetic scheme of MFstationary synthesis.

MF decomposition can be written roughly as

Z2CH3OCHO þ Z! 3Zþ 2COþ 2H2: ð17Þ

This reaction is surely more complicated. So far, theauthors have no solid information on its detailedmechanism.

Combining these proven scheme elements of MFformation and decomposition, scheme 3 can be drawn.

Figure 8. Contact time dependence of MF conversion into CO andH2

in a flow reactor at 167 8C (feed MF/He).

Table 3

Highly selective MF synthesis by methanol dehydrogenation over a Cu-based catalyst

T, 8CFeed rate,

l/g/h

Pressure,

P, atm

Pomethanol,

atm

Conversion

x, %

x into

MF,% x/x1

Selectivity,

S, %

190 21.7 8.5 1.4 21.5 20.7 73 96

240 20.3 8.5 1.3 44.1 42.0 99 95


As noted above, the right half of this scheme (MFdecomposition) can be realized only in the presence of afree adjacent center, i.e. under low partial pressure ofmethanol and MF. The whole scheme is effective only inthe absence of water in the reaction system. When wateris present, methanol transforms over Cu-based catalystsonly in terms of reaction (15), the reverse synthesisreaction.

The last conclusion does not lend credence to themethanol steam reforming scheme over Cu/Zn/Zr/Aland Cu/Zn/Zr oxide catalysts proposed by Takahashi etal. [54] and then by Breen and Ross [55]. This schemeconsists of the following reactions:

2CH3OH! CH3OCHOþ 2H2 ð12Þ

CH3OCHO þH2O! HCOOHþ CH3OH ð18Þ

HCOOH! CO2 þH2: ð19Þ

As follows from the discussion above, over Cu-basedcatalysts in the presence of water only two reactionsoccur: the reverse methanol synthesis

CH3OHþH2O! CO2 þ 3H2 ð3Þ

and reverse WGSR

CO2 þH2! COþH2O: ð�4Þ

Accordingly, we have never found MF or formic acidamong the products of methanol steam reforming overCu-based catalysts.

3.4. Kinetics of methanol steam reforming

Methanol steam reforming kinetics has been reviewedrecently [56,57]. In contrast to the approach of thoseworks, our approach is based on the reaction mechan-ism scheme proved by various independent methods.Other authors [56,57] also defend the proposition thatmethanol steam reforming involves the participation oftwo types of active centers. We believe that all the activecenters are equal in their activity, as directly proved by

experiments on the dynamics of transformations ofsurface species (see above).

As noted before, at moderate temperatures over Cu-based catalysts methanol transforms in the presence ofwater only by reaction (3), the reverse of its synthesis.The products, CO2 and H2; enter reverse WGSR inturn. Because the controlling step for direct and reversereactions is the same, the kinetic model of methanolsteam reforming can be made on the basis of scheme 2.Considering this scheme, one should take into accountthat intermediate ZCO2:H2 is in equilibrium withreactants (methanol and water), and that the lasthydrogen molecule release is the slowest step. Thus,under the conditions of methanol steam reforming, theformate species ZCO2:H2 should accumulate over activecenters. This is confirmed by various spectroscopicobservations (e.g., [57]).

The kinetic model of methanol steam reforming wasestablished based on the approach described aboveusing scheme 2. Calculations for our own kinetic datafor high pressures (0.5 and 2.0MPa) have shown a goodagreement between experimental and calculated data onthe rate of methanol steam reforming. An additionaltesting of the model was made using experimental dataobtained under atmospheric pressure by Kuznetsov etal. [58]. We have chosen their data to check our kineticmodel for two reasons. Firstly, the conditions areextremely different from that used in our studies. Forexample, the values of partial pressures of the reactantswere of the order 0.1 atm. Secondly, these data wereobtained in a circulating flow installation, where thereaction rate can be determined with the most precision.That is why the calculation of the methanol steamreforming kinetics using previous data [58] appears to bethe most radical way to check the theoretical kineticmodel.

The parameters of the kinetic model of methanolsteam reforming were found from the kinetics of thereverse reaction, methanol synthesis. If scheme 2 iscorrect, the proposed model should describe the kineticsof both methanol synthesis and methanol steamreforming adequately when the same parameter valuesare used.

Figure 9 demonstrates the dependence of methanolsynthesis rate on hydrogen partial pressure calculatedfrom previous data [58]. It is seen that the methanolsynthesis order on hydrogen under atmospheric pressureis also close to 1.

Calculations of methanol steam reforming kineticsunder atmospheric pressure are compared with previousresults [58] in figures 10 and 11. The water concentrationdependence of reaction rate (figure 10) and hydrogenpressure dependence of reaction rate at varying metha-nol concentrations (figure 11) are depicted correspond-ingly. It is seen that the calculated and experimentalvalues of reaction rate are close. Thus, the model reflectswell the influence of hydrogen, methanol and water

Scheme 3. MF formation and decomposition over Cu-containing

catalysts.


concentrations on the methanol steam reforming rateunder atmospheric pressure.

It seems that this method, the description ofmethanol steam reforming on the basis of data and themodel of methanol synthesis, can be considered valid atleast for Cu-based catalysts.

4. Conclusions

Various kinetic approaches were used to study thefundamentals of methanol synthesis and decomposition.Combinations of tracer technique, TPD after variouschemical pre-treatment procedures, data on dynamics ofsurface species reactions with gas-phase reactants,together with steady-state kinetics were employed. Onthis basis it became possible to build the kineticmechanistic schemes of methanol synthesis, of methanoldehydrogenation into methyl formate, of methanoldecomposition into carbon monoxide and hydrogen,and to determine the rate-limiting steps of these reactions.The approach, where some information on the reactionmechanism and kinetics can be obtained from studies ofthe reverse reaction mechanism, proved highly useful. Inthis way important information was obtained on themechanism of methanol dehydrogenation into methylformate and on methanol steam reforming.

Kinetic models of methanol synthesis and steamreforming were built and proved by comparison withexperiments. Highly selective synthesis of methyl for-mate was performed. It is shown that over Cu-basedcatalysts at moderate temperatures (up to 300 8C)methanol decomposition into a CO=H2 mixture pro-ceeds through methyl formate as an intermediateproduct. This pathway is realized when there is nowater in the reaction system. In the presence of waterthis reaction is suppressed and the reaction, which is thereverse to methanol synthesis from CO2 and water,proceeds instead, accompanied by reverse WGSR.

All the reactions studied are characterized by theformation of strongly (‘‘irreversibly’’) adsorbed pro-ducts with characteristic time of desorption markedlyexceeding that of catalysis. That is why the exchangebetween the adsorbed species and molecules from thegas phase proceeds in such systems by reactions of a newtype, namely by adsorption substitution. Such amechanism of the catalytic process results in a newgeneration of reaction kinetic models. It was shown thatthese models adequately describe methanol synthesisand methanol steam reforming kinetics.

Acknowledgments

This work was supported by the Russian Foundationfor Basic Research (Project No. 98-03-32186), theRussian Foundation for Support of Scientific Schools(Project No. 00-15-97389), and NEDO (Project No.

Figure 9. Hydrogen partial pressure dependence of methanol synthesis

rate at 200 8C over industrial catalyst SNM-1 in experiments [58]. Feed

composition (vol%): CO, 30; CO2, 5; remainder H2 and N2: The

calculations were performed using a kinetic model based on scheme 2.

Figure 10. Water partial pressure dependence of methanol steam

reforming rate at 200 8C over industrial catalyst SNM-1 in experiments

[58]. Methanol partial pressure varied from 0.44 to 0.66 kPa. The

calculations were performed using a kinetic model based on scheme 2.

Figure 11. Hydrogen partial pressure dependence of methanol steam

reforming rate at 200 8C over industrial catalyst SNM-1 in experiments

[58]. Partial pressure were: water, � 2kPa; methanol, from 0.067 to

0.59 kPa. The calculations were performed using a kinetic model based

on scheme 2.


98EA4). We would like to thank also I.N. Zavalishin foruseful discussions.

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Documents

Rozovskii Et Al. (2003) - Fundamentals of Methanol Synthesis and Decomposition