The influence of high methane concentrations on the stability of catalytic flammable-gas sensing elements

Sensors

and Actuators,

5 (1984)

229 - 238

229

THE INFLUENCE OF HIGH METHANE CONCENTRATIONS ON THE STABILITY OF CATALYTIC FLAMMABLE-GAS SENSING ELEMENTS

S J GENTRYand

P T WALSH

Health and Safety Executwe, Sheffield S3 7HQ (U K ) (Received

November

9, 1983,

Research

and Laboratory

m revlsed form January

Services 19, 1984,

Dtvrslon, accepted

Red January

Hall, 24,

1984)

Abstract

The influence of the form of the palladium catalyst used m catalytic flammable-gas sensing elements on their stability followmg exposure to high methane concentrations has been investigated Carbon deposltlon was found to have an adverse effect on the response of some types of element However, improvement m stabhty to coking effects can be achieved either by depontmg palladium on a high surface-area support or by using a copreclpltatlon of palladium and thorla For a senes of Pd + ThOz elements having different Pd ThO, ratios, the apparent hydrogen solublhty was determined by a temperature-programmed technique The solublllty of hydrogen m palladium 1s known to increase with decreasmg dispersion Good correlation was obtained between the dispersion inferred from the solublhty data and the rate of coking Thus the role of thorla as a coke prevention agent 1s to decrease the palladium particle size, resulting m a lower rate of methane decomposltlon

Introduction

Catalytic sensmg elements are widely used m instruments for the detection of flammable gases m air at concentrations approaching the lower explosive hmlt [l] These devices are small calonmeters (1 - 2 mm diameter) m which a catalyst 1s m contact with a temperature sensor, generally a renstance thermometer They are electrically heated using the resistance thermometer to about 550 “C, at which temperature catalytic oxldatlon of the flammable gas readily occurs The rise m temperature, with the correspondmg increase m the resistance of the thermometer produced by reaction on the catalyst, 1s then measured by mcorporatmg the element m a Wheatstone blndge network where the potential difference across the bridge forms the output of the device [ 21 For small changes m temperature, this output 1s 0250-6874/84/$3

00

0 Elsevler Sequola/Prmted

m The Netherlands

230

proportional to the rate of oxldatlon, which m turn 1s a function of the flammable gas concentration [ 21 The most commonly used catalyst m sensmg elements for the detection of flammable gas 1s palladium One such element [3], the ‘pelhstor’, consists of a palladium and thorla catalyst deposlted on the surface of an @-alumma bead encapsulatmg a platmum toll resrstance element On elements of this type, oxldatlon of fuel at high concentrations may be accompanied by decomposltlon of the fuel, resultmg m excessive carbon deposltlon (cokmg) Culhs et al [4] found that for the reactlon of methane over palladrum-sponge catalysts, carbon was deposrted even at low concentratlon (6% CHJalr), the cokmg rate mcreasmg as the methane concentration increased to 23% CH4/alr The cokmg of the catalyst may have a marked effect on the operation of the sensing element because (a) surface carbon may poison the oxldatlon reaction and (b) large deposits of carbon change the physical size and morphology of the element, and to a lesser degree Its emlsslvlty, which alters the rate of dlsslpatlon of heat from the calorimeter and thus changes the sensor output Even when the carbon IS subsequently burnt off, permanent damage to the sensor may have occurred, this IS mdlcated by a change m the zero-level of the sensor [ 51 (the output when the sensor IS exposed to clean an) Elements which mcorporate thorla m the catalyst layer, for example the ‘pelhstor’, are known to be slgmflcantly less susceptible to coking The present work mvestlgates the influence of the form of the palladlum catalyst on the stability of the sensmg element followmg exposure to high methane concentrations

ExperImental Prepara tlon of elements Four types of sensmg element were prepared (a) nPd + ThO,/a-Al,O, Palladium and thorla were deposited as a surface layer on to a bead of low porosity a-Al,O, [ 31 The bead (1 mm diameter) was formed on a co11 of platmum wire by deposltlon of a saturated solution of alummlum nitrate which was decomposed to a-Al,O, by electrical heating of the co11 Palladium and thorla were then deposlted from a mixed solution of ammomum chloropalladlte and thorium nitrate and subsequently decomposed A semes of beads was prepared m which the mole percent (n) of palladmm m the Pd + ThO, catalyst was varied between 6 and 100% whilst keeping the total amount of palladmm approximately constant (b) 13Pd + Al,O&-A1203 Beads were prepared as above except that the thorium nitrate solution was replaced by alummmm nitrate solution The fraction of palladium m the catalyst was 13 mol% (c) Pd/glass Palladium from 0 45 M ammomum chloropalladrte was deposited on to a bead of glass (type GSlO, m p 1210 “C, 1 mm diameter) encapsulatmg the platinum co11

231

(d) Pd/y-A1,03, 28Pd + ThOJy-A1,03 A porous, spherical bead of dlspersed catalyst (1 6 mm diameter) encapsulatmg the platinum co11 was formed by decomposmg a slurry of metal salt solution and fine-particulate y-A1203 [6] Solutions used were 0 45 M ammonmm chloropalladlte (Pd/ y-A1,03) and 0 45 M ammomum chloropalladlte plus 1 11 M thorium nitrate solution (28Pd + ThO&A1,03) Thus a series of elements was prepared havmg a range of palladium dlsperslons on different supports, with and without thorla Prior to operation, each sensor was pretreated by heatmg m air to approximately 600 “C, then a 12% CH4 + au- mixture (a net reducmg atmosphere) was passed over the sensor for 2 mm, the heat of oxldatlon causing the element temperature to rise to approxunately 800 “C! Finally, the sensors were returned to air for a further 2 mm This successive oxldatlonreduction-oxldatlon treatment produced sensing elements havmg stable catalytic activity Measurement

systems

The sensing elements were incorporated mto two forms of measurement clrcultry The first method, the non-isothermal or ‘out-of-balance voltage’ technique [ 23 1s commonly used m flammable-gas detection mstruments The method, outlined m the Introduction, requires a detecting element and a compensatmg element m series to form one half of a Wheatstone bridge network The out-of-balance voltage across the bridge constitutes the sensor output This technique was used to illustrate the effects of high methane concentratrons on the stability and sensltlvlty of sensmg elements used m field instruments In the second method (the ‘isothermal’ method), the temperature of the element 1s held constant dunng reactlon [7] Here the detector and a fixed resistor form one half of the Wheatstone bmdge A feedback curcult reduces the electrical power across the bridge to compensate for the chemical power generated during the oxldatlon The signal 1s derived from the difference m electrical power of the calonmeter m the non-reactmg (air) and reacting (fuel + a.~) condltlons and ISproportional to the rate of reaction on the element This method was used to obtain kmetlc data, at constant temperature, on the rate of carbon deposltlon on the nPd + ThOJcx-Al,03 elements A microcomputer was used to record and process the data Gas flow system The sensing elements were mounted m reactlon chambers (volume 0 4 ml) m the form of closed side-arms of a conventional gas flow system (total volume 5 ml) operated at atmosphenc pressure Gas muctures were composed from cylmders (BOG Ltd ) of 100% CH4, 1% CH4 + au and av The appropriate concentration of the mixture was obtwned using calibrated rotameters The gases were purified by passage through activated charcoal and magnesium perchlorate The test gases were supphed to the elements at a rate of 100 ml/mm through a set of valves controlled by the microcomputer Five elements could be tested simultaneously

232

Element characterzzatlon technzques Elements were exammed by X-ray diffraction (xrd) analysis and temperature-programmed reduction (tpr) Tpr has previously been used successfully to characterize various reducible materials [ 81 Furthermore, for catalysts contammg palladium metal where large amounts of hydrogen can be absorbed, mformatlon can be obtained regarding the metallic state m addition to the oxidized state Thus tpr has been applied here for the characterization of mdlvldual sensing elements where the amount of palladium 1s of the order of 100 pg The tpr apparatus has been described previously [ 91 A linear heating rate of 14 “C/mm was used from -50 to 200 “C The reducmg gas mixture was 10% HZ/N, (BOC Certified Grade) at a flow rate of 20 ml/ mm

Results Carbon depose tlon The sensmg elements were operated m the out-of-balance mode at a standard voltage of 1 2 V across the detector, correspondmg to a temperature of about 530 “C They were first exposed to an then 1% CH, + au test gas to determine their zero level and sensltlvlty respectively The elements were then exposed to 40% CH4 + air for one hour, followed by au for 30 mm, after which the zero level and sensltlvlty were redetermined The shifts are expressed as the difference between the final and mltlal values Several elements of each type were tested and typical results are shown m Table 1 A second batch of the same types of elements was treated m an identical way to the above except that followmg exposure to 40% CH4 + air, the elements were cooled to room temperature m that atmosphere and removed for analysis by xrd and tpr Palladium was present m the metallic form only TABLE Effect

1 of 40%

CH4 + air on zero level and sensltlvlty

Element

Initial sensitlvxty to 1% CH4 + air

of various elements Zero shift

Sensltlvlty

(mV)

(mv)

shift

(mv) 28Pd + ThO&-A1203 65Pd + ThO&-A1203 Pd/a-A1,03 13Pd + A120&-A1203 Pdjglass 28Pd + Th02/y-A1,03 (11 wt % Pd loadmg) Pd/y-AlzO, (11 wt % Pd loadmg)

27 30 30 20 20 25 30

3 2 12 -3 64 -12 3 1 Catalyst detached from support 2 1 5

1

233

Carbon deposltlon on elements 65Pd + Th02/ar-A1203, Pd/cu-Alz03 and Pd/glass was rapid and severe, the elements virtually doubled m size due to the growth of carbonaceous layers durmg exposure to methane The carbon was analysed by xrd and found to be graphite Upon exposure to air, some of the carbon burnt away and physical damage to the elements was observed Parts of the elements became detached from the platinum tolls, causing large posltlve shifts m the zero level and the decrease m sensitivity shown m Table 1 The compensatmg element, identical to the detecting element except that it contained no catalyst, did not coke-up during the exposure to methane The effect of thona on the rate of carbon deposltlon was then mvestlgated on the nPd + ThO,jcu-A1,03 elements A series of elements correspondmg to n = 6, 11, 28, 65 and 100 was exposed to flowmg 100% CH4 The sensors were operated m the isothermal mode and the power required to maintain the element at a fixed temperature of 500 “C! was monitored In the absence of cokmg this power reaches a constant value wlthm 1 mm of swltchmg from ar to 100% CH4 When there 1s decomposltlon of methane, resulting m carbon deposltlon, there IS an increase m size and emlsslvlty of the element Thus the power consumption at constant temperature mcreases with exposure time and 1s mdlcatlve of the rate of coking At least three elements of each type were tested and the times for a 10% increase m power ( tl,) noted Figure 1 shows the results for typlcal elements of each type and the results are summarized m Table 2 together with

.

.

11Pd

/

-.‘d

I 50

exposure time

ld

1

set

Fig 1 Effect of exposure to 100% CH4 on power consumption elements Temperature 500 “C

of nPd + Th0&~-A1~0~

234 TABLE Effect

2 of Pd ThOz ratlo on time taken to mcrease power

Element

Average (s)

t 10

Standard devlatlon

in 100% CH4 by 10% Sample number

(s)

Sensitivity to 1% CH4 + anat 400 “C (mw)

6Pd + ThOz/aA1203 1lPd + ThO&-Al,03 28Pd + ThO#-A1203 65Pd + ThOJa-A1203 1 OOPd/a-A120,

> 105 8 x lo4 6500 2700 187

2 x 104 2000 1000 42

5 6 5 5 3

8 12 15 12 17

the mltlal sensltlvlty of the elements to 1% CH,, + an- at 400 “C This temperature was chosen because dlffuslonal hmltatlons on senslt1vlt.y are slgmflcantly less severe than at temperatures above 500 “C Thus the relative sensltlvltles at lower temperatures are more mdlcatlve of the relative actlvltles of the catalysts In order to ensure that elements were of a slmllar size, those which had a power consumption between 350 and 390 mW (m air at 500 “C), corresponding to 500 - 550 mW m 100% CH4, were chosen Followmg the rapid rise m power consumption on changmg from air to 100% CH4, it can be seen from Fig 1 that there FS a small decrease m consumption for all elements except the 100% Pd This decrease m power consumption IS due to the shght mcrease m temperature of the reaction chamber arising from the increased thermal conductlvlty of the gas on changing from au to 100% CH, The absolute value of this decrease m power consumption and the rate of attainment of equlhbrlum depend on the exact thermal properties of the chambers and the size and posltlon of the mdlvldual elements wlthm them Consequently some varlatlon 1s observed m the results For the 65, 28 and 11% Pd elements, the power then gradually increases whereas no rise m power 1s observed for the 6% Pd element For the 100% Pd elements the cokmg process 1s so rapid that the mltlal decrease m power 1s not observed On visual exammatlon after time t10 had elapsed, the 100, 65, 28 and 11% Pd elements all showed large deposits of carbon whereas the 6% elements examined after 28 hours showed only small and very fme deposits, presumably too small to affect their power consumption m 100% CH4 Table 2 shows that, despite the large scatter m flO, decreasing the Pd ThOz ratro increases t 1o It was found that mcreasmg the temperature to 570 “C!decreased tl, for all elements except 100% Pd, where no slgmflcant change m t,, occurred

Characterlza

tlon by tpr

An ldentlcal set of elements to those described m Table 2 was analysed by tpr The elements were pretreated as previously described then heated m

235

heatlna 14Timln

cooling 14Timtn I

,,,I,,,,,,,,,,,,,,,,,,, 0

50 temperature

Fig 2 Typical temperature

100 T

150

2

programme profiles of nPd + ThO&-AlzOJ

element

air at 600 “C for 15 hours m the tpr apparatus to ensure complete oxldatlon of the palladium to PdO Each element was then cooled to -50 “C m air and exposed to the reducing gas The temperature was increased at 14 “C/mm to 200 “C, then allowed to cool at the same rate, the hydrogen concentration being monitored throughout A typical profile 1s shown m Fig 2 There ISfirst an uptake of hydrogen with a peak maxnnum between 65 and 85 “C, followed by desorptlon with a peak maximum between 110 and 125 “C As the temperature 1s then decreased there 1s a second hydrogen uptake at 75 - 60 “C The area of the first uptake (U,) 1s a measure of the total amount of hydrogen consumed by the reduction of PdO and sorbed on to the element The latter portion subsequently desorbs (area D) Consequently the amount of PdO present can be calculated by subtraction of the area of the desorptlon peak from that of the mltlal uptake, (U, - D) Using this method, good agreement was obtained with the amount of palladium determmed from the measurements of the weight of palladium deposited on the element [lo] The apparent solublllty of dlhydrogen m palladium was calculated by dlvldmg the area of the second uptake peak (U,) by ( U1 - D) Figure 3 shows the relatlonshlp obtained for the apparent solublllty of dlhydrogen (S) as a function of the percentage of palladium m the Pd + ThOz catalyst The error bars for S represent the scatter m a number of determmatlons It 1s apparent from the Figure that the apparent solublhty increases monotonically with the fraction of palladium m the catalyst, the rise being most rapid over the range 0 - 19% Pd Also shown m Fig 3 1s the coking time ( tlo), which decreases as the fraction of palladium increases

Fig 3 Effect of palladium concentration dlhydrogen solubIhty (0) and t10 (0)

m nPd + ThO+

AlzO,

elements

on apparent

Dlscusslon The results gven m Table 1 Illustrate the destructive effect of high methane concentrations on some palladium catalysts Large deviations m the zero level and sensltlvlty are confined to those elements containing a large proportion of palladmm m the catalyst, either deposited on a-Al,O, or glass The large posltlve shifts m zero level are rndlcatlve of a loss of catalyst and dlsmtegratlon of the alumma bead [ 51, whilst the decrease m sensltlvlty 1s due to a loss of catalyst This effect 1s accentuated m the Pd/ glass element where the catalyst becomes completely detached from the glass bead Improvement m the stablhty can be achieved by either dispersing the palladium throughout high surface-area ~-A1~0~ or, to even better effect, by mcorporatmg palladium m either a-Al,O, or ThOz The two competmg reactlons occurrmg when CH4 + air mixtures are passed over the elements are methane oxldatlon (AH = -803 kJ mol-I) and pyrolysis (AH = +75 kJ mol-I) Culhs et al [4] showed that even for CH4/ O2 ratios as low as 0 3 (equivalent to 6% CH4 + au), pyrolysis can still occur over palladmm Consldermg the effect of Pd ThO, ratlo on the resistance to coking, rt can be seen from Table 2 that increasing this ratlo increases the rate of coking whilst also mltlally mcreasmg the rate of oxldatlon Previous studies [ 11, 121, particularly on platinum reformmg catalysts, have shown the dependency of carbon deposltlon on the dlsperslon of the catalyst In the maJorlty of studies, an increase m the particle size of the

237

metal produces greater carbon deposltlon This 1s attnbuted to the structure sensltlvlty of the carbon deposltlon reactlon [ 13 3 Carbon formation proceeds more rapidly on smooth crystal faces, where metal atoms have high coordination, than on low coordmatlon sites found on edges and corners The addition of ThOz to the catalyst mixture may have a more dn-ect effect on the growth of carbon deposits than by solely mcreasmg the dlsperslon of palladium Baker and Chludzmskl 1141 mvestlgated carbon deposetlon on Nl-Fe surfaces and classlfled the roles of various oxide additives by then- method of mhlbltlon of carbon filament growth Small additions (- 1%) of oxide can strongly mhlblt the formatlon of carbon by reducing carbon solublhty and dlffuslvlty through the metal However, this mechamsm does not appear to be dominant for the Pd + ThO:, system investigated m the present work The rate of carbon deposltlon IS not strongly affected by small additions of ThOz (Fig 3) and there was no systematic shift m the tpr peak maximum (mdlcatlve of Pd-Th02 mteractlon) on the addition of Th02 to 100 Pd/a-A1,03 elements Thus the pnmary effect of ThOz may be to decrease the size of the palladium particles and thereby to prevent coking To test this hypothesis, the tpr method was adapted to provide mformatlon on the dlsperslon of palladium The solublhty of hydrogen m palladium, as determmed by a static, volumetric method, has been shown [ 151 to decrease with increasing dlsperslon The tpr data m Fig 3 show that the apparent solublhty measured by this technique increases with increasing Pd Th02 ratio The method therefore provides further evidence for the decrease m palladium dispersion as the Pd ThOa ratio increases However, It should be pointed out that the method used here underestimates the solublhty The apparent solublhty IS calculated from U,/( U1 -II), but because of the dynamic nature of the technique and the slow rate of absorption compared to the rate of reduction, UZ may be underestimated Furthermore, U1 IS the sum of uptakes due to reduction, absorption and adsorption Absorbed hydrogen can be easily removed and IS measured as D, whereas strongly adsorbed hydrogen requires higher temperatures to desorb [ 151 Therefore U, - D should be reduced by the amount of hydrogen strongly adsorbed Increasing the temperature to 600 “C at 20 “C/mm did not produce any slgnlhcant peaks due to adsorbed hydrogen Thus this correction 1s very small but would be expected to increase at high dispersion, the corrected hydrogen solublhty m relation to the mole fraction of palladium would fall less steeply than indicated over the 19 - 6% Pd region Nevertheless the techmque does show quahtatlvely the differences m dlsperslon m the semes of nPd + Th02 catalysts as reflected by their different hydrogen solubllltles There IS good correlation between palladium dlsperslon, as reflected by hydrogen solublhty, and the rate of cokmg shown in Fig 3 Therefore the role of ThOz as a coke prevention agent m Pd + ThOz catalytic sensing elements 1s to decrease the size of palladmm particles, resulting m a lower rate of methane decomposltlon

238

References 1 J G Forth, A Jones and T A Jones, The prmclples of the detectlon of flammable atmospheres by catalytic devices, Combust Flame, 21 (1973) 303 2 J G Frrth, Catalytic oxldatlon of methane on palladmm-gold alloys, Trans Faraday Sot , 62 (1966) 2566 3 A R Baker, Electrically heatable filaments, U K Patent 892530 (1962) 4 C F Culhs, D E Keene and D L Trlmm, Pulse flow reactor studies of the oxldatlon of methane over palladmm catalysts, Trans Faraday Sot , 67 (1971) 864 5 J G Forth and H B Holland, Stablhty of a porous catalyst subJected to carbon deposltlon, J Appl Chem Btotechnol, 21 (1971) 139 6 D W Dabill, S J Gentry, N W Hurst, A Jones and P T Walsh, Catalytic combustable gas sensors, UK Patent 2083630 (1982) 7 J G Frrth, Catalytic oxldatlon of methanol over platinum, Trans Faraday Sot , 67 (1971) 212 8 N W Hurst, S J Gentry, A Jones and B D McNrcol, Temperature programmed reduction, Cut Rev Scz Eng , 24 (1982) 233 9 S J Gentry and P T Walsh, Influence of silica and alumma supports on the temperature-programmed reduction of copper(I1) oxide, J Chem Sot , Faraday I, 78 (1982) 1515 10 S J Gentry and P T Walsh, Thloreslstant flammable gas sensing elements, m G Poncelet, P Grange and P A Jacobs (eds ), Preparcztzon of Catalysts III, Elsevler Amsterdam, 1983, p 203 11 P P Lankhorst, H C de Jongste and V Ponec, Particle size and carbon deposltlon effects m the hexane reforming reactions, m B Delmon and G Froment (eds ), Catalyst Deact~~tzon, Elsevler, Amsterdam, 1980, p 43 12 J Barbler, P Marecot, N Martin, L Elassal and R Maurel, Selective polsonmg by m B Delmon and G Froment (eds ), Catalyst Deactzua coke formation on Pt/A1203, tzon, Elsevler, Amsterdam, 1980, p 53 13 G A Somoqal and D W Blakely, Mechamsm of catalysis of hydrocarbon reactlons by platinum surfaces, Nature, 258 (1975) 580 14 R T K Baker and J J Chludzmskl, Fllamentous carbon growth on nickel-iron surfaces The effect of various oxide additives, J Catal, 64 (1980) 464 15 M Boudart and H S Hwang, Solublhty of hydrogen m small palladmm particles, J Catai, 39 (1975) 44

Biographies Stephen J Gentry received the BSc degree m Chemistry m 1969 and the PhD m 1973 from the Umverslty of Nottingham He Jomed the Safety m Mines Research Estabhshment m 1973 and has smce worked on the development of gas sensors with particular interest m catalytic devices He IS at present Head of the Catalytic Sensor Section at the Occupational Medicine and Hyaene Laboratories of the Health and Safety Executive Dr Gentry IS a Chartered Chemist and a Member of the Royal Society of Chemistry Peter 7’ WaZsh received the BSc degree m Chemistry m 1973 and the PhD in 1978 from the Uruverslty of Kent at Canterbury He Joined the Health and Safety Executive m 1976 and has worked on spectroscopic, semiconductor and catalytic gas sensors He 1s currently a Senior Sclentlflc Officer m the Catalytic Sensors Section