Flow injection-thermal analysis-mass spectrometry: Application to studies of carbon gasification reactions

Carbon Vol. 35, No. 2, pp. 217-225, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008~6223/97 $17.00 + 0.00

Pergamon PII: SOOO8-6223(96)00152-2

FLOW INJECTION-THERMAL ANALYSIS-MASS SPECTROMETRY: APPLICATION TO STUDIES OF CARBON GASIFICATION REACTIONS J. M. JONES,+ W. A. T. ELLYATT, F. E. NDAJI and K. M. THOMAS* Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K. (Received 30 May 1996; accepted in revisedform

19 August 1996)

Abstract-Flow

injection combined with thermal analysis-mass spectrometry is a useful technique for studying gasification reactions. It provides a rapid, powerful method of separating the variables of a particular reaction, such as reactant concentration, reactant composition and temperature, studying their influence on the reaction products and differential thermal analysis. The present work illustrates the technique using the following examples: (1) the reaction of NO with carbon, and (2) the reaction of NH, and NO over carbon. In both cases, >99% isotopically pure 13C was used, so that N, and CO, and also N,O and CO, could be mass-resolved. The results also illustrate the application of the technique to the study of the effect of oxygen concentration and gasification temperature on the formation of N,O and N, during the NO-carbon reaction. The results show that the N,O:N, ratio increases with oxygen concentration while the N,O concentration decreases with increasing temperature. 0 1997 Elsevier Science Ltd. All rights reserved Key Words-A.

Carbon,

B. gasification,

combustion,

In the study of heterogeneous gas-solid reactions, some of the most useful mechanistic information is that obtained in situ, under reaction conditions. Spectroscopic techniques such as FTIR, Raman, and Mbssbauer emission spectroscopy have proved useful for in situ reaction studies, particularly in catalytic systems. This type of mechanistic information is also useful in the study of coal and carbon gasification reactions, including combustion. Mass spectrometry offers distinct advantages for the detection of reactant, intermediate and product species during gasphase and solid&gas heterogeneous reactions. These include very rapid analysis, low detection limits and, in principle, the simultaneous detection of all reaction species. The technique of thermogravimetric analysis-mass spectrometry has proved valuable in the study of pollutant formation during the combustion of coal and coal chars [l-3]. Direct probe sampling of the gases near to the surface of the sample has enabled the detection of low concentrations of reactive intermediates [4-61. Similarly, by sampling the evolved gases at the exhaust of the thermogravimetric analyser, i.e. at room temperature, the relative concentrations of evolved gases at near-equilibrium conditions can be determined. Concurrent with the analysis of reaction species, additional information is obtained associated with the utilisation of TGA,

University

analysis.

i.e. weight loss, temperature, and differential thermal analysis (DTA) In the study of reactions involving coal and carbon, analysis is not simple because of the presence of a number of different species with the same nominal mass. In particular, it is important to resolve N, and CO at m/z 28 and N,O and CO, at m/z 44. In this regard, the use of isotopically labelled reactants is feasible. Char from isotopically pure (> 99%) 13C has proved a valuable model to study the release of N,, N,O and NO during combustion of coal using this technique [7,8]. 13C labelling has also proved useful in the study of catalysis of steam gasification by BaCO, [9]. In this study Ba13C03 was used, and the decomposition of the material was linked to the catalytic gasification effect. Studies of this type involving isotopes are often limited by cost, and so techniques which provide the maximum amount of information from the minimum amount of materials are useful. One such approach has been the use of step-response experiments, in which the gas phase is switched between reactant and inert gas so that transient kinetics are followed. This type of experiment has been applied to the study of the CO,-carbon reaction, and the decay of the labelled species in the gas phase provided useful mechanistic information [lo].

1. INTRODUCTION

*Corresponding author. +Present address: Department

C. thermal

1.1 Reaction of NO with carbon The release of nitrogen oxides into the atmosphere as a result of coal combustion is a major environmental problem. The amount of NO formed during coal

of Fuel and Energy,

of Leeds, Leeds LS2 9JT, U.K. 217

218

J. M. JONESet al.

combustion can be reduced by utilising low-NO, burners in which char nitrogen is the major contributor to the formation of nitric oxide [ 111. One approach to studying the release of nitrogen oxides during combustion is to use carbons derived from organic precursors as models for coal chars [ 12,131. In such systems complicating factors such as heteroatoms, metal salts and minerals are eliminated. The NO-carbon reaction has received some attention [l&20]. Physisorption is seen to occur at low temperatures, while both reversible and irreversible chemisorption occurs at temperatures well above room temperature [l&19]. The major products of the irreversible process are N,, CO and CO,. However, in the presence of oxygen and at higher temperatures, significant amounts of NzO are formed. N,O has also been detected as a minor product during the reaction of NO with a potassium-promoted carbon at temperatures in the range 200&400°C [20]. In addition, N,O is a major product during the combustion of ammonia-treated model carbons [ 7,8]. There exists a change in mechanism of the NO-carbon reaction at temperatures above approximately 650°C [ 191. It has been proposed that in the combustion of ammonia-treated model carbons, nitrous oxide is formed by the reaction of C(N0) and C(N) surface species [8]. Clearly, the formation of NO, N, or N,O during the combustion of carbon or chars has a complex dependence upon temperature, rate of combustion, and heteroatom content and functionality. In this paper applications of the technique of flow injection-thermal analysis mass spectrometry for the investigation of heterogeneous reaction systems are explored. 1.2 Reduction of nitric oxide with ammonia One approach to controlling NO emissions during coal combustion is by reducing the NO in the flue gas by the reaction with ammonia, either in the presence (selective catalytic reduction, SCR) or in the absence (selective non-catalytic reduction, SNCR) of a catalyst. Traditionally, SCR utilises a metaloxide catalyst at temperatures of 350-450°C. There has been interest recently in the use of carbon catalysts for the low-temperature SCR of nitric oxide [21-241. Good activity has been observed for 0- or N-functionalized carbons at temperatures as low as 150°C. The ammonia-NO reaction is also important from another perspective, namely, that this is thought to be a source of NzO formation during coal combustion [25,26]. The technique presented here is a modification of thermogravimetric analysis mass spectrometry (TGMS), in which small amounts of reactants are injected at the entrance to the TGA furnace into a flow of inert gas which then passes over the sample. In this manner, the perturbation of one or more species in the reactant stream on the composition of the gas phase near to the sample surface can be followed by mass spectrometry. In the experimental

arrangement used in this study thermogravimetric analysis was used as a check on the product analysis and differential thermal analysis was used to compare heats of reaction.

2. EXPERIMENTAL

2.1 Gene&

information

All gases were supplied by BOC Special Gases. Elemental analyses were carried out using a Carlo Erba 1106 Elemental Analyser (C, H, N) and a Carlo Erba 1108 Elemental Analyser (0). The carbon-l 3 material was supplied by the Aldrich Chemical Company and was subjected to heat treatment in argon (13C-Ar) or in ammonia (r3CN) to introduce more nitrogen into the carbon. These samples contained 0.6 and 1.7 wt% N, respectively. For the study of N,O and N, emission, the 13C was first heated to 1073 K at the rate of 15 K mini’ with 30 minutes soak time before treatment with NO and 0, at the required temperatures.

2.2 Flow injection-thermal spectrometry (FITAMS)

analysis-mass

Reactant gases were injected, through a GC septa in a stainless-steel cross-piece connector, into an argon flow (50 cm3 mini ‘) at the entrance to the furnace of a Stanton Redcroft 1500 thermogravimetric analyser (TGA) using a World Precision Instrument SP200 Series syringe pump and SGE gastight syringes. The experimental arrangement is shown in Fig. 1. In principle, the syringe pump allows variable flow rates in the range 3.3 x lO~‘~71 cm3 mini r, although a far more modest range of 0.1-5 cm3 min - ’ was used here. This translates to a volumetric flow rate of 0.002-0.091 cm3 mini’ (or 0.2-9.1 ~01% in argon). Up to four different gases could be injected simultaneously using this system. The TGA was connected to a VG Quadrupoles 300 amu mass spectrometer via a sampling probe, which consisted of a 1 mm diameter stainless-steel tube lined with a deactivated fused-silica capillary. Up to 15 different species were monitored simultaneously and scanned approximately every 20 seconds throughout the course of the experiments. Thus, gas composition profiles as a function of time were obtained for the injections. Two sampling positions were possible, as shown in Fig. 1: (1) with the probe sampling at a height of - 1 cm directly above the sample-“direct sampling”, and (2) with the probe sampling the exhaust gases of the TGA-“exhaust sampling” [4]. These two positions allow the detection of reactive (shortlived) intermediate species and near-equilibrium gas compositions, respectively. Previous work [4] has shown that HCN, C2N, and COS were detected using direct sampling in the temperatureprogrammed combustion ofchars (HTT 1OOOC). In this study, the direct sampling position was used so

Flow injection-thermal

analysis-mass

spectrometry

219

To mass spectrometer &

Capillary probe for direct sampling

TGA furnace cross section To mass spectrometer c

Ca illary probe for ex Raust sampling -L

1

Gas tight syringe

Gas out

Gas in Fig. 1. Schematic

diagram

of the modified

TGA

furnace

for flow-injection

experiments.

NH3 or 20% O,/Ar. The reproducibility of the peak areas was excellent with an error of < 1%. The contact time, 0, for a particular injection was calculated from the equation: 0 = m/F, where m is the average mass of carbon during the injection (g), and F is the volumetric flow rate of the reactant (cm” s-r). For the injection of NO only, the NO conversion and yields of products were calculated in

that reactive intermediate species could be detected, although both sampling positions could be used. Good linear response was observed by mass spectrometric detection. Figure 2 shows the NO profiles during FITAMS of nitric oxide at 2.0 cm3 min 1 and 115°C and linear regression of the peak area vs volume of NO injected gave r2 = 0.998. Similar results were seen for the injection of other gases such as

3.5x1o-5 3.ox1o-5 0

-

2

2.5~10~~ 1 3.0cm-~ 6.0cme3

3 &

2.ox1o-5

5;

12.0cme3

i

z

1.5x1o-s

-

t= i

1.ox1o-5

-

0 2;

9.0 crne3

5.0x106

-

r

0.0 t 0

L 20

I

40

60

*

15.0cme3

I I

I

80

100

,

120

Time (min) Fig. 2. Response

profile

at m/z 30 for the injection of different volumes of nitric oxide at a flow rate of 2 cm3 min-’ furnace temperature of 115°C.

and a

.I. M.

220 the following

JONES&~

way:

NO conversion

NO (%) =

BANG + AN*O

1.0x10-2

I

x 100

2.4~2 + AN,O + A,, N, yield (%) =

2.4~~2

x 100

2&+&o

N,O yield (%) =

A

N20 ~AK;L+&o

0

x 100

10

20

30

Time (min)

where A N, =N, peak area, ANzO=N,O peak area assume and A NO=NO peak area. These equations that N,O is formed by the reaction of NO with char nitrogen, while N, is formed by the reaction of two NO molecules. 3. RESULTS AND DISCUSSION

Time (min) 3.1 The carbon-nitric oxide reaction Typical gas-evolution response profiles during FITAMS of NO (25.0 cm3 at 5.0 cm3 mind’) are shown in Fig. 3. At this temperature of 700°C very little thermal decomposition of NO is observed. A signal at m/z 44 is present which is thought to be due to N,O impurity in the NO gas. In the presence of char ammonia-treated carbon- 13 model an (r3CN), significant reaction occurs. CO, and N, are the major products and are formed in a 1: 1 stoichiometry. It is interesting to note that a small amount of N,O was also formed during this reaction. The conversion of NO to N, and N,O is 18.9%, while the yields of N, and N,O are 98.2 and 1.8%, respectively. The conditions used for this experiment result in an approximate contact time of NO with the since carbon of 2.0 g s cmd3, (This is approximate the mass of the carbon changes during the course of the injection.) To explore further the relationship between contact time and conversion of NO to Nz and NzO, a number of injections of differing NO volumetric flow rates were made. To ensure that data of equal quality was obtained at each flow rate, the volume of NO was also varied, so that the actual time of each injection remained constant. As discussed in Section 1, a change in mechanism of the reaction of NO with carbon is seen at above approximately 650.‘C. Therefore, two reaction temperatures, 500 and 7OOC were studied. The response profiles are shown in Figs 4(a) and (b). Comparison of Figs 4(a) and (b) demonstrates the effect of temperature on this reaction. More N, (m/z 28), 13C02 (m/z 45) and i3C0 (m/z 29) are produced at the higher temperature, while the amount of N,O (m/z 44) formed is far less affected by temperature. Figure 5 shows the product distributions as a function of contact time for these two temperatures. In the chemical-controlled kinetic region, relatively large changes in conversion are expected as a function of contact time. In contrast, in the diffusion-controlled

0

10

20

3b

20

io

Time (min)

0

10 Time (min)

Fig. 3. Gas-evolution profiles during FITAMS of nitric oxide (25.0 cm3 at 5.0 cm’ min ‘) over -I -. no sample and -o-o-o-, carbon-13 at 700’C.

kinetic region much smaller changes in conversion with contact time are expected. Thus it is clear that at 5OO’C the change from chemical to diffusion time kinetics is at a contact controlled > 15 g s-l cm13, while at 700°C the change over is at much shorter contact times because of the increased rate of diffusion at higher temperatures. As expected, higher NO conversions are apparent at higher temperatures. The major nitrogen-containing product at both temperatures was N,, while N,O was only formed in very minor amounts. More N,O is formed at lower temperatures which is probably due to the fact that the N,O decomposition reaction is appreciable at the higher temperature. This is demonstrated in Fig. 6, which shows peak areas as a function of temperature during FITAMS of N,O (25 cm’, 5.0cm3 min-‘) in the absence of

Flow injection-thermal

7x1o-5

c (4

analysis-mass spectrometry

o,70.8 fl I.Ocsmih 7x1o-5

0.6

221

(b) 0.6

6~10~

&&$$

5x1o-5 $? 2 z & 4xl05

ti

N,xZ

B .9 2

3x1o-5

z” 2x1o-5

2x10S5

1 l-l ,-jJ

rl

mh29xX

1x1o-5 -

lxlo-5

c

0

^,

50

m/z 29 x2C

--7

,

m/z 44 x50 ,

-,-,,?-,”

100

150

0

200

100

1;o

Time (mitt) Fig. 4. Gas-evolution

profiles during

200

Time (mm)

FITAMS

of nitric oxide at varying

flow rates over carbon-13

at (a) 500°C and (b) 700°C.

0.10 0.08 cd

2 0.06 Kl ti 2 0.04 a IO

20

30

40

50

60

0.02

’ 0.00

500

600

Temperature

0

10

20

Contact

30

40

50

60

time (g.s.ml-‘)

Fig. 5. Effect of contact time on the percentage conversion of NO to N2 and N,O over carbon-13 at 500 (0) and 700 (M)“C.

700

800

(“C)

Fig. 6. Thermal decomposition of N,O; variation of peak area with temperature as seen by FITAMS (25.0 cm3, 5.0 cm3 min-I). -H-, N,O; -0-, N,; -X -, Oz. NO peak areas for analogous NO injections are also shown (A).

any sample. The analogous FITAMS results for thermal reactions of NO is also given for comparison. Interestingly, N,O is only apparent in the gasevolution profiles in the chemical control region. At lower flow rates (diffusion control) no NzO is detected. N,O is formed in minor amounts from the reaction of NO with ‘3C-N, as shown in Fig. 3. However, the gas-phase decomposition of N,O and the N&carbon reaction both occur at appreciable rates in this temperature range. These secondary

J. M.

222

reactions probably account for the decreasing N,O yield with increasing contact time. In any case, N,O appears to be produced in small but significant yield only under chemical control at low temperatures (5OO”C), while N, is the primary product under diffusion control at higher temperatures 700°C. Combustion of carbon [27] is usually considered to arise from the dissociative adsorption of oxygen molecules at active sites on the carbon surface. This reaction is thought to involve localised (1) and mobile (m) surface complexes which are involved in the following surface reactions: or m+-CrfCO

-C(O), -C(O),

0I m+ -C(O),-+

(1) -c,+co,

(2)

where C, is a free active site. An analogous model can be employed to explain the gasification of carbon by NO in terms of surface complexes. When diffusion is rate-limiting, N, and CO, are the major products. While under chemical control both N,, and N,O as well as CO, and CO are formed. Adsorption of NO onto carbon at high temperature is thought to be dissociative [ 171, resulting in the formation of surface complexes which desorb to form primarily NZ and co,: 2-C,+2NO-t2-C(N)+2-C(0) +N2+C02+

(3)

-C,

We proposed that at higher gas-phase concentrations of NO, N,O forms via an intermediate involving the associative adsorption of NO. -C(NO),+

-C(N)-+N,O+2-Cr

(4)

CO can also be formed by the desorption of surface oxide species. Note that C(N) can also represent nitrogen functionality inherent to the carbon, and it is intended to examine the effect of nitrogen content in the carbon on the formation of N,O. This simple model can explain the concentration dependence on the formation of N, and N20 in both the diffusion and chemical control regions. It would be interesting to explore this model further; at even higher concentrations of NO, N, formation may be expected to be favoured by the following reaction: -C(NO),

or 1-t -C(NO),

0r ,+Nz+CO,+

-Cr (5)

or -C((NO)&+Nl+C02+

-Cr

(6)

This type of process has been proposed to account for the release of N, during the gasification of carbons in oxygen [S]. Such a mechanism involving the (NO), dimer has been proposed for the low-temperature adsorption of NO [ 151. Subsequent reaction of N,O by: N,O+ or by thermal

-Cr+N,+

decomposition

-C(O) is probable.

(7) Both

of

JONES

et al.

these reactions are more significant at higher temperatures, as are other gas-phase reactions of N,O [28].

3.2 Eflect of oxygen on the jbrmation during the NO-char reaction

of N,O

In the absence of oxygen only very small amounts of N,O are formed in the NO-carbon reaction. The N20/N2 gasification product ratios obtained for injection of NO and NO/O, mixtures over 13C are given in Table 1. Carbon dioxide is the other major product of the reaction. Figure 7 shows the trend of increasing N,O/N, with increasing oxygen concentration. In these experiments the gases were sampled directly above ( - 1 cm) the sample so that contributions from gas-phase reactions were kept to a minimum. There is also a trend of decreasing N,O!N, with increasing temperature, illustrating the fact that the thermal decomposition of N,O and its reduction to N, by carbon are both enhanced by increasing temperature. The N,O/N, ratios obtained on a fresh sample in the absence of oxygen were similar to those obtained from the same sample after successive treatments with varying concentrations of NO in the presence of oxygen. For example, N,O.!NZ for a fresh sample was 0.043, while the value for a treated sample (carbon burn-off=27.5%) was 0.047. This shows that changes in the porous structure during the gasification reaction did not affect the N20/N, product ratio for the NO-carbon reaction in the presence of oxygen. Table 2 shows relative values of the heat of reaction (calculated from the DTA traces) for varying oxygen concentrations at each reaction temperature. The NO-carbon reaction is exothermic. A typical DTA trace for the NO-carbon reaction in the presence of oxygen is shown in Fig. 8. The DTA trace for a blank run, also shown, indicates that any interaction between reactant gases has not affected the heat of reaction obtained for the NO-carbon reaction. The heat evolved and amount of carbon gasified per injection at a given reaction temperature increase with increasing oxygen content, However when the heats of reaction are normalised to the amount of carbon gasified as a result of the injection, the trend is less clear The results suggest a possible minimum, but further refinement of the technique is required to establish this trend unequivocally. The heat of reaction decreases with increasing temperature for the range of NO/O2 ratios studied. The extent of burnoff varies with each injection, and while the heats of reaction have been normalised to the weight loss for a particular injection, some differences may arise with carbon burn-off. It is apparent that this technique can also be used in the simultaneous measurement of relative heats of reaction. The NO reduction probably involves the following surface reactions (3))( 6) in Section 3.1. The presence of oxygen favours the higher concentration of

Flow injection-thermal

analysis-mass spectrometry

223

Table 1. The variation of N,O:N, with the oxygen and NO concentrations Gas composition (%) (balance argon)

Temperature (K)

NO

0,

823

873

953

973

1073

4.76 4.29 3.80 3.33 2.86 1.90 1.43 0.95 0.48

0.00 0.48 0.95 1.43 1.90 2.86 3.33 3.81 4.29

0.040 0.127 0.120 0.170 0.133 0.223 0.297 0.320 0.810

0.045 0.063 0.073 0.067 0.055 0.164 0.250 0.220 0.724

0.047 0.029 0.021 0.048 0.055 0.069 0.106 0.177 0.333

0.042 0.042 0.048 0.040 0.040 0.059 0.118 0.210 0.261

0.041 0.029 0.021 0.021 0.020 0.025 0.068 0.112 0.217

0.8

Concmration

Of oxygen(%)

0

Fig. 7. The variation of N,O:N* with oxygen concentration during the NO-carbon

reaction at different temperatures.

Table 2. Values of the relative heats of reaction (arbitrary units) for the NO-carbon reaction at different gas concentrations and temperatures Gas composition (%) (balance argon)

Temperature (K) 873

953

973

1073

Relative heats of reaction

NO

02

4.76 4.29 3.80 3.33 2.86 1.90 1.43 0.95 0.48

0.48 0.95 1.43 1.90 2.86 3.33 3.81 4.29

0.00

8.80 7.02 6.14 6.54 6.10 6.25 5.93 7.85 6.92

7.96 5.63 5.12 5.00 5.34 5.16 5.06 6.44 5.84

7.01 5.15 4.65 5.51 4.90 5.03 4.50 6.08 6.26

3.65 3.77 3.12 3.68 3.92 4.07 3.53 3.75 4.08

The required amounts of nitric oxide and oxygen were injected simultaneously into the reaction furnace over a period of 6 min. -C(NO)

by the surface

-C(O)+ CARBOH 35-2-o

reaction

-C(N)-+-Cf+

-C(NO)

(8)

Fig. 8. Typical DTA traces for the NO-carbon reaction in the presence of oxygen. A, typical DTA trace for the NO-carbon reaction in the presence of oxygen. B, typical DTA trace when NO and oxygen are injected into the reaction furnace without carbon.

This favours the formation of N,O by surface reaction (4). Although there are reports of the effects of oxygen concentration on the conversion of coal-nitrogen to NsO during coal combustion [29-341, the inherent

J.M.Jo~~setal

224

contribution of the NO-carbon reaction to the overall N,O formation has not been fully investigated. It is evident from the results presented here that an increase in oxygen concentration increases the formation of N,O during the reduction of NO by carbon.

3.3 Reduction

of NO by ammonia

A number of experiments have been conducted using FITAMS to investigate the reaction of NO and NH, over carbon. At 200°C only SNCR of NO was apparent, N, and H,O were both detected at m/z 28 and m/z 18, respectively. This is thought to be due to firstly the small amount of carbon (< 5 mg) present in the TGA furnace, and secondly the configuration of the system, in which the gas phase passes over the surface of the sample rather than through its bulk. At higher temperatures, the results were quite different, however. The results of both the individual and the simultaneous injection of NO and NH, at 4OO”C, both in the absence (thermal) and presence of a carbon- 13 (13C-N) sample, are shown in Fig. 9. No reaction of the carbon was detected at

(a>

this temperature, i.e. no peaks at m/z 45 or m/z 29 due to 13C0, and 13C0 were detected. The detection of H,O during the injection of NH, only may be due to either the “NH, isotope, the tail of the intense m/z 17 (NH,) signal, or water impurity in the ammonia. Note that the m/z 18: m/z 17 ratio is 2.4 x lo-*, which is approximately ten times that expected on the basis of the natural isotopic abundance of 15N. Thus, it appears that the latter two explanations must account for the signal at m/z 18. Some thermal conversion of ammonia to N, is apparent from the small peak at m/z 28. The signal at m/z 44 observed during the injection of NO only is thought to be due to a small N,O impurity, as discussed in Section 3.2. In the case of simultaneous injection of NO and NH3, both SNCR and SCR are apparent. In the absence of a carbon sample, both N, and H,O are detected (Fig. 9(a)). Note that the amount of H,O detected is far greater than that seen upon injection of NH, only, demonstrating that it is a product of the gas-phase reaction of NO and NH,. Similarly, a

N2

Time (min) Fig. 9. Gas-evolution

profiles during FITAMS of NO (-O-O-O-), NH, (-O-O-O-) and NO+NH, (thermal reactions) and (b) carbon-13 at 400°C.

(-)

over (a) no sample

Flow injection-thermal

peak at m/z 28 due to Nz is also observed. This is thought to be formed by the reduction of NO with NH, radicals [35]. Interestingly, the gas-phase reaction of NO and NH3 also results in substantial N,O formation. These results can be explained by the following reaction sequences [22 1: NH,+NH,+H NH, + NO+N,

+ H,O

H,O-+OH+H NH,+NH+H NH+NO-+N20+H In the presence of the carbon- 13 sample (Fig. 9 (b)) similar peaks at m/z 28 and 18 are seen during the injection of NH, only, which are assigned on the same basis as the analogous peaks observed during thermal injections. Similarly, for the injection of NO only, the m/z 44 peak is still present, although slightly smaller, which presumably arises from the reaction of the N,O impurity with the carbon. Simultaneous injection of NH, and NO produces peaks at m/z 18 and m/z 28. The m/z 28 peak is 100 times the size of that formed during thermal, SNCR, reaction. This is indicative of SCR over the carbon-13 sample. The catalytic activity of carbon is thought to be due to activation of NO towards reduction by ammonia through its adsorption on heteroatom sites on the carbon surface. 4. CONCLUSIONS

Flow injection - thermal analysis - mass spectrometry (FITAMS) has been shown to be a useful technique for probing complex heterogeneous chargasification reactions. Step-wise or simultaneous injection of different reactants allows the formation and reaction of surface species to be probed. Changing reaction variables is facile using this method, so that a vast amount of information can be obtained on a small amount of sample in a short period of time. Also, the contributions of homogeneous and heterogeneous reactions can be distinguished using different sampling positions. The small amounts of solid and gaseous reactants required and the ease of containment of materials means that FITAMS also lends itself to isotopic labelling studies. This was shown by a study of the effect of oxygen concentration and reaction temperature on the product distribution in the NO-13C reaction. The N,0:N2 ratio increases with increasing oxygen concentration but decreases with increasing temperature. The relative heats of reaction were also measured simultaneously to the product analysis and weight loss using this technique. Acknowledgements-The authors would like to thank the EPSRC and ECSC for funding this work under grants CR/479426 and 7220-ED/016, respectively.

analysis-mass

spectrometry

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