Non-noble metal environmental catalysts: Synthesis, characterization and catalytic activity

A. Frennet and J.-M. Bastin (Eds.) Catalysis and Automotive Pollution Control I11 Studies in Surface Science and Catalysis, Vol. 96 9 1995 Elsevier Science B.V. All rights reserved.

487

NON-NOBLE METAL ENVIRONMENTAL CATALYSTS: SYNTHESIS, CHARACTERIZATION AND C A T A L Y T I C A C T I V I T Y

Philip G. Harrison

Nicholas C. Lloyd and W a n Azelee

Department of Chemistry, University of Nottingham, UniversityPark, Nottingham NG7 2t?D (U.K.) ABSTRACT

The preparation, characterization, and catalytic activity of Cr(VI)- and Cu(II)doped tin(IV) oxide catalysts are described. The catalysts, particularlythe mixed SnCu-Cr-O catalysts, exhibit comparable activity to conventional platinum catalysts for CO and hydrocarbon oxidation.

1 INTRODUCTION

The control of noxious emissions resulting from either the combustion of fossil fuels or from other industrial activities is one of the most immediate and compelling problems faced by nearly every cotmtry in the world. Environlnental problems caused by two sources, automobile exhaust emissions and flue emissions from coal and oil fired power stations, have received much publicity over recent decades. Emission problem arising from both automobile and stationary industrial internal combustion engine give rise to similar pollutants: carbon monoxide (CO), hydrocarbons (HC's), and nitrogen oxides (NOx). For automobiles, these are now the subject of ever increasingly stringent legislation controlling the maximum permitted levels of emissions of each substance [1,2]. Platinum group catalysts currently represent the state-of-the-art in internal combustion engine emission teclmology. Current usage of

488 rhodium and platinum in these catalysts exceeds the Rh/Pt mine ratio, mad hence it is essential to reduce consumption in order to conserve the limited noble metal supply. It would be very beneficial, therefore, to reduce dependence on noble metals for catalytic converter usage and to seek viable alternative catalytic materials. The driving force for the development of non-platinmn exhaust emission catalysts is the price, strategic importance and low availability of the platinum group metals. Our studies have shown that catalysts based on tin(IV) oxide (SnO2) promoted with chromium and/or copper (CrSnO2 and Cu-Cr-SnO2 catalysts) which exhibit excellent three-way catalytic activity - activity which is comparable to that shown by noble metals dispersed on alumina. This family of materials offers tremendous promise as cheap and efficient catalyst systems for the catalytic conversion of noxious emissions from a variety of sources. In this paper we describe three aspects of this family of enviromnental catalysts: (1) the synthesis, (2) their characterisation, and (3) their catalytic activity. Knowledge of the solid-state chemistry of catalysts is invaluable to an understanding of their catalytic behaviour. Characterisation of this type of mixed oxide presents a major problem and is notoriously difficult. No single technique can yield anything but a very small amount of information, and a complete picture can only be gained by using a combination of methods. We report data obtained from a number of techniques including gas adsorption, FT-Raman, EPR m~d ESEEM (electron spin echo envelope modulation), XRD, mid electron microscopy and electron diffraction.

2 CATALYST PREPARATION The catalysts have been prepared by a combination of coprecipitation and impregnation techniques, and also by sol-gel methods. Although coprecipitation constitutes one of the most widely applied methods for the preparation of oxide catalysts, more recently it has become apparent that sol-gel techniques offer several advantages including (a) greater control of catalyst stoichiometry and homogeneity, (b) more efficient and intimate particulate mixing at the nanometer level, (e) greater thermal stability towards deleterious solid state processes such as sintering, segregation of metal components to grain boundaries, and phase separation and (d) greater control of catalyst dispersion on inert supports.

489

Tin(IV) Oxide Gel and Sol-gel: Tin(IV) oxide gel was obtained by the ammonia precipitation method. Conversion to a stable sol-gel modification was effected by peptisation using choline.

Chromium(Vl)- and Copper(ll)-doped Tin(IV) Oxide To a suspension of tin(IV) oxide gel in triply distilled water was added a solution of chromium(VI) oxide and the solution stirred for 24h. The resulting yellow mixture was filtered and the yellow solid air dried at 60~ for 24h. If desired, the powdery material was then washed with distilled water until the washings were colourless. The resulting material was again air dried. Copper(II)-doped tin(IV) oxide was either prepared in a similar manner using a solution of copper(II) nitrate, or by coprecipitation from an aqueous solution containing tin(IV) chloride and copper(II) nitrate in the required ratio. Both chromiun(VI)- and copper(II)-doped catalysts were also prepared by treatment of tin(VI) oxide sol-gel using aqueous solutions of chromium(VI) oxide or copper(II) nitrate. Mixed Sn-Cu-Cr-O catalysts were obtained by the solgel route.

3 FUNDAMENTAL CHEMISTRY UNDERLYING CATALYST PREPARATION

3.1 The Nature of Tin(IV) Oxide Sols: Photon Correlation Spectroscopy The effect of added chromium(VI) oxide and copper(II) nitrate on the aggregation of a choline-stabilised tin(IV) oxide sol is illustrated in Figure 1. The choline-stabilised tin(IV) oxide sol exhibits an average particle size of ca. 700ran. However, addition of copper(II) nitrate to the tin(IV) oxide sol results in a decrease in the mean particle size. At a Cu:Sn ratio of 0.001 the mean particle size is 220ran rising steadily to a value of 400ran at a ratio of 0.02. In contrast, addition of even small amounts of chromium(VI) oxide to the sol has a dramatic effect. With a Cr:Sn ratio of 0.001, the mean particle size increases to 1250nm. Increasing the Cr:Sn ratio filrther results in a steady increase in the mean particles size until at a Cr:Sn ratio of 0.025, the mean particle size is 1850ran. At high metal:Sn ratios (>0.025 the tin sol is destabilised). 3.2 The Sorption of Chromate Species Onto Tin(IV) Oxide Gel Exposure of tin(IV) oxide gel to aqueous solutions of CrO3 result in the sorption of chromium species onto the oxide giving orange powders

490 after filtering and drying in air. The amotmt of chromium species sorbed onto the oxide is dependent on several variables including the concentration of the CrO3 solution, the temperature, the time of exposure, and the separation and washing procedures adopted. Figure 2 illustrates the effect of CrO3 solution concentration on the loading achieved after stirring the slurry for 16 hours at ambient temperature. For CrO3 solution concentrations in excess of ca. O.1M followed by filtering and drying but no washing, the loading achieved is linearly proportional to the concentration up to the high concentration studied (1M). Increased time of exposure results in an increased loading, whereas washing the catalyst dramatically decreases the loading thus indicating the weak nature of the adsorption. FT-Raman spectra in the range 700-1100 cm-1 (the v(Cr-O) region) of three Sn-Cr-O catalysts with Cr:Sn atom ratios of 1:0.12, 1:0.23 and 1:0.38 are illustrated in Figure 3. All three spectra are similar in form exhibiting two principal maxima together with several shoulders. However, the positions of the peaks maxima shift with the Cr:Sn ratio. At the lowest chromiuln loading the spectrum exhibits maxima at 886 and 946 cm-1, the former corresponds to the presence of CrO4- ions whilst the latter indicates the presence of Cr2072- m~ions (Table 1, assignments are by consistent with those made previously[3-5]. Other bands due to these

Table 1. Assignment of FT-Raman bands for chromate anions (cm-1)a. CrO4-

Cr2072-

Cr3Olo 2987

904

Vas(CrO2) vs(CrO2) Vas(CrO4)/(Cr03) vs(CrO4)/(CrO3)

844

8as(Cr'OCr")

956 886

942

848

904

Assignment

(a) Refs. 3-5. species are present as shoulder features, mad it is also probable that the shoulder at high wavenumber is due to a small amount of Cr3Olo 2- ion.

491 As the chromimn loading is increased, however, the peak maxima shift to 890/901 and 952 cm -1 for ratio 1:0.23 and 901 and 959 cm-1 when the ratio is 1:0.38. In both cases pronounced shoulder features are present both to higher and lower wavenumber. This observed shift to higher wavenumber is readily rationalised by an increased concentration of Cr2072- and Cr3Olo 2- anions in these catalysts, and CrO4- ions are present only in small amounts. Higher polychromate species cannot be excluded at high ratios. 3.3 The Physical Nature of the Sn-Cr-O Catalysts and the Effect of Calcination Nitrogen adsorption isotherms for the freshly prepared Cr(VI)/SnO2 catalyst and after calcination for 24h. at various temperatures up to 1000~ are shown in Figure 4. Numerical data are collected in Table 2.

Table 2. Nitrogen adsorption data calculated by the BET and as methods. Calcination BET Data

ot Data

Vp/cc g-1 d //l,

Temp./~

A s / m 2 g-1 C

A s / m 2 g-1 Vmic /cc g-1

60

114

167 125

0.007

0.057

18

300

96

425 114

0.008

0.053

19

600

58

16

0.059

40

1000

0.4

0.011

42

59

The specific surface area ( A s ) of the uncalcined Cr(VI)/SnO2 catalyst (BET, 114 m 2 g-l; as, 125 m 2 g-l) is considerably lower than that of tin(IV) oxide gel itself (185 m2 g-l). The specific surface area of the neat oxide decreases steadily with increase in temperature to a value of 40 m2 g-1 after calcination at 1000~ In contrast, that of the Cr(VI)/SnO2 catalyst decreases by ca. 15% to ca. 100 m2 g-1 after calcination at 300~ by ca. 50% to 58 m 2 g-1 after calcination at 600~ and falls drmnatically to nearly zero after calcination after calcination at 1000~

492 The BET isotherm plots for the Cr(VI)/SnO2 catalyst (Figure 4) show that the isotherm for uncalcined catalyst is typical for adsorption onto a mieroporous solid (type 1 isotherm). This form of isotherm is retained after calcination at 300~ but after calcination at higher temperatures the isotherm changed drastically in form. Calcination at 1000~ resulted in an isotherm characteristic of a type III non-porous solid. Whilst after calcination at 600~ the material exhibited intermediate behaviour and was mesoporous. The powder XRD spectra of freshly prepared (dried at 60 ~ samples of the Sn-Cr-O catalysts exhibit only broad diffuse lines characteristic of nanoparticulate tin(IV) oxide. These peaks are seen to gradually sharpen on calcination, but are still relatively broad after calcination at 600 ~. Only after calcination at high temperatures (ca. 1000 ~ is a characteristically sharp spectrum due to crystalline SnO2 obtained. When the concentration of chromium in the catalyst is low (eg for a Sn:Cr ratio of 0.015), the chromium in the catalyst remains amorphous and no chromium-containing phase can be detected. Only at high ratios (eg 1:0.13) and at high calcination temperatures (1000 ~ does a chromium-containing phase, Cr203, separate (XRD 2theta values at 24.5, 36.2, 41.56, 50.2, 63.5). Transmission electron microscopy of the Sn-Cr-O catalyst corroborates these observations. For example, the TEM micrograph of the catalyst with a Sn:Cr ratio of 0.015 at 600 ~ shows only the presence of small crystallites of tin(IV) oxide and no chromium-containing phase although chromium is detected by EDXa analysis. We deduce therefore that the chromium is present in an amorphous surface layer of an as yet m~mown composition on the crystallites of the tin(IV) oxide. 3.4 Preparation of Sn-Cu-O Catalysts - Coprecipitation or Sorption?

A key issue in the activity of these catalysts concerns the specific role of the Cu and Cr active sites which are expected to be located on the stLrface of the catalyst particles. Since both Cu(II) and Cr(III) are paramagnetic, EPR spectroscopy can be used to identify the nature of the heterometallic species within the catalyst matrix after different thermal and chemical treatments. However, a detailed picture of the local environment of the transition metal center calmot be obtained by conventional continuous wave EPR spectroscopy alone. These can, nevertheless, be obtained by pulsed EPR methods, namely the electron spin echo envelope modulation (ESEEM) teclmique.

493 Two types of catalyst, prepared by (1) impregnation of SnO2-gel by Cu 2+, and (2) coprecipitation have been examined. Both have the same composition of a Sn:Cu 2+ ratio of 1250:1. Although EPR is very sensitive to the surroundings of the paramagnetie cation, it cannot provide directly information concerning the fine interactions with the framework. Such interactions can, however, be measured by ESEEM. This teclmique is particularly useful for the measurement of weak hyperfine interactions. The modulation frequencies are the NMR frequencies of the coupled nuclei. In disordered systems, the modulation frequencies are essentially the Lannor frequencies of the coupled nuclei, which serve to identify the coupled nuclei. The modulation depth can be related to the distance between the electron spha and the coupled nuclei, and to their number. The ESEEM measurements described here were carried out at 4 K using an operating frequency of 9.1 GHz. Both two-pulse and three-pulse sequences were employed. The EPR spectra recorded at 120K of the SnO2-Cu 2+ catalyst sample prepared by impregalation (a) as prepared and (b) after calcination at 600 ~ show that a single Cu 2+ species with a cylindrical enviromnent is present in each case although with differing g and A values. At intermediate calcination temperatures both species exist. When a compositionally identical sample prepared by coprecipitation is examined, both species are observed in the freshly prepared sample even prior to calcination. Three peaks are observed in FT-ESEEM spectra of the SnO2-Cu 2+ catalyst sample prepared by impregnation, one at the Lannor frequency for hydrogen at ca. 13 MHz together with two peaks at the first overtone value. These spectra are very similar to the spectra of the aquo [Cu(H20)6] 2+ cation found in aqueous solution indicating that this is the species initially sorbed on to the tin(IV) oxide gel most probably v i a hydrogen bonding. In tile FT-ESEEM spectra for the stone sample calcined at 300~ The three features due to echoes from hydrogen still persist, but in addition new frequencies at the Lannor frequency of tin119 at ca. 5 MHz and overtones appear. This indicates that, unlike for the uncalcined sample where no echoes from tin are observed and hence the copper is at a distance >5A from any tin atoms, the result of calcination is to incorporate the copper ions into the tin oxide lattice. Calcination of the same sample at 600~ essentially removes all the initial [Cu(H20)6] 2+ sorbate, and only the lattice-bound copper species is present. The

494

01-12 H

H

H

~0

H

H

.0

H

H20" ..... [ ,,,,,,,' OH 2

_q I OI-t2 Cu.~,

H20~" H 9 H O

:o

I

l

.o.

H

H

H

~.

O %

.oo.o

9

[Cu2+]aq

H O

H O

H O

I

I

I

I

H

H

I

I

~,..9 >5A

Sn O Sn O Sn O

Sn O Sn O Sn O Sn

IIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIII IIIIIIIIHIIIIIIIIIIIIIIIIIII

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII HIIIIIIHIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

room temperature

calcination at 300~

OH 2

H20%' ! '"'""OH2 H20~ ~'~' [ u ~ O H 2

.o.

H 0

0

0

0

I

I

I

I

calcination at 600~ -~

I:I

H

O

()

O

O

O

O

I

I

I

I

I

I

Sn O Sn O Sn O Sn O Sn O Sn .,.,,.,,,,,,.,.,a,,,,.,,,,.,,,,,,,,,,,,,,.,,,,..,.,,,,

Sn O Sn O Sn O Sn

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

IIIIII IIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIII IIIIIIIII

Sn O Sn O Sn O Sn O C u O S n IIIUlIIIIIUlIUlIIIIIIIIIIIIUlIIIIUlIIIIIIIIIIIIIIIIUlIUlII

Sn O Sn O C u O S n

III I IIIIIIII IIIIIIIIIIIIII II III IIIIIIII I I

Scheme 1

495

processes are illustrated schematically in Scheme 1. No evidence for any intermediate covalently bound surface copper species could be detected. In contrast, FT-ESEEM spectra of the second sample, that prepared by coprecipitation, exhibit both hydrogen mad tin-119 frequencies for the freshly prepared sample. The conclusion, therefore, is that preparation of SnO2-Cu2+ catalyst smnples by coprecipitation results in the incorporation of copper into the tin(IV) oxide lattice as well as sorption of [Cu(H20)6] 2+ cations on to the oxide particle surface.

4 CATALYTIC ACTIVITY

Catalytic activity studies were carried out using a typical catalytic microreactor. Catalyst sample sizes were 0.5g for CO oxidation studies and 2.0g for n-propane oxidation studies. Flow rates of the gas mixtures, which varied from stiochiolnetric to oxygen-rich, were in the range 90100 ml min -1. Table 3 SUlmnarises the activity of selected Sn-Cu-O, SnTable 3. Temperatures necessary for the complete removal of CO and n-propane over Cu-Cr-Sn-O catalysts together with specific activities

Catalyst

SnO2

T100(CO) /oca

TI00(C3H8 /ocb

Spec. Act.

(co)c

Spec. Act. (C3H8) c

0

360

525

0.06

71Sn 29Cu-O

150

400

0.53

0.82x10-3

77Sn-23Cr-O

250

280

0.072

8.4x10-3

62Sn-19Cu-19Cr

175

310

0.43

5.9x10-3

200

400

0.064

1.6x10-3

Pt/AI203

(a) (b) (c)

Temperature required for the complete removal of CO. Temperature required for the complete removal of C3H8. Moles converted/g catalyst/h at 300~ at complete conversion.

496 Cr(VI)-O and Sn-Cu-Cr(VI)-O catalysts for the oxidation of CO mid npropane compared with tin(IV) oxide itself and a comparable aluminasupported platinum catalyst. Activity towards oxidation of CO of the catalysts is good if copper is present with specific activities which are approximately ten times that of a comparable alumina-supported Pt catalyst. Light-off temperatures tend to be lower than 100 ~ with complete conversion occurring below c a . 150 ~. In contrast, catalysts containing chromium(VI) enhance the conversion of propane, and the Sn-Cu-O catalysts performed less well than either the Sn-Cr(VI)-O or Sn-CuCr(VI)-O catalysts. Again, specific activities compare favourably with the alumina-supported Pt catalyst. The best performance is exhibited by the mixed Sn-Cu-Cr(VI)-O catalysts which are efficient for the removal of both CO and propane. Figures 5 and 6 illustrate the comparative performances of the tin(IV) oxide catalysts together with data for commercial copper chromite for both the conversion of CO and propane. REFERENCES

EC Directive Dir. 88/76/EEC, December 1987. See also Directives Dir. 88/436/EEC, 16 June 1988, and Dir. 89/458/EEC, 18 July 1989. EC Communication COM (89) 662, 2nd February 1990. G. Michel and R. Machiroux, J. Ramma Spectrosc., 1983, 14, 22; 1986, 17, 79. F. Gonzalez-Vilchez and W.P. Griffith, J. Chem. Soc., Dalton Trans., 1972, 1417. M.A. Vuunnan, Doctoral Dissertation, University of Amsterdam, 1992.

ACKNOWLEDGEMENTS: We thank the Science and Engineering Research Council and the Govenunent of Malaysia for support. Dr. Daniella Goldfarb mid Mr Khalid Matar for the EPR and ESEEM measurements, and Dr. Carole C. Harrison for assistance with the electron microscopy.